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WET Weather Treatment and Disinfection in the City of Welland

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SUMMARY The results of characterization and testing of CSO at the Welland WWTP determined the following and are summarized below:

Fifteen high flow wet weather events were sampled through the course of the study. Analysis was performed on the CSO from all fifteen events.

Both flow and rainfall data were collected to investigate the hydrologic response of the Welland WWTP. A range of rainfall and corresponding WWTP flow rates were captured over the study period, including WWTP storm flow events resulting from spring melt, summer thunderstorms, and spring/fall showers. Rainfall volumes triggering WWTP storm flow ranged from 0 mm (melt) to 38 mm over a wide range of intensities and durations.

Generally, the CSO quality observed at the Welland WWTP are within the range observed in other studies. Concentrations of CBOD5, nitrogen, phosphorus, TSS and bacteriological results are comparable to the values observed in other studies.

1Assessment of Advanced Physical-Chemical Treatment Options for CSO, the Municipality of Metropolitan Toronto, 1992.

2 cfu – coliform forming units.
Parameter Units Welland WWTP CSO Average Quality (Untreated) Other Studies Typical CSO Ranges
Total Suspended Solids mg/L 274 7 – 522
Total CBOD5 mg/L 67 2 – 357
Total Kjeldahl (TKN) mg/L 16.4 0.6 – 30
Total Ammonia (NH3) mg/L 6.6 0.1 – 48
Dissolved Phosphorus mg/L 0.5 1.9 – 5.3
Total Phosphorus mg/L 2.8 1.4 – 8.2
Fecal Coliform cfu/100 mL 851,000 15 – 4,500,000


Results from CSO treatability settling tests for 60 minute settled sample with and without chemical enhancement showed that chemically enhanced settling produced effective removals of nearly all conventional pollutant parameters. As expected and observed in previous studies, the removal efficiency for ammonia was negligible. Bacteria reduction is minimal with 60 minute settled sample; however, chemically treated 60 minute settled sample yielded a more substantial 2 log reduction. Removal percentages for Welland WWTP CSO are shown for 60 minute settled sample along with 60 minute settled sample with chemical enhancements for TSS, CBOD5 and TP.
Welland WWTP CSO Average Water Quality
Parameter Units 60 Minute Settled CSO 60 Minute Settled with Chemical Addition (Aluminium Sulphate as Primary Coagulant)
Total Suspended Solids mg/L 85% 99%
Total CBOD5 mg/L 64% 94%
Total Phosphorus mg/L 54% 97%


Optimum coagulant dosages established during CSO treatability settling tests for chemically enhanced 60 minute settled sample for aluminium sulphate were in the range of 4-6 mg/L (as aluminium). Optimum coagulant aid (polymer) dosage used during testing was in the range of 0.1–0.2 mg/L.

Chlorine disinfection experiments were carried out on 60 minute settled CSO samples and chlorination proved an effective method of disinfection – a desired 3-log target reduction was achieved on 60 minute settled CSO sample with a CT product of 60–80 mg/L*min.

UV dosage requirements to achieve target reduction is in the range of 15–25 mWs/cm2 for chemically enhanced 60 minute settled CSO using aluminium sulphate as the primary coagulant.

Ferric chloride used as the primary coagulant appears to have the typical effects of having iron salts interfering with UV disinfection.
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Document Type: Research Article

Publication date: 2005-01-01

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