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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

1.1 Wastewater characteristics (Topic a)

Waste from urban and dense settlements generally reaches the larger water environment in four waste streams:

  • Sewage waste

  • Grey water

  • Stormwater, and

  • Solid waste /litter

In Africa, the smaller proportion of the population who have in-house water supplies generate more grey water per capita. The more affluent communities generate even more grey water. Moreover, the very tiny ratio of Africans who are connected to piped sewerage system produce more sewage wastewater. Seepage from pit latrines in areas of poor sanitation services may pollute groundwater. In Southern Africa, the following figures shown in Table 1.3 appear typical for both human excreta (to VIPs) and grey water (disposed of to the ground surface).

Table 1.3: Typical figures for human excreta and greywater disposal in Southern Africa
Variable Human excreta Grey water
COD 100g/person/day 200mg/l
Total N 10g/person/day 20mgN/l
Total P 2.5g/person/day 31mgP/l
Water consumption  30l/person/day  -

Three chemical indicators, chemical oxygen demand (COD), nitrogen and phosphorus were traced by modelling from their source in human waste to their final destination in groundwater or surface water. The approximate loadings by sanitation type are shown in Table 1.4.

 

Table 1.4: Average chemical loadings* (kg/annum) depicting relative impacts of sanitation systems in South African urban areas.
Sanitation system COD Nitrogen Phosphorus
Water-borne 280 100 60
Septic tank 400 155 19
VIP 450 169 20
VIP + GW 640 180 52
Source:
Note:
Water Research Commission (1993)
*The loadings represent model estimates for a typical household.

In Kenya, the characteristics of sewage for Nairobi and Nakuru are shown in Table 1.5. The per capita BOD5 loading is also shown for Kenya and Zambia in Table 1.6.

Table 1.5: Sewage characteristics (mg/l), Kenya and Zambia, 1972
Chemical/physical characteristic Nairobi Nakuru
BOD5 448 940
Ss 550 662
TDS 503 611
Chloride 50 62
Ammoniacal N
67 72
Source: Water Research Commission (1993)

 

Table 1.6: Sewage characteristics (BOD5 per capita), Kenya and Zambia
Location Grams
Zambia 36
Kenya 23
Source: Water Research Commission (1993)


In Lagos, Nigeria, the 1986 edict eliminated the pail/bucket latrine and shifted the emphasis from an obnoxious collection system to holding of waste in cesspools. There is thus a respite for the population between filling and emptying of such cesspools. Millions of Nigerians living in other towns and cities still depend on unacceptable systems. Millions who live in the squatter settlements of many African cities use pail/bucket latrines. They are used extensively in the low-income high density households of Accra and Gaborone. Some 2 million South Africans still have to rely on the bucket system. In Accra the bucket/pail/pan latrine dominates that category of households (it is patronized by 22% of the low-income and 11% of the medium-income group), with night soil being collected regularly and dumped into the ocean.

In Omdurman (the Sudan) some latrines have been in use for decades; some reaching >20m in depth. In Tanzania, the pit is often 1m wide, 2m long and 3-4m deep. Muslims and most people along the coast, and in other Muslim dominated areas, use water for anal cleaning. Other people use banana leaves or other leaves, grass, corn cobs, paper, etc. The excreta and cleaning materials go into the pits. The variety of materials affects the characteristics of the resulting wastewater.

Wastewater is derived from various sources and is closely related to the quantity of water use. Put simply it represents return flow resulting from water use. In residential areas domestic wastewater is predominant, and its volume is greatly influenced by the income group and the population density. For example, the per capita wastewater flow is estimated at 260 litres per day in low density high income areas of Lagos Metropolis (Nigeria), 77 litres in medium density areas and 54 litres in high density low income areas where average density is 700 persons per ha. The average wastewater flow for the metropolis is only 115 litres/capita/day (compared to Cairo where it is 310 litres/capita/day). In 1995 the total wastewater emptied into Lagos Lagoon was 811,300 m3 per day, of which domestic wastewater accounted for 54%. The volume of wastewater generated is expected to increase to 1,663,090 m3 per day by 2010.

In Africa about 80% of water consumption of those connected to the sewer ends up discharged into the municipal sewer, but in semiarid and drought prone Gaborone the return flow is 50-65%. Thus, if poor people are connected in great numbers, the resultant reduced sewer flows could upset the operation of the sewer system because of too little water being used to keep waste flowing. Actually, sewer flow in Gaborone is 18,000 to 75,000 m3 per day by a population of some 220,000. It is envisaged however that from an average water consumption of 63,100 m3 by 2010, a wastewater of 57,180 m3 will be generated, giving an optimistic return flow factor of 90% (UN DESD, 1996).

Estimated wastewater flows per hectare are comparatively light in most of the 21 sewerage districts of Lagos; meaning that on-site sanitation methods could cope with these flows presently. However, some three to five districts have significant wastewater flows (40 to 55 m3/day /ha), and measures must be taken to remove the flows by appropriate disposal and/or treatment facilities. Some form of sewerage system is likely to be the only technically viable option.

Accra’s formal and informal drains are filled with stormwater, septage, sullage (or greywater), and solid waste. They flood and overflow in the rainy season and become informal waste dumps, or stagnant cesspools in the dry. The result of this practice is periodic flooding that destroys properties, wastes time and causes loss of life, in addition to the environmental pollution and health hazards they present. Nearly 20% of Accra is subject to flooding. This picture of Accra typifies the mixed nature of wastewater in many African cities.

For example, in medium to high density residential areas of several African cities, such as Lagos, Dar es Salaam, Addis Ababa, open storm drains are common and in many cases act as open sewers, particularly for the conveyance of sullage (or greywater). Often too, septic tanks act as vaults, with the overflow of sullage water being discharged direct to open stormwater drains. In addition most industrial wastewater is discharged into these same drains. This practice tends to change the characteristics of wastewater drastically, and invariably compromises the function of such drains.

In Lagos, wastewater is discharged to the Lagos lagoon and even sullage (or greywater) water is discharged in open drains throughout some of the urban catchments. In the largest peri-urban settlement of Nairobi known as Kibera, drainage is virtually non-existent, and during the rains in April/May and December, the areas can hardly be walked as stormwater and sullage are of particular nuisance.

The work of Schneider (1994) in one of Lake Victoria’s watersheds that houses Kenya’s Kisumu town, estimates the per capita BOD5 load from septic tanks in the town as 30 g/day, that from pit latrines as 19-22 g/day, and 23 g/day for domestic sewerage, a value that seems to typify Kenya’s domestic effluents. The load associated with leachate from solid waste is also estimated at 9.0 g/c/day or 1.7 tons/day. Thus for Kisumu town a total of 6,374.11 g/day is generated and much of it subsequently released into Lake Victoria to contribute to the lake’s severe eutrophication problem (Figure 1.1).

 

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