Patent Application
20250059076
Ammonia (NH3), phosphorus (P), and potassium (K)-based fertilizers play a vital role in agricultural production to meet the food demand for more than 3 billion people worldwide. The production processes of these critical substances are, however, energy-intensive and account for significant CO2 emission. In particular, ammonia production via the conventional Haber-Bosch process accounts for ˜1.4% global CO2 emission. Apart from ammonia, phosphorus and potassium are also essential micro-nutrients for the growth and yield of plants and crops. Raw materials for phosphorus- and potassium-based fertilizer are produced by mining different non-renewable rocks and mineral ores, and the mining processes also emit substantial amounts of CO2. Meanwhile, excessive amounts of nutrients discharged to the environment from wastewaters can result in harmful algal blooms in receiving waters, causing adverse impacts on aquatic ecosystems, fisheries, recreation, water supplies, and public health. In a watershed, nutrient sources may be identified as a point of discharge (i.e., point sources) or diffuse in nature (i.e., nonpoint sources). Nutrient loads of effluents discharged from wastewater treatment plants represent a significant point source and can be reduced through nutrient recovery from the wastewaters. In particular, digestate waters from the anaerobic digestion (AD) processes have high ammonia (primarily as ammonium, NH4+), phosphorus (P), and potassium (K) contents from which nutrients can be effectively recovered. The recovered nutrients can therefore be a sustainable alternative source that helps reduce energy consumption of fertilizer production and decarbonize the agricultural sector which is associated with 11-15% of total GHG emissions.
A developed nutrient recovery method for ammonium and phosphorus from concentrated wastewater involves chemical precipitation of struvite (MgNH4PO2·6H2O) under alkaline conditions (pH 8.5-9.5). However, this approach requires pH-adjusting alkaline chemicals as well as Mg and P salts due to excessive NH4+. There is currently no developed technology for recovering potassium (K+) from concentrated wastewater.
Vivianite (Fe3(PO4)2·8H2O) is a valuable product which has simple formation conditions and can be potentially used as a P fertilizer. Vivianite precipitation/crystallization has been observed in municipal wastewater treatment plants under circumneutral pHs and reducing environment and accounted for 40%-50% of total P precipitation in an iron-based removal process. Vivianite has slow oxidation kinetics and can be readily recovered by a magnet. Therefore, using vivianite precipitation could be a cost-effective method for P recovery from AD digestates. However, reaction conditions optimal for vivianite precipitation (e.g., pH and Fe/P stoichiometry) from digestate and their effects on minerology remain uninvestigated.
In addition to ammonium and phosphorus, AD digestates also contain a significant amount of organic carbon and therefore a technology that could selectively recover ammonium (as NH3—N) while excluding organic carbon is preferred. In fact, recovering ammonium, phosphorus and potassium in separate products provides multifaceted benefits including flexibility in fertilization at an optimal N:P:K ratio that suites the soil and crops, and less nutrient losses to runoff. Membranes, via their size and charge-based exclusion mechanisms, could enable such separation. In addition, the energy consumption of membrane-based separation is significantly lower than other competing separation techniques. Membrane-based separation can also provide a sustainable approach to recover ammonium (NH3—N) from organic carbon in a manner that requires no additional chemical dosing/pH adjustment. Previous studies have shown that nanofiltration (NF) membranes (e.g., NF270) can be used to recover ammonia from wastewater, however, the selective separation of ammonia from waste streams with high organic carbon content (measured in the form of total organic carbon, TOC) remains uninvestigated.
Usually, pH of the concentrated wastewater or digestate is close to 7 and recovering nutrients N, P, and K at this pH condition provides multifaceted benefits such as preservation of high nutrient contents when utilized in soil, no pH-changing chemical costs, etc. Alteration of pH increases soil toxicity under both acidic and basic pH conditions. Although limited research has been conducted to recover nutrients (N, P, K) by utilizing commercial NF membranes under both acidic and basic conditions, such membranes are not suitable at neutral pH conditions and therefore, rejects significantly high amounts of both NH4+ and K+.
Despite advances in wastewater treatment research, there is still a scarcity of systems and methods for removing ammonium, phosphorus and potassium from waste streams which contains high organic carbon content. An ideal method would also enable facile and efficient recovery of valuable products, including vivianite, ammonium and potassium, following separation, without the need for additional chemical manipulation. An ideal method could further make use of commercially available membranes following surface modification rather than building entirely new devices, and could operate at neutral pH. These needs and other needs are satisfied by the present disclosure.
In accordance with the purpose(s) of the present disclosure, as embodied and broadly described herein, the disclosure, in one aspect, relates to system for recovering ammonium, phosphorus and potassium from a waste stream, methods of using the system to precipitate phosphorus as vivianite and to separate ammonium and potassium from total organic carbon in the waste stream, methods of modifying a nanofiltration membrane to exhibit selectivity for ammonia passage relative to total organic carbon passage, and compositions including fertilizers produced using recovered ammonia, phosphorus, and optionally potassium from the waste stream. The nutrients N, P, and K can be recovered at neutral pH, which can be advantageous as it preserves high nutrient contents for agricultural and other activities.
Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. In addition, all optional and preferred features and modifications of the described embodiments are usable in all aspects of the disclosure taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.
Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
Additional advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or can be learned by practice of the invention. The advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
Disclosed herein is a hybrid method (
System for Recovering Ammonia and Phosphorus from a Waste Stream
In one aspect, disclosed herein is a system for recovering ammonia and phosphorus from a waste stream, the system including at least the following two components:
Herein “ammonia” and “ammonium” are used interchangeably to refer to uncharged (NH3) and charged (NH4+) inorganic ammonia and/or ammonium species.
In some aspects, the system further includes a pre-filter to remove large particles from the wastes stream. In a further aspect, the filter can include a 0.45 μm polyethersulfone membrane.
In a further aspect, the system includes a means for separating and recovering the precipitated vivianite such as, for example, a filter, a magnet, or any combination thereof. In an aspect, the ferrous salt can be or include ferrous sulfate heptahydrate.
In another aspect, the second stage can be operated as a dead-end (batch) filter or a cross flow (continuous) filtration system.
In one aspect, the one or more polyelectrolytes can be or include poly(diallyl dimethylammonium) chloride (PDAC), poly(sodium 4-styrenesulfonate) (SPS), poly(allylamine hydrochloride) (PAH), poly(acrylic acid) (PAA), or any combination thereof. In some aspects, the membrane further includes sodium chloride in contact with the one or more polyelectrolytes. In any of these aspects, the one or more polyelectrolytes may be present on only a first side of the membrane.
In an aspect, the one or more polyelectrolytes can be PDAC and SPS in alternating layers. In a further aspect, an outermost layer of the alternating layers can be PDAC and can be positively charged, or is SPS and can be negatively charged. In an alternative aspect, the one or more polyelectrolytes can be PAH and PAA in alternating layers. Further in this aspect, the outermost layer can be PAH and can be positively charged. In a still further aspect, the outermost layer can be modified with glutaraldehyde to form a dense polyelectrolyte selective layer and reduce the effective porosity of the selective layer. In some aspects, surface charge analysis can be conducted and may show a negative surface charge.
In any of these aspects, the nanofiltration membrane can be constructed from a support fabric, a porous layer having a first side in contact with the support fabric, and a polymer coating in contact with a second side of the porous layer. In one aspect, the support fabric can be a nonwoven fabric. In another aspect, the porous layer can be made from or include polyethersulfone, polysulfone, or any combination thereof. In an aspect, the porous layer can be about 50 μm thick. In a further aspect, the polymer coating can be or include polypiperazine, polyamide, or any combination thereof, and can have a thickness of less than about 200 nm.
