Sustainability of Domestic Sewage Sludge Disposal

Large-scale cropland application of municipal wastewater was first practiced about 150 years ago after flush toilets and sewer systems were introduced into cities in Western Europe and North America. Wastewater was discharged without any treatment and receiving watercourses became heavily polluted [1]. Many “sewage farms” were designated as a preferred alternative to the direct discharge of raw sewage into waterways [2]; in 1875, this “land treatment” served England and major cities in Europe and with the new century it has also started in the United States. Sewage farms played a role in decreasing pollution in the receiving streams, and also creating several environmental sanitation problems: hydraulic and pollutant land overloading caused clogging of soil pores, soil water logging, odors and contamination of food crops [1]. This procedure was gradually abandoned with the development of more effective technologies and building municipal sewage treatment systems. Land application of sewage sludge starts with the treatment of municipal wastewater and the consequent production of an end product, a solid waste, consisting in a concentrated suspension of solids high in organics and biodegradable compounds. One of the most widely used process for the treatment of wastewaters from medium to large populations that has found application in almost all of the countries of the world is the activated sludge process. It is a biological treatment which uses a mass of microorganisms to aerobically treat wastewater. It is widely accepted that the original process is attributed to the experimental work undertaken by Dr. Edward Ardern and Mr. William Lockett and carried out at the Davyhulme Sewage Works, which at that time were operated by the Manchester Corporation, with the cooperation of Dr. Fowler in 1914 [3]. The process was developed for the treatment of domestic wastewater and it has since been adapted for removing biodegradable organics from industrial wastewaters [4]. Owing to the physical-chemical processes involved in its treatment, sewage sludge tends to concentrate heavy metals and poorly biodegradable trace organic compounds, as well as potentially pathogenic organisms (viruses, bacteria, etc.) presented in wastewaters [5]. Since the late 1970s, source control and industrial wastewater pretreatment programs were applied to limit the discharge of industrial constituents into municipal sewers resulting in a consistent reduction of trace elements in wastewater and sewage sludge. Over the past 30 years and until now, many studies were started to understand and predict the toxicity and the fate of toxic substances and pathogens in sewage sludge when they are applied to soils [1]. Results constituted a benchmark for the development of guidelines in the United States and in western European countries. For more than 20 years, the sewage sludge directive 86/278/EEC has encouraged the use of sewage sludge in agriculture, suggesting at the same time regulation of its use to prevent possible harmful effects on soil, vegetation, animals and humans. The key concept of the directive is to consider sewage sludge as a valuable resource: in fact it is rich in plant macro and micronutrients and its application can, in the long term, improve soil fertility [6]. In Europe the most pragmatic and environmentally sustainable approach to manage sludge from wastewater treatment plants is actually recycling it on agricultural land [7].