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With rising energy costs and concerns over global warming, sustainable treatment with a reduced carbon footprint is fast becoming the goal of major wastewater treatment utilities. Sustainability goals are driving the industry to take on the challenge of transforming wastewater treatment from an energy-consuming and waste-producing activity to one with positive net energy production and minimal residuals. Raw wastewater contains ten times the energy needed to treat it (Shizas and Bagley, 2004). However, most of the soluble organic compounds that contribute to the measured energy are mineralized to carbon dioxide or synthesized into new cell matter during treatment. The remaining energy is recoverable from biosolids either as steam or waste heat following thermal processes or as a methane-rich gas following anaerobic digestion. To be able to recover energy from biosolids for meeting energy needs onsite or for electricity generation and to displace the fossil-based energy used is very central to the theme of sustainability. Methods of energy recovery from biosolids are at more advanced stages of development than technologies for energy recovery from the liquid stream. This paper provides an overview of some of the newer thermal conversion technologies and their potential for energy recovery in comparison with the conventional anaerobic digestion pathway.

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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.