Proceedings

Digester Outage Management

Winter, P., Linsell, D., Onanuga, A., Waghorn, A., and Williams A., Thames Water

(free)

Abstract

Thames Water operates 25 AD sludge centres with some 120 anaerobic digesters. Operating the digesters efficiently is an important factor in helping us hit our target of generating 30% of our own electricity by 2020. Taking a digester out of service is expensive and has significant impact on selfgeneration, sludge processing and operational practice. It is therefore imperative to manage digester outage effectively and to minimise the associated downtime of an asset. The paper outlines Thames Water’s AMP6 digester cleaning programme and discusses issues that potentially result in extended downtime. Digester outage involves a significant number of business units – asset management, operations, H&S, sludge recycling and other stakeholders must be involved in the process. As well as the strategic perspective, the paper gives examples of good operational practice for digester outage management and discusses an innovative approach of digester degritting without taking the asset out of service.

Keywords Anaerobic digestion; biogas; digester cleaning; de-gritting

Introduction

Thames Water operates 25 AD sludge centres with some 120 anaerobic digesters. The company treats 342,000 tonnes dry solids (tds) of sludge per annum (2014-15), looking to release 380-420 Nm3 /TDS of biogas generating 800-1000 kWh per TDS in a fleet of some 50 combined heat and power (CHP) generators totalling 50 MWe installed capacity at 24 sites in 2015. Thames Water’s strategic direction and investment plans are focusing on reducing the wet mass of sludge recycled to land, improving sludge quality and increasing the proportion of sludge that is treated through enhanced digestion such as thermal hydrolysis (THP). This strategy will simultaneously address requirements for larger throughput due to population growth and support the drive to improve renewable energy generation from sewage sludge through AD CHP (Pearce et al., 2014). As a result, the amount of sludge treated by conventional digestion will be reduced in AMP6 as more sludge is treated through THP (Figure 1).

The efficient operation of the digester fleet is an important aspect of our strategy and in helping us hit our target of generating 30% of our own electricity by 2020. The company recognises sludge as a resource and Thames Water’s innovation department is increasingly involved in developing options to unlock the full energy potential from sludge.

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