CATIONIC POLYMER

BACKGROUND OF THE INVENTION

Field of the Invention

[0001] This invention is related to a method of using a cationic flocculant to enhance anaerobic bioprocess, such as biogas production rate and increasing chemical oxygen demand (COD) removal efficiency.

Description of Related Art

[0002] Anaerobic treatment is a common way to treat solid and liquid waste streams. The process is advantageous in that it is suitable for treating wastes with high strength wastewater content or difficulties in incineration, but also produces methane as a source of clean energy and reduces the volume of residual solids requiring processing and disposal. However, anaerobic bioprocess is carried out by a consortium of microorganisms and depends on various factors like pH, temper ature, hydraulic retention time (HRT), biomass concentration, carbon/nitrogen (C/N) ratio, etc. Further, it is a relatively slow process with a large HRT in conventional anaerobic treatment plants. This leads to a large volume of the digester and hence high cost of the system. Further, the decrease in gas generation during the winter season or during shock external conditions poses serious problems. Additionally, the variance in biogas production and unexpected effluent quality in the practical application of this technology will cause an increase in pressure on the downstream treatment, and add additional costs. All of these reasons have resulted in restricted popularization of anaerobic treatment technology.

[0003] Prior attempts to solve these problems have focused on the need to improve the overall efficiency of the anaerobic digestion process in the anaerobic treatment plants. This has been done by utilizing several methods, such as pretreatment, optimizing the various operational parameters, satisfying the nutritional requirements of microbes and using different biological and chemical additives. Pretreatment by physical, chemical or biological means is a well-investigated process for ethanol production and biogas production enhancement. The pretreatment can enhance the biodigestibility of the wastes for ethanol and biogas production and increase accessibility of the enzymes to the materials. It results in enrichment of the difficult biodegradable materials, and improves the yield of ethanol or biogas from the wastes. However, these methods do not achieve significant system performance improvement with minimum investment and/or cost.

[0004] The current invention allows for a significant system performance improvement with minimum investment and/or cost by providing a method that significantly quickens the anaerobic treatment process, enhances the anaerobic treatment performance and produces more biogas. The method increases the synergy of the microbes, improves the microbe consortia and facilitates the granular sludge formation through coagulation and flocculation in the anaerobic digester.

SUMMARY OF THE INVENTION

[0005] The present invention concerns a method of biogas production enhancement for an anaerobic digestion process. The method of biogas production enhancement uses a cationic polymer to improve the efficiency of converting organic waste into biogas, such as a methane containing gas. The cationic polymers typically used are a copolymer of

acrylamide/dimethylaminoethyl acrylate methyl chloride. Biochemical methane potential (BMP) assay was carried out with varying amounts of cationic polymer. The results indicated much earlier and higher biogas production rates compared to the control and increasing COD removal efficiency with the application of a cationic polymer at a certain range of concentration.

[0006] In some embodiments, the low concentration of cationic polymer additive can condition the charges on the surface of the anaerobic microbe and enhance synergy of the microbe community resulting in better kinetics of hydrolysis, acidogenesis and methanogenesis, meanwhile showing minimum bio-toxicity of the additive to the anaerobic microbes. Further, the low concentration of the cationic polymer can enhance the tolerance of the microbes to the toxicity of the wastewater by conditioning the microbial community.

[0007] The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] These and other aspects of the invention will be understood from the description and claims herein, taken together with the drawings showing details of construction and illustrative embodiments, wherein:

[0009] Fig. 1 depicts a plot of biogas volume vs. time.

DETAILED DESCRIPTION OF THE INVENTION

[0010] Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about" , is not limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Range limitations may be combined and/or interchanged, and such ranges are identified and include all the sub-ranges stated herein unless context or language indicates otherwise. Other than in the operating examples or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, reaction conditions and the like, used in the specification and the claims, are to be understood as modified in all instances by the term "about" .

[0011] "Optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, or that the subsequently identified material may or may not be present, and that the description includes instances where the event or circumstance occurs or where the material is present, and instances where the event or circumstance does not occur or the material is not present.

