Patent Application
20060006121
The present invention relates to a method and composition for inhibiting the production and release of gaseous compounds, which result in odors, from organic waste produced by metabolic processes, including human and animal waste, as well as industrial wastes, effluents, sewage, and the like.
The biogenic production of volatile compounds which cause objectionable odors is one of the problems associated with the collection and treatment of various waste materials. Domestic sewage is the largest source of such odorous compounds. Because of the magnitude of domestic sewage that is collected and treated and the associated odorous compounds, the present invention is particularly directed, but not limited to the control of sulfidic compounds in municipal or industrial waste. As used herein, the term “sulfidic compounds” also includes hydrogen sulfide (H2S), mercaptans (RSH), and other related odoriferous sulfidic compounds.
The mixed biological population common to municipal or industrial waste utilizes the compounds found in the waste as a source of nutrient. In this process, oxygen is the preferred terminal electron acceptor, and the nutrient, commonly an organic compound, is oxidized. In highly nutrient loaded systems such as municipal sewage, bacterial action can result in a rapid consumption of oxygen in the water. In the absence of oxygen, bacteria require an alternate terminal electron acceptor.
In general, bacteria will utilize the terminal electron acceptor that provides them with the greatest amount of energy. Thus, there is a preferred selection order of a terminal electron acceptor by bacteria. This order is shown below:
O2>NO3−>Fe+2>SO4−2>C3−2
As nitrate is not typically found in natural waters, the sulfate ion (SO4−2) is generally the preferred alternate. In the absence of oxygen, unless nitrate is added supplementally, those bacteria which can utilize sulfate as a terminal electron acceptor in their respiration process will predominate. The best known and most highly characterized bacteria of this type is Desulfovibrio desulfuricans. These bacteria are most commonly referred to as sulfate-reducing bacteria, SRB. SRB are known to metabolize sulfate ion with organic matter to form H2S as shown in the following equation.
SO4−2+organic matter+SRB—H2S+CO2+H2O
H2S, responsible for the characteristic odor from rotten eggs, is toxic in low concentrations. Citizen complaints are often the driving force behind efforts to control odor. Such odors are generally regarded as a public nuisance and a health hazard.
H2S is also corrosive towards steel and concrete.
Although H2S is a gas, H2S in water can dissociate with increasing pH as shown in the following equations. Thus at a given pH, the relative amount of dissolved H2S species can be predicted. The sulfide ion, S−2, and bisulfide ion, HS—, being ionic, remain in the aqueous phase.
H2S+OH−→HS−+H2O
HS−+OH−→S−2+H2O
(H2S— gas phase & aqueous phase, HS− & S−2 aqueous phase)
Various compounds, including hypochlorite (sodium or calcium), potassium permanganate, nitrate (calcium, sodium and ferric), ferrous and ferric salts, hydrogen peroxide, chlorine, chlorine dioxide, sodium chlorite, and more recently, magnesium oxide or magnesium hydroxide have been used for the control of odor in wastes, and sewage waste in particular.
Sodium chlorite has been used alone for odor control. Several references to such use follows:
It is also known that nitrates added to sewage effect reduction in BOD and even suppress the formation of hydrogen sulfide gas via bacterial action.
Even nitrite has been used to control sulfate reducing bacteria and associated odors:
In addition, the use of some sulfide reactive chemicals in combination with nitrates is known:
A combination of nitrate and microorganisms is taught in the following patent:
Other compounds reactive with sulfide are known:
The use of compounds to elevate pH to convert sulfide species to ionic species which remains in solution and minimizes H2S gas evolution is taught in the following patents:
Some of the treatments using specific chemicals have advantages in certain applications. However, they also suffer from various drawbacks, some of which are listed below. The chemicals are separated by their rate of reaction with sulfide or H2S.
In the control of odors, there are three primary types of odor control chemistries that are required. These are listed in chronological order of appearance. The first type is for municipal sewer lines, or similar applications. In such applications, the biogenic generation of H2S occurs in pipes with residence times on the order of hours or days, and thus any chemical added has a substantial amount of time to work. Those chemicals which act in a slow to moderate manner are acceptable in such applications.
The second type is for sludge dewatering, where the H2S must be consumed immediately. Generally chemistries which are slow to react with H2S are not acceptable in such applications.
The third type of odor control chemistry is for dewatered sludge, where the sludge may sit for days or weeks prior to being landfilled or moved. Chemistries that are very slow to react are acceptable in such applications.
