28 Anaerobic digestion of solid waste

Dr. Yogalakshmi K. N

 

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Objectives:

  • To understand the concept and process of anaerobic digestion
  • To generate awareness about the types of reactors used in anaerobic digestion of solid waste To gain knowledge about the factors influencing the process of anaerobic digestion
  • To realize the merits and demerits of the anaerobic digestion process

 

1.0 Introduction

 

Municipal solid waste consists of around 70% of organic fraction of which 53% is biodegradable. Instead of disposing the organic fraction in landfills, they can be efficiently utilized to recover energy and convert it into a manure. Organic fraction of waste is carbon rich and hence can be easily degraded and converted into energy. Anaerobic digestion technology is one option by which the organic biodegradable fraction of municipal waste is digested in the absence of oxygen to produce methane (CH4), carbon dioxide (CO2) as biogas and residue that can be used as manure.

 

2.0 Anaerobic digestion of solid waste

 

Anaerobic digestion can be performed only with the biodegradable fraction of the waste. It is essential to separate the biodegradable waste from the other components of the comingled waste. Mechanical and air component separators are used to remove ferrous, non-ferrous metals and paper from the waste. Post segregation the biodegradable fraction of the municipal waste is brought to the reception area where it is stored till for the processing. Sometimes the biodegradable fraction of the municipal waste is co-digested with sewage sludge or agricultural waste.

 

2.1 Pretreatment: It is process by which the contaminants are removed and the waste is homogenized to enable efficient digestion. Homogenization protects the other downstream process. Three main process namely sorting, chopping and mixing is used for pretreating the waste before anaerobic digestion. Waste is sorted to carefully remove small stones silt and other gritty material to avoid wear and tear of the equipment. Sorting also avoids the accumulation of the gritty material in the digester.

 

Chopping or shredding of waste is done to maintain uniform size and increase the surface area which will facilitates uniform digestion. As mention earlier mixing increases the homogeneity of the waste.

 

2.2 Digestion: After pretreatment the waste is transfer to the digester. A digester is an enclosed and controlled reactor into which the waste slurry is introduced. The slurry can be made with water , sewage sludge or liquid extracted from the digestate. The inoculum can be cow dung, biogas digester slurry or digestate from other anaerobic digesters. Within the digestor the waste is heated and continuously mixed till the generation of methane and carbon dioxide. The anaerobic digestion occurs in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. The organic components of the waste which is generally represented by carbohydrates, proteins and lipids are degraded by the action of anaerobic microorganisms. In addition to the biological process certain interrelated physical and chemical processes also occurs in the digester. The first step of degradation (i.e.) hydrolysis uses facultative anaerobes and converts the carbohydrates into sugars; proteins to amino acids and ammonia via de-aminisation; and lipids to fatty acids and glycerol. The gas production at the stage can rise upto 80% carbon dioxide and 20% hydrogen. The second stage converts the organic acids also termed as volatile fatty acids into acetic acids and its derivatives, carbon dioxide and hydrogen. This stage is generally terms as the acid stage that involves acetogenic bacteria. Some microorganism converts carbohydrates into acetic acid in the presence of carbon dioxide and hydrogen. The end of the stage is marked by the decrease in hydrogen and carbon dioxide levels.

 

C6H12O6 → 2C2H5OH + 2CO2

 

The last stage of anaerobic digestion, methanogenesis plays an important role in generation of methane gas. It is the phase during which methanogenic bacteria convert the acetic acids and its derivative into carbon dioxide and methane. The methane generating microorganisms are promoted during low hydrogen levels. Two groups of microorganisms are active during this stage: mesophilic bacteria which are active in the temperature range of 30-350C and thermophilic bacteria in the range of 45-65-0C. The gas produce during the methanogenesis stage comprises of 60% methane and 40% carbon dioxide.

 

2.3 Post treatment: The completion of methanogenesis indicates the end of digestion process. The digestate is withdrawn and subjected to post treatment like, separation composting and storage.

 

Undigested material or components like glass and plastic if any are sorted manually. The digestate is fed into a filter press for dewatering. The residue is then stabilized by the process of composting. The composted product is stored, packed and sold as a manure. In certain times if the residue contains hazardous substance they are disposed off safely in landfills.

