Implications of detachment promoting agents, disinfectants and flow hydraulics on growth and dispersion of bacterial biofilms in drinking water distribution systems

Abstract

Biofilms in water distribution systems (WDS) can adversely affect the effluent water quality and structural integrity of pipes. Current disinfectants and flushing strategies often prove insufficient to mitigate biofilm growth. Long-term monitoring of biofilm growth under varied WDS flow and disinfection conditions is needed (and often overlooked) for improving biofilm management in the WDS. An alternative approach to counter biofilms in the WDS over the traditional approach of using antimicrobials is to target and weaken the extracellular polymeric substances (EPS) instead of just the microorganisms in the biofilms - via the use of 'detachment promoting agents' (DPAs) to weaken the EPS matrix and cause biofilm dispersion. The growth of fresh biofilm and reattachment of biofilm flocs were investigated by time-lapse imaging in capillary flow cells under varying temperatures, fluid shear, and chlorine dosage. Biofilm disruption efficacy of potential DPAs was investigated for single and multi-species biofilms over varying durations (72 h - 4 months) and on different surfaces (e.g., glass, copper, stainless steel, cement, HDPE). The similar biofilm growth rates with non-chlorinated and 0.2 mg/L chlorine indicate the ineffectiveness of low chlorine dosages for biofilm inhibition. Substantial biofilm growth occurred despite using higher chlorine dosages (0.2-1 mg/L), demonstrating the disinfection resistance of microorganisms in biofilm and detached clusters. The enhanced initial biofilm growth kinetics in the reattachment systems compared to the fresh flow cell reactors indicates the significant role of existing biofilms (and likely their detached clusters) to stimulate biofilm growth in a newer WDS section. The lower reattachment rate at the early stage with hot water (50°C) increased over time likely due to interaction with a high residual chlorine dosage. Multi-species biofilms grown for both long and short duration showed higher resistance to DPA treatment. The maximum dispersal of single-species biofilms was achieved using the DPA agent EDTA (45 ± 32%). The highest observed protein in the effluent with EDTA treatment indicates the effectiveness of DPAs for weakening EPS. Sodium tripolyphosphate (STP) reduced 75% of biomass and half of biofilm thickness on HDPE coupons. However, the formation of disinfection by-products by STP treatment (0.32-0.67 mg/L bromoform) raises regulatory concerns for DBPs. The use of DPAs as anti-EPS agents for biofilm dispersal, rather than the traditional antimicrobial approach yielded promising results for different plumbing materials. Overall, the results from this study demonstrate the potential of non-invasive imaging-based research in understanding the growth mechanisms of biofilm in various flow conditions, highlighting the widespread application of online water monitoring systems. Overall, this research offers new insights and perspectives on the impact of fluid shear and disinfectants on biofilm growth and reattachment, as well as the implications of these processes for water safety and public health.