ANAEROBIC DIGESTION

Anaerobic digestion promotes the bacterial decomposition of the volatile solids in animal wastes to biogas, thereby reducing lagoon loading rates and odour. The primary component of an anaerobic digestion system is the anaerobic digester, a waste reactor containing bacteria that digest the organic matter in waste under controlled conditions to produce biogas.

Anaerobic digestion is widely applied in the treatment of high strength municipal sludges and industrial wastes, where the BOD is more than 1500mg/lt. It is the oldest processes used for the stabilization of solids and bio-solids. This seminar discusses about the anaerobic digestion processes, design and some applications, in detail.

Organic waste has the potential to cause significant pollution through atmospheric methane, polluted groundwater and the spread of diseases. Anaerobic digestion - a technology that has traditionally been viewed as simply 'energy-from-waste' technique - can provide a range of benefits in addition to the valuable renewable energy from biogas.

Anaerobic digestion is a technology that is generally perceived of as a waste treatment technology. However, in its broadest sense, it is a natural process harnessed by mankind. Anaerobic digestion (AD) is a biological process in which biodegradable organic matters are broken-down by bacteria into biogas, which consists of methane (CH₄), carbon dioxide (CO₂), and traces of other gases. Absence of Oxygen is the primary requirement of AD to occur. Anaerobic digestion provides an effective method for turning residues from livestock farming and food processing industries into:

·         Biogas (rich in methane), which can be used to generate heat and/or electricity

·         Fibre which can be used as a nutrient-rich soil conditioner, and

·         Liquor, which can be used as liquid fertilizer.

Anaerobic processes could either occur naturally or in a controlled environment such as a biogas plant. Organic wastes such as livestock manure and various types of bacteria are put in an airtight container called digester so that the process could occur. Depending on the waste organic matter and the system design, biogas is typically 55% to 75% pure methane. State-of-the-art systems report producing biogas that is more than 95% pure methane.

SOURCES OF WASTE ORGANIC MATTERS

Food Waste: Fruits, vegetables, dairy products, bakery waste, meat, pre/post consumer food wastes (liquid or solid), processed and frozen foods.

Yard Waste: Leaves, grass, plant material, etc.

Fibre Waste: Paper products, polycoat, boxboard, towels, napkins, tissues, waxed plates/cups, books, directories, magazines, newsprint, disposable diapers, feminine hygiene products, incontinence Pads etc.

Miscellaneous Waste: Pet wastes, hair, vacuum cleaner bags, sawdust, etc.

Industrial, Commercial & Institutional Waste: Agricultural waste (food, straw, chaff , manures), organic sludges and slurries (label and brewery pulps, filtration etc), glycol and other alcohols, slaughterhouse slurries, food processing residues. Industrial waste and wastewater usually come from food/beverage industry, starch industry, sugar industry, paper processing, slaughterhouse (gastrointestinal wastes), chemical industry, pharmaceutical industry, dairies, cosmetic industry, fish oil and fish processing residues.

PRINCIPLE OF THE ANAEROBIC DIGESTION:

The Digestion Process

Anaerobic decomposition is a complex process. It occurs in three basic stages as the result of the activity of a variety of microorganisms. Initially, a group of microorganisms converts organic material to a form that a second group of organisms utilizes to form organic acids. Methane-producing anaerobic bacteria utilize these acids and complete the decomposition process.

The anaerobic digestion process can be carried out in quite different conditions. All of these conditions have specific influences on the biogas production. Additionally, from a technological viewpoint, the biological process can also be carried out in more than one reactor, which has some, mainly economical, implications. 

"Dry" digestion and "wet" digestion

In the digestion processes, water is an important parameter. Water is needed for life in general and for digestion bacteria too. It is an important medium for the transport of nutrients, for products and it is a very good reaction medium for digestion.

Digestion is practiced in two different ranges of water content: dry digestion, with a typical dry solids content of 25-30% and wet digestion, with a dry solids content of less than 15%. These ranges have technological and economic reasons: higher solid contents leads to smaller reactors, lower solid contents (more water) lead to much better mixing possibilities but to a higher energy input (more water to be heated) and a bigger reactor.

Natural wastes from plants (like greenhouse residues), have an estimated dry solids content of 25%. This dry solids content opens the possibility to perform the digestion without addition of water.

Batch processes and continuous processes

In process technology the two main types of process (models) are used, the batch process and the continuous process. In the batch process the substrate is put in the reactor at the beginning of the degradation period after which the reactor is closed for the entire period without adding additional substrate. In the continuous process, the reactor is filled continuously with fresh material and also emptied continuously.

