Health risks in wastewater irrigation: comparing estimates from quantitative microbial risk analyses and epidemiological studies

The combination of standard quantitative microbial risk analysis (QMRA) techniques and 10,000-trial Monte Carlo risk simulations was used to estimate the human health risks associated with the use of wastewater for unrestricted and restricted crop irrigation. A risk of rotavirus infection of 10(-2) per person per year (pppy) was used as the reference level of acceptable risk. Using the model scenario of involuntary soil ingestion for restricted irrigation, the risk of rotavirus infection is approximately 10(-2) pppy when the wastewater contains < or =10(6) Escherichia coli per 100ml and when local agricultural practices are highly mechanised. For labour-intensive agriculture the risk of rotavirus infection is approximately 10(-2) pppy when the wastewater contains < or = 10(5) E. coli per 100ml; however, the wastewater quality should be < or = 10(4) E. coli per 100ml when children under 15 are exposed. With the model scenario of lettuce consumption for unrestricted irrigation, the use of wastewaters containing < or =10(4) E. coli per 100ml results in a rotavirus infection risk of approximately 10(-2) pppy; however, again based on epidemiological evidence from Mexico, the current WHO guideline level of < or =1,000 E. coli per 100ml should be retained for root crops eaten raw.     

Estimation of Ascaris infection risks in children under 15 from the consumption of wastewater-irrigated carrots

Ascaris lumbricoides, the large human roundworm, infects ~1,200 million people, with children under the age of 15 being particularly at risk. Monte Carlo quantitative microbial risk analyses were undertaken to estimate median Ascaris infection risks in children under 15 from eating raw carrots irrigated with wastewater. For a tolerable additional disease burden of 10^-5 DALY (disability-adjusted life year) loss per person per year (pppy), the tolerable Ascaris infection risk is ~10^-3 pppy, which can be achieved in hyperendemic areas by a 4-log unit Ascaris reduction. This reduction can be easily achieved by wastewater treatment in a 1-day anaerobic pond and 5-day facultative pond (2 log units) and peeling prior to consumption (2 log units).     

Estimation of norovirus infection risks to consumers of wastewater-irrigated food crops eaten raw

A quantitative microbial risk analysis—Monte Carlo method was used to estimate norovirus infection risks to consumers of wastewater-irrigated lettuce. Using the same assumptions as used in the 2006 WHO guidelines for the safe use of wastewater in agriculture, a norovirus reduction of 6 log units was required to achieve a norovirus infection risk of ~10^-3 per person per year (pppy), but for a lower consumption of lettuce (40–48 g per week vs. 350 g per week) the required reduction was 5 log units. If the tolerable additional disease burden is increased from a DALY (disability-adjusted life year) loss of 10^-6 pppy (the value used in the WHO guidelines) to 10^-5 pppy, the required pathogen reduction is one order of magnitude lower. Reductions of 4–6 log units can be achieved by very simple partial treatment (principally settling to achieve a 1-log unit reduction) supplemented by very reliable post-treatment health-protection control measures such as pathogen die-off (1-2 log units), produce washing in cold water (1 log unit) and produce disinfection (3 log units).     

Quantification of the health risk associated with wastewater reuse in Accra, Ghana: a contribution toward local guidelines

Quantitative Microbial Risk Assessment (QMRA) models with 10,000 Monte Carlo simulations were applied to ascertain the risks of rotavirus and Ascaris infections for farmers using different irrigation water qualities and consumers of lettuce irrigated with the different water qualities after allowing post-harvest handling. A tolerable risk (TR) of infection of 7.7 × 10^-4 and 1 × 10^-2 per person per year were used for rotavirus and Ascaris respectively. The risk of Ascaris infection was within a magnitude of 10^-2 for farmers accidentally ingesting drain or stream irrigation water; ~10⁰ for farmers accidentally ingesting farm soil and 10⁰ for farmers ingesting any of the irrigation waters and contaminated soil. There was a very low risk (10^-5) of Ascaris infection for farmers using pipe-water. For consumers, the annual risks of Ascaris and rotavirus infections were 10⁰ and 10^-3 for drain and stream irrigated lettuce respectively with slight increases for rotavirus infections along the post-harvest handling chain. Pipe irrigated lettuce recorded a rotavirus infection of 10^-4 with no changes due to post harvest handling. The assessment identified on-farm soil contamination as the most significant health hazard.     

