Potentially hazardous agents in land-applied sewage sludge: human health risk assessment

3.6. Human and animal pathogens

3.6.1. Background

In the UK and other developed countries, the application of sewage sludge to land is strictly regulated (ADAS, 2001; USEPA, 1993; Carrington, 2001; EPA Victoria, 2004; Pritchard et al., 2010; CEC, 1986; Defra, 2017). Under these legislative frameworks, sludge treatment processes are required to minimise or eliminate the presence of pathogens. In the UK, to ensure microbiological safety and consumer confidence, sewage sludges are usually treated and applied according to the requirements of the Safe Sludge Matrix. This sets out a risk-based 'table of crop types, together with clear guidance on the minimum acceptable level of treatment for any sewage sludge-based product which may be applied to that crop or rotation' (ADAS, 2001).

After application to land, depending on the soil type and rainfall, a proportion of the microorganisms associated with sewage sludge are retained in the soil surface layers. The practice of incorporating sewage sludge into the topsoil provides an initial immediate dilution on pathogens when expressed as a concentration. Numbers of pathogens in surface soils will further decline if the pathogens are transported to depth via percolation. Sewage sludge contains a range of pathogens (generally reflecting the prevalence of pathogens in the local human population), some of which are better adapted to survival in the soil environment than others. Depending on the pathogen, the soil physical conditions (i.e. temperature, soil type and moisture content) and soil biotic processes, there will be varying degrees of pathogen decay in the soil (Pritchard et al., 2010; Gerba et al., 1975; van Elsas et al., 2012, McLaughlin et al., 2011).

Whilst studies in the US have provided limited evidence of health impacts associated with the application of sewage sludge (treated to EPA standards) to land (Dorn et al., 1985; NRC, 2002), it has been acknowledged that there are significant knowledge and research gaps. These include the occurrence, fate and potential health risks of known and emerging pathogens (WEAO, 2010; Sidhu & Toze, 2009; Viau et al., 2011). This section of the report aims to identify current and emerging risks to public health associated with pathogens in sewage sludges that are applied to land.

3.6.2. Emerging pathogens of concern

Bacteria

Previous reviews of pathogens in sewage sludge have identified the range of potential bacterial pathogens of concern (Sidhu & Toze, 2009; Pepper et al., 2006).

Escherichia coli is an important species in the context of the microbiological quality of sewage sludge. E. coli is a natural and ever-present member of the microbial community in the healthy human gut. Quality assurance approaches such as the Safe Sludge Matrix incorporate the use of E. coli as a faecal indicator organism (FIO) i.e. an indicator of the sanitary quality of the product to assess the effectiveness of treatment processes and by association the likely effect on pathogen reduction. The use of FIO bacteria for this purpose is well established in environmental microbiology and public health protection. There are, however, well-founded criticisms of this approach which surround the critical question of whether it is possible to infer reliably anything about pathogen risk from the presence of E. coli. The ability of E. coli to reappear (i.e. regrow or be resuscitated from an unculturable state) in dewatered sludge or in soil to which it is applied (Gerba et al., 2002) adds to the questions about the meaningfulness of the standards that rely on it.

Certain serotypes of E. coli are enteric pathogens in their own right. E. coli O157 emerged in the 1980s and has since been considered as an important bacterial hazard in risk assessments. The emergence of new pathogenic E. coli serotypes should be expected. In 2011 the novel E. coli O104 strain caused an outbreak of disease which resulted in more than 50 deaths and was eventually traced back to the consumption of beansprouts grown from fenugreek seeds imported from Egypt. This emphasises both the potential for new strains to emerge and for global transport to play a role in public health incidents. Infected people would have released E. coli O104 to sewer and it could have conceivably appeared in sewage sludge. However, it would be reasonable to expect that the controls in place to protect the public from E. coli O157 would also provide protection from newly emergent bacteria via the application to land route.

The potential for the release of zoonotic bacteria into the sewerage network poses a hypothetical risk for the environmental spread of pathogens via sewage sludge. A large outbreak of Q-fever caused by the bacterium Coxiella burnetii occurred between 2007 and 2010 in the Netherlands and affected 4000 people, most likely due to inhalation of aerosolised bacteria from goat farms (van der Hoek et al., 2011). Schets et al. (2013) detected C. burnetii DNA at sewage works receiving discharge from Q fever positive goat farms; 36% of activated sludge samples were positive for C. burnetii DNA but at low levels. The probability of C. burnetii surviving sludge treatment in sufficient numbers to expose workers and local residents as a result of aerosolisation of sewage sludge is unknown. The bacteria can exist as small cell variants and small dense cells (SDC). SDC have a greater physical stability (related to heat, pressure and chemical agents) and are believed to be the persistent forms in the host and environment (ACDP, 2007). Survival in soil has also been demonstrated (Evstigneeva et al., 2007).