 

2.4 Microorganisms involved

 

Anaerobic digestion involves a mixed population of microorganisms. The biogas generation during anaerobic digestion occurs due to the synergistic action of the microorganisms. Several microorganisms are involved in the process of gas generation. Hydrolytic bacteria are mostly used for breakdown of complex substances especially cellulose and hemi cellulosic fibers into simple substances. Among the microbial population, 90% population is acidogenic bacteria. They degrade the polymers to acids and acetates to facilitate easy utilization of substrate by methanogens. Acid-formers grow vigorously and tolerate a wide variety of environmental conditions. Due to their wide tolerance, they rarely act as a rate-limiting factor in anaerobic digestion process. The major microorganism involved in the acetogenesis include Syntrophobacter wolinii, a propionate decomposer and Sytrophomonos wolfei, a butyrate decomposer. Other acid formers include Clostridium spp., Peptococcus anerobus, Lactobacillus, and Actinomyces.

 

Unlike the acid-formers, the methanogens are slow growers and require less nutrition. Simple organic compounds are sufficient to provide nutrients for methanogenic bacteria. They rely on polymer and acid stage for carbon and ammonia for nitrogen needs. Methanogens are more sensitive to environmental factors. Atmospheric oxygen is one main factor that affects the methanogens even at low concentrations. Likewise, nitrites and nitrates also inhibit the growth of the methanogens. Optimum pH level required for the survival of methanogens is 7. However, the tolerance level extends from pH 4.5 or 8.0. Methanobacterium, Methanobacillus, Methanococcus and Methanosarcina are the methanogenic bacteria commonly found in the digester. Methanogens can also be divided into two groups: acetate and H2/CO2 consumers. Methanosarcina spp. and Methanothrix spp. (also, methanosaeta) are considered to be important in anaerobic digestion both as acetate and H2/CO2 consumers.

 

Methane producing bacteria (methanogens) transforms the acids by two types of reactions: fermentation of short-chain fatty acids and some alcohols; and a respiration in which H2, CO2, and certain simple organic compounds are oxidized anaerobically, coupled with the reduction of CO2 to CH4.

 

Acetic acid:

 

CH3COOH → CH4 + CO2

 

Methyl alcohol:

 

4CH3OH → 3CH4 + CO2 + 2H2O

 

The production of CH4 through respiration involving the incomplete oxidation of alcohol to acetic acid, coupled with the reduction of CO2 to CH4, can be exemplified by the reaction by Methanobacterium omelianski.

 

2CH3CH3OH + CO2 → 2CH3COOH + CH4

 

The reduction of CO2 with molecular hydrogen is:

 

4H2 + CO2 → CH4 + 2H2O

 

3.0 Types of reactors used in anaerobic digestion of solid waste

 

Based on the feed and flow anaerobic digestion is classified into five types of system or reactors. The details of the process are elaborated below:

 

3.1 Dry continuous digestion: The waste with a dry matter content of 20-40% is digested in this system. The digester is operated in continuous fed mode. Both completely mixed and plug flow system are available for dry continuous digester. To initiate digestion in the plug flow system the digested is mixed with the incoming feed stock.

 

3.2 Dry batch digestion: Dry batch system are operated in batch mode where the digester is filled with the waste material, sealed and left aside till complete digestion. The digestion in this system occurs naturally and the leachate generated during the process is recirculated to maintain the moisture with in the digester. Leachate recirculation also helps in maintaining the inoculum of microorganism and nutrients in the system. The digester is opened at the end of the digestion process. The digested material is used as in inoculum for other digesters.

 

3.3 Leach bed or sequencing batch process: Leach bed process is similar to dry batch process except for the fact that the leachate obtained during biodegradation is exchanged between previously established and new batches of waste. Exchange is done to facilitate the start up of biodegradation process. Waste in which methanogenesis process is complete is termed as established waste. After the completion of methanogenesis the digester is disconnected and loaded with fresh solid waste. In certain cases, the fresh waste is loaded into a second digestor and connected to the first digester to facilitate complete digestion.

 

3.4 Wet continuous digestion: The waste to be digested by wet continuous method should contain a large proportion of water and 10% of dry solid. Briefly the waste is made into a slurry and fed to the digester. The digester can be conventional or completely mixed. At the completion of the digestion process the digestate is removed from the digester and supplied to filter press. The liquid from the wet digestate is removed hydraulically by the compressive force applied by the filter press. The liquid is used for feed preparation. This would minimize the discharge of excessive volume of the liquid into the environment. Sometimes the sewage sludge is used as an alternative for solid waste slurry preparation. Care should be taken to remove glass, stone and other abrasive material from the solid waste to avoid its accumulation in the digestor.

 

3.5 Multistage wet digestion: Wet digestion is canalized on multiple stages. It is similar to wet digestion where waste is made into a slurry with water or recycled liquor. The slurry is then fermented using hydrolytic and fermentative microorganisms. The resulting volatile fatty acids is converted into gas in high rate anaerobic digester specially designed for this purpose.

