Chlorine demand removal by biological treatment in cold water

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Abstract
A major strategy to reduce or prevent the formation of undesirable by‐products is to reduce chlorine demand before the application of chlorine. The major objective of this study was to evaluate the performance of different water treatment processes at full scale, specifically the combination of ozonation and biological activated carbon (BAG) filtration, for the removal of chlorine demand over a range of natural organic matrix and water temperature (l°‐23 °C). Both long term and short term chlorine demand were measured using a 2:1/C12:TOC ratio and pH 7,0 phosphate buffer at 20 °C in darkness. Results show that chlorine demand increases over a period of 200 hours and that long term chlorine demand can be modeled using a second order relation. Chlorine demand can be reduced by 55% for short term (4h) and by 61% long term (168h) by coagulation, floculation and sedimentation. The net effect of ozonation is variable depending on the prevalent oxidation conditions and on the organic matrix. In warm temperatures ( > 11 °C) ozonation slightly increases short term and long term chlorine demand. In cold water (1 °C) ozonation decreases significantly long term and short term chlorine demand. Biological activated carbon filtration will always reduce short term (25–80%) and long term chlorine demand (15–43%). Maximum removal of chlorine demand will require EBCT of about 20 minutes in BAG filters. Removal of chlorine demand could not be correlated with the removal of AOC. Some removal of refractory carbon was observed in the saturated BAG filters (0–80% of DOC removed) especially after backwashing. This removal accounts for some of the chlorine demand reduction. The results show that, if sufficient contact time is provided in BAG filters, biological filtration is an efficient process to reduce short term and long term chlorine demand.