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Aug 25

Anaerobic Digestion of Biomass

Posted in Biofuels | Waste to Energy

Anaerobic Digestion The generation and disposal of organic waste without adequate treatment result in significant environmental pollution. Besides health concerns for the people in the vicinity of disposal sites, degradation of waste leads to uncontrolled release of greenhouse gases (GHGs) into the atmosphere. Conventional means, like aeration, is energy intensive, expensive and also generates a significant quantity of biological sludge. In this context, anaerobic digestion offers potential energy savings and is a more stable process for medium and high strength organic effluents. Waste-to-Energy (WTE) plants, based on anaerobic digestion of biomass, are highly efficient in harnessing the untapped renewable energy potential of organic waste by converting the biodegradable fraction of the waste into high calorific gases. Apart from treating the wastewater, the methane produced from the biogas facilities can be recovered, with relative ease, for electricity generation and industrial/domestic heating.

An Attractive Option for Renewable Power

Anaerobic digestion plants not only decrease GHGs emission but also reduce dependence on fossil fuels for energy requirements. The anaerobic process has several advantages over other methods of waste treatment. Most significantly, it is able to accommodate relatively high rates of organic loading. With increasing use of anaerobic technology for treating various process streams, it is expected that industries would become more economically competitive because of their more judicious use of natural resources. Therefore, anaerobic digestion technology is almost certainly assured of increased usage in the future.

Benefits of Anaerobic Digestion

Anaerobic digestion provides a variety of benefits. These may be classified into three groups viz. environmental, economic and energy benefits:

The environmental benefits include:

1. Elimination of malodorous compounds.
2. Reduction of pathogens.
3. Deactivation of weed seeds.
4. Production of sanitized compost.
5. Decrease in GHGs emission.
6. Reduced dependence on inorganic fertilizers by capture and reuse of nutrients.
7. Promotion of carbon sequestration
8. Beneficial reuse of recycled water
9. Protection of groundwater and surface water resources.
10. Improved social acceptance

Anaerobic digestion is advantageous in terms of energy in the following manner:

1. Anaerobic digestion is a net energy-producing process.
2. A biogas facility generates high-quality renewable fuel.
3. Surplus energy as electricity and heat is produced during anaerobic digestion of biomass.
4. Anaerobic digestion reduces reliance on energy imports.
5. Such a facility contributes to decentralized, distributed power systems.
6. Biogas is a rich source of electricity, heat, and transportation fuel.

The economic benefits associated with a biomass-to-biogas facility are:

1. Anaerobic digestion transforms waste liabilities into new profit centers.
2. The time devoted to moving, handling and processing manure is minimized.
3. Anaerobic digestion adds value to negative value feedstock.
4. Income can be obtained from the processing of waste (tipping fees), sale of organic fertilizer, carbon credits and sale of power.
5. Power tax credits may be obtained from each kWh of power produced.
6. A biomass-to-biogas facility reduces water consumption.
7. It reduces dependence on energy imports.
8. Anaerobic digestion plants increases self-sufficiency.

Feedstock for Anaerobic Digestion Plants

A wide range of feedstock is available for anaerobic digesters. In addition to MSW, large quantity of waste, in both solid and liquid forms, is generated by the industrial sector like breweries, sugar mills, distilleries, food-processing industries, tanneries, and paper and pulp industries. Out of the total pollution contributed by industrial sub-sectors, nearly 40% of the total organic pollution is contributed by the food products industry alone. Food products and agro-based industries together contribute 65% to 70% of the total industrial wastewater in terms of organic load. Poultry waste has the highest per tonne energy potential of electricity per tonne but livestock have the greatest potential for energy generation in the agricultural sector.

Most small-scale units such as tanneries, textile bleaching and dying, dairy, slaughterhouses cannot afford effluent treatment plants of their own because of economies of scale in pollution abatement. Recycling/recovery/re-use of products from the wastes of such small-scale units by adopting suitable technology could be a viable proposition. Generation of energy using anaerobic digestion process has proved to be economically attractive in many such cases. The urban municipal waste (both solid and liquid) – industrial waste coming from dairies, distilleries, pressmud, tanneries, pulp and paper, and food processing industries, etc., agro-waste and biomass in different forms – if treated properly, has a tremendous potential for energy generation. Fig 1 lists the possible feedstock for waste-to-energy plants based on anaerobic digestion of biomass.

