Implications of organic carbon in the deterioration of water quality in reclaimed water distribution systems

Changes in water quality in reclaimed water distribution systems are a major concern especially when considering the potential for growth of pathogenic microbes. A survey of 21 wastewater process configurations confirmed the high quality effluent produced using membrane bioreactor (MBR) technology, but suggests that other technologies can be operated to produce similar quality. Data from an intensive twelve-month sampling campaign in four reclaimed water utilities revealed the important trends for various organic carbon parameters including total organic carbon (TOC), biodegradable dissolved organic carbon (BDOC), and assimilable organic carbon (AOC). Of the four utilities, two were conventional wastewater treatment with open reservoir storage and two employed MBR technology with additional treatment including UV, ozone, and/or chlorine disinfection. Very high BDOC concentrations occurred in conventional systems, accounting for up to 50% of the TOC loading into the system. BDOC concentrations in two conventional plants averaged 1.4 and 6.3 mg/L and MBR plants averaged less than 0.6 mg/L BDOC. Although AOC showed wide variations, ranging from 100 to 2000 μg/L, the AOC concentrations in the conventional plants were typically 3-10 times higher than in the MBR systems. Pipe-loop studies designed to understand the impact of disinfection on the microbiology of reclaimed water in the distribution system revealed that chlorination will increase the level of biodegradable organic matter, thereby increasing the potential for microbial growth in the pipe network. This study concludes that biodegradable organic carbon is an important factor in the microbial quality and stability of reclaimed water and could impact the public health risk of reclaimed water at the point of use.