Disinfection of Viral Indicators and Treatability Implications for WRRFs

Last Modified: Sep 04, 2018


  • Thomas Worley-Morse, Melanie Mann, Wendell Khunjar - Hazen and Sawyer
  • Raul Gonzalez - Hampton Roads Sanitation District

The United States Environmental Protection Agency (USEPA) is conducting the 5-year review of the 2012 Recreational Water Quality Criteria (RWQC), and the USEPA is planning to develop RWQC for coliphages, as class of bacteriophages that infect Escherichia coli. Motivations for coliphage RWQC include the use of an indicator that better mimics the fate and transport of enteric viruses through wastewater treatment plants, as the fate of enteric viruses and bacterial indicators is different in wastewater treatment (Rose, et al., 2004). Additionally, the upcoming coliphage RWQC is being designed to protect users of recreational water (Wiedenmann, et al. 2006).

Secondary effluent samples were collected from Facilities A and B (Utility A). Facility A is a biological nutrient removal (BNR) wastewater treatment plant with integrated fixed-film activated sludge reactor (typical effluent ammonia concentration is less than 0.1 mg/L). Facility B is a conventional three cell activated sludge treatment with a swing anoxic/aerobic zone. Both facilities A and B use chlorination and dechlorination for disinfection.

Samples were stored at 4 degrees C until jar testing was conducted. Pre-disinfected samples were collected before adding the initial dose for quantifying the initial concentration. Jar testing for sodium hypochlorite (free and combined chlorine) was conducted at the central laboratory for Utility A. Jar testing for UV was performed at Trojan’s and Wedeco’s laboratories (Bolton and Linden, 2003). Jar testing for PAA was performed at USP’s laboratory. And ozone was performed at Wedeco’s Laboratories. Coliphages were enumerated using USEPA method 1602, a single-layer agar procedure that evaluates plaque formation on a host lawn of E. coli specific to the targeted type of coliphage, male specific or somatic (USEPA 1602). E. coli was enumerated with the Modified mTEC method, USEPA 1603; enterococci was enumerated with the mEI agar method, USEPA 1600. Serial dilutions were used to obtain the proper concentration for coliphage plating, and all bacterial and coliphage data was reported in CFU or PFU/100 mL.

Indicator reduction with sodium hypochlorite (combined chlorine) in the presence of ammonia varied. E. coli was the most susceptible followed by enterococci, somatic coliphages, and male specific coliphages. This suggests that facilities with chlorine disinfection and significant ammonia in their effluents will likely see minimal reductions of somatic and male specific coliphages. For example, with the male specific coliphages a chloramine CT of greater than 600 mg-min/L provided less than 1 log of reduction, and for the somatic coliphages a chloramine CT of greater than 600 mg-min/L provided ~ 1.5 logs of reduction.

Indicator reduction for sodium hypochlorite (with free chlorine or breakpoint chlorination) was rapid with CTs of less than 5 mg-min/L reducing all indicators to concentrations less than 10 CFU or PFU/100 mL (data not shown). Indicator log reductions ranged from 2 to 3 logs; however, it is likely that higher log reductions would have been reportable if the concentration of indicators in the secondary effluent of this facility were higher.

Indicator reduction for UV was similar to previously reported results with somatic coliphage being more susceptible to UV than male specific coliphages (Hijnen, et al. 2006). A difference from previously reported results was that the indigenous coliphages exhibited tailing, whereas the typical coliphage used for UV reactor validation, T1UV and MS2, have more linear inactivation curves.

Indicator reduction with ozone was tested from with CTs from 0 to 4 mg-min/L. E. coli, somatic coliphage, and male specific coliphages all exhibited rapid inactivation (~3.5 logs at 1 mg-min/L); however, enterococci was more resistant to ozone achieving a total log reduction of 2 after 4 mg-min/L (data not shown).

PAA disinfection for the viral indicators was similar to UV in that PAA had faster disinfection kinetics with somatic coliphages than male specific coliphages. However, both somatic and male specific coliphages exhibited significant tailing with male specific coliphages having ~ 1 log of reduction at 200 mg-min/L and somatic coliphages having 2.5 logs of reduction at 200 mg-min/L (data not shown).

These results suggest viral and bacterial inactivation kinetics are not consistent from indicator to indicator. Furthermore, because most facilities have been designed to remove bacterial indicators and not viruses, potential coliphage RWQC may require that facilities shift not only their disinfectants but also their secondary processes if low levels of both bacterial and viral indicators are required.

For more information, please contact the author at twmorse@hazenandsawyer.com.

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