1.1 Dual water distribution system

Technical description

Water reuse, and the reuse of wastewater in particular, is receiving increasingly wide attention, even though it is often considered to be of marginal quality. Use of treated wastewater through dual water distribution and plumbing systems can provide a secondary source of water for purposes such as irrigating private gardens and toilet flushing. The provision of waters of lesser quality through a separate distribution system for non-potable purposes from alternative sources of supply can help lower the demand for potable freshwater. Application of this technology is largely a matter of cost, acceptance and practice, as the distribution technology involved is not significantly different in dual distribution systems compared to conventional single distribution systems.

In the Kathmandu Valley, where water scarcity is increasing, conjunctive use of shallow groundwater sources for toilet flushing and washing of clothes, together with potable water supplied through the municipal water supply system for other purposes, is being practised in residential areas. A rower pump (hand pump) is fitted to a 3.8 cm diameter polyvinylchloride pipe ranging from 6 m to 15 m in length which is driven to the ground. Water from the groundwater source thus tapped is pumped manually whenever needed.

Extent of Use

This technology is widely used in the Kathmandu Valley in locations where the groundwater table is within 15 m of the ground surface. It is also popular in locations where the municipal water supply is intermittent. People of low income are increasingly using this technology in preference to the higher cost municipal supply.

Operation and Maintenance

The operation and maintenance of dual distribution systems is simple, and involves keeping the pipe and the pump clean. Maintenance consists of changing the pump washer once a year or whenever it starts leaking. No additional maintenance of the municipal supply system is required, and no changes in municipal distribution system operation are necessary.

Level of Involvement

Providing dual distribution systems at the household level requires no external involvement relative to the groundwater sourced portion of the system. The municipal sourced portion of the system is generally constructed and operated by the local governmental unit.

Costs

The total cost of the groundwater sourced portion of the system is about $55 for a 10 m deep well.

Effectiveness of the Technology

This technology has helped alleviate the problem of water scarcity in Kathmandu. For an average household, more than 60% of the annual household water requirements is met by using shallow groundwater, which is of lower quality than the municipal water, for non potable purposes.

Suitability

Water supplied from the rower pump can be used for toilets, bathing, gardening, car washing, and similar purposes. The dual distribution system is most suitable for use in areas where the groundwater is within 15 m of the ground surface; otherwise, a mechanical pump will be necessary, adding to the cost of the groundwater sourced portion of the system.

Advantages

This technology is inexpensive and can be constructed using locally-available technology. Water is made available whenever needed, and use of the dual sourced system eases the water scarcity problem not only at the household level but also throughout the entire city.

Disadvantages

Conjunctive use of dual sourced water may be limited as a result of poor water quality. Water drawn from alternative sources may not be used for drinking even after boiling because of odours and tastes associated with groundwater. Further, such water may pose severe health hazards if the abstraction point is located too close to septic tank outflows. The possible high nitrate concentrations and bacterial levels that may be present in surfacial groundwaters may also lead to health hazards when used by poor people and children for drinking purposes. Widespread use may contribute to a decline in the groundwater table (due to over exploitation). Also, water logging and mosquito breeding in the pump area may occur if proper drainage is not provided.

Cultural Acceptability

No cultural problems have been noted, although use may be limited due to odours associated with the groundwater.

Further Development of the Technology

This technology will be more attractive and useful if simple and inexpensive electric motors are attached to the tubewell and the groundwater sourced portion of the system is fully incorporated into the household water supply system.

Information Sources

Dirgha Nidhi Tiwari, Koteswar, c/o Post Office Box EPC 4000, Kathmandu, Nepal. Tel. 977 1 410249, E-mail: [email protected].