Thickening and Dewatering By: Ed Fritz, P.E., Sig Hausegger, Rashi Gupta, P.E
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FACT SHEET Thickening and Dewatering By: Ed Fritz, P.E., Sig Hausegger, Rashi Gupta, P.E. Introduction Thickening The wastewater treatment process is generally What is thickening? separated into the “liquid” train and the “solids” train. Solids that are allowed to settle in primary A process designed to increase the solids and secondary clarifiers are conveyed to solids concentration in residuals by removing a portion processing systems. These systems separate of the liquid.¹ Feed solids generally range from water from solids and increase the concentration 2,000 mg/L (0.2% solids) to 15,000 mg/L (1.5% of solids for subsequent stabilization or hauling solids). Thickening typically produces a thickened offsite. Facilities may also provide some type of solids product of 2-8% solids concentration stabilization of the solids to reduce pathogens (Figure 2). The product is still flowable and and vector attraction potential when the solids handled as a liquid. The water separated from are intended for beneficial use. Solids the solids is typically captured and returned for management represents a critical part of the treatment in the liquid train. Depending on the overall treatment process. While stabilization thickening process, this returned water stream is processes are discussed in other WEF documents, called filtrate, overflow, underflow, centrate, or this fact sheet provides a basic overview of solids permeate. This stream is referred to as “filtrate” in thickening and dewatering, as highlighted in this fact sheet. Figure 1. Figure 1. Typical Water Resource Recovery Facility Schematic highlighting the Thickening and Dewatering Processes Figure 2. Photograph of typical thickened waste activated sludge. Page 1 of 4 Copyright © 2019 Water Environment Federation. All Rights Reserved. WSEC-2019-FS-001, RBC Solids Separation Subcommittee Why thicken solids? Determination of the number and size of units required should account for both hydraulic and “…to increase the solids concentration of waste solids loading limits. solids and reduce the hydraulic loading (total volume) to subsequent solids treatment What else do I need for thickening? processes….”² Some types of thickening processes require The benefits of reduced hydraulic loading conditioning of the solids with chemicals (e.g. include: polymer) upstream of the thickening equipment. • Smaller downstream basins and equipment; This helps flocculate the solids and aids in the • Increased storage or detention time of thickening process by producing clearer “filtrate” downstream basins; and thicker solids. When chemicals are required, • Reduced energy needs (i.e. heat) for a chemical system that includes storage, processes such as anaerobic digestion blending, and make-down (for polymer), and chemical feed pumps are necessary. Mixing Where to thicken solids? valves or static mixers may also be needed to promote mixing of the chemical with the solids The thickening process is located within the stream, upstream of the thickening equipment. treatment train depending on the desired effect. Monitoring and optimizing chemical dose is Often the thickening process is placed on the recommended to minimize operating costs. solids stream discharged from clarifiers, upstream of the digestion process or solids storage tanks, to Sludge pumps are often necessary to feed the reduce the volume required in these tanks. The thickeners and to convey thickened solids from most common municipal solids streams that are the thickening system to downstream processes. thickened include primary sludge and waste While the feed solids are thin and able to be activated sludge (WAS). pumped by a variety of pump types, the thickened solids may reach concentrations that What equipment is used for thickening? necessitate the use of positive displacement • Gravity thickeners (or primary clarifiers if used pumps. for in-tank thickening) Most thickening processes require wash water for • Dissolved air flotation thickeners elutriation or to keep the thickener screening • Gravity belt thickeners elements clean and functional. Depending on • Centrifuge thickeners the type of feed solids and odor sensitivity at the • Rotary drum/screw thickeners treatment plant, odor control systems may be • Membrane thickeners necessary. Dissolved air flotation and membrane • Disc thickeners thickeners also require a pressurized air source. • Volute thickeners It is also important to consider the impact of The advantages and disadvantages of thickener recycle streams, such as the “filtrate” stream, types should be considered when selecting and the constituents of those streams on the equipment. Table 1 gives an overview of the facility’s processes. most common mechanical thickening devices and their relative operational parameters. (See Dewatering note at end of fact sheet.) What is dewatering? Drum Gravity Belt Thickening Parameter Thickener Thickener Centrifuge “Dewatering is the removal of water from 100-400 200-900 30-1,500 wastewater treatment plant (WWTP) solids to Capacity gpm gpm gpm achieve a volume reduction and to produce a material for further processing or disposal.”