Anaerobic lagoon

An anaerobic lagoon or manure lagoon is a man-made outdoor earthen basin filled with animal waste that undergoes anaerobic respiration as part of a system designed to manage and treat refuse created by concentrated animal feeding operations (CAFOs). Anaerobic lagoons are created from a manure slurry, which is washed out from underneath the animal pens and then piped into the lagoon. Sometimes the slurry is placed in an intermediate holding tank under or next to the barns before it is deposited in a lagoon. Once in the lagoon, the manure settles into two layers: a solid or sludge layer and a liquid layer. The manure then undergoes the process of anaerobic respiration, whereby the volatile organic compounds are converted into carbon dioxide and methane. Anaerobic lagoons are usually used to pretreat high strength industrial wastewaters and municipal wastewaters. This allows for preliminary sedimentation of suspended solids as a pretreatment process.

Anaerobic lagoons have been shown to harbor and emit substances which can cause adverse environmental and health effects. These substances are emitted through two main pathways: gas emissions and lagoon overflow. Gas emissions are continuous (though the amount may vary based on the season) and are a product of the manure slurry. The most prevalent gasses emitted by the lagoon are: ammonia, hydrogen sulfide, methane, and carbon dioxide. Lagoon overflow is caused by faulty lagoons, such as breaches or improper construction, or adverse weather conditions, such as increased rainfall or strong winds. These overflows release harmful substances into the surrounding land and water such as: antibiotics, estrogens, bacteria, pesticides, heavy metals, and protozoa.

In the U.S., the Environmental Protection Agency (EPA) has responded to environmental and health concerns by strengthening regulation of CAFOs under the Clean Water Act. Some states have imposed their own regulations as well. Because of repeated overflows and resultant health concerns, North Carolina banned the construction of new anaerobic lagoons in 1999. There has also been a significant push for the research, development and implementation of environmentally sound technologies which would allow for safer containment and recycling of CAFO waste.

Background

Beginning in the 1950s with poultry production, and then later in the 1970s and 1980s with cattle and swine, meat producers in the United States have turned to CAFO as a way to more efficiently produce large quantities of meat. This switch has decreased the price of meat. However, the increase in livestock has generated an increase in manure. In 2006, for example, livestock operations in the United States produced of manure. The high quantity of manure produced by a CAFO must be dealt with in some way, as improper manure management can result in water, air and soil damage.

In order to manage their waste, CAFOs have developed agricultural wastewater treatment plans. To save on manual labor, many CAFOs handle manure waste as a liquid. In this system, the animals are kept in pens with grated floors so the waste and spray water can be drained from underfloor gutters and piped to storage tanks or anaerobic lagoons. Studies have shown that in fact the lagoons typically leak at a rate of approximately per day, with or without a clay liner, because it is the sludge deposited at the base of the lagoon that limits the leakage rate, not the clay liner or underlying native soil.

Anaerobic lagoons are not heated, aerated or mixed. Anaerobic lagoons are most effective in warmer temperatures; anaerobic bacteria are ineffective below . Lagoons must be separated from other structures by a certain distance to prevent contamination. States regulate this separation distance. The overall size of the lagoon is determined by addition of four components: minimum design volume, volume of manure storage between periods of disposal, dilution volume and the volume of sludge accumulation between periods of sludge removal. The process of anaerobic digestion has been shown to release over 400 volatile compounds from lagoons. The most prevalent of these are: ammonia, hydrogen sulfide, methane, and carbon dioxide.

Ammonia

In the United States, 80% of ammonia emissions come from livestock production. A lagoon can vaporize up to 80% of its nitrogen<nowiki/>through the reaction: NH4+-N ⇌ NH3 + H+. This volatilized ammonia has been implicated in widespread ecological damage in Europe and is of growing concern for the United States.

Hydrogen sulfide

With averages greater than 30 ppb, lagoons have high concentration of hydrogen sulfide, which is highly toxic. Methane is combustible at high temperatures, and explosions and fires are a real threat at or near lagoons. Additionally, methane is a greenhouse gas. The U.S. EPA estimated that 13% of all the methane emissions came from livestock manure in 1998, and this number has grown in recent years.

Water-soluble contaminants

Contaminants that are water-soluble can escape from anaerobic lagoons and enter the environment through leakage from badly constructed or poorly maintained manure lagoons as well as during excess rain or high winds, resulting in an overflow of lagoons.

Pathogens

More than 150 pathogens in manure lagoons have been found to impact human health. This reduction would decrease the pathogen's presence in both meat and waste of the cattle, and decrease the E. coli population found in anaerobic lagoons.

Cryptosporidium

Cryptosporidium is a parasite that causes diarrhea, vomiting, stomach cramps and fever. It is particularly problematic because it is resistant to most lagoon treatment regimens

Other common pathogens

Other common pathogens (and their symptoms) include: Seventy percent of all antibiotics and related drugs are given to animals as feed additives.

Heavy metals

Manure contains trace elements of many heavy metals such as arsenic, cadmium, copper, iron, lead, manganese, molybdenum, nickel, and zinc. Sometimes these metals are given to animals as growth stimulants, some are introduced through pesticides used to rid livestock of insects, and some might pass through the animals as undigested food. Ronnie Kennedy, county director for environmental health, said that of 310 private wells he had tested for contamination since the storm, 9%, or three times the average across eastern North Carolina, had fecal coliform bacteria. Normally, tests showing any hint of feces in drinking water, an indication that it can be carrying disease-causing pathogens, are cause for immediate action.

Regulation

Anaerobic lagoons are built as part of a wastewater operation system. As such, compliance and permitting are handled as an extension of that operation. Therefore, manure lagoons are regulated on the state and national level through the CAFO which operates them. In recent years, because of the environmental and health effects associated with anaerobic lagoons, the EPA has increased regulation of CAFOs with a specific eye towards lagoons. North Carolina banned the construction of new anaerobic lagoons in 1999 and upheld that ban in 2007.

Further research

Some research has been done to develop and assess the economic feasibility of more environmentally superior technologies (ESTs). Five main alternatives which have been implemented in North Carolina are: a solids separation/nitrification–denitrification/soluble phosphorus removal system; a thermophilic anaerobic digester system; a centralized composting system; a gasification system; and a fluidized-bed combustion system. These systems were judged based on their ability to: reduce impacts of CAFO waste in the surface and groundwater, decrease ammonia emissions, decrease the escape of disease-transmitting pathogens, and lower the concentration of heavy metal contamination.