Environmental Engineering Reference
In-Depth Information
infiltration flows increase significantly the volume of the influent. During high intensity
storm events, hydraulic loads are very high, forcing the plant operators to by-pass the
plant and discharge a considerable amount of the untreated wastewater into the rivers in
order to protect the plant from process failure. The overflow volumes are retained for a
short period of time in designated ponds, and in many cases, discharged directly into the
River. This practice has also been reported by Nhapi et.al. (2001). Thus, the wastewater
treatment plants become a source of diffuse pollution through illegal discharges. The
diffuse character of the pollution in this case is not related to the location of the
discharge, which is well defined, but to the irregularity of the discharges and the variation
of the effluents quality. In the cases of overloaded stabilization ponds systems, the result
is a reduced quality of the effluent, leading to the overloading of the disposal sites and
indirect pollution of the soil, ground water and surface water. A more detailed study of
the impacts of diffuse pollution from storm water in different types of land use practices
is presented in the consequent chapters.
3 ABOUT THE LAKE CHIVERO AND TRIBUTARIES
3.1
About the lake water quality
Since the year of its construction in 1953, Lake Chivero was a mesotrophic impoundment
similar in many respects to the existing reservoirs along the Manyame River. Nitrogen
and phosphorus concentrations were relatively low. Species diversity was great and the
fish fauna of the lake had the predominantly riverine characteristics of the newly created
man-made lake (Marshall 1982). Throughout this formative period (1960-1964), rainfall
was good and the lake levels were reasonably constant, although drought years did occur
in 1960 and 1964, when water levels dropped by several meters. During the same period,
the volume of municipal wastewater being discharged by the local sewage works, rose
considerably and the first manifestations of eutrophication were detected. (Balllinger &
Thornton 1982).
Marshall and Falconer (1973) conveyed a study to investigate the Lake water quality,
which reflected the trend of increased nutrients concentrations. The most important
parameters, which were influencing the trophic status, were nitrates, ammonia nitrogen
and soluble reactive phosphorus (SRP). Their concentration rose considerably during the
late sixties due to point discharges from Crowborough and Firle STWs, which at this time
were treating wastewater by biofiltration. As a result of this investigation, the
municipality decided to divert the effluent from discharging into the rivers and to reuse it
for pasture irrigation. Also, the water quality regulatory instruments restricted the
discharge of nutrients rich effluents into natural water bodies. Later studies (Thornton
1980, Thornton & Nduku 1982c) reported a considerable reduction of SRP into the Lake
water after these measures were implemented. It was found that nitrite nitrogen and
particulate phosphorus were of lesser importance and were varying slightly during the
year. The concentrations of these parameters, measured at mid-lake station, during the
period 1969 to 1979 were usually less than 0.005 mg/l, whilst concentrations of nitrate,
ammonia and SRP were generally in excess of 0.010 mg/l. (Thornton & Nduku 1982c).
Calculations regarding the pollution load before and after the implementation of the new