Estimation of norovirus and Ascaris infection risks to urban farmers in developing countries using wastewater for crop irrigation

A quantitative microbial risk analysis—Monte Carlo method was used to estimate norovirus and Ascaris infection risks to urban farmers in developing countries watering their crops with wastewater. For a tolerable additional disease burden of10^-4 DALY loss per person per year (pppy), equivalent to 1 percent of the diarrhoeal disease burden in developing countries, a norovirus reduction of 1–2 log units and an Ascaris egg reduction to 10–100 eggs per litre are required. These are easily achieved by minimal wastewater treatment—for example, a sequential batch-fed three tank/pond system. Hygiene improvement through education and regular deworming are essential complementary inputs to protect the health of urban farmers.     

Bactericidal activity of Ag nanoparticle-impregnated fibreglass for water disinfection

A new bactericidal system composed of fibreglass impregnated with silver (Ag) nanoparticles was developed and tested. Silver content, particle size and distribution were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The antibacterial effectiveness was evaluated against Escherichia coli (E. coli, ATCC 29055). The minimum inhibitory loading was determined to be less than 1.8 wt% of silver nanoparticles per gram of fibreglass. In a 1 h immersion test, using a 0.1 mg fibreglass mat ml^-1, with 2.9 wt% loading of silver nanoparticles completely disinfected 100 ml of 10⁶ CFU ml^-1 of E. coli, dramatically outperforming activated carbon fibres impregnated with silver. Inactivation rate studies of 0.05 mg fibreglass mat ml^-1 (Ag 1.8 wt%) with 10¹² CFU E. coli displayed a 7 log reduction in 5 minutes. The activation and reuse of fibreglass (Ag 4.3 wt%) maintained its full effectiveness after two cycles of use and thermal regeneration at 350°C.     

Water- and wastewater-related disease and infection risks: what is an appropriate value for the maximum tolerable additional burden of disease?

The maximum additional burden of water- and wastewater-related disease of 10^_6 disability-adjusted life year (DALY) loss per person per year (pppy), used in the WHO Drinking-water Quality Guidelines and the WHO Guidelines for Wastewater Use in Agriculture, is based on US EPA's acceptance of a 70-year lifetime waterborne cancer risk of 10^_5 per person, equivalent to an annual risk of 1.4×10^_7 per person which is four orders of magnitude lower than the actual all-cancer incidence in the USA in 2009 of 1.8×10^_3 pppy. A maximum additional burden of 10^_4 DALY loss pppy would reduce this risk to a more cost-effective, but still low, risk of 1.4×10^_5 pppy. It would increase the DALY loss pppy in low- and middle-income countries due to diarrhoeal diseases from the current level of 0.0119 pppy to 0.0120 pppy, and that due to ascariasis from 0.0026 pppy to 0.0027 pppy, but neither increase is of public-health significance. It is therefore recommended that the maximum additional burden of disease from these activities be increased to a DALY loss of 10^_4 pppy as this provides an adequate margin of public-health safety in relation to waterborne-cancer deaths, diarrhoeal disease and ascariasis in all countries.     

Hepatitis A virus in surface water in South Africa: what are the risks?