In one aspect, the system can be operated at a temperature of up to about 45° C. In a further aspect, this represents an advantage over current methods for production of ammonia, which require temperatures up to 400-500° C. and pressures of over 100 bar as in the Haber-Bosch process. In another aspect, the system can be operated at a pressure of up to about 41 bar, or can be operated at about 6.9 bar. In still another aspect, the system can be operated at a pH of from about 2 to about 11. In a further aspect, although the system can successfully operate in this pH range, operation at neutral pH allows for successful operation of the system and completion of the disclosed method and thus reduces the need for pH adjustment of the waste stream allows for operation of the system under milder conditions than those known for struvite precipitations.
Method for Recovering Ammonium and Phosphorus from a Waste Stream
In one aspect, disclosed herein is a method for recovering ammonium and phosphorus from a waste stream, the method including at least the steps of:
Further in this aspect, the method can include anaerobically digested municipal sewage sludge. In one aspect, the waste stream includes from about 350 to about 685 mg/L of total ammonium, up to about 100 mg/L of total phosphorus, and from about 250 to about 750 mg/L of total organic carbon. In some aspects, in the disclosed method, the molar ratio of ferrous salt to total phosphorus in the waste stream can be about 2:1. In one aspect, substantially all of the phosphorus in the waste stream can be recovered using the disclosed system and method.
In one aspect, the method rejects at least about 75% of the total organic carbon in the waste stream, or at least about 85% of the total organic carbon in the waste stream. In another aspect, the method rejects less than about 50% of the total ammonium in the waste stream, or less than about 30% of the total ammonia in the waste stream. In a further aspect, the method recovers at least about 80% of the total ammonia in the waste stream. In some aspects, the method is at least about 3 times more selective for ammonia passage through the membrane than for TOC passage through the membrane, or is at least about 4.5 times more selective for ammonia passage through the membrane than for TOC passage through the membrane.
In some aspects, the membrane further recovers at least a portion of dissolved potassium in the waste stream. In one aspect, the method is at least about 1.25 times more selective for ammonium passage through the membrane than for potassium passage through the membrane. In any of these aspects, the method can recover at least about 70% of the dissolved potassium from the waste stream.
Also disclosed herein is a composition including ammonium, phosphorus, potassium, or any combination thereof recovered using the disclosed methods. In a further aspect, the composition can be or include a fertilizer.
Many modifications and other embodiments disclosed herein will come to mind to one skilled in the art to which the disclosed compositions and methods pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the disclosures are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. The skilled artisan will recognize many variants and adaptations of the aspects described herein. These variants and adaptations are intended to be included in the teachings of this disclosure and to be encompassed by the claims herein.
Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure.
Any recited method can be carried out in the order of events recited or in any other order that is logically possible. That is, unless otherwise expressly stated, it is in no way intended that any method or aspect set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not specifically state in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including matters of logic with respect to arrangement of steps or operational flow, plain meaning derived from grammatical organization or punctuation, or the number or type of aspects described in the specification.
All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided herein can be different from the actual publication dates, which can require independent confirmation.
While aspects of the present disclosure can be described and claimed in a particular statutory class, such as the system statutory class, this is for convenience only and one of skill in the art will understand that each aspect of the present disclosure can be described and claimed in any statutory class.
It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosed compositions and methods belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly defined herein.
Prior to describing the various aspects of the present disclosure, the following definitions are provided and should be used unless otherwise indicated. Additional terms may be defined elsewhere in the present disclosure.
As used herein, “comprising” is to be interpreted as specifying the presence of the stated features, integers, steps, or components as referred to, but does not preclude the presence or addition of one or more features, integers, steps, or components, or groups thereof. Moreover, each of the terms “by”, “comprising,” “comprises”, “comprised of,” “including,” “includes,” “included,” “involving,” “involves,” “involved,” and “such as” are used in their open, non-limiting sense and may be used interchangeably. Further, the term “comprising” is intended to include examples and aspects encompassed by the terms “consisting essentially of” and “consisting of.” Similarly, the term “consisting essentially of” is intended to include examples encompassed by the term “consisting of.
As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a membrane,” “a waste stream,” or “an electrolyte,” include, but are not limited to, mixtures or combinations of two or more such membranes, waste streams, or electrolytes, and the like.
It should be noted that ratios, concentrations, amounts, and other numerical data can be expressed herein in a range format. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as “about” that particular value in addition to the value itself. For example, if the value “10” is disclosed, then “about 10” is also disclosed. Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms a further aspect. For example, if the value “about 10” is disclosed, then “10” is also disclosed.
When a range is expressed, a further aspect includes from the one particular value and/or to the other particular value. For example, where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure, e.g. the phrase “x to y” includes the range from ‘x’ to ‘y’ as well as the range greater than ‘x’ and less than ‘y’. The range can also be expressed as an upper limit, e.g. ‘about x, y, z, or less’ and should be interpreted to include the specific ranges of ‘about x’, ‘about y’, and ‘about z’ as well as the ranges of ‘less than x’, less than y’, and ‘less than z’. Likewise, the phrase ‘about x, y, z, or greater’ should be interpreted to include the specific ranges of ‘about x’, ‘about y’, and ‘about z’ as well as the ranges of ‘greater than x’, greater than y’, and ‘greater than z’. In addition, the phrase “about ‘x’ to ‘y’”, where ‘x’ and ‘y’ are numerical values, includes “about ‘x’ to about ‘y’”.
It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a numerical range of “about 0.1% to 5%” should be interpreted to include not only the explicitly recited values of about 0.1% to about 5%, but also include individual values (e.g., about 1%, about 2%, about 3%, and about 4%) and the sub-ranges (e.g., about 0.5% to about 1.1%; about 5% to about 2.4%; about 0.5% to about 3.2%, and about 0.5% to about 4.4%, and other possible sub-ranges) within the indicated range.
As used herein, the terms “about,” “approximate,” “at or about,” and “substantially” mean that the amount or value in question can be the exact value or a value that provides equivalent results or effects as recited in the claims or taught herein. That is, it is understood that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art such that equivalent results or effects are obtained. In some circumstances, the value that provides equivalent results or effects cannot be reasonably determined. In such cases, it is generally understood, as used herein, that “about” and “at or about” mean the nominal value indicated ±10% variation unless otherwise indicated or inferred. In general, an amount, size, formulation, parameter or other quantity or characteristic is “about,” “approximate,” or “at or about” whether or not expressly stated to be such. It is understood that where “about,” “approximate,” or “at or about” is used before a quantitative value, the parameter also includes the specific quantitative value itself, unless specifically stated otherwise.
As used herein, the term “effective amount” refers to an amount that is sufficient to achieve the desired modification of a physical property of the composition or material. For example, an “effective amount” of a ferrous salt refers to an amount that is sufficient to achieve the desired improvement in the property modulated by the formulation component, e.g. achieving the desired level of precipitation of phosphorus as vivianite from a waste stream. The specific level in terms of wt % in a composition required as an effective amount will depend upon a variety of factors including the amount of phosphorus in the waste stream, volume of water, other purifications method used in series with the precipitation, and other compounds present in the waste stream.
As used herein, the terms “optional” or “optionally” means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.
Unless otherwise specified, temperatures and pressures referred to herein are based on ambient temperature and atmospheric pressure (i.e. one atmosphere).
Now having described the aspects of the present disclosure, in general, the following Examples describe some additional aspects of the present disclosure. While aspects of the present disclosure are described in connection with the following examples and the corresponding text and figures, there is no intent to limit aspects of the present disclosure to this description. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of the present disclosure.
The present disclosure can be described in accordance with the following numbered aspects, which should not be confused with the claims.
Aspect 1. A system for recovering ammonium and phosphorus from a waste stream, the system comprising:
Aspect 2. The system of aspect 1, further comprising a pre-filter to remove large particles from the waste stream.
