Sand trap: By slowing flow into the facility, the sand trap precipitates sand which falls to the bottom and is periodically removed.
This trap also provides first oxygenation of water after it enters the facility via an underground inlet pipe.
Biodigesters perform five principle functions:
- substantial odor reduction
- up to half of pathogen elimination
- up to two thirds of BOD and sludge reduction
- converting nutrients so they are
- digestible by algae
- biogas production
The value of biodigesters for wastewater recycling is often under-estimated and under-used. Many earlier digesters did not work properly or were too expensive to construct. Today, several efficient digester designs are available.
Biodigesters are used at three locations:
- Pre-treating wastewater at decentralized locations prior to entering the central collection system;
- at the end of the collection system prior to entering the wastewater recycling facility;
- on site to treat wastewater from livestock.
Biodigester systems include:
- collection system from waste source to the digester
- the biogas digester,
- feeder line to storage or gas-use, cooking stove, gas lights, or pumps for aerating
- outlet to wastewater recycling facility
The basic system operates this way:
- Anaerobic digestion of organic waste, produces methane which builds up pressure in the gas chamber. A pipe with a valve is installed at the gas outlet. The gas is fed through a hose or storage tank to a stove, light or, pump or generator.
- After certain intervals, depending on the size of the piggery and digester, sludge is removed and spread on gardens for fertilizer.
- For more detail on operations, consult a biodigester expert.
This is where many biochemical changes occur quickly due to greater concentrations than elsewhere on the site. Normally, and especially during dry periods it is possible to see a green layer of algae on top of the basin. This is part of the normal process of conversion of nutrients. Algae flow rate depends on BOD results in the fish pond. Careful regulation of algae flow via macrophytes and mechanical filtration is required in the beginning. This needs advice of a fish expert.
The algae/settlement pond is where odors may emerge, due to high wastewater concentrations. To keep odor and concentrations manageable, check flow through this basin. Compare BOD entering the facility with BOD exiting the algae basin. If BOD is reduced by around 40 percent it shows that mineralization is occurring and the flow is working.
The large open water surface and high nutrient concentration also causes breeding of mosquitoes. Ducks significantly eliminate mosquito larvae. Between algae basin and fish ponds it may be necessary to have a shallow macrophyte pond for mechanical and biological control of algae before the fish pond. This requires advice from a fish expert.
Unlike most freshwater fisheries, this system uses deep ponds of 2.5 - 3 meters. This depth is required to:
- increase retention time in a smaller area
- provide several levels where different fish species live
- increase productivity per surface area.
However, due to the depth, oxygenation of lower levels is very important. Sufficient oxygen levels are key to keeping fish alive in the ponds. Oxygen readings are taken on a daily basis for first months. Shape and width of ponds plus slope of dykes are key to assuring fish harvesting without entering the ponds. Before stocking the ponds it is necessary to determine if they are free of other fish species, particularly predator species which may eat the fish fingerlings and smaller fish. Predators are not necessarily bad, as they may help to control over-breeding. However it is essential to balance predators with the target species to assure optimal yield.
The ponds are normally stocked with fish from a local fish farm. Special care must be taken to make sure dates of completing construction coincide with dates when fish fingerlings are available at the fish farm for stocking ponds. Harvesting occurs every two or three months depending on growth rates. A fish net is dragged from one end of the pond to the other. Before fish are harvested it is advisable to slap the water surface with bamboo poles so the fish jump and empty their intestines into the ponds. Thus, valuable nutrients are kept in the ponds and the fish keeps fresh. Testing of fish must be done for parasites and some toxins before they could be used for human consumption. If fish are unsuitable for human consumption they are cooked and used for feed for livestock.