I'm familiar with a plant that uses both DAF and belt filter presses.  My understanding is the coagulant often aids in thickening and higher solids percents are achieved (or less chemicals are needed during dewatering).

Adding chemicals before the dewatering machines can effect the performance of the polymer used for dewatering.  Before adding any chemical, jar testing of the sludge with the polymers you presently use would be a good idea to see how it effects the reaction.  I ran a dewatering test trailer for one of the centrifuge manufacturers for many years as well as commissioning the equipment and there were occasions where chemicals added upstream effected the dewatering performance.  Sometimes too much chemical is added and it decreases the performance of the polymer used at the dewatering equipment, but I did a test at one location where coagulation of the sludge was required to get good performance.  There really doesn't seem to be a correct answer to this, every plant is different and bench testing seems to be the only way to get a good grasp of what will happen in the machines before actually testing.

As for the DAF units, I have found they tend to retain air in the sludge and require more polymer in centrifuge dewatering since this then causes some of the sludge to float resulting in poor capture unless more polymer is used then would be needed otherwise.

Feel free to let me know if you have any more questions.

Mike Bensema
www.dutchmenservices.com