Historically anaerobic digestion (AD) has been considered as an essential process to stabilise raw sludge and thus reduce its smell. Biogas production was a secondary benefit and combined heat and power (CHP), known as cogeneration was used in larger projects. It was generally accepted that 45% organic matter (VS) reduction was attainable and that this would ensure the sludge was sufficiently stabilised.

However the production of digested sludge had lost popularity in some countries because of concerns over whether digested sludge would continue to find applications in agriculture in the future. This has led to an increased interest in drying raw sludge and incineration projects with some interest also in gasification.It has been considered then that AD is an unnecessary step in such cases.

In many countries this thinking has changed in the last year or two. The current and inevitable increasing cost of energy combined with the value of Green Certificates has meant that energy is now a major driver for sludge projects and that digestion is thus a process that warrants greater understanding.

In some countries such as the UK, Benelux and Germany, the combined value of avoided power purchase and green certificates means that there is a premium in designing systems that give significantly higher VS conversion. In such cases digesters becomes a profit centre and the owners are seeking to maximise their throughput by importing raw sludge from other sites as the value of green electricity produced exceeds the overall cost of sludge treatment and biosolids recycling.

Also the volume reduction after dewatering is a plus whether the product is recycled or disposed by thermal methods. In the last 5 years the science of digestion has moved on dramatically and it is now widely accepted that digestion is best with some form of pretreatment.

One of the first advanced sludge digestion plants in the world was built in 1995 for the Norwegian Water and Wastewater Company, HIAS. The plant uses the Cambi Thermal Hydrolysis Process (THP).

The sludge is pressure cooked in the THP and fed to a below ground 1300m3 digester at about 11% dry solids.

The THP process makes the sludge non-viscous so it can be mixed easily in a conventional mesophilic digester. The plant, serving about 100,000 people, has been operated successfully for 10 years and processing about 2800 dry tonnes of sludge per year.

Three years ago HIAS manager, Tor Fjaergaard, decided that there was enough biogas to install a 320kW engine: “The decision was helped by the fact that the long term low cost supply expired that year” said Tor. “The engine’s waste heat is used to supply heat for the boiler that drives the THP process, so it’s very energy efficient”

The product made at HIAS is a pasteurised cake at about 30% DS and has been approved by the Norwegian government as a product and is therefore free of waste regulations. The reliability and success of the plant has led to it becoming an import centre for other organic wastes and sludges in the area and the feed quantity has risen progressively to 3400 dry tonnes per year.