Aspect 3. The system of aspect 2, wherein the filter comprises a 0.45 μm polyethersulfone membrane.
Aspect 4. The system of any one of aspects 1-3, further comprising a means for separating and recovering the precipitated vivianite.
Aspect 5. The system of aspect 4, wherein the means for separating and recovering the precipitated vivianite comprises a filter, a magnet, or any combination thereof.
Aspect 6. The system of any one of aspects 1-5, wherein the ferrous salt comprises ferrous sulfate heptahydrate.
Aspect 7. The system of any one of aspects 1-6, wherein the second stage operates as a dead-end or a cross flow membrane filtration system.
Aspect 8. The system of any one of aspects 1-7, wherein the one or more polyelectrolytes comprise poly(diallyl dimethylammonium) chloride (PDAC), poly(sodium 4-styrenesulfonate) (SPS), poly(allylamine hydrochloride) (PAH), poly(acrylic acid) (PAA), or any combination thereof.
Aspect 9. The system of any one of aspects 1-8, further comprising sodium chloride in contact with the one or more polyelectrolytes.
Aspect 10. The system of any one of aspects 1-9, wherein the one or more polyelectrolytes are present only on a first side of the membrane.
Aspect 11. The system of any one of aspects 8-10, wherein the one or more polyelectrolytes comprises PDAC and SPS in alternating layers.
Aspect 12. The system of aspect 11, wherein an outermost layer of the alternating layers comprises PDAC and is positively charged.
Aspect 13. The system of aspect 11, wherein an outermost layer of the alternating layers comprises SPS and is negatively charged.
Aspect 14. The system of any one of aspects 8-10, wherein the one or more polyelectrolytes comprises PAH and PAA in alternating layers.
Aspect 15. The system of aspect 14, wherein an outermost layer of the alternating layers comprises PAH and is positively charged.
Aspect 16. The system of aspect 14, wherein an outermost layer of the alternating layers is modified with glutaraldehyde to form a dense polyelectrolyte selective layer and decrease an amount of positive charge present on the outermost layer.
Aspect 17. The system of any one of aspects 1-16, wherein the nanofiltration membrane comprises a support fabric, a porous layer having a first side in contact with the support fabric, and a polymer coating in contact with a second side of the porous layer.
Aspect 18. The system of aspect 17, wherein the support fabric comprises a nonwoven fabric.
Aspect 19. The system of aspect 17 or 18, wherein the porous layer comprises polyethersulfone, polysulfone, or any combination thereof.
Aspect 20. The system of any one of aspects 17-19, wherein the porous layer has a thickness of about 50 μm.
Aspect 21. The system of any one of aspects 17-20, wherein the polymer coating comprises polypiperazine, polyamide, or any combination thereof.
Aspect 22. The system of any one of aspects 17-21, wherein the polymer coating has a thickness of less than about 200 nm.
Aspect 23. The system of any one of aspects 1-22, wherein the system is operated at a temperature of up to about 45° C.
Aspect 24. The system of any one of aspects 1-23, wherein the system is operated at a pressure of up to about 41 bar.
Aspect 25. The system of aspect 24, wherein the system is operated at a pressure of about 6.9 bar.
Aspect 26. The system of any one of aspects 1-25, wherein the system is operated at a pH of from about 2 to about 11.
Aspect 27. A method for recovering ammonium and phosphorus from a waste stream, the method comprising:
Aspect 28. The method of aspect 27, wherein the waste stream comprises anaerobically digested municipal sewage sludge.
Aspect 29. The method of aspect 27 or 28, wherein the waste stream comprises from about 350 to about 685 mg/L of total ammonium.
Aspect 30. The method of any one of aspects 27-29, wherein the waste stream comprises up to about 100 mg/L of total phosphorus.
Aspect 31. The method of aspect 30, wherein a molar ratio of ferrous salt to total phosphorus is about 2:1.
Aspect 32. The method of any one of aspects 27-31, wherein the method recovers substantially all of the phosphorus in the waste stream.
Aspect 33. The method of any one of aspects 27-32, wherein the waste stream comprises from about 250 to about 750 mg/L of total organic carbon.
Aspect 34. The method of any one of aspects 27-33, wherein the method rejects at least about 75% of the total organic carbon in the waste stream.
Aspect 35. The method of aspect 34, wherein the method rejects at least about 85% of the total organic carbon (TOC) in the waste stream.
Aspect 36. The method of any one of aspects 27-35, wherein the method rejects less than about 50% of the total ammonium in the waste stream.
Aspect 37. the method of aspect 36, wherein the method rejects less than about 30% of the total ammonium in the waste stream.
Aspect 38. The method of any one of aspects 27-37, wherein the method recovers at least about 80% of the total ammonium in the waste stream.
Aspect 39. The method of any one of aspects 27-38, wherein the method is at least about 3 times more selective for ammonium passage through the membrane than for TOC passage through the membrane.
Aspect 40. The method of aspect 39, wherein the method is at least about 4.5 times more selective for ammonium passage through the membrane than for TOC passage through the membrane.
Aspect 41. The method of any one of aspects 27-40, wherein the membrane further recovers at least a portion of dissolved potassium in the waste stream.
Aspect 42. The method of aspect 41, wherein the method is at least about 1.25 times more selective for ammonium passage through the membrane than for potassium passage through the membrane.
Aspect 43. The method of aspect 41 or 42, wherein the method recovers at least about 70% of the dissolved potassium from the waste stream.
Aspect 44. A composition comprising ammonium, phosphorus, potassium, or any combination thereof recovered using the method of any one of aspects 27-43.
Aspect 45. The composition of aspect 44, wherein the composition comprises a fertilizer.
The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, articles, devices and/or methods claimed herein are made and evaluated, and are intended to be purely exemplary of the disclosure and are not intended to limit the scope of what the inventors regard as their disclosure. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in ° C. or is at ambient temperature, and pressure is at or near atmospheric.
Materials. Poly(diallyl dimethylammonium) chloride (PDAC, 20 wt % in H2O), poly(sodium 4-styrenesulfonate (SPS, MW˜70,000 g/mol), and sodium chloride (NaCl, 99% w/w) were purchased from Sigma-Aldrich. Two commercial NF membranes, NF90 and NF270, were purchased from DuPont Water Solutions (MN). NF270 was used as the support for fabricating surface-modified polyelectrolyte multilayer membranes. Sodium phosphate monobasic (NaH2PO4·2H2O), ammonium chloride (NH4Cl), sodium acetate anhydrous (C2H3NaO2), ethanol (C2H6O), lactose monohydrate (C12H22O11·H2O), and ferrous sulfate heptahydrate were purchased from HACH (Loveland, Colorado).
Preparation of synthetic digestate. A literature review was conducted on the composition of digestate from anaerobic digestion of municipal sewage sludge (Table 1). The data showed large variations in TOC, NH3 and P concentrations. These variations were attributed to differences in the wastewater and sludge composition, as well as treatment processes used at the different treatment plants. In this study, synthetic digestates containing these major constituents were used (Table 2). The synthetic digestate solutions were prepared using sodium phosphate monobasic (NaH2PO4·2H2O, 0.6 mmol/L or mM), ammonium chloride (NH4Cl, 11.2 mM), sodium acetate anhydrous (C2H3NaO2, 12.2 mM), ethanol (C2H6O, 0.17 ml/L), and lactose monohydrate (C12H22O11·H2O, 2.8 mM). Ferrous sulfate heptahydrate was used as the iron source for chemical dosing. The real digestate samples were first filtered with 0.45 μm polyethersulfone membrane to remove large, suspended particles and microorganisms before using them for P and NH3 separation.
