Posts Tagged ‘lagoon covers’

Basic Types Of Anaerobic Digesters

November 2, 2010

Anaerobic Digesters

Basic Types:

While many different types of biogas recovery systems are available, the three designs most commonly used at U.S. farms are described below.

Covered anaerobic lagoon

An anaerobic lagoon is sealed with a flexible cover, and the methane is recovered and piped to the combustion device. Some systems use a single cell for combined digestion and storage.

Plug flow digester

A plug flow digester has a long, narrow concrete tank with a rigid or flexible cover. The tank is built partially or fully below grade to limit the demand for supplemental heat. Plug flow digesters are used only at dairy operations that collect manure by scraping.

Complete mix digester

A complete mix digester is an enclosed, heated tank with a mechanical, hydraulic, or gas mixing system. Complete mix digesters work best when there is some dilution of the excreted manure with water (e.g., milking center wastewater). The photo on the left shows an externally mounted mixer.

Additional digester types:

Several other digester types have also been constructed in recent years, such as induced blanket reactors, fixed film digesters and batch digesters.

  • Induced Blanket Reactors are digesters in which a blanket of sludge develops that retains anaerobic bacteria, providing a bacteria-rich environment through which influent must pass.
  • Fixed film digesters contain plastic media (e.g., pellets) on which bacteria attach and grow, instead of relying solely on suspended bacteria to break down the digester influent.
  • A batch digester is the simplest form of digestion, where manure is added to the reactor at the beginning of the process in a batch and the reactor remains closed for the duration of the process.

The most common means of collecting and storing the gas produced by a digester is with a floating cover—a weighted pontoon that floats on the liquid surface of a collection/storage basin.

Source: epa.gov


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Anaerobic Digester Types and Designs

October 28, 2010

Anaerobic Digesters

Factors to consider when designing an anaerobic digestion system include cost, size, local climate, and the availability and type of organic feedstock material.

Anaerobic digesters—also known as biodigesters—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 anaerobic digestion system designs incorporate the same basic components:

  • A pre-mixing area or tank
  • A digester vessel(s)
  • A system for using the biogas
  • A system for distributing or spreading the effluent (the remaining digested material).

There are two basic types of digesters:

  • Batch

Batch-type digesters are the simplest to build. Their operation consists of loading the digester with organic materials and allowing it to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated.

  • Continuous

In a continuous digester, organic material is constantly or regularly fed into the digester. The material moves through the digester either mechanically or by the force of the new feed pushing out digested material. Unlike batch-type digesters, continuous digesters produce biogas without the interruption of loading material and unloading effluent. There are three types of continuous digesters: vertical tank systems, horizontal tank or plug-flow systems, and multiple tank systems.

Proper design, operation, and maintenance of continuous digesters produce a steady and predictable supply of usable biogas. They may be better suited for large-scale operations.

Many livestock operations store the manure they produce in waste lagoons, or ponds. A growing number of these operations are placing floating covers on their lagoons to capture the biogas. They use it to run an engine/generator to produce electricity.

Floating cover applications

  • any type of gas collection from water basin
  • keep rain & snowmelt water separate from wastewater under the cover

Floating cover advantages:

  • provide a true “floating” cover, keeping the cover on the water surface avoiding damage from wind due to an inflated cover without lateral floats
  • accommodate fluctuation in water level
  • can be installed without interruption in basin use
  • can be installed on tanks or lagoons
  • eliminates rainwater pooling problems
  • eliminates inflating the cover and gas ballooning
  • hatches provide access under the cover for equipment

Benefits and Challenges of Biogas Technology

October 14, 2010

Biogas Technology

Anaerobic digestion can convert organic wastes into profitable byproducts as well as reduce their environmental pollution potential. Anaerobic digestion offers the following benefits to an animal feeding operation and the surrounding communities:

  • Electric and thermal energy.
  • Stable liquid fertilizer and high-quality solids for soil amendment.
  • Odor reduction.
  • Reduced groundwater and surface water contamination potential.
  • Potential revenue from sales of digested manure (liquid and solids) and excess electricity and/or processing off-site organic waste.
  • Reduction of greenhouse gas emissions; methane is captured and used as a fuel.
  • Revenue from possible reuse of digested solids as livestock bedding.
  • Potential revenue from green energy and carbon credits.

