Combined Sewer Overflow pretreatment with chemical coagulation and a particle settler for improved peracetic acid disinfection
R. K. CHHETRI, Dept. of Environmental Engineering, Technical University of Denmark, Miljøvej, B113, DK-2800 Kgs. Lyngby, Denmark
A. BONNERUP, Bonnerup Consult APS, Fynsvej 56, 5500 Middelfart, Denmark
H. R. ANDERSEN, Dept. of Environmental Engineering, Technical University of Denmark, Miljøvej, B113, DK-2800 Kgs. Lyngby, Denmark
Many old cities are drained by combined sewer systems in which wastewater mix with rainwater for transport to the wastewater treatment plant. When intense rainfalls occurs the design capacity of combined sewer systems exceeds resulting in discharge of combined sewer overflows (CSO) to nearby surface water resulting in contamination with various pathogenic organisms, suspended solids and chemicals.
The European Union has stated that good microbial quality for bathing water should not exceed 500 MPN/100 mL for E. coli and 200 MPN/100 mL for Enterococcus. Bathing water quality can be maintained by disinfecting the CSOs.
This study was conducted to characterize the disinfection by peracetic acid in combination with chemical coagulation in a HydroSeparator® system for CSO. HydroSeparator® CSO system is a patented and specialized system consisting lamella settler and mess filter (20 microns). The entire system was installed in Middelfart to treat the CSO from the towns of Båring and Asperup (Denmark) and contains a traditional CSO structure before the HydroSeparator and a constructed wetland to polish the disinfected effluent.
Samples for experiment was collected from inlet and outlet of HydroSeparator at different flow to optimize the PAA dose without coagulation. In experiment II, samples were collected from the inlet of the HydroSeparator to optimize the coagulation dose in a Jar test using PAX-XL 100. Experiment III was performed in full scale by applying PAX-XL 100 as flocculent (5 mg-Al/L) followed by disinfection with 10 mg/L PAA in the HydroSeparator. In order to confirm the PAA dose delivered in the field, comparable PAA treatments were made in the laboratory on samples collected after coagulation.
Turbidity and phosphorus was reduced by applying increasing flocculent doses, but higher than 5 mg-Al/L achieved insignificant improvements. In experiment III the removal of turbidity was 92%, COD 28%, total nitrogen 61% and phosphorus 27% with 5 mg-Al/L.
The stability of PAA increased after the HydroSeperator treatment, but was markedly further improved by the coagulation. Consistently with this, PAA disinfection was more efficient after the HydroSeparator, and further improved by the coagulation. In experiment III, removal of Enterococcus was 2.2 log for onsite disinfection and 2.4 log for the laboratory disinfection, which confirms the field dosing considering the analytical uncertainty.
Overall, it is evident that disinfection efficiency of PAA was more effective in the flocculated and HydroSeparator treated water, but as long as the HydroSeparator was applied efficient disinfection could be achieved by PAA dosing towards preserving bathing water quality.