Viruses

It is well known that viruses pathogenic to humans are released in human faeces. They can therefore be expected to be found in untreated sewage entering WWTPs and in raw sludge by consequence of attachment to faecal solids and biomass generated in the wastewater treatment process. Adenoviruses are commonly detected in wastewater effluent (Enriquez et al., 1995) and are more resistant to thermal treatment than other viruses (Gerba et al., 2002). However, the infectivity of adenoviruses in sewage sludges can be effectively eliminated by lime treatment (Wei et al., 2009), although this practice does have implications for generation of malodour (Section 3.1).

Hepatitis A and E have been detected in raw sewage (Casas & Suñén, 2002) and in treated wastewater and sewage sludge (Clemente-Casares et al., 2009). It was reported that Hepatitis A RNA was significantly degraded after 60 days at 20C in sewage sludge (Wei et al., 2010) and the Hepatitis A virus is rapidly inactivated at high pH brought about by lime treatment (Wei et al., 2010; Katz & Margolin, 2007). Hepatitis E virus (HEV) is an RNA virus that causes liver inflammation in humans, predominantly in developing countries. HEV genotype 3 was detected in 17% of samples studied from the Meuse River in the Netherlands which was inferred to have originated from sewage (Rutjes et al., 2009). This may in turn indicate a connection between piggery wastewater and sewage catchments, and incomplete wastewater treatment of the viral load. In Canada, Brassard et al. (2012) detected HEV of animal origin, and norovirus and rotavirus of human origin, on strawberries irrigated with river water. Whilst samples of irrigation water proved to be negative, the authors inferred that wastewater and animal faeces contaminating the river were the likely source and indicating a cause for concern regarding the environmental survival of these viruses.

Rotavirus and norovirus are the most common causes of acute gastroenteritis (Sidhu & Toze, 2009) especially in children and immunocompromised individuals (Pepper et al., 2006). As a result, these pathogens can be expected to be present at large sludge treatment facilities. Norovirus were found at a level of 105 norovirus L^-1 in raw sludge and remain at a relatively high level of 103 L^-1 even after treatment (van der Berg et al., 2005). Wei et al. (2010) studied the infectivity of murine norovirus spiked into sludge over time and found that the virus can maintain some infectivity after 60 days in both 4C and 20C conditions. However, this needs to be put into the context of the quantitative risk assessment by Gale (2005) which considered the risk of infection from enteroviruses to be very low.

Protozoa

Several species of protozoa cause disease in humans, including Giardia and Cryptosporidium (Straub et al., 1993). Large quantities of Cryptosporidium oocysts and Giardia cysts are frequently found in treated sewage sludge (Robertson et al., 1992). They are known to be environmentally resistant, with cysts remaining viable for almost 2 months at 0 - 2C (deRegnier et al., 1989) and oocysts for nearly 6 months at 4C (Robertson et al., 1992). Gavaghan et al. (1993) assessed the inactivation of Giardia cysts during anaerobic digestion and showed that 99.9% of the cysts were inactivated within an 18-hour exposure period at 37C. Risks associated with Cryptosporidium parvum and Giardia were assessed by Gale (2005) and were considered low if the multiple barriers built into schemes such as the Safe Sludge Matrix were applied. Since then, Amorós et al. (2016) have detected Cryptosporidium oocysts and Giardia cysts in digested sludge in Spain, but there was no update on the risks of exposure or health outcomes.

Antibiotic resistance

There is widespread concern that the effectiveness of antibiotics is in such rapid decline that their future utility is under threat in the short term (Wellington et al., 2013). Soil is a reservoir of naturally-occurring resistance genes, but there is growing interest in the way that anthropogenic activities, such as the application to land of animal and human faecal material, can contribute to the evolution of antibiotic resistance in the environment (Wellington et al., 2013). Sewage sludge contains antibiotics, antibiotic-resistant bacteria (ARBs) and antibiotic resistance genes (ARGs), which can be released into the environment via application to land (Bondarczuk et al., 2016). Pruden, (2014) stated that "It is now clear that human activities, including WWTPs, have a strong influence on the distribution of ARGs in the aquatic environment". There is no evidence to suggest that ARBs present in soil to which sewage sludge has been applied are more likely to infect an exposed person than a susceptible bacterium, but the concern is that the soil becomes a reservoir for the development and spread of ARGs (Wellington et al., 2013). The principal concern is that the conditions may be created in the wastewater-land application system by which ARBs may develop as a result of the transfer of ARGs and/or the selection and proliferation of ARBs due to the presence of co-selecting agents in sewage sludge and sludge-amended soils such as biocides and metals (Wellington et al., 2013; Tezul & Pavlostathis, 2011; Ashbolt et al., 2013). Gaze et al. (2011) investigated the prevalence of mobile genetic elements known as integrons carrying antibiotic and quaternary ammonium compound (QAC) resistance genes that confer resistance to detergents and biocides. Studies of class 1 integron prevalence in sewage sludge amended soil showed measurable differences compared with controls, although prevalence dropped sharply after a month. This study concluded that by selecting for class 1 integrons, detergents and biocides co-select for antibiotic resistance in sewage sludge.