The aim of this study was to assess the potential risk of infection constituted by HAV to persons using surface dam and river water for domestic and recreational purposes. It estimates the potential risk using a deterministic exponential risk assessment model with mean values and conservative assumptions. Hepatitis A virus was detected in 17.5% of river and 14.9% of dam water samples tested. The number of indicator organisms in these sources exceeded drinking and recreational water quality guidelines set by the United States Environmental Protection Agency (US EPA), indicating possible health risks to recreational water users. Based on the available data and taking all the assumptions into consideration, the probability of infection (P_inf) to the higher socio-economic population using the river water for recreational purposes was 1.1 × 10^-3 per day and 3.3 × 10^-1 per annum if 100 ml was ingested per day. For recreation in the dam water the P_inf value was 1.2 × 10^-4 per day and 4.2 × 10^-2 per annum. For the lower socio-economic population, risk values for drinking purposes (2 L day^-1) were ten-fold greater. These surface waters therefore did not conform to the US EPA guidelines of 1 infection per 10,000 consumers per year for drinking water or eight gastrointestinal illnesses per 1,000 bathers per day in environmental waters used for recreational purposes. This is the first risk assessment study addressing the risk of infection by HAV in surface water to different socio-economic populations in South Africa.     

Microbiological quality of fish grown in wastewater-fed and non-wastewater-fed fishponds in Hanoi, Vietnam: influence of hygiene practices in local retail markets

Mean water quality in two wastewater-fed ponds and one non-wastewater-fed pond in Hanoi, Vietnam was ~10⁶ and ~10⁴ presumptive thermotolerant coliforms (pThC) per 100 ml, respectively. Fish (common carp, silver carp and Nile tilapia) grown in these ponds were sampled at harvest and in local retail markets. Bacteriological examination of the fish sampled at harvest from both types of pond showed that they were of very good quality (2-3 pThC g^-1 fresh muscle weight), despite the skin and gut contents being very contaminated (10²-10³ pThC g^-1 fresh weight and 10⁴-10⁶ pThC g^-1 fresh weight, respectively). These results indicate that the WHO guideline quality of 1000 faecal coliforms per 100 ml of pond water in wastewater-fed aquaculture is quite restrictive and represents a safety factor of ~3 orders of magnitude. However, when the fish from both types of pond were sampled at the point of retail sale, quality deteriorated to 10²-10⁵ pThC g^-1 of chopped fresh fish (mainly flesh and skin contaminated with gut contents); this was due to the practice of the local fishmongers in descaling and chopping up the fish from both types of pond with the same knife and on the same chopping block. Fishmonger education is required to improve their hygienic practices; this should be followed by regular hygiene inspections.     

Inactivation of Nitrosomonas europaea and pathogenic Escherichia coli by chlorine and monochloramine

The purpose of this study was to measure the chlorine and monochloramine inactivation kinetics of Nitrosomonas europaea at 21°C in the presence and absence of particles. The inactivation kinetics rates were compared with those obtained with Escherichia coli O157:H7. The results show that, in pure water, the use of free chlorine produced 4 log₁₀ of N. europaea inactivation at a CT value of 0.8 mg.min l^-1, whereas monochloramine yielded 4 log₁₀ of inactivation at CT values of approximately 9.9–16.4 mg.min l^-1. With E. coli, chlorine produced approximately 4 log₁₀ of inactivation at a CT of 0.13 mg.min l^-1, whereas monochloramine resulted in 4 log₁₀ of inactivation at a CT of approximately 9.2 mg.min l^-1. These results suggest that N. europaea is more resistant to monochloramine and chlorine than E. coli. Corrosion debris, soil material and wastewater had no statistically significant (p < 0.05) impact on the inactivation of N. europaea by either chlorine or monochloramine. It seems likely that the CT values present in distribution systems would be sufficient to control suspended cells of these two organisms, especially under conditions of breakpoint chlorination, which could be used to control nitrification. Adequate disinfection should prevent the growth of these organisms in a distribution system.     

