Boosting Polymer Power
A new mixing regime optimizes the work value of polymer serving the biosolids dewatering operations at the Lancaster, Pa., wastewater treatment plant
- By Barry Bracken
- Sep 01, 2004
Sludge dewatering operations at the Lancaster wastewater treatment plant run continuously five and a half days every week, processing an average of 95 cake tons per day. Before it adopted a new polymer preparation approach to more fully activate cationic polymer, belt-press dewatering at the facility had become highly expensive and inefficient.
When the 30-million-gallon-per-day (mgd) plant was expanded and upgraded in 1988, the new advanced treatment design included the activated sludge process with preliminary sedimentation and separate sludge digestion. Following screening and grit removal, the wastewater passes through enclosed primary clarifiers to settle out solids. After primary clarification, the wastewater is biologically treated to remove the remaining organic material, as well as being treated for nutrient removal. Here, the technology utilized in this treatment phase employs the A/O® process, which uses pure oxygen for the biological removal of phosphorus. The A/O process is designed to improve upon the activated sludge process by using an anaerobic selector to develop a selective biomass.
Following biological treatment, the mixture of wastewater and active biological solids flows to the final clarifiers, where the biosolids settle to the bottom of the tank while the clarified liquid is decanted over the top. The biosolids are either returned to the A/O process or sent to dewatering.
Inefficient Dewatering Operations
Up until recently, sludge dewatering efficiencies at the Lancaster facility had been in steady decline. Biosolids produced in the primary and final clarifiers, averaging 1 percent to 3 percent solids, were being blended in a 600,000-gallon transfer tank, mixed with anionic polymer, and sent to a sludge thickener. The thickened sludge was then sent to a holding tank prior its dewatering on four 2.5-meter belt filter presses.
Sludge coming off the belt-filter presses was averaging only 15 percent to 17 percent. Management, looking for ways to effectively increase liquid-solid separation, determined that two factors were contributing to the low percentage of dry solids coming off the belt presses.
One factor was the plant's post sludge thickening operation. For example, when the blended activated and primary sludge from the transfer tank contained 3 percent dry solids, it would be thickened to 5 percent dry solids and then stored in a 150,000-gallon holding tank prior to going to the presses. But the thickened sludge would average only 2 percent dry solids when removed from storage for dewatering. This was attributed to the lack of effective blending between the primary and secondary sludge.
A second major factor contributing to poor throughput in the plant's sludge processing operations was the inefficient work value of the cationic flocculant being added to the sludge prior to thickening and again prior to belt-press dewatering. Polymer performance is dependent upon the degree of its activation prior to its introduction to sludge. Polymer that is thoroughly activated conditions sludge so that it quickly passes through the dewatering process with a high percentage of dry solids. Less thoroughly activated polymer, evident in the Lancaster plant's dewatering operations, was resulting in higher polymer usage and power consumption, decreased efficiency of dewatering units, and more trips to the landfill.
Polymer Activation: The Key
To obtain their optimum effectiveness, polymers must be thoroughly dissolved in water before use. The polymer molecules, originally in a highly entangled form, absorb water in these solutions, which allows them to untangle. The objective of polymer activation is to untangle and fully hydrate the polymer, because fully activated polymer chains attach on more than one particle to maximize the efficiency of particulate removal in filtration.
At the Lancaster facility, the four conventional batch-mixing systems used to prepare and feed polymer proved highly inefficient. Polymer was mixed with water in freestanding 2,000-gallon batch mix tanks equipped with large impellers. Once mixed, the polymer solution was sent to a second 2,000-gallon tank for aging prior to sludge application.
Insufficient initial mixing energy in the batch tank created a high degree of agglomerations that were ineffective for flocculation or coagulation. Due to the low mixing energy applied by the impellers when polymer first contacting water, it was difficult to obtain a homogeneous solution rapidly because a film of concentrated polymer would build up around individual polymer gels. In addition, the high velocity and lack of uniform agitation intensity in the mix tank following initial wetting allowed the untangling polymer molecule chains to fracture, eliminating their effectiveness as flocculants.
Minimizing the generation of agglomerates and fractures during polymer activation is paramount to optimizing polymer performance. Because this minimization was not taking place at the Lancaster facility, excessive polymer was needed to adequately dewater sludge.
Taking a New Approach
Plant management realized that sludge dewatering costs could be reduced if higher performance from the polymer was attained, and this would require a change in the plant's polymer activation method.
As part of its on-going investigation into different polymer activation technologies, plant management visited the Reading, Pa., wastewater treatment plant, which had recently replaced a batch mixing system for dry polymer preparation and feed with a custom PolyBlend® DP2000 automated system from USFilter's Stranco Products. Based on its on-going investigation, as well as the positive observation of the performance of the Reading plant's new systems, Lancaster plant management elected to replace its four old polymer-delivery systems with two custom PolyBlend DP2000 automated systems.
With the new units installed at the facility, polymer and water come together in a high-energy disperser, where initial wetting of the polymer occurs. Polymer and water are subjected to high energy created by mechanical mixing.
In the disperser, the polymer is subjected to a relatively high shear environment. Then, the partially hydrated polymer enters a low-energy mix tank -- a low shear zone where it is further mixed. With this system, uniform, controlled dispersion energy at the moment of initial polymer wetting in the disperser helps to prevent the formation of agglomerations and eliminates the need to expose the polymer to extended aging time. The subsequent entry into a lower shear zone helps to prevent damage to the extending polymer molecules. From the mix tank, the polymer is sent to a holding tank, and from there to the final feed skid to the point of application. Lancaster's custom polymer feed system is equipped with larger holding tanks (750-gallon capacity) that are situated side-by-side, rather on top of each other, to meet the specific footprint requirement of the site.
Shortly after the adoption of the new polymer feed system, in-house testing determined that better dewatering performance was achieved when sludge thickening was by-passed. The plant has since discontinued thickening operations. Now, polymer solution is just added to the sludge prior to belt-press dewatering.
Since new equipment startup in May 2001, the modifications to polymer preparation and feed operations have clearly improved polymer performance and, in turn, sludge dewatering efficiency at the Lancaster facility. Polymer consumption has been reduced by more than 70 percent -- now averaging 1.5 pounds per dry ton. Budgeted polymer expenses, which had been $110,000 per year, are now only $30,000 per year.
The sludge cake coming off the belt presses now contains an average of 27 percent solids, compared to the 15 to 17 percent figure that had been common before the new equipment was put in place. This has reduced sludge hauling costs to the landfill significantly because fewer trips are required.
The switch to the new polymer feed system has also reduced labor time significantly. The plant's previous dry polymer preparation and feed system was a manual, batch-feed unit that blended polymer for approximately one hour before it was sent to a day tank. It was a time-consuming operation that required constant adjustment, and an operator had to attend to the prior system almost full time. With the new automated system, the only routine requirement by the operator is to maintain dry polymer in the unit's hopper. The switch to the automated system has reduced the total employee hours required at the plant for polymer preparation and feed by more than 90 percent.
Big Savings, Fast Payback
With the reduction in polymer, employee-hour requirements, and trips to the landfill; reduced power consumption due to the lower horsepower requirements of the new polymer feed systems; and the elimination of sludge-thickening operations, plant management estimates the facility has saved more than $200,000 annually since the switch in polymer feed equipment. This savings brought a payback for the new equipment in its first few months of operation.
This article originally appeared in the 09/01/2004 issue of Environmental Protection.
Barry Bracken is supervisor of biosolids management at the city of Lancaster, Pa., wastewater treatment plant. He can be reached at (717)293-5541.