Wastewater, Volumes and Composition
The ratio between various substances in wastewater influences the selection and
function of treatment processes. Table 1.12. shows typical ratios. A high COD /BOD
ratio indicates organic matter difficult to degrade. A high COD /TN ratio favours
denitrification and a high VSS/SS ration indicates a high fraction of organic matter
in the suspended solids.
Ratio
Low
Typical
High
COD/BOD
1.5-2.0
2.0-2.5
2.5-3.5
COD/TN
6-8
8-12
12-16
COD/TP
20-35
35-45
45-60
BOD/TN
3-4
4-6
6-8
BOD/TP
10-15
15-20
20-30
COD/VSS
1.2-1.4
1.4-1.6
1.6-2.0
VSS/SS
0.4-0.6
0.6-0.8
0.8-0.9
COD/TOC
2-2.5
2.5-3
3-3.5
Table 1.12.
Ratios in domestic wastewater
1.2.2 Variations
Daily, weekly and monthly fluctuations in the loads of water and pollutants are
important for the design, operation and control of the treatment plant. Fig 1.9 shows
the diurnal variations of organic matter (measured as BOD7) for the Lundtofte
treatment plant. It is seen that there is a factor 10 between the lowest and the highest
hourly influent.
An example of variations in the daily influent of nitrogen is shown in Fig 1.10. In
this case no distinct weekly variation is observed. Often the daily influents of
municipal wastewater show a clear weekly variation. It may be particularly important for the operation of a biological treatment plant. For example the COD IN-ratio
will often be lower on Saturdays and Sundays than during the rest of the week. This
may cause operational problems in connection with denitrification.
In Fig 1.11 a fractile diagram for the influent of BOD to a treatment plant is shown.
The temperature of wastewater varies over the year. The result is that the temperature also varies in the tanks. In Fig 1.12 the temperature variation in two activated
sludge plants is shown.
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