1. Wastewater, Volumes and
Composition
by Mogens Henze
1.1. The volumes of wastewater
Wastewater flows are not steady or uniform, but vary from one hour to another,
from day to day, from month to month and from year to year. When building a
treatment plant, it is important to know the volumes of wastewater and their
variations, now as well as in future. Based on the knowledge of the wastewater, the
design of the treatment plant can be determined, taking into account the wastewater
to be treated. In this connection measurements are useful; if such measurements do
not exist, an estimate should be made. In respect of the volume of future wastewater,
the development should of course be taken into account, i.e. a prognosis should be
made.
1.1.1 Measurements
Measurements of the volume of wastewater will be either in the form of curves or
in the form of figures (metering). Fig 1.1 shows a diurnal variation for wastewater
in the influent entering a treatment plant. The curve is the sum of domestic,
industrial and public institution wastewater, infiltration and exfiltration. There is no
reason to consider the size of the individual contributions as the curve shows what
reaches the treatment plant. In case of a projection of the volumes of wastewater and
their variations, however, it is recommended to analyze the curve and the catchment
area for the purpose of splitting up the sub-contributions as it will be easier to
project these separately. This is briefly dealt with in Sections 1.2 and 1.5. Sampling
and measurements at treatment plants are difficult. Watch out for flows of return
water (such as supernatant) which are often mixed into the raw wastewater before
reaching bar screen and grit chamber, hence making correct measurement on the
raw wastewater difficult.
The curve shown in Fig 1.1 can be used to find the maximum hourly flow (190
m 3 /h) and the average hourly flow (78 m 3 /h) on the day in question. If a sufficient
number of diurnal measurements are available, two important quantities can be
calculated which form part of the design of the treatment plant, i.e.
Qh,max
the average of the maximum hourly flow rate of the individual days
(m 3 /h)
Oh,av
the average hourly flow rate for many days (m 3 /h)
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Composition
by Mogens Henze
1.1. The volumes of wastewater
Wastewater flows are not steady or uniform, but vary from one hour to another,
from day to day, from month to month and from year to year. When building a
treatment plant, it is important to know the volumes of wastewater and their
variations, now as well as in future. Based on the knowledge of the wastewater, the
design of the treatment plant can be determined, taking into account the wastewater
to be treated. In this connection measurements are useful; if such measurements do
not exist, an estimate should be made. In respect of the volume of future wastewater,
the development should of course be taken into account, i.e. a prognosis should be
made.
1.1.1 Measurements
Measurements of the volume of wastewater will be either in the form of curves or
in the form of figures (metering). Fig 1.1 shows a diurnal variation for wastewater
in the influent entering a treatment plant. The curve is the sum of domestic,
industrial and public institution wastewater, infiltration and exfiltration. There is no
reason to consider the size of the individual contributions as the curve shows what
reaches the treatment plant. In case of a projection of the volumes of wastewater and
their variations, however, it is recommended to analyze the curve and the catchment
area for the purpose of splitting up the sub-contributions as it will be easier to
project these separately. This is briefly dealt with in Sections 1.2 and 1.5. Sampling
and measurements at treatment plants are difficult. Watch out for flows of return
water (such as supernatant) which are often mixed into the raw wastewater before
reaching bar screen and grit chamber, hence making correct measurement on the
raw wastewater difficult.
The curve shown in Fig 1.1 can be used to find the maximum hourly flow (190
m 3 /h) and the average hourly flow (78 m 3 /h) on the day in question. If a sufficient
number of diurnal measurements are available, two important quantities can be
calculated which form part of the design of the treatment plant, i.e.
Qh,max
the average of the maximum hourly flow rate of the individual days
(m 3 /h)
Oh,av
the average hourly flow rate for many days (m 3 /h)
11