Disinfection byproduct formation during biofiltration cycle: Implications for drinking water production

Insights for booster chlorination strategy based on DBPs control in a large-scale water supply system

Based on a one-year field investigation of disinfection by-products (DBPs) in large scale water distribution system (LSWDS), the various characteristics of DBPs together with their correlation with booster chlorination were elaborated through ArcGIS model. Furthermore, the effects of booster chlorination on DBP formation were investigated through simulated experiments. Residual chlorine showed a strong relationship with occurrence of different DBPs in LSWDS, and the yield of DBPs increased significantly after booster chlorination. The simulated chlorination experiments showed that diminution of the ratio of primary to secondary (booster) chlorination dosage, and delaying the secondary chlorine addition reduced the generation of DBPs during water conveyance. The yield concentrations of THMs and HAAs obviously increased after booster chlorination. The correlation between HAAs and chlorine dosage is weaker in the field research than in the simulation experiment while THMs had a positive correlation with the chlorine addition in both field research and simulation experiment.

The effect of operational conditions on the disinfection by-products formation potential of exopolymeric substances from biofilms in drinking water

Biofilms are ubiquitous in drinking water systems due to their external matrix of exopolymeric substances (EPS) that provide them protection and adaptability. They are even more common in low flow conditions where hydraulics favor their growth. EPS are organic substances (i.e., proteins, carbohydrates and humic substances) that can react with disinfectant, forming disinfection byproducts (DBP), some of which are controlled by water regulation. However, there is little information available on biofilm-disinfectant interaction and the effect of operational conditions such as biofilm age, water velocity, chlorine and pipeline length on the DBP formation potential of EPS (DBP_fpEPS). Using experimental setup and studies of two different biofilms: Biofilm 1 (2.6 ± 0.8 mg Cl/L) and Biofilm 2 (0.7 ± 0.2 mg Cl/L), the DBP_fpEPS was studied and compared to the DBP_fp of filtered water (FW). The DBP studied were trihalomethanes (THM), haloacetic acids (HAA), haloacetonitriles (HAN), chloropropanones (CP) and chloropicrin (CPK). The DBP concentration trend in both EPS and FW was HAA > THM > CP > HAN > CPK. Biofilm age only increased chloroform (CF)_fpEPS in Biofilm 1, while other DBP_fpEPS decreased. A direct relationship between water velocity and CF_fp in Biofilm 1 was found, probably related to higher chlorine diffusion and the production of a more reactive matrix. Chlorine positively affected DBP_fpEPS, increasing Cl-HAA, Cl-THM, CPK and Br-HAN. Biofilm 2 produced higher quantities of EPS per meter of pipeline, this constituting a precursor of intermediary DBP 1,1 dichloropropanone (1,1, DCP). The study compared DBP in chlorinated water in contact with biofilm (BCW) and without (CW). Biofilm 1 increased levels of Cl-HAA, Cl-CP and dichloro-acetonitrile, while Biofilm 2 diminished Cl-HAA and Cl-HAN. Biofilm 1 reduced some Br-HAA in BCW, whereas Biofilm 2 promoted Br-HAA and 1,1, DCP in BCW. EPS and biofilms were significant in terms of their effect on DBP formation.

Anammox attachment and biofilm development on surface-modified carriers with planktonic- and biofilm-based inoculation

This study investigates the kinetics, attachment, biofilm development and anammox bacteria enrichment of a novel detached anammox biofilm inoculation method on non-modified virgin MBBR carriers and pre-seeded denitrifying carriers. The study compares these results to the more common use of attached anammox carriers for anammox MBBR inoculation. The anammox bacteria specific attachment-growth rates for virgin carriers inoculated with detached anammox biofilm mass were 38.1% greater for the first 25 days, leading to approximately 30% less time required to achieve complete biofilm coverage than those measured in attached biofilm carrier inoculated systems during the attachment and early biofilm growth stages. The biofilm thickness increase rate was also 52.3% higher for virgin carriers with detached biofilm inoculum. Further, inoculation using pre-seeded denitrifying carriers compared to virgin carriers demonstrated a 13.8% preferential increase in anammox bacteria specific attachment-growth rate and a corresponding 47.2% higher NH₄⁺-N removal rate at the time of biofilm maturation.