The cost of installing an anaerobic digester depends on the type and size of system, type of livestock operation, and site-specific conditions (EPA AgStar, 2006).  In general, consider the following points when estimating installation/operating costs:

  • Estimate the cost of constructing the system.
  • Estimate the labor and cost of operating the system.
  • Estimate the quantity of gas produced.
  • Estimate the value of the gas produced.
  • Compare operation costs to benefits from operation (include value as a waste-treatment system and the fertilizer value of the sludge and supernatant).

The main financial obligations associated with building an anaerobic digester include capital (equipment and construction and associated site work), project development (technical, legal, and planning consultants; financing; utilities connection; and licensing), operation and maintenance, and training costs.

In making a decision to install a digester, one must realize that the system will require continuous monitoring and routine maintenance and repair that should not be underestimated.  Components should be maintained as recommended by the manufacturers because manure and biogas can be corrosive on metal parts.  In fact, the majority of digester failures over the past few decades were the result of management, not technological, problems.

Maintenance Free Patented Design Floating Covers

August 17, 2010

Maintenance Free Floating Covers

Floating covers are a very economical method of protecting precious water.  The cost can be 20% of that for other cover options.  Industry-recognized guidelines exist for the design, installation and maintenance of floating cover systems.  An example is the American Waterworks Association (AWWA) standard D130-87, which contains the most current standard practices observed for system compliance to US national health and safety regulations.

The California-Nevada Section American Water Works Reservoir Floating Cover Guidelines dated March 1999 offers an excellent source of design, operations and maintenance guidelines.

Floating Covers are customized for each client’s lagoon or tank size and shape.  A series of lateral floats provide pathways for gas to flow underneath to the perimeter collection pipe and on to a single collection point.  The lateral floats keep the entire cover floating on the waters’ surface, and eliminate cover inflation, and exposure to wind damage.

Floating covers that inflate (without lateral floats) expose the geomembrane cover to vibration and geomembrane fatigue from wind constantly blowing over the inflated cover.  This significantly reduces the life span of the cover.  Ballast weights on top of the floating cover channel rainwater and snow melt to sumps to remove all rainwater from the surface of the floating cover.

Properly designed reservoir-floating cover systems prevent fluid loss due to evaporation, reduce chemical demand and improve water quality by preventing contamination from bird droppings, airborne particulates, dead animals, pollen and other pollutants.  Floating covers block off sunlight preventing algae bloom.  They also reduce the production of trialomethane (THM) type compounds such as chloroform from forming that result from the combining of organic substances with chlorine due to reductions in chlorine demand.

Floating cover systems were introduced over 30 years ago.  Many have provided a service life beyond 20 years.  When first introduced, materials and designs were not developed and in some cases had limited success. Today, with advancements in design and materials, floating covers offer the low cost quality solution of choice where water quality standards require potable water reservoirs be covered.

Patented Design Floating Covers, Tank Systems, Storage Lagoon Covers and Liners

July 6, 2010

IEC, a company with 16 years experience designing, fabricating, and installing industrial cover and liner systems, designs its products to provide many years of lasting service in a variety of environments and applications.  Since 1993, the company has designed, fabricated and installed more than 250 projects involving odor control, gas collection, pond liner systems and tank liner systems.

IEC’s Odor Control & Gas Collection Covers are specifically designed for each client utilizing a variety of material options.  Cover applications can be used with any type of gas collection from water basin and keep rain and snowmelt water separate from wastewater under the cover. Advantages of a cover include installation without site interruption, use on tanks or lagoons, elimination of rainwater ponding problems, elimination of gas ballooning, provides high buoyancy and rigidity, hatches can provide access to in-basin equipment, improved quality with pre-manufactured panels and are fabricated at IEC’s plant, so field welding is not required.

