Patent Application
20200061541
Anaerobic treatment of organic wastes has become widespread due to benefits including reduction of biological oxygen demand (“BOD”) in the waste, production of methane, which can be burned to generate electricity, and fertilizer value in the digester effluent. Capturing the full fertilizer value of the effluent is challenging because effluent production is year-round while, in temperate zones, fertilizer is only needed during the growing season. In some climates, land application of digester effluent during the winter is prohibited because it causes run-off of nutrients into waterways.
Digestate volumes from biomass digesters can be large. For example, a digester producing two megawatts of power from food and farm waste may generate 100 tons of effluent per day. If all waste is applied to land only during the growing season, storage for 250 days of digestate is required. Tankage to hold the full 25,000 m3 is very capital intensive. The fertilizer value of most digestate is less than 5000 ppm nitrogen, which does not justify the expense of storage tanks.
Fertilizer value in the digestate arises from potassium or phosphorus fed to the digester, as well as ammonia formed during digestion. In anaerobic digesters, bacteria convert oxygen-containing organic compounds such as carbohydrates and cellulose to carbon dioxide and methane gas. Nitrogen in organic compounds such as protein or chlorophyll is converted to ammonia Ammonia in digestate generally reacts with carbon dioxide to form ammonium bicarbonate in solution.
Digester effluent from biomass digesters fed by waste from dairies or animal feedlots is particularly high in ammonia due to the breakdown of urea. Land spreading of such biomass effluent is a major cause of nitrogen contamination of lakes, rivers, and groundwater.
Environmental hazards from phosphorous also restricts the land application of biomass effluent. Precipitation and collection of phosphorous as struvite from single-strength effluent is often not economical due to low phosphorous concentrations.
Accordingly, there exists a need to recover nutrients from digestate by economically viable and environmentally friendly methods.
Embodiments disclosed herein are directed to methods and systems for concentrating effluent, removing nitrogen from effluent, and producing fertilizers. In an embodiment, a method of concentrating digestate from a biomass digester is disclosed. The method includes filtering solid matter from the digestate to produce a filtered digestate. The filtered digested has an amount of ammonia in it. Forward osmosis is performed using a draw solution and the filtered digestate as a feed solution. A digestate concentrate, which has an amount of ammonia in it, is produced from forward osmosis. The amount of ammonia in the digestate concentrate is less than amount of ammonia in the filtered digestate.
In an embodiment, a method of removing nitrogen from a feed solution is disclosed. A feed solution is provided, which includes nitrogen in the form of at least ammonia and ammonium ions. A draw solution is also provided. Forward osmosis is performed with the feed solution and the draw solution to transfer ammonia from the feed solution to the draw solution. The method reduces the amount of nitrogen in the feed solution by up to about 50%.
In another embodiment, a method of producing fertilizer is disclosed. Forward osmosis is performed with a feed solution and a draw solution. A diluted draw solution is produced from the draw solution. The draw solution has a percentage of ammonium ions that is greater than a percentage of ammonia. Reverse osmosis is performed on the diluted draw solution to produce a concentrate. Ammonium ions in the diluted draw solution react with an acid in the diluted draw solution to produce an ammonium salt in the concentrate, which forms a fertilizer.
In another embodiment, a method of processing digestate from a biomass digester is disclosed. The method includes filtering solid matter from the digestate to produce a filtered digestate. Forward osmosis is performed using a draw solution and the filtered digestate as a feed solution. A digestate concentrate and a diluted draw solution are produced from the filtered digestate and the draw solution. Reverse osmosis is performed on the diluted draw solution to produce a concentrate. Ammonium ions in the diluted draw solution react with an acid in the diluted draw solution to produce an ammonium salt in the concentrate, which forms a fertilizer. Magnesium salt is added to the feed solution to produce struvite, which forms a fertilizer. The amount of ammonia in the filtered digestate is greater than the amount of ammonia in the digestate concentrate. The amount of ammonia in the diluted draw solution is greater than an amount of ammonia in the draw solution. The diluted draw solution has an amount of ammonia that is less than the amount of ammonium ions in the diluted draw solution.
Features from any of the disclosed embodiments may be used in combination with one another, without limitation. In addition, other features and advantages of the present disclosure will become apparent to those of ordinary skill in the art through consideration of the following detailed description and the accompanying drawings.
The drawings illustrate several embodiments of the present disclosure, wherein identical reference numerals refer to identical elements or features in different views or embodiments shown in the drawings.
