Digesting change

Monsal and United Utilities recently held a conference on advanced digestion technology. Aidan Cumiskey from Monsal reports on some of the key findings


The first UK conference and workshop on advanced digestion technology was held on June 24, 2005 recognising the need within the water industry for more advanced technology which will meet the future requirements of sludge processing in the UK. Water industry delegates met at the event in the north-west to discuss the latest innovations and application of advanced technology in this specialist sector.

UK water utilities are looking for more efficient solutions for sludge processing whilst ensuring sustainability of agricultural recycling and maximising green credits for electricity production. The anaerobic digestion process still treats over 700,000TDS or about 50% of all sewage sludge in the UK and is a key asset to the water industry for sludge stabilisation, odour management, pathogen reduction and renewable power generation. However, this existing asset base is coming under increasing pressure from the new demands of modern sludge processing such as thicker sludges, polymer thickened sludges and legislative demands for increased levels of pathogen reduction. New market drivers such as renewable energy production are becoming more important as wastewater sludges becomes an increasingly important revenue stream for many sludge-processing facilities.

The conference and workshop, jointly hosted by United Utilities (UU) and Monsal, invited speakers representing industry experts and technology vendors to discuss the latest technology advances. Approximately 40 key industry specialists attended the event,
including sludge managers, energy managers, process engineers and specialist project managers. All parties were actively involved in implementation of sludge management capital programmes and operations within the water utilities.

Jeff Lang, UU chief operating officer for wastewater, opened the event and welcomed speakers and delegates from the water utilities and technology companies. Lang confirmed UU's commitment to advanced technology and innovation in the sludge sector. He also highlighted construction of the new enzymic hydrolysis digestion
facility at Blackburn as an example of innovation in what will become an established European centre of excellence in the area.

Dr Tim Shea from the US consulting firm CH2MHill gave a review of the key process variables and benchmarks to look for with advanced digestion systems. Dr Shea, a senior technical specialist in the areas of residuals and biosolids management, has worked on some of the latest advanced projects being implemented in the US. One of the most significant points he raised was that the benchmark of VS destruction is moving from a recognised norm of 40% with conventional digestion systems towards an upper band of 70% for advanced systems. A family of processes now exists in which operations occur with mesophilic or thermophilic and single or multiple stages, with reactors in series or parallel. These processes seek to deliver better performance than conventional digestion technology.

The drivers for advanced digestion now also include greater pathogen reduction, digestion of thicker sludge, greater throughput and biogas production, improved dewaterability, control of foaming, greater product stability and ultimately a higher quality product.

Multi-stage digestion is now being considered for some of the largest sites in the US. The trend is towards more optimal pre-treatment reactors. A number of large projects are currently at the design or construction stage in the US, and operational plants now exist in Baltimore, Du page, Duluth, Vancouver and both Terminal Island and Hyperion in Los Angeles. Back River WwTW in Baltimore, Maryland, has had a single-vessel acid hydrolysis system in operation since 2003 as a full scale demonstrator.

Commenting on the differences between the US and UK, Dr Shea said, "The development of anaerobic digestion systems in support of sustainable land application of biosolids is being pursued differently in the US than in the UK. In the US the trend is to use complex and costly thermophilic anaerobic digestion systems, whereas in the UK, and especially at several works owned by UU, efficient acid hydrolysis systems preceding mesophilic anaerobic digestion systems are used to accomplish similar levels of disinfection, but at a much lower capital and operating cost."

An assessment of the current digestion asset loading was reviewed during the conference. This pointed to a conventional digester-loading band reported in the UK at 2-3.2 kgVS/m3.day. The 50% increase in capacity in the last ten years has mainly been the result of increased use of mechanical thickening and has seen DS% to digesters increase from 2.0-5.0% to 5.5-6.0%. Traditional refurbishment techniques such as upgrade of key processing equipment including digester feed pumping, heating, mixing and biogas management systems have been employed to allow this increase in sludge processing capacity for many of the older configured digestion assets.

The resultant loadings implemented during the AMP3 (2000-2005) investment period pushed the digesters loadings towards the higher band. For example, both UU and Severn Trent Water adopted 2.8 kgVS/m3.day (15 days at 5.5% DS) as their asset standards.
Conventional anaerobic digestion of sewage sludge centred on delivering all three digestion phases in a single completely mixed reactor. Anaerobic digestion involves three discrete steps:
n hydrolysis - large molecules/polymers such as polysaccharides and proteins are converted to smaller compounds,
n acidogenesis - conversion of small compounds to volatile fatty acids (VFA) by anaerobic digestion,
n methanogenesis - VFA broken down by methanogenic bacteria to methane and CO2.