An evaluation of the mobility of pathogen indicators, Escherichia coli and bacteriophage MS-2, in a highly weathered tropical soil under unsaturated conditions

Laboratory column experiments were conducted to study the effects of anionic polyacrylamide (PAM) polymer and surfactant linear alkylbenzene sulfonate (LAS) on the movement of Escherichia coli and the FRNA phage MS-2. The study was designed to evaluate if PAM or PAM + LAS would enhance the mobility of human pathogens in tropical soils under unsaturated conditions. No breakthrough of phage was observed in a 10 cm column after passing 100 pore volumes of solution containing 1×10⁸ plaque-forming units (PFU)/ml. In later experiments, after passing 10–20 pore volumes of influent containing 1×10⁸/ml MS-2 or E. coli through 15 cm columns, the soil was sliced and the organisms eluted. Phage moved slightly deeper in the polymer-treated column than in the control column. There was no measurable difference in the movement of E. coli in either polymer-treated or control columns. The properties of the soil (high amounts of metal oxides, kaolinitic clay), unsaturated flow conditions, and relatively high ionic strengths of the leaching solution attributed to significant retention of these indicators. The impacts of PAM and LAS on the mobility of E. coli or MS-2 phage in the chosen soils were not significant.     

An approach for developing quantitative risk-based microbial standards for fresh produce

A key objective of the Good Agricultural Practices (GAP) program is to reduce the potential for produce to become contaminated with microbial pathogens, such as through irrigation water. Without microbial standards, however, it is impractical to decide whether there is a need to disinfect, a need to institute watershed protection programs, or a need to institute post-harvest disinfection regimes. To develop such standards, quantitative microbial risk assessments can be performed using pathogen monitoring data for produce. This paper presents an approach which can be used towards the application of a risk assessment framework to developing microbial standards for fresh produce. Risks of infection are estimated using typical monitoring data of Salmonella detected on carrots and assuming various scenarios of the likelihood of an individual consuming a contaminated serving of carrots in a given year. Estimated annual risks of infection range from 2.20 × 10^-5 to 2.16 × 10^-3, assuming 1% and 100% of an individual's carrot servings are contaminated, respectively. In addition, critical factors are identified which need to be incorporated in such a risk assessment approach as well as their impact on risk estimates to provide growers with benchmarks which may be targeted to reduce health risks.     

Wastewater disinfection and organic matter removal using ferrate (VI) oxidation

The use of iron in a +6 valence state, (Fe (VI), as FeO₄^-2) was tested as a novel alternative for wastewater disinfection and decontamination. The removal of organic matter (OM) and index microorganisms present in an effluent of a wastewater plant was determined using FeO₄^-2 without any pH adjustment. It was observed that concentrations of FeO₄^-2 ranging between 5 and 14 mg l^-1 inactivated up to 4-log of the index microorganisms (initial concentration c.a. 10⁶ CFU/100 ml) and achieved OM removal up to almost 50%. The performance of FeO₄^-2 was compared with OM oxidation and disinfection using hypochlorite. It was observed that hypochlorite was less effective in OM oxidation and coliform inactivation than ferrate. Results of this work suggest that FeO₄^-2 could be an interesting oxidant able to deactivate pathogenic microorganisms in water with high OM content and readily oxidize organic matter without jeopardizing its efficiency on microorganism inactivation.     

Health implications of lipopolysaccharide endotoxins in domestic container water used by rural households in South Africa

This study assessed the occurrence of endotoxins, cyanobacteria and enterobacteria in untreated drinking water stored in domestic water containers by rural households in South Africa. Endotoxins, cyanobacteria, total coliforms and Escherichia coli were measured in the following numbers and ranges in container-water samples: 4–54 mg l^-1, 69–64,505 cells ml^-1, 9,000–280,000 CFU/100 ml and 90–1,100 CFU/100 ml, respectively, in source water and 0.23–24.7 mg l^-1, 1–501,187 cells ml^-1, 25–1,584,893 CFU/100 ml and 1–25,118 CFU/100 ml, respectively, in water from containers. The concentrations of these contaminants in water often exceeded guidelines. Container type, especially those that permit light into the vessel, played a significant role in the occurrence of these contaminants. Limited guidelines, as well as the absence of health evidence, make it uncertain whether the high levels of endotoxins in the containerised drinking water could cause a health effect in healthy persons. Most importantly, in the context of exposure to endotoxins potentially derived from high levels of cyanobacteria and enterobacteria such as coliforms in the water, a case is made for possible health effects in immune-compromised individuals exposed to water containing endotoxins and the bacteria that potentially produce it.     

