Sludge minimization technologies have been available for several decades; however recent developments have brought some sludge minimization technologies to the forefront. All of the technologies utilize one or more of three basic approaches to minimize the amount of waste activated sludge produced by an activated sludge process: cell lysis, cyclic oxic environments, and long solids retention time. This paper will discuss the three basic mechanisms, will review the development of several sludge minimization technologies, and will report on the current viability of each technology as well as current research needs for each. 506 WEFTEC®.06
The handling and disposal of wastewater sludge is an increasingly costly portion of the operation of a wastewater treatment plant and is developing into an even greater future risk given the trend of decreasing availability and increasing costs of ultimate disposal options. Recently several technologies and process innovations have been proposed to reduce or eliminate the waste activated sludge (WAS) fraction resulting from treatment, or to render it more amenable to anaerobic digestion, a common sludge stabilization step, in order to maximize biogas production and the corresponding potential for energy cogeneration. Several of these technologies are also claiming significant improvements in the dewatering characteristics of the stabilized biosolids as a result of these pre-conditioning steps. The above claims are especially relevant for industrial facilities and smaller municipal facilities which frequently have no primary sludge but generate large quantities of difficult-to-dewater waste biomass. WERF's 05-CTS-3 Evaluation of Processes to Reduce Activated Sludge Solids Generation andDisposal, which started in the Spring of 2007, seeks to establish a comprehensive evaluation methodology for WAS reduction processes based on the in-depth consideration of a select number of technologies considered to be representative of the many options currently available in the marketplace. These technologies will cover both those intended to reduce the generation of residual sludge from the liquid treatment process, as well as those designed to pre-condition whatever sludge is produced in order to make it more amenable to a subsequent stabilization process such as anaerobic digestion. This paper presents the results of a literature review task conducted in the initial stages of the project, and is intended to be the first one of a series of papers in which the results of this important project are presented to the Residuals and Biosolids community of practitioners. This literature review places into perspective the basic mechanism behind the different WAS Reduction technologies, finding applications worldwide, identifying their application point within the treatment facility (i.e., digestion pretreatment, treatment of activated sludge recycle streams), and defining its development status.
The application of high intensity ultrasound has significant potential for improving biosolids management at treatment plants across North America. This paper provides a review of how the technology works, the potential applications at wastewater treatment plants, the different approaches being used by various suppliers, the state of development of the technology, and on-going work in North America and around the globe. Ultrasound is sound above the range of human hearing, with frequencies between 20 kHz and 10 MHz. At the lower end of this range, ultrasound generates cavitation bubbles when applied in a fluid, which implode creating high mechanical shear forces. This can be used for disintegrating solids in the fluid.For wastewater applications it has been shown that ultrasound is most beneficial when applied on biological secondary solids, where rapid hydrolysis can be induced. Hydrolysis of cellular material is usually the rate-limiting step in digestion. This provides advantages in anaerobic digestion, with increased volatile solids destruction and gas production. Improved dewaterability has also been observed at some installations. Improvements through reduction in biosolids volume and increased gas production can provide significant savings. Other applications of ultrasound include the potential for prevention of foaming caused by filamentous organisms.Most of the work on ultrasound in wastewater applications has been done in Europe. In North America, California has been at the forefront of ultrasound development, with demonstrations conducted by Orange County Sanitation District and the Los Angeles County Sanitation District. The California Energy Commission has also supported a demonstration project at the City of Riverside for comparison of performance and cost effectiveness of two ultrasound manufacturers that use different treatment philosophies. Ultrasound technology is also being implemented in Australia and Singapore. This paper provides an overview of ultrasound system suppliers, existing installations, continuing developments in this field and guidance on when ultrasound technology may prove beneficial for improved solids management.
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