Centrifuge Operational Adjustments Result in Cost Saving Opportunities at NEORSD

FINDINGS & OBJECTIVE Northeast Ohio Regional Sanitary District's (NEORSD's) nine (9) dewatering centrifuges can handle a surprisingly diverse array of operating conditions and are capable of significant operational cost savings. These are findings based on unique full-scale centrifuge tests performed at its Southerly Wastewater Treatment Center (WWTC). The tests were done to understand how the centrifuges will work as upstream process conditions are stressed including wet-weather events and hydraulicly overloaded primary sludge gravity thickeners (GVTs). INTRODUCTION | BACKGROUND Wet-weather events greatly impact sludge operations at the WWTC. Operators try to maintain a >5-foot sludge blanket in the GVTs for downstream sludge storage and centrifuge operation purposes. However, wet weather events flush thinner sludge through the process and can overload GVTs. NEORSD engineering and operations wanted to determine if a consistently thinner sludge into the dewatering centrifuges would have any negative impacts to its operations. Allowing thinner sludge blankets at the primary clarifiers and GVTs would reduce hydraulic overloading incidents during wet-weather events. The goal of the testing was to simulate a lower solids sludge concentration (thin sludge) into a centrifuge and determine any needed setpoint changes for successful performance. A secondary goal of the testing was to optimize centrifuge operation including polymer dose and energy use. Six different tests were performed. The first two tests evaluated the impact of feeding low solid concentration sludge. Four additional tests were performed to optimize polymer use, recognize energy savings, and abilities to control percent cake total solids. The testing revealed that thin sludge down at least 0.8% can be successfully fed to dewatering without negative impacts to polymer, centrate quality, or cake total solids. In addition, at much as a 30% polymer dose savings could be recognized and electrical energy savings are possible. Process Flow Description The WWTC utilizes four (4) Sludge Storage Tanks to hold Primary Sludge (PS) and Excess Activated Sludge (EAS) prior to centrifuge dewatering and incineration. At the Sludge Storage Tank complex, three (3) Centrifuge Feed Pumps (CFPs) feed the sludge to the Renewable Energy Facility (REF) for centrifuge dewatering. The REF includes three (3) equipment trains, each consisting of three (3) centrifuges, design to operate at around 200 GPM each. The target minimum solids concentration of the centrifuge cake is 28%. The target for centrate capture rate is greater than 95%. Test Preparation A rigorous testing protocol was developed to create clarity in roles, responsibilities, sampling points, measurement locations, data recording. One unique aspect of the testing setup was how to introduce Non-Potable Water (NPW) into the centrifuge feed piping to simulate a desired thin sludge concentration. NPW introduction was made via a temporary piping system with clamp-on ultrasonic flow meter and globe valve for flow control. Polymer bench testing with feed sludge was performed to observe if any deleterious polymer consumption impacts were created with the introduction of NPW. FINDINGS OF SIGNIFICANCE Test #1 - Thin Sludge, Low Solids Loading Rate Testing included introduction of NPW into the feed sludge pipeline for a single centrifuge. The goal of this test was to confirm stable operations of thinned sludge, as low as 0.5% TS, down to the machine's minimum rated total solids throughput of 1,500 lbs/hr and determine if parameters need to be adjusted for stable operations. Operation of the centrifuge was unaffected with the NPW addition, feed sludge as low as 0.84% total solids, and machine loading rate to its minimum of 1,500 lbs/hr. Centrifuge performance maintained its Torque, Centrate Quality, and Cake Total Solids. No operational impacts were identified. These results add confidence to NEORSD's operations group that the dewatering system will handle thin sludge if it is encountered in the future due to wet weather events. Test #2 - Thin Sludge and High Solids Loading The goal of this test was to confirm stable operations of thinned sludge, with a 350 GPM hydraulic limitation, up to the machine's maximum rated total solids throughput of 4,500 lbs/hr and determine if parameters need to be adjusted for stable operations. Feed sludge total solids were sampled and tested prior to the beginning of the test. The sludge flow rate was then increased to the calculated maximum solids loading rate of 4,500 lbs/hr. Once stable centrifuge operation was confirmed at 4,500 lbs/hr, NPW water was introduced to achieve thinned sludge without exceeding the centrifuge maximum hydraulic loading rate. Operation of the centrifuge was unaffected with the NPW addition, operating at the maximum hydraulic feed rate of 350 GPM, and with a thinned sludge down to as low as 1.79% TS during this test. Centrifuge performance maintained its Torque, Centrate Quality, and Cake Total Solids. No operational impacts were identified. These results confirm the solids capacity of the machine, the machine's ability to run at the hydraulic maximum, and ability to handle changing conditions without impacting normal operation parameters. Test #3 - Polymer Reduction Optimization Polymer optimization is the process of adjusting the polymer dose without losing performance in cake total solids or centrate capture rate. The goal of this test was to determine if lower polymer dosing rates can produce similar centrifuge performance. The result of lower polymer dose at similar performance is a cost saving measure that NEORSD may consider in future operations. The process of reducing polymer dose on an existing centrifuge is an operator adjustment via the SCADA system. During the testing, polymer was reduced to as low as 5.2 lbs/dt active polymer dose while still achieving a 95% solids recover/capture rate. This represents a 30% polymer savings potential versus current full-scale operations and an opportunity for significant cost savings with limited impacts to operations. Test #4 - Increasing Centrifuge Bowl Speed for Cake Solids Control The centrifuge's normal operating speed was increased from the current setpoint of 2300 RPM up to 2500 RPM to observe any operational change or improvements. The centrifuge manufacturer's O&M Manual indicated that 2500 RPM is the recommended rotational bowl speed; higher than the 2300 RPM that the bowls have operated since they were commissioned. During testing, centrifuge operations were basically unchanged while the bowl speed was increased. The higher bowl did not reduce flocculation stabilization, nor did it provide any observable value to operations. The downside of operating at the higher bowl speed is the energy cost, increasing wear on the bearings, and increasing potential for vibration issues. Test #5 - Centrifuge Lower Bowl Speed The centrifuge's normal operating speed was decreased from the current setpoint of 2300 RPM to as low as 2000 RPM to observe operational changes or improvements. The speed at which cake total solids concentration decrease was at 2000 RPM. This offers and opportunity for energy savings if the bowl speed is reduced permanently. Data collected during the testing showed an 11-amp energy savings. In addition, if a lower bowl speed can be implemented less wear-and-tear on the centrifuge bearings and lower vibration offers an opportunity to extend equipment life. Test #6 - Centrifuge Lower Torque This test was performed to observe how a lower torque setting will decrease cake total solids. This could be used as a tool by operators if feed solids are too dry for the incinerator. Centrifuge torque was reduced from its current setpoint of 50% down to as low as 42% with performance impacts. Lowering the torque setpoint created a wetter cake as predicted. Operations may use this adjustment to manage their cake total solids going to the incinerator, especially if the solids are dry and the incinerator is overheated. DISCUSSION & RESULTS Six different centrifuge tests were performed, each of them provided additional insight to NEORSD operations staff. The robustness of the centrifuge operating system was confirmed with differing sludge concentrations and throughputs to handle future scenarios. Significant cost savings potentials and operational flexibility with polymer use and operating parameters were also confirmed. NEORSD is continuing to improve their processes with the knowledge of full-scale testing. The testing effort allows improvements to their system by providing reliable, efficient, and robust solids processing. Attendees of this presentation can gain detailed knowledge of centrifuge operation and understanding optimization opportunities at their facilities.