Pretreatment process puts heat on sludge disposal
15 Jan 2000
A joint venture between sludge dewatering specialists Simon-Hartley and Norwegian thermal engineers Cambi is introducing a new sludge treatment process to the UK.
Currently being installed at Thames Water's Chertsey treatment works in Surrey, the process uses thermal hydrolysis to disinfect sludge prior to disposal to agricultural land. Over half of the million tonnes plus of dry solids sludge produced annually in the UK currently goes to agriculture, with this figure estimated to rise to 900 000tpa by 2002.
The Cambi thermal hydrolysis process is claimed to more effective than the biological hydrolysis that normally occurs in digestion. Hydrolysis sterilises the sludge, so that the methanogenic bacteria active in the digesters are not disturbed by varying bacterial content of the incoming sludge.
The patented technique uses direct steam injection to heat the sludge to 130 C at residence times of around 30minutes. By providing consistent temperatures and pressures in the process, the system is said to produce ideal feed sludge for anaerobic digestion.
The result is that biogas production from the digesters can be increased to the extent that the Cambi process could be run entirely on recycled energy from a CHP system powered by the biogas.
The first plant using the process at HIAS in Norway, treating sewage from a 70 000 population (2660 tons of dry solids/year), produces on average 2560Nm3/day of gas with a 65 per cent methane concentration. This equates to 15000kWh/day as net energy in steam at 10bar. Consuming only 2725kWh/day, the plant is a net energy exporter.
The Cambi process is being retrofitted into Chertsey where a centralised treatment centre handles sludge arising from six different locations. The plant was previously only taking 4000 tonnes a year, but this will be increased to 8000 when the thermal hydrolysis plant goes on-line in December.
