Enhanced settling in activated sludge: design and operation considerations

Abstract

Settling of activated sludge particles has long been the key to successfully achieving secondary treatment. While soluble products can be converted to particulate components via microbial reactions in the activated sludge process, it is the subsequent removal of these particulate components that is the key to achieving ultimate water quality criteria. An understanding of the operating parameters for selecting good settling activated sludge particles was first documented in the 1970s and 1980s. An understanding of the growth pressures that can be imposed on filamentous organisms, and the impacts of selector zones in general, allowed the design and operation of activated sludge processes to routinely achieve good sludge settleability. More recently, research has identified what could be the next evolution in flocculant growth, with the growing interest in aerobic granular sludge. Aerobic granular sludge is purported to provide superior settling properties, and many of the growth pressures identified for aerobic granular sludge are also present in activated sludge systems. These enhanced settling sludge systems are gaining significant interest, but the factors leading to enhanced sludge settleability could be present in historical and existing systems. Three facilities were evaluated that exhibited enhanced settleability (i.e. sludge volume indices of less than 70 mL/g the majority of the time) to determine how these enhanced settling sludges compare to typical settling curves from the literature. The enhanced settling sludge facilities exhibit key differences related to surface overflow rate, return activated sludge (RAS) pumping requirements, and sensitivity to solids concentration that are critical for developing effective settling designs for enhanced settling sludge facilities. As more facilities aim to achieve enhanced settling sludge for intensification of infrastructure, it will be important to carefully consider historic settling curves and to develop site-specific settling criteria when possible.

Pilot-scale comparison of biological nutrient removal (BNR) using intermittent and continuous ammonia-based low dissolved oxygen aeration control systems

“…Alternatively, should SVI levels reach such high values during full-scale operation, other corrective actions may need to be implemented, such as polymer dosing, RAS chlorination, tertiary filtration or selective sludge wasting, that will increase operational costs. Thus, methods to improve settleability through selective sludge wasting (Liu et al 2005(Liu et al , 2012Sun et al 2019), selector zone design (Daigger et al 2018), or other factors that may influence dense floc formation and retention during low-DO BNR operation remains an issue that requires further investigation.…”

Development and applications in computational fluid dynamics modeling for secondary settling tanks over the last three decades: A review

“…There are a number of settling velocity functions and Li and Stenstrom () reviewed 15 of them, but the Takács et al () (Equation ) and the Vesilind () (Equation ) are the most widely applied sub‐models coupled in SST CFD models. Some recent developments of sludge settling models are based on these two models (e.g., Daigger, Redmond, & Downing, ; Guo, Zhang, Xu, & Chen, ; Ramin et al, ).…”

“…An alternative method which is widely used in field and consulting projects (e.g. Chu et al, ; Esping et al, ; Griborio & McCorquodale, ; Winkler et al, ) is to correlate r h with easily measurable parameters in WWTPs (Daigger et al, ), including SVI, stirred sludge volume index (SSVI), and diluted sludge volume index (DSVI).…”

Exposure of the static magnetic fields on the microbial growth rate and the sludge properties in the complete-mix activated sludge process (a pilot-scale study)

“…An important point in the operation of AS process is the higher gravity settling of bio ocs in secondary clari ers. Higher removal of ocs in the clari er basins has a basic effect on the raise of the sludge in the e uent and in the other processes in the continuous wastewater treatment steps (Daigger et al, 2018).…”