As explained before, digestion consists of several consecutive steps. In a batch reactor, all these reaction steps occur more or less after each other. The production of biogas (end product) is non-continuous: at the beginning only fresh material is available and the biogas production will be low. Halfway through the degradation period the production rate will be highest and at the end, when only the less easily digestible material is left, production rate will drop again.

In a continuous process, fresh substrate is added continuously, and therefore all reactions will occur at a fairly constant rate resulting in a fairly constant biogas production rate. Several mix forms of these two models are developed in process technology including the so-called plug-flow reactor and the sequencing batch-reactor all of which try to combine the advantages of the two extremes. 

Environmental Factors affecting the digestion

The environmental factors affecting anaerobic digestion include solid retention time, nutrients, pH, temperature, Volatile fatty acids and toxic materials.

Mixing of the digesting material: Pre-sizing and mixing of the feed material for a uniform consistency allows the bacteria to work more quickly. Occasional mixing or agitation of the digesting material can aid the digestion process.

Retention time: Retention time is the time needed to achieve the complete degradation of the organic matter. The retention time varies with process parameters, such as process temperature and waste composition. The retention time for waste treated in a mesophilic digester ranges from 15 to 30 days and 12 to 14 days for thermophilic digester.

The three reactions (hydrolysis, fermentation, and methanogenesis) are directly related to solid retention time (SRT= mass of solids in the reactor / mass of solids removed daily). An increase or decrease in SRT results in an increase or decrease in the extent of each reaction. There is a minimum SRT for each reaction. If the SRT is less than the minimum SRT, bacteria cannot grow rapidly enough and the digestion process will fain eventually.

Nutrients: Nutrients must be present in sufficient quantities to ensure efficient digestion. The nutrients required in highest concentration are nitrogen and phosphorous. Nitrogen is used in synthesis of proteins, enzymes. Phosphorous is required to synthesis energy storage compounds ATP (adenosine tri-phosphate) RNA and DNA. Other nutrients required are Iron, Nickle, Cobalt, Sulphur and Calcium.

Temperature: Anaerobic digestion can occur under two main temperature ranges:

Mesophilic conditions, between 20-45ºC, usually 35ºC.

Thermophilic conditions, between 50-65ºC, usually 55ºC. digestion:

Thermophlic is much faster than mesophilic. Advantages cited for thermophilic include increased solids destruction capability, improved dewatering, and increased bacterial destruction. Disadvantages of thermophilic are higher energy requirements for heating, poorer quality supernatant containing larger quantities of dissolved solids, odours and less process stability. Thermophilic systems offer higher methane production, faster throughput, better pathogen and virus ‘kill’, but require more expensive technology, greater energy input and a higher degree of operation and monitoring. However, the process is highly sensitive to disturbances such as changes in the feed materials and temperature.

To optimize the digestion process, the digester must be kept at a consistent temperature, as rapid changes will upset bacterial activity. The sterilization of the waste is also linked with the temperature. The higher it is the more effective it is in eliminating pathogens, viruses and seeds.

pH: The optimal pH values for the acidogenesis and methanogenesis stages are different. During acidogenesis, acetic, lactic and propionic acids are formed and, thus the pH falls. Low pH can inhibit acidogenesis and pH below 6.4 can be toxic for methane-forming bacteria. The optimal pH range for all is between 6.4 and 7.3.

Volatile fatty acids: Anaerobic reactor instability is generally manifested by a marked and rapid increase in volatile fatty acids concentration. Concentration of acetic acid of 1000mg/l has no inhibitory effect on methanogens. However, at low concentration 100mg/l of the propionic acid are inhibitory to anaerobic digestion.

Carbon to Nitrogen ratio (C:N): The relationship between amount of carbon and nitrogen present in organic materials is represented by the C: N ratio. Optimum C: N ratios in anaerobic digesters are between 20 and 30. A high C: N ratio is an indication of a rapid consumption of nitrogen by the methanogens and results in a lower gas production. On the other hand, a lower C: N ratio causes ammonia accumulation and pH values exceeding 8.5, which is toxic to methanogenic bacteria. Optimum C: N ratio of the organic matter materials can be achieved by mixing waste of low and high C:N ratio, such as organic solid waste mixed with sewage or animal manure.

Toxicity: The natural of toxicity in biological waste treatment is often misunderstood especially to anaerobic digestion. Whether a substance is toxic to a biological system is a matter of the nature of substance concentration and acclimation. Many substances will stimulate the reactions in low concentrations. However, as concentration increases the effect becomes inhibitory.