Embodiments disclosed herein are directed to methods and systems of concentrating effluent, such as a digestate from a biomass digester; removing nitrogen from a feed solution, which may be a digestate; and producing fertilizer, such as from a digestate.
With reference to
A starting material for use in the disclosed systems and methods may be a digestate, leachate, or effluent from other biomaterial processing streams. In the examples of
The raw digestate 202 may include undigested fibrous or particulate matter such as cellulose, clay, sand, and bone fragments. The digestate 202 may be passed through one or more filters 204 to remove the solid matter and to produce a filtered digestate 208 and sludge 206. The step 102 of filtering the digestate 202 may help to reduce or prevent plugging of a forward osmosis membrane. Filtration may be stepwise to remove solid matter in stages of decreasing particle size down to about 100 μm or less, about 50 μm or less, about 25 μm or less, about 20 μm or less, about 1 μm to about 100 μm, about 5 μm to about 100 μm, about 10 μm to about 100 μm, about 20 μm to about 100 μm, about 50 μm to about 100 μm, about 1 μm to about 80 μm, about 1 μm to about 60 μm, about 1 μm to about 40 μm, about 1 μm to about 20 μm, or about 5 μm to about 25 μm.
The filtering step 102 may be performed at any time during performance of the methods disclosed herein. Filtering the digestate 202 before performing forward osmosis, as shown in
The sludge 206, which may include the undigested fibrous or particulate matter, may be composted as shown in
Methods disclosed herein include performing forward osmosis 210 on a digestate 202 to transfer nitrogen and water from the digestate 202 to a draw solution 214, thereby concentrating the digestate 202 and reducing the amount of nitrogen in the digestate 202. In embodiments, the forward osmosis membrane permits water and neutral nitrogen species to cross, but does not permit charged nitrogen ion species to cross, which aids in concentrating and reducing the nitrogen content of the digestate 202. In embodiments, a pH differential between the digestate 202 and draw solution 214 aids in the transfer of nitrogen as a neutral species from the digestate 202 to the draw solution 214 and the retention of nitrogen as a charged species in the draw solution 214.
In the system or process of forward osmosis 210, two solutions are brought into contact with opposite sides of a semipermeable membrane. Water can diffuse freely through the membrane while most dissolved species are substantially blocked. If one of the two solutions has a higher concentration of dissolved species, water will move from the solution with fewer dissolved species into the solution with more dissolved species.
In some commercial forward osmosis systems, a large area of membrane is rolled into an element and the solution to be dewatered or concentrated (referred to as a “feed solution”) is pumped over one side of the membrane. A highly saline solution (referred to as a “draw solution”) is pumped across the other side of the membrane and water is drawn by osmosis from the feed solution into the draw solution. During the forward osmosis process, the feed solution is concentrated and the draw solution is diluted. The draw solution may be re-concentrated by a separate reverse osmosis process 230, as described in detail below.
The presently disclosed forward osmosis processes 210 may employ membrane modules such as those disclosed in PCT International Application No. PCT/US2016/053321 filed on 23 Sep. 2016, which is incorporated herein, in its entirety, by this reference.
Membranes used herein may be permeable to neutral nitrogen species, such as ammonia (NH3), and impermeable to charged nitrogen species, such as ammonium cations (NH4+). For example, the membrane may have a permeability to ammonia up to about 10,000 times higher than a permeability to ammonium ions. The membrane may permit passage of ammonia at a rate similar to that of water. The membrane may be constructed of a cellulose ester.
In the presently disclosed methods, the forward osmosis feed solution 212 may be any one or more of the digestate 202, filtered digestate 208, or aerobically digested digestate, which is described in more detail below. In embodiments, as shown in
The feed solution 212 may have a nitrogen content of about 0.1% to about 0.8%, about 0.12% to about 0.8%, about 0.15% to about 0.8%, about 0.2% to about 0.8%, about 0.4% to about 0.8%, about 0.1% to about 0.7%, about 0.1% to about 0.6%, about 0.1% to about 0.5%, about 0.1% to about 0.4%, or about 0.2% to about 0.5%.
The feed solution 212 may have a pH of about 7 to about 9, about 7.5 to about 8.5, about 7.5 to about 8, or about 8. The pH of the feed solution 212 may be adjusted by the addition of acid or base, as desired, or may be processed by forward osmosis without adjusting the pH.
