Anaerobic Digesters - there are many types, lots of ideas, and several applications. They are as varied as the number of different stomachs living things have. Here is a list of them that I am compiling.
The bag design originated in the 1960’s in Taiwan, as an answer to problems with other digester types. Nowadays it is also used extensively in other areas, for example in Central America. The design is very simple, as the digester is basically a tube made of a flexible material, placed in a trench in the ground (slightly deeper than the digester radius).
Used materials are a membrane called Red Mud Plastic (RMP), but also PVC is widely used. As a result, these digesters are very lightweight and easy to install. Furthermore, the bag- type digesters can be successfully installed in areas with a higher groundwater level, where application of brick or concrete digesters is very difficult.
In Taiwan the digester was mainly developed for swine manure, and also in China, Korea and Fiji this is the most common substrate. Typical retention times for swine waste are 60 days at 15°C-20°C and 20 days at 30°C-35°C. The thin construction material allows for easy heating of the digester content, for example by the sun, and in practice the temperature has been found to be 2-7°C higher than in fixed dome digesters. In Korea the following specific gas yields were found: 0.1 in winter (8°C) and 0.7 in summer (volumes of gas per volume of digester per day). In China these digesters are used as batch digesters, filled with manure and straw and operated for half a year. The costs for this digester are very low in comparison to other digester types, and the digesters can either be prefabricated and transported or built on site. At present, the costs for a tubular polyethylene bag digester with PVC piping and plastic hosing are varying from $34 in Vietnam to $150 in Costa Rica. Digester sizes vary in length for from 10 to 20 m, with a 5 m circumference. The wastewater to biogas ratio is approximately 1:3. In Vietnam (around Ho Chi Minh City) the average digester length was 10.2 m with an estimated digester volume of 5 m3.
The material is in principle very durable, but both weather conditions and mechanical failure can be a problem. A survey in Vietnam revealed that most technical failures occur because of exposure to the sun and damage by falling objects, animals or people. Total exposure to the sun led to breaking of the material in two years. Most users could fix their digesters without external help.
On their website, Biorealis Systems provides calculations and a construction manual for a home built slurry digester. The information given by Biorealis includes digester operation and a discussion forum. Detailed drawings are available on the website, the pictures in Figure 24 given a first impression.
A system can be designed to digest a wide variety of organic wastes, from kitchen scraps to sewage, to livestock manure, to industrial wastes. The ideal feedstock is a 6-8% slurry with a Carbon to Nitrogen ratio of about 30:1. Incoming waste material should be macerated, and as close to the operating temperature (95 degF) of the digester as possible. The small scale system described below will handle the toilet wastes produced by a family of 6, each flushing a 1-1/2 Pint/flush toilet 5 times per day.
Gas can be stored in low pressure gas bags (i.e. truck tire inner tubes, etc.), rigid tank(s) with floating cover and water seal, compressed and stored in pressure tank, and/or burned as it is produced, (minimizing storage requirements). For safety reasons, it is recommended that the gas be burned as soon as possible, avoiding the requirement to store and handle larger quantities of flammable gas. The gas produced typically consists of about 30% CO2 and about 60-65% methane, depending on the content of the wastes. Small amounts of hydrogen, hydrogen sulfide, and nitrogen gas will also be produced, as well as water vapor.
Effluent from the digester of about 60 gallons/ day is assumed. Estimated solids loading will be about 35 lbs/day. Assuming that volatile solids will be reduced by about 60-70% in the digester, additional volatile solids entering the sewer system will be about 35 - (35 x .65) ~ 12-15 lbs/day.
The volume of sludge solids accumulating in the digester will depend on the digestibility of the influent material and the extent to which digester contents are mixed (i.e. kept in suspension and discharged with effluent), or allowed to settle. Tanks are designed to facilitate sludge removal (e.g. quick disconnect fittings provided for connection to vacuum pump, etc.).