It is often presumed that methane-producing bacteria are dead once gas production goes to zero. This is not necessarily true. It was found that maintenance of proper sludge retention time (SRT) is extremely important in allowing for acclimation to toxicants tend to alter their growth kinetics by increasing bacterial generation time. If SRT is too low, dormant methane bacteria will be washed out of the system prior to acclimation or metabolism of the toxicants.

Chlorinated hydrocarbons are extremely toxic to anaerobic digestion. Ethyl benzene concentration can reduce the activity by 25% to 60%. The solvent ethylene dichloride is found to severely inhibit methane fermentation. Also antibiotics in livestock feed have been known to kill the anaerobic bacteria in digesters. So the Complete digestion, and retention times, depends on all of the above factors.

How does AD work?

The digestion process takes place in a warmed, sealed airless container (the digester), which creates the ideal conditions for the bacteria to ferment the organic matter in the absence of oxygen. The digestion tank needs to be warmed and mixed thoroughly to create the ideal conditions for the bacteria to convert organic matter into biogas.

Anaerobic digestion is a complex process, which consists of three stages of

1) hydrolysis, 2) Acidogenesis, and 3) Methanogenesis.

1)      Hydrolysis: During hydrolysis insoluble complex organic matter such as cellulose is decomposed into simple soluble organic molecules such as fatty acids, amino acids and sugar, using water to split the chemical bonds between the substances. Then, it can be utlised by the bacteria, which is achieved by extracellular, hydrolytic enzymes produced and excreted by the bacterial population for this specific purpose. Chemicals can be added during this stage in order to decrease the digestion time and provide a higher methane yield.

2)      Acidogenesis: In this stage, acetogenic bacteria degrade amino acids, sugars, and some fatty acids further into simple organic acids, carbon di oxide and hydrogen. The principle acids produced are acetic acid, butyric acid, propionic acid and ethanol.

3)      Methanogenesis: Methanogenesis is carried out by a group of organisms known as methanogens. Two groups of methanogenic organisms are involved in methane production. One group, termed aceticlastic methanogens, split acetate into methane and carbon dioxide. The second group, termed hydrogen-utilizing methanogens, the acetic acid will be converted to methane. Waste stabilization occurs during the methanogenic phase of conversion of the acetic acid into methane, which is insoluble in water. So approximately 72% of the methane formed in the anaerobic digestion of wasterwater comes from acetate cleavage.

CH₃ COOH -------- CH₄ + CO₂

            The remaining 28% results from reduction of carbon dioxide using hydrogen as energy source by CO₂ – reducing methanogens.

C0₂ ± H₂ ----- CH₄ + H₂O

Biogas produced in anaerobic digestion is primarily composed of methane (CH₄) and carbon dioxide (CO₂), with smaller amounts of hydrogen sulphide (H₂S) and ammonia (NH₃). Trace amounts of hydrogen (H₂), nitrogen (N₂), carbon monoxide (CO), saturated carbohydrates and oxygen (O₂) are occasionally present in the biogas. The composition of biogas is different from the one of natural biogas but it is quite similar to landfill gas.

Types of Anaerobic Digesters

There are three basic digesters. All of them can trap methane and reduce fecal coliform bacteria, but they differ in cost, climate suitability, and the concentration of manure solids they can digest.

A covered lagoon digester , as the name suggests, consists of a manure storage lagoon with an impermeable cover. The cover traps gas produced during decomposition of the manure. Covered lagoon digesters are used for liquid manure (less than 2 percent solids) and require large lagoon volumes. This type of digester is the least expensive of the three.

Covered lagoon digesters typically have a hydraulic retention time (HRT) of 40 to 60 days. The HRT is the amount of time a given volume of waste remains in the treatment lagoon. A collection pipe leading from the digester carries the biogas to either a gas treatment system such as a combustion flare, or to an engine/generator or boiler that uses the biogas to produce electricity and heat. Following treatment, the digester effluent is often transferred to an evaporative pond or to a storage lagoon prior to land application.

Climate affects the feasibility of using covered lagoon digesters to generate electricity. Covered lagoon digesters are most appropriate for use in warm climates if the biogas is to be used for energy or heating purposes.

A complete mix digester  is suitable for manure that is 2 percent to 10 percent solids. Complete mix digesters process manure in a heated tank above or below ground. A mechanical or gas mixer keeps the solids in suspension. However, complete mix digesters are expensive to construct and cost more than a plug-flow digester to operate and maintain.