A novel stochastic wastewater quality modeling based on fuzzy techniques

Measurement and prediction of wastewater quality parameters are crucial for evaluating the risk to the receiving waters. This study presents new methods for the identification of outlier data and smoothing as an effective pre-processing technique prito to modelling. This new data processing method uses a combination of the autoregressive integrated moving average (ARIMA) model and -the adaptive neuro fuzzy inference system with fuzzy C-means clustering (FCM) (ANFIS-FCM). These new pre-processing methodsare compared to previously employed non-linear approaches for modelling of wastewater influent/effluent 5-day biochemical oxygen demand (BOD₅), chemical oxygen demand (COD) and total suspended solids (TSS). Linear modelling of each parameter, 242 linear models, were investigated, and a linear model for each parameter was selected. The results of the non-linear models led to an acceptable prediction for qualitative parameters so that the high coefficient of determination (R ² ) was observed for the influent and effluent BOD and TSS, respectively. The range of the R ² for all models was recorded as 0.8-0.87 and 0.83-0.89, respectively. By a combination of the linear and non-linear mothods a hybrid model was introduced. The proposed hybrid model for the influent BOD with the highest correlation between the observed and predicted values, and limited scattering was identified as the optimal model (R² = 0.95). The use of hybrid models to predict wastewater quality parameters improved the performance and efficiency of the models. In addition, a comparison of the hybrid model with the recently developed models in the literature indicates that the developed ARIMA-ANFIS-FCM outperformed other models.

THMs, HAAs and NAs production from culturable microorganisms in pipeline network by ozonation, chlorination, chloramination and joint disinfection strategies

Disinfection is an indispensable process to inactivate pathogens, while unexpected disinfection by-products (DBPs) would also be formed between the reaction of residual disinfectants and microorganisms in the water distribution system (WDS). However, there are few studies referring to the formation of DBPs and DBPs-associated toxicity under various disinfection methods based on microorganisms in the real WDS. In addition, the main contributors of bacterial communities or components that generate DBPs are unclear. In this study, the formation of trihalomethanes (THMs), halo-acetic acids (HAAs), nitrosamines (NAs) from culturable microorganisms in pipeline network by ozonation(O₃), chlorination (Cl₂), chloramination (NH₂Cl) and joint disinfection methods were compared, meanwhile, their calculated toxicities under different oxidation scenarios were also discussed. Moreover, 16S ribosomal ribonucleic acid (rRNA) gene sequencing was used to identify the main microbial communities. The results demonstrated that THMs and HAAs increased with increasing disinfectant dosages, while the quantity of NAs (mainly nitroso dimethylamine (NDMA)) was not significantly related to disinfectant dosages for each disinfection strategy. Chloroform (TCM) and dichloroacetic acid (DCAA) were the dominant THMs and HAAs species, respectively. NDMA existed in the samples before disinfections, which may due to the metabolic activity of microorganisms. Pre-O₃ increased THMs formation during subsequent Cl₂ and NH₂Cl treatment. However, pre-O₃ effectively reduced HAAs produced by subsequent chlorination. O₃/Cl₂ disinfection had the highest DBPs formation potential (DBPFP) (883.6 nM), while its calculated toxicity was similar to that in Cl₂ disinfection treatment. Pseudomonas was the most abundant bacterial genus in biofilm of WDS pipeline. This study can aid in an optimal disinfection strategy for water treatment plants to reduce the toxicity of DBPs caused by biomass in pipelines and ensure water quality safety.

GAC to BAC: Does it make chloraminated drinking water safer?

Biological activated carbon (BAC) is widely used as a polishing step at full-scale drinking water plants to remove taste and odor compounds and assimilable organic carbon. BAC, especially with pre-ozonation, has been previously studied to control regulated disinfection by-products (DBPs) and DBP precursors. However, most previous studies only include regulated or a limited number of unregulated DBPs. This study explored two full-scale drinking water plants that use pre-chloramination followed by BAC and chloramine as the final disinfectant. While chloramine generally produces lower concentrations of regulated DBPs, it may form increased levels of unregulated nitrogenous and iodinated DBPs. We evaluated 71 DBPs from ten DBP classes including haloacetonitriles, haloacetamides, halonitromethanes, haloacetaldehydes, haloketones, iodinated acetic acids, iodinated trihalomethanes, nitrosamines, trihalomethanes, and haloacetic acids, along with speciated total organic halogen (total organic chlorine, bromine and iodine) across six different BAC filters of increasing age. Most preformed DBPs were well removed by BAC with different ages (i.e., operation times). However, some preformed DBPs were poorly removed or increased following treatment with BAC, including chloroacetaldehyde, dichloronitromethane, bromodichloronitromethane, N-nitrosodimethylamine, dibromochloromethane, tribromomethane, dibromochloroacetic acid, and tribromoacetic acid. Some compounds, including dibromoacetaldehyde, bromochloroacetamide, and dibromoacetamide, were formed only after treatment with BAC. Total organic halogen removal was variable in both plants and increases in TOCl or TOI were observable on one occasion at each plant. While calculated genotoxicity decreased in all filters, decreases in overall DBP formation did not correlate with decreases in calculated cytotoxicity. In three of the six filters, calculated toxicity increased by 4-27%. These results highlight that DBP concentration alone may not always provide an adequate basis for risk assessment.