 

Based on the process stage the anaerobic digestion is divided into batch, single and multi-stage digesters

 

3.6 Single stage reactors: Total solids (TS) of the solid waste play an important role in determining the digestion efficiency. Waste containing 10% and 15-20% solids is categorized as low and medium solids, respectively. High solid waste contains a TS of 22-40%. Reactors operated with low solids require high reactor volume as it requires large quantity of water for slurry preparation. Also, low solid waste requires dewatering as post treatment which further adds to the cost of treatment. High solid waste reactors are robust and smaller in size with high organic loading rate. The use of sophisticated pumping system makes the process costly. Depending on the solid concentration, single reactors are subclassified into single stage low solids (SSLS) and single stage high solids (SSHS) reactors. In single stage reactors, all the phases of anaerobic digestion occur in single reactor.

 

3.6.1 Single stage low solid reactor: SSLS are simple and economical reactors that are in use for many decades. The feedstock (i.e) municipal solid waste is pretreated and made into a slurry with sewage. The slurry is then introduced into the reactor and retained for 14-28 days. The rate of feed loading into the digester is same as that of effluent withdrawal. SSLS are operated in mesophilic and thermophilic temperature range. However, the retention time varies for the both the process. Mesophilic SSLS requires 20 days retention unlike thermophilic reactors that require 10 days. Formation of scum layer and settling of dense material at the bottom affects reduces the gas yield and minimizes the hygenization of waste. Shortcircuiting (i. e.) i.e. a fraction of the feed passes through the reactor at a shorter retention time than the average retention time of the total feed is an important drawback of the system. As mentioned earlier the cost of operation is high as it requires post treatment.

 

3.6.2 Single stage high solid reactor: Around 23% of solids can be maintained in SSHS reactors. They yield high levels of biogas. The reactor uses the plug flow model digester. The reactor is provided with agitator for mixing. Gas generated during the process is also recirculated for mixing. at certain times, the digested residue is mixed with the feedstock and fed into the reactor. Due to the high solid level, mixing is very difficult. SSHS reactors require minimum pretreatment of waste materials. During the process the scum layer is not removed and hence it leads to higher biogas generation. The water consumption for SSLS is higher than SSHS.

 

3.7 Multistage reactors

 

In single stage reactors, hydrolysis and growth of methanogenic bacteria limits the success of the process. Thus, multistage reactors are designed with the purpose of carrying out the different phases of anaerobic digestion in two reactors. This might improve digestion and biogas generation, and favor parameter optimization for each phase. Multistage reactors comprise of two reactors: First for hydrolysis/liquefaction-acetogenesis and the second for methanogenesis. Microaerophilic conditions are maintained to achieve better hydrolysis of waste materials. Likewise, optimum growth conditions such as long retention were provided to retain the methanogenic bacteria and improve the anaerobic digestion process. Moreover, removal of suspended solids after hydrolysis process will also favor better methanogenesis. The solid percent, pretreatment and water requirement are similar to that of single stage reactors.

 

3.7.1 Multistage low solid reactors (MSLS): The pretreated municipal solid waste is made into a slurry and fed into the first reactor. Mostly, total solid concentration of 10% was maintained in the first reactor. Agitation in the reactor was achieved through gas circulation. Both the reactors were operated at mesophilic conditions. The digestate of the first reactor is dewatered and the liquid obtained is fed to the second reactor for methanogenesis. Sometimes, the digestate is recirculated back with fresh waste. They are also used as inoculum for the first reactor. At times, to maintain the pH of the first reactor at 6-7, the product water from the second reactor is added to the first one. The investment cost of these reactors is very high due to their technical complexity. Similar to SSLS, MSLS also faces problems of scum formation, foaming and short-circuiting.

 

3.7.2 Multistage high solid process (MSHS): MSHS reactors too consists of hydrolysis reactor and methanogenesis reactor. Microaerophilic condition in hydrolysis reactor favored better digestion of waste. Certain MSHS process use attached biofilm in the second reactor to achieve higher gas generation and digestion. Higher biomass retention with attached biofilm increases the resistance of methanogens to high ammonium concentrations. These reactors are operated under high organic loading rates. Unlike single stage high solid digesters, MSHS does not show high biogas generation. This is due to solids removal from digestate of first reactor that might have led to the loss of biodegradable matter.