There are additional factors which may impact the choice of odor control chemistries. One such factor is the impact such chemical has on corrosion of piping or equipment. Sometimes this factor is paramount in the selection of odor control chemistry, and some of these chemistries are significantly more corrosive than others.
One final factor which is just beginning to be recognized is the impact monovalent and polyvalent cations have on floc formation in sludge, and its ultimate impact on sludge dewatering. In fact, researchers have identified that there is an optimum ratio of monovalent to divalent cations in the formation and stability of floc formed, and ultimately in the dewaterability of the sludge formed. A dryer sludge cake translates directly to reduced shipping costs of the dewatered sludge.
In view of the disadvantages and additional factors cited above, there is a need in the art for a method and composition for abating odor in waste materials that is cost effective, has the capability of consuming aqueous sulfide or H2S immediately, adjusts the pH into a range where the volatile form of sulfide species is very low, and provides long term control of biogenic sulfide production. Additionally the method and composition should be less corrosive and should have stable floc formation and as a result the sludge should dewater easily. Accordingly, it is an object of the present invention to address this need in the art. This and other objects of the present invention will become more apparent in light of the following summary and detailed description of the invention.
This invention relates to a method and composition for the elimination or reduction of sulfidic odors in sewer systems, municipal waste treatment plants and in other industrial waste applications, sludge dewatering, and in the dewatered sludge. The composition comprises a synergistic combination of a rapid sulfide-consuming chemical and a chemical which both increases and buffers the pH, a chemical which acts as an organic biocide, and a chemical which provides long term biogenic prevention.
Existing sulfides are consumed immediately by the sulfide-consuming chemical. The pH-elevating compound adjusts the pH into a range of >7.5, where the form of volatile sulfide is minimized. In addition, adjustment of the pH into this range promotes growth of sulfide-consuming bacteria and inhibits the growth of sulfide-producing bacteria. The organic biocide also consumes H2S while at the same time targeting sulfide-producing bacteria to both prevent and minimize H2S. The chemical preventing long term biogenic H2S production acts as a terminal electron acceptor so that the bacteria metabolize the chemical rather than sulfate, thus preventing the formation of H2S. In addition, the chemical promotes the growth of competing bacteria which produce metabolites that are inhibitory to sulfide producing bacteria.
The sulfide consuming chemical is selected from the group comprising an iron salt, a hypochlorite, a permanganate, a persulfate, a perborate, a periodate, a percarbonate, a chlorite, a nitrite, a chlorate, a perchlorate and a peroxide. The pH elevating and buffering chemical is magnesium oxide, magnesium hydroxide, magnesium carbonate, or magnesium peroxide. The organic biocide is selected from the group comprising glutaraldehyde and tetrakis (hydroxymethyl) phosphonium sulphate (THPS). The chemical for long term prevention of biogenic sulfide is selected from the group comprising metal and alkali metal nitrates. The method according to the present invention comprises the step of contacting the waste products or their surrounding airspace with the composition.
Unexpected benefits are predicted to arise also from application of the composition. Application of chlorite has been found to enhance dewaterability of produced sludge in some applications. In addition, application of Mg(OH)2 alone has also been found to promote the dewaterability of sludge, although the mechanisms by which these two chemicals act are different. There is an expected synergy when both of these chemicals are applied simultaneously in that the dewaterability of sludge is enhanced over that which would be seen by application of each chemical separately.
Another unexpected benefit is that long term odor of dewatered sludge, which is troublesome in land application of such sludge, is predicted to be controlled much better by the composition than by each component of the composition when applied separately.
A third unexpected benefit is that the corrosivity of the preferred embodiment is significantly reduced.
A final unexpected benefit is that the level of pathogenic bacteria found in the dewatered sludge is significantly reduced over untreated sludge. This is very important in situations where the sludge is applied to land to act as a fertilizer.
The composition for controlling odor from waste products according to the present invention comprises a combination of a rapid-acting sulfide-consuming material and a longer acting material which elevates and buffers pH >7.5. At this pH range, the solubility of H2S in the aqueous phase is increased substantially, and the growth of sulfide-producing bacteria is inhibited, as shown in Table 2.
