Environmental Engineering Reference
In-Depth Information
Ammonia
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TP
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Nitrate
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TDS
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indicates that the whole central part of the town could be accepted to be of uniform
quality with respect to its runoff. The area drained by SP2, includes industrial complexes
and is of a mixed pattern because commercial areas and medium-density housing
developments are drained to this point too. However, medium and low-density residential
areas are not expected to contribute considerably to the runoff flow rate at this sampling
point, as they consist of single story buildings with gardens and the main portion of the
roof runoff is infiltrated into the ground before it reaches the collectors. Consequently,
the influence from the industrial sites runoff, which has a high percentage of impervious
covers, could be substantial, and this has been reflected by the results of this study, as
well as in earlier studies (Jarawaza 1997). Also, Heather et al. (1996) reported that some
industries in this area, both small and large, do not have on site pre-treatment facilities or
do not bother to ensure that the wastewater and storm water generated by them should
meet standards set by the Harare City Council.
The Coventry Rd channel, between points SP2 and SP3, collects runoff from
residential areas, small-scale enterprises and open spaces. In general, results show that at
SP3 (the discharge point of the Coventry Rd drainage channel), the water quality has
improved, compared to SP2 and is similar in strength to the one from the CBD. Thus, it is
expected that the runoff from this drainage area is diluting the more concentrated flow
coming from up-stream. It should be noted that the dilution might be due to groundwater
flow, as along this section, between SP2 and SP3, the channel is not lined.
4 ESTIMATING POLLUTION LOADS
The evaluation of pollution loads plays an important role in the process of pollution
control and abatement. It allows the estimation of the expected impacts of pollutants on
the water environment and the evaluation of the assimilative capacity of receiving water
bodies by means of models. Also, comparing loads from different sources allows to rank
them and to identify the critical ones, which should have a priority during the process of
planning and the implementation of pollution control and abatement measures.
The estimation of pollution loads from point sources, which are related to treatment
plants, is a relatively simple process, as, usually, data regarding quantities and qualities
discharged are available. It forms part of the treatment plant operation and is collected on
a regular basis. In the case of discharge points of storm water channels, as it is the case in
this study, the procedure of pollution loads estimation is more complex, because usually
data is limited. In addition, the range of variability of the parameters is much higher,
compared to discharges from treatment plants, and also the character of the diffuse
pollution implies a much higher level of uncertainty in the correct estimation of
qualitative parameters. Storm water channels, discharging into natural water bodies could