IEC also has a patented Modular Cover System comprised of a series of individual casings connected together to form a complete floating cover system.  Each individual casing consists of a panel of closed cell insulation encapsulated between two sheets of durable geomembrane.  The result is a unique floating cover system that provides insulation values ranging from R-2 to R-30; and is engineered and manufactured to specific dimensions/basin requirements.

The Modular Cover System offers the following advantages over conventional covers systems:
• maintenance free
• can be installed on tanks or lagoons
• adepts to varying water levels
• individual casings are removable,
• installed without site interruption
• shorter installation time, no field welding required
• installation requires less heavy equipment
• eliminates rainwater ponding problems
• eliminates gas ballooning
• high buoyancy and rigidity
• hatches can provide access to in-basin equipment

Wastewater Lagoons

June 22, 2010

Lagoons

Wastewater lagoons have been used as a process for wastewater treatment for centuries.  In the 1920’s artificial ponds were designed and constructed to receive and stabilize wastewater.  By 1950, the use of ponds had become recognized as an economical wastewater treatment method for small municipalities and industries.  As of 1980, approximately 7,000 waste stabilization lagoons were in use in the U.S. Today, one third of all secondary wastewater treatment facilities include a pond system of one type or another.  Of these, just over 90% are for flows 1 MGD or less.  But ponds can be used for larger cities for wastewater treatment as well.  Some of the largest pond systems in this country are in Northern California, serving such cities as Sunnyvale (pop. 105,000), Modesto (pop. 150,000), Napa (pop. 175,000), and Stockton (pop. 275,000).

Floating tank covers can be custom designed for lagoon or tank applications.   Floating covers are a cost-effective alternative to many aluminum or fiberglass dome applications and do not require venting (non-gas-collection applications), recirculation or explosion proof equipment.  Tank covers can adapt to varying water levers and in-basin equipment.  They also install easily and quickly.

Types of Anaerobic Digesters Part 1

June 1, 2010

Types Of Digesters

There are three basic digester designs. 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 a cover.  The cover traps gas produced during decomposition of the manure.  This type of digester is the least expensive of the three.

Covering a manure storage lagoon is a simple form of digester technology suitable for liquid manure with less than 3-percent solids.  For this type of digester, an impermeable floating cover of industrial fabric covers all or part of the lagoon.  A concrete footing along the edge of the lagoon holds the cover in place with an airtight seal.  Methane produced in the lagoon collects under the cover.  A suction pipe extracts the gas for use.  Covered lagoon digesters require large lagoon volumes and a warm climate.  Covered lagoons have low capital cost, but these systems are not suitable for locations in cooler climates or locations where a high water table exists.

A complete mix digester converts organic waste to biogas in a heated tank above or below ground.  A mechanical or gas mixer keeps the solids in suspension.  Complete mix digesters are expensive to construct and cost more than plug-flow digesters to operate and maintain.

Complete mix digesters are suitable for larger manure volumes having solids concentration of 3 percent to 10 percent.  The reactor is a circular steel or poured concrete container.  During the digestion process, the manure slurry is continuously mixed to keep the solids in suspension.  Biogas accumulates at the top of the digester.  The biogas can be used as fuel for an engine-generator to produce electricity or as boiler fuel to produce steam.  Using waste heat from the engine or boiler to warm the slurry in the digester reduces retention time to less than 20 days.

Plug-flow digesters are suitable for ruminant animal manure that has a solids concentration of 11 percent to 13 percent.  A typical design for a plug-flow system includes a manure collection system, a mixing pit and the digester itself.  In the mixing pit, the addition of water adjusts the proportion of solids in the manure slurry to the optimal consistency.  The digester is a long, rectangular container, usually built below-grade, with an airtight, expandable cover.

New material added to the tank at one end 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.”  Average retention time (the time a manure “plug” remains in the digester) is 20 to 30 days.

Anaerobic digestion of the manure slurry releases biogas as the material flows through the digester.  A flexible, impermeable cover on the digester traps the gas.  Pipes beneath the cover carry the biogas from the digester to an engine-generator set.

