Historically it has been shown on many projects that centrifuges can outperform belt presses in dewatering digested sludge. However, results from a number of projects where sludege has been treated before digestion with Cambi thermal hydrolysis process (THP) hase 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 extracellular polymer (ECP) that binds water manily to secondary sludge and limits dewaterability and compressibility of the mixed sludge. The digested sludge after THP is also has much finer in structure and this can lead to problems with capture rate, higher polymer rate with some centrifuges. The aim of this work was to aid process selection for final dewatering for future projects to ensure a cake of greater that 30% DS and with a good capture rate.
At Cotton Valley WWTP in the UK, Cambi THP was retrofitted to an existing digestion plant in order to increase throughout 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 tetrofit. One new centrifuge was added. The site historically had a poor dewatering performance - average 22% DS and it was expected that the dewaterability would increase to >20% DS based on experience of retrofitting Cambi in other projects where 10% points, increase was typical. However to date the plant has averaged 26.7% DS and has not been able to achieve greater than 29% DS even with the new centrifuge and has problems achieving good capture rate. The site has a polymer limitation of 6kg/tds which does not help the situation, but increasing polymer seems to lead to unstable results.
Trails of other dewatering rechnologies were arranged to ascertain whether the problem was slude related or equipment related. Full scale trials of a commercially available belt press and a commercially available centrifuge were carried out in the fall of 2008 and the spring of 2009.
A 1 meter high performance belt press was tested for a one week period at Cotton Valley in November 2008. The belt press gave 28-32% DS with the onsite polymer and up to 36% DS with a new polymer. Cambi digested sludge is at 6% so that at normal belt feed rates the loading on hte 1 meters belt was up to 850 kg dry solids/h/meter belt. By reducing the load rate to 525 kgs/h/meter belt the very high dewaterability was achieved. Average polymer does was about 3 kgs/tds. Observed filtrate quality was higher than the existing centrifuge operation. The cake was visibly of much higher quality and would store more easily and be more useful as a soil improver.