Technical drawings of a fixed dome anaerobic digester (includes a bill of quantity)
Digesters of the fixed dome type have been built since 1936 in China and it is the most common type applied in developing countries. The digester is usually built of bricks, stone and/or poured concrete. Many variations to the fixed dome digester have been developed. Different variations of the so-called Chinese type make use of a removable manhole cover in the top of the dome. In India the two most common fixed dome digester models are the Janata and the Deenbandhu design, both without a manhole in the top. These digesters are usually between 5 m3 and 10 m3.
The gas tight chamber is formed by a hemispherical top sealed by layers of mortar. Feeding is done semi-continuously (e.g. once a day) through the inlet pipe, displacing the same volume of effluent. Biogas is stored under the dome, leading to quite high gas pressures (between 1 and 1.5 m of water), which is why the top and bottom are hemispherical. Leakage of gas was a main problem in older dome digesters and can still be an issue in newer ones. In 1992 there were about 5 million family- sized fixed dome digesters operating in China, with volumes of 6, 8 and 10 m3.
The design and construction of these digesters is well-known and a lot of experience is available, also freely accessible through the internet. Construction material is chosen on site to keep costs low. In China the design of these digesters is standardised based on several design aspects: gas pressure, average rate of gas production, gas storage, digester size, geometric forms, loads and forces. At ambient pressure the water level is 95% of the total volume, and the gas pressure should be equal to, or below, 120 cm of water. The diameter to height ratio is usually 2:1. Both cow and pig manure are common substrates, and for both materials an HRT of 35 - 40 days is adopted, applying a solids concentration of less than 10%.
Leaking of gas through the dome is the main problem with standard fixed dome biogas plants, especially in areas where skilled labour and good quality materials are scarce. Polyethylene domes are available for the 2 m3 Deenbandhu type digester, making installation easy and avoiding gas leakage. The construction time can be reduced from 3 weeks for a conventional plant to only 6 days with a PE dome. It is also suitable for repairing existing digesters.
Diligent energy systems
This particular digester has a capacity of 60 cubic metres, and produces 60 cubic metres of biogas each day, using effluent from a nearby toilet block and 40 kg of crop residue from jatropha plants and human waste. The base of the digester is lined with aggregate and cement. Brick walls surround the digester. The bricks are finished inside with concrete. The digester is covered with sand and soil to counter the pressure from the gas inside. The gas is enough to cook lunch for 300 employees a day.
Constructed as a circle with a biogas plant in the center. Low-flush pour-flush squatting toilets (design: 2 L, actual: 4 L per flush with cleansing). Biogas: 2-3 cylinders of biogas per month (only) – used for cooking. Quantity low (could be increased by adding kitchen waste and cow manure). Digestate: drying bed, composting, used as compost.
- 300 students on the campus (established 1999)
- New ecosan sanitation system inaugurated August 2006
- Project review Sept. 2008: working satisfactorily
The Indian floating dome digester was designed in 1950 by Patel, and promoted by the Khadi and Village Industries Commission (KVIC) of Bombay in the 1960’s. Since then this digester type has been improved and the technology has been disseminated by the KVIC, and the particular type promoted by them is known as the KVIC type digester. Other designs exist, for example the Pragati model. The most distinguishing characteristic of floating drum digesters is the floating gas holder, which moves up and down with the production and usage of the biogas.
The digester walls are usually made of brick or reinforced concrete, and the gas holder of fiberglass reinforced plastic. Steel was used in the past, but because of corrosion problems FRP is now more frequently used, even if its costs are higher than for a steel drum. The weight of the gas holder determines the gas pressure inside the digester (Marchaim 1992). This feature can be used to increase the gas pressure when needed, by placing extra weight on the holder.
As a feed mostly cattle dung is used, sometimes mixed with nightsoil, a.k.a. bucket collection, (in community applications), agricultural residues and other substrates such as water hyacinth. If needed, the influent is diluted to a dry matter content of around 10%.
