Sewage sludge is the organic by-product of urban wastewater treatment. The production of sewage sludge in Ireland, similar to other European countries, has generally increased: in 2015, for example, more than 58,000 tonnes of sewage sludge was produced. This is largely attributable to the implementation of the Urban Waste Water Treatment Directive and other legislative measures. In Ireland, approximately 80% of treated sewage sludge (‘biosolids’) is reused on agricultural land (Table 1), and is done so in compliance with the Waste Management (Use of Sewage Sludge in Agriculture) Regulations, which prescribe standards and limits on sludge used in agriculture subject to the carrying out of nutrient management plans. In addition, the Department of Environment, Community and Local Government Code of Good Practice for the Use of Biosolids in Agriculture sets out mandatory guidelines for producers, end-users and local authority regulators of sewage sludge used in agriculture. They must also comply with the provisions of the European Union (Good Agricultural Practice for Protection of Waters) Regulations, 2014, which specify the periods when land application of fertilisers is prohibited and limits on the land application of fertilisers.

Agriculture Composting Landfill Other Total
Tonnes dry solids 46,697 10,946 94 650 58,387
(80%) (18.7%) (0.2%) (1.1%)
Table 1: Destination routes for the national load of sewage sludge in 2015 (EPA, 2016)
Despite these strict regulations, there remain several issues associated with the reuse of sewage sludge on agricultural land. While many of these are issues of perception, there is considerable concern over the presence of metals, nutrients, pathogens, pharmaceutical and personal care products (PPCPs), and other endocrine-disrupting and synthetic compounds in biosolids, which may cause environmental and human health problems. An Environmental Protection Agency-funded (EPA-funded) research project, led by Dr Mark Healy, principal investigator of the Geo-Environmental Engineering Research Group (GENE) and senior lecturer in civil engineering at the College of Engineering and Informatics in NUI Galway, and comprising researchers from the university, Teagasc and UCD, set out to examine these questions.

Sewage sludge production and agricultural land

The project aimed to examine all aspects of sewage sludge production and application to agricultural land. The main aims of this research were to:
  • Quantify the range of concentrations of metals and two of the most abundant PPCPs in the world, the antimicrobials triclosan (TCS) and triclocarban (TCC), in biosolids from a range of wastewater treatment plants (WWTPs) in the Republic of Ireland;
  • Undertake a field-scale experiment to assess losses of nutrients (nitrogen and phosphorus), metals, TCS and TCC, and microbial matter following successive rainfall events on grassland onto which biosolids had been applied, and to compare the results with another commonly spread organic fertiliser, dairy cattle slurry;
  • Measure the uptake of metals by ryegrass for a period of time after the application of biosolids; and
  • Conduct a risk assessment of potential hazards of human health concern based on the experimental data. To read the published EPA report, click here.
The metals in the biosolids from the WWTPs examined were below the maximum allowable concentrations of metals for use in agriculture in the EU (Healy et al., 2016a). Some priority metals such as antimony and tin, which are potentially harmful to human health, were identified in some of the samples analysed. As these parameters are not currently regulated, this means that a number of toxic metals, which may be several times higher than their baseline concentrations in soils, may be applied to land without regulation. In the WWTPs examined, concentrations of TCS and TCC were 0.61 and 0.08 µg g-1, which were below the concentrations for these parameters measured in other countries. Working with colleagues in the Marine and Freshwater Research Centre in the Galway-Mayo Institute of Technology, synthetic polymers measuring less than 5 mm in diameter, called Microplastics, were also found in the treated sewage sludge (Mahon et al., 2017). As these have the potential to adsorb persistent organic contaminants and priority metals, significant risks to the environment may arise following their application to land. The possibility exists that these potentially harmful, unregulated contaminants, for which no international standards currently exist for recycling in agriculture, may accumulate in the soil upon repeated application.

