Leading edge

One of the first power plants, at Kings Lynn, Norfolk

A new treatment process designed to eliminate pathogens from sewage sludge will save the UK’s Anglian Water £3 million a year in operating costs.

Farmers have for years been spreading treated sewage sludge on their fields. Anglian Water generates 180,000 tonnes of so-called biosolids per annum.

“Anglian Water’s strategy is to recycle as much of its biosolids product as possible to agriculture,” explains the company’s special projects delivery manager Ian Turner.
Some 90% of treated sewage sludge goes to land. “Though biosolids are a treatment byproduct that we have to get rid of, it’s not really fair to call it a disposal route – farmers want it as a soil improver. Recycling biosolids to agriculture is also more environmentally sustainable than alternative disposal routes, such as landfill or incineration,” Turner adds.

Much of Anglian Water’s sludge is currently treated by dosing it with lime, which elevates its pH, so killing off nearly all pathogens – disease causing bacteria, protozoa, viruses and parasitic eggs. But lime treatment increases the volume of biosolids to be disposed of. Quarrying, manufacturing and transportation of lime is also environmentally costly. Anglian Water has therefore set out to replace lime treatment.

“To safeguard the agricultural disposal route, we decided to treat our sludge to the highest standard, which is 99.9999% pathogen free,” Turner says.

"Biosolids are a treatment byproduct that we have to get rid of, but farmers want it as a soil improver." Turner

Anglian Water is achieving this by replacing the existing sludge treatment at four works with new hydrolysis processes. “Sludge from primary and secondary sewage treatment is currently fed into digesters,” explains Mott MacDonald water director Mark Enzer, who has played a key part in delivery of the new plants. Digesters are large tanks where, over the course of 12 days, naturally occurring bacteria anaerobically break down the sludge particles, producing methane in the process. Hydrolysis is being introduced before sludge is put into the digesters. Two types of hydrolysis are being used. Enzymic hydrolysis is a two stage process, in which heat is used to stimulate indigenous bacteria to break down molecules. Thermal hydrolysis involves cooking sludge at 160°C and 8 bar of pressure. The great strength of hydrolysis is in making cellular material easier to digest.

The predicted results are spectacular, Enzer says. As well as enabling pathogens to be all but eliminated through the digestion process, hydrolysis results in the conversion of a higher proportion of solid matter to gas. Methane will be used to fuel combined heat and power plants, making each of the four sludge treatment centres heat and energy self-sufficient, and providing enough surplus to sell to the national electricity grid.

Improved efficiency of the digestion process also increases the solids content of treated sludge. Together with the elimination of lime treatment, the new processes will reduce the volume of material to be recycled by 25%.

Energy self-sufficiency and the reduction of vehicle movements needed to bring lime to site and to remove biosolids is helping deliver operating expenditure savings of £3 million per annum, Enzer reports. Though the capital cost of the hydrolysis processes is greater than for alternative options, the premium will be paid back through reduced operating costs within the first three years.

Key sustainability facts

• Maintains environmentally sustainable sewage sludge disposal route
• Sludge treatment centres are energy self-sufficient
• Lowest whole life cost – increased capital cost is more than offset by reduced operating cost
• Reduced transport mileage