Anaerobic Lagoon

The anaerobic lagoon is most commonly used to treat livestock waste. A livestock lagoon contains manure diluted with building washwater, rainfall, water wastage, and surface runoff. In the earthen and pondlike lagoon, the waste becomes partially liquefied and stabilized by the biological reactions. Anaerobic bacteria can decompose more complex organic matters per unit lagoon volume than aerobic bacteria. The process is predominantly used for treatment of concentrated wastes. Because the anaerobic lagoon does not require DO, it can be much deeper and permit a small surface area for a given volume. Anaerobic decomposition of livestock waste can lead to emission of a series of odorous gases, which are mainly carbon dioxide, methane, hydrogen sulfide, and ammonia. The advantages of lagoon systems for treatment of livestock waste include storage and disposal flexibility, less land requirement, liquid recycling for pit waste removal, land application by simple irrigation, and lower labor and operational costs. However, it has disadvantages such as loss of nutrient value, offensive odors (H2S and NH3), and possible groundwater pollution.

Slightly different from the conventional anaerobic processes, biological treatment in a lagoon follows four steps: sedimentation of manure solids at the botto m of the lagoon; biological conversion of the settled solids into organic acids and other byproducts; conversion of organic acids to methane and carbon dioxide; and emission of unpleased odorous byproducts, such as hydrogen sulfide, ammonia, and volatile organic compounds (VOC).

A lagoon performs these functions in five zones as shown in Figure 6. In the operation, liquid levels are not allowed to drop below the minimum drawdown to maintain treatment and sludge storage functions. In addition, liquids must be maintained below the maximum operating level to prevent overflow and to protect the embankment from waves. The volume sandwiched between the minimum drawdown and maximum operation levels is used as effluent storage.

Anaerobic lagoon design is based on the following considerations: loadings, volume, operating levels, shape, site investigation, land application, irrigation equipment, sludge removal, and solids separation [12]. A typical volatile solids loading rate [VSLR, gram of volatile solids (VS) fed per liter reactor volume per day] for a nonheated covered anaerobic lagoon is 0.24 g L-1 day -1; hydraulic retention time is 65 days [14].

Two-stage lagoons provide certain advantages over single primary lagoons as demonstrated in Figure 7. The main advantage of the secondary treatment is that it can reduce odors and the possibility that disease may be transmitted when the lagoon water is used for flushing gutters. Multistage lagoons work well for livestock manure treatment, especially when the treated manure is used for irrigation or recirculation in a flush-type handling system [1,6].

Secondary lagoons provide temporary storage prior to land application. A second stage also allows a maximum liquid volume to be maintained in primary anaerobic lagoons for stabilizing incoming wastes. It also provides some insurance against disease organisms being returned from the primary lagoon before a reasonable die-off period. Pumping from a secondary

Figure 6 Schematic illustration of anaerobic lagoon.

lagoon reduces the solids pickup common in primary lagoons due to seasonal water turnovers and biological mixing [5,6].

Anaerobic digestion in lagoons is operated at normal temperatures ranging from 3 to 35°C (sometimes termed as psychrophilic). The retention time ranges from 30 to 90 days. Higher temperatures in the operational areas would cause lower retention time. Removal efficiencies for COD, TSS, TN, phosphorus, and potassium are 70-90, 75-95, 25-35, 50-80, and 30-50%, respectively. Higher removal of pathogen from the animal manure may also be achieved [4-6].

Covered anaerobic lagoons are used for manure digestion as well as for collection of biogas. Normally, methane is dominant in the biogas. Its percentage is 70-85%, depending on the chemical compositions of the manure and operational conditions. Carbon dioxide is the second dominant species (20-30%); H2S and NH3 are present in lower levels. When applied, air pollution can be virtually eliminated.

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