Many SVI tests at waste water treatment plants are done in a 2-L settleometer that has a larger diameter than 1-or 2-L graduated cylinders( see Fig. 7-6). To eliminate well effects on solids settling in a small-diameter test apparatus,use of a slow-speed stirring device is encouraged(Wahlberg and Keinath 1988). The test is called a stirred SVI when a stirring device is used(see Standard Methods, WEF, 1998) The stirred sVi test is used frequently in Europe Secondary Clarification. Several approaches are used in the design of secondary clarification facilities The approach used most commonly is to base the design on a consideration of the surface overflow rate and the solids loading rate. Because steady-state operations seldom occur due to fluctuations in wastewater flowrate, return activated-sludge flowrate, and mLss concentrations, attention to the occurrence of peak events and use of safety factors are important design considerations Overflow rates are based on wastewater flowrates instead of on the mixed-liquor flowrates because the overflow rate is equivalent to an upward flow velocity. The return sludge flow is drawn off the bottom of he tank and does not contribute to the upward flow velocity. Selection of a surface overflow rate is influenced by the target effluent requirements and the need to provide consistent process performance The solids loading rate on an activated-sludge settling tank may be computed by dividing the total solids applied by the surface area of the tank The commonly used units for SlR are kilograms per square meter per hour(kg/m"h). If peak flowrates are of short duration, average 24-h values may govern; if peaks are of long duration, peak values should be assumed to govern to prevent the solids from overflowing the tank In effect, the solids loading rate represents a characteristic value for the suspension under consideration. In a settling tank of fixed surface area, the effluent quality will deteriorate if the solids loading is increased beyond the characteristic value for the suspension. Higher rates should not be used for design without extensive experimental work covering all seasons and operating'variables While the surface overflow rate has been the historical clarifier design parameter, the solids loading rate is considered by some to be the limiting parameter that affects the effluent concentration. Parker et al. (2001) have shown that with a proper hydraulic design and management of solids in the sedimentation tank, the overflow rate has little or no effect on the effluent quality over a wide range of overflow rates, and the design can be based on the solids loading rates. Wahlberg(1995) supports Parker's position and, based on the evaluation of secondary clarifier performance for a number of facilities, found no effect of using surface overflow rates up to 3. 4 m/h Use of Selectors. Because solids separation is one of the most important aspects of biological wastewater treatment, a biological selector(a small contact tank) is often incorporated in the design to limit the growth of organisms that do not settle well. Selectors are naturally incorporated into the biological nitrogen- and phosphorus. removal processes described. For BOD removal only or BOD removal and nitrification processes, an appropriate selector design can be added before the activated-sludge aeration basin Effluent Characteristics. The major parameters of interest that determine effluent quality from biological treatment processes consist of organic compounds, suspended solids, and nutrients as indicated by the following four constituents 1. Soluble biodegradable organics a. Organics that escaped biological treatment 2.. Cellular components(result of cell death or ly)ological degradation of the waste b. Organics formed as intermediate products in the Biomass produced during treatment that escaped separation in the final settling tank b. Colloidal organic solids in the plant influent that escaped treatment and separation 3. Nitrogen and phosphorus a. Contained in biomass in effluent suspended solids b, Soluble nitrogen as NH4-N, NO3-N, N2-0, and organic N c. Soluble orthophosphates 4. Nonbiodegradable organics a. Those originally present in the influent b. Byproducts of biological degradation In a well-operating activated-sludge process treating domestic wastes with an SrT->4 d, the soluble carbonaceous BOD of a filtered sample is usually less than 3.0 mg/L. With a proper secondary clarifier design and good settling sludge, the effluent suspended solids may be in the range of 5 to 15 mg/L 7-87-8 Many SVI tests at wastewater treatment plants are done in a 2-L settleometer that has a larger diameter than 1- or 2-L graduated cylinders (see Fig. 7-6). To eliminate well effects on solids settling in a small-diameter test apparatus, use of a slow-speed stirring device is encouraged (Wahlberg and Keinath, 1988). The test is called a stirred SVI when a stirring device is used (see Standard Methods, WEF, 1998). The stirred SVI test is used frequently in Europe. Secondary Clarification. Several approaches are used in the design of secondary clarification facilities. The approach used most commonly is to base the design on a consideration of the surface overflow rate and the solids loading rate. Because steady-state operations seldom occur due to fluctuations in wastewater flowrate, return activated-sludge flowrate, and MLSS concentrations, attention to the occurrence of peak events and use of safety factors are important design considerations. Overflow rates are based on wastewater flowrates instead of on the mixed-liquor flowrates because the overflow rate is equivalent to an upward flow velocity. The return sludge flow is drawn off the bottom of the tank and does not contribute to the upward flow velocity. Selection of a surface overflow rate is influenced by the target effluent requirements and the need to provide consistent process performance. The solids loading rate on an activated-sludge settling tank may be computed by dividing the total solids applied by the surface area of the tank. The commonly used units for SLR are kilograms per square meter per hour (kg/m2 .h). If peak flowrates are of short duration, average 24-h values may govern; if peaks are of long duration, peak values should be assumed to govern to prevent the solids from overflowing the tank. In effect, the solids loading rate represents a characteristic value for the suspension under consideration. In a settling tank of fixed surface area, the effluent quality will deteriorate if the solids loading is increased beyond the characteristic value for the suspension. Higher rates should not be used for design without extensive experimental work covering all seasons and operating 'variables. While the surface overflow rate has been the historical clarifier design parameter, the solids loading rate is considered by some to be the limiting parameter that affects the effluent concentration. Parker et al. (2001) have shown that with a proper hydraulic design and management of solids in the sedimentation tank, the overflow rate has little or no effect on the effluent quality over a wide range of overflow rates, and the design can be based on the solids loading rates. Wahlberg (1995) supports Parker's position and, based on the evaluation of secondary clarifier performance for a number of facilities, found no effect of using surface overflow rates up to 3.4 m/h. Use of Selectors. Because solids separation is one of the most important aspects of biological wastewater treatment, a biological selector (a small contact tank) is often incorporated in the design to limit the growth of organisms that do not settle well. Selectors are naturally incorporated into the biological nitrogen- and phosphorus. removal processes described. For BOD removal only or BOD removal and nitrification processes, an appropriate selector design can be added before the activated-sludge aeration basin. Effluent Characteristics. The major parameters of interest that determine effluent quality from biological treatment processes consist of organic compounds, suspended solids, and nutrients as indicated by the following four constituents: 1. Soluble biodegradable organics a. Organics that escaped biological treatment b. Organics formed as intermediate products in the biological degradation of the waste c. Cellular components (result of cell death or lysis) 2. Suspended organic material a. Biomass produced during treatment that escaped separation in the final settling tank b. Colloidal organic solids in the plant influent that escaped treatment and separation 3. Nitrogen and phosphorus a. Contained in biomass in effluent suspended solids b. ,Soluble nitrogen as NH4-N, NO3-N, N2-O, and organic N c. Soluble orthophosphates 4. Nonbiodegradable organics a. Those originally present in the influent b. Byproducts of biological degradation In a well-operating activated-sludge process treating domestic wastes with an SRT -> 4 d, the soluble carbonaceous BOD of a filtered sample is usually less than 3.0 mg/L. With a proper secondary clarifier design and good settling sludge, the effluent suspended solids may be in the range of 5 to 15 mg/L. Fig. 7-6 Field test for determining sludge volume index(SVI)