As with the fixed dome digester, feeding is done semi-continuously through an inlet pipe, displacing the effluent. A vertical baffle can be installed when the height:diameter ratio is high and might allow for short-circuiting of the influent and effluent flow. Obtained gas yields roughly range between 0.2 and 0.6 volume of gas per volume of digester in the cold and warm areas respectively. Especially in India but also elsewhere in the world this digester type is undergoing continuous experimental improvements, for example in the geometric configuration but also with respect to mixing, insulation and heating.
Compact floating dome
ARTI, the Appropriate Rural Technology Institute, is an NGO based in Maharashtra, India. The ARTI compact biogas plant is a floating dome digester made from two cut-down high-density polythene (HDPE) water tanks, typically of 1 m3 and 0.75 m3, with the larger tank being the digester body and the smaller one the gas holder (see Figure 15). This digester was developed for producing biogas from food waste, and to be sufficiently compact to be used in an urban environment. It has won the 2006 Ashden Award for Sustainable Energy in the Food Security category.
The substrate (all kinds of food waste, mixed with water or digester liquid) is fed through an inlet pipe and displaces a similar amount of effluent via an overflow. Biogas can be led directly to the kitchen, and the gas pressure can be increased by placing weights on the floating dome. Installation of the digester is easy and takes only a few hours when ordering the plant as a kit, with a space requirement of about 2 m2 by 2.5 m (ARTI 2009). The digester specifications are given in Table 7. The costs, including a biogas stove, are around $200. ARTI also provides a manual for a complete do-it-yourself building and installation of the plant.
Water jacket floating dome
As the floating drum is submerged in the digester content, it becomes dirty and it can even get stuck in cases of severe scum layer formation. An improved design is the water jacket biogas plant, with a floating drum that is not in contact with the digester liquid but rests in a water jacket around the top of the plant. The water jacket involves an extra cost but the hygiene of this design is superior to the standard floating drum plant.
LeAF Northern European Micro System
Proposed digester system. (Simplified process flow diagram, not to scale, dashed connecting lines: slurry/liquid flow can be operated manually).
The micro anaerobic digester (AD) plant is a small-scale system designed to treat kitchen waste, in this report also referred to as food waste. It is highly biodegradable material and therefore in principal suitable for anaerobic digestion. For this case study, it is assumed that there is sufficient kitchen waste available for feeding the digester. The digester is designed such that it is relatively easy to build, operate and maintain and that it fulfils the requirements set by legislation. The overall system includes the anaerobic digester and its required auxiliaries.
The digester has a volume of 1 m3. When operating at a conservative retention time (40 days), the digester would process around 25 kg kitchen waste/d (approximately waste of 10-15 households), and produce about 3 m3 biogas/d. It should be emphasized that the design in Figure 2-1 is conceptual. For example the way to keep a certain liquid level and gas pressure in the digester depends on the way the digester is configured.
Read the complete description in The Phase Two CCN Micro AD report.
The MuckBuster is a self-contained anaerobic digester, built inside a repurposed shipping container. It can produce renewable energy from any organic materials — animal waste, grass clippings or the stuff bound for the septic tank. In a month-long process, bacteria break down the organic materials and produce methane, a gas that can then be burned to produce electricity. This kind of technology is usually found on large-scale farms or municipal solid-waste sites where fuel is, well, plentiful. But SEaB has shrunk it down, allowing you to squeeze two kilowatt hours of power — about half of what a typical American home consumes — from 100 gallons of organic waste. If you’re wondering what you’ll need to eat to get that much waste, it’s about the “production” equivalent to what 10 horses would “process” from munching hay.
The Supergas plant consists of two essential components - a pressure equalisation system and a plant for processing organic waste. The organic material used in the system is effectively mixed by means of the pressure arising as a result of the gas production. Thus the pressure equalisation system is designed on the basis of hydrolic principles without application of mechanics.