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Complete mix digester systems consist of a mix tank, a complete mix digester and a secondary storage or evaporative pond. The mix tank is either an aboveground tank or concrete in-ground tank that is fed regularly from under-floor waste storage below the animal feedlot. Waste is stirred in the mix tank to prevent solids from settling in the waste prior to being fed to the digester. The complete mix digester is essentially a constant-volume covered lagoon that is fed daily from the mix tank. Complete mix digesters with in-ground lagoons often employ covers similar to those used in covered lagoon digesters. In the digester, a mix pump circulates waste material slowly around the heater to maintain a uniform temperature. Hot water from an engine/generator co-generation water jacket or boiler is used to heat the digester. A cylindrical above-ground tank, optimizes biogas production, but is more capital intensive than in-ground tanks.

Complete Mix Digesters – A Methane Recovery Option for All Climates. Complete mix digesters have an HRT of 12 to 20 days, which means that complete mix digesters can reduce the overall lagoon volume required for waste storage and treatment. This makes complete mix digesters comparable to covered lagoon digesters in cost, despite the increased complexity of stirring, mixing and plumbing components. In addition, biogas production rates, and therefore heat and electricity production, are greater and more consistent than for covered lagoons. This can help reduce system payback periods compared to covered lagoon systems. Like covered lagoon systems, digester effluent from complete mix digesters is frequently stored in evaporative ponds or storage lagoons.

Plug-flow digesters, are suitable for ruminant animal manures having a solids concentration of 11% to 14%. In a plug-flow digester, raw manure slurry enters one end of a rectangular tank and decomposes as it moves through the tank. New material added to the tank pushes older material to the opposite end. Coarse solids in ruminant manure form a viscous material as they are digested, limiting solids separation in the digester tank. As a result, the material flows through the tank in a "plug." Anaerobic digestion of the manure slurry releases gas as the material flows through the digester. A flexible, impermeable cover on the digester traps the gas.

A plug-flow digester requires minimal maintenance. Inside the digester, suspended heating pipes allow hot water to circulate. The hot water heats the digester to keep the slurry at 25°C to 40°C, a temperature range suitable for methane-producing bacteria. The hot water can come from recovered waste heat from an engine generator fueled with digester gas or from burning digester gas directly in a boiler.

The plug-flow digester design offers a high-temperature variation. High temperature speeds the digestion process and reduces the required volume of the tank by 25% to 40%. However, there are more species of anaerobic bacteria that thrive in the temperature range of a standard design (mesophillic bacteria), than there are species that thrive at higher temperatures (thermophillic bacteria). High-temperature digesters also are more prone to upset because of temperature fluctuations, and their successful operation requires close monitoring and diligent maintenance.

 

Digester Designs

The primary purpose of these anaerobic digesters is waste (sewage) treatment and fertilizer production. Biogas production is secondary. Anaerobic digesters are made out of concrete, steel, brick, or plastic. They are shaped like silos, troughs, basins or ponds, and may be placed underground or on the surface. All designs incorporate the same basic components: a pre-mixing area or tank, a digester vessel(s), a system for using the biogas, and a system for distributing or spreading the effluent (the remaining digested material).

For the optimum design and efficient operation of the anaerobic digester following factors helps:

1.      Optimum Retention Time.

2.      Adequate mixing.

3.      Proper pH (6.5 –7.6)

4.      Proper Temperature Control.

5.      Adequate concentration of proper nutrients (BOD: N: P = 100: 2.5: 5).

6.      Absence of toxic materials, and

7.      Proper feed characteristics.

 

EFFLUENT/ DIGESTATE UTILISATION

The material drawn from the digester is called sludge, or effluent or digestate. Anaerobic digestion draws carbon, hydrogen and oxygen from the organic matter. Essential plant nutrients (N, P and K) remain largely in the digestate/effluent. The composition of the fertilizing agents depends on the organic matter and can therefore vary. The availability of nutrients is higher in digestate than in untreated organic waste, also it benefits the humus balance in the soil. Thus it can be used fertilizer or soil amendment in agriculture, landscaping. Such use permit the creation of a nutrient cycle and maintains or improves soil structure due to the application of organic matter.

The digestate may have to be dewatered and thus separated into two fractions: the fibre and the liquor.

The fibre is bulky and contains a low level of plant nutrients. It can be used as a soil conditioner and as a low-grade fertilizer and also as alternative to peat as well. Further processing of the fibre, such as through composting could produce good quality compost.

