316
G. Söderlund
Discharge from one downpour
The curves in Fig. 2 have been obtained as the product of water flow and the
corresponding concentration; the curve in Fig. 3 has been obtained by integration of the
curve in Fig. 2 with respect to time. As seen from these figures the load on the recipient is
very heavy during a downpour. From the 25 mm storm nearly 3000 kg of suspended
matter was discharged, and from the 8 mm rainfall about 800 kg. Figs. 4-5 give the
values for BOD, oil and lead.
Whether such a load over a short time can cause
damage to a recipient has not been established, but in connection with heavy storms the
dissolved oxygen content in some recipients has been found to fall to zero.
150
B O D , k g
lOOh
50 h
400
600
T i m e , min
Fig. 4.
Total amount of BOD during a 25 mm storm (circles). The thick line shows the
estimated discharge from treated effluent from the district with a population of 11.000.
OIL
0
02 04 06 08
10 12 14
CLOCK TIME (HOUR)
Fig. 5.
Total amount of oil and lead during a rainfall of 8 mm.