The pressure equalisation system between the reactor and the higher displacement chamber can be regarded as the heart of the Supergas biogas system. The biogas pressure is converted into potential energy which is released at regular time intervals by means of a pressure equalisation system controlled by the gas pressure in the biogas reactor. This process releases remarkable amounts of energy which normally cannot be achieved manually nor mechanically. In this way the break-downs and energy loss which is normally associated with mechanical parts are avoided and at the same time the stirring process optimize the efficiency of the plant considerably.
The device consist of two small chambers placed on top of each other. The chamber below is connected to the reactor whereas the chamber above is connected to the gasometer. The chambers are interconnected by a level tube and a reflux tube. The chamber below is provided with a filler tube. The level tube is set to blow out approximately 4 hours of gas production. The chamber below is approximately 3/4 full at the beginning of a new cycle. The liquid column in the level tube reflects the blow-off pressure. The level tube is calculated for a gas speed of 120 m/sec during blow-off. Within approximately 10 seconds the pressure between the reactor and the gasometer will be equalised and the substrate will flow back from the higher level of the displacement chamber. In a 5 m3 reactor the reflux substrate within the same time interval will be approximately 1,000 l. The system operates without any supply of external energy for control or heating. The plant produces 3-5 m3 gas per day, which is enough for approximately 10 hours use. This is sufficient to provide a family of 10 members with gas for cooking and gas light purposes. The plant is raised without any mechanical or motor-driven tools.
Upflow anaerobic sludge blanket (UASB)
Consolidated Management Services Nepal (CMS) P. Ltd. with the financial assistance of United Nations High Commissioner for Refugees (UNHCR), constructed a latrine-attached biodigester in 1998 at Jantemod of Ward No. 1 of Pathari Village Development Committee (VDC). Users of this public pay-latrine are the residents of Jantemod, passengers of bus services and the people who come there twice a week during "Hatiya" (local marketing day). The ten latrines (four for females and six for males) together with three urinals for males provide a daily input needed for the 15 m3 fixed dome Chinese model biodigester. Users of the latrines pay a reasonable fee for the services used.
The community latrine was put for common use with effect from 15 March 1998. It is visited by 250 regular and 35 occasional users per day. With about daily available 115 kg of human excreta as raw material for feeding the biodigester, about 5.75 m3 of gas production is expected per day. Currently, the gas is being used to provide illumination inside the latrines. Thus, two lamps consuming each 0.07 m3 gas per hour are lit for about 10 hours during night. Although it was envisioned to provide the excess gas to the interested household (s) located close to the project site for cooking purposes as a substitude to firewood, this plan has yet to materialize as conservative belief system considered thus cooked as "impure".
Two compost pits were constructed beside the biodigester for storage, treatment and utilization of digested effluent from biodigester. A part of slurry is being used by the watchman to fertilize his kitchen garden. Effluents (called "slurry") from biogas are safe for handling compared to raw excreta as pathogens are killed in course of anaerobic digestion process Such slurry has been proved to be a high quality organic fertilizer for plant nutrition. But still there seems to be a need for practical and demonstrative type of training to local people to acquaint them in managing the biodigester for proper utilization of the slurry. However, due to cultural or religious belief, people are reluctant to use the sludge produced from human excreta. This belief can be overcome by educating the people.
Environment and sanitation training programme was simultaneously implemented with the participation of the selected leaders/community workers and personnel of institutions of the refugee affected areas of Ward No. 1. This programmer has helped the local mass, community and institutions to understand and appreciate the appropriate and multiple uses of human excreta for environment-friendly local sustainable rural development. Such a situation indicates a clean need of an intensive programme for the biogas users.