Advanced digestion technology
The impending change in legislation in 2002 led to some water utilities questioning whether a single stage of operation could deliver the benefits required whilst meeting the relevant legislation.

UU was the first utility in the UK to invest in advanced digestion
technology that sought to separate the digestion phases and optimise each separately. The construction of the first advanced digestion facility at Ellesmere Port in 1997 employing the use of ATAD pre-treatment marked a change in the industry. More fundamental research into the digestion process resulted in UU developing its own process which optimised the hydrolysis step by using a plug flow reactor. UU developed the technology in partnership with the specialist sludge
technology company Monsal.

The resulting enzymic hydrolysis process was tested in 2000 at Macclesfield with the first commercial plant operational at Bromborough. At least three more plants will be in operation by the end of AMP4 with construction at Crewe and the largest plant, Blackburn (21,600TDS) already at an advanced stage. Some of the early development work and performance of the recent full-scale plants were described by Dr Son Le from UU, who reported reliable operation and VS% destruction in excess of 60% for both the Macclesfield and Bromborough plants.

The author reported a review of the sewage sludge digestion market in the UK. 18 full-scale sites will employ advanced digestion processes such as enzymic hydrolysis, thermal hydrolysis and acid gas pre-treatment prior to mesophilic digestion. More than 66 digesters will be upgraded using bolt-on advanced technology at the end of 2005. The estimated UK capital investment in advanced digestion technology to date is in excess of £25M at the technology level. This figure can be doubled for the enabling works and reflects that the technology cycle has moved out of the early adopters and into mainstream implementation in less than ten years.

Design and operation of advanced reactors was explored in the final session. The session involved a discussion of three technologies (cambi thermal hydrolysis, acid gas and enzymic hydrolysis) that now have good operational experience in the UK.
Keith Panter (EBCOR) provided an update on the latest developments with the cambi thermal hydrolysis process. He stated that a major advantage with advanced processes, and in particular with high VS destruction, was that when they were used in conjunction with thermal dryers the overall energy balance was substantially improved, resulting in smaller dryers with resultant reduced operational costs. A case study for Dublin highlighted this, where the evaporation rate was reduced from 11.2t water/h to 5.7t water/h for the cambi/digestion/drying option.

Presentations from Dr Manocher Asaadi (RWE Thames Water) and Dorian Harrison (Monsal) involved discussion of their experiences in reactor design, equipment selection and process performance for the advanced systems. Operational feedback from the Swindon acid gas plant demonstrated that the plant has enabled high sludge throughputs, high organic digester loading (above 4.0kgVS/m3.day), stable digester performance and improved E.Coli kill which guarantees the treated sludge standard on this site. Key engineering aspects of a plug flow reactor were outlined by Dorian Harrison and it was demonstrated that very high levels of VFA generation and pathogen destruction can be achieved by a plug flow configuration. Key engineering considerations for these advanced systems such as biogas management, sludge pumping, and heating and mixing systems are critical to the success of these units. Expert application knowledge is being developed in the UK, as illustrated by the advanced heat exchanger concepts employed in the enzymic hydrolysis plants, which allow for high DS%, efficient heat transfer, avoidance of blockage and anti-sludge baking control on packaged heat exchanger modules.

A panel discussion followed the conference with many questions relating to comparisons between the US and UK markets. Integration issues including biogas management, digestate liquor strength and treatment, and the cost of the various systems were also discussed. This was followed by a site tour facilitated by UU to their enzymic hydrolysis digestion plant at Bromborough. The plant produces a high quality sludge product and enables digester operation above 4.0kgVS/m3.day. The plant has been fully operational since January 2002, processing on average 250-275m3/d up to a peak of 375m3/d.

In summary, the conference highlighted the new multi-stage processes and their capacity to deliver higher VS destruction and renewable energy production than the equivalent single-stage process. The optimisation of reactions allows the digestion process to be carried out much faster and hence allows higher loading of existing digester assets whilst producing a high quality product.

Addition of bolt-on advanced technology is now a viable means of upgrading the performance and processing capacity of existing anaerobic digestion assets. This approach can be adopted by providing stand-alone reactor configurations with dedicated equipment whilst minimising any refurbishment of existing digesters.

The trends emerging from the UK, with clear supporting evidence discussed at the conference from overseas, was that the emphasis on digestion technology will now shift to pre-treatment and reactor configuration, which will be critical to the success of these plants in the future. There is now a proven suite of technologies available which will meet the demanding conditions set for modern sludge processing in the future. nnn
The author would like to thank Dr Tim Shea (CH2MHill), Dr Son Le (United Utilities) and Dr Manocher Asaadi (RWE Thames Water) for their contributions in the preparation of this article.

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