Environmental Engineering Reference
In-Depth Information
4.3
Hydraulic and pollution loads
The flow values were obtained by averaging the daily flows from the plant operation
records during the study period. The dry weather flows were obtained by averaging the
flow rates for October and November 1998, while wet weather flows were averaged for
the period December 1998 up to March 1999. During the dry months an average
incoming raw sewage of 50,230 m
3
/day was measured, which is still within the design
limits of the treatment plant (54,000 m
3
/day ADWF). An average of 19,406 m
3
/day
effluents and 921 m
3
/day wasted and digested sludge, were pumped to the pastures. There
was no overflow from the ponds into the Marimba River during the dry season, which is
in line with the actual observations on site. An average flow of 3790 m
3
/day was being
released to the river from the BNR treatment units (Fig.8.5).
The wet weather flow was found to be 123,120 m
3
/day on average. The volumes
pumped to the pastures were 21,030 m
3
/day and did not vary considerably from the dry
weather values. The ponds showed a very high average overflow value into Marimba
River of 50,183 m
3
/day, while the BNR plant was releasing 15,905 m
3
/day. The flow to
the trickling filters treatment units was 53,246 m
3
/d on average, which was quite near the
design wet weather flow value of 54,000 m
3
/d. On the ground, however, during heavy
rains, the actual peak flows exceeded the design flows resulting in overflows occurring at
the distribution box. The average annual hydraulic load to the farm was 17,670
m
3
/ha.year, with a dry component of the mixture between 3.5-4.5%.
In Harare, the sewer system receives wastewater from residential, commercial and
industrial sites, and in the vast majority of the cases, the industrial discharges do not
undergo preliminary treatment. This leads to a considerable amount of heavy metals in
the row water, which are accumulated in the sludge during the treatment process. The
data presented in Table 8.2 supports this statement. The values have been calculated on
the basis of the average pumped rates and the average concentrations of ponds effluent
and sludge mixture during the study period. Consideration has been given to the
operational practice of different pumping rates and times of the effluent and sludge
respectively. The limit values (maximum permissible values) with respect to South Africa
and USA are based on literature data (WRC 1997), while the Zim-babwe limits are based
on WWEDR (2000).
According to Reed et al. (1988) the annual nutrients uptake rates for different types of
grass vary between 150 and 670 kg/ha for nitrogen and between 24 and 84 kg/ha for
phosphorous. Results in Table 8.2 show that the phosphorus load exceeds the maximum
value of plant uptake capacity by about 50%.
Nitrogen rates, expressed as TKN load, exceed the maximum permissible value (high
hazard) for sludge land application of 600 kg/ha.year (WWEDR 2000). Comparison with
Canadian and USA sludge quality criteria for land application (Roy & Couillard 1998)
show that Ni concentrations exceed twice the maximum USA limits of 400 mg/kg and Cd
concentrations exceed the Canadian maximum limit of 15 mg/kg. The application rates
regarding Cd are within the WWEDR (2000) requirement, but Ni application rates
exceed all stated guidelines and limits. It has been reported that sludge containing more
than 7 mg Cd/kg may present a risk of groundwater contamination even at an annual
sludge application rate of 200 kg available N/ha (Roy & Couillard 1998). The stated
application rate of nitrogen is expressed as TKN but not as available N and it could be
expected that the loadings with respect to available N should be lower. A study