Sanitation and community health of the camps inhabited by about one hundred thousand Bhutanese refugees in Nepal in the eastern Jhapa and Morang districts of Nepal as well as in areas affected by these refugees is one of the main concerns of United Nations High Commissioner for Refugees (UNHCR). It is essential to treat human waste properly to prevent infestation of various water-borne diseases that spread through faecal contamination such as worms (hook worms, round worms), bacterial (typhoid, paratyphoid, dysentery, cholera) and viral infections (gastro-enteritis resulting in diarrhea and hepatitis), especially in crowded areas like refugee camps and refugee affected areas.
Anaerobic digestion technology is one of the appropriate methods for treating human waste wherein more than 95 percent of the pathogens found in human faeces get killed in the process. Furthermore, biogas and stabilized compost (which are almost pathogens free are obtained as byproducts of the anaerobic digestion process. The slurry has been proved to be high quality organic materials for plant nutrition. The technology is economically viable, technically feasible, environmentally sound and socially being increasingly more acceptable [1, 2, 4, 6].
With above backdrop, Consolidated Management Services Nepal P. Ltd (CMS) with financial assistance of UNHCR have implemented an integrated project namely (i) Installation of Community Latrine-cum-Biogas plant and (ii) Conduction of Training in Environment and Sanitation. This inter-related project was implemented between October 1997 and February 1998.
The principal objectives of the project were as follows:
- To improve sanitary conditions of the community members residing near Ward No.1 of Pathari VDC;
- To educate the local community on health, hygiene and sanitation aspects so that they can improve their quality of life;
- To produce and utilize pathogen-free stabilized manure (bio-fertilizer) from faecal sludge to improve soil fertility, and
- To use biogas as an alternate energy for domestic cooking and lighting to reduce/replace kerosene and firewood.
INSTALLATION OF LATRINE-CUM-BIODIGESTER
Installation of Community Latrines
The design of latrine-cum-biodigester is based upon the drawings and the sketches of the public latrine and biogas plant which has been found operating successfully in the seashore of “Cox Bazzar” in Bangladesh. The relevant drawings are presented in Figure 2 and 3. The project has been implemented by the Slum Improvement Project of Local Government Engineering Department, Dhaka for the benefit of Burmese refugees and the areas affected by these refugees.
Originally, it was envisioned that the waste of the latrine would be collected into a settling tank and the excess of liquid would be drained into soak pit. But realizing the high ground water table of the proposed site, the system was somewhat modified. The settling tank which was conceived as pre-storage tank was substituted by a manhole chamber. Thus, the faecal raw sludge from the latrines is first collected into this humanhole compartment from where it is led into the biodigester (see Photo-1). Considering that excess of urine can create toxicity to the methanogenic bacteria, a provision was made to drain it into a soak pit particularly from the three urinals constructed in the male section of the latrines. The concept of the integrated production and use of biogas and stabilized manure from the community latrine-cum biogas plant has been given in Figure 4.
Installation of 15 m3 Biodigester
A 15 m3 capacity GGC (Gobar Gas and Agricultural Equipment Development Company) model biodigester of a fixed dome Chinese type as approved by Biogas Support Program (BSP) of the Netherlands Development Organization (SNV/Nepal) was installed to produce stabilized manure and biogas.
High ground water table greatly hampered the construction of community latrine in the beginning. Therefore for achieving quality masonry work in the process of building the biodigester, continuous dewatering was done to remove accumulated water inside the digester with the help of a pumping machine. This process was completed cautiously and methodologically for achieving desired quality work.
Construction of Compost Pits
As planned, two compost pits with dimension of 1.5 m x 1.5 m x 8 m were constructed close to the biodigester for collection, treatment and utilization of slurry as fertilizer. The design and construction of twin compost pits permit to fill and empty each pit by turn. Thus, in case the first pit is filled in completely, it is left for drying for couple of week and the use of the second one is done (Photo-2).
