Challenge

Sludge dewatering efficiencies at the 30 million gallons per day (MGD) Lancaster, PA., wastewater treatment facility had long been in steady decline, with sludge coming off the belt filter presses averaging only 15-17 percent solids.  Plant management, looking for ways to increase liquid-solid separation, determined that two factors were contributing to the problem.  One factor was the lack of effective blending between the primary and secondary sludge before thickening.  The second factor contributing to poor throughout was the inefficient work value of the cationic flocculant being added to the sludge prior to thickening and again prior to belt press dewatering.

"The four conventional batch-mixing systems used to prepare and feed polymer proved highly inefficient," says Barry bracken, supervisor of biosolids management.  Insufficient initial mixing energy in the batch tank created a high degree of agglomerations and 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.  After investigating a variety of polymer activation technologies, Lancaster management elected to replace the plant's batch mixing system with a dry polymer preparation and feed system that they had seen in action at the nearby Reading, Pa., wastewater treatment plant.

Solution

Lancaster management replaced the plant's four batch mixing systems with two custom PolyBlend^®  DP2000 automated polymer preparation and feed systems from Siemens Water Technologies, as well as eliminated thickening operations.  With the new units, polymer and water come together in a high-energy disperser, where 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.  This system helps prevent the formation of agglomerations and eliminates the need to expose the polymer to extended aging time, and the subsequent entry into a lower shear zone helps to prevent damage to the extending polymer molecules.

Results

The modifications to polymer preparation and feed operations have greatly improved sludge dewatering efficiency at the Lancaster facility.

• Polymer consumption has been reduced by more than 70 percent.
• Budgeted polymer expenses, which had been $110,000 per year, are now only $30,000 per year.
• Due to the low horsepower requirements of the Polyblend DP2000 units, power consumption for the dewatering process has decreased
• In addition, solids were increased to 27 percent, thereby reducing sludge hauling costs because fewer trips to the landfill are required.

With the reduction in polymer, man-hour requirements, trips to the landfill, and power consumption, plus the elimination of the sludge thickening operations, the Lancaster wastewater treatment plant has saved more than $200,000 annually since the switch to the new polymer feed and preparation equipment.  "The monetary savings brought a payback for the Polyblend DP2000 systems in their first few months of operation," says Bracken.