Follow the energy
Optimizing energy consumption at wastewater treatment plants
A wastewater treatment plant’s (WWTP) total energy consumption constitutes a large portion of the plant’s operation costs and ecological footprint. Intense focus has thus been put
on methods for energy conservation. The selected treatment process, the physical design
of the plant and the control strategy of the plant all have significant influences on the total
energy consumption. Purenova Miljöteknik AB has conducted a study on the effect of these
factors in order to improve the overall plant performance.
The process
The imperative question posed when designing a new WWTP is that of what type of treatment process to select. From an energy point of view, processes using carrier media require higher oxygen levels than activated sludge processes. A theoretical study has shown that
carrier based processes requires roughly 40 % more oxygen and over 200 % more air, see
table 1. Processes with carrier media tend to use coarse diffused aeration systems which
result in lower oxygen transfer efficiencies from air to wastewater.
SOR (kg
O2/d)
% compared
to AS
Nm3/d
% compared
to AS
AS
Fine
aeration
IFAS
Coarse
aeration
IFAS
Coarse
aeration
AS+ARP
Fine
aeration
2100
3000
3700
2000
-
+43 %
+75 %
-5 %
27 000
103 000
47 000
26 000
-
+280 %
+75 %
-5 %
Table 1. Theoretical oxygen- and air consumption for treatment plant with 15 000 PE.
Furthermore, a decision needs to be made on how to achieve denitrification. Pre denitrification which is common in activated sludge plants requires much energy due to recirculation. Intermittent aeration in the biological tank counteracts the need for recirculation of
nitrate since both nitrification and denitrification occur in the entire volume, leading to a
significant decrease in energy use.
Control strategy - case study
Crucial for the energy consumption and the treatment efficiency is that the processes are
optimally controlled for the prevailing conditions.
Ljungby WWTP plant serves approximately 25 000 PE and was originally designed solely
for BOD removal, but has since been extended to perform nitrogen removal. An Advanced Online Control (AOC) system for control of the aeration system and return sludge flow
has been installed in order to meet the effluent total nitrogen requirement during winter.
Furthermore, the biological process was converted from recirculation to intermittent
aeration.
The implementation of the AOC system resulted in significantly increased SS concentration
of the return sludge, see figure 1, and a reduction of total nitrogen in the effluent with 5
mg / l.
Figure 1. Increase of SS in return sludge after implementation of online control
This was achieved while the plant’s total energy consumption was reduced by 170 000
kWh/year, i.e. approximately 13 % of the annual electricity consumption, see table 2.
The return of investment (850 000 SEK) will thus be five years.
Physical design
The depth of the biological process tank has significant influence on the oxygen transfer.
The general rule is that the deeper the tank - the higher the oxygen transfer efficiency.
However, at a certain tank depth a different type of blower is required, which offsets the
energy conservation of higher oxygen transfer efficiency.
In addition, energy can be conserved by redesigning existent u-channels to circular channels. The kinetic energy of the incoming water is then utilized to circulate the mixed
liquor in the channel.
Energy gain/year
Aeration
123 000 kWh
Mixing
13 000 kWh
Return sludge
34 000 kWh
Table 2. Energy savings at Ljungby WWTP. The largest gains were made for aeration.