Comparing peracetic acid and hypochlorite for disinfection of combined sewer overflows: Effects of suspended-solids and pH
Graphical abstract
Introduction
Combined sewer overflows (CSOs) are a common problem in the United States and throughout the world (NRDC, 2011). In the United States, the EPA has made CSO abatement a priority (USEPA, 1999, USEPA, 1994). However, complete elimination of CSOs is often economically prohibitive. Regulations increasingly require disinfection of the occasional CSO discharges, which contains high total suspended solids (TSS) and potentially has large particle sizes. The ideal disinfectant would achieve at least 4-log reduction (99.99% removal) of bacterial concentrations with contact times of 15 to 30 min, and avoid the formation of toxic by-products (Coyle et al., 2014).
A popular option for CSOs is chlorine-based disinfectants, which are effective against bacteria, viruses, and protozoan cysts, and are commonly used for disinfection of wastewater final effluents (De Luca et al., 2008, Lazarova et al., 1998, Veschetti et al., 2003). However, chlorine derivatives often have high rates of decay during storage, are sensitive to changes in pH, can form toxic disinfection by-products upon reaction with organics, and require quenching to remove residual disinfectants prior to discharge. These characteristics make them less desirable for CSOs, which are intermittent and more variable in volume and composition. Also, CSO discharges often include higher amounts of suspended solids, which may shelter pathogens and impact the chlorine disinfection efficiency (Rossi et al., 2007).
Peracetic acid (PAA), a non-conventional disinfectant, has shown potential as an environmentally-friendly and cost-effective alternative for CSO disinfection (Chhetri et al., 2014, Coyle et al., 2014, Kitis, 2004, Luukkonen and Pehkonen, 2017). PAA is a relatively strong oxidant, and is highly effective for disinfection of bacteria; it has also demonstrated some antiviral properties (Baldry and French, 1989a, Baldry et al., 1991, Koivunen and Heinonen-Tanski, 2005).
PAA is commercially available in a stable, quaternary equilibrium mixture of acetic acid, hydrogen peroxide, peracetic acid, and water (Kitis, 2004):
As a weak acid, the dissociation constant, pKa, of PAA is 8.2 (Coyle et al., 2014, Santoro et al., 2005). When the pH is below 8.2, PAA is thought to spontaneously decompose into reactive oxygen, which can disrupt sulfhydryl (SH) and sulfur (SS) bonds within enzymes and proteins in cell membranes. As a result, transport across the cell membrane is hindered and cellular function is impaired (Gehr et al., 2003, Lefevre et al., 1992). Other oxidizing agents, including chlorine derivatives, have disinfection mechanisms similar to those proposed by Lefevre, involving disruption of cell membranes by reactive oxygen (Kitis, 2004). However, the mechanism of disinfection by PAA remains uncertain, with some studies suggesting that PAA releases hydroxyl radicals, which readily oxidize proteins and lipids to compromise bacterial cell walls and to denature cellular DNA (Coyle et al., 2014, Koivunen and Heinonen-Tanski, 2005, Lubello et al., 2002).
A major advantage of PAA over chlorine-based disinfectants is that it reacts and decomposes quickly, possibly eliminating the need for quenching prior to discharge and potentially requiring shorter contact times (Coyle et al., 2014, De Luca et al., 2008, Kitis, 2004). It is believed to produce little to no toxic by-products upon reaction with wastewater or natural organic matter (Baldry and Fraser, 1988, Monarca et al., 2002).
PAA has been demonstrated as an advantageous alternative for treating effluent wastewaters with low doses and short contact times, and could feasibly be used to treat CSOs (Baldry et al., 1995, De Luca et al., 2008, Rossi et al., 2007, Wagner et al., 2002). However, past PAA research has mainly addressed the disinfection of pure cultures of bacteria, or wastewater effluents with few or very small particles.
Several bench-scale studies have addressed PAA disinfection of wastewater, but researchers have found that the effective PAA dosage and contact times vary widely based on water quality (Baldry and French, 1989b, Coyle et al., 2014, Sanchez-Ruiz et al., 1995). Also, a major unknown is the effect of wastewater particle quantity and size on PAA disinfection efficiency (Falsanisi et al., 2008). While it has been demonstrated that high disinfection efficiency can be achieved with PAA in wastewater containing TSS up to 100 mg/L, suspended solids in CSOs can range from 10 to over 1000 mg/L (Lefevre et al., 1992, USEPA, 1999). The particle size of solids in CSOs, ranging from a few microns to hundreds of microns, may allow larger solids to shelter pathogens from disinfection. Also, disinfectants may react with organics in the solids, limiting the available biocide concentration (Coyle et al., 2014).
Previous work on PAA disinfection has shown that disinfection efficiency decreases with increasing TSS (Kitis, 2004). Studies have also looked into bacterial sheltering and tailing phenomena produced by suspended solids during UV and chlorine disinfection (Liang et al., 2013, Yong et al., 2009). Tailing occurs when a group of microorganisms survives disinfection and the population stabilizes with extended disinfection time (Liang et al., 2013). Despite previous studies into the TSS effect, little has been done to correlate suspended solids sizes with disinfection efficiency and to study the direct effect of disinfection on particles using microscopy (Falsanisi et al., 2008). The solids present in CSOs may provide a diffusive barrier that shelters bacteria from disinfection.
The objective of this study was to determine the effect of suspended solids on PAA disinfection efficiency, and to compare PAA disinfection to disinfection by free chlorine as hypochlorite, a conventional chlorine-based disinfectant. Tests were carried out on a pure culture of E. coli and on wastewater solids.
Section snippets
Disinfection of E. coli
Initial disinfection tests were performed on suspended E. coli cultures in a synthetic medium. E. coli were grown to exponential phase on 2% Luria Bertani broth (LB) in batch culture. Cells were centrifuged for 5 min at 6,700 g, washed, and re-suspended with physiological saline solution (0.85% NaCl in deionized water) to a cell count of approximately 108 to 109 CFU/mL. A 1 mL aliquot was inoculated into tubes containing 9 mL of PAA in saline solution, with concentrations ranging from 0.5 ppm to 2.0
Disinfection kinetics of E. coli
To demonstrate the relative susceptibility of the pure E. coli culture to disinfection by PAA or hypochlorite, inactivation curves were plotted as the log of the fraction of culturable bacteria (log (N/N0)) versus Ct (disinfectant concentration × contact time) (Fig. 1). The Ct (mg-min L− 1) was changed by varying the disinfectant concentration for the same 10-min contact time. Results indicated that, after a given “lag Ct” of inactivity, E. coli was susceptible to rapid disinfection by both PAA and
Conclusions
This study provides a direct comparison of two disinfectants, PAA and hypochlorite, on CSO-like wastewater and for a water matrix containing a pure E. coli culture. While hypochlorite reached a higher maximum inactivation rate for the pure E. coli culture, hypochlorite required much higher CT values than PAA before effective disinfection could begin.
Inactivation of CSO-like water containing suspended solids was studied at pH values of 6.5, 7.5, and 8.5. In these tests, disinfection by PAA was
Acknowledgements
Funding was provided by the Ganey Foundation for Community-Based Research and the Clare Boothe Luce Research Fellowship. The FTIR analyses were conducted at the Center for Environmental Science and Technology (CEST) at University of Notre Dame.
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