Biosolids

2.7.2.1 Background

The term ‘biosolids’ was created in 1991 by the Water Environment Federation (WEF) to differentiate between the usable ‘solids’ from municipal waste water or sewage treatment plants and the untreated raw sewage sludge from households, commerce and industry (Jenson, 1993 as cited by Moffet et al., 2005). Treatment can involve either aerobic digestion (i.e. settling ponds) or anaerobic digestion. The anaerobically digested biosolids may be dewatered by presses, a centrifuge and/or polymers to reduce the

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volume. After dewatering, biosolids can still have a consistency that varies from custard to moist soil (Shammas and Wang, 2007), with further treatment processes adopted such as pasteurisation and lime stabilisation to control odours and reduce pathogens (Brown et al., 1997). Refer to Plate 2.1. Once treated, biosolids may then be disposed of by ocean dumping, landfill or incineration, or used beneficially for composting, remediation of contaminated mining sites or agricultural land application. Disposal methods of biosolids are controlled in order to reduce pollution, while beneficial re-use of biosolids has been defined as a sustainable practice that protects environmental, public and agricultural health while delivering economic, social and environmental benefits (Bethel, 1999).

Plate 2.1 Weighing lime amended biosolids prior to application to trial site at Cambridge

20 2.7.2.2 Biosolids disposal

Prior to 1990, the preferred method for biosolids disposal from coastal cities around the world was ocean dumping: preferred because disposal cost through a long pipeline was considerably less than sophisticated land treatment systems (Wood et al., 1993). Since that time, this disposal method has been outlawed by the United States (Schroder et al., 2008), Australia and the European Union (EU). However, like some developing parts of the world, countries such as Canada continue to dump raw sewage into waterways and oceans unabated (Maclean, 2005).

Landfilling continues to be a preferred method of biosolids disposal despite the EU enacting a directive in 1999 to limit potential negative environmental impacts of this practice. Landfilling occurs because either the land is not available for surface

application or the quality of the biosolids is below environmental protection authority (EPA) standards for beneficial re-use. In the US, landfilling of biosolids can accumulate carbon credits with methane extraction from the site providing additional sequestration benefits (Brown and Leonard, 2004).

Incineration of biosolids occurs where space for disposal is limiting, provided stringent environmental controls over toxic and particulate emissions are adhered to. However, more recently, incineration is gaining interest not only for disposal but for energy generation (Englande and Reimers, 2001).

2.7.2.3 Biosolids - beneficial re-use

Composting and Site Remediation

Composting is a process of mixing and incubating different organic materials together in fixed proportions for use as a soil amendment. Maintained at a specific temperature and moisture content for a certain period, composting can convert materials to a form more readily incorporated into the soil (Crawford, 2006). Composting of many types of organic wastes, including biosolids, is a preferred treatment to reduce the overall mass to landfill, for reclamation of infertile soils (Raviv, 1998; Stratton and Rechcigl, 1998), and to reduce the risk of transmitting human pathogens (Sarooshi et al., 2002). It has also been viewed as an option for degrading pharmaceuticals and personal care products

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(PPCPs) and other organic contaminants found in biosolids (Xia et al., 2005), and to stabilise the organic matter contained in the product (Brown and Leonard, 2004). Composting of biosolids may also prevent the bioaccumulation of pharmaceuticals in earthworms after application of biosolids (Kinney et al., 2008). However, uptake of a contaminant by plants does not necessarily follow its accumulation in the soil (Wrigley et al., 2008).

Surface land application of composted biosolids has been shown to stabilise soil after forest fires (Meyer et al., 2004), and to prevent leaching of heavy metals (Gove et al., 2002). However, Wrigley et al. (2008) observed that mixing composted biosolids with plant potting media at various ratios resulted in leaching of zinc and cadmium.

Conversely, lime stabilised biosolids has been found to reduce phytoavailability of zinc and cadmium in smelter contaminated soils (Basta et al., 2001). In the former example, leaching may be due to soluble complexes being formed between dissolved organic matter and metals ions contained in both the potting media and the composted biosolids. In the latter example, phytoavailability of heavy metals can reduce with an increase in pH (Basta and Sloan, 1999), but also the metal ions may be adsorbed to the introduced carbonates in the soil and subsequently immobilised. Plants have also assisted in

removing heavy metals after application of biosolids to contaminated sites. Case studies outlined by Adriano et al.(2004) showed that the use of biosolids enhanced natural remediation of contaminated sites by increasing vegetative growth, which in turn removed soil contaminants via bioaccumulation.

Agricultural Land Application

Biosolids contain many of the macro and micronutrients required for plant growth as well as organic matter for maintaining or improving soil physical characteristics. Most countries where biosolids are re-used beneficially have EPA guidelines stipulating the application rates for agronomic benefit. Many states in Australia use the nitrogen limiting biosolids application rate (NLBAR) together with contaminant limiting

biosolids application rate (CLBAR) to determine application rates, along with other soil constraints including hydraulic conductivity and pH.The level of final treatment and subsequent grade of biosolids also dictates the potential end use (Dettrick and McPhee, 1999).

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Extensive laboratory and field trials were conducted between 2005 and 2009 across five mainland states of Australia under the banner of the National Biosolids Research

Program, in which the benefits and risks to human and environmental health associated with applying biosolids to agricultural land was investigated (Broos et al., 2007; McLaughlin et al., 2008). Although this and other research has been undertaken espousing the benefits of biosolids for land application (Barbarick et al., 2004; Cogger et al., 2006), concerns continue to be raised about excess plant nutrients (i.e.

phosphorus) and leaching (Alleoni et al., 2008; Shober et al., 2003), potential soil contamination from heavy metals (Oliver et al., 2005; Stehouwer and Macneal, 2004), and accumulation of PPCPs (Kinney et al., 2008; Xia et al., 2005).

2.7.2.4 Biosolids in Tasmania

About 86 wastewater treatment plants (processing >100 kilolitres/day) operate in Tasmania (DPIPWE, 2009) servicing a population of around 500,000 people and producing approximately 10,000 dry tonnes of biosolids per year. As of 1st July 2009, these treatment plants came under the authority of three regional jurisdictions. The larger treatment plants servicing the cities of Hobart and Clarence produce

anaerobically digested biosolids (ADB) and lime amended biosolids (LAB). Both products undergo anaerobic digestion and are dewatered with polymers and either a centrifuge or a belt press. For the LAB, the lime is added to the ADB (in the form of calcium carbonate or calcium oxide) just prior to entering a screw type conveyor contained in a tube, after which the final product is discharged into reuse containers for distribution (B. Hanigan pers comms).