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.

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.

No abstract

In 1998, the Madison Metropolitan Sewerage District (MMSD) initiated facility planning for the 10 th addition to the 50 MGD Nine Springs Wastewater Treatment Plant (NSWWTP). The modifications comprised of several upgrades including a new headworks facility, modifications to the existing digestion process, and construction of new biosolids processing facilities, revisions to energy utilization and energy recovery facilities and other miscellaneous improvements. The anaerobic digestion facilities at the plant were included in the evaluation because the digester loadings were exceeding design values, resulting in a nominal 14-day solids retention time. The digester capacity related issues were being further accentuated by recurrent foaming problems in the gas-mixed digesters.Though not mandated, it was thought prudent to ensure Class A compliance with the capacityrelated digester improvements as increased public scrutiny and growing resistance to land application of Class B biosolids in many parts of the country have raised questions about the continued reliance on this practice. Temperature-Phased Anaerobic Digestion (TPAD) and Acid/Methane phased digestion emerged as the leading candidates, as research studies have indicated the potential of these processes to reduce pathogens. Laboratory studies conducted at the Nine Springs laboratory by Reusser et al. (2001) found that TPAD was capable of higher volatile solids (VS) destruction with total detention times of 10 and 12.5 days than any of the other systems at 15 to 20 days. But MMSD recognized that it would need to operate the thermophilic stage of TPAD in a batch, sequential batch, or similar mode to ensure that "every particle" has been exposed to conditions stipulated under CFR Part 503 Regulations for Class A operational compliance. Operation in a sequential-batch mode appeared to be a workable solution for meeting the time-temperature requirements for Class A pathogen reduction at NSWWTP. Lab-scale studies at Iowa State University sponsored by MMSD's design consultant, Black & Veatch, confirmed the stability and performance of the sequencing-batch TPAD scheme.With the critical process questions answered, the next task was working out the design details and completing plans and specifications for bidding the 10 th Addition project.

No abstract