14 March 2007

Zimbabwe: Sewage Management: Challenges and Solutions


Harare — Sewage Conveyance (Reticulation) THE sanitation system that Zinwa took over from Harare City Council refers to the whole sewerage reticulation network, trunk sewers, sewage pump stations and sewage treatment works infrastructure and sewage billing.

The sewage reticulation infrastructure for Harare comprises of pipe work in excess of 5 000 kilometres ranging in size from 100mm to 1 400mm with pipe materials of various types.

Sewage Treatment Plants

There are five sewage treatment works in Harare, namely Firle near Glen View, Crowborough, Hatcliffe, Marlborough and Donnybrook, which presently are overloaded and almost collapsing. Harare also has sewage pump stations such as North-eastern in Mt Pleasant, Avonlea, Chisipite, Borrowdale Brooke and Budiriro which all need to be equipped with pumping plants as they are either operating below capacity or are totally run down leading to raw sewage spilling into the river systems.

The Sewage Reticulation System: General Overview

All waste water generated as a result of water usage by all stakeholders should be safely collected, conveyed and treated prior to discharge of the final effluent into the river systems and this should be done in compliance with the country's environmental management, water and effluent discharge regulations.

Generally, when we talk about sewage reticulation we will be referring to the cycle where used water moves from the household, office building or factory warehouse to the sewage treatment plants and ponds and back into the rivers and the water treatment works where it is treated again to produce potable water.

Wastewater gravitates from people's homes and industrial areas through sewage pipelines and in certain terrains where gravity is not possible sewage is pumped to sewage treatment works.

At the treatment works sewage passes through various stages of treatment. The first stage of sewage treatment is known as screening. This is where big inorganic materials which are greater than 10 millimeters in size -- the rugs, spoons, logs, stones and others -- are trapped. The trapped materials are known as screenings and are buried in trenches at the treatment plant.

The next stage is grit removal where small inorganic materials from sewage settle down by gravity. Screening and grit removal is done at the inlet works after which sewage passes on to the primary settling tanks where biological oxygen demand removal takes place. Most of the sludge and organic matter settles down in these tanks and is later pumped to digesters for stabilisation.

Grit and screenings that escape from the inlet works damage pump impellers and end up filling the digesters. The digesters are designed for organic material not screenings and grit so the filling up of the digesters causes disruptions to the sewage treatment process.

The treatment works are equipped with biological filters and biological nutrient removal systems which operate by re-circulating activated sludge where bacterium is active. The industrial waste destroys the bacteria if the pH is high or if other toxic substances are present.

Depending on the quantity, concentration, toxicity and presence of non-biodegradable organics in an industrial waste, the treatment process may consist of any one or more of the following processes: equalisation, neutralisation, physical treatment, chemical treatment and biological treatment.

Our treatment process uses physical treatment which refers to the physical separation of suspended inorganic matter like in the screening, grit removal and primary settling tanks that we talked about above. Our sewage treatment plants are not designed for the use of equalisation, neutralisation and chemical treatment processes that are extremely expensive but were designed for the biological treatment process which is affordable and user friendly.

The discharge of highly toxic, corrosive sewage with high chemical content renders the simple biological treatment process in use ineffective and in need of neutralisation and chemical processes that are extremely expensive. This is the reason why we intend to measure the chemical oxygen demand at industrial points and arrest the problem at source.

The last stage in the sewage treatment process is the final effluent pond system. Final effluent refers to the treated product (which is raw water after activated sewage treatment and humus for the biological filter system) that is discharged into the rivers after adding oxygen to make it fresh. The final effluent from biological filters is pumped to farms where it is used for irrigating a special type of grass, star grass or kikuyu, which has a high uptake of nutrient content.

Final effluent ponds are widely used for fish farming worldwide and the presence of aquatic life in the ponds is an indicator of the successful completion of the treatment process.

Residents' Contribution to Sewerage Problems

Residents contribute a lot to sewerage problems by using sand for washing their kitchen utensils. Theft or vandalism of manhole covers by residents also contributes to Harare's sewerage problems as it leads to rubbish being thrown into sewers.

