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2. Local collection system of PVC or concrete pipes which collect sewage from a source and pipe it to a natural waterway. Normally these are badly constructed and require repair. In this case it is necessary to budget the cost of collection system improvement.
As part of this, determine if it is possible to reform the system to separate sewage from rainwater. If yes, this allows installation of biodigesters and improves functioning of the overall wastewater recycling system.
If no, locate the wastewater facility far enough from households that the social aspects are not a problem, and also install sand traps at the entrance to the facility to gather the substantial amounts of sand which normally infiltrate these systems.
3. Centralized collection system piping sewage to a stabilization pond for primary treatment. In this case the primary treatment question is already solved, unless the stabilization ponds are overloaded, which is a common occurrence when communities grow above the designed level of the conventional sewage treatment system.
Collection systems leading to the centralized facility are often infiltrated and it is very expensive to solve this, but even if funds are unavailable for such repairs, it is still possible to recycle wastewater which makes it into the facility.
In any case, if the wastewater recycling system is added to a conventional stabilization pond system, some problems such as sludge are much easier to deal with.
4.4.2. Gathering Data to Determine the Choice of Technology
Space of 0.25 to 1.5 ha. surface area is required to service 1,500 - 2,500 inhabitants. Variation depends on:
- water usage rates per household - this is usually 50-200 liters per inhabitant
- dilution rate of wastewater
Tests and calculations are required establish dimensions to process existing and planned wastewater volume:
- Factors affecting this include washing machines, very young children, office facilities etc. To determine these do a feasibility survey (see Workplan) and community survey, if statistics are not available.
- Determine nutrient and pathogen levels. Wastewater recycling requires a certain level of phosphorous because phosphorous is a limiting nutrient for agriculture. Also determine concentrations of organic material, industrial toxins and pathogens in the water. To determine these, tests are needed. A list of these tests is found in 'Guide to Wastewater Recycling in Tropical Regions' (see Homepage).
- Determine soil and slope parameters for the site - see Workplan.
Once above procedures are done, use a formula provided by a wastewater recycling specialist for determining facility dimensions.
This is a very important calculation. A wrong calculation here is extremely expensive later, so the basic data and calculations need to be done carefully.
Only after the above process is it possible to choose technology for a site.
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