Posts Tagged ‘pond 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

Part 1 of Biogas and Anaerobic Digestion

October 21, 2010

Biogas is formed solely through the activity of bacteria, unlike composting in which fungi and lower creatures are also involved in the degradation process.  Microbial growth and biogas production are very slow at ambient temperatures.  They tend to occur naturally wherever high concentrations of wet organic matter accumulate in the absence of dissolved oxygen, most commonly in the bottom sediments of lakes and ponds, in swamps, peat bogs, intestines of animals, and in the anaerobic interiors of landfill sites.

The overall process of anaerobic digestion (AD) occurs through the symbiotic action of a complex bacteria consortium as show in diagram.  Hydrolytic microorganisms, including common food spoilage bacteria, break down complex organic wastes.  These subunits are then fermented into short-chain fatty acids, carbon dioxide, and hydrogen gases.

Syntrophic microorganisms then convert the complex mixture of short-chain fatty acids to acetic acid with the release of more carbon dioxide, and hydrogen gases.  Finally, methanogenesis produces biogas from the acetic acid, hydrogen and carbon dioxide.  Biogas is a mixture of methane, carbon dioxide, and numerous trace elements.  According to some, the two key biological issues are determining the most favorable conditions for each process stage and how non-optimal circumstances affect each stage as a whole, and the governing role of hydrogen generation and consumption.

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.

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.


Stages Of Anaerobic Digestion Part 1

May 6, 2010

Stages of anaerobic digestion

As with aerobic systems, the bacteria in anaerobic systems, and the growing and reproducing microorganisms within them require a source of elemental oxygen to survive.  In an anaerobic system there is an absence of gaseous oxygen.  Gaseous oxygen is prevented from entering the system through physical containment in sealed tanks.  Anaerobes access oxygen from sources other than the surrounding air.  The oxygen source for these microorganisms can be the organic material itself or alternatively may be supplied by inorganic oxides from within the input material.  When the oxygen source in an anaerobic system is derived from the organic material itself, then the ‘intermediate’ end products are primarily alcohols, aldehydes, and organic acids plus carbon dioxide.  In the presence of specialized methanogens, the intermediates are converted to the ‘final’ end products of methane, carbon dioxide with trace levels of hydrogen sulfide.  In an anaerobic system the majority of the chemical energy contained within the starting material is released by methanogenic bacteria as methane.

Populations of anaerobic microorganisms typically take a significant period of time to establish themselves to be fully effective.  It is therefore common practice to introduce anaerobic microorganisms from materials with existing populations, a process known as “seeding” the digesters, and typically takes place with the addition of sewage sludge or cattle slurry.
Stages

The key process stages of anaerobic digestion

There are four key biological and chemical stages of anaerobic digestion:

  1. Hydrolysis
  2. Acidogenesis
  3. Acetogenesis
  4. Methanogenesis

In most cases biomass is made up of large organic polymers.  In order for the bacteria in anaerobic digesters to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts.  These constituent parts or monomers such as sugars are readily available by other bacteria.  The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis.  Therefore hydrolysis of these high molecular weight polymeric components is the necessary first step in anaerobic digestion.  Through hydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids.

Acetate and hydrogen produced in the first stages can be used directly by methanogens.  Other molecules such as volatile fatty acids (VFA’s) with a chain length that is greater than acetate must first be catabolised into compounds that can be directly utilized by methanogens.

The biological process of acidogenesis is where there is further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here VFAs are created along with ammonia, carbon dioxide and hydrogen sulfide as well as other by-products.   The process of acidogenesis is similar to the way that milk sours.

The third stage anaerobic digestion is acetogenesis.  Here simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid as well as carbon dioxide and hydrogen.

The terminal stage of anaerobic digestion is the biological process of methanogenesis.  Here methanogens utilize the intermediate products of the preceding stages and convert them into methane, carbon dioxide and water.  It is these components that makes up the majority of the biogas emitted from the system.  Methanogenesis is sensitive to both high and low pHs and occurs between pH 6.5 and pH 8.  The remaining, non-digestible material which the microbes cannot feed upon, along with any dead bacterial remains constitutes the digestate.

A simplified generic chemical equation for the overall processes outlined above is as follows:

C6H12O6 → 3CO2 + 3CH4

refer to “what is anaerobic digestion” article and “what do floating covers do?”

What Do Floating Covers Do?

April 15, 2010

A floating membrane cover installed over the surface of a lagoon or tank rises and falls with the changing level of the liquid.  When used on water reservoirs, floating covers protect the contents from contamination and evaporation.  Used to cover chemical, industrial waste and wastewater lagoons they protect the surrounding area from the material contained.  Floating covers can also collect methane, which is released when anaerobic digestion takes place in concentrated animal feeding operations (CAFO) and food processing plants.  The methane (biogas) is often used to help power the facility, or it may be sold to utility companies for resale.

The cover shown includes a series of floats arranged in strings of parallel pairs with weights centered between them, forming troughs.  The floats that are laid out around the cover create sufficient tension to allow workers to walk on it.  That tension also channels rainwater to the troughs, then to dewatering pumps for removal.

While material selection for floating covers is site-specific, in all cases it must be resistant to UV degradation, tears, and punctures; it must have excellent seam strength—enough to hold up to foot traffic; and it must be very flexible.  The type of liquid contained, its chemical composition, its temperature range, the ambient temperature range, environmental conditions, and the intended function of the cover are some of the variables that must be considered.  For example, reinforced polypropylene was used for the water reservoir shown.

Floating covers are an economical way to protect water resources in ponds and reservoirs. Compared to building a tank, a pond with a floating cover is a much less expensive option. Floating covers have many uses from the storage of potable water to the collection of biogas from waste water.

Hello world!

March 18, 2010

Our History

Industrial & Environmental Concepts, Inc., (IEC), is widely recognized as the worldwide leader in the design, fabrication, and installation of industrial cover and liner systems.  Incorporated in 1993, IEC has established itself as the industry leader in the modular and insulated cover market.  Examples include pond, tank, lagoon and floating covers.

IEC’s patented modular cover is unmatched in terms of quality, flexibility and durability.  This unique cover system offers a tremendous amount of advantages over traditional covers and provides long term value for floating, pond, tank and lagoon covers.