Vivianite precipitation for P recovery: ferrous dosing and pH effects. Predetermined amounts of ferrous (0.07-7 mM) were added to 1 L of the synthetic digestate to obtain Fe/P molar ratios ranging from 0.02 to 2.1, while phosphate concentration was kept at 3 mM. For each Fe/P molar ratio, three batch reactors (1 L glass bottles) conditioned to pH 6.5, 7, and 7.5 with a NaOH solution (0.5 N) were used to study the pH effects. The bottles were sealed immediately after the ferrous addition. These bottles were then placed in a shaker for 5 min, followed by 30 min without mixing to allow continued chemical precipitation. Each solution was then filtered through a polyethersulfone membrane (0.45 μm) and the aqueous Fe(II) and phosphate concentrations of the filtrate were measured. The filtered solids were collected for spectroscopic analyses. TOC analysis was also done before and after chemical precipitation.
Estimation of solubility product (Ksp) of precipitated solid. Thermodynamic solubility product (Ksp) of the precipitated solids was determined by extrapolating the solubility products at non-zero ionic strengths to standard state of zero ionic strength.
The solubility product of vivianite,
where Kc is the experimentally determined solubility product and was calculated as [Fe2+]3[PO43−]2 (where [ ] denotes molar concentration). In Equation 1, Y− denotes the activity coefficient and was estimated following the Guntelberg approximation of the Debye-Huckel limiting law
where I is the ionic strength (mol/L). Combining Equations (1) and (2) gives the following relationship:
where A′=3z2Fe2++2z2PO43−; pKsp or −log Ksp can be determined from the intercept of the fitted line of the plot of −log Kc versus (√I)/(1+√I) at I=0.
To obtain different values of Kc and ionic strength (I) or developing a plot for the Ksp estimation (Equation 3), the measured Fe(II) and phosphate concentrations from the batch vivianite precipitation experiments for pH 7 were used. Visual Minteq 3.1 (KTH, Sweden, 2013) was used to determine Fe2+ and PO43− activities from the measured concentrations. The ionic activity (I) was determined using Equation 4. The data points from the −log Kc versus (√I)/(1+√I) graphs were used for linear extrapolation to determine the value of pKc at I=0.
where C is the molar concentration of ferrous/phosphate, which was calculated as added concentration subtracted by the filtrate concentration, and Z is the charge number.
Membrane filtration. The digestates after chemical precipitation and solid removal were used in batch (dead-end) and continuous (cross flow) membrane filtration for liquid ammonia recovery.
Preparation of polyelectrolyte membranes. Layer by layer (LbL) dip coating experiments were performed on the sur-face of NF270 membrane using the alternative layers of PDAC (+) and SPS (−) as described by Sanyal et al. Prior to LbL deposition, the NF270 membrane substrates were soaked in DI water overnight. Prior to the LbL experiments, four alternate sheets of parafilm and aluminum foil were used to cover the permeate sides of the membranes to prevent the deposition of polyelectrolytes on the permeate side. Polyelectrolyte solutions were prepared at a concentration of 10 mM by dissolving each polyelectrolyte in 18.2 MΩ DI water with and without 0.5 M NaCl. The covered membrane sample was dipped in PDAC solution for 10 min followed by three DI water washing steps for 2+2+1 min. Subsequently, the membrane was dipped in SPS for 10 min, followed by the same three DI water washing steps. This process of dipping in alternate PDAC and SPS polyelectrolytes was repeated for 5 or 5.5 cycles, resulting in 5/5.5 bilayer (BL) surface modified membranes ending with negatively charged SPS or positive charged PDAC respectively. On completion of the LbL process, a required portion of the membrane was cut out and dipped in DI water overnight, prior to the filtration tests.
LbL-modified membranes with several different polyelectrolyte compositions were developed in this work.
Set 1—(PDAC/SPS 5BL W NaCl coated NF270): 5 bilayers of PDAC and SPS (both in 0.5 M NaCl) were deposited on NF270 substrate, therefore ending with a negatively charged (SPS) outermost layer. Note that in some aspects of this disclosure, this membrane was also designated as “NF270[PDAC/SPS w NaCl]5”.
Set 2—(PDAC/SPS 5.5BL W NaCl coated NF270): 5.5 bilayers of PDAC and SPS (both in 0.5 M NaCl) were deposited on NF270 substrate, therefore ending with a positively charged (PDAC) outermost layer. Note that in some aspects of this disclosure, this membrane was also designated as “NF270[PDAC/SPS w NaCl]5.5”.
Set 3—(PDAC/SPS 5.5BL W/O NaCl coated NF270): 5.5 bilayers of PDAC and SPS (both in pure DI without NaCl) were deposited on NF270 substrate, therefore ending with a positively charged (PDAC) outermost layer. Note that in some aspects of this disclosure, this membrane was also designated as “NF270[PDAC/SPS w/o NaCl]5.5”.
Dead-end filtration setup. A HP 4750 stirred dead-end membrane permeation cell (Sterlitech, Kent, Washington) system was used for the preliminary experiments. The volume of this filtration system is ˜300 mL and the effective membrane filtration area is ˜14.6 cm2. A nitrogen cylinder was used to pressurize the water/solution across the membrane, providing the effective driving force. The filtration system was placed over a magnetic stirring plate, and the solution was stirred throughout the entire duration of the experiment to reduce concentration polarization effects. Before testing with synthetic digestate, pure DI water was run through the membranes at 100 psi until the flux became constant, from which pure water permeance was calculated. For synthetic digestate, flux and permeance were recorded after collecting 60 mL permeate. All dead-filtration experiments were performed in triplicates and the results reported represent an average of three samples along with standard deviations.
Cross flow filtration setup. A CF 042 cross flow unit (Sterlitech, Kent, Washington) was used for analyzing the long-term performances and determining the fouling behavior of modified polyelectrolyte membranes. The effective surface area of the cross flow system is around 42 cm2. A positive displacement pump was used to transfer the feed sample from conical feed tank (˜19 L) to mem-brane module. A portion of the feed stream was divided and recycled back to the feed tank through a bypass needle valve. For controlling the pump speed, a variable speed controller was used. Temperature of the feed solution was controlled through a digital chiller (Polysciences, Inc.). Transmembrane pressure between membrane modules was controlled by adjusting a control valve connected to the retentate site as well as a bypass needle valve connected to the feed site. Retentate stream flow rate was measured using a Site Read Panel Mount Flowmeter connected to the retentate line. A closed circuit filtration experiment was performed from which both the retentate and permeate streams were recycled back to the feed tank. All the components of the cross flow setup was bought from Sterlitech, Kent, Washington and assembled in the lab. The detailed diagram for this setup is shown in
Membrane transport property evaluation. Water permeance was calculated by using the following equation:
Here, P is the water permeance (L/m2 h bar or LMH/bar); JW is the water flux (L/m2 h or LMH) calculated using the volumetric flow rate (L/h) over effective filtration area (m2); and ΔP is the transmembrane pressure difference (bar).
Rejection (%) was calculated by using the following equation:
Here, CF is the sample feed concentration (mg/L); CR is the sample retentate concentration (mg/L); and CP is the sample permeate con-centration (mg/L).
Selectivity of ammonia (NH3) over TOC was determined by using the following equation:
Here, CP,NH3 is the concentration of ammonia in permeate stream (mg/L); CP,TOC is the concentration of total organic carbon in permeate stream (mg/L); CF,NH3 is the concentration of ammonia in feed stream (mg/L); and CF,TOC is the concentration of total organic carbon in feed stream (mg/L).
