An investigation into the biodegradability of natural organic matter (NOM) and the resulting potential of biodegradable NOM fractions to form disinfection by-products

Abstract

Not only does the presence of excessive levels of natural organic matter (NOM) in surface waters affect the raw water quality, but it also impacts the water treatment and supply processes. Other notable challenges caused by NOM is its contribution to bacterial regrowth and the formation of ‘toxic’ disinfection by-products (DBPs). DBPs are nuisance chemicals in water systems as they lead to the production of inferior water quality which may affect human health, the eruption of toxins and various disease-causing microorganisms. Most conventional water treatment plants (WTPs) insufficiently remove NOM, primarily the biodegradable dissolved organic carbon (BDOC) fraction. In the presence of bio-available fractions of NOM, conditions are created for opportunistic pathogens to regrow. While chlorination is crucial for the control of microbial contaminants, the co-existence and interaction of residual chlorine and residual NOM in the WTP lead to the introduction of DBPs such as trihalomethanes (THMs). To maintain the quality of potable water during conveyance, the system must be optimized with adequate control and monitoring, particularly of disinfection and microbial control. A better understanding of the biodegradability of NOM fractions and their potential to form DBPs due to interactions with chlorine residues is required. This study investigated the character of NOM and its fractions in water treatment plants as well as their biodegradability and influence of these fractions on the THM formation potential (THMFP). The aim was achieved through a combination of conventional and advanced NOM characterization techniques. Raw and treated water from a conventional WTP was characterized through specific ultraviolet absorbance (SUVA) (L/mg.m) to define the NOM composition in terms of aromaticity. The water was further isolated into 3 NOM fractions (i.e. Hydrophilic [Hpi], transphilic [Tpi] and Hydrophobic [Hpo]) through the application of the modified polarity rapid assessment method (m-PRAM). Then, the biodegradability was assessed through the BDOC method, which measures the change in DOC of a NOM sample attached to biologically activate sand over a given period. The THMFP assessment was also conducted on each NOM fraction. Lastly, due to the significant correlation between BDOC and biomass production, the impact of the biodegradability of each fraction on bacterial regrowth potential (BRP) was investigated. This was concurrently together with the BDOC studies by monitoring the concentrations of heterotrophic plate counts (HPC) and Total coliforms (TC) on the first and last day of the experiment. The BRP of each fraction was calculated as the difference between the initial and the final concentration of HPC or TC, and only a ≥1x103 increase in the bacterial counts was considered positive for the BRP. The raw water SUVA ranged between 3.88 L/mg.m and 4.11 L/mg.m, with an even distribution of the Hpi and Hpo NOM obtained through the m-PRAM fractionation. In terms of biodegradability, the Hpi and Tpi fractions were the most biodegradable fractions, with BDOC values of >32% and >29%, respectively. The relatively high BDOC on the Hpi and Tpi fractions substantially contributed to BRP, thereby increasing the HPC to ranges between 121.4 x103 cfu/mL to 197.4 x103 cfu/mL, respectively, while their impact was less significant to THMFP. The Hpi fraction can be confirmed as the primary cause of bacterial regrowth. The strong correlation (i.e. R2= >0.9) between BDOC and BRP allows for the prediction of the BRP in a water sample using the BDOC of each of the NOM fractions. In terms of THMFP, chloroform (CHCl3) was the most abundant, increasing up to 708 µg/L and 611 µg/L for the raw water and treated water, respectively, while bromodichloroform (CHBrCl2) were detected in very low concentrations (<21µg/L) both in raw and treated water. The formation of CHBrCl2 and CHCl3 was mainly ascribed to the Hpo fraction. The high proportion of the NBDOC to the BDOC observed on the HWM Hpo fraction can be attributed to the higher potential of the Hpo fraction to form TTHMs. Significant correlations (R2) ranging from 0.83 to 0.91 were observed between SUVA and TTHM, confirming that SUVA alone can be successfully used to predict TTHM formation. A relationship between the biodegradability of NOM and DBPFP exists, the less biodegradable the NOM fraction, the more influence they have on the formation potential of DBPs. The enhanced BDOC method has been successfully optimized for NOM biodegradation studies. The various ways in which systems can be retrofitted to effectively deal with biodegradable NOM can be accomplished through this method. The BDOC is an excellent tool for BOM quantification and is thus crucial in the development of an effective NOM removal strategy. Now that the link between BDOC and TTHM formation has been established, there is a need to conduct an assessment for N-nitrosodimethylamine formation potential (NDMAFP), particularly in the chloraminated distribution network where NDMA is more likely to occur. The study also recommends an investigation into the other NOM fractions such as Hpi-Acids, Hpi-Neutral, Hpo-Base etc., with respect to biodegradability and how they can impact the mechanisms for bacterial regrowth and DBPFP in distribution systems.