Agricultural Origin

  • Livestock manure
  • Agricultural residues
  • Animal mortalities
  • Energy crops

Industrial Origin

  • Wastewater
  • Industrial sludges
  • Industrial by-products
  • Slaughterhouse waste
  • Spent beverages
  • Biosolids

Municipal Origin

  • Sewage sludge
  • Municipal solid waste
  • Food residuals

Anaerobic Digestion of Livestock Manure

The livestock industry is an important contributor to the economy of any country. More than one billion tons of manure is produced annually by livestock in the United States. Animal manure is a valuable source of nutrients and renewable energy. However, most of the manure is collected in lagoons or left to decompose in the open which pose a significant environmental hazard. The air pollutants emitted from manure include methane, nitrous oxide, ammonia, hydrogen sulfide, volatile organic compounds and particulate matter, which can cause serious environmental concerns and health problems.

Anaerobic digestion is a unique treatment solution for animal agriculture as it can deliver positive benefits related to multiple issues, including renewable energy, water pollution, and air emissions. Anaerobic digestion of animal manure is gaining popularity as a means to protect the environment and to recycle materials efficiently into the farming systems. Waste-to-Energy (WTE) plants, based on anaerobic digestion of biomass, are highly efficient in harnessing the untapped renewable energy potential of organic waste by converting the biodegradable fraction of the waste into high calorific gases.

Potential biogas yield from various animals

Animal – Biogas Yield/Ton Manure (ft3/ton/day)

  • Dairy – 920
  • Beef – 1148
  • Swine – 741
  • Poultry – 2266

The establishment of anaerobic digestion systems for livestock manure stabilization and energy production has accelerated substantially in the past several years. There are more than 111 digesters operating at commercial livestock facilities in the United States which generated around 215 million kWh equivalent of useable energy. Besides generating electricity (170 million kWh), biogas is used as boiler and domestic fuel. Many of the projects that generate electricity also capture waste heat for various in-house requirements.

In the past, livestock waste was recovered and sold as a fertilizer or simply spread onto agricultural land. The introduction of tighter environmental controls on odor and water pollution means that some form of waste management is necessary, which provides further incentives for biomass-to-energy conversion.

Important Factors to Consider

The main factors that influence biogas production from livestock manure are pH and temperature of the feedstock. It is well established that a biogas plant works optimally at neutral pH level and mesophilic temperature of around 35o C. Carbon-nitrogen ratio of the feed material is also an important factor and should be in the range of 20:1 to 30:1. Animal manure has a carbon – nitrogen ratio of 25:1 and is considered ideal for maximum gas production. Solid concentration in the feed material is also crucial to ensure sufficient gas production, as well as easy mixing and handling. Hydraulic retention time (HRT) is the most important factor in determining the volume of the digester which in turn determines the cost of the plant; the larger the retention period, higher the construction cost.

An emerging technological advance in anaerobic digestion that may lead to increased biogas yields is the use of ultrasound to increase volatile solids conversion. This process disintegrates solids in the influent, which increases surface area and, in turn, allows for efficient digestion of biodegradable waste.

Process Description of WTE Facility Based on Livestock Manure

The layout of a typical biogas facility using livestock manure as raw material is shown in Fig 3. The fresh animal manure is stored in a collection tank before its processing to the homogenization tank which is equipped with a mixer to facilitate homogenization of the waste stream. The uniformly mixed waste is passed through a macerator to obtain uniform particle size of 5-10 mm and pumped into suitable-capacity anaerobic digesters where stabilization of organic waste takes place.

In anaerobic digestion, organic material is converted to biogas by a series of bacteria groups into methane and carbon dioxide. The majority of commercially operating digesters are plug flow and complete-mix reactors operating at mesophilic temperatures. The type of digester used varies with the consistency and solids content of the feedstock, with capital investment factors and with the primary purpose of digestion.

