A technology developed to provide better treatment as science advanced the knowledge of aquatic life mechanisms and human health effects, and the need for purer water was identified. Heavy metals, toxic chemicals, and other pollutants can be removed from domestic and industrial wastewater to an increasing degree. Methods of advanced treatment include microfiltration, carbon adsorption, evaporation /distillation, and chemical precipitation.
In sludge management, the greatest uncertainty about the future trends lies in the prospects for recycling sewage sludge in agriculture due to political and environmental concerns about this practice and tightening legislation. This is strategically the most important outlet for sludge and is the most environmentally sustainable from the point of view of recycling resources. However, the future of agricultural re-use will depend on adopting pragmatic controls that facilitate land application and protect the environment based on sound scientific principles and risk assessment. It will also crucially depend on counter-acting prejudicial perceptions about sludge and gaining consumer confidence that land application is a safe and acceptable practice that, on balance, represents the best overall approach to sludge management. As the availability of outlets for sludge diminishes, the ability to provide secure, cost-effective and environmentally acceptable approaches to sludge management becomes ever more challenging. This emphasizes the importance of maintaining the land application route to allow beneficial reuse of sludge otherwise the only remaining viable alternative will be the construction of more sludge incinerators in urban areas and for ash disposal to landfill.
The treatment and management of sewage sludge present a range of technical and perception challenges, and the costs associated with this represent more than 50% of total wastewater treatment costs (Kroiss, 2004). Therefore, sludge production is often considered as a disadvantage and should be minimized as far as possible. However, sludge produced from wastewater treatment presents a range of resource recovery opportunities, some of which are well established, such as the reuse of sludge biosolids as fertiliser products in agriculture, whereas others have yet to be fully maximized, for example, the generation of biogas and renewable energy from sludge anaerobic digestion (AD). There are also other resource recovery options which have yet to be realized and, taken together, all of these can offset the resource demands of the AS process. Recycling sludge by land application (for agricultural use or other purposes, such as restoration), or disposal of sludge in a landfill are the two principal, final destination routes for sewage sludge management in Europe.
Separate statistics are collected on the incineration of sludge, but this is not a disposal route per se; incineration is effectively a sludge treatment process that achieves the maximum solids reduction by thermal conversion and the residual ash is disposed of in a landfill. Energy production from sludge incineration processes is consumed for water evaporation and to meet the parasitic load of the process, therefore, incineration does not usually contribute to improved energy management (Mininni et al., 1997; Thierbach and Hanssen, 2002; Kroiss, 2004). In general, the disposal of whole sludge in a landfill is limited and is regarded as unsustainable in the long-term and inconsistent with EU landfill policy to reduce disposal of biodegradable municipal waste by this route. Nevertheless, it represents a significant disposal route in some cases. There is also uncertainty about the final destinations for sludge in some European countries as suggested by the large ‘others’ category in some report.