Compared with observations in clinical settings, antibiotic resistance profiles are often detected at low percentages in wastewaters. Current knowledge on the prevalence and types of antibiotic resistance in wastewater and sewage sludge is limited (Rizzo et al., 2013). Biosolids samples were reported in several studies to contain a high concentration of ARBs in a range between 6.78 × 105 -4.46 × 10⁸ CFU g^-1 (Munir et al., 2011; Brooks et al., 2007; Auerbach et al. 2007; Munir & Xagoraraki, 2011). Prado et al. (2008) studied the presence of ESBL (extended-spectrum β-lactamase)-producing Klebsiella pneumoniae in the effluents and sludge of a hospital sewage treatment plant, evaluating the plant's potential to remove these microorganisms. They found antibiotic resistant Klebsiella penumoniae, some of which were multiple drug resistant, at all stages of sewage treatment including in sludge. Galvin et al., (2010) also found that ESBL-producing E. coli survived treatment in a modern secondary treatment facility although they took no samples from sludge. Munir et al. (2011) found that ARBs and ARGs were reduced by wastewater and sludge treatment processes, with significant differences observed in ARGs and ARB concentrations between anaerobic digestion/lime stabilization and dewatering and gravity thickening methods. Ju et al. (2016) reported that anaerobic digestion can achieve a 20–52% removal efficiency of total ARGs.

Burch et al. (2014) demonstrated that ARGs in sewage sludge decay relatively slowly following their application to agricultural soils (half-lives > 2 weeks). Although the removal efficiency is moderate, it was found that when treated with aerobic/anaerobic digestion, air-drying etc, ARGs decay in soil after land application at much faster rates of <1 week (Burch et al., 2013a; Burch et al., 2013b).

Prions – Risks from sewage sludge spreading to land

Prion diseases or transmissible spongiform encephalopathies (TSEs) are progressive neurodegenerative disorders that affect both humans and animals. The causative agents of TSEs are prions, which are abnormal, transmissible agents capable inducing abnormalities in normal cellular proteins called prion proteins found most abundantly in the brain. The UK public is most familiar with the prion disease Variant Creutzfeldt-Jakob disease (vCJD). There is strong evidence to suggest that the agent responsible for a prion disease in cows, bovine spongiform encephalopathy (BSE or 'mad cow' disease), is the same agent responsible for vCJD in humans, and that in the UK cross-species transmission occurred in the 1980s due to the entry of bovine offal into the food chain (Gale, 2006). BSE has been reduced to extremely rare and isolated cases in the Scottish herd and there is no known risk of spread of this prion via any route including through sewage sludge.

Hypothetically, if a country had cases of prion disease, the transmissible agents could enter WWTPs from slaughterhouses, laboratories, or landfill leachate containing infected carcasses and other materials (Hinckley et al., 2008; Miles et al., 2011). Prions are insoluble in water and many detergents and are resistant to chemical and thermal degradation (Taylor, 2000). There are a few studies that have investigated prion survival in sewage sludge. Hinckley et al. (2008) found no significant degradation of prions during activated sludge treatment of the WWT process. Kirchmayr et al. (2006) found no reduction of prion survival under mesophilic conditions, but 20-40% reduction after 302 hours of incubation under thermophilic conditions. These studies strongly suggest that if prions were to enter the wastewater treatment system, most could survive mesophilic anaerobic digestion, and be present in treated sewage sludge.

With respect to the historical case of BSE in UK, Gale and Stanfield (2001) assessed the risk of prion-diseases from land application of sewage sludge and found it to be very low. The study adopted a Source-Pathway-Receptor approach to quantify the risk to humans through consumption of vegetable crops grown in sludge-amended soil and found it to be acceptably low, at 1.32 x 10^-9 persons infected year^-1. More recently, assessments have been completed using similar approaches for TSE risks associated with the land spreading of mammalian meat and bone meal which support the reinstatement of this practice subject to a range of controls (Cummins & Adkin, 2007). Adkin et al. (2013) reappraised the TSE risk posed by the irrigation to pasture land of wastewater from facilities processing livestock. The results indicated that the predicted number of new TSE infections arising from the spreading of wastewater on pasture over one year would be low, with a mean of one infection every 1,000 years for BSE in cattle, and one infection every 30 years and 33 years for classical and atypical scrapie, respectively.

In conclusion, the risks to humans from prions in sewage sludge are negligible.