Abstract
Sewage sludge from sewage treatment plants has soil-forming and fertilising properties. However, sewage sludge cannot always be used in nature, including agriculture. One of the main reasons is the concentration of heavy metals. Sludge from wastewater treatment plants operating in MBR (membrane biological reactor) and SBR (sequential batch reactor) systems was analysed. Studies comparing the risk analysis of the natural use of sludge from MBR and SBR treatment plants were performed for the first time, due to the fact that more and more MBR plants, which are a BAT technology, are being developed in Poland, displacing the classical SBR plants. MBR technology uses a combination of activated sludge and filtration with microfiltration membranes. Wastewater treated in these reactors meets the highest quality standards, both in terms of physicochemical and microbiological aspects. This paper presents studies on the mobility of heavy metals in sewage sludge carried out using the BCR sequential extraction method. Geo-accumulation index (GAI), potential environmental risk index (ER), risk assessment code (RAC), and environmental risk determinant (ERD) were calculated. Heavy metals dominated the stable fractions in all cases. Furthermore, an increased content of copper and cadmium was observed in the MBR sludge. This fact is favourable in view of the efforts to eliminate heavy metals in the environment.
Highlights
Sewage sludge is formed as a by-product of processes carried out during the treatment of municipal and industrial wastewater
Sewage sludge from wastewater treatment plants mostly met the requirements of acceptable heavy metal content for agricultural purposes according to the Regulation of the Minister of Environment of 6 February 2015 [9] and EU Directive 86/278/EEC [13]; the exceptions were cadmium for WWTP1 and zinc for WWTP6
Sewage sludge collected from wastewater treatment plants operating in membrane biological reactor (MBR) systems was characterised by an increased content of copper and cadmium, compared to sludge from sequencing batch reactor (SBR) treatment plants
Summary
Sewage sludge is formed as a by-product of processes carried out during the treatment of municipal and industrial wastewater. With the development of civilisation, an increase in the amount of sewage sludge is observed worldwide [1,2,3]. There are many methods to manage the sludge, but the most beneficial is to use it for agricultural or natural purposes, due to the fact that it has high land-forming properties [4]. The choice of how a particular sludge can be used is strongly dependent on its properties [5,6,7]. The use of sludge for agricultural and environmental purposes is highly dependent on the content of micropollutants, especially heavy metals and parasite eggs [11,12]
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