PAC Dosage
PAC dosage to flocculation filtration of WWTP Kloten/Opfikon to improve micropollutant removal
(Complementary investigations to the national MicroPoll project)
Ozonation or powdered activated
carbon (PAC) addition after biological treatment are promising technologies for
complementing nutrient removal plants to strongly reduce micropollutants and
ecotoxicity in the receiving water.
Sand filtration units of WWTP Kloten/Opfikon in direct neighbourhood of airport Zuerich
Due to the stringent P removal
conditions a lot of WWTP in Switzerland are already equipped with a
flocculation filtration step. WWTP Kloten/Opfikon near the city of Zuerich operates
a sand filtration to reach effluent suspended solids concentration smaller than
4 mg/l. Flocculants addition to the flocculation tank
in front of the sand filter is not in use due to a very efficient secondary
clarifier. The aim of the project is to evaluate the significance of PAC
addition directly to the filtration step using the flocculation tank as contact
tank for PAC addition. The filter is normally back flushed every 24 hours
giving a few hours contact time for the accumulated PAC in the filter.
To evaluate general operation parameters for
PAC and flocculant addition, the flow regime and the hydraulic retention time in
the contact tank as well as the back flush interval for the planned full scale
application at WWTP Kloten/Opfikon, a pilot sand filtration system was built in
the experimental hall of Eawag in summer 2008. The lay out as well as the dimensions
of the pilot plant comply with the full scale facilities. The full scale trials
at the WWTP were conducted in autumn 2008.
First results in 2008 for the
retention of PAC in the filter showed inconsistent behavior between the pilot
and full scale application. In addition micropollutants measurements by BfG
showed a heterogeneous picture with elimination rates up to 80% in maximum. To
optimize the process and to enhance the elimination rates additionally trials
are planned at Eawag and at WWTP Kloten/Opfikon for 2009. The investigation are
closely linked to the national Bafu-project MicroPoll.
Initial results [pdf, 135.1 k]