Problematik vid höga flöden - Gästrike Vatten AB

the hopper of the primary sedimentation tanks to the inlet of the tanks, and allowing it to mix
with the influent wastewater. In this way, VFA produced in primary sludge is washed out and
fed to the anaerobic reactor (method modified from Tykesson et al., 2005). Additional
diffuser blocks were also installed in the last quarter of the anaerobic tank to allow for a
reduction of the anaerobic retention time during low-flow conditions. The EBPR process has
since then been stable during normal operating conditions, and reducing the phosphorus
content to an extent of approximately 95 % without any use of chemical precipitation.
However, after long periods with high-flow conditions and low organic loadings in the order
of weeks, the process has been deteriorating leading to increased effluent phosphorus
concentrations and a need for chemical precipitation. Moreover, from the year 2012 the
effluent requirements will become more stringent with an effluent discharge limit of 0.3 mg
P/l instead of the present 0.4 mg P/l. To meet these new demands, and at the same time
minimize the use of chemical precipitation, the problems with insufficient phosphorus
removal at prolonged high-flow conditions must be solved.
Several studies of the EBPR process during high-flow starving conditions have been
conducted at a laboratory scale. However, there is a lack of knowledge from full scaleperformances
at these conditions. The purpose of this paper has been to translate the
knowledge from laboratory investigations in the literature, to a complex full-scale practice. A
study of the full scale treatment plant Duvbacken has been made to identify critical keyfactors
for the current process during prolonged high-flow conditions. The result from this
study was then used to develop different operating control strategies and to implement them
in full scale, with the purpose of preventing process failure and the need for chemical
precipitation during and after high-flow events.
METHOD
The WWTP treats the wastewater for about 88,000 population equivalents (p.e.) and consist
of a simple AO-configuration (anaerobic-aerobic) with a highly loaded activated sludge
treatment system without nitrification. The flow scheme of the plant is plotted in figure 1.
Influent
pretreated
wastewater
Returned
primary
sludge
By-pass
Sedimentation tanks
with primary sludge
hydrolysis
Return flow
Excess primary sludge to
sludge treatment
Additional diffusers
for low-flow
conditions
AN A1 A2
Figure 1. Flow scheme of the WWTP. AN=anaerobic reactor, A1=aerobic reactor 1, A2=aerobic reactor 2, A3=aerobic
reactor 3.
55
A3
Return of activated sludge
Flow-equalization tanks
Settling tanks
Sludge
tank
Excess sludge to
sludge treatment
Effluent during prolonged
high-flow conditions
(chemically treated)
Effluent