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

3.2. Heavy metals and inorganics

3.2.1. Background

In Europe and North America, regulations have been in place to limit concentrations of heavy metals in sewage sludge intended for land application. The USEPA adopted regulations in 1993 (USEPA, 1993) to set concentration limits for nine metals (Arsenic (As), Cadmium (Cd), Copper (Cu), Lead (Pb), Mercury (Hg), Molybdenum (Mo), Nickel (Ni), Selenium (Se), and Zinc (Zn)). In Europe, EU sewage sludge Directive 86/278/EEC sets regulatory limits for seven heavy metals (Cd, chromium (Cr), Pb, Ni, Cu, Hg and Zn) in soil and sewage sludge itself (Kelessidis & Stasinakis, 2012). In different countries, however, various models are adopted for regulating the application of sewage sludge to land. In the UK and US a risk-based approach has been adopted (Smith, 2009), whereas a precautionary principle is preferred in the majority of EU member states (Schoof & Houkal, 2005). This divergence has resulted in more stringent concentration limits (either proposed or enacted) in Europe than in the US for metals (McGrath et al., 1994).

The presence of heavy metals in sewage sludge is primarily due to the mixing of industrial wastewater with domestic sewage (Cornu et al., 2001). Even though there are legislative frameworks in place to regulate concentrations of heavy metals in sewage sludge, there remains scepticism over the safety of land application of sewage sludge due to potential risk of metal accumulation in the soil (Mamindy-Pajany et al., 2014) and in food crops (Latare et al., 2014), or of transport to groundwater and eventually the food chain (Kidd et al., 2007).

3.2.2. Risk analysis

Due to the potential risks of heavy metals entering food chain, many studies have investigated their uptake by food crops grown on land where sewage sludges have been applied (e.g. Latare et al., 2014; McBride et al., 1999; 2004). The results of these studies have confirmed metal accumulation in crops, therefore the authors concluded that uptake of metals by crop plants is a potential route of exposure to humans via the food chain. Under some circumstances (localised food production and consumption combined with long-term elevates use of sewage sludge, and high levels of consumption by vulnerable individuals), this might represent a health risk (Hough et al., 2003). However, many of these studies were laboratory-based, with the plants exposed to unrealistically high concentrations of heavy metals, or the plants were cultivated in media that favoured the uptake of heavy metals (e.g. aqueous growth media, low pH), or that had been purposely enriched with heavy metals and are therefore not representative of standard agricultural practices (Prosser & Sibley, 2015). Other studies based on field and greenhouse trial are more realistic compared with agriculture practise, however due to crop varieties, soil characteristics and many other factors, the measured uptake of heavy metals can vary significantly (Harrison et al., 1999). These experimental difficulties therefore lead to uncertainties in the environmental and human health risk assessment.

The SNIFFER report (SNIFFER 2008) reported minimal risks associated with the presence of heavy metals in sewage sludge. Hosseini Koupaie & Eskicioglu (2015) commented that most of the previous risk assessments of heavy metals in biosolids were implemented using a deterministic (non-probabilistic) approach, which may not be realistic due to the highly variable parameters involved in the risk assessment. Using a probabilistic risk analysis approach, the authors concluded that potential risks to human health were acceptable even when considering a long-term application of sewage sludge to land at an annual rate of 100 t ha^-1.

In addition, under usual agricultural soil conditions, potential risks from heavy metals are directly related to the concentrations of these contaminants in sewage sludges. In a European scale study conducted by the European Commission's (EC's) Joint Research Committee (JRC), 114 chemicals including 21 metal elements were evaluated in 63 sewage sludge samples originating from 15 different countries. The study found that concentrations of all regulated heavy metals were well below the legislative limits. Therefore, the study concluded that monitored concentrations did not justify the introduction of new threshold limits for considered parameters within the Directive as no unacceptable risk had been identified (EC, 2012). This is in agreement with a recent UK based study assessing the statutory limits of HM set by the UK Sludge (Use in Agriculture) Regulations in a long-term sludge experiment (Charlton et al., 2016).

Due to these findings, the low risk reported by SNIFFER 2008 and the fact that the regulation of heavy metals in sewage sludge has a long history and is well developed, no formal risk analysis was undertaken in this study.