The rapid-acting sulfide consuming material is selected from the group which includes an iron salt, or a hypochlorite, a permanganate, a persulfate, a perborate, a periodate, a percarbonate, a chlorite, a nitrite, a chlorate, a perchlorate and a peroxide of ammonium, metal or alkali metal. The longer acting chemical which raises and buffers the pH is magnesium oxide or magnesium hydroxide. The pH is adjusted and buffered into a range where the concentration of the volatile sulfide species is minimized, and where the formation of biogenic sulfide is minimized because the pH is adjusted to a range where growth of sulfide-producing bacteria is inhibited. The organic biocide is chosen from the group comprising glutaraldehyde and tetrakis (hydroxymethyl) phosphonium sulphate (THPS). The biocide aids in sulfide consumption and in particular targets sulfide producing organisms, inactivating them and preventing their production of volatile sulfide over a longer time period. The chemical used for prevention of long term biogenic sulfide production is selected from the group comprising metal or alkali metal nitrates. The nitrate acts as a terminal electron acceptor and is preferentially metabolized by bacteria over sulfate, and as a result, biogenic sulfide production is eliminated or greatly reduced. As used herein, the term “controlling odor” means reducing and/or eliminating odor that is offensive to humans. Such odors are usually caused by volatile sulfides and other volatile odorous substances.
The waste products treatable with the present invention include, but are not limited to organic waste produced by metabolic processes, including human and animal waste, as well as industrial wastes, effluents, sewage, and the like. The preferred aqueous composition includes sodium chlorite at a weight percent of 0.01-31%, magnesium hydroxide which may range in concentration from 0.01-65% weight percent, THPS, which may range in concentration from 0.01-10%, sodium nitrate which may range in concentration from 0.1-40% by weight and water at 4%-99.98 wt %. The preferred dry composition includes sodium chlorite at concentrations of 0.01-75% and magnesium hydroxide at 25-99.99%.
The aqueous solution or the dry composition according to the invention can be employed to destroy and prevent the malodorous characteristics of odor causing compounds such as sulfides or volatile fatty acids found in sewage and other waste products. The solution can be pumped into the material to be treated (liquid, sludge, or solid) or sprayed onto the surface or into the airspace surrounding the material. The dry material can be mixed into a slurry or solution at the point of application and applied in a similar manner.
This method and composition involve three primary application areas. These include treatment at fairly low levels into municipal sewer lines and the like, sludge dewatering applications, and sludge dewatering applications where the sludge is to be held for several days. Each area will be discussed in some detail.
Municipal Sewer Lines: Sodium chlorite reacts very slowly, if at all, with the vast majority of compounds found in sewage at the pHs normally encountered in sewage. It will, however, react rapidly with sulfidic compounds. Thus, the vast majority of the chlorite added to the sewage will consume sulfidic compounds, which generally allows low concentrations to be used. The sodium chlorite provides rapid control of low levels of sulfides commonly found at upstream points early in the sewer line distribution system. The treatment concentration is directly dependent upon the amount of odor causing compounds with chlorite demand that are present in the waste.
The magnesium hydroxide buffers pH to a range sufficiently high so that sulfate reducing bacteria do not thrive, and thus are prevented from producing H2S. The arrest of H2S production using magnesium hydroxide alone is not immediate, and in some situations, if the sulfide is sufficiently high, has been shown to be ineffective at controlling sulfide odors. The concentration of magnesium hydroxide present in the treatment solution may vary depending upon the amount of residual control of malodorous compounds that is required. Magnesium hydroxide is less costly than chlorite and thus lowers the cost per pound of the treatment solution.
The preferred embodiment of such a composition for treatment of municipal sewer lines would include a 45-55 wt % Mg(OH)2/0.1-2 wt % NaClO2 blend. The preferred application rate would be to apply the composition at the rate of 2-10 part of chlorite per part by weight of sulfide, with sufficient Mg(OH)2 to achieve a pH of at least 7.5.
Sludge Dewatering: Chlorite reacts very rapidly with odor causing sulfidic compounds. For applications such as sludge dewatering, sufficient chlorite should be fed to consume the relatively high levels of sulfide which are present in such applications. The Mg(OH)2 also buffers the pH upward into a range where the volatile form of sulfide is converted to the soluble form, thus providing some benefit in terms of odor control. In addition, the floc formed in the presence of the blend is predicted to be more stable and more easily dewatered than in the presence of either component alone. Finally, the presence of Mg(OH)2 has been found to inhibit corrosive properties of chlorite when the Mg(OH)2/chlorite ratio is around 3:1 on a weight: weight basis. A study of the corrosive properties of Mg(OH)2/chlorite at varying ratios, was compared to the corrosive properties of straight chlorite, and chlorite/nitrate mixtures.
