The costs for disposal consist of the disposal of Sewage sludge, screenings, sand and municipal waste. The disposal costs can differ between 15 and 50 % of the total operation costs. Due to very low part on the total operation costs for screenings, sand and municipal waste, only sludge disposal will be discussed in the following.

Generally, disposal costs depend to a large degree on

• the size of the treatment plant,

• national regulations for the disposal of organic materials like Sewage sludge,

• local conditions and market price conditions respectively.

Sludge originates from the process of treatment of waste water. Due to the physical-chemical processes involved in the treatment, the sludge tends to concentrate heavy metals and poorly biodegradable trace organic compounds as well as potentially pathogenic organisms (viruses, bacteria etc) present in waste waters. Sludge is, however, rich in nutrients such as nitrogen and phosphorous and contains valuable organic matter that is useful when soils are depleted or subject to erosion. In particularly the organic matter and nutrients are the two main elements that make the spreading of this kind of waste on land as a fertilizer or an organic soil improver suitable.

Sludge treatment and disposal operations on a local or regional basis need careful planning to ensure that the strategy undertaken is environmentally acceptable, reliable and cost-effective. Sludge quantities and quality have to be assessed now and into the future. Disposal options for sludge have to be analyzed by an environmental assessment approach which studies the accessibility of all outlets, environmental legislation and attitudes of collaborating agencies and the public at large. Other wastes which may compete with sludge for disposal outlets must be considered.

Outlets which involve recycling and beneficial use of sludge are advantageous but may not be practical. Sludge treatment must be evaluated in relation to the disposal options available. Sludge treatment centres, treating sludge from several surrounding wastewater plants, may be required especially if thermal drying or incineration are likely options. Economic evaluation has to consider capital and operating costs of sludge treatment and transport and other costs associated with disposal.

Practical experience with sludge disposal in different European countries (S, DK, G, F, CH) can be summarized as follows [EWA 2001, European Commission 2002]:

• Policy aspects as decision making procedures and continuity in political strategy play a key role in the different countries. There is a strong relation between these political factors and the costs for sludge disposal. Even very elaborated legal frameworks with very stringent quality criteria can have detrimental effects on the selection of sludge disposal routes.

• For large treatment plants in agglomerations sludge disposal after incineration can be an economically and ecologically sound solution. The development of low cost small incineration plants meeting advanced off gas standards is in progress.

• Co-incineration of sludge with cement and solid waste can result in economically sound solutions. In regard to P-recovery it does not represent a sustainable solution.

• Sludge disposal of dewatered sludge in landfills should be avoided as it results in long term monitoring requirements especially for ammonia leaching.

• For the very large number of small treatment plants (e.g. <20.000 PE) landspreading / agricultural use of Sewage sludge seems to be the most economical and sustainable solutions as long as source abatement of possibly hazardous substances is successful. Landspreading of semi-solid and landspreading of solid sludge entail on average the lowest total cost (110 – 160 €/ton dry matter incl. external&internal costs).

• Landspreading of composted sludge, use of sludge in land reclamation and use of sludge in silviculture record intermediate total costs (210 – 250 €/ton dry matter).

• Landfilling, mono-incineration and co-incineration of sludge with other wastes entail the highest costs (260 – 350 €/ton dry matter).

Sludge Disposal Regulations

Sewage sludge, also known as biosolids, is what is left behind after water is cleaned in waste treatment works. It is high in organic content and plant nutrients and, in theory, makes good fertilizer. However, most developed countries regulate its use because it also can contain a multitude of metals, organic pollutants, and pathogens. The application of Sewage sludge to land, especially on agricultural lands, has been contentious since the late 1980s, when national and international clean water regulations prohibiting the ocean dumping of sludge were first enacted.

Millions of tons of Sewage sludge generated each year must go somewhere. If not applied to land, most sludge would have to be burned in incinerators or landfilled. U.S. total annual production of sludge is stable or only growing slowly; however, in Western Europe, where tougher clean water laws are beginning to take effect, sludge production is growing significantly, as small communities build and improve waste treatment plants to comply. Although opponents of sludge use have many grievances, one of their main concerns is the long-term buildup of heavy metals in the soil. Over time, they argue, metals such as cadmium, zinc, and copper could build up to levels high enough to damage agricultural soils. Some opponents advocate a full-scale ban on the use of sludge as fertilizer. But for others, who acknowledge its benefits, the question is: At what levels do heavy metals cause harmful effects?

The European Union (EU) is beginning work on a new sludge directive that will lower permissible limits for heavy metals. Another EU directive, which sets absolute values for contaminants in food, could also drive down permitted levels of metals in sludges. Draft standards for some metals taken up by plants, in particular, cadmium in wheat, are set so low that to meet them, sludge cadmium levels would have to be significantly lower than current EU requirements. U.S. regulations for metals in Sewage sludge are also slated for scrutiny. EPA is planning to commission a review of the science behind the regulations.

Table 1: Sewage sludge generation rates and disposal methods in different countries [EPA, 1999]