Analyzing the performance of biological versus conventional drinking water filtration under warm and cold water conditions: A pilot scale study

Conventional filtration practices typically operate with a pre-chlorination step for the disinfection of harmful pathogens. Without pre-chlorination, biofilters will develop, which are capable of reducing the formation of disinfection by-products (DBPs) and can decrease bacterial regrowth in the distribution system. However, concerns, particularly in North America, still exist with the implementation of biofiltration and there is a lack of side-by-side comparisons with pre-chlorinated filters in literature. A pilot scale study comparing conventional and biological dual media anthracite/sand filters was conducted to assess their performance under warm (15-25 °C) and cold (0-5 °C) water conditions. The filters were operated under various backwash conditions, including the addition of air scour and extended terminal subfluidization wash (ETSW). The biofilter effluent exhibited significantly lower (p < 0.05) DBP formations under both temperature conditions, with trihalomethanes (TTHM) and haloacetic acids (HAA9) concentrations 33-35% and 36-46% lower than the conventional filters, respectively. There was no significant difference when it came to particle passage or turbidity during ripening for both filter types, however the conventional filter proved to be more vulnerable to changes in terms of particles between backwashes. The biofilter exhibited greater average headloss development by 8.8 cm and 4.5 cm under warm and cold water conditions, respectively. Biofilter headloss was found to be minimized by 18% when applying air scour and ETSW during backwash compared to water only, under warm temperature conditions.

Partial nitritation at elevated loading rates: design curves and biofilm characteristics

There is a need to develop low operational intensity, cost-effective, and small-footprint systems to treat wastewater. Partial nitritation has been studied using a variety of control strategies, however, a gap in passive operation is evident. This research investigates the use of elevated loading rates as a strategy for achieving low operational intensity partial nitritation in a moving bed biofilm reactor (MBBR) system. The effects of loading rates on nitrification kinetics and biofilm characteristics were determined at elevated, steady dissolved oxygen concentrations between 5.5 and 7.0 mg O₂/L and ambient temperatures between 19 and 21 °C. Four elevated loading rates (3, 4, 5 and 6.5 g NH₄⁺-N/m² days) were tested with a distinct shift in kinetics being observed towards nitritation at elevated loadings. Complete partial nitritation (100% nitrite production) was achieved at 6.5 g NH₄⁺-N/m² days, likely due to thick biofilm (572 µm) and elevated NH₄⁺-N load, which resulted in suppression of nitrite oxidation.

Pilot-scale comparison of cyclically and continuously operated drinking water biofilters: Evaluation of biomass, biological activity and treated water quality

The objective of this pilot study was to evaluate the impact of cyclical (operated 8-12 h per day) and continuous biofilter operation with respect to biomass development, biological enzyme activity and treated water quality (in terms of organics, nutrients and disinfection by-product (DBP) formation potential). Continuously operated biofilters developed greater densities of biomass, as measured by ATP, when compared to cyclically operated filters; reducing the empty bed contact time (EBCT) increased biomass density under continuous flow conditions. However, once normalized to biomass, it was shown that cyclically operated filters exhibited higher enzyme activity, indicating that this method of operation may improve bacterial function. Reduction of organics was generally similar for both continuous and cyclical filters with the same EBCT, however, cyclical filters demonstrated higher variability during the first 4 h following start-up. Overall, HAA formation potential was better controlled by continuously operated filters, due to poor performance by the cyclical filters upon start-up while THM precursors were removed equally well by all filters. To understand the removal capacity for NDMA precursors through biological filters, both naturally occurring NDMA FP and NDMA FP resulting from spiked anthropogenic precursors was monitored through the filter depth. All the filters removed 90% of the naturally occurring NDMA FP within the first 45 cm; cyclical operation resulted in higher reduction of spiked anthropogenic NDMA precursors (50% higher than continuously operated) demonstrating the advantage of routine shut down on overall microbial activity. Tools to monitor and predict biofilter performance are in high demand. Here we present an "effective activity" term which combines enzyme activity with contact time (EBCT). Effective esterase activity was strongly correlated to DOC reduction as a function of filter operation (cyclical or continuous) and EBCT; effective phosphatase activity was indicative of phosphate removal. The results of this study indicate that routine shut down of the filters as this location improved enzyme activity without compromising control of chlorinated DBPs (THMs and HAAs) or NDMA derived from natural and anthropogenic precursors.

