Biofilm growth and chlorine stability in the recycled water distribution system

Abstract

Wastewater recycling is widely practiced to solve water crises created by increasing demand due to rapid population growth and scarcity of resources arising from climate change. Certain treatment is always provided to meet the appropriate health guidelines of the recycled water. When water is distributed over the pipelines and tanks, microbes can regrow and deteriorate water quality, and hence a disinfectant, usually chlorine, is added to the water. Chlorine can still decay while in transport and the problem can exist. Optimal chlorine levels are therefore to be maintained after the treatment to ensure the water quality does not deteriorate. For such fundamental understanding of factors impacting chlorine residuals are needed in the recycled water. While there are a lot of studies on drinking water systems, there is a scarcity of information on recycled water chlorine stability. This work is aimed to fill the knowledge gap by investigating the Sydney Olympic Park Authority (SOPA) recycled water pipelines and pilot-scale biofilm reactor system set up at the water treatment plant to elucidate the fundamentals. An investigation was carried out to examine the water quality parameters that may degrade chlorine residuals in the recycled water distribution system of SOPA. Physicochemical parameters such as free chlorine, total chlorine, DOC, ammonia, nitrite, nitrate and pH were measured in the bulk water samples. Biofilm growth potential of two different pipe materials under the similar environment, especially chlorine residuals of the storage reservoir before supplying to the distribution system was investigated to determine the role of biofilm components in decaying chlorine. Three flow cell columns of bioreactors containing biofilm coupons of polyvinyl chloride (PVC) and high-density polyethylene (HDPE) pipes were continuously operated for 105 days. Results were obtained from the examination of chlorine stability in the bulk water samples shows organic chloramine has reduced the effectiveness of chlorination. Chlorine decay in the water system occurred mainly due to inorganic and organic compounds such as metals deposition and bacterial activities, which were supported by the results of biofilm development data from the pilot-scale bioreactor system. Biofilm thickness, volume, mass and visualized images contribute an important role on understanding the decay of chlorine residuals. Over 15 weeks of biofilm development, the fluctuating chlorine trend of the recycled water in Sydney Olympic Park system has an adverse impact on biofilm constituents, bacteria cells and extracellular polymeric substance (EPS) production. As compared to traditional measurements, confocal laser scanning microscope (CLSM) provided different recording of multiple biofilm parameters with their subsequent visualization and quantification. In addition, discoloured water factors such as metals such as Fe, Mn deposition within biofilms were observed and the results illustrate that the discoloured water event could be related to presence of Fe and Mn in the recycled chlorinated systems. In all the samples, organic chloramine was found to be the dominant chlorine species in the recycled water distribution system. According to monitoring data, biofilms did not grow as fast as expected due to the presence of chlorine, organic chloramine, other unknown inhibitors and/or high flow rate. Free chlorine and slow-growing biofilms may oxidise Fe or Mn and influence the retention of these elements within the biofilm. Both PVC and HDPE had the same trend of increasing biofilm thickness as well as the biomass. HDPE pipe surfaces were more susceptible towards biofouling than PVC. EPS volume was usually higher than the bacterial cell volume in both pipe materials whereas EPS volume was higher in HDPE than PVC. The highest volume of EPS was approximately 4000 I¼m3/m2 compared to the highest volume of the bacterial cell about 2400 I¼m3/m2. The biofilm is not enough in the pipe materials to show the impact on decaying chlorine at concentrations range between 1 - 3 mg/L. Organic chloramine possibly plays a critical, but an unknown, role in determining the growth of biofilm and dirty water complaints through release of metals.

Date of Award	2018
Original language	English