Summary of key findings
Direct disinfection of established Pseudomonas aeruginosa biofilms by electrochemically activated solutions [ECAS] exhibited significantly greater anti-biofilm activity compared to NaOCl and HOCl at free chlorine concentrations ≥ 50 mg L-1. When disinfectants were dosed in-situ at a free chlorine concentration of 50 mg L-1 HOCl was the only disinfectant where no environmental biofilms were recovered after 48 hours. However, in the presence of NaOCl or ECAS low density environmental biofilms were recovered after 48 hours.
Background and relevance
Conventional chlorination (sodium hypochlorite (NaOCl)) is effective in centralised drinking water treatment/distribution at maintaining biological drinking water quality through residual chlorine. However, decentralised drinking water treatment systems do not necessarily require residual disinfection as many systems are designed for point-of-use, negating the need for distribution networks. Decentralised water treatment systems require reliable disinfectants that can effectively reduce microbial loads in water, ensuring high quality, biologically safe drinking water for communities. Through the reduction of microbial load in waters, this will reduce the formation potential of biofilms on drinking water treatment infrastructure (e.g. pipework and filtration membranes).
This study investigated the comparative efficacy of NaOCl, HOCl and ECAS at reducing the density of established Pseudomonas aeruginosa biofilms (48 hours) of through direct disinfection. This study also investigated whether NaOCl, HOCl and ECAS manage environmental bacterial biofilm formation on polyethersulfone [PES] coupons in a modified in-situ dosing biofilm reactor model.
ECAS exhibited significantly higher antimicrobial activity against P. aeruginosa biofilms, grown on polycarbonate [PC] coupons at free chlorine concentrations ≥ 50 mg L-1, compared to NaOCl and HOCl (Figure 1.1). When using a multi-species environmental water source, biofilms formed on polyethersulfone [PES] coupons after 24 and 48 hours (no disinfectant dosing). All disinfectants (NaOCl, HOCl and ECAS) resulted in significantly reduced biofilm density in comparison to the control (p < 0.05), see Figure 1.2. Environmental biofilms were recovered from PES coupons in the presence of ECAS and NaOCl after 48 hours, whilst no environmental biofilm was recovered in the presence of HOCl.
The greater anti-biofilm activity of ECAS against established biofilms is thought to be a result of
a high oxidation reduction potential (relative to HOCl and OCl-), indicative of its increased oxidation ability and metastable antimicrobial species (Gil et al., 2015; Martínez-Huitle et al., 2008). This disrupts the protective EPS barrier, composed of a complex mixture of polysaccharides, proteins and DNA (Flemming and Wingender, 2010), through the destruction of covalent bonds within DNA, RNA and proteins (Cloete et al., 2009; Zinkevich et al., 2000). In addition, it is postulated that the active chlorine species within ECAS, predominantly HOCl, have no charge and therefore can penetrate through the EPS and react with the embedded bacterial cells (Fukuzaki, 2006). However, HOCl was the most efficacious when the disinfectants were dosed at a free chlorine concentration of 50 mg L-1 into environmental source water. The reactive nature of NaOCl (OCl-) and ECAS in environments with high organic concentrations results in reduced antimicrobial efficacy, as reaction with organic material inhibits reactions with microorganisms. HOCl is known to be more efficacious in environments with high organic loads, as free chlorine is released at a slower rate when associated with chlorinated isocyanurates (Bloomfield and Uso, 1985).
Clayton, G., Thorn, R., & Reynolds, D. (in press). Biofilm management; Investigating the inhibitory effects of chlorine-based disinfectants on biofilms