Disinfection of tertiary wastewater effluent prior to river discharge using peracetic acid; treatment efficiency and results on by-products formed in full scale tests

This is an investigation of chemical disinfection, with peracetic acid (PAA), in a tertiary sand filter at a full scale activated sludge plant with nitrification/denitrification and P-removal. The reduction efficiency of Escherichia coli and intestinal enterococci in the sand filter is reported. E. coli log reductions of between 0.4 and 2.2 were found with contact times from 6 to 37 min and with dosing from 0 to 4.8 mg L⁻¹. The average log reduction was 1.3. The decomposition products, bromophenols, chlorophenols and formaldehyde and residual H₂O₂ were measured before and after the sand filter. The residual H₂O₂ concentration in the effluent was critical at short contact times and high dosages of PAA due to the discharge limit of 25 μg L⁻¹. The other three products could not be detected at 0.1 μg L⁻¹ levels. The chemical cost of PAA dosing is estimated to be 0.039 US$ m⁻³ treated wastewater.