Dewatered Cake Lime Stabilization at the Ann Arbor WWTP

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1 Dewatered Cake Lime Stabilization at the Ann Arbor WWTP Keith Sanders, City of Ann Arbor WWTP Jennifer Drinan, P.E., Greeley and Hansen Jared Buzo, Greeley and Hansen Background AA plant was first created in 1930s Numerous expansions and renovation projects occurred over the years with the major plant improvements in 1950 and1978 AVG flow 18 MGD. Activated sludge utilizing the A/O process for phosphorus removal. Undergoing multi-phase, plant wide upgrade Currently beginning $93 million upgrade to West Plant Treatment Process Recently completed $43 million upgrade to Residuals Handling Facilities 1

2 AA WWTP Aerial View Background cont. Residuals created during the treatment of wastewater are disposed of using two methods: Land application of lime stabilized liquid biosolids when available Landfill of dewatered cake during the off season Previously used plate and frame press for dewatering Ferric and slaked lime used for preconditioning Currently use centrifuges as part of residuals handling upgrades. Polymer used as dewatering agent Lime not compatible with centrifuges 2

3 Residuals Handling Improvements Project The project was to renovate the existing Solids Handling Building and upgrade the process equipment. The Solids Building was home to a mothballed multiple hearth incinerator and plate and frame presses and ancillary equipment which had outlived its useful life. RHI Project continued Design began in 2006 Construction bid in late 2008 Construction commenced in 2009 Major concerns addressed: Maintain operational flexibility dewatering biosolids liquid land application of lime stabilized sludge 3

4 RHI Project continued Major concerns continued: Improve thickening and dewatering efficiencies Address odor control to improve and limit complaints from nearby neighbors Upgrade the truck loading facilities for both disposal methods Plate and Frame Press 4

5 Plate and Frame Presses Operated presses totaling 60 hours/day 5 days/week Ferric Chloride and slaked lime were used as conditioning agents. Local landfills were happy to receive the cake from the presses as they were approx. 27% lime by weight and could even use it as cover material. Dewatering Flow Diagram 5

6 Centrifuges Centrifuges Selected over belt presses as centrifuges are enclosed and limit odors in the operational area Pilot tested two manufacturers: Andritz Westfalia Emulsion polymer for dewatering 6

7 Centrifuges Normal operation between 3800 to 4200 lb/hr Polymer dosing 11 to 12 gph (20 active lb/dry ton) Operate one centrifuge 3 days per week. Why a Pug Mill? Concerns about landfill acceptance of polymer conditioned dewatered cake Heard about issues encountered by a neighboring WWTP Staff at AA came from Wayne County WWTP and familiar with cake odor issues 7

8 Why a Pug Mill? Reasons to include: Odor control Local Landfill Less chemical dosing Possibility to land apply Class B cake in the future Provide wide spot in the process Dewatering Flow Diagram 8

9 Pug Mill System Lime Silos 9

10 Pug Mill Operation Batch system Uses load cells to measure the amount of cake in the hopper and lime in the bin Through calibration, establish dosing rates (by weight)of the lime into the mixing chamber Pug Mill Operation cont. Sizing of the rotolok valve to provide the correct amount of lime Set % of lime by weight as a ratio of the weight of the cake Loading too quickly caused the Pug Mill to plug Too many hands in the pot? Settings set by people not familiar with equipment limitations 10

11 Pug Mill Spills Lime Type Considered using pebble lime- redundant with slaking system still used in land app season Pebble lime would not mix as thoroughly Dust lime used- better mixing and lower price Dust lime less strenuous on rotary valve Recently started using lime kiln dust Equal efficiency and half the price of dust lime 11

12 Comparison of Limes Pebble Dust Kiln Dust Available CaO% Size (in.) powder powder Cost $/ton $ $ $ Lime Dosing About 6-9% lime added to dewatered cake Previous dewatered cake contained approx. 35% chemical Without lime, fish smell from polymer and centrifuging Too much lime caused ammonia odors Balance required 12

13 Odor Issues Non-stabilized cake not suitable for landfill Lime addition to cake caused ammonia odors, impacting: Plant operations Truck loading activities Plant neighbors Training Operators trained prior to using equipment Greeley and Hansen Operational Specialist on site for first few months of use Second training session 90 days later 13

14 Cost Savings Dewatering $13,498.57/month average chemical savings $16,023.07/month average trucking savings Totals $29,521.64/month of savings! Thickening $3,754.80/month additional chemical cost $38,365.74/month average trucking savings $34,610.94/month Net Savings! Lessons Learned Programmed interlocks with upstream equipment Use of level trending in SCADA to improve operations Stretch out time of batches to limit start/stops. Improve consistency of lime addition Smaller lime particles, better mixing 14

15 Questions? 15