PILOT-SCALE POLYMER-ENHANCED LAGOON DEWATERING OF DIGESTED SLUDGE

A pilot-scale study was conducted at the StickneyWater Reclamation Plant (WRP) of the Metropolitan Water Reclamation District of Greater Chicago (District) with a view to investigating whether the application of cationic polymer would improve dewatering and settling of anaerobically digested sludge, and thereby increase the sludge processing and resulting storage capacity of two existing lagoons which are held in reserve for emergency storage of liquid digested sludge in the event of catastrophic failure or prolonged shutdown of the existing centrifuge installation.Three separate test conditions were studied during each of three experimental runs. During the first of the three runs, the sludge was fed a few inches above the bottom of pilot-scale lagoons (1,000 gallon volume), while during the second and third runs, the sludge was fed from the top of the pilot-scale lagoons. Also, during the third run, a fifty-percent reduction in polymer was used to examine the performance at a lower polymer dosage. During all three runs, two experimental lagoons were fed with digester draw conditioned with dilute cationic emulsion polymers – referred to as Polymers 1 and 2 - (the polymers were selected from laboratory-scale experiments) and the third lagoon (control) was fed with unconditioned digester draw, in increments of approximately 200 to 300 gallons a day. The pilot-scale lagoons were filled over three to four days to approximately 900 gallons of liquid volume. After having filled the pilot-scale lagoons, they were allowed to remain quiescent.All pilot-scale lagoons were observed for water-solids interface every twenty-four hours. The separated water was withdrawn from the lagoons when a minimum of approximately 200 gallons or more had formed. An equivalent volume of conditioned or unconditioned sludge was then added to replace the withdrawn water. Feed sludge, thickened solids, and separated water matrix samples from each lagoon, raw and dilute polymer samples, along with dilution water samples, were collected and analyzed for percent total solids and other analytical parameters. Hydrogen sulfide measurements were taken above the lagoon surface before and after each withdrawal of separated water, and after each fill, during each experimental run.The polymer-fed pilot-scale lagoons produced a much drier sludge cake than the control. During all three runs, the polymer dosed lagoons showed water separation almost instantaneously with a measurable quantity within a few hours after polymer addition. The separation rate was much faster than the control lagoon. The separated water in the polymer-fed pilot-scale lagoons had much lower suspended solids and ammonia nitrogen concentrations than the control lagoon.Hydrogen sulfide measurements made above the pilot-scale lagoons during each test were found to be elevated compared to the background levels. The highest values occurred after water withdrawal and ranged from approximately 16 to 2,800 ppb. These measurements do not show a significant difference between the polymer-fed pilot-scale lagoons and the control pilot-scale lagoon during all three runs indicating that polymer addition did not contribute to increased odor levels.The results of the study showed that the addition of cationic emulsion polymer would appreciably improve the dewatering of anaerobically digested sludge and thereby increase the lagoon's sludge processing and emergency storage capacity without increasing objectionable odor levels. It is estimated that 26 to 40 percent more sludge could be processed in the full size lagoons using polymer treatment. This increased storage capacity could prove critical during a catastrophic failure or prolonged shutdown of the StickneyWRP's centrifuge dewatering facility.