Dewatering Treatment Process:

SEDIMENTATION & CENTRIFUGATION NEWSLETTER December 1994, No. 180 Ferrous Chloride Improves Cake Dryness by 5 Percent at Hyperion The City of Los Angeles Hyperion Treatment Plant conducted a digested sludge dewatering study with two full high-speed centrifuges — a Humboldt Wedag CP 4-1.1 and an Alfa Laval Sharples DS 706. To determine operating parameters for a new future sludge dewatering facility, testing included changing sludge flow rate, temperature and polymer dosage once per day while adjusting centrifuge torque several times per day. According to Jeffrey S. Berk of HYA Consulting Engineers, Roger Haug of the City of Los Angeles, and David Harrison of Montgomery Watson, there was a surprising result of the 6-month study. Analysis showed that the ferric chloride which is normally added to the digesters to reduce the formation of hydrogen sulfide gas, actually had a significant effect on both the wet cake solids and the centrifuge capture efficiency. Choice: Should I choose ferric chloride as an addition to enhance the effectiveness of the polymer when operating centrifuges for biosolids dewatering in waste treatment plants? The answer, according to the speakers, is yes. Enhancing the effectiveness of polymer by ferrous chloride addition can allow the operator to run the centrifuge at higher torques while maintaining high capture efficiency and higher cake solids concentration. The data indicates that ferrous chloride dosages to the digested sludge below 300 mg/l coincide with a capture efficiency of 70 to 97 percent capture. At ferrous chloride dosages greater than 300 mg/l, the capture efficiency of the centrifuge remains almost consistently above 90 percent. For ferrous chloride dosages higher than 300 mg/l, centrifuge torque can be adjusted to as high as 80 percent of maximum while maintaining a 90 percent capture efficiency. At this torque, the centrifuge was able to produce cake that had a 33.3 percent cake solids concentration with a polymer dosage of only 8 g/kg. At a sludge flow rate of 200 gpm, sludge temperature of 35° C and polymer dosage of 8 g/kg increasing the ferrous chloride by 187 mg/l produced cake solids that were as much as 5 percentage points higher. This could amount to a significant saving in trucking costs for off-site beneficial reuse.