A plug-flow digester requires minimal maintenance.  Waste heat from the engine-generator can be used to heat the digester.  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 (77°F to 104°F), 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.

There are three basic digester designs.  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 a cover.  The cover traps gas produced during decomposition of the manure.  This type of digester is the least expensive of the three.

Covering a manure storage lagoon is a simple form of digester technology suitable for liquid manure with less than 3-percent solids.  For this type of digester, an impermeable floating cover of industrial fabric covers all or part of the lagoon.  A concrete footing along the edge of the lagoon holds the cover in place with an airtight seal.  Methane produced in the lagoon collects under the cover.  A suction pipe extracts the gas for use. Covered lagoon digesters require large lagoon volumes and a warm climate.  Covered lagoons have low capital cost, but these systems are not suitable for locations in cooler climates or locations where a high water table exists.

A complete mix digester converts organic waste to biogas in a heated tank above or below ground.  A mechanical or gas mixer keeps the solids in suspension. Complete mix digesters are expensive to construct and cost more than plug-flow digesters to operate and maintain.

Complete mix digesters are suitable for larger manure volumes having solids concentration of 3 percent to 10 percent. The reactor is a circular steel or poured concrete container. During the digestion process, the manure slurry is continuously mixed to keep the solids in suspension. Biogas accumulates at the top of the digester. The biogas can be used as fuel for an engine-generator to produce electricity or as boiler fuel to produce steam. Using waste heat from the engine or boiler to warm the slurry in the digester reduces retention time to less than 20 days.

Plug-flow digesters are suitable for ruminant animal manure that has a solids concentration of 11 percent to 13 percent. A typical design for a plug-flow system includes a manure collection system, a mixing pit and the digester itself. In the mixing pit, the addition of water adjusts the proportion of solids in the manure slurry to the optimal consistency. The digester is a long, rectangular container, usually built below-grade, with an airtight, expandable cover.

New material added to the tank at one end 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.” Average retention time (the time a manure “plug” remains in the digester) is 20 to 30 days.

Anaerobic digestion of the manure slurry releases biogas as the material flows through the digester. A flexible, impermeable cover on the digester traps the gas. Pipes beneath the cover carry the biogas from the digester to an engine-generator set.

A plug-flow digester requires minimal maintenance. Waste heat from the engine-generator can be used to heat the digester. 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 (77°F to 104°F), 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.

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The Process of Anaerobic Digestion

The process of anaerobic digestion occurs in a sequence of stages involving distinct types of bacteria. Hydrolytic and fermentative bacteria first break down the carbohydrates, proteins and fats present in biomass feedstock into fatty acids, alcohol, carbon dioxide, hydrogen, ammonia and sulfides. This stage is called “hydrolysis” (or “liquefaction”).

Next, acetogenic (acid-forming) bacteria further digest the products of hydrolysis into acetic acid, hydrogen and carbon dioxide. Methanogenic (methane-forming) bacteria then convert these products into biogas.

The combustion of digester gas can supply useful energy in the form of hot air, hot water or steam. After filtering and drying, digester gas is suitable as fuel for an internal combustion engine, which, combined with a generator, can produce electricity. Future applications of digester gas may include electric power production from gas turbines or fuel cells. Digester gas can substitute for natural gas or propane in space heaters, refrigeration equipment, cooking stoves or other equipment. Compressed digester gas can be used as an alternative transportation fuel.

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Manure Digesters

Anaerobic digestion and power generation at the farm level began in the United States in the early 1970s. Several universities conducted basic digester research. In 1978, Cornell University built an early plug-flow digester designed with a capacity to digest the manure from 60 cows.

In the 1980s, new federal tax credits spurred the construction of about 120 plug-flow digesters in the United States. However, many of these systems failed because of poor design or faulty construction. Adverse publicity about system failures and operational problems meant that fewer anaerobic digesters were being built by the end of the decade. High digester cost and declining farm land values reduced the digester industry to a small number of suppliers.

