Operation of BAC Filters in Cold Water - An Orange WTP Study

Biological activated carbon (BAC) filters are often used in conjunction with ozonation to reduce biodegradable dissolved organic carbon (BDOC) release into the distribution system. BDOC is associated with higher chlorine demand, biofilm growth and taste and odour issues within the distribution system and therefore a target of 0.2 mg BDOC/L is desired after the BAC filters. Orange WTP in Australia experiences high variations in water temperature from summer to winter. It was discovered, through regular BDOC monitoring, that optimisation of the BAC filters’ operating conditions - in particular backwashing frequency – significantly influences BAC filter performance. It also raised the question as to whether the BDOC test should be conducted at 21oC as per the standard method or whether it should be conducted closer to the temperature of the water in the distribution system. This paper will describe the differences in BDOC concentration in relation to the backwashing frequency of the BAC filters, water temperature, initial dissolved organic carbon (DOC) levels, ozone residuals and temperature of the BDOC test itself in the water collected from Orange WTP over the year 2012.