The technology of anaerobic digestion, or the biogas technology, is based on the natural breakdown of organic material under bacteriological attack in the absence of oxygen, producing a combustible mixture of gas with up to 70 % methane. Biogas can be used in cooking stoves, piglet heaters, direct thermal heat processes and - in larger systems - electricity generation.

It is expected that the anaerobic fermentation programme will have the following impact on the national economy:

Methane release in animal farms

Methane release, which contributes to the greenhouse effect, is diminished when farmers use biogas to the highest extent. The 70,000 m3 biogas programme of both partners, DoAE and BAU/CMU, will result in a CH4 reduction of about 6,000 tons per annum. In addition, using the biogas substitute some of fossil fuel, does not increase the concentration of CO2 in the atmosphere.

Cleaning and Recycling of waste water from pigsties

The organic loading rates of waste water (BOD/COD) can be reduced by 90% or more. Waste water entering the overall system usually has a COD about 20,000 mg/lit. After leaving the system, it will only be between 200 and 500 mg/lit COD. The final success of the treatment process, however, depends very much on the farm management’s efforts and motivation, to operate the system in a reliable and proper manner.

The cleaned water is suitable for fish ponds, as well as recycling and cleaning purposes in pig pens; thus, reducing the water consumption on animal farms. The amount of water treated in the 50,000 m3 BAU-programme is around 2,000,000 m3 per year (180 lit/day * 365 days * 30,000 livestock units).

Provision of a valuable organic fertilizer

The application of effluents from the biogas digester (liquids and dried fermented sludge) provides many farms with a valuable source of organic fertilizer and supports the recycling of biomass. The main nutrients (N, P, K) in animal manure are preserved. It has to be mentioned, that the fertilizing value of fermented slurry is not, or only under certain conditions, higher compared to composted unfermented manure. However, harmful effects on plants from untreated pig manure cannot be observed after it undergoes an anaerobic treatment process. The 50,000 m 3 BAU-Programme can provide up to 380,000 tons of fertilizer (Mc. 20%) in 20 years.

Employment

The technology provides employment for craftsmen and private enterprises in rural areas. The 70,000 m3 Biogas construction programme for Thailand can create employment opportunities of about 2,400 men years for skilled craftsmen and technicians and about 1,100 men years for unskilled labour (both figures include construction work, repairs and maintenance).

Technological capacity building

It is possible to start with small agricultural systems as a proven technology. Then, develop solutions applicable to larger farms, for waste control in agro-industries, and residential waste water treatment with sophisticated input requirements.

The technology has a high potential for direct 'south-south' technological cooperation and know-how transfer. As a considerable part of the biogas technology research & development was carried out in countries such as China and India, consultancy services and improvements appropriate to specific conditions in the developing world can be implemented directly without inter-mediation. This contributes to national capacity building in the field of technology development and management skills.

Savings in foreign exchange

The provision of an autonomous and renewable source of energy, the supply of an organic fertilizer and the characteristics of the technology in using mainly local construction material, are favourable to savings in foreign exchange.

SYSTEM HIGHLIGHTS

The BAU Biogas System will typically include the design and application of the following:

a) a waste water collector connecting to the main ditch canal, a waste water pump may be needed,

b) a channel digesters with plastic covered gas storage of 600 - 1000 m3 or their combination,

c) Upflow Anaerobic Sludge Blanket (UASB) reactors of 100 - 200 m3,

d) sand bed filters with two layers of sand and gravel to separate solid from the fermented slurry(solid separator may be used in rainy season)

e) gas utilization devices, e.g. engines, electricity generators, piglet heaters, hot-water devices, refrigerators, stoves and lamps,

f) a series of constructed wetlands and a fish pond as life indicator.

An actual system for each particular case will comprise a combination of the above components, as appropriate to the site, the farm size, available land area, gas utilization needs, and the fertilizer handling preferred.

The overall structure of the Pig Waste Treatment System is outlined in the flow chart in Figure 1. Waste water treatment system for medium and large pig farms.The system consists of primary, secondary and tertiary treatment:

I) Primary treatment in a low rate channel digester. The digester also functions as a separator for the liquid and solid fraction of the waste water. For the solid fraction, a hydraulic retention time in the low rate digester from 20 to 30 days is calculated. The liquid fraction (75 % of waste water; 35 % of volatile solids) will go to the UASB(high rate reactor). II) Secondary treatment. This treatment consists of mainly two components: an Upflow Anaerobic Sludge Blanket (UASB) reactor and a Slow Sand Bed Filter (SSBF). The sand bed filter is connected to the low rate biogas plant and receive directly the discharged fermented slurry from the plant.

The solid waste from the Slow Sand Bed Filter can be dried further or composted and sold as organic fertilizer to crop farms.

The bulk of waste water with a high soluble organic matter content is treated in the UASB reactor together with filtered water from the sand bed filter. The ratio of digester volume between the low rate biogas digester and the UASB is 5 - 6 : 1, it is up to the condition of the wastewater. After passing the UASB, a remaining COD of 600 mg/l waste water can be achieved.

III) As part of the final and tertiary treatment, the water should be released into an oxidation pond for further cleaning. The incoming water will flow out surfacely through a series of wetlands to enhance natural treatment. Fish pond serves at the end of the wetland as life indicator providing sign of water treatment quality. Finally COD of less than 120 mg/l is expected. From there, the treated water can be used again as cleaning water for pig stables.