The Tillamook Digester Facility (MEAD Project) began operation in 2003. The facility is located on a site once occupled by a Navy blimp hanger on property owned by the Port of Tillamook Bay. The facility consists of two 400,000-gallon digester cells. The facility uses the biogas to run two Caterpillar engines, each coupled to a 200 kilowatt generator. The facility sells its electric output to the Tillamook PUD. Manure is brought to the facility by truck from participating dairy farms in the Tillamook area.

Anaerobic Digesters For Lagoons Part 1

May 25, 2010

What Is An Anaerobic Lagoon?

An anaerobic lagoon is an earthen impoundment receiving manure from an animal feeding operation in which manure is stored and stabilized by bacterial activity operating without oxygen (compare with an aerobic structure). The statute specifically provides that an anaerobic lagoon does not include a confinement feeding operation structure such as an earthen manure storage basin; a basin connected to unroofed operations (feedlots) which collects and stores runoff produced by rain or a system which collects and treats off gases.

Covered lagoon digesters are the simplest AD system.  These systems typically consist of an anaerobic combined storage and treatment lagoon, an anaerobic lagoon cover, an evaporative pond for the digester effluent, and a gas treatment and/or energy conversion system.  Figure 1 shows a typical schematic for a floating covered anaerobic lagoon.

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.  Engine/generator systems typically do not produce sufficient waste heat to maintain temperatures high enough in covered lagoon digesters in the winter to sustain consistently high biogas production rates.  Using propane or natural gas to provide additional heat for the lagoon contents is typically not an economically viable option.  Without that additional heat, most covered lagoon digesters produce less biogas in colder temperatures, and little or no gas below 39 FACE= “Symbol”>° F.  As a result, covered lagoon digesters are most appropriate for use in warm climates if the biogas is to be used for energy or heating purposes.

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 underfloor 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 aboveground tank or in-ground 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 cogeneration water jacket or boiler is used to heat the digester.  A cylindrical aboveground tank, such as that shown in Figure 2, optimizes biogas production, but is more capital intensive than in-ground tanks.
Source: EPA. Manual for Developing Biogas Systems at Commercial Farms in the United States

Types of Geosynthetic Materials

May 20, 2010

Geosynthetic Materials

Geotextiles – Textiles in the traditional sense, they consist of synthetic fibers so that biodegradation is not a problem.  They make up one of the two largest groups of geosynthetics.  These synthetic fibers are made into a flexible fabric by standard weaving machinery or are matted together in a random, or nonwoven, manner.  The fabric is porous to water flow across its manufactured plane and within its plane.  There are at least 80 specific applications for geotextiles, but the fabric always performs at least one of five discrete functions: separation, reinforcement, filtration, drainage, or barrier to moisture.

Geomembranes – These are the other largest group of geosynthetics.  In sheer sales volume, they are probably larger than geotextiles because their growth has been stimulated by government regulations enacted in 1982.  The materials themselves are impervious thin sheets of rubber or plastic material used primarily for linings and covers of liquid- or solid-storage facilities.  Thus, the primary function is always as a liquid or vapor barrier.  The range of applications, however, is very great, and at least 30 applications in civil engineering have been developed.

Geogrids – Plastics formed into very open, gridlike configurations, geogrids have at least 25 applications, but they function almost exclusively as reinforcement materials. They represent a rapidly growing segment within the geosynthetics family, says Drexel University Professor Grace Hsuan.

Geonets – Also called “geospacers,” these products are usually formed by a continuous extrusion of parallel sets of polymeric ribs at acute angles to one another.  When the ribs are opened, relatively large apertures are formed into a netlike configuration.  Their design function is completely within the drainage area, where they have been used to convey fluids of all types, explains Hsuan.

Geosynthetic Clay Liners – Rolls of thinly layered bentonite clay sandwiched between two geotextiles or bonded to a geomembrane, these products are seeing use as a composite component beneath a geomembrane or by themselves as primary or secondary liners.

