Controlling the Influent Load to Wastewater Treatment Plants

Sammanfattning: The need for control of the influent load to a wastewater treatment plant
(WWTP) is becoming more important. One reason for this is that there are
a number of things that cannot be achieved with plant-focused control. For
instance it is hard to avoid sludge loss as a result of poor settling or
reducing a too high influent flow rate by in-plant control actions. It is also
difficult to reduce the effects of a toxin in the influent, if the entire influent
is to be biologically treated. Optimisation of the various parts of the
collection system, with respect to locally defined objectives, may be
counter-productive as it may increase the effluent loads when taking the
whole system into account. This is typically the case as optimisation of the
control of the sewer net with respect to combined sewer overflows (CSOs)
leads to an increased flow to the WWTP. Equalization basins are used to
control the flow rate or the load in the sewer net as well as at the WWTPs.
The focus has recently been shifted from only reducing the amount of
CSOs to reduce the effluent load from the sewer and the WWTP. To
minimize the total load from the system the methods previously used to
optimise the individual sub-systems must be used together and information
from various parts of the system should be available system wide.
Due to the cost associated with the construction of equalization basins, the
current approach is to increase storage volume by constructing and
controlling gates in the sewer net. The potential of system wide control is
difficult to estimate, which is exemplified by a discussion on some existing
implementations. In this thesis an equalization basin is modelled and used
with an existing model of a WWTP. This system is operated with some
commonly applied control strategies of equalization basins to estimate the
result of control during ideal conditions. Without control of the basin, the
possible benefit of construction, or providing an equal amount of storage
capacity in the sewer net, is evaluated.