be practiced, but incineration operation and emission control are subiect to greater regulatory restriction and adverse public scrutiny. Alternatives to landfills and incineration include land application of liquid or dried biosolids and composting for distribution and marketing. Land application of biosolids is used extensively to reclaim marginal land for productive uses and to utilize nutrient content in the biosolids Composting, although a more expensive alternative, is a means of stabilizing and distributing biosolids for use as a soil amendment. Alkaline stabilization of biosolids for land applications also used but to a lesser extent Over the last 30 years, the principal focus in wastewater engineering has been on improving the quality of treated effluent through the construction of secondary and advanced wastewater treatment plants. With improved treatment methods, higher levels of treatment must be provided not only for conventiona wastewater constituents but also for the removal of specific compounds such as nutrients and heavy metals. A byproduct of these efforts has been the increased generation of solids and biosolids per person In many cases, the increase in solids production clearly taxes the capacity of existing solids processing and dispos In addition to the shear volume of solids that has to be handled and processed, management options continue to be reduced through stricter regulations. Limitations that affect options are: (1)landfill sites are becoming more difficult to find and have permitted, (2 )air emissions from incinerators are more closely regulated, and()new requirements for the land application of biosolids have been instituted. In large urban areas, haul distances to landfill or land application sites have significantly affected the cost of solids rocessing and disposal. Few new incinerators are being planned because of difficulties in finding suitable sites and obtaining permits. Emission control regulations of the Clean Air Act also require the installation More communities are looking toward (1)producing Class a biosolids to improve beneficial reuse opportunities or(2)implementing a form of volume reduction, thus lessening the requirements for disposal The continuing search for better methods of solids processing. disposal, and reuse will remain as one of the highest priorities in the future Additionally, developing meaningful dialogue with the public about health and environmental effects will continue to be very important New solids processing sy stems have not been developed as rapidly as liquid unit operations and processes Anaerobic digestion remains the principal process for the stabilization of solids. Egg-shaped digesters. developed in Europe for anaerobic digestion, are being used more extensively in the United States because of advantages of easier operation. lower operation and maintenance costs, and, in some cases, increased eases the prod as). Other developments in anaerobic and aerobic digestion include temperature autothermal aerobic digestion(ATAD). another process developed in Europe. These processes offer dvantages of improved volatile solids destruction and the production of stabilized biosolids that meet Class a requirements High solids centrifuges and heat dryers are expected to be used more extensively. High solids centrifuges extract a greater percentage of the water in liquid sludge, thus providing a dryer cake. Improved dewatering nor only reduces the volume of solids requiring further processing and disposal, but allows composting or subsequent drying to be performed more efficiently. Heat drying provides further volume reduction and improves the quality of the product for potential commercial marketing Fig. 1-5 Egg-shaped digesters used for the anaerobic treatment of biosolids1-10 be practiced, but incineration operation and emission control are subject to greater regulatory restriction and adverse public scrutiny. Alternatives to landfills and incineration include land application of liquid or dried biosolids and composting for distribution and marketing. Land application of biosolids is used extensively to reclaim marginal land for productive uses and to utilize nutrient content in the biosolids. Composting, although a more expensive alternative, is a means of stabilizing and distributing biosolids for use as a soil amendment. Alkaline stabilization of biosolids for land applications also used but to a lesser extent. Over the last 30 years, the principal focus in wastewater engineering has been on improving the quality of treated effluent through the construction of secondary and advanced wastewater treatment plants. With improved treatment methods, higher levels of treatment must be provided not only for conventional wastewater constituents but also for the removal of specific compounds such as nutrients and heavy metals. A byproduct of these efforts has been the increased generation of solids and biosolids per person served by a municipal wastewater system. In many cases, the increase in solids production clearly taxes the capacity of existing solids processing and disposal methods. In addition to the shear volume of solids that has to be handled and processed, management options continue to be reduced through stricter regulations. Limitations that affect options are: (1) landfill sites are becoming more difficult to find and have permitted, (2)air emissions from incinerators are more closely regulated, and (3)new requirements for the land application of biosolids have been instituted. In large urban areas, haul distances to landfill or land application sites have significantly affected the cost of solids processing and disposal. Few new incinerators are being planned because of difficulties in finding suitable sites and obtaining permits. Emission control regulations of the Clean Air Act also require the installation of complex and expensive pollution control equipment. More communities are looking toward (1) producing Class A biosolids to improve beneficial reuse opportunities or (2)implementing a form of volume reduction, thus lessening the requirements for disposal. The continuing search for better methods of solids processing, disposal, and reuse will remain as one of the highest priorities in the future. Additionally, developing meaningful dialogue with the public about health and environmental effects will continue to be very important. New solids processing systems have not been developed as rapidly as liquid unit operations and processes. Anaerobic digestion remains the principal process for the stabilization of solids. Egg-shaped digesters, developed in Europe for anaerobic digestion, are being used more extensively in the United States because of advantages of easier operation, lower operation and maintenance costs, and, in some cases, increased volatile solids destruction (which also increases the production of reusable methane gas). Other developments in anaerobic and aerobic digestion include temperature-phased anaerobic digestion and autothermal aerobic digestion (ATAD),another process developed in Europe. These processes offer advantages of improved volatile solids destruction and the production of stabilized biosolids that meet Class A requirements. High solids centrifuges and heat dryers are expected to be used more extensively. High solids centrifuges extract a greater percentage of the water in liquid sludge, thus providing a dryer cake. Improved dewatering nor only reduces the volume of solids requiring further processing and disposal, but allows composting or subsequent drying to be performed more efficiently. Heat drying provides further volume reduction and improves the quality of the product for potential commercial marketing. Fig. 1-5 Egg-shaped digesters used for the anaerobic treatment of biosolids