Water and mineral analyses, and thermodynamic modeling. The phosphate concentration of the solutions were determined using an ascorbic acid method (Standard Method 4500 E; Rodger et al.). Ferrous iron concentration was measured using the 1,10 phenanthroline method (Standard Method 3500 B). The ammonium (NH4+) concentration was measured using a phenol method with a UV/Vis spectrophotometer (Perkin Elmer 3100). TOC concentration was measured by using a total organic carbon analyzer (Shimadzu TOC-L series). Solids collected from the filtration (i.e., chemical precipitates) were analyzed to characterize the morphology and chemical composition using a scanning electron microscope equipped with an energy dispersion spectroscopy (SEM-EDS, Hitachi S 4700). A closed desiccator filled with calcium sulfate and flushed with N2 gas was used to dry out the precipitated solid while avoiding oxidation. The dried solids were ground and mounted on aluminum stubs and then coated with Au—Pd using a sputter (Denton Desk V) to avoid surface charging. An accelerating voltage of 10.0 KV was used for SEM analysis. Elemental analysis was conducted using EDS spectrometry under an accelerating voltage of 15 kV. In addition, X-ray photoelectron spectroscopy (XPS, PHI 5000 Versaprobe) analysis was used to determine the chemical states of Fe and P in the powdered solid precipitate mounted on a sample holder with a zero reflective quartz plate. A monochromatized Al Kα X-ray source (1487 eV) was used to obtain XPS spectra while the base pressure in the analytical chamber was maintained on the order of 10−7 Pa. Thermodynamic evaluations were conducted with Visual Minteq to study the stable minerals that can be formed with availability of Fe2+,PO43−, and NH4+ as major ions that represent the digestate environment with the ferrous dosage. The presence of organics was not considered in thermodynamic evaluation as their interference has not been observed during solid precipitation.
Herein, the factors involved in vivianite precipitation that led to optimum phosphorus recovery as well as the role of the membrane selective layer chemistry in achieving high NH3—N/TOC selectivities are analyzed. At first, digestates (containing P, NH3, TOC) were treated with ferrous iron to recover phosphorus as vivianite. No observable change in TOC level was found after chemical precipitation. The filtrate (P-free digestates) was then tested in a batch (dead-end) and continuous (cross flow) membrane filtration system for ammonia recovery.
Phosphorus recovery using vivianite precipitation: Thermodynamic analysis. Solubility product (Ksp): The pKc values of the precipitated solid showed almost a linear relationship (R2=0.88) with [I0.5/(1+I0.5)] for the range of Fe/P molar ratio 0.02-2.1 (
Effects of pH and ferrous dosage on P recovery. Thermodynamic modeling using the Visual Minteq 3.1 software indicated that with Fe2+,PO43−, and NH4+ availability for Fe/P=0.5 and 2.1, vivianite was the major precipitated solid under pH from 6.5 to 9.0, while ferrous hydroxide precipitation occurred along with vivianite under higher pH (
P removal/recovery from the digestates were low for Fe/P molar ratio of <0.5 (
Effects on mineralogy of precipitated solids. The color of the precipitated solids was observed to vary with ferrous dosage (
Our analysis of Fe2+ and PO43− activities also indicates possibility of ferric mineral coprecipitation at the lower Fe/P molar ratios.
XPS analysis showed Fe, O, and P as the major elements in precipitated solid. Two major peaks were identified in the Fe2p spectrum at 710.0 and 711.8 eV (for Fe/P=0.5 and 1.0), which corresponds to the binding energies of vivianite and goethite respectively (
Design of membrane-based processes with high NH3—N/TOC selectivity: Performance of commercial nanofiltration membranes. Two commercial NF membranes, NF90 and NF270, were tested under a dead-end filtration system with the P-free synthetic anaerobic digestate. NF270 is a “loose” NF membrane with a polypiperazinamide based selective layer (˜14-80 nm in thickness), while NF90 is classified as a “tight” NF membrane with a polyamide-based selective layer (134-214 nm). NF membranes were selected due to their ability to demonstrate high ion selectivities based on both size and charge-based exclusion mechanisms between similar-sized solutes.32 To identify the optimum operating conditions, the membranes were tested under three different pressures at 50, 100, and 150 psi, and their performances were compared in terms of wastewater permeance and NH3—N/TOC selectivity. The permeance and NH3—N/TOC selectivity of both membranes were essentially independent of the applied pressure. Under all pressure conditions, NF90 and NF270 membranes had similar selectivities (
Performance of surface-modified membranes. Modification of the membrane selective layer via LbL was envisioned to offer opportunities to tune the NH3—N/TOC selectivity of the overall membrane. Since NF270 showed the highest NH3—N permeance, it was selected as the substrate for polyelectrolyte surface modification. In prior work, such modified membranes showed high performance and lower fouling propensity than commercial membranes with similar intrinsic performance when tested with real wastewater effluents.
The three sets of polyelectrolyte compositions based on PDAC and SPS described in “Preparation of Polyelectrolyte Membranes” under “Materials and Methods” were used for membrane surface modification, with the goal to understand the effects of the outermost surface layer charge as well as surface charge density on enabling high NH3—N/TOC selectivities. Selection of this polyelectrolyte combination (i.e., PDAC/SPS) was also based on results from prior studies in which they were shown to significantly enhance the performance of the underlying NF membrane.
The organic compounds used in the synthetic solution are charge-neutral which explains the TOC rejection being largely independent of the surface charge. At neutral pH, ammonia exists as a combination of ammonium complex (charged) and neutral ammonia and its transport through charged layers is clearly complex and not well understood. Nevertheless, the results in both
Series-resistance analysis of modified NF270 membrane. The schematic in
Equations for calculation of ammonia and TOC permeability of PDAC/SPS 5.5BL W NaCl (without NF270 backing layer) using a series resistance model are shown below:
While this explains the high performance of the modified membrane, it is clear that such polyelectrolyte-based coatings are very suited for such selective separations. In fact, if such coatings are applied on membranes with low-resistance selective layers, even higher NH3 recovery could be achieved compared with the modified NF270 case described here.
Long-term filtration experiments for fouling and selectivity analysis. PDAC/SPS 5.5BL W NaCl coated NF270 membranes were tested under across flow filtration system for analyzing the long-term performance in terms of fouling and selectivity. Membranes were tested for ˜48 h with synthetic and real digestate. At the end of the 48 h experiment, permeate and retentate samples were collected for calculating rejections (%), and selectivities. For both synthetic and real digestate, two sets of experiments were performed (run 1 and run 2), where nutrients' feed concentrations were the same for each individual digestate.
This study depicted the potential of 100% P recovery as pure vivianite from digestate with the ferrous dose at Fe/P molar ratio of 2.1. With P-free digestate, surface modified PDAC/SPS 5.5BL W NaCl coated NF270 membrane can achieve ˜2× higher NH3—N/TOC selectivity compared with commercial NF (NF90 and NF270) membranes. The series resistance model revealed that such polyelectrolyte multilayer coatings could provide a high NH3—N and low TOC permeance in comparison to commercial NF270 membranes. The results further demonstrated the potential of this combined strategy to recover both P and NH3 under neutral pH conditions. In this respect, this integrated chemical precipitation and membrane separation technique could provide a sustainable route for phosphorus and ammonia recovery by eliminating the chemical cost for pH adjustment and reducing the energy requirements and CO2 emissions with respect to traditional approaches.
