by Peter Harvey, member
At Durrington Community Centre – 8th March, 1989.
Mr. Harvey spoke from his experience as a water engineer both at home and abroad in third World countries.
The treatment of water for human consumption depends on the source of supply. That from a borehole generally only needs chlorine sterilization. River supplies contain suspended solids and need pre settlement, clarification (filtration) disinfection, and, sometimes, extra treatment for taste, odour etc, Bankside storage capacity for seven days normal use is constructed for river supplies, particularly where rivers are subject to drought and irregular flow; this also provides some settlement treatment.
In desert areas water, found at Oases flows, underground naturally from mountainous areas, subsequently man made open ducting to inhabited areas collects sediment.
A number of schemes were described where the source was a sluggish river carrying a lot of silt, such as the Indus (Pakistan) and the Middy (Nigeria). In the Inugu (Nigeria) the heavy silt contains iron, with the disadvantage that it stains. The iron is settled out by a fountain – a picture of which showed the stain clearly.
The river Caroon in Iran meanders a lot – moves its course quickly. This presents a problem in locating the take-off point, and settlement ponds. Build in one spot, and next year the river has moved! One installation pictured was on the inside of a bend, where the flow caused the bank to build up, leaving the plant on dry land! Engineering to overcome this is difficult and expensive – some sort of portable plant has been ingeniously designed.
One very difficult problem was to get the local staff to operate the plant properly. An example was a complete failure to operate the filter and settlement plant with serious effects in the supply. Often routine cleaning of filters or other jobs are ignored or done when someone feels like it! The fact that the water may be unfit for human consumption as a result does not seem to register as being the operator’s responsibility!
Some examples of reservoirs nearer home were given, and mention made of amenity considerations leading to some very attractive countryside. Whereas at one time public access to reservoirs was denied to prevent pollution, to-day most are available for controlled recreation such as fishing and sailing. Water exposed to oxygen for about 30 days kills bacteria and needs no further treatment – a benefit too, of river bankside storage.
In answer to questions, Mr. Harvey said that about 25% of water supply authorities in Britain are private. Building a national water grid to provide additional security of supply is ruled out by the extreme cost involved, although there are inequalities in supply capacity and demand between parts of the country.
Report by S.J. Morley