³ N/A Washwater 25-60 (required for Feed solid concentrations can range widely, 20-60 gpm Requirement gpm clean-in- depending on upstream treatment processes place cycle) utilized. Some facilities directly dewater WAS, Odor No, unless which is typically between 5,000 mg/L (0.5% Yes Yes Containment enclosed solids) and 10,000 mg/L (1% solids). Facilities with digestion generate solids ranging from 1.5% to Power ≈0.005 ≈ 0.005 ≈ 0.2 3.5% solids. Dewatering typically produces a Consumption kW/gpm kW/gpm kW/gpm dewatered cake product of 15-40% solids concentration (Figure 3). This product is semi-solid Table 1. Common thickeners and operating parameters and can be handled as a solid. Page 2 of 4 Copyright © 2019 Water Environment Federation. All Rights Reserved. WSEC-2019-FS-001, RBC Solids Separation Subcommittee Similar to thickening, the dewatering process What equipment is used for dewatering? generates a filtrate, pressate, or centrate that is returned for upstream treatment in the liquid • Air drying beds processes. As with thickening, this stream is • Belt filter presses referred to as “filtrate” in this fact sheet. • Recessed-plate (plate and frame) filter presses • Recessed chamber and membrane filter presses • Screw presses • Volute presses • Rotary fan presses • Centrifuges • Hydraulic piston presses The advantages and disadvantages of dewatering systems should be considered when selecting equipment. Table 2 shows some of the most common dewatering devices and their relative operational parameters. (See note at end of fact sheet.) Screw Belt Filter Dewatering Parameter Press Press Centrifuge 5-200 40-300 Capacity 20-1,200 gpm gpm gpm Figure 3. Photograph of dewatered cake. N/A Washwater 25-45 (required for 20-60 gpm Requirement gpm clean-in- place cycle) Why dewater solids? Odor Yes No Yes Volume reduction and to produce a material for Containment further processing or disposal. Power ≈0.05 ≈ 0.05 ≈ 0.2 The benefits of volume reduction include: Consumption kW/gpm kW/gpm kW/gpm • reduction in the size of downstream Table 2. Common dewatering equipment and operating equipment and basins; parameters • reduction in volume of material to be transported and associated costs Producing a material for further processing or Determination of the number and size of reuse/disposal means that the dewatering mechanical units required should account for process makes the downstream processes or both hydraulic and solids loading limits. reuse/disposal more efficient. Some processes Performance parameters for dewatering is that benefit from upstream dewatering include dependent on the characteristics of the solids thermal hydrolysis; chemical conditioning and stream. Laboratory bench testing and onsite pilot stabilization; drying; incineration; and testing are recommended to determine composting. expected equipment performance based on a facility’s unique solids. Where to dewater solids? What else do I need for dewatering? The dewatering process is typically located Many of the same types of ancillary processes downstream of all biological and digestion required for thickening are also required for processes. Reuse/disposal, thermal hydrolysis, dewatering. Most mechanical dewatering drying, or incineration processes typically follow processes require conditioning of the solids with dewatering. chemicals (e.g. polymer) upstream of the equipment. This helps neutralize charge and flocculate the solids, ultimately aiding in the Page 3 of 4 Copyright © 2019 Water Environment Federation. All Rights Reserved. WSEC-2019-FS-001, RBC Solids Separation Subcommittee dewatering process by producing clearer Acknowledgments “filtrate” and thicker solids. When chemicals are WEF Residuals and Biosolids (RBC) Solids required, a chemical system that includes Separation Subcommittee storage, blending, and make-down (for polymer) is necessary. Also, for some types of polymer, aging and chemical feed pumps are necessary. Authors Mixing valves or static mixers may also be Ed Fritz, P.E. needed to promote mixing of the chemical with Sig Hausegger the solids upstream of the dewatering equipment. Monitoring and optimizing chemical Rashi Gupta, P.E. dose is recommended to minimize operating costs. Reviewers James Hanson, P.E. Sludge pumps are necessary to feed the dewatering units. If the feed solids stream is David Oerke, P.E., BCEE relatively thin, then it is able to be pumped by a Richard Tsang,