 

3.8 Batch reactors

 

In batch reactors, the feedstock is loaded and kept idle for digestion and biogas generation for 25-30 days. After digestion, the digestate is removed and fed with new feedstock. Batch reactors symbolizes the landfills but show higher reaction rate and 50-100% higher gas production than landfill systems. This is due to leachate circulation and controlled temperature conditions. Single stage batch system and sequential batch system are the two types of batch reactors used in anaerobic digestion process. In single stage batch reactors, the waste is loaded and the leachate generated is recirculated back to the top of the batch reactor. The waste is retained in the batch reactor for 40 days or more till generation of biogas. The gas generated in single stage batch reactors is 40% less than SSLS reactors. Sequential batch reactors consist of two or more reactors, where the organic rich leachate from first reactor is introduced to the second reactor (i. e.) methanogenic reactor. Further, the leachate from the methanogenic reactor is combined with buffering agent and recirculated back to first reactor. The leachate from the second reactor does not contain acid. Recirculation favors inoculum maintenance and provides complete degradation.

 

3.8.1 Advantages:

  • Simple
  • Inexpensive Robust
  • Works efficiently for low organic loading rate

 

3.8.2 Disadvantages:

  • Large land requirement
  • Waste settles down and thereby affects digestion
  • Risk of explosion is high especially during unloading

 

4.0 Products of anaerobic digestion

 

The products of anaerobic digestion are biogas and residue. The biogas is composed of 55% to 65% methane and 34% to 44% is carbon dioxide. H2S, N2, and H2O are other gases present in the biogas. Methane, hydrogen and carbon monoxide can be used as a fuel. The heating value of raw biogas ranges from 18,630 to 26,080 kJ/m3. The biogas should be free of hydrogen sulphide to avoid corrosion in the engine. The biogas can be cleaned, compressed and used as natural gas. Biogas is termed a methane when they are cleaned and brought to natural gas standards.

 

Table 1 Composition of biogas

Component Concentration (by volume)
Methane 55-60%
Carbon dioxide 35- 40%
Water 2-7%
Hydrogen sulfide 20-20,000 ppm (2%)
Ammonia 0-0.05%
Nitrogen 0-2%
Oxygen 0-2%
Hydrogen 0-1%

 

 

Digestate is a nutrient rich by-product of AD and can be used as a fertilizer and soil improver.

 

5.0 Factors influencing anaerobic digestion: The efficiency of the anaerobic digestion is determined and controlled by various factors discussed below.

 

  1. Temperature:  Temperature  plays  an  important  role  in  successful  generation  of  biogas.  A temperature in the range of 30-350C and 45-650C is required for mesophilic and thermophilic bacteria respectively. Higher the temperature more is the gas production. Thermophilic bacteria play a major role in gas generation. Heating of the digester is usually done to enhance gas production.
  2. pH: The ideal pH range for the existence and growth of methanogenic bacteria is 6.8- 7.5. An optimum pH of 7 should exist in the reactor for good gas production. The reactor is prone to acidic condition during the acido and acetogenesis phase. Acidic pH generated due to organic acids inhibit the activity of the methanogens.
  3. Nature of waste: The gas generation depends upon the biodegradability and complexity of the waste. Simple waste can be degraded faster resulting in more gas generation unlike complex waste.
  4. Moisture: High moisture levels might affect the growth of microorganisms. Moreover, lower moisture content can reduce the liquid effluent from the digester. Municipal waste contains around 60% of moisture. When codigested with sewage sludge the moisture percentage increases to 90%.
  5. Shredding: Size reduction of the waste will increase the surface area and provide better contact of substrate with microorganisms. This will lead to improved gas generation and better biodegradability.
  6. C/N ratio: High C/N ratio leads to high acid content and low methane generation. Municipal waste contain C/N ratio above 50 whereas sewage sludge contains C/N ratio below 10 therefore co-digestion can result in better gas generation.

 

6.0 Advantages of anaerobic digestion

  • Generation of valuable by-products biogas and residue that can be used as manure Reduces greenhouse gas emissions through recovery of methane
  • Can efficiently treat varied type of organic waste and wastewater
  • Reduces solid (i.e) less excess sludge. Hence problem of sludge handling is minimized Removes pathogen
  • Process stability (high-loads can be treated but anaerobic sludge can also be preserved for prolonged periods without any feeding)

 

7.0 Disadvantages of anaerobic digestion

  • Skilled operators are required
  • Use of energy produced during the process is still under development Sensitive to chemical compounds
  • Presence of sulphurous compounds might create odour problem

 

8.0 Summary

  • To summarize, in this module we have discussed about
  • anaerobic digestion process
  • types of reactors used in anaerobic digestion factors influencing anaerobic digestion
  • advantages and disadvantages of anaerobic digestion process

 

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