Leach, K.1 and Edgington, R.M.2, 1United Utilities PLC, 2 United Utilities(free)
Lancaster sludge treatment centre treats the indigenous and imported sludges from the surrounding area. In 2011, an enhanced enzymic hydrolysis plant (EEH) was commissioned in front of the existing mesophilic digesters. The plant is designed to treat 7080tdspa and to meet an enhanced treated quality standard.
This paper will discuss the design, commissioning and the performance of the EEH plant and the key lessons learned.
Keywords Enhanced Enzymic Hydrolysis, Safe Sludge Matrix, CHP, log kill, E.Coli, Digestion
Introduction – Enhanced Enzymic Hydrolysis
In the UK, the main advanced digestion processes are Enhanced Enzymic Hydrolysis (EEH) and thermal hydrolysis (TH) to produce an enhanced treated sludge. EEH, consists of Stage 1 (three reactors in-series at 32-42O C), followed by Stage 2 (one heating reactor, and two holding tanks all at 55O C), before feeding into the parallel operated mesophilic anaerobic digester (MADs). This is shown in Figure 1
The first EEH plant was installed at United Utilities Blackburn WwTW in 2005 and showed a volatile solids destruction (VSd) of around 55% and excellent E.Coli destruction (Le 2006). Later plants have been installed at Cambridge (2007), Kings Lynn (2008), Great Billing (2009) and Eign (2009) (Bungay 2008); the same paper gives details on their design. The sites show a VSd of about 50 to 55% (Asaadi 2008, Riches 2012). With greater VSd than conventional mesophilic anaerobic digestion, more biogas is produced which leads to greater energy generation potential; at Kings Lynn 0.75MWh/TDS is generated (Riches 2012).
Heating in stage 1 has always been by hot water heat exchangers. Initially, heating in stage 2 was obtained using hot water heat exchangers. However, more recent plants from Kings Lynn onwards have used direct steam injection into the sludge to reduce the risk of viviantite scaling (due to Iron salts being used to remove phosphorous on the treatment works).