Full scale evaluation of combined sewer overflows disinfection using performic acid in a sea-outfall pipe

• Combined sewer overflow was disinfected with performic acid in a sea-outfall pipe.
• Dosing 1–4 ppm performic acid removed indicator bacteria by 1–5 log units.
• Laboratory and field disinfection agreed fraction wise and detected known errors.
• Laboratory disinfection correlation with dose for ⩽4 ppm initial performic acid.
• Performic acid water quality limits upheld through fast degradation, high dilution.

Pollution of surface waters with pathogens from combined sewer overflows limits recreational use of surface waters. Large retention basins are a satisfactory solution but they are rarely sufficient for economic or space reasons. Fast disinfection during the overflow is an alternative, but few methods are known and each has problems. This work evaluated for the first time the full-scale disinfection using performic acid by the removal of the two currently regulated indicator bacteria for bathing water quality, Escherichia coli and Enterococcus. Experiments were performed at a sewage bypass through a sea-outfall pipe with a minimum hydraulic retention time of 24 min.The disinfection efficiency in the field was measured by analyzing samples taken before and after the treatment. Samples were also treated with performic acid in the laboratory to measure the disinfection effectiveness and kinetic of degradation of performic acid. Doses of 1–8 ppm of performic acid achieved 1.0–3.5 log removal of E. coli and 1.0–2.44 log removal of Enterococcus in the field, but were somewhat higher in laboratory conditions at 1.69–4.38 and 1.0–4.27 log units, respectively. Studies of the degradation of performic acid in collected real samples showed more than 50% was degraded in 20 min, and mostly degraded by 120 min. Comparison of field and laboratory dosed samples detected that performic acid synthesis did not start in one event and clogging of the sampler in another event.Overall the tests showed that the treatment was successful but it is indicated that online control could benefit treatments efficiency.

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