Environmental Engineering Reference
In-Depth Information
Chitungwiza by a conventional trickling filter scheme. After that, it is pumped to six
stabilization ponds, which can be operated in a series or in parallel. Five of the ponds
have a depth of 2 m and a size in plan of 75 m by 75 m (ponds number 2, 3, 4, 5 and 6).
The first pond is designed as anaerobic pond with a depth of 5 m. The ponds were
designed with a total mean retention time of nine days. However, due to rapid
urbanization and the increase in the population of Chitungwiza, as well as due to irregular
desludging, the ponds have been filled with sediments and the retention time has been
reduced considerably. The primary function of the pasture and irrigation facilities has
been the disposal of the effluents, rather than intentional reuse. Based on data provided
by the operators, during the period of study, the ponds received 36 × 10
3
m
3
/day of
wastewater against a design flow of 21.75 × 10
3
m
3
/day.
Most of the effluent has been used on the farm, whilst a relatively small portion goes
to Boronia Farm and Ellerton Farm to the north and east respectively. Farm records and
JICA (1997) showed that the effluent had TKN of 30-40 mg/l and 60 mg/l for the rain
and the dry seasons respectively. Recently, after this study was performed, a new BNR
plant was commissioned, parallel to the trickling filter line, in order to alleviate the
hydraulic and pollution loads at ZSTW and the pastures.
Farm records during the initial stages of the pasture operation, showed that the cattle
stocking rate for the irrigated pasture was seven livestock units (LU) per hectare
compared to one livestock unit per 3 hectares for veld grazing. The total irrigated area
under furrow and sprinkler irrigation was 390 ha giving a total of 1820 LU grazing on the
pasture for 8 months. Approximately 15 × 10
3
m
3
/ha yr of effluent was applied during
this time. This hydraulic load, together with the average annual rainfall of approximately
