Anaerobic Digesters For Lagoons Part 1

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


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One Response to “Anaerobic Digesters For Lagoons Part 1”

  1. Troy Says:

    Great site ya got here!

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