Poly(allylamine hydrochloride) (PAH, molecular weight ˜17500 g/mole), poly(diallyl dimethylammonium chloride (PDAC, 20 wt % in H2O), Polyethylenimine (PEI, branched, molecular weight ˜25000 g/mole) and poly(sodium 4-styrenesulfonate) (SPS, molecular weight ˜70,000 g/mol) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Poly(acrylic acid, sodium salt) (PAA, molecular weight ˜225,000 g/mol) (20 wt % in water) was purchased from Polysciences Inc. (Warrington, PA, USA). Sodium chloride (NaCl) (99% w/w), hydrochloric acid (HCl) (37% v/v), sodium hydroxide (NaOH) (≥97% w/w), Glutaraldehyde (GA) (50 wt % in water), N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide (EDC) (297% w/v), and poly(ethylene glycol) (PEG) molecular weight 300 and 400 g/mol were purchased from Sigma-Aldrich. 2-(N-morpholino) ethanesulfonic acid (MES) (≥99% w/v) buffer was purchased from VWR. Commercial NF270 nanofiltration membrane was purchased from DuPont Water Solutions (Edina, MN, USA) and used as the support for the fabrication of polyelectrolyte multilayer membranes. Polyethersulfone, PES (MWCO 5000 Da) and polysulfone, PSF (MWCO 10000 Da) ultrafiltration membranes were purchased from Alfa Laval Inc. (Richmond, VA, USA). Glycerol (99% W/V) and D(+)-Glucose was purchased from Thermo Scientific (Waltham, MA, USA). Sodium phosphate monobasic (NaH2PO4·2H2O), ammonium chloride (NH4Cl), sodium acetate anhydrous (C2H3NaO2), ethanol (C2H6O), and ferrous sulfate heptahydrate were purchased from HACH (Loveland, CO, USA). D-lactose monohydrate (C12H22O11·H2O) was purchased from Research Product International (RPI) (Mount Prospect, IL, USA). Potassium chloride (KCl) was purchased from Fisher Scientific (Hampton, NH, USA). Spherical silica gel (10 μm diameter) was purchased from VWR International (Radnor, Pennsylvania, USA). All purchased chemicals were used as received.
Sodium phosphate monobasic (NaH2PO4·2H2O), ammonium chloride (NH4Cl) and Potassium chloride (KCl) were used sources of phosphorus, ammonium and potassium, respectively. For organic pollutant source, three different molecular weight organic compounds—ethanol (C2H6O, MW˜46 g/mole), sodium acetate anhydrous (C2H3NaO2, MW˜82 g/mole), and D-lactose monohydrate (C12H22O11·H2O, MW˜360 g/mole) were utilized. The synthetic wastewater was prepared by conducting literature reviews of AD supernatant from various municipal wastewater plants and a local municipal wastewater treatment facility located in Morgantown, WV, USA and following feed concentrations of nutrients and organic pollutants were maintained throughout the study: Phosphorus: 100 mg/L; organic pollutant: 650-750 mg/L; NH4+: 550-650 mg/L and K+: 60-80 mg/L. Feed solution pH was unchanged since it remained within the neutral range (pH 6˜7).
To assess the rejection performance of individual organic pollutants, four distinct organic components with molecular weights ranging from 92 to 400 g/mole were employed. These components include glycerol (92 g/mole, 1000 μL/L), glucose (180 g/mole, 1000 mg/L), PEG (300 g/mole, 1000 μL/L), and PEG (400 g/mole, 1000 μL/L). Individual samples were prepared by using each organic component and then subjected to testing using both commercial NF270 and modified PEM membranes.
Polyelectrolyte multilayer membrane was fabricated using layer by layer (LbL) dip-coating of positive and negatively charged polyelectrolyte on top of NF270 membrane selective layer (feed side) following a similar protocol as previous studies. Prior to the LbL experiment, NF270 membranes were immersed in DI water overnight. Before starting the experiment, permeate side of the membrane was covered with four alternate sheets of parafilm and aluminum foils to prevent coating from occurring on that side. All polyelectrolyte solutions were used at a concentration of 10 mmol/L (mM) with respect to their repeat unit (molecular weights of monomer). Solutions were prepared in 18.2 MΩ DI water with and without 0.5 M NaCl. NF270 membrane support was at first dipped in the positively (+) charged polyelectrolyte solution for 10 min followed by three consecutive DI water washing steps through dipping for 2+2+1 min or 1+1+1 min. Then the membrane was dipped in negatively (−) charged polyelectrolyte for 10 min, followed by the same three DI water washing steps through dipping for the same 2+2+1 min or 1+1+1 min. Deposition of these two alternatively charged polyelectrolytes was repeated 11 times, resulting in 5.5 bilayers of PEM membranes ending with positively charged polyelectrolytes. Some membranes were dipped for 24 hours and for these membranes, only 1 layer of polyelectrolyte films were deposited followed by three DI water washing steps for 1+1+1 minutes. Upon completion of LbL deposition process, the membrane samples were soaked in DI water overnight prior to the filtration experiments.
The polyelectrolyte combinations used in this work can be broadly classified into (a) polyelectrolytes with ionic (or electrostatic) bonding and (b) polyelectrolytes with hydrogen bonding and covalent crosslinking.
Polyelectrolytes with ionic bonding. Poly(diallyl dimethylammonium chloride (PDAC) (+) and Poly(sodium 4-styrenesulfonate (SPS) (−) (
Polyelectrolytes with hydrogen bonding or covalent crosslinking. poly(allylamine hydrochloride) (PAH) (+) and poly(acrylic acid) (PAA) (−) (
The following polyelectrolyte combinations have been used herein:
In this study, membranes were tested with three different synthetic digestate after phosphorus recovery. Initially, filtration experiments were performed using the synthetic digestate containing ammonium, and organic carbon without the presence of potassium to observe ammonium recovery performance and organic carbon rejection capability.
From these experiments, best-performed membranes were picked, and these membranes were used to observe nutrient recovery (ammonium and potassium) performance from the digestate containing ammonium (NH3—N), potassium (K+) and organic carbon and results were listed in Table 11. Some membranes were also tested with binary mixtures of ammonium (NH3—N) and potassium (K+) to observe NH3—N/K+ selectivities without any presence of organics and results were shown in Table 12 and Table 13.
Performance of commercial nanofiltration membranes. Two commercial NF membranes, NF90 and NF270 (“loose” NF membrane), were selected due to their ability to demonstrate high ion selectivities based on both size and charge-based exclusion mechanisms between similar-sized solutes. As seen from the table, the performance of the polyelectrolyte-coated membranes have the ability to surpass that of commercial membranes when optimized.
NF90 membrane. NF90 had a tight polyamide layer with lower effective porosity. When tested with AD containing ammonium and organic carbon, this type of coated membrane can achieve ˜91% TOO and ˜71% ammonium rejection with a NH3—N/TOC selectivity of ˜3.2 (Table 10).
NF270 membrane. NF270 had a polypiperazinamide layer with higher effective porosity. When tested with AD containing ammonium and organic carbon, this type of coated membrane can achieve ˜83% TOO and ˜43% ammonium rejection with a NH3—N/TOC selectivity of ˜3.0 (Table 10).
Organic pollutant rejections of commercial and coated membranes (type-1 and type-2). Rejections of organic pollutants in presence of either ammonium (NH3—N) or mixtures of ammonium (NH4+) and potassium (K+) were shown in Tables 11 and 12. Both commercial and type-1 membranes had similar organic pollutants rejection whereas cross-linked type 2 membrane had high organic retention (>90%).
NH3—N/K+ selectivity of commercial and coated (type-1 and type-2) membranes. In this work, two types of surface-modified membranes were discovered (
In the case of cross-linked membrane (PAH/PAA xGlu 5.5BL coated NF270 membrane) (type 2), NH3—N rejection is significantly lower (˜1.5×) than K+ and it is possible to achieve a high NH3—N/K selectivity (˜1.26 for NH3—N/K+/organic mixture and ˜1.78 for binary NH3—N/K mixtures). Although ammonium and potassium have the exact same hydrated radii (0.33 nm), the differences between the high N/K selectivity is attributed to the differences in ion solvation properties (local water structure in the nearest solvation shell of ammonium is surrounded by lower water molecules than potassium) between ammonium and potassium ions. It is believed that the crosslinked membranes create confinement domains for both ions (
Prior to undergoing treatment with surface-modified PEM membranes, synthetic wastewater was subjected to a predetermined ferrous (II) solution with a molar ratio of 2.1:1 Fe/P, facilitating the formation of vivianite through chemical precipitation and effectively removing all phosphate content. The detailed experimental procedure can be found in an earlier study. This solution was then filtered using a polyethersulfone (0.45 m) microfiltration membrane and solid precipitate as a form of vivianite was separated. The remaining wastewater solution comprises a ternary mixture of ammonium (NH4), potassium (K+), and organic pollutants (TOC), and was used as feed solution for all the membranes.