Comparative assessment of ceramic media for drinking water biofiltration

Media type is a critical design consideration when implementing biofiltration for drinking water treatment. Granular activated carbon (GAC) has been shown to provide superior performance when compared to a wide range of media types, largely due to its higher surface area. Engineered ceramic media is an attractive alternative to GAC as it has a similar surface area but at a lower cost. This pilot-scale biofiltration study compared the performance of GAC, anthracite and two different effective sizes of ceramic (CER) media (1.0 mm and 1.2 mm), in terms of dissolved organic carbon (DOC), head loss, turbidity, and disinfection by-product formation potential (DBPFP). Biological acclimation was monitored using adenosine tri-phosphate (ATP) measurements; biomass was further examined using laccase and esterase enzyme activity assays. When compared to other media types examined, biological GAC had higher (p > 0.05) removals of DOC (9.8 ± 3.8%), trihalomethane formation potential (THMFP, 26.3 ± 10.2%), and haloacetic acid formation potential (HAAFP, 27.2 ± 14.0%). CER media required 6-7 months to biologically acclimate, while filters containing GAC and anthracite were biologically active (>100 ng of ATP/g media) following 30-45 days of operation. Once acclimated, ATP values of 243 and 208 ng/g attained for CER 1.0 and 1.2, respectively, were statistically comparable to GAC (244 ng/g) and higher than anthracite (110 ng/g), however this did not translate into greater organics removal. Esterase and laccase enzyme kinetics were highest for GAC, while CER was shown to have greater biodegradation potential than anthracite. The four media types attained similar turbidity reduction (p > 0.05), however ceramic media filters were observed to have run times which were 1.5-2.3 times longer when compared to anthracite, which could represent potential cost savings in terms of energy for pumping and backwash requirements. Overall, ceramic media was shown to be a potential alternative to anthracite when considering biofiltration, especially during cold water conditions (T < 10 °C).

The control of disinfection byproducts and their precursors in biologically active filtration processes

While disinfection provides hygienically safe drinking water, the disinfectants react with inorganic or organic precursors, leading to the formation of harmful disinfection byproducts (DBPs). Biological filtration is a process in which an otherwise conventional granular filter is designed to remove not only fine particulates but also dissolved organic matters (e.g., DBP precursors) through microbially mediated degradation. Recently, applications of biofiltration in drinking water treatment have increased significantly. This review summarizes the effectiveness of biofiltration in removing DBPs and their precursors and identifies potential factors in biofilters that may control the removal or contribute to formation of DBP and their precursors during drinking water treatment. Biofiltration can remove a fraction of the precursors of halogenated DBPs (trihalomethanes, haloacetic acids, haloketones, haloaldehydes, haloacetonitriles, haloacetamides, and halonitromethanes), while also demonstrating capability in removing bromate and halogenated DBPs, except for trihalomethanes. However, the effectiveness of biofiltration mediated removal of nitrosamine and its precursors appears to be variable. An increase in nitrosamine precursors after biofiltration was ascribed to the biomass sloughing off from media or direct nitrosamine formation in the biofilter under certain denitrifying conditions. Operating parameters, such as pre-ozonation, media type, empty bed contact time, backwashing, temperature, and nutrient addition may be optimized to control the regulated DBPs in the biofilter effluent while minimizing the formation of unregulated emerging DBPs. While summarizing the state of knowledge of biofiltration mediated control of DBPs, this review also identifies several knowledge gaps to highlight future research topics of interest.

Potential risk and control strategy of biofilm pretreatment process treating raw water

An enhanced lab-scale biofilm pretreatment process treating raw water obtained from eutrophicated water bodies was established and started up with a novel strategy of low-level nutrients addition and effluent recirculation. Results showed that the startup strategy was useful for biofilm formation and pollutants removal, but it had the risks of increasing substrate affinity constant (Ks) and biofilm decay in treating raw water. Fortunately, the increased Ks value did not affected the NH4(+)-N removal performance via keeping the NH4(+)-N loading rate larger than 6.29 mg L(-1)d(-1). In addition, lower hydraulic retention time (HRT) favored the removal of organic matters, and the maximum TOC removal rate of 76.5 mg L(-1)d(-1) were achieved at HRT of 2h. After long-term acclimatization at oligotrophic niche, the decrease of Ks value and increase of biomass, extracellular polymeric substances, bioactivity were achieved. Finally, the stable operation of biofilm pretreatment process was realized in treating polluted raw water.