Geopipe – Perhaps the original geosynthetic material still available today is buried plastic pipe.  Plastic pipe is being used in all aspects of geotechnical, transportation, and environmental engineering with little design and testing awareness, probably because of a general lack of formalized training.  The critical nature of leachate collection pipes coupled with high compressive loads makes geopipe a bona fide member of the geosynthetics family.  Its function is clearly drainage.

Geocomposites – Combinations of geotextile and geogrid; geogrid and geomembrane; geotextile, geogrid, and geomembrane; or any one of these three materials with another material (e.g., deformed plastic sheets, steel cables, or steel anchors) are geocomposites.  This exciting area brings out the best creative efforts of the engineer, manufacturer, and contractor.  The application areas are numerous and growing steadily and they encompass the entire range of functions for geosynthetics: separation, reinforcement, filtration, drainage, and liquid barrier.

“Geo-Others” – Innovations in geosynthetics have created products that defy categorization.  These “geo-others” include such products as threaded soil masses, polymeric anchors, and encapsulated soil cells.  The geo-other name is not one specific area, although similar to geocomposites, its primary function is product-dependent and can be any of the five major functions of geosynthetics.  The category is ‘temporary housing,’ if you will, for any new products.  When it is determined the appropriate family, it is moved to “permanent housing.'”

Stages Of Anaerobic Digestion Part 2

May 11, 2010

Stages Of Anaerobic Digestion 2

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 (methanogenic) anaerobic bacteria utilize these acids and complete the decomposition process.

Anaerobic digestion, which takes place in three stages inside an airtight container, produces biogas. Different kinds of micro-organisms are responsible for the processes that characterize each stage.

A variety of factors affect the rate of digestion and biogas production.  The most important is temperature.  Anaerobic bacteria communities can endure temperatures ranging from below freezing to above 135°F (57.2°C), but they thrive best at temperatures of about 98°F (36.7°C) (mesophilic) and 130°F (54.4°C) (thermophilic).  Bacteria activity, and thus biogas production, falls off significantly between about 103° and 125°F (39.4° and 51.7°C) and gradually from 95° to 32°F (35° to 0°C).

In the thermophilic range, decomposition and biogas production occur more rapidly than in the mesophilic range.  However, the process is highly sensitive to disturbances, such as changes in feed materials or temperature.  While all anaerobic digesters reduce the viability of weed seeds and disease-producing (pathogenic) organisms, the higher temperatures of thermophilic digestion result in more complete destruction.  Although digesters operated in the mesophilic range must be larger (to accommodate a longer period of decomposition within the tank [residence time]), the process is less sensitive to upset or change in operating regimen.

To optimize the digestion process, the biodigester must be kept at a consistent temperature, as rapid changes will upset bacterial activity.  In most areas of the United States, digestion vessels require some level of insulation and/or heating.  Some installations circulate the coolant from their biogas-powered engines in or around the digester to keep it warm, while others burn part of the biogas to heat the digester.  In a properly designed system, heating generally results in an increase in biogas production during colder periods.  The trade-offs in maintaining optimum digester temperatures to maximize gas production while minimizing expenses are somewhat complex.  Studies on digesters in the north-central areas of the country indicate that maximum net biogas production can occur in digesters maintained at temperatures as low as 72°F (22.2°C).

Other factors affect the rate and amount of biogas output.  These include pH, water/solids ratio, carbon/nitrogen ratio, mixing of the digesting material, the particle size of the material being digested, and retention time.  Pre-sizing and mixing of the feed material for a uniform consistency allows the bacteria to work more quickly.  The pH is self-regulating in most cases. Bicarbonate of soda can be added to maintain a consistent pH; for example, when too much “green” or material high in nitrogen content is added.  It may be necessary to add water to the feed material if it is too dry or if the nitrogen content is very high.  A carbon/nitrogen ratio of 20/1 to 30/1 is best.  Occasional mixing or agitation of the digesting material can aid the digestion process.  Antibiotics in livestock feed have been known to kill the anaerobic bacteria in digesters.  Complete digestion, and retention times, depend on all of the above factors.