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Liquid Stream Fundamentals: Sedimentation

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FACT SHEET Liquid Stream Fundamentals: Sedimentation Sedimentation is one of the processes most commonly used in wastewater treatment and in many cases the last barrier before the effluent leaves the water resource recovery facility (WRRF). Despite being vital components of the WRRF, sedimentation processes are sometimes overlooked. This fact sheet discusses the key design and operational considerations for the sedimentation processes and their role in achieving optimal plant performance. Introduction Sedimentation refers to the physical process where gravity forces account for the separation of solid particles that are heavier than water (specific gravity > 1.0). The common sedimentation unit processes in a wastewater liquid treatment train include grit removal, primary sedimentation, secondary sedimentation and tertiary sedimentation. This fact sheet primarily focusses on the primary and secondary sedimentation processes. Tanks dedicated to primary sedimentation are typically referred to as primary sedimentation tanks, primary settling tanks or primary clarifiers. The tanks dedicated to secondary sedimentation are typically referred to as secondary sedimentation tanks, secondary settling tanks or secondary clarifiers. Within this factsheet these terms are used interchangeably. Particles in sedimentation tanks/clarifiers settle in four distinct settling regimes basically dependent on the concentration of particles and their tendency to coalesce (Table 1). Discrete Settling In this regime, particles settle as independent units with little interaction with neighbor- (Type I) ing particles. Discrete settling typically occurs for total suspended solids (TSS) concen- trations less than 600 mg/L. Flocculent Set- Flocculent settling typically occurs in the TSS range of 600 mg/L to 1,200 mg/L where tling (Type II) particles start interacting with each other through collision and differential settling re- sulting in formation of larger particles through flocculation. Hindered or Zone Hindered or zone settling refers to settling where strong inter-particle forces result in Settling (Type III) formation of a matrix of particles that settle together. This settling regime is predomi- nant at TSS concentrations typically between 1,200 mg/L and 5,000 mg/L. Compression Set- This type of settling is observed at very high solids concentrations where particles by tling (Type IV) means of their physical contact are able to compress the matrix releasing the water in between particles. This type of settling behavior is typically observed at TSS concentra- tions greater than 5,000 mg/L. Table 1—Four Types of Settling Regimes The dominant types of settling in a particular clarifier tend to depend on the type of influent (e.g. raw wastewater, mixed liquor suspended solids) and are also characteristic of certain regions within the clarifier (e.g. compression settling is dominant in the bottom region of the clarifier due to high solids concentrations). These regimes are discussed in further detail in the primary and secondary sedimentation sections. Copyright © 2017 Water Environment Federation. All Rights Reserved. 1 WSEC-2017-FS-022—Municipal Resource Recovery Design Committee— Liquid Stream Fundamentals: Sedimentation Clarifier Functions Surface Overflow Rate (SOR) Generally, clarifiers in a wastewater treatment plant are SOR is defined as the rate of flow of clarifier effluent per unit designed to serve four main functions discussed below. of clarifier surface area and is theoretically the upflow rate that the settling solids in the clarifier need to overcome in • Flocculation: Clarifiers should promote the aggregation order to settle before the liquid is carried out of the clarifier. of dispersed particles and prevent floc breakup. Typical design SORs for primary clarifiers range from 800 to • Clarification: Separation of solid and liquid fractions in 1,200 gallons per day per square foot (gpd/sf) at average the influent stream to produce a clarified effluent. flow conditions and 2,000 to 3,000 gpd/sf at peak flows. Most state regulatory agencies specify limits for SORs, which • Thickening: Production of thickened sludge streams. may govern the design basis unless the design engineer • Storage: Clarifiers should be able to accumulate solids, provides a clear rationale for higher SOR loadings. particularly during storm events. In general, clarifiers Primary Sludge Flow should operate with low sludge blankets and the Primary sludge removal rates generally should be optimized accumulation of solids should only occur during high to provide high sludge solids concentrations, while avoiding flow periods. any undesirable biological activity and resultant septic The extent of each function/role