Corrosion Experiment
The experimental conditions were determined by the DOE software, DOE PRO-XL, Air Academy Associates, corrosion rates obtained from weight loss of coupons. The data was then entered and the program generated the contour surface plot from the best fit to the experimental data.
The data indicate that in the absence of chlorite, the corrosion rate is reduced linearly with increasing nitrate concentration. With increase in chlorite concentration, the corrosion increases. There does appear to be a very slight improvement in corrosive effects of chlorite toward SS 316.
Corrosion Experiment
A number of things can be identified from this information. First, the impact NaNO3 has on the corrosive effects of chlorite on the carbon steel corrosion rate is shown in the second plot.
In this plot, chlorite is 500 mg/L, and SQ (Sea Quest™, a mixture of 77% poly-phosphate) and CaCO3 are both held to 0. This plot shows that as the Mg(OH)2 is increased, a minimum is reached at about 1100-1200 mg/L. On the other hand, when Mg(OH)2 is held to zero, as NaNO3 increases from 0 to 500 mg/L, there is almost no reduction in corrosion effects of chlorite. As CaCO3 (effects not shown) and NaNO3 showed very little improvement in corrosion rate, they were ignored in subsequent experiments.
Corrosion Experiment
It is clear from the results of Experiment 2 and 3 that Mg(OH)2 has a substantial corrosion inhibiting properties on the corrosive effects of NaClO2, the corrosion rate being reduced by almost an order of magnitude when Mg(OH)2/chlorite ratio is around 2.5-3.5:1. Also, the polyphosphate product, SQ, shows even greater improvement in the corrosion rate, effectively stopping corrosion altogether with SQ at 10 mg/L and Mg(OH)2 at 1300-1500 mg/L.
The preferred embodiment of a composition for sludge dewatering would include a 25-45 wt % Mg(OH)2/6-25 wt % NaClO2 blend. The preferred composition would vary depending upon sludge pH, concentration of monovalent cations, and concentration of odor causing sulfidic compounds. The application rate would be to apply the composition at the rate of 2-10 part of chlorite per part by weight of sulfide, with sufficient Mg(OH)2 to achieve a pH of at least 7.5.
Unexpected benefits are predicted to arise also from application of this composition. Application of chlorite has been found to enhance dewaterability of produced sludge in some applications. In addition, application of Mg(OH)2 alone has also been found to promote the dewaterability of sludge, although the mechanisms by which these two chemicals act are thought to be different. There is an expected synergy when both of these chemicals are applied simultaneously in that the dewaterability of sludge is enhanced over that which would be seen by application of each chemical separately.
We predict an improvement in the sludge dewaterability from 5-50% or more, as measured by Capillary Suction Timer or Time-to-Filter tests as described in ‘Standard Methods,’ over the application of each component individually.
Dewatered Sludge: The treatment of the sludge during dewatering application by the method and composition described above will generally be sufficient to accomplish the immediate goal of oxidation of sulfidic compound and odor control by such method. In some cases, the sludge produced from the dewatering application is stored for later removal or land fill application. Odors have been found to emanate from such dewatered sludge, and so supplemental chemicals may be utilized during the dewatering application to extend the time of odor control. In addition, there may be a requirement for pathogen destruction of the sludge prior to landfill.
The preferred embodiment for odor control during sludge dewatering applications would include a 25-45 wt % Mg(OH)2, 6-25 wt % NaClO2, 0.1-10 wt % THPS, and 0.1-40 wt % NaNO3. The preferred composition would vary depending upon sludge pH, concentration of monovalent cations, and concentration of odor causing sulfidic compounds. The application rate would be to apply the composition at the rate of 2-10 part of chlorite per part by weight of sulfide, sufficient Mg(OH)2 to achieve a pH of at least 7.5, sufficient THPS to accomplish at least a 1 log reduction in pathogenic bacteria, and sufficient NaNO3 to retard the growth of sulfide-producing bacteria so that odors attributable to sulfidic compounds are not obvious for at least 24 hours.
An unexpected benefit of this method and composition is that long term odor of dewatered sludge, is predicted to be controlled much better by the composition than by each component of the composition when applied separately.
This application refers to Provisional Application Ser. No. 60/586,349 filed Jul. 8, 2004.
| Number | Date | Country | |
|---|---|---|---|
| 60586349 | Jul 2004 | US |
