Faecal contamination of greywater and associated microbial risks

Abstract

The faecal contamination of greywater in a local treatment system at Vibyåsen, north of Stockholm, Sweden was quantified using faecal indicator bacteria and chemical biomarkers. Bacterial indicator densities overestimated the faecal load by 100–1000-fold when compared to chemical biomarkers. Based on measured levels of coprostanol, the faecal load was estimated to be 0.04 g person⁻¹ day⁻¹. Prevalence of pathogens in the population and the faecal load were used to form the basis of a screening-level quantitative microbial risk assessment (QMRA) that was undertaken for rotavirus, Salmonella typhimurium, Campylobacter jejuni, Giardia lamblia and Cryptosporidium parvum. The different exposure scenarios simulated—direct contact, irrigation of sport fields and groundwater recharge—gave unacceptably high rotavirus risks (0.04<P_inf<0.60) despite a low faecal load. The poor reduction of somatic coliphages, which were used as a virus model, in the treatment was one main reason and additional treatment of the greywater is suggested. Somatic coliphages can under extreme circumstances replicate in the wastewater treatment system and thereby underestimate the virus reduction. An alternative QMRA method based on faecal enterococci densities estimated similar risks as for rotavirus. Growth conditions for Salmonella in greywater sediments were also investigated and risk modelling based on replication in the system increased the probability of infection from Salmonella 1000-fold, but it was still lower than the risk of a rotavirus infection.

Introduction

The interest in the separation and reuse of different wastewater fractions (i.e. storm-, grey- and blackwater) has increased in recent years, largely due to economical, structural and ecological considerations [1], [2]. Greywater, here defined as wastewater without input from toilets (i.e. wastewater from laundries, showers, bathtubs, hand basins and kitchen sinks) is often extensively treated in the combined systems or separately in spread settlings. The later treatment often consists of a settling tank followed by a soil infiltration system, a sandfilter trench or a subsurface flow wetland providing a 0.7–3 log reduction of thermotolerant coliforms [3]. The high-grade treatment of greywater has been questioned since it constitutes a large fraction of the actual wastewater flow, but has a low degree of faecal contamination [4] and local systems are often ill adapted for reuse. Attention has been paid to the possibility of reusing greywater, especially in arid areas [5], [6], [7]; however, the potential risk with such reuse need to be systematically addressed. Greywater contains many chemicals used in households of which some may lead to soil degradation. Christova-Boal et al. [8] have addressed the risks of a pH-rise and zinc accumulation in the soil as well as excessive phosphorus leakage to groundwater in sandy soils. Furthermore, a risk of increased transmission of hazardous microorganisms may also occur depending on the faecal content and contamination of the greywater.

Since toilet waste is not included in greywater, faecal contamination should be minimal. Some activities, such as washing faecally contaminated laundry (i.e. diapers), childcare and showering may add minor amounts. Occasionally gastro-intestinal bacteria, such as Salmonella and Campylobacter, can be introduced by food-handling in the kitchen [9]. Greywater may have an elevated load of easily degraded organic material, which may favour growth of enteric bacteria such as faecal indicators and such growth in wastewater systems has been reported [10]. Hence, focus on bacterial indicator numbers will lead to an overestimation of faecal loads and thus risk. Several studies have reported high numbers of traditional faecal indicators in greywater, that from “a regulatory point-of-view” would indicate substantial faecal contamination (Table 1).

If greywater should be reused, there is a need to differentiate between the actual faecal loads and potential regrowth of indicators if actual risks should be assessed.

Potential areas of greywater reuse are irrigation of golf courses and parks as well as fertilisation of crops [17], toilet flushing [18] and groundwater recharge [1]. The guidelines based on thermotolerant coliform-counts range from <1 to 10,000 colony forming units (c.f.u.) 100 mL⁻¹ depending on its designated use [19], [20], [21]. These guidelines result in a substantial treatment requirement of the greywater, which does not necessarily reflect the potential risks of faecal contamination.

A risk estimate approach has been discussed for the evaluation of reuse alternatives of wastewater with quantitative microbial risk assessment (QMRA) [22], [23], [24], where it is easier to measure or calculate the potential pathogen load than in greywater. Since indicator bacteria may overestimate the faecal load and thereby the risks of enteric pathogens, chemical biomarkers, such as coprostanol, may be used as an alternative faecal indicator also applicable to other systems [25]. Coprostanol is formed by the intestinal microflora from the cholesterol. Both substances are excreted with the faeces [26]. The risk models however need to take into account the potential regrowth of some pathogenic bacteria such as Salmonella and Campylobacter.

Human health risks are dependent on both the source of the pathogens, the treatment applied and the exposure routes. In the current study, a pond system is included as post treatment where direct human exposure to the water is possible. Since the water may be recirculated for irrigation purposes or used for groundwater recharge, these scenarios have also been considered in an evaluation of the microbial risks. The coliform indicator bacteria probably overestimate the risks due to their potential regrowth within the system. We have therefore chosen to make a comparative validation of the greywater system based on three different approaches: (1) evaluation of faecal load based on coprostanol concentrations and using epidemiological data to estimate the pathogen load, (2) based on faecal enterococci numbers and a dose–response model derived from bathing water exposure [27] and (3) based on faecal enterococci as an index organism for Salmonella. The overall aim of this study is to provide a broad view on microbial risks associated with reuse of source-separated greywater based on a screening level QMRA approach.