performed by a clarifier is conditions. However, the optimum primary sludge solids dependent on the type of unit process (primary, concentration range will depend on downstream sludge secondary, tertiary, etc.). handling facilities (e.g., pumping, thickening, degritting, Primary Sedimentation fermentation, and/or digestion). Primary sludge concentrations are typically much higher than secondary Primary sedimentation is one of the key processes for TSS sludge concentrations, resulting in higher effective viscosities and BOD removal in the liquid treatment train and for that must be accounted for when selecting the scraper reducing the load to the downstream biological process mechanism and sludge pumps. (es). Primary sedimentation is principally governed by Influent TSS discrete settling (Type I) along with minimal flocculation/ flocculent (Type II) settling (Griborio et. al, 2014). The extent of TSS/BOD removal achieved in primary clarifiers Compression settling (Type IV) occurs in the sludge blanket is dependent on primary influent characteristics, such as the region of the primary clarifiers. Settling in the primary ratio of non-settleable to settleable TSS, influent TSS clarifiers is targeted to maximize removal of the settleable concentration, settling characteristics of the settleable fraction of the influent TSS and biochemical oxygen solids, soluble BOD/COD concentration and ratio of demand (BOD) (or chemical oxygen demand (COD)). particulate BOD/COD to TSS. In general, primary clarifiers Performance design goals for primary clarifiers are typically behave as solids equalization units where higher percent quantified based on TSS removal efficiency, which normally removals are achieved at higher influent TSS concentrations. ranges between 50 and 75 percent, and BOD removal Temperature Effects efficiency, which normally ranges between 25 and 40 percent (Tchobanoglous et. al, 2003). To maximize Temperature differences between the water in the tank performance, the design of primary clarifiers must include and the primary influent can cause density currents to features to promote flocculation and prevent hydraulic form. The rotational direction of the density current will short-circuiting. Additionally, the design of primary clarifiers tend to be counter-clockwise when cooler influent enters should include considerations for removal of floatables the tank and clockwise when warmer influent enters. The (scum). impact of these density currents should be taken into account when designing the clarifiers and appropriate Design Considerations considerations to reduce them should be analyzed, such Design considerations for primary clarifiers can be divided as baffling, and appropriate sizing of the center well. into: 1) influent loading and wastewater characteristics- Chemically Enhanced Primary Treatment (CEPT) and High based design considerations, and 2) geometry-based Rate Clarification (HRC) Processes considerations. This section discusses the design considerations based on influent loading and wastewater CEPT can be used to increase TSS and BOD/COD characteristics, while the geometry features of clarifiers are removals for primary clarifiers operating at typical SORs discussed in a later section. and to allow effective operation at higher SORs. CEPT processes seek not only to capture settleable suspended solids, but also a fraction of colloidal suspended solids. Copyright © 2017 Water Environment Federation. All Rights Reserved. 2 WSEC-2017-FS-022—Municipal Resource Recovery Design Committee— Liquid Stream Fundamentals: Sedimentation Essentially, the use of chemicals (typically metal salts [i.e., secondary clarifier system. This section discusses the ferric chloride or alum] and/or polymers) causes non- design considerations based on influent loading and settleable solids to coagulate and flocculate with characteristics, while geometry features are discussed in a settleable solids, causing more solids to settle and at later section. higher settling velocities. The basic CEPT process, while Surface Overflow Rate operating at average SORs for primary clarifiers, achieves TSS removals of 60 to 90 percent and BOD removals of 50 SOR is a traditional parameters in secondary clarifier sizing to 60 percent. and is commonly used to define the required clarifier surface area at average and peak flow conditions. The SOR HRC processes include plate or tube settlers, ballasted typically ranges between 400 gpd/sf and 700 gpd/sf at flocculation processes, and solids contact/sludge average flow and between 800 gpd/sf and 1,600 gpd/sf at recirculation clarifiers. These HRC processes allow peak flow conditions (WEF MOP 8, 2005). Several state/local operation at substantially higher SORs than conventional regulatory agencies may recommend limits on the primary sedimentation
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