Dual-chemical conditioning for dewatering municipal wastewater sludges

The Regina Wastewater Treatment Plant presently uses Percol 757, a cationic polymer, to condition its anerobically digested wastewater sludge. This study was initiated to examine the feasibility of using ferric chloride in conjunction with either of two polymers---Percol 755 or Percol 757---to condition the sludge. Specific resistance to filtration (SRF), capillary suction time (CST), and time-to-filter (TTF) were the three parameters used to evaluate conditioning performance and dewaterability. The role of charge neutralization in the conditioning process was examined by zeta potential measurements. Optimum dosages of ferric chloride, Percol 755 and Percol 757 were determined to be 80.6, 2.9 and 2.7 kg/T of dry solids respectively. Optimum dosing resulted in complete charge neutralization. It was concluded that zeta potential measurements could not be reliably used to predict dewaterability. Sludge conditioning experiments done using different combinations of optimal dosages of ferric chloride and polymers indicated that all combinations of ferric chloride and polymer conditioning resulted in better dewaterability than conditioning with ferric chloride or polymer alone. Dual-chemical conditioning was cheaper than ferric chloride conditioning, but more expensive than polymer conditioning. It was shown that dual-chemical conditioning using a combination of 50% of optimum dosage of ferric chloride and 50% of optimum dosage of Percol 757 had the lowest CST, SRF and TTF values. It was noticed that a percentage of the optimum dosage of polymer could be replaced by a similar percentage of the optimum dosage of ferric chloride without adversely affecting the dewatering efficiency. Both CST and TTF were found to have excellent correlation with SRF data.