[0012] As used herein, the terms "comprises", "comprising" , "includes" , "including", "has" , "having" , or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article or apparatus that comprises a list of elements is not necessarily limited to only those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0013] The singular forms "a" , "an", and "the" include plural referents unless the context clearly dictates otherwise.

[0014] Disclosed is an improved method of biogas production enhancement using a cationic polymer for an anaerobic digestion process. Anaerobic treatment is suitable for treating wastes with high strength wastewater content or difficulties in incineration, but also produces methane as a source of clean energy and reduces the volume of residual solids requiring processing and disposal. However, anaerobic bioprocess is carried out by a consortium of microorganisms and depends on various factors like pH, temperature, hydraulic retention time (HRT), biomass concentration, carbon/nitrogen (C/N) ratio, etc. The improved method of biogas production enhancement uses a cationic polymer to improve the efficiency of converting organic waste into biogas, such as a methane containing gas. Typically, the cationic polymer is a copolymer of acrylamide/dimethylaminoethyl acrylate methyl chloride, but could be any suitable cationic polymer as is known in the art without affecting the overall concept of the present invention.

[0015] Specifically, the cationic polymer is a copolymer of acrylamide and

dimethylaminoethyl acrylate methyl chloride. Additional polymers that can be used include, but are not limited to, a polymer of (meth)acrylamide and one or more cationic monomers selected from the group consisting of: dimethylaminoethyl (meth)acrylate methyl

chloridequaternary salt, dimethylaminoethyl (meth)acrylate methyl sulfate quaternary salt, dimethylaminoethyl (meth)acrylate benzyl chloride quaternary slat, dimethylaminoethyl (meth)acrylate sulfuric acid salt, dimethylaminoethyl (meth)acrylate hydrochloric acid salt, dialkylaminoalkylacrylamides or methacrylamides and their quaternary or acid slats, acrylamidopropyltrimethylaninionium chloride, dimethylaminopropyl (meth)acrylamide methyl sulfate quaternary salt, and dimethylaminopropyl (meth)acrylamide hydrochloric acid salt, diethylaminoethylacrylate, diethylaminoethylmethacrylate, diallyldiethylammonium chloride, and diallyldimethyl ammonium chloride.

[0016] A biochemical methane potential (BMP) assay was carried out using varying amounts of the cationic polymer. The effective amount of cationic polymer is from about 1 ppm to about 100 ppm active polymer in the anaerobic digester. The results indicated much earlier and higher biogas production rates compared to the control and increasing COD removal efficiency with the application of a cationic polymer.

[0017] Additionally, enhancement of the anaerobic bioprocess will also result in shortening start-up time and shorter HRT, as well as lower effluent COD, total suspended solids (TSS) and less sludge production. The addition of the low concentration of cationic polymer facilitates the granular sludge formation through coagulation in the anaerobic digester. The addition of the cationic polymer can increase the synergy of the microbes and shorten the duration of the healthy microbe consortia formation in the system which is the critical step to the successful digester start-up. The enhancement of the anaerobic bioprocess gives rise to much earlier and higher biogas production rates and increasing COD removal efficiency compared to the control. Thus, the system could complete the treatment within less time, which results in shorter HRT.

[0018] The low concentration of cationic polymer additive can condition the charges on the surface of the anaerobic microbe and enhance synergy of the microbe community resulting in better kinetics of hydrolysis, acidogenesis and methanogenesis. Specifically, as bacteria have negatively charged surfaces under normal pH conditions, the addition of the cationic polymer at low concentrations can reduce the electrostatic repulsion between the negatively charged bacteria. Furthermore, the cationic polymer may form a bridge between cells, and this would facilitate the collaboration of different kinds of microbes that have different functions including hydrolysis, acidogenesis and methanogenesis. Accordingly, the low concentration of cationic polymer additive can facilitate the granular sludge formation through coagulation and flocculation in the anaerobic digester. Further, at low concentrations the cationic polymer showed no toxicity to the microbes in the anaerobic reaction system. Instead, the low concentration of the cationic polymer can enhance the tolerance of the microbes to the toxicity of the wastewater by conditioning the microbial community.

[0019] The present disclosure will now be described more specifically with reference to the following examples. It is to be noted that the following examples are presented herein for purpose of illustration and description; they are not intended to be exhaustive or to limit the disclosure to the precise form disclosed.