In embodiments, anaerobic digestion of biomatter produces an effluent (digestate) having a pH of about 8. The digestate 202 is processed by forward osmosis without adjusting the pH. Without being limited to any mechanism or mode of action, the anaerobic digestion produces ammonia and carbon dioxide and the two compounds combine in solution to form ammonium bicarbonate. The gas/solution equilibrium of ammonium bicarbonate drives the digestate 202 to a pH of about 8. Ammonia has a pKa of 9.4; at a pH of about 8, about 88% is in the NH4+ form and about 12% is in the NH3 form. Carbon dioxide has a pKa of 6.4; at a pH of about 8, about 97% is in the HCO3− (bicarbonate) form and about 3% is in the H2CO3 form (carbonic acid). Accordingly, a digestate 202 having a pH of about 8 has a greater percentage of its nitrogen content in the form of ammonium ions than in the form of ammonia.
The forward osmosis draw solution 214 includes at least one acid or salt of an acid. The acid may be organic or inorganic. Examples of acids include acetic acid, citric acid, hydrochloric acid, maleic acid, nitric acid, phosphoric acid, sulfuric acid, and combinations thereof. In one embodiment, the draw solution 214 includes sulfuric acid and/or ammonium sulfate.
The draw solution 214 may have a pH of about 5 or less, about 4 or less, or about 3 to about 5. In an embodiment, the draw solution 214 has a pH of about 4.3. The pH of the draw solution 214 may be adjusted by the addition of acid. At a pH of about 5 or less, the ammonium/ammonia equilibrium is about 99.9995% Nhd 4+ and about 0.0005% NH3. At a pH of about 5 or less, the carbon dioxide/bicarbonate equilibrium is about 99.5% CO2 and about 0.5% HCO3−.
A feed solution 212 having a pH of about 7 to about 9 has a greater percentage of its nitrogen content in the form of ammonia (e.g., 12% NH3) than does a draw solution 214 having a pH of about 5 or less (e.g., 0.0005% NH3).
The feed solution 212 and draw solution 214 may have a pH differential of about 2 to about 6, about 3 to about 6, about 4 to about 6, about 2 to about 5, about 2 to about 4, about 3 to about 4.5, or about 3.5 to about 3.9. A pH difference between the solutions may help achieve or maintain higher percentage of ammonia in the feed solution 212 than in the draw solution 214. In an embodiment, a feed solution 212 having a pH of about 7 to about 9 has a greater percentage of its nitrogen content in the form of ammonia (e.g., 12% NH3) than does a draw solution 214 having a pH of about 5 or less (e.g., 0.0005% NH3). A pH different between the solutions may allow ammonia in the feed solution 212 to move to the draw solution 214, thereby reducing the nitrogen load in the feed solution 212.
In embodiments, the concentration of ammonia is greater in the feed solution 212 than in the draw solution 214 and ammonia in the feed solution 212 moves across the ammonia-permeable forward osmosis membrane to the draw solution 214. In the draw solution 214, the pH is acidic and ammonia converts to the ammonium cation. The ammonium cation cannot return to the feed solution 212 because the membrane is relatively impermeable to ammonium cations. Nitrogen is thereby transferred from the feed solution 212 to the draw solution 214.
In embodiments, the concentration of bicarbonate anions is greater in the feed solution 212 than in the draw solution 214 and bicarbonate anions in the feed solution 212 move across the bicarbonate-permeable forward osmosis membrane to the draw solution 214. In the draw solution 214, the pH is acidic and HCO3− coverts to H2CO3. Carbon dioxide has a solubility of about 1000 ppm in the draw solution 214 and excess CO2, such as that entering from the feed solution 212, is released as a gas from the draw solution 214.
During forward osmosis, acid 216 may be added to the draw solution 214 to maintain a desired pH, such as about 5 or less or about 4.3. Maintaining the desired pH may help to maintain the concentration of ammonia in the draw solution lower than the concentration of ammonium ions in the draw solution. Maintaining the desired pH may help to maintain the concentration of ammonia in the draw solution lower than the concentration of ammonia in the feed solution 212. Maintaining the desired pH may help to permit the diffusion of nitrogen from the feed solution 212 to the draw solution 214. The acid 216 may be any acid described above. The acid 216 may be the same acid as that used to acidify the draw solution 214 prior to the start of forward osmosis. In one embodiment, the acid 216 is sulfuric acid.
In the methods disclosed herein, the step 104 of forward osmosis reduces the volume of the feed solution, reduces the nitrogen content of the feed solution 212, increases the volume of the draw solution 214, and/or increases the nitrogen content of the draw solution 214.
Forward osmosis produces a digestate concentrate 228 from the digestate 202 or filtered digestate 208 feed solution 212. The volume of the digestate concentrate 228 may be less than about 40% of the volume of the feed solution 212 prior to performing forward osmosis, less than about 30%, less than about 20%, less than about 10%, about 5% to about 40%, about 10% to about 30%, or about 20% of the volume of the feed solution 212 prior to performing forward osmosis.
The nitrogen content of the digestate concentrate 228 may be about 0.25% to about 1.8%, about 0.3% to about 1.8%, about 0.5% to about 1.8%, about 0.75% to about 1.8%, about 1% to about 1.8%, about 1.25% to about 1.8%, about 1.5% to about 1.8%, about 0.25% to about 1.5%, about 0.25% to about 1%, about 0.25% to about 0.75%, about 0.25% to about 0.5%, or about 0.5% to about 1.25%.
The nitrogen content of the digestate concentrate 228 may be about 1% to about 50% of the nitrogen content of the feed solution 212 prior to performing forward osmosis, about 1% to about 40%, about 1% to about 30%, about 1% to about 20%, about 1% to about 10%, about 2% to about 50%, about 5% to about 50%, about 10% to about 50%, about 20% to about 50%, about 30% to about 50%, or about 5% to about 30% of the nitrogen content of the feed solution 212 prior to performing forward osmosis.
Forward osmosis produces a diluted draw solution 218 from the draw solution 214. The volume of the diluted draw solution 218 is greater than the volume of the draw solution 214 prior to performing forward osmosis. The volume may increase due to any one or more of the diffusion of water from the feed solution 212 to the draw solution 214, the diffusion of ammonia from the feed solution 212 to the draw solution 214, or the addition of acid 216 to the draw solution 214.
The nitrogen content of the diluted draw solution 218 may be greater than the nitrogen content of the draw solution 214 prior to performing forward osmosis. The increase in nitrogen content may approximate the decrease in nitrogen content from the feed solution 212 to the digestate concentrate. 228
The digestate concentrate 228 may rich in potassium and/or phosphate. The digestate concentrate 228 may be applied to land as potassium- or phosphate-rich solution. Alternatively or additionally, the digestate concentrate 228 may be further processed, such as by subjecting it to aerobic digestion or removing phosphorus from it, each as described below.
The digestate 202 may include large amounts of undigested cellular material such as macromolecules including cellulose, lignin, proteins, and extracellular polysaccharides. Such macromolecules can cause high viscosity levels in digestates 202 and can set to form a gel-like solid at low temperatures or stagnant flows. High viscosity reduces the ability to concentrate the digestate 202.
The macromolecules are generally amenable to aerobic but not anaerobic digestion. The digestate 202 may be anaerobically digested to break down undigested material so as to reduce the viscosity of the digestate 202, which may permit further concentration of the digestate 202. Anaerobic digestion may lower the biological oxygen demand of the digestate 202, and release phosphate from some macromolecules.
The aerobic digestion 232 may be performed by bubbling gas through the digestate 202 or digestate concentrate 228. The step 106 of aerobic digestion may be performed for a period of time short enough to reduce or limit the stripping of ammonia from the digestate 202 or digestate concentrate 228 by air utilized in the aerobic digestion process. The aerobic digestion may be performed for less than 48 hours, less than 36 hours, less than 24 hours, or less than 12 hours.
The aerobic digestion step 106 may be performed at any time during performance of the methods disclosed herein. Aerobic digestions may be performed on the digestate 202, filtered digestate 208, or digestate concentrate 228. With reference to
Aerobic digestion may produce sludge 234, which may include undigested or indigestible material from the digestate 202. The sludge 234may be returned to digester feed as shown in
The digestate 202 may include phosphorus, which may be present at levels too high to apply to the land in some jurisdictions, such as due to regulatory restrictions. The phosphorus, as well as some nitrogen, may be removed in the form of struvite 240 (magnesium ammonium phosphate; MgNH4PO3). Generation and collection of struvite 240 may help reduce phosphorus or nitrogen concentrations such that the digestate 202 can be applied to land without causing environmental damage. Generation and collection of struvite 240 may help extract phosphorus and nitrogen from the digestate 202 to produce an economically valuable struvite fertilizer.
Struvite 240 is an insoluble salt and will precipitate out of the digestate 202. Struvite 240 may be produced by the addition of a magnesium salt, such as magnesium carbonate, to the digestate 202, which may include little or no magnesium. Addition of magnesium 238 may result in the precipitation of all or nearly all of the phosphorus in the digestate 202. In some implementations, the pH of the digestate 202 may be increased to aid in the generation and precipitation of struvite 240.
Magnesium 238 may be added to, and struvite 240 may accordingly be removed from, the digestate 202, filtered digestate 208, digestate concentrate 228, or aerobically digested concentrate 236. In the embodiments depicted in
After removal of phosphorus and some nitrogen in the form of struvite 240 from the aerobically digested concentrate 236, some or all of the remaining aerobically digested concentrate 236 may be returned to the feed stream of the digester, such as along with farm and food waste. Returning the aerobically digested concentrate 236 to the digester reduces or eliminates the need to store the aerobically digested concentrate 236, which reduces costs.
Prior removal of phosphorus and some nitrogen in the form of struvite 240 helps produce a aerobically digested concentrate 236 that is suitable for environmentally safe land application. Some or all of the aerobically digested concentrate 236 may be applied to land, with or without storage. Applying the aerobically digested concentrate 236 to land reduces the buildup of salts in the digestate 202. Applying the aerobically digested concentrate 236 to land may reduce or eliminate the need to store the aerobically digested concentrate 236, which reduces costs.
In the methods disclosed herein, a step 110 of reverse osmosis may be performed, which may produce an ammonium salt fertilizer. In the system or process of reverse osmosis 230, water under pressure is pushed across a semi-permeable membrane to separate water and dissolved material, such as salts. With reference to
The diluted draw solution 218 includes ammonium ions. In embodiments, the diluted draw solution 218 has a percentage of ammonium ions greater than a percentage of ammonia. Without being limited to any mechanism or mode of action, ammonia in the forward osmosis feed solution 212 diffuses into the draw solution 214, where the acidic pH converts the ammonia to ammonium ions. The ammonium ions react with an acid 216 in the diluted draw solution 218 to produce ammonium salts 224 in the concentrate 222. Examples of ammonium salts 224 include ammonium acetate, ammonium chloride, ammonium citrate, ammonium maleate, ammonium nitrate, ammonium phosphate, and ammonium sulfate. In an example, ammonium ions react with sulfuric acid to produce ammonium sulfate.
Ammonium salts 224, such as ammonium sulfate, may be concentrated to greater than 150,000 ppm during reverse osmosis, greater than 100,000 ppm, greater than 75,000 ppm, greater than 50,000 ppm, from about 50,000 ppm to about 150,000 ppm, from about 100,000 ppm to about 150,000 ppm, or from about 50,000 ppm to about 100,000 ppm during reverse osmosis.
The concentrate 222 may include ammonium salts 224 and water. The concentrate 222 may include about 8% to about 25% ammonium salts 224, about 10% to about 25%, about 15% to about 25%, about 20% to about 25%, about 8% to about 20%, about 8% to about 15%, or about 8% to about 10% ammonium salts 224. In an example, the concentrate 222 includes about 15% to about 17% ammonium sulfate and about 83% to about 85% water.
The concentrate 222 may be dried to remove water. The concentrate 222, with or without drying, may be stored until needed or until application to the land will not result in nutrient runoff. The concentrate 222, with or without drying, may be used as an ammonium salt fertilizer.
The water 226 produced by reverse osmosis may be used as process water, digester feed water, irrigation water, or the water may be land applied without also applying an excess of nutrients.
Compared to other methods of treating effluent from biomatter digesters, the presently disclosed methods remove more water and remove more ammonia. The ability to concentrate effluent more than other methods helps dairy farmers maintain herd size without exceeding limits on nutrient production and application to the land. The presently disclosed methods help the agricultural industry comply with environmental regulations regarding nutrient application to the land and limiting nutrient runoff. The presently disclosed methods may provide a revenue source from selling ammonium salt fertilizers and struvite fertilizers.
While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting.
This application claims priority to U.S. Provisional Application No. 62/423,666 filed on Nov. 17, 2016, the disclosure of which is incorporated herein, in its entirety, by this reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2017/062066 | 11/16/2017 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62423666 | Nov 2016 | US |
