UK advances on digestion

In just two years, the UK has seen a shift away from incineration and towards more sustainable methods of sludge management. Ebcor's managing director, Keith Panter, shows how a global shift is now under way

An article in WWT July 2009, by Ebcor, on the shift from drying raw sludge to advanced digestion (AD), concluded: “These projects demonstrate a new paradigm in the biosolids world. In an energy dependent world sludge should be an energy provider not an energy user. Future world energy prices and a general trend to green electricity credits around the world will most likely spawn more projects of this kind where energy cost/ benefit rather than disposal becomes the major project driver.”

Since then, there has been a new trend – the closing or deferral of incineration capacity:

  • United Utilities changed its approach and abandoned plans for an incinerator near Preston in favour of advanced digestion with thermal hydrolysis (ADTH) at Davyhulme
  • Severn Trent has cut back on incineration dramatically in favour of land application
  • Thames Water is deferring expenditure at its two London incinerators in favour of ADTH
  • Kelda plans to decommission its Bradford incinerator in favour of ADTH and has long term intentions to phase out incineration.

Why is this happening? Obviously, cost is one factor – building and operating incinerators is very expensive. In addition, incinerators are poor power generators compared with AD, especially with the bonus of Renewable Obligation Certificate (ROC) subsidies. But the other factor is the success of UK water companies in recycling biosolids.

The UK has reasonable sludge regulations and skilled biosolids managers. The gradual improvement in biosolids quality, with the advent of advanced anaerobic digestion (AAD), has led to more confidence in the market place and respect for biosolids management. It is no coincidence that the other trend in AMP4 and AMP5 is the move away from lime stabilization projects in favour of AAD, once again driven partly by cost but also the realisation that quality biosolids products are key to a sound sludge strategy.

All told we calculate that the move to AAD in AMP4 and AMP5 accounts for more than 20% of all sludge treated in UK. This is a paradigm shift.

This trend in the UK has been noticed in other parts of the world and there have been many visitors coming to see what we have been doing in the past few years. There are a high proportion of ADTH projects using the Cambi process and the success of the UK projects has led on to further projects in construction or development in many parts of the world.

The most notable recent success for Cambi is to be selected as the designated pre-treatment technology for DC Water’s Blue Plains digestion project in Washington DC, US. DC Water currently produces more than 1,000 tonnes of class B lime-treated cake per day.

The biosolids programme has been successful, but is very expensive to operate. An exhaustive review of technology was carried out to look at the long term sludge strategy. The basic choices came down to some form of digestion with or without thermal drying. DC Water’s general manager George Hawkins says: “In developing our plans further in recent years, we recognized that we needed to produce a biosolids product that was Class A (pathogen free) and that had maximum flexibility for multiple reuse markets. And we needed a digestion process that could fit within our very small available footprint.

“We had tracked the development of thermal hydrolysis process (THP) for many years, but in 2006 we began a more concerted effort to confirm the process could be of major value to DC Water. Because thermal hydrolysis of sludge is a new process that has not been implemented in North America DC Water conducted thorough evaluations that included several years of pilot testing with our material.

“We also investigated many of the 20 plants overseas, and confirmed successful operation with process operators and owners. By 2009, we completed our due-diligence and assessment work and were ready to proceed with detailed planning, design, and implementation. We found that the thermal hydrolysis process developed by Cambi has by far the most experience, especially with large plants like Blue Plains.”

Blue Plains’ 300t/d of raw dry sludge will, after AAD, produce biogas to generate 13MW of electricity in high efficiency gas turbines. There is sufficient hot gas from the turbine exhaust to produce all the medium pressure steam required for the Cambi process. This electricity saves the plant £6.2M per year and goes towards meeting DC Waters drive to reduce reliance on fossil fuels – currently 32-35MW.

Another benefit is that the plant can make a cake of about 30% dry solids (DS) using low energy belt presses. The final amount of cake will be half of current and the expected saving in recycling a further £6.2M per year. The main process train, consisting of pre-dewatering, 24 reactor THPs and four very large digesters, is being delivered as a design-build project that is expected to be operational by 2014. At the other extreme, the latest innovation in the Cambi THP range is the development of a compact version of the THP aimed at medium sized wastewater treatment plants.

This compact version is applicable to treatment plants in the 100,000 to 500,000 population equivalent (PE) range. The Cambi compact plant in Drammen, Norway, is designed to treat about 20,000 tonnes of liquid sludge (3-4% DS) per year from municipalities in the Drammen region (about 200,000 PE), together with some grease and septic waste. Construction started mid 2010 and the plant is scheduled to be finished in the second half of 2011.

The UK has been a test bed for AD over the past 10 years. The combination of green energy policies, good biosolids practice and regulation, and good commercial management has made the UK a world leader in this area of wastewater treatment.

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