Community and household determinants of water quality in coastal Ghana

Associations between water sources, socio-demographic characteristics and household drinking water quality are described in a representative sample of six coastal districts of Ghana's Central Region. Thirty-six enumeration areas (EAs) were randomly chosen from a representative survey of 90 EAs in rural, semi-urban and urban residence strata. In each EA, 24 households were randomly chosen for water quality sampling and socio-demographic interview. Escherichia coli per 100 ml H₂O was quantified using the IDEXX Colilert^® system and multi-stage regression models estimated cross-sectional associations between water sources, sanitation and socio-demographic factors. Almost three quarters, 74%, of the households have >2 E. coli /100 ml H₂O. Tap water has significantly lower E. coli levels compared with surface or rainwater and well water had the highest levels. Households with a water closet toilet have significantly lower E. coli compared with those using pit latrines or no toilets. Household size is positively associated, and a possessions index is negatively associated, with E. coli. Variations in community and household socio-demographic and behavioural factors are key determinants of drinking water quality. These factors should be included in planning health education associated with investments in water systems.     

Microbial groundwater quality and its health implications for a border-strip irrigated dairy farm catchment, South Island, New Zealand

Intensification of dairying on irrigated pastures has led to concern over the microbial quality of shallow groundwater used for drinking purposes. The effects of intensive dairying and border-strip irrigation on the leaching of E. coli and Campylobacter to shallow groundwater were assessed over a three-year period in the Waikakahi catchment, Canterbury, New Zealand. Well selection excluded other sources of contamination so that the effect of dairying with border-strip irrigation could be assessed. Groundwater samples (135) were collected, mostly during the irrigation season, with E. coli being detected in 75% of samples. Campylobacter was identified in 16 samples (12%). A risk assessment of drinking water with these levels of Campylobacter was undertaken. A probability distribution was fitted to the observed Campylobacter data and the @RISK modeling software was used, assuming a dose response relationship for Campylobacter and consumption of 1 L/day of water. The probability of infection on any given day in the study area was estimated at 0.50% to 0.76%, giving an estimated probability of infection during the irrigation season of 60% to 75%. An epidemiological assessment of the Canterbury region comparing areas encompassing dairy within major irrigation schemes (~55% border-strip irrigation) to two control groups was undertaken. Control group 1 (CG1) encompasses areas of dairying without major irrigation schemes, and a second larger control group (CG2) comprises the rest of the Canterbury region. Comparisons of the subject group to control groups indicated that there was a statistically significant increase in age-standardised rates of campylobacteriosis (CG1 Relative Risk (RR)=1.51 (95% CI = 1.31-1.75); CG2 RR = 1.51 (1.33–1.72)); cryptosporidiosis (CG1 RR = 2.08 (1.55–2.79); CG2 RR = 5.33 (4.12–6.90)); and salmonellosis (CG2 RR = 2.05 (1.55–2.71)).     

Large-scale freshwater microbiological study: rationale, results and risks

A fifteen-month fortnightly survey of microbial health risk indicators and pathogens has been carried out at 25 freshwater recreational and water supply sites distributed throughout New Zealand, for: E. coli, Clostridium perfringens spores, F-RNA bacteriophage, somatic coliphage, human enteroviruses, human adenoviruses, Cryptosporidium oocysts, Giardia cysts, Salmonella and Campylobacter. Sites were selected to represent five geographical areas covering New Zealand and five categories of predominant environmental impact: birds, dairy farming, forestry/undeveloped, municipal, and sheep/pastoral farming. Six of the sites were also source waters for treated drinking-water supplies. Of the indicators, E. coli was detected in 99 % of all samples, with somatic coliphage being detected most of the time (89 %). Of the pathogens tested, Campylobacter and human adenoviruses were inferred to be the most likely to cause human waterborne illness to recreational freshwater users. Using data from all sites, an estimated 5 % of notified campylobacteriosis cases in New Zealand could be attributable to water contact recreation. The critical value for E. coli as an indicator of increased Campylobacter infection is in the range of 200–500 E. coli per 100 ml. This result has been used to derive new national water quality guidelines for recreational fresh water in New Zealand.     