Land application of biosolids on surface runoff of contaminants

[caption id="attachment_35625" align="alignright" width="250"]Figure 1 Fig 1: Field scale plots used in this study (prior to application of biosolids and dairy cattle slurry)[/caption] To address the second aim of the project, a field-scale study examined the impact of land application of biosolids on surface runoff of contaminants (Peyton et al., 2016). Three different types of biosolids, all originating from the same WWTP but subject to different types of treatments (anaerobic digestion, lime stabilisation and thermal drying), were applied to small field plots at the maximum permissible rate allowed in Ireland (Figure 1). To give context to the results, dairy cattle slurry was also applied to separate plots at the same rate. All plots were then subject to numerous simulated rainfall events. During each rainfall event, the water flowing over the soil surface (‘runoff’) was collected and analysed for a range of water quality parameters that could be detrimental to the environment and human health. This study found that nutrient concentration in runoff following land application of dairy cattle slurry was far greater than the concentrations arising from the application of biosolids. Furthermore, the metals and microbial matter present in the runoff from the biosolids-amended plots were, in general, of the same order as the dairy cattle slurry plots. Therefore, in these respects, biosolids did not pose a greater risk than dairy cattle slurry. Furthermore, there was no significant difference in metal bioaccumulation of the ryegrass between plots that received biosolids and those that did not, over the study duration (Healy et al., 2016b). Various exposure assessment models were developed, which considered exposure to metals and E. coli through surface water abstracted for drinking, taking account of surface runoff, dilution and water treatment effects, and the likelihood of illness arising from exposure and the severity of the resulting illness. In general, the results indicated that the risk of illness was negligible for healthy individuals (Clarke et al., 2016, 2017); however, care is required with immunocompromised individuals, where the annual risk was greater than the threshold risk of illness. The overall conclusion from this study is that although, in general, land applied biosolids pose no greater threat to water quality than dairy cattle slurry and cattle exclusion times from biosolids-amended fields may be overly strict (within the context of current exclusion criteria), a matter of concern is that unlegislated metals, PPCPs and microplastics, found to be present in biosolids originating from a selection of WWTPs examined in this study, may be inadvertently applied to land. With multiple applications over several years, these may build up in the soil and may enter the food chain, raising concerns over the continued application of biosolids to land in Ireland. Author: Dr Mark Gerard Healy BE, MEngSc, PhD, Eur Ing, CEng, FIEI, chartered engineer, Senior lecturer in civil engineering, College of Engineering and Informatics, National University of Ireland, Galway References: Clarke, R., Peyton, D.P., Healy, M.G., Fenton, O., Cummins, E. 2016. 'A quantitative risk assessment for metals in surface water following the application of biosolids to grassland.' Science of the Total Environment 566-567: 102-112. Clarke, R., Peyton, D., Healy, M.G., Fenton, O., Cummins, E. 2017. 'A quantitative microbial risk assessment model for total coliforms and E. coli in surface runoff following application of biosolids to grassland.' Environmental Pollution 224: 739 – 750. EPA. 2016. Urban waste water treatment in 2015. EPA, Co. Wexford. http://www.epa.ie/pubs/reports/water/wastewater/2015%20urban%20waste%20water%20report_Web%20Version.pdf Healy, M.G., Fenton, O., Forrestal, P.J., Danaher, M., Brennan, R.B., Morrison, O. 2016a. 'Metal concentrations in lime stabilised, thermally dried and anaerobically digested sewage sludges.' Waste Management 48: 404-408. Healy, M.G., Ryan, P.C., Fenton, O., Peyton, D.P., Wall, D., Morrison, L. 2016b. 'Bioaccumulation of metals in ryegrass (Lolium perenne L.) following the application of lime stabilised, thermally dried and anaerobically digested sewage sludge.' Ecotoxicology and Environmental Safety 130: 303-309. Mahon, A.M., O’Connell, B., Healy, M.G., O’Connor, I., Officer, R., Nash, R., Morrison, L. 2017. 'Microplastics in sewage sludge: effects of treatment.' Environmental Science and Technology 51(2): 810 – 818. Peyton, D.P., Healy, M.G., Fleming, G.T.A., Grant, J., Wall, D., Morrison, L., Cormican, M., Fenton, O. 2016. 'Nutrient, metal and microbial loss in surface runoff following treated sludge and dairy cattle slurry application to an Irish grassland soil.' Science of the Total Environment 541: 218-229.