Decisive information on the different polluting agents as well as the prevailing wastewater flows and load variaS.
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tions are important both in the planning stage as well as during the operation. A very clear circumstance for a
municipal wastewater treatment plant is a time related variation of the pollution loads. The “classic” variation
often addressed is the variation over 24 hours. For smaller communities the wastewater flow variation at the
treatment plant is characterized by two peak flows, one in the morning and a second one in early evening. The
night flow is normally found to be minimal and mainly composed of infiltration of water into the sewers. In
larger communities with a vast collection system along with a number of pumping stations the peak flows will
be attenuated and the night flow will be relatively higher. On the other hand quite different patterns may be
found even more relevant for a plant. In Figure 1 is shown the load variation during a week at a suburban plant
near Stockholm, Sweden.
The pattern reflects the fact that the town is handling wastewater from day-migrants, working in central
Stockholm weekdays, but they use the water facilities at home to a “full” extent only on weekends. Even more
“dramatic” fluctuations may be found at for instance skiresort facilities, with a peak flow during a few months
during wintertime. Thus the pollution pattern may not be clearly discovered until a thorough investigation of the
actual conditions has been performed. The example demonstrates that the question “when should the sampling
take place” sometimes has a fundamental importance.
A second example is found in Dhaka, Bangladesh. One of the very few wastewater treatment plants in operation
in the country is found in the south-eastern part of the capital. The plant is called “Pagla”. The operators exercised
a grab sampling model, taking samples at the inlet, upstream the screen and at the discharge point from
the primary sedimentation stage. The grab samples were taken at 09:00 hours every day. The analysis at the
plant laboratory included both SS (Suspended Solids) and BOD5. The typical results from the sampling and
analysis are illustrated in Figure 2.
The demonstrated pattern in the figure is based on in all 44 observations, and the pairs of incoming and discharge
BOD observations are from the same day of operation. The observations are sorted with the maximum
inlet figure to the left and the minimum inlet figure at the right end of the curve. Thus a direct “effect” of a
BOD5-removal at the primary treatment at the plant may be found by comparing the inlet and outlet concentrations?
The very clear answer to this question is however “NO!” The superficial conclusion from the given data
(and a rather astonishing one) would be that a primary sedimentation is capable of performing a BOD-removal