Biogas contain significant amount of hydrogen sulfide (H2S) gas which needs to be stripped off due to its highly corrosive nature. The removal of H2S takes place in a biological desulphurization unit in which a limited quantity of air is added to biogas in the presence of specialized aerobic bacteria which oxidizes H2S into elemental sulfur.

Gas is dried and vented into a CHP unit to a generator to produce electricity and heat. The size of the CHP system depends on the amount of biogas produced daily. The digested substrate is passed through screw presses for de-watering and then subjected to solar drying and conditioning to give high-quality organic fertilizer. The press water is treated in an effluent treatment plant based on activated sludge process which consists of an aeration tank and a secondary clarifier. The treated wastewater is recycled to meet in-house plant requirements. A chemical laboratory is necessary to continuously monitor important environmental parameters such as BOD, COD, VFA, pH, ammonia, C:N ratio at different locations for efficient and proper functioning of the process.

The continuous monitoring of the biogas plant is achieved by using a remote control system such as Supervisory Control and Data Acquisition (SCADA) system. This remote system facilitates immediate feedback and adjustment, which can result in energy savings.

Utilization of Biogas and Digestate

An anaerobic digestion plant produces two outputs, biogas and digestate, both can be further processed or utilised to produce secondary outputs. Biogas can be used for producing electricity and heat, as a natural gas substitute and also a transportation fuel. A combined heat and power plant system (CHP) not only generates power but also produces heat for in-house requirements to maintain desired temperature level in the digester during cold season. CHP systems cover a range of technologies but indicative energy outputs per m3 of biogas are approximately 1.7 kWh electricity and 2.5kWh heat. The combined production of electricity and heat is highly desirable because it displaces non-renewable energy demand elsewhere and therefore reduces the amount of carbon dioxide released into the atmosphere.

In Sweden, the compressed biogas is used as a transportation fuel for cars and buses. Biogas can also be upgraded and used in gas supply networks. The use of biogas in solid oxide fuel cells is being researched.

The surplus heat energy generated may be utilized through a district heating network. Thus, there is potential scope for biogas facilities in the proximity of new housing and development areas, particularly if the waste management system could utilise kitchen and green waste from the housing as a supplement to other feed stock.

Digestate can be further processed to produce liquor and a fibrous material. The fiber, which can be processed into compost, is a bulky material with low levels of nutrients and can be used as a soil conditioner or a low level fertilizer. A high proportion of the nutrients remain in the liquor, which can be used as a liquid fertilizer.

Conclusions

Anaerobic digestion of biomass offer two important benefits of environmentally safe waste management and disposal, as well as the generation of clean electric power. The growing use of digestion technology as a method to dispose off livestock manure has greatly reduced its environmental and economic impacts. Biomass-to-biogas transformation mitigates GHGs emission and harness the untapped potential of a variety of organic waste. Anaerobic digestion technology affords greater water quality benefits than standard slurry storage due to lower pollution potential. It also provides additional benefits in terms of meeting the targets under the Kyoto Protocol and other environmental legislations.

The livestock industry is a vitally important contributor to the economy of any country, regardless of the degree of industrialization. Animal manure is a valuable source of renewable energy; additionally, it has soil enhancement properties. Anaerobic digestion is a unique treatment solution for animal agriculture as it can deliver positive benefits related to multiple issues, including renewable energy, water pollution, and air emissions. Anaerobic digestion of animal manure is gaining popularity as a means to protect the environment and to produce clean energy. There is an urgent need to integrate the digester with manure management systems for effective implementation of the anaerobic digestion technology to address associated environmental concerns and to harness renewable energy potential of livestock.

By Salman Zafar, Renewable Energy Expert

  • Gail Feddern

    Bio gas made from sewage and lawn, weeds, and tree trimmings is the answer. We should not be wasting corn, which is so valuable as food and feed, on manufacturing bio fuel. The whole corn-bio fuel thing is a big fiasco, causing world famine and economic strife. The real solution is always to take a problem (sewage pollution) and flip it over to make it a viable solution (a new source of energy). People should be thinking “How can I turn this disadvantage into an advantage?” rather than trying to dispose of it. Yes, find new uses for the unwanted bi-product. That is the proper way to eliminate the problems of pollution without causing other, often greater, problems.