The liquor (liquid effluent) contains a large proportion of nutrients and can be used as a fertilizer. Its high water content facilitates its application through conventional irrigation methods, representing an advantage over compost as it can be applied throughout the crop cycle. The liquor is generally used on the farms on which it was produced. It has advantage over raw manure applications, as the ammonia uptake by plans is higher than for organic nitrogen.

ADVANTAGES OF AD

AD contributes to reducing the greenhouse gases. A well-managed AD system will aim to maximize methane production, but not release any gases to the atmosphere, thereby reducing overall emissions. AD also provides a source of energy with no net increase in atmospheric carbon which contributes to climate change.

The organic matter for AD also a renewable source, and therefore does not deplete finite fossil fuels. Energy generated through this process can help reducing the demand for fossil fuels. The use of the digestate also participates to this reduction by decreasing synthetic fuels use in fertilizer manufacturing, which is an energy intensive process.

AD creates an integrated management system, which reduces the likelihood of soil and water pollution to happen, compared to disposal of untreated animal manure/slurries. The treatment can also lead to a reduction up to 80% of the odour and it destroys virtually all weed seeds, thus reducing the need for herbicide and other weed control measures.

On a financial aspect, the advantage of AD is to convert residues to potentially saleable products: biogas, soil conditioner, and liquid fertilizer. It can also contribute to the economic viability of farms by keeping costs and benefits within the farm if the products are used on-site.

DISADVANTAGE OF AD

AD projects, as with many developments, will create some risks and have some potential negative environmental impact. These need to be removed wherever possible or at least minimized.

AD has significant capital and operational costs. It is unlikely that AD will be viable as an energy source alone and therefore must be seen as an integrated system. It is likely to be cost effective for those who can use the other products of AD: better waste management fertilizer.

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All waste management systems create traffic movement. This can become a problem in centralized AD plants and alternative methods of transport should be investigated as transport greatly influences costs and emissions. The location of the plant should be chosen carefully so that distances traveled are minimized between the production of the organic matter, the storage tanks and the digester. Nuisance for the neighborhoods has also to be taken into account.

About health and safety, there may be some risks to human health with the pathogenic content of the organic matter but it can be avoid with an appropriate plant design and organic matter handling procedures. There may also be some risks of fine and explosion, although no greater than for natural gas installation.

APPLICATIONS

Applications such as: waste treatment, reducing pollution, odours and disease; recycling of nutrients back to the soil (thereby reducing the requirements for artificial fertilizers); improved soil quality by recycling the organic matter as humus, thus preserving fragile top soils; sanitization of the compost, reducing the spread of soil-borne pathogens and weeds.

Anaerobic digestion is particularly suited to wet, organic wastes. As such it has been used in the water industry for sewage treatment since the end of the last century. Most large sewage treatment works use the process, recovering the biogas to meet on-site heat and power requirements.

Household waste is made up of a range of materials including paper, card, plastics, textiles, glass, metal and putrescibles (organic material that is easily degraded such as fruit and foodstuffs). Of these, only putrescibles and paper are ideally suited to anaerobic digestion and the process will be easier to manage if these wastes comprise the only feed to the system.

Garden waste may also be treated by anaerobic digestion, but the extent of degradation will vary according to the type of material.

At Freiburg, Germany, cow manure is scraped and fed into a plug flow digester. The biogas produced is used to fire an 85 kW gas engine. The engine operates at 35 kW capacity levels and drives a generator to produce electricity. Electricity and heat generated is able to fulfill all dairy energy demand. The system has been in operation since 1982.

Fig 5: Anaerobic digestion plant (Freiburg, Germany) processes 36,000 tonnes waste per year, producing 3 million m³ gas and 15,000 tonnes compost.

Anaerobic digestion technology is available to produce and recover biogas from biodegradable organic matters. Three commercial technologies are available and have been operated successfully throughout the world to treat a wide range of industrial wastes.

The Anaerobic digestion process can only degrade the organic fraction of the organic matter. The implication of this is that not all waste streams are amenable to anaerobic digestion. In addition, anaerobic digestion is not the technology that can solve all the problems of waste.

It is a biological process and this places constraints on the operation of the process and how quickly it can respond to changes. The process requires conditions within the process to be stable, with either mesophilic (35°C) and thermophilic (55°C) temperatures and at moisture contents above 60%. The micro-organisms can be killed off by extremes of process conditions or toxic chemical and also require time to grow to sufficient populations to carry out the treatment. However, as a biological process the methane product is much purer than could be achieved by chemical means.

Thus, anaerobic digestion is an engineered process that requires management to obtain the best from the process.