Testing of Commercial Defoamant for Anaerobic Digester Froth Control

Routine froth potential tests conducted on the Hunts Point Wastewater Treatment Plant (WWTP)anaerobically digested sludge revealed that Digesters 2 and 3 had a froth potential index in the ‘frothing’ to ‘severe frothing’ range on August 4, 2011. On Monday, August 8, 2011, the PO-88 team began full-scale digester defoamant addition at Hunts Point that concluded on Tuesday, August 16, 2011. When both digesters again showed high froth potentials in early December 2011, a second demonstration was conducted from Monday, December 19, 2011, to Thursday, December 29, 2011.The full-scale defoamant addition demonstrations were carried out on Digesters 2 and 3, with Digester 2 as the experimental digester (receiving defoamant) and Digester 3 as the control (no defoamant addition). On Day 1 of the demonstration, 18 gallons of ESP FC Rel® defoamant were pumped into the suction side of the recirculation pump on Digester 2 at a rate of 6 gallons/hr via a chemical dosing pump, resulting in a defoamant concentration of 15 ppm in the digester. Subsequent doses were added to Digester 2 to attempt to maintain this concentration over the test period. Samples of digester sludge from Digesters 2 and 3 were collected in the morning and afternoon, and thickener sludge samples were collected daily. All samples collected were tested for stable and unstable froth potential. Additionally, a microbial analysis was performed on samples taken from Digesters 2 and 3 on the first and last test days to determine if the froth was biological in nature.The results from the August 2011 test indicate that the ESP FC Rel® defoamant was effective in significantly reducing the frothing potential in Digester 2 as compared to Digester 3, the control digester. December 2011 testing confirmed that ESP FC Rel® could be an effective defoamant, but only showed effects immediately after addition.To further examine the reduced impact of defoamant on frothing potential observed in the December 2011 full-scale testing, several bench scale tests were conducted. These tests used varying dosing levels of defoamant in order to re-evaluate the optimal defoamant dosing target. The results of the dosing experiments showed that a higher dose of defoamant than that used during the August and December 2011 testing was needed to effectively reduce frothing.It is recommended that a target defoamant dose be confirmed with bench scale tests prior to the use of defoamant. This will ensure that sufficient defoamant is added to produce a measureable impact on frothing potential measurements. Additionally, continuous addition of the defoamant, beginning when frothing conditions are first noted, is recommended to maximize the effectiveness of the defoamant. The impact of changes in digester sludge composition and natural frothing potential fluctuations could be dampened with continuous defoamant addition.Additional full-scale defoamant addition tests should be conducted to confirm the observed results and investigate continuous dosing. The PO-88 team will provide monitoring of the Hunts Point WWTP digesters when requested by DEP and will initiate another round of testing when another frothing opportunity arises.