    Ram Bux Singh in India invented the Bio Gas Digester back in the 60s. Villages in China run their electric lights from crude bio gas technology, consisting of a pit dug in the ground with a floating lid & pipe on it to collect the methane gas. Rudimentary technology is there now; it’s usable as is, or we can refine it so every backyard has a bio digester. The big question is WHY AREN’T WE DOING ANYTHING WITH IT NOW????

  • GaltKnows

    You’re absolutely RIGHT!!!! One American man is doing something about it. He is teaching a workshop about how individuals can make their own biomass or trash gasifier. Check it out in Treehugger.

    http://www.treehugger.com/files/2008/08/build-do-it-yourself-biomass-gasifier.php

    Great article and the guy is showing people how to build one hands on. If people are real and want to walk the walk instead of just talk green, then they should attend his workshop. Tickets are available according to treehugger by going to VictoryGasworks.com

  • Chloe

    Hiya my name is Chloe, I’m a student at Glasgow University I’m doing a project on utilization of woody materials. I found this article very interesting and I was wondering if there are any other biological methods that can be exploited to release feedstocks for biofuel?

  • Salman

    Hi Chloe
    Thanks for your comment.
    Apart from anaerobic digestion, there are aerobic as well as fermentation processes to obtain biofuels from woody biomass. It would be more convenient if you can write to me at salman.alg[AT]gmail.com so that I can send you some study material in your field of interest.

    Best wishes
    Salman

  • Michel

    I think there should be more anaerobic digesters in the agricultural community. They should also be more prevalent in the cities where everyone is producing a fuel source each and everyday.

  • Salman

    Thats true, Michel. Alternative energy systems are inherent to achieve sustainable development, energy-security and address environmental issues in any country. Biochemical methods of waste-to-energy conversion, like anaerobic digestion, should be promoted in a big way.

  • vijayaillu ruralcoophome support, India

    We are thankful if you could support how much financing it may attract, and who will fund it. As you say it produces is renewable energy, but the power derived from seem to mot much useful, however under CDM methane generation, is 21 times co2, does this not build higher vlaues of CDM? We are thankful if you can furnish such data also. Where have such installations have taken place? Who has funded it? Please give a few concrete instances, where one can educate further.

  • Tony Catignani

    I have a few questions about this process.
    1. What is the production rate?
    How much waste is put in to produce 1 days energy?
    2. What does it cost to produce 1 days energy?
    3. How small can the plant process be?
    4. Can it be fitted into a 2 meter cube or less?
    5. Can this all be done without compromising vital crops?

  • Marlene

    I am trying to find a web site that help me to build a an anaerobic digestion system for my home. Does anyone knows where I can find a site?

  • Zung Young

    13.5% GHGs is from Agricultural Remains in the world, within these GHGs, most of them are CH4 (Methane), build the Methane recovery system like biodigester to reduce the carbon emission reduction and utilize biogas as power generation fuel or CHP.

  • alan

    Could somebody tell me some of the companies that have completed the Anaerobic Digestion system?

  • Salman

    hello

    I am a student in UK and am now on a project to generate max 6MW of electricity through anaerobic digestion for a paper processign plant. I was wondering how much waste material we need and can anyone tell me how to size my digester(s)? Besides I wonder what type of process I shall choose? like Single stage or multistage?

    Thank you
    salman

  • Salman

    I would appreciate if you can write to me directly at salman.alg [AT] gmail.com and discuss your project. I assure all help.

  • pikant loulou

    hello

    I am working on an anaerobic digestion plant. i am using a process similar to varlorga process. I have not been able to find the price of a valorga digester till now. anyone has an idea or link where i can get the information. I have tried on the valorga international website endless time with no success.

    thanking you in advance.
    pikanto

  • http://www.smithgill.com Christopher Drew

    I’m looking for an AD plant that can serve a community of 2250 people (individuals not homes). The food waste from the community would be added to the sewage network through in-sink grinders. Digestate would be composted and waste water would go through an MBR and polishing facility.

  • rishi gupta

    i’m looking for a solution to use the vegetable waste as a source of electricity. can you give me details?


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