|Urban Wastewater Projects - A Layperson's Guide (EEA, 1998, 124 p.)|
|Chapter 2. An Introduction to Urban and Rural Wastewater Management|
2.7.1 What are the options for effluent disposal?
Treated effluent from a wastewater treatment plant is normally discharged into the nearest water body capable of accepting it without detrimentally affecting it. This can be a drainage ditch, a river, stream or torrent, a lake or the sea.
Fig. 2.5 Agricultural re-use of treated effluent
In some cases, to ensure adequate and effective dilution of the effluent in the receiving water, an underwater pipeline must be constructed, equipped at its discharge end with a diffuser system. This is particularly so, when treated effluent is discharged into the sea and acceptable bacterial levels are required at the shoreline and inshore waters without resorting to disinfection. The issue of mixing zones is important and the environmental/economic balance of several small discharges versus one large discharge needs careful assessment.
In addition to providing dilution, a long offshore outfall provides time for natural bacterial die-off to occur before the considerably diluted effluent reaches the shoreline. Techniques exist for predicting the dilution, dispersion and die-off that will be achieved.
2.7.2 Under what circumstances is it worthwhile to reuse treated effluent for beneficial use?
Recycling treated effluent for further use is unlikely to be either necessary or worthwhile where natural water resources are sufficient to satisfy all normal demands placed upon them by an area, e.g. satisfactorily serving the needs of the population, commerce and industry, the public services, landscaping and agriculture.
However, if water resources are either periodically or continually unable to satisfy water demands in the area, the recycling of treated effluent for beneficial use should be considered.
It is generally possible to reuse effluent for use as irrigation water in agriculture, landscaping and forestry, although, unless it has been either disinfected or stored for some time, care should be taken in its use on crops eaten raw, or where spray irrigation techniques are used.
Treated sewage effluent can be used for secondary industrial purposes such as cooling and quenching without treatment other than disinfection and dosing with algaecides.
Some water deficient areas have insisted on dwellings and public buildings being provided with dual plumbing systems for the water supply, one for toilet flushing and the other for all other water uses. It is possible to use treated effluent in the toilet flushing system as long as suitable precautions have been taken against cross-connection with the potable supply, e.g. colour coding of pipes and labelling of the effluent, and the effluent has been disinfected and dosed with algaecide.
If other uses are under consideration, such as in industrial processes or for drinking purposes, it will be necessary to subject the effluent to considerable further treatment as for any other primary quality potable supply. As effluent is generally of lower quality than natural water sources, this can be very expensive and has been practised in very few locations.