All membranes were tested using a HP 4750 stirred dead-end filtration cell system (Sterlitech, Kent, Washington). The volume of the filtration system is approximately 300 mL, and the effective membrane filtration area is around 14.6 cm2. Ultra-pure N2 was used to provide the required transmembrane pressure (100 psi/6.9 bar). Prior to testing with the synthetic mixture, all membranes were compacted with DI water for at least 2 hours, and the pure water permeance values were computed once the membranes reached steady state. The solution permeances of the membranes were estimated after collecting 60 mL of permeate for each membrane. All experimental results reported are based on at least three independent data points, except the results related to
Water permeance (pure or wastewater) of the membrane was evaluated by using the following equation:
where is the water permeance of the membrane in L/m2 hr bar or LMH/bar; JW (L/m2 hr or LMH) is the water flux (pure/wastewater) of the membrane measured using water flowrate (L/hr) over effective membrane filtration area (m2); and ΔP (bar) is the applied pressure (6.9 bar) used in this experiment.
Membrane rejection, R (%) was calculated by using this equation:
Here, Ca (mg/L) is the average concentration of feed and retentate solution, whereas CP (mg/L) is the permeate concentration.
Membrane permeance, P (%) was calculated by using this equation:
Membrane selectivity of component “A” over component “B” was measured using the following equation:
[component A or B can be ammonium, potassium, or organic pollutant]
Here, Cp,A (mg/L) is the permeate concentration of component A; Cp,B (mg/L) is the permeate concentration of component B; CF,A is the feed concentration of component A; and CF,B is the feed concentration of component B.
Functional groups of polyelectrolyte multilayer membranes were analyzed using FTS 7000 Fourier Transform Infrared Spectrometer (FTIR) coupled with Varian Resolution Pro software within mid infrared region (4000-400 cm−1). Polyelectrolyte multilayer (PEM) film thickness of membrane was measured using a Bruker DektakXT Stylus Profilometer. For this, PEM films were deposited on a glass substrate (Carolina Biological Supply Company, Burlington, North Carolina, USA). Prior to LbL deposition, glass slides were cleaned with ethanol for 20 minutes and exposed to oxygen plasma treatment (March PX-250 Plasma Asher) for 5 minutes at 50 W power to induce hydrophilicity and negative surface charge. Immediately after plasma treatment, the glass slides were subject to LbL dip coating experiments as per the method explained previously. PEM samples were fabricated in triplicates. Ten thickness measurements were taken for each polyelectrolyte sample, resulting in a total of 30 measurements for three replicates of each polyelectrolyte coating, from which the average and standard deviation values were reported (Table 14).
In the context of membranes while direct measurements of the charge of stem plane—located between the interface of membrane surface and shear plane—is challenging, electrokinetic potential or zeta potential (charge of shear plane) is considered to be sufficient to predict membrane surface charge. Usually for the membranes, streaming potential analysis is conducted to determine the zeta potential for assessing charge-dependent separation performance. However, the widespread adoption of instruments dedicated to analyzing membrane streaming potential is hindered by their high cost, resulting in limited accessibility within laboratory settings. Fortunately, there exists an alternative method to predict membrane zeta potential by measuring the zeta potential of particles coated with similar materials used to fabricate the membrane itself. Membrane streaming potential is typically generated by the convective movement of charge at zero current, driven by the pressure gradient across the membrane fixed to a specific support, while for particles, zeta potential is measured due to the electrophoretic mobility of particles under an applied electric field. These two techniques, differing in their experimental setup—where streaming potential measurement involves a fixed membrane support and particle zeta potential measurement is based on the mobility of particles—may result in different magnitudes of potential values. However, their trends are very similar with values from both methods being closely aligned. In this study, zeta potential measurement of polyelectrolyte coated silica particle was employed to analyze the charge-dependent separation performance of polyelectrolyte membranes. Note that while standard deviations exist for these measurements, these are very usual and in fact may sometime result lower values than streaming potential measurements which was done on fixed membrane surfaces.
A Zetasizer Nano Series (Malvern Panalytical Ltd., UK) was used to determine the ζ potential of different polyelectrolyte coatings by taking average and standard deviation of three measurements. Polyelectrolyte multilayer was developed on top of silica microparticles (10 μm, ζ potential—16.8±3.9 mV). For the coating of polyelectrolytes, a vacuum filtration (PES 0.45 μm, Thermo Scientific, USA) method was developed, with the detail experimental procedure was shown in
NH4+ concentration was determined using a phenate method with a UV/Vis spectrophotometer (Perkin Elmer 3100 model). K+ concentration was measured using an Agilent Inductive Coupled Plasma Optical Emission Spectrometer (ICP OES 720). A Shimadzu TOC-L CSN series total organic carbon (TOC) analyzer was used to measure the organic pollutant concentration. The same TOC analyzer was also used to measure the concentration of different molecular weights organic components (glycerol, glucose, PEG300, and PEG400) used to observe rejections of modified and commercial NF membranes.
A wide range of polyelectrolyte multilayer (PEM) membranes were fabricated on commercial NF 270 substrate using the LbL deposition technique, by varying the type of polyelectrolytes, pH used for deposition, and the nature of interaction between them. In this work, 5.5 bilayers of polyelectrolytes (1 bilayer=1 layer of polycation+1 layer of polyanion) were found to be optimum. Previous results have shown that when ended with positively charged outer layers, PEM membranes show desirable separation performance in terms of nutrient permeance and nutrient/organic selectivity and therefore, for this application, all multilayers were capped with positively charged outer layer. Previous work effectively demonstrated that in comparison to commercial NF membranes, polyelectrolyte modified NF membranes provide higher nutrient permeance and higher nutrient/organic selectivities under neutral pH conditions. These PEM membranes were primarily based on PDAC/SPS polyelectrolyte combination and the best permeability-selectivity trade off was achieved under the optimum conditions of using 5.5 bilayers with 0.5M added salt in the polyelectrolyte solutions. While the feasibility of PEM membranes has been shown in prior work, this paper is focused on identifying the underlying mechanisms of simultaneous nutrient permeance and organic rejection using this platform. For this work, the range of polyelectrolytes used, and their associated deposited conditions were expanded. Four specific sets of polyelectrolytes, as described previously, were used in this work utilizing a range of inter-polyelectrolyte interactions such as electrostatic, hydrogen bonding and covalent interactions. In addition to nutrient-organic separation, some proof of concept analyses on intra-nutrient (i.e. NH4+/K+) separation is also discussed, by considering K+ as a co-nutrient with NH4+ in the feed mixture. Intra-nutrient separation, as elaborated in detail later, is an extremely interesting scenario which involves an unusual desolvation-based separation mechanism for distinguishing between two same-sized nutrient ions (NH4+ and K+).
