Problematik vid höga flöden - Gästrike Vatten AB

clarifiers and were measured for VFA. The results showed that this sludge could be used as an
extended resource for substrate for the primary sludge hydrolysis during high-flow conditions.
The VFA content of the effluent water from the sedimentation tank was instantaneously
increased by 46 % for the activated sludge, and by 81 % for the primary sludge. The mean
value of VFA in the supernatant from the activated sludge was 559 mg/l and, from the
primary sludge, 952 mg/l.
A change in the programming of the surveillance system was implemented in order to pump
less primary sludge to the clarifier during high-flow conditions. This was done to assure a
certain amount of substrate during these conditions. Samples taken at the effluent showed a
correlation between sludge volume in the tank hoppers and the production of VFA. This is
shown in figure 6 and illustrates the importance of having enough substrate for sufficient
VFA-production. A correlation was also found between the pH and the production of VFA.
Measurements of pH could hence be used for an indication of sufficient VFA-production.
VFA (kg/h)
20
16
12
8
4
0
(a) VFA-feed in correlation to sludge
volume
R² = 0,6336
0 50 100 150
Sludge volume (m 3 )
Figure 6. VFA-feed to the biological stage in correlation to (a) sludge volume in the hopper of the sedimentation
tank and (b) pH in the effluent from the sedimentation tank.
The inlet to the biological stage was rebuilt to prevent turbulent flow. Subsequently, DO
concentrations were measured during high-flow conditions and showed that the anaerobic
retention time had been secured.
High DO-peaks in the aerobic reactors were prevented by implementing another change in the
programming of the surveillance system. An automated change to a compressor with lower
capacity during high hydraulic load was programmed. Another setup made it possible to
choose different set points for the DO-level during normal and high-flow conditions, both in
A1 and in A2. A further programming change was implemented to allow for different DOlevels
in the beginning and the end of A2 by using two oxygen probes individually. The
change to a high-flow set point was done depending on the airflow in the aerator-system in
A1 which reflects the current load of oxygen demanding substances. The purpose of these
changes was to reduce the concentration of DO during prolonged high-flow and endogen
starvation conditions to possibly conserve PHA pools and thereby make process failures less
likely. Proper control of aeration has also been shown to reduce the biomass decay (Siegrist et
al., 1999).
The point for bypass was moved from after the primary sedimentation tanks to before them, to
be able to control the influx-flow and thereby avoiding the risk for sludge wash out from the
60
VFA (kg/h)
25
20
15
10
5
(b) VFA-feed in correlation to pH
R² = 0.7099
0
6.8 7.2 7.6
pH