Heavy metal fate in pilot-scale sludge drying reed beds under various design and operation conditions

Abstract

Thirteen pilot-scale sludge drying reed bed (SDRB) units have been constructed and operated under various settings. The beds included a cobbles lower layer, where perforated PVC aeration tubes were placed, and two gravel layers on top. The setup included planted beds with common reeds and control units. Three sludge loading rates (SLR) were examined: 30, 60 and 75 kg dm/m²/yr. Heavy metal (HM) accumulation in the residual sludge layer was negligible or low, and was found to increase with sludge layer depth. Plant uptake was low; the belowground biomass accumulated significantly more HMs compared to the aboveground biomass. Less than 16% of the influent HM left the bed through drainage. HM accumulation in the gravel layer was the major metal sink in the mass balance. On the whole, the HM content of the residual sludge was below the legal limits proposed by the EU for land application.

Introduction

The operation of wastewater treatment plants (WWTPs) produces significant amounts of sludge, which contains a large number of valuable constituents, such as nutrients and organic matter, when disposed of on agricultural land. However, it may also contain undesired pollutants, such as heavy metals (HMs), synthetic organics and pathogenic microorganisms, suggesting that its final disposal may have adverse environmental and/or health impacts. Therefore, sludge treatment and disposal is of major concern around the world, and depends on various economic, environmental and legal parameters. Particularly in Greece, sludge utilization in agriculture is very limited; the majority (>90%) of produced sludge is sent to landfills, while the respective mean value in EU countries is only 27% [1].

The main EU Directives dealing with sludge management are 86/278/EEC [2] and 91/271/EEC [3]. The first one describes the necessary measures and precautions for the safe sludge use in agriculture, and defines limiting values for HMs in soil and sludge. The 3rd Draft EC Working Document on Sludge of 2000 [4] recommends stricter HM limit values for land application. According to a European Commission Report of 2001 [5], many EU countries have adopted comparable HM limit values to those of the EU Directives (e.g., Greece, Italy, Spain, the UK), while other countries adopted stricter limits (e.g., Denmark, Finland, the Netherlands). An EC Working Document on Sludge and Bio-waste of 2010 [6] proposes even stricter limit values to incorporate bio-waste. In Greece, the EU Directive 86/278/EEC [2] was adopted by Ministerial Decision 80568/4225/91 in 1991, without any alterations, except of the addition of the following chromium limits: 500 mg/(kg dm) for Cr(III) and 10 mg/(kg dm) for Cr(VI).

Sludge drying reed beds (SDRBs) or sludge treatment wetlands have been successfully used over the last two decades [7]. They appear as an effective, safe, environmentally friendly, and economical alternative technology (low investment, operation and maintenance cost, low energy consumption) [8], [9], especially appropriate for small and medium size communities or remote areas. This technology results in high water volume reduction and good sludge stabilization [10]. SDRBs seem quite effective in sludge dewatering, mainly through evapotranspiration and draining, and also in sludge mineralization [9], [11], [12], [13], [14]. Residual sludge from these facilities is a well-composted and beneficial product, which can possibly be used as an organic fertilizer or for land application [14]. The key parameter in SDRBs is the presence of reeds (Phragmites australis). These plants possess a high transpiration capacity and are quite tolerant to wet/dry conditions [8], [13]. They contribute to the creation of aerobic microzones around their extensive root system, while they provide the necessary attachment area for microbes [15].

Although metal concentrations have been briefly reported in limited SDRB studies [11], [16], [17], [18], the fate of metals allocated in various SDRB components has not been well presented. The objective of this study is to evaluate heavy metal fate in various pilot-scale SDRBs with various design configurations under Mediterranean conditions, and quantify the effect of design and operational parameters in order to evaluate sludge suitability for land application.