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Digester foaming is poorly understood. An increasing number of wastewater utilities are experiencing significant disruption of both liquid stream treatment and solids processing operations as a result of foaming episodes and, in some cases, costly structural damage to their digesters. A review of the literature indicates that we should anticipate increased foaming problems in the future as the use of biological nutrient removal (BNR) and membrane (MBR) processes become more prevalent, and as utilities operate at lower DO concentrations to contain operating costs and reduce their carbon footprint. This paper reviews research findings and actual operating experience related to digester foaming and describes foam formation and its behavior inside the digester. Measures to reduce the impacts of digester foaming such as changes in digester operational practices and facility and equipment modifications that can make a digestion complex more "foam tolerant" are reviewed, along with the prospects for their success based on observations at operating plants. Merely optimizing operational practices may not be sufficient in all cases, and some wastewater utilities have opted to make major process changes in an effort to reduce digester foaming. Early operating experience with two phase digestion, in particular, has shown considerable promise at several treatment plants with long histories of foaming in conventional digestion systems. Short of undertaking major capital improvements to effect process changes, there are more modest steps that can be taken to reduce the potential for facility damage and disruption from digester foaming.

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

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Recent research provides ample indication that a conventional, single-stage flow train is not the optimal configuration for realizing the full potential for solids stabilization and pathogen reduction in mesophilic anaerobic digestion--and it is now time to put these research findings into practice. Temperature Phased Anaerobic Digestion, two-phase digestion and other advanced digestion process trains may be too ambitious for some, but simple staging of anaerobic digesters would be a good start in improving digestion performance. Staging coupled with draw-and-fill feeding would be even better. Encouragement to do so is provided by a twostage mesophilic digestion system in Rockford, Illinois that has been beating performance expectations in over four years of operation. This paper discusses the Rockford installation, plus a new two-stage digestion system currently in design for the Minneapolis-St. Paul metropolitan area. The anticipated system operation and the design adjustments required to accommodate future operation in a two-stage mode are reviewed.