The Advantages of Advanced Anaerobic Digestion with Thermal Hydrolysis Over Sludge Drying

In an energy dependent world, sludge should be an energy provider not an energy user. Keith Panter of Ebcor argues in favour of projects where energy cost/benefit rather than disposal becomes the major project driver. He also promotes the raised AD conversion rates achieved by what he and Cambi describe as "advanced digestion with thermal hydrolysis".

The following article has is based upon an article in Water and Wastewater Treatment August 2009 edition. In that article Keith Panter has highlighted the need for energy as the UK finally passes from the age of UK energy sufficiency into dependency upon supplies from overseas, and not only that but dependency on supplies from nations which have been politically unstable in quite recent times.

He highlights the welcome fact that following a recent review of energy policy, Dwr Cymru Welsh Water has decided to reduce its reliance on energy intensive thermal drying of raw sludge at three Wastewater Treatment Plants in South Wales in favour of anaerobic digestion (AD) and power generation at Cardiff and Afan.

The energy these AD plants will generate is high compared with farm waste type AD plants, and the aim is to generate 5MW of green power by March 2011 to grandfather the value of the Renewable Obligation Order Credits (ROCs). Apparently a technical and commercial review of pre-treatment technologies for AD has led to the selection of Cambi as the THP provider. The operator is Kelda who and they were consulted as part of the decision.

What I find remarkable is that he informs us that when complete, eight Cambi plants in total then operating in the UK and Ireland will be treating sludge from the equivalent of a 6M population. That’s almost a tenth of the United Kingdom population!

So, it reinforces me in the view that already in a short period of 5 years since I started to write about Anaerobic Digestion in my www.anaerobic-digestion.com web site, Anaerobic digestion has progressed from a novelty seen as largely just a sludge treatment and farm waste disposal process, to a main stream renewable energy producer.

Of course, it has been the rapid development of economic driving forces (drivers) which have moved the agenda forward at such a pace, and in recent years government incentives such as ROCs that have given the final push to propel AD into the limelight.

Back in March 1999, the Economist magazine (UK, London) predicted that energy prices would remain stable for the foreseeable future. How wrong could they have been?

Water treatment professionals got closer to reality, Keith says, when in a straw poll at the 1999 European Biosolids conference they were asked what they thought, and predicted a doubling of energy prices within ten years.

In reality the wholesale price of natural gas that was about £6/MWh at that time rose to over £20/MWh in the time period.

Ebcor has clearly for 10 years and longer, been looking in depth at the process cost model for sludge drying. He also evidently demonstrated at that same conference 10 years ago that investment in raw sludge drying would be misplaced if the price of energy even only doubled.

He explains that it takes about 1 MWh of energy to evaporate 1 tonne of water - so in general at that high an energy cost it is much cheaper to haul water in un-dried sludge cake than to evaporate it in a dryer.

The same model has apparently shown that the magnitude of the carbon footprint is more or less related to process cost as most sludge process costs are primarily energy related. Ebcor concluded at the time that advanced digestion (such as using the Cambi process with thermal hydrolysis) would give the best whole life cost and the most flexibility and would safeguard utilities from future energy price hikes.

The non financial justification of raw sludge drying is the flexibility offered by producing a potential fuel. Keith points out that this surely applies if markets do not exist for biosolids products but in reality these markets have remained stable and in some cases improved as biosolids quality has improved.

The UK safe sludge matrix was developed about the same time and has been a big help in securing biosolids' place in agriculture.

So, there it is. Yet another Anaerobic Digestion expert is in effect demonstrating very clearly that the day for AD has arrived. Are you using Anaerobic Digestion, or still drying WWTW sludge?