If you wondered just what has been happening at Practically Green over the past year, well here is the answer!
This is Goddards Green Sludge Treatment Centre. Practically Green was sub-contracted by Siemens PLC to do the process engineering and commissioning. The site building team was one of the best we have ever worked with and the results speak for themselves.
Here is a brief description of the project.
Building began in April 1998
Construction Completion - May 1999
28 day testing programme - June 1999
Handover to client was immediate!
Sited between London's Gatwick airport and Brighton, the Sludge Treatment
Centre enables the purchaser, Southern Water, to process surplus sludges
from 81 Sewage works into biogas and fertiliser.
Until recently, this sludge would have been sent to sea for dumping or ploughed into agricultural land. However, the Urban Wastewater Directive from the European Community and the pressures from Supermarket chains have removed or eroded these traditional avenues for sludge disposal for many water authorities and alternative 'land based mechanisms had to be put into place to process sludges on land. Goddards Green is one of the first of a new generation of flexible processing plants that convert both pressed sludge cake and imported liquid sludges into biogas and a wonderful black bread crumb structured compost like fertiliser which is being purchased by local farmers.
The process revolves around two very effecient
anaerobic digesters with a total capacity close to 4000 cubic meters. The
digesters feed off thickened sludge and convert it into high quality biogas
and a digested effluent which contains half the input solids and consists
mainly of digester bacteria. These digester bacteria die on contact with
air and form the high quality fertiliser. 
There are two input streams, one is for sludge cake. Sludge is mainly the dead bacteria grown on sewage in small remote works). The sludge cake is made by adding a small amount of polyelectrolyte and then pressing the cake on a belt press which removes the water and leaves stackable solids which cost only 10% of the cost of liquid sludge to transport.
Upstream of Goddards Green, many small works produce 500 tonnes of sludge
a day and this is caked to form just over 50 tonnes a day of cake for processing
at the new treatment centre.
In addition, 200 tonnes of liquid sludge is imported (this may rise to
332 tonnes in the future). The liquid sludge is measured for accounting
purposes, screened in very highly efficient strain presses and then thickened
in drum thickeners prior to digestion.
Some of the imported sludges are also used to blend with cake to form thickened sludge for digestion. This thickened sludge stream is also screened in a strain press and thus the digesters receive only screened thickened sludge.
After digestion, the sludge is stored in secondary digesters and then
centrifuged - this dewaters the fine microbical mass into 25% TS cake for
sale to the local farmers. 
Over 4000 cubic meters of biogas per day is made in the process and after the plant has settled down and the final biogas flowrate has been established this may be used for one of a number of options. At present chp, pasteurisation and other energy uses are no doubt being considered.
