Historically it has been shown on many projects that centrifuges generally outperform belt presses in dewatering digested sludge. However results from a number of projects where sludge has been treated before digestion with thermal hydrolysis process (THP) have shown good results for belt press dewatering – generally greater than 30% DS. The authors propose that the main mechanism in THP is in changing the structure of water binding extra cellular polymer (ECP) that binds water mainly to secondary sludge and limits dewaterability and compressibility of the mixed sludge.
At Cotton Valley WWTP in the UK, Cambi THP was retrofitted to an existing digestion plant in order to increase throughput from less than 10,000 tds per year to over 20,000 tds by importation of other sludges and to make a class A product. Existing centrifuge dewatering was retained as the plant had to keep producing digested cake during the retrofit. One new centrifuge was added. The site had poor dewatering performance historically – average 21% DS and it was expected that the dewaterability would increase to >30% DS based on experience retrofitting Cambi in other projects where 10% points increase was typical. However to date the plant has not been able to achieve greater than 29% DS with the new centrifuge and has problems achieving good capture rate. The older centrifuges give variable results and site is struggling to optimise the dewatering. Average polymer dose is 6kg/tds.
Trials of other dewatering technologies were arranged to ascertain whether the problem was sludge related or equipment related. Three tests have been carried out: on site belt press, on site filter press, laboratory filter press. At the time of writing results are available from the belt press trial the remaining results will be in the main paper.
A 1 meter high performance Ashbrook belt press was tested for a one week period at Cotton Valley on October 2008, initially using the on-site polymer, Ciba Zetag 8180. This was subsequently changed to 8587 that was more suited to the smaller particles produced in the THP. The belt press gave 28-32% with the onsite polymer and up to 36% DS with the new polymer. Cambi digested sludge is at 6% so that at normal belt feed rates the loading on the 1 meter belt was up to 800 kgs dry solids/h/meter belt. By reducing the load rate to 500kgs/h/meter belt the very high dewaterability was achieved.
Average polymer dose is about 3 kg/tds. Observed filtrate quality was higher than the centrifuge operation – results to follow. The cake was visibly of much higher quality and would store more easily and be more useful as a soil improver.
Filter press results will follow – but the belt press results suggest that the sludge is highly compressible and comparable or better results should be obtained with the filter press trial. More work is being done on optimising the centrifuges as it appears that the sludge should dewater much better.
The annual payback of belt press dewatering is approximately: power £40, Polymer £50 k and biosolids recycling £80k k (based on 10,000 tds digested). Unless the performance of these centrifuges can be improved there is a financial case changing them to belt presses.
The trial clearly demonstrated that the Cotton Valley sludge is highly compressible and that in this case that belt presses can achieve very good dewatering results.
Keywords: Dewatering, Digestion, Thermal Hydrolysis, Polymer