Guidelines for the microbiological quality of treated wastewater used in agriculture: recommendations for revising WHO guidelines

Three different approaches for establishing guidelines for the microbiological quality of treated wastewater that is reused for agriculture are reviewed. These approaches have different objectives as their outcomes: the absence of faecal indicator organisms in the wastewater, the absence of a measurable excess of cases of enteric disease in the exposed population and a model-generated estimated risk below a defined acceptable risk. If the second approach (using empirical epidemiological studies supplemented by microbiological studies of the transmission of pathogens) is used in conjunction with the third approach (using a model-based quantitative risk assessment for selected pathogens) a powerful tool is produced that aids the development of regulations. This combined approach is more cost-effective than the first approach and adequately protects public health. The guideline limit for faecal coliform bacteria in unrestricted irrigation (< or = 1000 faecal coliform bacteria/ 100 ml) is valid, but for restricted irrigation < or = 10(5) faecal coliform bacteria/100 ml is recommended when adult farmworkers are exposed to spray irrigation. A limit of < or = 10(3) faecal coliform bacteria/100 ml is recommended if flood irrigation is used or children are exposed. The guideline limit for nematode eggs for both types of irrigation is adequate except when conditions favour the survival of nematode eggs and where children are exposed; in these cases it should be reduced from < or = 1 egg/l to < or = 0.1 egg/l.     

Unsafe tap water in households supplied from groundwater in the Salento Region of Southern Italy

Although the fractured aquifer of the Salento supplies over 80% of the drinking water requirements of the local population, its exposure to pollution has recently increased. In recent years, owing to the arid climate and droughts, the spreading of wastewater on soil for irrigation has become much more frequent. Consequently, hazardous and pathogenic microorganisms released with wastewater have been transported into the subsoil and have contaminated groundwater. An elaboration of epidemiological data has shown that the local population has the highest exposure to endemic gastroenteritis in Italy. In order to reduce human exposure to unsafe groundwater, the setback distance for drinking wells necessary to achieve the ‘natural disinfection’ criteria, has been determined experimentally at the Nardò aquifer (Salento region), supported by groundwater monitoring results and a mathematical transport model able to determine the apparent pathogenic microorganism pathways in fractures. The results also provided valuable inactivation constants of cultural indicators (coliforms, enterococci, Clostridium spores and somatic coliphages) and viruses in the wastewater that have been injected into the fractured aquifer since 1991. Furthermore, the efficacy of chlorine to remove viral indicators from water in a well 500 m from wastewater injection was tested. Hypochlorination reduces somatic coliphages and Clostridium spores in groundwaters but did not achieve complete inactivation in all tests. Complete disinfection of groundwater samples was possible only when there was an initial Clostridium spores count of 10 CFU 100 ml^-1.     

Assessment of a low-cost, point-of-use, ultraviolet water disinfection technology

We describe a point-of-use (POU) ultraviolet (UV) disinfection technology, the UV Tube, which can be made with locally available resources around the world for under $50 US. Laboratory and field studies were conducted to characterize the UV Tube's performance when treating a flowrate of 5 L/min. Based on biological assays with MS2 coliphage, the UV Tube delivered an average fluence of 900±80 J/m² (95% CI) in water with an absorption coefficient of 0.01 cm^-1. The residence time distribution in the UV Tube was characterized as plug flow with dispersion (Peclet Number = 19.7) and a mean hydraulic residence time of 36 s. Undesirable compounds were leached or produced from UV Tubes constructed with unlined ABS, PVC, or a galvanized steel liner. Lining the PVC pipe with stainless steel, however, prevented production of regulated halogenated organics. A small field study in two rural communities in Baja California Sur demonstrated that the UV Tube reduced E. coli concentrations to less than 1/100 ml in 65 out of 70 samples. Based on these results, we conclude that the UV Tube is a promising technology for treating household drinking water at the point of use.