OPERATION OF BIODIGESTER
As it was planned to feed the biodigester initially with animal excreta as raw materials, a mixture of cow dung and water was used for loading the digester. To activate and enhance the process of fermentation in order to shorten the time of gas production, the digester was inoculated with 150 litres of the effluents from the operating digester to which 5 kg of molasses were added. Thus, addition of inoculum that furnished methanogenic bacteria and molasses that provided source of energy to multiply these bacteria resulted into the production of first combustible gas (methane) on ninth day after loading the digester. The production of biogas was tested by using a biogas lamp and a burner in presence of the Chairman of Pathari Village Development Committee (VDC), his staff and the local people. The latrine was put for general public use with effect from 15th March 1999. Since then animal manure has not been added to the digester and it is fed only with latrine wastes.
At present, the latrines are being daily visited by 250 regular and 35 occasional users per day. From 285 latrine visitors, about 115 kg of human excreta is available per day as raw materials for feeding the biodigester (0.4 kg of faeces per person). The actual gas production is about 5.75 m3 per day.
Two biogas lamps consuming each 0.07 m3 gas per hour are equipped inside the community latrine and they are lighted for illumination during night, while two burners are being tested currently. Presently, the watchman of the installation has been demonstrating the gas for cooking (e.g. preparation of tea). After completing the test, the gas produced is planned to be distributed by the Management Committee to the interested household (s) in the area.
SUSTAINIBILITY OF THE PROJECT
Role of Latrine-cum-Biodigester Management Committee
In order to create local capacity to manage the installed toilet and biodigester and to ensure its operation and maintenance, repair and durability, a Latrine-cum-Biodigester Management Committee was formed on October 20, 1997 under the chairmanship of Mohan Tumbapo, the local VDC Chairman. This committee consists of a total of 12 members selected from various local institutions and intellectuals including representation from UNHCR, Refugee Coordination Unit (RCU) and CMS. The Management Committee had assigned a watchman as a care-taker of the latrine and biodigester. For proper care, maintenance and repair of the latrine, reasonable fees are charged to the latrine users. The total income generation from the latrine users amounts to Rs 2,700 per month (i.e. Rs 2,500 from the latrine users and Rs 200 from the sale of kitchen garden; about US $50). The monthly salary of the watchman is around Rs 2,000 per month and the occasional maintenance charge is Rs 400. The users' pay system covers the expenses for maintenance of the utilities. For the security of the latrine users, the Committee has completed the fencing around the premises of the utilities. For long-term sustainability, the Committee has agreed to carry out necessary repair and maintenance of the toilet and to resolve all the problems that may arise in the future. The VDC plans to launch an awareness campaign among the local people for more scientific and maximum use of the utilities.
Environment and Sanitation Training
An environment and sanitation training programme was implemented from 21 to 26 December 1997 with support from UNHCR. Selected leaders/community workers and personnel of local rural institutions of the refugee affected areas of Ward No. 1 of Pathari VDC in this interactive programme.
FUTURE ACTION PLAN
Proper Use of Gas: The future action plan proposed by the CMS focuses on the proper use of gas produced from the utilities. Although about 40 percent of the family size biogas plants so far installed in Nepal are attached with latrines, considerable number of population has still reservation to use the gas generated from human faeces. Therefore, the rural people residing close to the project site need to be educated and motivated so that they could accept the gas for cooking food and burning lamps.
Proper Use of Effluent: The local people need to be educated about the proper use of effluent. As originally envisioned in this project, the effluent from biogas plants was planned to be mixed with plant wastes such as leaves, rice straw and or other biodegradable materials in order to make the compost to fertilize agricultural crops, horticultural plants and to raise nursery. For doing that, it is an imperative to motivate and convince the farming community to use locally produced manure from the utility.
Practical-cum-Demonstrative Workshop: In view of resisting the above bottlenecks, CMS has put forth suggestions to UNHCR to conduct demonstrative training-cum workshop on operation and maintenance of latrine-cum-biodigester and proper utilization of slurry by involving the local rural community and institutions of Ward No. 1 of Pathari VDC.