Composting of domestic solid waste – fixing of heavy metals

Over half of domestic solid waste is compostable and represents a significant potential for reduction of waste to landfill and for production of good quality compost. The composting process can be carried out by the householder using a home composting unit, or at a centralised facility treating separately collected garden and organic waste or whole waste. Each can have its niches of application depending on local factors, and can contribute to efforts to reduce waste to landfill by more than 50%. \n \nSeveral different types of home composting units are now available on the market and many local councils are promoting their use. There is an advantage in waste being composted where it is produced, saving transport, treatment and landfilling cost. It can contribute significantly to reduction in waste to landfill, with an average of 350kg/year/household of waste that can be composted in the backyard. Only mesophilic aerobic composting is generally achieved at best. Unless carried out properly anaerobic conditions can develop resulting in odour and fly infestation problems. \n \nSeparately collected garden and organic waste can produce high quality compost, but will necessitate an additional collection bin. An advantage with a centralised composting facility is the ability to ensure operating conditions in the thermophilic range ensuring destruction of weed seeds and pathogens. \n \nComposting of whole waste will necessitate pretreatment at the centralised plant to remove and recover metals, plastics, glass and other non-compostables. The compost may still be contaminated by heavy metals. A novel clay-composting process has been developed at Murdoch University to fix the heavy metals making them less available for leaching and plant uptake. The clay component also increases moisture and fertiliser holding capacity of the compost.