Changes in Escherichia coli to enteric protozoa ratios in rivers: Implications for risk-based assessment of drinking water treatment requirements

Highlights

• Parametric models were developed to assess source water E. coli to protozoa ratios.

• Ratios were 1.0 to 2.0-log lower in small rural rivers than in large urban rivers.

• Low E. coli concentrations in small rivers during winter drove these differences.

• There were no proportionalities between mean E. coli and mean protozoa concentrations.

• An E. coli trigger level would have limited value for defining treatment requirements.

Abstract

Minimum treatment requirements are set in response to established or anticipated levels of enteric pathogens in the source water of drinking water treatment plants (DWTPs). For surface water, contamination can be determined directly by monitoring reference pathogens or indirectly by measuring fecal indicators such as Escherichia coli (E. coli). In the latter case, a quantitative interpretation of E. coli for estimating reference pathogen concentrations could be used to define treatment requirements. This study presents the statistical analysis of paired E. coli and reference protozoa (Cryptosporidium, Giardia) data collected monthly for two years in source water from 27 DWTPs supplied by rivers in Canada. E. coli/Cryptosporidium and E. coli/Giardia ratios in source water were modeled as the ratio of two correlated lognormal variables. To evaluate the potential of E. coli for defining protozoa treatment requirements, risk-based critical mean protozoa concentrations in source water were determined with a reverse quantitative microbial risk assessment (QMRA) model. Model assumptions were selected to be consistent with the World Health Organization (WHO) Guidelines for drinking-water quality. The sensitivity of mean E. coli concentration trigger levels to identify these critical concentrations in source water was then evaluated. Results showed no proportionalities between the log of mean E. coli concentrations and the log of mean protozoa concentrations. E. coli/protozoa ratios at DWTPs supplied by small rivers in agricultural and forested areas were typically 1.0 to 2.0-log lower than at DWTPs supplied by large rivers in urban areas. The seasonal variations analysis revealed that these differences were related to low mean E. coli concentrations during winter in small rivers. To achieve the WHO target of 10⁻⁶ disability-adjusted life year (DALY) per person per year, a minimum reduction of 4.0-log of Cryptosporidium would be required for 20 DWTPs, and a minimum reduction of 4.0-log of Giardia would be needed for all DWTPs. A mean E. coli trigger level of 50 CFU 100 mL⁻¹ would be a sensitive threshold to identify critical mean concentrations for Cryptosporidium but not for Giardia. Treatment requirements higher than 3.0-log would be needed at DWTPs with mean E. coli concentrations as low as 30 CFU 100 mL⁻¹ for Cryptosporidium and 3 CFU 100 mL⁻¹ for Giardia. Therefore, an E. coli trigger level would have limited value for defining health-based treatment requirements for protozoa at DWTPs supplied by small rivers in rural areas.

Introduction

Concentrations of reference enteric protozoa (Cryptosporidium, Giardia) in source water must be adequately estimated to define health-based protozoa treatment requirements for drinking water safety (WHO 2016). However, data are not always available because of the difficulty and cost associated with analyzing waterborne protozoa in environmental samples. In these situations, fecal indicator bacteria (FIB) such as Escherichia coli (E. coli) are commonly used as indicators of pathogen occurrence. Although FIB monitoring data sets can provide important information on fluctuations of fecal contamination in source water, it is essential to keep in mind that this indicator has limitations for predicting concentrations of enteric protozoa (Wu et al. 2011, Health Canada 2017). Meteorological and environmental factors can have different effects on the fate and transport of indicators and pathogens in water. Moreover, indicators can originate from other sources than pathogens.

Monitoring of protozoa in raw water from drinking water treatment plants (DWTPs) is recommended in Canada (Health Canada 2017). Still, protozoa monitoring is not mandatory in most Canadian provinces (Government of Manitoba 2007, Gouvernement du Québec 2012). In the United States and Alberta, Canada, protozoa monitoring is mandatory for large community water supplies and small community water supplies when E. coli concentrations are low (USEPA 2010, Government of Alberta 2012). Most of these regulations rely on the assumption that drinking water sources exposed to high E. coli concentrations have a greater probability of protozoa occurrence, independently of their concentration (Payment and Locas 2011). However, a quantitative relationship between concentrations of E. coli and protozoa needs to be established to define health-based minimum treatment requirements for protozoa using quantitative microbial risk assessment (QMRA).

To determine whether E. coli data can be used to support the implementation of health-based treatment requirements, E. coli to protozoa ratios can be evaluated over a given period at multiple DWTPs supplied by different types of drinking water sources. A meta-analysis of E. coli/Cryptosporidium ratios in primary sources of fecal contamination suggested that E. coli is generally a good indicator for predicting Cryptosporidium occurrence for urban pollution sources (raw and treated wastewater) but not for agricultural runoff (Lalancette et al. 2014). In this study, E. coli/Cryptosporidium ratios were also evaluated at drinking water intakes from 13 DWTPs in Quebec, Canada, using data from Payment et al. (2000). As estimated in their meta-analysis, ratios at drinking water intakes were lower for sources influenced by agricultural runoff than those influenced by municipal sewage. The present study was designed to validate these findings using recent data collected at 27 surface DWTPs supplied by rivers dominated by urban, agricultural, or wildlife sources of fecal pollution in Quebec, Canada. The mathematical model of Lalancette et al. (2014) was also extended to evaluate the uncertainty associated with arithmetic mean E. coli/Cryptosporidium and E. coli/Giardia ratios.

The objectives of this study were (i) to derive the arithmetic mean ratio of two correlated lognormal distributions and use this model to characterize E. coli/Cryptosporidium and E. coli/Giardia ratios in source water for 27 DWTPs supplied by rivers; (ii) to investigate the influence of river types and seasons on the variation in the magnitude of the mean ratios; and (iii) to evaluate whether E. coli trigger levels provide valuable information for defining health-based treatment requirements for pathogenic protozoa at these DWTPs.