Environmental Engineering Reference
In-Depth Information
5 MANAGING URBAN STORM WATER QUALITY
5.1
The present management practice
This section illustrates the opinion of the two major institutions responsible for
controlling storm and natural water quality in Harare's catchment basin-the city's
municipality and DWR. A questionnaire was prepared and submitted for completion to
these authorities, in order to present their experience and opinion on two questions:
i. What are the main sources of diffuse pollution specific for the area?
ii. What are the major problems associated with the diffuse pollution control and
abatement?
A summary of the answers regarding the first question are listed below:
• Discharge from sewage blockages - the questionnaire results show that the local
authorities recognizethis to be one of the serious causes of diffuse pollution in the city.
The number of reported and cleaned sewage blockages during the year 2002, supports
this statement, with the data presented as follows: 70-80 blockages per month in high-
density areas; 30 blockages per month in the city centre; and less than 5 blockages per
month in industrial, medium and low - density areas.
• Oils and grease spillage from food enterprises (irregular cleaning of oil and grease
traps);
• Illegal discharges of process industrial water into rivers/storm sewers by some
industries;
• Leachate from dumpsites/landfill sites;
• Illegal dump of paint, oil, antifreeze and other used chemicals into storm drains;
• Oil and chemicals from garages-detergents from car washing, paints, anti-skid
compounds are washed out from the surface into the storm water, especially in cases
of informal open-space, low cost garage services;
• Car exhausts and wearing of vehicle parts;
• Erosion of roof material;
• Storm water from industrial enterprises with poor yard hygiene;
• Spills from haulage companies of the goods that they transport.
• Religious/church gatherings in open spaces, where there is no provision for toilet
facilities and refuse collection.
The summarized answers of the second question are as follows:
• Control of point sources of pollution - Despite the fact that the emphasis of the
pollution control measures has been placed on sewerage treatment, there are treatment
plants, which either by-pass substandard flows directly to natural water bodies, or
discharge a poor effluent quality that does not meet the Zimbabwe standards for safe
discharge into receiving waters. This is due to the fact that some of the treatment
facilities are overloaded and are forced to discharge part of the influent directly to the
river, in order to protect the treatment plant, especially during or after high intensity