Of the different types of PEM membranes tested, the NF270[PAH(8.5)/PAA(3.5)]5.5×GA membrane is the only kind which involves an additional post-deposition crosslinking step involving Glutaraldehyde (GA). The latter is known to form imines (C≡N) through a Schiff-base reaction with the primary amines of PAH. An FTIR analysis (
Separation performance of polyelectrolyte multilayer membranes. Commercial NF membranes such as NF 270 and NF 90 show nutrient (NH4+) permeances of ˜57% and ˜29% respectively, as discussed in prior work. When modified with [PDAC/SPS w NaCl]5.5 multilayers, the NH4+ permeance increased ˜1.4× vs. bare NF 270 membrane, also resulting in ˜2× increase in NH4+/TOC selectivity. Below, results from three other polyelectrolyte systems are discussed in terms of pure water flux, organic rejection and nutrient (NH4+ and K+) permeation. As expected, surface modification of NF 270 membrane led to added resistance to permeation which resulted in decreased water and solution permeances as shown in Table 15. The extent of permeance decrease is however different, depending on the type of polyelectrolyte coatings. The electrostatically crosslinked combinations typically showed higher permeances than the hydrogen bonded multilayers and the covalently crosslinked multilayers showed the lowest permeances among all the membranes tested. These results align well with expectations, as the pure water and solution permeances follow an inverse trend to that of the thickness of the multilayer films. As reported in Table 13, the electrostatic systems are generally thinner (≤50 μm) whereas the covalent and hydrogen bonded systems are the thickest (≥130 μm). In addition to thickness, the effective porosity also plays an important role. Modifying the NF 270 surface with the covalent polyelectrolyte systems results in a marked reduction in effective porosity which reduces the water flux through the membranes but improves the organic rejection and nutrient/organic selectivities.
The goal of the membrane process is to minimize organic transport, while permeating the majority of the nutrients (NH4+ and K+). To develop a fundamental analysis of the underlying mechanisms, the complex process was deconvoluted into the two separate components—organic rejection and nutrient permeance, as discussed below.
Organic rejection. To assess the rejection of organic pollutants (as TOC), three different organic compounds with varying number of carbon atoms (C2-C12), and a wide range of molecular weights (46 g/mole to 342 g/mole) were used. The molar ratios used were representative of actual AD supernatants from local wastewater plant at Morgantown, WV, USA. The bare NF 270 membrane, by itself, shows a substantially high (˜82%) organic rejection. Polyelectrolyte modification did not result in significant enhancement in organic rejection in most cases, and remained comparable to the bare NF270 membrane (rejection ˜82%) (
The organic rejection results suggest that covalent crosslinked membranes have lower effective pore sizes or a tighter pore size distribution vs uncrosslinked, ionic crosslinked and bare NF 270 membranes and showed the highest % organic removal via size exclusion. To validate this, direct, high-fidelity pore size measurements to differentiate among these PEM membranes could be performed but are challenging in nature. Therefore, as a more reliable alternative, a molecular weight cut off experiment (MWCO) was designed for validation. All membranes were tested with of 4 specific organics of different molecular weights: glycerol (92 g/mole), glucose (180 g/mole) and PEG molecules of two different molecular weights—300 g/mole and 400 g/mole, following the protocol discussed previously. As
In this work, only neutral organics were considered, however, charged organics such as personal care products, perfluoro-components, organic acids, and bases could also be present in the feed matrix. It is anticipated that beyond a certain molecular weight, size exclusion effects will be the dominant rejection mechanisms even for charged organics. This is based on the fact that rejection rates for neutral organics were already >80% for all membranes considered here. Charge-based effects could however play a role in case of substances with lower molecular weight, since except the GA-crosslinked membranes, all membranes have lower % rejections for neutral organics.
Nutrient permeance. While the GA-crosslinked membrane provided high organic rejection by virtue of its size exclusion abilities, the NF 270[PDAC/SPS w NaCl]5.5 membrane showed the highest nutrient (NH4+, K+) permeance (
While NH4Cl and KCl were used in this study to investigate nutrient permeance mechanisms, the results are expected to be different, if the corresponding sulfates ((NH4)2SO4 and K2SO4) were used instead of chlorides. Unmodified NF membranes with negatively charged outer surfaces, typically reject divalent ions such as SO42− via Donnan exclusion, in the process also increasing the retention of NH4+ and K+. Positively charged PEM membranes could provide an advantage in this case by reducing the possibility of charge-based repulsion of SO42− ions. In any case, for a number of feed systems including municipal wastewater, the SO42− concentration (92-320 mg/L) is 4-5× lower than the typical Cl− concentration (391-900 mg/L).
In
Nutrient organic separation trade-off in PEM membranes and insights into new membrane design. All PEM membranes tested in this work exhibited higher nutrient (NH4+, K+)/TOC selectivity (˜4 to ˜6) compared to commercial NF270 membrane (˜3), as shown in
As discussed previously, the nutrient permeance mechanism is distinct from that of organic rejection—the former being influenced by surface charge and the latter by size-based exclusion. A trade-off nature is evident wherein membranes that display high nutrient permeance show lower nutrient (NH4+, K+)/organic selectivities and vice-versa.
Permeability-selectivity trade off curves are very common in case of gas separations and ion separations and they serve as important frameworks to guide the design of membrane materials. In fact, the upper bound trade-off relationship for gas separation led to the conclusion that both chain stiffness and inter-chain free volume are necessary for polymer membranes to overcome the permeability-selectivity trade-off, and this principle has continued to guide gas separation membrane material design till today. The results in this paper allow for developing the first-of-its-kind nutrient-organic trade-off curve and is similarly aimed at guiding rational membrane design for nutrient recovery. The origin of this trade-off nature, especially for polyelectrolyte-based membrane arises from the fact that strong surface charge is necessary for high nutrient permeance, while crosslinking of polyelectrolytes leads to high nutrient/organic selectivities. Covalent crosslinked polyelectrolyte membranes had lower surface charge compared to ionic-crosslinked counter parts as evident from the results pertaining to
Intra-nutrient selectivity of PEM membranes. Herein, the intra-nutrient (NH4+/K+) selectivities that are exhibited by the NF 270[PAH (8.5)/PAA (3.5)]5.5×GA membranes are highlighted. While the selectivities (˜1.3-1.5) are low (
This study demonstrates the potential of polyelectrolyte membranes to achieve concurrent nutrient permeance and organic pollutant retention from nutrient-rich resources under neutral pH conditions. The findings of this study revealed that the mechanism governing nutrient permeance differs from organic pollutant rejection, with the former being affected by surface charge and the latter by size-based exclusion. In particular, ionically crosslinked PEM membranes, modified with PDAC/SPS polyelectrolytes, facilitates higher nutrient permeation (˜30% higher compared to commercial NF270 membranes) due to their high positive surface charge. In addition, covalently crosslinked PEM membranes, modified with PAH/PAA and glutaraldehyde, demonstrates enhanced nutrient/organic selectivity (up to ˜2× compared to NF270) owing to their size-exclusion mechanism and increased organic pollutants rejection. The findings of this study helps with the development of a first-of-its-kind nutrient-organic trade-off curve aiming to rational membrane design for nutrient recovery applications. These results suggest that achieving a synergistic combination of surface charge and size exclusion properties in a specific membrane material is essential to overcome this permeability-selectivity trade-off barrier. Finally, herein are provided insights into the intra-nutrient separation mechanism, demonstrating how covalently crosslinked membranes effectively separate similarly sized nutrients (NH4+ and K+) based on their distinct desolvation properties. The conclusions drawn from this study will benefit the membrane community in directing the design of membranes, specifically tailored for nutrient recovery applications. This approach eliminates organic pollutants and allows for the recovery of adjustable proportions of nutrients tailored to specific fertilizing needs, all without the need to alter wastewater pH and thereby minimizing additional chemical costs.
It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.
This application claims the benefit of U.S. Provisional Application No. 63/519,369 filed on Aug. 14, 2023, and U.S. Provisional Application No. 63/634,989 filed on Apr. 17, 2024, each of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63519369 | Aug 2023 | US | |
| 63634989 | Apr 2024 | US |
