Kendir, E., Sanin, F.D., Kentel, E., Middle East Technical University, Turkey(free)
Abstract Biosolids which are treated and processed sewage sludge are valuable products which include 40-80 % dry weight organic matter and nutrients such as nitrogen and phosphorus. Due to these constituents, land application is considered to be a beneficial removal mechanism in some countries. Various management alternatives for biosolids are being evaluated in Turkey. Utilization of biosolids in agriculture is among these alternatives. However, due to the presence of inorganic, organic and biological pollutants in biosolids, serious effects on human and environmental health are possible. In order to evaluate health effects associated with land application of biosolids, risk-based approaches are commonly used in the world. Pollutants in the biosolids applied to land may reach different receptors through a number of pathways. Ingestion of biosolids by children is identified as one of the most critical pathways by the U.S. Environmental Protection Agency (US EPA). This study aims to investigate the health risks associated with the ingestion of biosolids originating from Ankara Central Wastewater Treatment Plant (ACWWTP) by a child for five heavy metals (Cr, Cu, Ni, Pb, and Zn). Eightmonthly sewage sludge samples were collected from ACWWTP and analysed for the stated pollutants using Atomic Adsorption Spectroscopy (AAS). Then, the associated health risks are calculated using US EPA’s approach. Calculated hazard index values indicated that there is no appreciable risk that non-cancer health effects will occur.
Keywords: Biosolids, Health Risk Assessment, Heavy Metals, Land Application
Introduction Sewage sludge is the solid or semisolid residue generated during treatment of domestic sewage (U.S. EPA, 2002). In order to distinguish raw sludge from processed sludge which can be used in land application, the term ‘biosolids’ was firstly used in wastewater treatment industry (Jenkins et. al., 2007). Then, this term was adopted by U.S EPA and defined as “the primarily organic solid product yielded by municipal wastewater treatment processes that can be beneficially recycled” (National Research Council, 2002). Biosolids include organic matter (up to 80%) and nutrients such as nitrogen and phosphorus (Schowanek et. al., 2004). These contents of biosolids make its beneficial usage possible. Land application of biosolids as a beneficial disposal route has many advantages for soil and crop. For instance, organic matter in sludge achieved by land application enhances water infiltration, water holding capacity and soil granulation. Additionally, land application of sludge decreases soil compaction and erosion of soil. It also has benefits f organisms in soil and plants such as being a source for food, energy and nutrients (Jacobs and McCreary, 2001).
Land application of biosolids is commonly preferred in the world. In USA 6,514 million tons of biosolids were produced in 2004 and main disposal route for biosolids is beneficial use on land (agriculture, silviculture and land restoration) (55%). In E.U., 9 million tons of biosolids are produced annually and the most preferablble disposal route is land application (45%). In Turkey municipal sludge production is 580,000 tonnes dry matter /year and the most common disposal route for biosolids is landfilling (Global Atlas of Excreta, 2008). Due to beneficial contents of biosolids and poor soil conditions in Turkey, there is potential for agricultural use of biosolids.
On the other hand, biosolids can pose potential health risks to human receptors and ecological receptors such as animals, plants and organisms since it contains metals and trace elements, PCB’s, dioxins, steroids, pharmaceuticals, pathogens, bacteria, viruses and vectors (National Research Council, 2002). The pollutants may result in contamination of the environment and this leads to consideration of potential health and safety implications in order to prevent adverse effects (WHO, 2002). Therefore, health risk biosolids are applied to land (National Research Council, 2002).
For this step, US. EPA initially listed 200 pollutants for review and then 50 of them were chosen based on the probability of toxicity, the likelihood of human and environmental exposure, available toxicity and exposure data and professional judgment to be reviewed further (U.S. EPA, 1995). After this screening, hazard index values are calculated for each pollutant using the formula below (U.S. EPA, 1995):
A hazard index value less than 1 means the pollutant concentration in the environment is less than the limit pollutant concentration that is toxic to receptors for the evaluated pathway and no adverse non-cancer effects are expected to occur for the receptors. On the other hand, a hazard index value more than 1 means the pollutant concentration exceeds the limit value and is potentially toxic to the receptors for the evaluated pathway. The pollutants with hazard index values less than 1 were eliminated and finally, 24 pollutants were chosen to be regulated for land application of biosolids in USA (U.S. EPA, 1995).
In exposure assessment, exposure pathways and receptors are determined for land application of biosolids, incineration and landfilling operations. For land application, both human (child, home gardener) and ecological receptors (soil organisms, animals and plants) were determined for risk assessment studies and a total of 14 pathways were considered (U.S. EPA, 1995).
In this step, U.S. EPA used reference doses (RfD) and cancer potency factors (q1*) in order to determine toxic effects of pollutants on humans. Cancer potency factors are used to evaluate human cancer risks when exposed to a pollutant during 70 years life-time. Reference doses (RfDs) are used to determine the threshold effects of the pollutant other than cancer effects (U.S. EPA, 1995). Both RfD and q1* are listed for a number of pollutants in U.S. EPA Integrated Risk Information System (IRIS) database.
In risk characterization, first of all, pollutant limits named as RSC (Reference Pollutant Concentration in Biosolids, µg-pollutant/g-biosolids dry weight) or RP (Reference Cumulative Application Rate of Pollutant, kg-pollutant/hectare) were calculated for each pathway for land application of biosolids (U.S. EPA, 1995). In addition, ceiling concentration limits which are the maximum allowable concentration that can be applied to land were determined using 99th percentile approach for the data in National Sewage Sludge Survey (NSSS) for the pollutants. After calculating pollutant limits for a particular pollutant listed for each pathway, the lowest pollutant limit and most limiting pathway were chosen to identify the pollutant limit. These are listed for the 10 heavy metals of concern in Part 503 Rule (Table 1).
In this, study we used the same approach to evaluate non-cancer risks associated with ingestion of sewage sludge originating from ACWWTP by a children. The results of this study are important because utilization of sewage sludge for agricultural purposes is currently being evaluated in Turkey.