Anaerobic Digestion of CAMBI-treated Sludge at Ringsend WWTP: The Impact of Changing Sludge Blends on Key Process Parameters

McCausland, C. and McGrath, S., Celtic Anglian Water


The sludge stream at Ringsend WWTP in Dublin was recently expanded to include a third CAMBI hydrolysis stream and a fourth anaerobic digester. The expanded sludge stream has the capacity to treat 120 tonnes/day of sludge on a dry matter basis. In addition to this, the capacity of the drum thickening plant was doubled by the installation of 3 new drum thickeners, increasing the capacity to thicken surplus activated sludge (SAS). This reduced the co-settling of large volumes of SAS in the primary settlement tanks which was historically the case. The sludge processed at Ringsend comprises of a blend of primary sludge and thickened SAS (TSAS). Since January 2010, the ratio of Primary : TSAS in the sludge blend for CAMBI-digestion has changed toward a more TSAS-rich blend. On a volumetric basis, sludge blends of 0,10,20,30,40, and 50% TSAS have been digested, with minimal impact on the stability of the anaerobic digestion process. This paper outlines the performance of the sludge stream at Ringsend (since the sludge stream expansion) in terms of the following key performance indicators: dewatering efficiency, biogas composition and biogas yield
Keywords: CAMBI, Biosolids, Anaerobic Digestion.
Ringsend Overview
Celtic Anglian Water operate Ringsend WWTP in Dublin. The plant treats all of Dublin’s municipal wastewater and is the largest treatment plant in Ireland. Ringsend WWTP is a tertiary treatment plant, with 12 lamella plate settlement tanks for primary treatment, 24 SBRs (secondary treatment) and UV treatment during the summer bathing season. There is also an extensive sludge treatment works at Ringsend. This incorporates a CAMBI THP plant, 4 mesophilic digesters, a CHP and 3 thermal dryers. The plant in its current form has been operating since 2003, though there has been wastewater treatment at the site since the early 20th Century.
Ringsend WWTW was designed with projected loads of 101 tonnes TSS by 2020. However, due to rapid population growth and infrastructural expansion in Dublin during the last fifteen years the plant was accepting 2020 design loads on opening in 2003. These loads tested the capacity of the plant, especially in its capacity to process the volumes of sludge produced, and a sludge stream expansion programme was thus put in place.
In 2008/2009 the sludge stream at Ringsend was expanded. This brought the CAMBI process from 2 streams to 3 streams. In addition to this a 4th anaerobic digester was built, and 3 extra drum thickeners (bringing the total to 6) were commissioned. The extra thickeners allowed for all SAS to be thickened, thus stopping (or minimising) the practice of co-settling. The new CAMBI THP stream allowed for duty/duty/standby operation and the ability to run 3 streams in times of increased sludge production, or when reduced capacity in the thermal drying plant necessitates processing more through the CAMBI plant. The 4th anaerobic digester allowed for the capacity to deal with the increased sludge loading from the 3rd CAMBI stream without compromising sludge retention times.

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