Problematik vid höga flöden - Gästrike Vatten AB

efficiency can be achieved by withdrawing the excess of sludge formed, containing the
biomass of PAO whith high phosphorus content within its cells.
Sweden and other northen countries have experienced an increased level of precipitation
during the last decades and both the level and intensity of these events can, according to
present climate models, be expected to increase further during the decades to come. This has
an impact on the wastewater treatment systems of the communities which receives a higher
level of stormwater both to the sewers and to the wastewater treatment plants (WWTP). This
occurs due to drainage from houses and leakage into the pipe-system. Another reason for high
influx flows can be that storm drains are not always operated independently from sanitary
sewer systems, especially in older parts of the cities, where the pipe-system can be more than
a hundred years old. Apart from the risk of discharging untreated sewage into the
environment, the increased flow can also lead to serious failures at the WWTP, especially
when biological treatment methods are being applied.
An activated sludge system with EBPR is very sensitive to external disturbances, such as high
influx flows. This can either deteriorate the phosphorus removal efficiency instantaneously or,
if the disturbance is prolonged, cause the ecosystem of the activated sludge to shift in favour
of other microorganism types than the essential PAO. The latter can affect the ability to
remove phosphorus biologically for long periods, in the order of weeks to months, until the
microbial population of PAO is re-established in the system. The weakened overall system
performance can also render the need for chemical precipitation of phosphate. This can have a
further inhibiting effect on the recovery of the EBPR process, since the biologically available
phosphate becomes chemically bound and therefore limits the ability to rebuild the
endogenous poly-P pools (Tykesson et al., 2003).
One of the most crucial factors related to high influx flows is the availability of easily
biodegradable substrates in the influent wastewater feeding the PAO in the anaerobic stage of
the process. A correlation has been found between the VFA-feed and the phosphate release in
the anaerobic reactor and hence the overall phosphorus removal performance (Carlsson et al.,
1996, Lie et al., 1997). During high-flow conditions the WWTP temporary receives a diluted
sewage at a high hydraulic load. The concentration of VFA has been shown to decrease and,
at the same time, the VFA to phosphate ratio is reduced during these conditions (Carlsson et
al., 1996). Depending on the duration of the highflow, this may result in a partial or complete
depletion of the endogenous carbon reserves, and leads to high effluent phosphate loadings
(Temmink et al., 1996). During these conditions microbial growth becomes substrate limited,
and the PAO tend to focus on maintenance using utilized energy for survival processes and
excluding the uptake of phosphate. If the conditions are prolonged the active biomass of PAO
can be reduced (Lopez et al., 2006).
When the conditions have been normalized after a high-flow event the effluent phosphorus
concentration has been reported to peak (Pitman et al., 1983). This is due to a temporary
imbalance between phosphate-release and uptake. The phosphate release is recovered almost
instantly while the phosphate uptake depends on a slowly rising level of PHA in the cell, and
is therefore delayed (Temmink et al., 1996, Miyake et al., 2005).
The WWTP Duvbacken in Gävle was rebuilt for EBPR during 2004. During the first years the
phosphorus removal capacity was unstable. The main reason to this was found to be a lack of
organic substrate for the process. A sufficient level of VFA was achieved by introducing a
main stream primary sludge hydrolysis in 2007. This was achieved by pumping sludge from
54