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© IWA Publishing 2014 Water Science & Technology
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69.3
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2014
Treatment of waste activated sludge together with agrowaste by anaerobic digestion: focus on effluent quality C. Cavinato, C. Da Ros, P. Pavan, F. Cecchi and D. Bolzonella
ABSTRACT Waste activated sludge production and management plays an important role in wastewater treatment plants (WWTPs), especially from an economic point of view. One possible approach is the anaerobic co-digestion of waste activated sludge with others organic substrates in mesophilic and thermophilic conditions in order to exploit the spare volume of existing reactors, recover energy from biogas production, and obtain a fertilizer as final product. The anaerobic trials were carried out at pilot scale, applying two organic loading rates (2.8 and 4.5 kg chemical oxygen demand (COD)/(m3·d)) with a hydraulic retention time of 16 and 21 days. Among agro-wastes, wine lees were chosen because of their continuous availability throughout the year, and their high COD content (up to 200–300 g/l, 70% soluble, on average). The addition of wine lees to activated sludge determined a higher biogas production (best yield was 0.40 Nm3/kgCODfed) improving the energetic balance of the sludge line of the WWTP. The characterization of both substrates fed and digester effluents was carried out in terms of heavy metals; comparison with EC proposed limits showed that, due to high content of Cu in wine lees, the loading rate of this agro-waste should be limited to maintain good characteristics of final biosolids. Key words
| agro-waste, anaerobic co-digestion, waste activated sludge, wine lees
C. Cavinato (corresponding author) C. Da Ros P. Pavan Department of Environmental Sciences, Informatics and Statistics, University Ca’ Foscari of Venice, Calle Larga Santa Marta, Dorsoduro 2137-30123, Venice, Italy E-mail:
[email protected] F. Cecchi D. Bolzonella Department of Biotechnology, University of Verona, Strada le Grazie 15-37134, Verona, Italy and Interuniversity National Consortium ‘Chemistry for the Environment’, Via delle Industrie, 21/8-30175 Venice, Italy
INTRODUCTION The need to fulfil stringent effluent standards for chemical oxygen demand (COD) (or biochemical oxygen demand), nitrogen and phosphorus in wastewater treatments has determined in recent years the adoption of advanced activated sludge processes for nutrient removal. The biological nutrient removal (BNR) processes, either for nitrogen or both nitrogen and phosphorus removal, can be performed only when the necessary amount of carbon in the treated wastewater is available. A typical solution for preserving COD in the wastewater is avoiding the primary settling tanks. Moreover, in BNR wastewater treatment plants (WWTPs), in order to support the nitrification capability of the activated sludge, high solids retention times (SRTs) are applied to the activated sludge (>10 days). As a consequence, a partial sludge stabilization occurs in the activated sludge process itself, and the following anaerobic stabilization of waste activated sludge (WAS) can result in low efficiency both from a processing and an economic standpoint since this substrate shows a low biomethanization potential (Bolzonella et al. ). This fact results in a decrease in biogas production so that the energetic doi: 10.2166/wst.2013.736
balance of the anaerobic digester is often negative if sludge is not properly thickened, especially in winter. In these WWTPs anaerobic digesters are typically low loaded (OLR
