Introduction
For human consumption, water needs to be of potable quality; however for many other purposes the water required can be of an inferior quality. Treated domestic wastewater can often provide a viable alternative water source when water is required for purposes such as:
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Agricultural irrigation;
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Garden irrigation;
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Landscape irrigation (plant nurseries, parks, residences, urban greenery etc.);
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Aesthetic uses (ponds and fountains); and
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Industrial uses (cooling towers, process water and dust control).
The concept of water re-use implies the recovery, treatment (where necessary) and re-use of water that has already been used. Water re-use can be applied at various levels, from household to industrial scale. The degree to which the wastewater needs to be treated depends both on the quality of the used water and the purpose for which it is to be reused.
The Colours of Wastewater
The sewers of many cities serve to collect a mixture of different types of wastewater, for example storm water run-off from streets and domestic wastewater from households. This domestic wastewater in itself is made up of a mix of different flow streams that are often described in the following terms:
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Grey water - domestic water, mainly from showers, kitchen sinks, washing machines etc., without faeces and urine;
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Black water – a mixture of human faeces and urine from toilets either with or without toilet flushing water;
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Yellow water - urine only or mixed with flushing water from toilets; and
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Brown water – black water with no urine.
In sewered urban areas with wastewater treatment plants, large volumes of treated wastewater can be recovered. However, within the basin there are no significant volumes of treated wastewater which would be available for re-use. With the exception of limited sewer networks in Huambo and Lubango, which serve only parts of both towns and are not connected to a treatment plant, almost all households in the basin use on-site facilities to collect and dispose of both grey water and black water (faecal sludge and urine).
As wastewater in the basin occurs at household level, re-use is most practicable within households, in garden or for animals. Recovering and using wastewater at home would enable drinking water to be saved. It would mean water from bathing and washing would be collected and used for other purpose, such as watering vegetables or trees. Wastewater which has no soap in it can be given to chickens. If there is not too much fat or soap in the water it may be suitable to water plants. If the water is particularly soapy or fatty, simple means can be used to treat it and make it suitable for irrigation – such as using simple leaf mulch beds, or grease traps as part of a simple treatment and re-use system. Water re-use can be combined with Water Harvesting to make the most efficient use of water resources in arid areas.
Household grey water can be treated using a simple grease trap (at the front of the picture) before the water flows into the garden (towards the back of the picture).
Source: SANDEC 2006
( click to enlarge )
Planning Approaches for Re-use Options
The potential re-use options depend largely on water demand and quality requirements. There are two planning approaches: ‘Bottom-up’ and ‘Top-down’:
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The bottom-up approach defines the intended re-use options first followed by the required treatment technologies, thus allowing the structured planning of future infrastructure within the context of a broader wastewater management master plan. If economically feasible, this approach offers the highest flexibility of re-use options, tailored to the specific local conditions by integrating future end users in the planning process.
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The top-down approach takes the available quality of existing treated wastewater as a starting point and defines possible re-use options on that basis - a pragmatic approach considering the existing sanitation infrastructure. This approach, however, limits the potential re-use options considerably.
One particular planning approach that has been developed by the Water Supply and Sanitation Collaborative Council (WSSCC) to promote such a circular system of resource management is the Household Centred Environmental Sanitation (HCES) approach.
The Household Centred Environmental Sanitation Approach
The HCES is a multi-sector, multi-actor approach to delivering integrated urban environmental services. It is designed to respond to household needs and priorities, since the household is the level at which decisions on investments (or its use) are made and where behaviour change begins. Its strength is that it offers the possibility of providing economic and non-economic benefits, an integrated, affordable and sustainable package of services meeting the users’ priorities. The two central components of the HCES are:
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The focal point of environmental sanitation planning should be the household, reversing the customary order of centralised top-down planning. The user of the services should have a deciding voice in their design, and sanitation issues should be dealt with as close as possible to the site where they occur. With the household as the key stakeholder women are provided with a strong voice in the planning process, and the government’s role changes from that of provider to that of enabler;
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A Circular System of Resource Management should be used emphasising the conservation, recycling and re-use of resources, in contrast to the current linear sanitation service system.
A special challenge is that it requires collaboration and coordination between multiple agencies which may have different capabilities and little commitment to working together. Therefore HCES should only be considered where there is a strong political commitment to the sustained effort essential to success. The HCES approach attempts to avoid the problems resulting from either ‘top-down’ or ‘bottom-up’ approaches, by employing both within an integrated framework. The HCES approach tries to combine the benefits and reduce the negative aspects of both approaches by focusing planning on household demand and by including all stakeholders in the process, from planning to implementation.
Source: Eawag - Swiss Federal Institute of Aquatic Science and Technology, 2005
Legal Aspects
The World Health Organisation published “Guidelines for the Safe Use of Wastewater, Excreta and Grey Water in Agriculture and Aquaculture” in 2006 which present system approaches, treatment technologies and recommended treatment efficiencies for wastewater, excreta and grey water for re-use from household to city level. The guidelines contain only recommendations only and have no legal status under any jurisdiction. Governments can either use them as a basis for national standards or simply refer to them as non-enforceable recommendations, giving authorities the flexibility to set health-based targets in line with what is realistic in the national socio-economic context.
