Introduction

In municipal and industrial wastewater treatment plants (WWTPs) the removal of biodegradable compounds and organic or inorganic particulate matter by means of settling and filtration, produces large amounts of waste sludge for disposal. Most WWTPs are based on activated sludge systems, in which sludge is produced in two ways:

– primary sludge: originates from the initial physical separation of settleable solids in the primary settlers;

– secondary sludge: produced in the final settlers where activated sludge is separated from the treated effluent. Secondary sludge is the result of net biological growth and the accumulation of inert and organic refractory compounds.

Sludge is characterised by a high percentage of volatile solids and a high water content even after dewatering (>70–80% by weight), which results in extremely large volumes of sludge, even though the volume of sludge produced in a WWTP only represents 1% of the volume of influent wastewater to be treated.

As WWTPs become more widespread the volume and mass of sludge generated is expected to increase continuously in the next decade, due to the increase of the population connected to the sewage network, the building of new WWTPs and the upgrading of existing plants because of more stringent local effluent regulations (Ødegaard, 2004; Paul and Debellefontaine, 2007).

In Europe 7.7 and 8.4 million tonnes per year (dry solids) were produced respectively in 2001 and 2003. An increase in sludge production of about 40% from 1998 to 2005 has been observed, reaching a current sludge production of 9.4 million tonnes per year (dry solids). In 2010, production is expected to exceed 10 million tonnes. In Europe, particularly among the new EU members or candidate countries, the increasing choice of biological treatment is leading to higher sludge production. Sludge production increased by about 65% in a decade in the Czech Republic, while in Poland the current half a million tonnes per year of sewage sludge (dry solids) is expected to double by 2015 due to the extension of services (Jenicek, 2007).

The trend is also similar in North America: in the USA sludge production is estimated at 6.9–7.6 million tonnes (dry solids) per year in the period 2005–2010 (Turovskiy and Mathai, 2006; Dentel, 2007), while in Canada sludge generation is approximately 0.4 million tonnes and in Japan it has reached more than 2 million tonnes per year (Okuno, 2007). In China, the current amount of sewage sludge production is expected to increase dramatically to around 11.2 million tonnes of dry solids per year by 2010 (State Environmental Protection Administration of China, Chu et al., 2009).

So far the main options for sludge disposal are agricultural use, landfill, incineration or composting. One of the major destinations for sludge in many areas, especially in the past, was the nearby ocean or sea, although recently many countries have introduced laws for marine pollution control, which do not permit sea dumping.

The disposal of sludge in landfill site will be limited in Europe as a consequence of stricter regulation, aimed to ban the disposal of biodegradable waste in landfills. Among the many drawbacks in sludge disposal in landfill are: gaseous emissions which contribute to global warming, hazardous compounds in leachate to be treated, nutrients and organic matter lost to recycling. For example, in Switzerland the disposal in landfill is already banned and sludge has to be dewatered, dried, incinerated and only the ash disposed of in landfills (Böhler and Siegrist, 2004). In Germany only material with a loss on ignition 55% may be disposed of in landfill. Sewage sludge, even after anaerobic treatment, still shows a loss on ignition of about 50% (Scheminski et al., 2000). Furthermore the selection of suitable sites for landfill is becoming more and more problematic due to the ever more scarce land availability and consequent increasing costs.

In Central and Eastern Europe the amount of sludge disposed of in landfill is also expected to decrease, while a slow increase in the adoption of more expensive technologies, such as incineration, can be expected (Jenicek, 2007).

Recently, there is a broad consensus of opinion that sludge must be managed in a more sustainable way to recover some energy or to make use of organic and nutrient content on land, but both these options present problems due to public concern.

The amount of sludge used on land or farmland may vary widely between countries, depending on the need to provide an important source of fertilisers or on the need to prevent the overloading of the soil and the environment. For example the percentage of sludge used in agriculture varies between 0 and 66% in European countries: ~0% in the Netherlands, 9% in Sweden, 28% in Austria, 32% in Italy, 33% in Germany, 58% in France, 59% in Denmark, 61% in the UK, 63% in Ireland, 66% in Spain (Müller, 2007). The future use of sludge in agriculture in Western Europe will be strongly influenced by EU directives on sewage sludge use in agriculture, which, notwithstanding the general awareness that agricultural applications could be a better way of recycling valuable compounds from sludge, will always include restrictive maximum values for many hazardous compounds. In fact, the use of stabilised sludge in agriculture has several advantages to reintegrate the progressive loss of organic matter and nutrients in soils, but its use is often strictly regulated due to the potential health risks associated with the presence of pathogens and contaminants such as heavy metals, micro-pollutants, antimicrobial agents or contaminants of pharmaceutical origin. In many areas, the application of sludge to land is regulated by specific guidelines developed at national level, which represent a conservative approach to the management of the health risk. For example, agricultural use of sludge has been limited in Sweden since 1999 and a total ban has been proposed in Switzerland, due to eco-toxicological considerations (Böhler and Siegrist, 2004; Lundin et al., 2004).

Another limiting factor for the agricultural use of sewage sludge in developed countries is the declining public acceptance of crops produced using sludge, a factor which seems to be becoming more important than the development of legal regulatory standards (Müller, 2007). The pros and cons of agricultural use are currently under...