EXAMPLE 1

[0020] This example demonstrates that the low concentration of a cationic polymer additive can significantly speed up biogas production, increasing biogas production rate by about 80%- 112% (compared to the control) and increasing COD removal efficiency by > 25% with the application of a cationic polymer at a certain range of concentration.

[0021] In this BMP test example, HCL was used to adjust the seed sludge to a pH of 7.0 before adding the sludge to the bottles. The seed sludge was from a Continuous Stir Tank Reactor (CSTR) system with the characteristics of 60 days solids retention time (SRT), 13 days hydraulic retention time (HRT), 1.4 g/L/day organic loading rate (OLR), pH 7.2 and 2000ppm alkalinity. After the pH adjustment, sludge, media and a cationic polymer additive (CI) was added into the bottles sequentially according to Table 1. The media composition in the BMP test system included sucrose and glucose (COD 1920ppm), meat extract, and NELCl, K ₂ HP0 ₄ , FeCl ₂ , CaCl ₂ , MgS0 ₄ , ZnCl ₂ , CuCl ₂ , A1C1 ₃ , NiCl ₂ , MnS0 ₄ , (NH ₄ ) ₆ Mo ₇ 0 ₂ (COD: N: P = 230: 10: 1) 2000ppm NaHC0 ₃ , pH 7.47. The CI polymer comprises the copolymer of 60mol% acrylamide and 40mol% dimethylaminoethyl aery late methyl chloride.

[0022] Then, N ₂ is used to remove air from every bottle (0.5m ³ /hour N ₂ for 5 minutes of every bottle). Data collection of the accumulated biogas production volume was performed periodically. At the end of the test, Mixed Liquor Suspended Solids (MLSS), Mixed Liquor Volatile Suspended Solids (MLVSS), Total Chemical Oxygen Demand (TCOD), Filtered Chemical Oxygen Demand (FCOD), Soluble Chemical Oxygen Demand (SCOD), pH and CH ₄ % in biogas was measured.

Table 1. Experimental Conditions

Sample Name Sludge concentration CI Chemical: sludge

ratio

bottle g L ppm ppm/(g/L sludge)

Control 8 0 0

4ppm 4 0.5

8ppm 8 1

20ppm 20 2.5

40ppm 40 5

Table 2. Accumulated Biogas Production Volume and CH ₄ % in Biogas

Accumulated Biogas volume (mL) and CH ₄ %

Time (hour) Control (mL) 4 ppm (mL) 8 ppm (mL) 20 ppm (i

0 0 0 0 0 0

18 68 76 1 18 74 82

48 82 148 185 94 98

88 102 180 250 108 116

112 118 216 120 142

Biogas 100% 183% 212% 102% 120% enhanced

efficiency

COD removal 100% 84% 128% 126% 130% improve efficiently

Methane % 49.90% 53.60% 45.90% 57% 53.50%

[0023] The results were then plotted as the accumulated biogas production volume comparison in Figure 1. As can be seen, BMP test results show that the low concentration of cationic polymer will significantly speed up biogas production: produced more biogas volume about 10% -80% more than the control at the first 18 hours. The results also show an increased biogas production volume by 80%-112% at < =20ppm. Further, the results show an improved COD removal efficiency by > 25% and minimum bio-toxicity of chemical to anaerobic sludge. It should be noted that the effective dosage of the polymer additive depends on various factors like biomass concentration, pH, temperature, hydraulic retention time (HRT), polymer molecular weight, etc. Overdosing the polymer additive could lead to over tight aggregation of the biomass and result in poorer performance of the anaerobic digestion performance in terms of biogas production and COD removal.

[0024] While this invention has been described in conjunction with the specific

embodiments described above, it is evident that many alternatives, combinations, modifications and variations are apparent to those skilled in the art. Accordingly, the preferred embodiments of this invention, as set forth above are intended to be illustrative only, and not in a limiting sense. Various changes can be made without departing from the spirit and scope of this invention. Therefore, the technical scope of the present invention encompasses not only those embodiments described above, but also all that fall within the scope of the appended claims.

[0025] This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated processes. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. These other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

[0026] What is claimed is: