Three-step Piggery Wastewater Treatment (TPWT)

The three-step piggery wastewater treatment system is based on a typical continuous plug-flow design, and the volume of raw wastewater remains constant over each 24-hour period. Under optimal operation conditions, wastewater thus flows into the system and is discharged continuously. Anaerobic treatment is conducted after solid/liquid

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separation, and occurs inside of anaerobic basins covered with “red-mud plastic cover” (1.2~1.8mm of thickness), made of a kind of PVC material, which is corrosion-resistant and gas-and-water impermeable. Anaerobic treatment is generally slower than forced aeration, but consumes less energy. The anaerobic treatment system of TPWT process can also salvage a part of chemical energy content of wastewater by generating methane, a useful fuel. The optimal hydraulic retention time is around 4–6 days, and BOD removal is expected to be more than 80%.

2.2.1 Solid-liquid Separation

Separation of the solid fraction from the wastewater is to reduce the content of solids for subsequent handling and treatment, and to recover the solids for using as fertilizer, etc. This physical process is accomplished by using various kinds of screens. The efficiency of this treatment is a 15-30% decrease in BOD and a 50% decrease in SS. The moisture content of the separated solids is 70-80%. An extruder is often added to reduce the water content of the solids to 70% or below so that the material is suitable for composting.

2.2.2 Anaerobic Treatment

Since hog wastes are biodegradable, biological treatment is generally an economical way of handling them. The horizontal tent-type anaerobic fermenter is a modification of the Red Mud Plastic (RMP) bag fermenter which was also developed by the Taiwan Livestock Research Institute.

Among its merits are the fact that it is easy to construct, has a low investment cost, is easy to maintain, and can be separated into several divisions as desired. These fermenters can be sealed from either inside or

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outside. The four sides of RMP sheet, which make up the top of fermenter, are in tubular shape, so that PVC pipes may be inserted inside them to give extra strength. The strengthened sheet is then fixed to the wall of the fermenter with hooks.

The hydraulic retention time (HRT) is calculated according to the amount of water used to wash the pig houses, as the following the formula:

HRT =

A 1:3 ratio of manure to washing water is suggested, which can easily be achieved with a flushing tank system. According to the work by Hong (1985), the daily excreta of a 100-kg pig is around 5 liters, so the total wastewater from one pig may be estimated as 20 liters. A HRT of 12-15 days is common for hog wastewater treatment.

A tent-type fermenter should consist of no fewer than two digesters.

The volume of the first digester is usually 1.2 times of the daily wastewater. Both acidogenesis and sedimentation take place in this first digester. Most of the methanogensis reaction occurs in the rear digester(s).

Biogas may be collected for the use as fuel. The excreta of each pig can generate 0.1-0.3 m³ of biogas per day. Biogas may be used in cooking stoves, water heaters, water pumps, electric power generators, gas lamps, warming piglets, vehicles, mowers, and incinerators for animal bodies, etc.

2.2.3 Aerobic Treatment

There are many kinds of aerobic treatments that may be utilized for livestock wastewater. Considering the environmental conditions of

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Taiwan in subtropical climate, activated sludge processing and oxidation ditches are recommended. In aerobic treatment, organic matter is decomposed solely through aerobic oxidation.

Activated sludge processes are versatile and flexible. Effluent of any desired quality can be produced by varying the processing parameters.

These processes require less land but more skilled management than simpler processes, such as oxidation ditches. Activated sludge is a complex biological mass, resulting from when organic wastes are aerobically treated. The sludge will contain a variety of heterotrophic microorganisms, including bacteria, protozoa, and higher forms of life.

The relative abundance of any particular microbial species will depend on the type of waste that is being treated, and the way in which the process is operated. For optimum treatment, raw waste must be balanced nutritionally. In three-step treatment, most of the easily biodegradable matter has already been decomposed in the anaerobic digester, therefore, operating an activated sludge treatment requires intensive care for good performance. It is best to control the BOD of anaerobic effluent at around 1000 mg/L. The growth conditions for microorganisms in activated sludge tanks are usually measured according to the mixed liquor suspended solids (MLSS) and sludge volume index (SVI). The HRT for an aerobic tank is normally 1.0-1.5 days.

While activated sludge tanks have a water depth of 2-5 m, this should not exceed 1.5 m in oxidation ditches. Oxidation ditches, therefore, require a larger land area, but have the advantages of being easy to operate and of generating less sludge.

A final clarifier to settle the activated sludge before the discharge of

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treated water is required in aerobic treatment. The settled sludge may be removed by mechanical methods for return to the aerobic unit, or be dehydrated for disposal. Usually the HRT in the clarifier should not exceed 6 hours.