The use of temporary toilets with squat pans, which allow storm water to flow into them, use of inappropriate sanitary material and improper use of kitchen sinks and outdoor washing basins by throwing household utensils, soaps and rags also contribute to sewerage problems. These substances eventually choke the sewage pipelines and increase the wear and tear of pump impellers leading to the underperformance of treatment plants.

The other major problem that we are facing in Harare is that of inappropriate use of the sewerage infrastructure and deliberate vandalism of the sewerage trunk mains for diverting and use of sewage as manure on fields and for irrigation purposes on fields on Harare's peri-urban fields, especially on the outskirts of Budiriro, Glen Norah and Rugare.

Effects of Above on Sewage Treatment

Once rubbish and sand is carried to the sewage treatment plants or ponds the following problems will be experienced:

  • Pumps worth millions of dollars, with a design life of 10 years wont last one year or even a month in some cases. On average a pump costs US$50 000.00
  • Pipes and treatment channels are choked with rubbish and sand leading to spillage of sewage into the Mukuvisi, Marimba, Gwebi and Mazowe rivers and subsequently into raw water sources, Lakes Chivero and Manyame, Seke and Harava dams.
  • A survey conducted by the Authority revealed that most of the pipes are operating at half capacity with sand filling up the other space. The situation of sand accumulation in pipes has deteriorated to such an extent that out of a total of five sewage treatment units at Firle Treatment Works, only one is functional. From a total of 20 sludge stabilisation digesters at Firle Sewage Treatment Works only one is functional. We are receiving in excess of 150 wheelbarrows of sand instead of around 30 wheelbarrows that should be received under normal circumstances.
  • Raw sewage and partially treated effluent, which under normal circumstances should be retreated or pumped onto farms, is being discharged into the Mukuvisi River and subsequently into Lake Chivero due to broken down pumps at Firle, Crowborough and Budiriro sewage treatment works.
  • The breakdown of equipment and dysfunctional state of sludge digesters at Firle is causing the country to lose out on methane gas that should have been produced at the treatment works, trapped and used as a source of energy.
  • At Crowborough sewage treatment works only one out of a possible three treatment units is functional.
  • At Budiriro pump station spoons, sand and rags frequently damage all the pumps. The pump impellers which are supposed to last at least 10 years are only lasting six months due to wear and tear.

Industry's Contribution to Sewerage Problems

Industries and commercial properties are discharging sub-standard effluent into the sewers, which is causing corrosion of pipes and equipment such as pumps, mixers/stirrers and aerators.


Zinwa has embarked on a programme to refurbish or rehabilitate all sewage treatment works in Harare to reduce pollution of raw water sources and the Authority's water treatment costs, replacing pumps at all sewage mains and pump stations with highly resistant pumps suitable for pumping raw sewage to control sewer blockages, and informing residents on the proper use of the sewerage infrastructure.

But upgrading the sewage treatment plants and replacement of old pipes is not the only solution to the sewerage problems as the problems being faced in Harare and other cities with regard to sewage management are both self-inflicted and technically driven.

While Zinwa is working hard to address the technical aspects of the problem the residents must also complement the Authority's efforts with behaviour change. Residents can help reduce the problems of sewer blockages and bursts by not using substances such as sand for washing their dishes or throwing cutlery items, rags or other household stuff into the city's sewage drainage system.

Efficient operation of the sewerage reticulation system will benefit residents in the reduction of mosquito breeding grounds, transmission of water-borne diseases and will also help in maintaining the integrity of our buildings and other infrastructure.

Efficient functioning of the treatment plants will also benefit the country in the form of energy. If well treated the sludge from Firle and Crowborough is estimated to produce about 25 500 cubic metres of biogas per day, a figure that can be increased by improving the treatment process. Gas produced at our treatment plants has 66,4 percent methane, 32,2 percent carbon dioxide and 1,4 percent hydrogen sulphide and other traces.

Imagine what we can do with all that energy. So next time you buy your potatoes from the market, please don't wash them in the sink and like we always say, water management is everyone's responsibility. Play your part today!

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