O’Reilly C, Bickerstaffe J and Adam J
Celtic Anglian Water Ltd


The Ringsend WwTP is Dublin city’s main wastewater treatment facility. Prior to its 2-phase upgrade, beginning in 1997, the plant was a primary only system with settled sewage discharged directly into Dublin Bay and sludge disposal at sea. Now, the plant caters for a population equivalent of 1.7 million people with secondary and tertiary wastewater treatment and advanced sludge management in the form of thermal hydrolysis, anaerobic digestion and thermal drying. A complete solids treatment facility has been installed at the works capable of processing an average of 105 tonnes dry solids/d. Co-settled primary and surplus activated sludge, scum liquors, and digester drain down are held in two holding tanks prior to processing. From these tanks, the solids and liquor are screened and pumped to one of two solids buffer tanks for storage. Screened solids are dosed with polymer before being fed to five belt presses for thickening and dewatering.

A ‘Cambi’ thermal hydrolysis plant (THP) operates prior to anaerobic digestion in a dual role of sludge sterilization and sludge hydrolysis (Figure 1). Hydrolysis is usually the rate limiting stage in anaerobic digestion so utilization of the THP results in a break down of cells and release of soluble COD for digestion. Soluble COD, mainly in the form of volatile fatty acids (VFAs) can by as high as 130, 000 mg l -1 in the digester feed. Sludge is initially dewatered to c 14% DS, heated and pumped into reactors where steam is added at 6 bar and the temperature is raised to 165o C. After 24 minutes hydrolysis retention, the sludge is subjected to a rapid pressure drop, cooled and fed to 3 x 4250 m3 mesophilic digesters. Hydrolysed sludge feed to the digesters can be as high as 11% DS. Conventional anaerobic digestion would require significantly increased digester volume as well as significantly lower %DS feed. The process enables digestion at very high dry solids feed and low hydraulic retention time. The net effect is that the plant can produce 3.5m3 /d/m3 of digester capacity. Heat for the mesophilic anaerobic digestion process is provided in the form of heated hydrolysed feed.

Average daily biogas production is 27,000 m3 /d with the methane content of the biogas averaging 65%. There are 4 dual powered 1mW ‘Jenbacher’ engines operating mainly on this produced biogas, contributing electrical power to the site load. Each pair of engines are also fitted with a waste heat boiler with a capacity of one tonne of steam per hour. These boilers have sufficient capacity to provide 70% of the steam required for the THP. There are also 2 biogas boilers for top up steam.

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