Recently there has been increasing rhetoric throughout the media on both the opportunities and risks posed by recycled and non-recycled organic wastes being spread to farmland stimulated, in part, by the adoption of more ‘plate to paddock’ approaches. Last month’s inaugural National Symposium of Recycled Organics (NRSO) in Brisbane, facilitated further discussion on this subject, from experts including RMIT’s Professor Bradley Clarke.
Up to 50 percent of domestic waste is organic materials with other organic wastes including sewage sludge, food wastes and industrial food processing wastes. Biosolids are generated at the sewage treatment plant from the sewage treatment process, with production estimated anywhere between 30-50kg dry solids per equivalent person per day (equivalent to 150kg of dry cake per annum). The biosolids are produced as either a thickened slurry or a dewater cake and, more recently, pellets. And contain useful quantities of organic matter, and nutrients such as nitrogen (N), phosphorus (P) and potassium (K), and lead to improvements in soil characteristics such as improved microbial activities and oxygen consumption.
Beneficial use is now a prerequisite for disposal of biosolids in many developed countries which has necessitated formulation of guidelines to prevent environmental contamination with heavy metals and pesticides, and infection of human and animal populations with pathogenic organisms; resulting in different classifications (such as Grades A to E) to control the end uses for the material. While it is commonly accepted that the utilisation of nutrients in biosolids at or below agronomic loading rates are highly beneficial, there is increasing concern related to contamination of biosolids from both known and new sources; and how the concentrations or leaching potential of these new contaminants will be impacted through further pre-treatment (such as pelletisaion to improve handling and reduce the amount of water being transported) or co-composting (to value-add to lower nutrient organic waste streams such as green waste).
As an example, over 85 percent of the total biosolids in Queensland are produced within 50km of Brisbane, with the most recent waste and recycling report (2016) highlighting the use of biosolids across the State, showing that 56,000 tonnes of biosolids (dry solids equivalent, DSE) was beneficially reused. The majority of this use occurring in SEQ with nearly 48,000 tonnes (DSE) utilised in the agricultural region of Darling Downs – Maranoa.
These biosolids are utilised primarily in the production of fibre to the west of Brisbane, as the application of biosolids to food crops or grazing land is strictly controlled. The restrictions on the surface application of biosolids to pasture for grazing and the promotion of the immediate incorporation into the soil reduces the likelihood of any organic contaminant accumulation by grazing animals.
While the End of Waste Code (currently transitioning from the former General Beneficial Use Approval) methodology sets strict guidelines on the level of stabilisation required, limits of pathogens and concentrations of heavy metals and pesticides, many of the emerging organic contaminants are not included in the required analysis.
Every year thousands of new contaminants enter the market in common consumer products and are washed down our drains, ending up in drinking water, the marine environment and in the resulting waste-water sludges. These contaminants are comprised of lawfully produced and retailed chemicals and pharmaceuticals through to illicit drugs, and more recently, high profile contaminants such as microbeads which are used as exfoliating agents in hundreds of personal care products globally.
The perceived impact of these physical contaminants and chemical compounds, particularly those which persist in the environment, bioaccumulate in both humans and the environment (particularly as they become concentrated in higher quantities and move up the food chain), and/or there is evidence of ecotoxicity, is growing.
Contaminants from everyday products like shampoos, toothpaste and makeup are almost impossible to manage once used by the householder; while new organic contaminants arising from manufacturing and processing practices all end up down the drain, where the burden of dealing with them falls onto the wastewater systems.
These newer contaminants have arisen from microbeads and nanoparticles in cosmetics, to microthreads or cancer-causing nonylphenol ethoxylates (NPE) and pthalates in synthetic clothing (see Greenpeace’s 2012 report which found NPEs in 63% of the new clothing items it tested and phalates in 100% of the samples) through to perfluorinated chemicals. They also include antimicrobials and endocrine disruptors from medications and, more recently, potentially toxic levels of more mainstream chemicals, such as caffeine (see the ‘Coffee Catch-(C)up’ at http://www.insidewaste.com.au/general/features/1051039/waste-opportunist-coffee-catch).
There are over 143,000 chemicals registered for use in the European Union (EU) alone. It is then no surprise that researchers in the EU have identified over 140,000 chemical contaminants in wastewater sludge; whilst in the United States research has identified over 80,000 contaminants. Given that Australian consumers buy and use similar products to both the Americans and Europeans, we could assume broadly similar levels.
A recent review has identified a list of ‘chemicals of concern’, including new organic contaminants (OCs) in biosolids used for agricultural use. These include, perfluorinated chemicals (PFOS and PFOA), polychlorinated alkanes (PCAs), polychlorinated naphthalene’s, organotins, triclosan, through to antibiotics and pharmaceuticals. This review specifically noted a number of emerging OCs (namely PFOS, PFOA and PCAs) for priority research attention as they are ‘environmentally persistent and potentially toxic with unique chemical properties or presence in large concentrations in biosolids, that make it theoretically possible for them to enter human and ecological food-chains from biosolid-amended soils’.
But before we panic, it is important to acknowledge the significant volume of high quality scientific analysis around the beneficial application of biosolids to land which has been conducted over 35+ years. This research has unanimously determined that the majority of compounds studied do not place human or animal health at risk under current application rates and procedures. Much of this evidence and regulatory assessment has concentrated on total elemental content or single-step leaching procedures as the basis for its risk assessment. More recent studies have investigated leaching from soils associated with repeated applications of biosolids (and biosolids amended composts) across a range of soils, even considering convection and diffusion effects from different application scenarios. At least one study has even considered the effect on soils and leachability from repeated applications of biosolids with a secondary material (such as flyash). While other studies have determined that many persistent organic pollutants are not readily bioavailable to microorganisms and for plant uptake. There is certainly more work to be done on the actual impacts by heavy metals to plant uptake, soil quality and leaching to both surface and ground waters and, in particular, the transport and availability of new and emerging organic contaminants entering the market.
Future research and regulatory assessment/conditioning must consider long-term impacts as well as immediate environmental risks. All stakeholders must continue to be vigilant to monitor and determine the significance of emerging OCs in biosolids. This research is essential for ensuring the long-term sustainable agricultural use of biosolids and in compost manufacture; whereas mitigating the risks of individual contaminants will require a range of possible policy, industry and consumer responses.