Anaerobic digesters continue to be a focus for processing wastewater solids to produce a renewable fuel source, digester gas. Effective mixing of the digester contents is important for process stability, maximizing gas production, minimizing scum and foam formation, and preventing solids deposition in the digester. Mixing requires electrical power which is often generated by coal-fired power plants. With the concern regarding electrical power cost and the interest in reducing greenhouse gas emissions, there is a renewed focus on reducing the power required for digester mixing. This paper reports the test results of a new mixing system called the Vertical Linear Motion Mixer (VLMM) manufactured by Enersave Fluid Mixer, Inc which was shown to have comparable performance to other more conventional mixing systems (impeller draft tube mixers) with substantially less electrical power needed. The test results represent testing conducted at the Ina Road Wastewater Reclamation Facility (WRF), located in Tucson, Arizona. One new mixer was retrofitted to a conventional "pancake" digester and compared to an identical digester using five impeller draft tube mixers.

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In recent years publicly owned treatment works (POTWs) have increasingly turned to thermal drying to treat municipal biosolids. Thermal drying significantly reduces the mass and volume of the biosolids which must be handled and typically produces a Class A product suitable for beneficial reuse. However, thermal drying systems can be sensitive to changes in operating conditions. If the percent solids of either the feed to the dryer or the material inside the dryer move outside of acceptable ranges, dryer process malfunction can result, causing significant operational and maintenance problems. Therefore, establishing acceptable ranges of the percent solids of the material and monitoring those ranges during operation is critical to establishing smooth drying system operation.This paper examines the commissioning and optimization of the thermal drying system installed as part of the Alternative Solids Project (ASP) at the Morris Forman Wastewater Treatment Plant (WWTP), which is owned and operated by the Louisville and Jefferson County Metropolitan Sewer District. During commissioning of one of the largest drying systems in the country, operational difficulties were encountered for several reasons. Some of the difficulties were the normal process of learning acceptable operating conditions through trial and error, while others were related to temporary upstream facilities, which resulted in inconsistent feed characteristics to the dryers and undigested sludge to process. To address process malfunctions, a set of optimization procedures were established to monitor solids concentrations of the feed to the dryer and at several internal dryer locations. Procedures were also established to correlate the solids concentrations data with process malfunctions to establish acceptable operating ranges for each of the locations monitored. The optimization plan was implemented and proved to be a successful tool in predicting when process malfunctions would occur. Use of the plan has significantly reduced the number of process malfunctions, which in turn has increased the availability of the dryers and reduced maintenance and landfill costs. BACKGROUND