4.4.1. The Collection System
and soil conditions at the site.
Here are variants of wastewater collection systems:
- 1. No collection system, where sewage goes directly into streams and rivers or the ground, or where new housing is in planning stages and sewerage services are not yet installed. In this case consider using dry compost toilets with graywater irrigation at each household. These are cheaper than installing a central collection system. Check if dry compost toilets are socially acceptable in the area. If dry composting is not an option, make sure that the sewage system separates rainwater from sewage water, and that the collection system is properly constructed to minimize contamination from infiltration.
- 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
- collection system type
- soil type
- dilution rate of wastewater
- choice of technology
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.
4.4.3. Choice of Technology
Variations on the technology include:
- Dry compost toilets with graywater irrigation
- Soil filter with integrated recycling system attached
- Decentralized biodigesters in the community, with effluent pipes leading to one or several wastewater recycling facilities
- Separated sewage collection system leading to centralize biodigester with wastewater recycling system
- System without biodigester, including sand trap at inlet, oxygenation basins, macrophyte tanks, fish tanks, garden areas
- Conventional stabilization ponds (if already present), with recycling system attached
- Fully integrated variant; the previous system plus fish, poultry, meat-producing animals, and biogas reactor.
Choosing technology is only possible after a site and community assessment with a wastewater recycling expert. Factors include:
- technical capability and interest in community
- available site security
- type of agriculture and aquaculture in the area
- feasibility of farm animals (noise, odor etc.)
- site slope & groundwater level
- sewage dilution rate
- population density, density of structures
4.4.4. Procedure for Determining Property Dimensions
Avoid committing land until these questions are settled:
- A general location is available and suitable for a facility.
- Water, soil and technology questions are answered.
- Dimensioning occurs with wastewater recycling consultant.
- Final negotiations commence on the size of property required.
- Legal processes are finalized.
- After that, construction planning starts.