Water Journal : Water Journal April 2011
refereed paper wastewater treatment water APRIL 2011 145 wall depth of the tank. It is not common practice to provide for an inlet disturbance zone with circular horizontal flow settling tanks. For circular tanks, the clarification function is more dependent on factors such as the type and design of the inlet and centre feedwell, tank depth and outlet weir design and location, rather than the provision of an inlet disturbance zone. With relatively deep tanks, the separation distance between the inlet and the bottom of the tank means that the full plan area is available for thickening and sludge storage (Ekama et al., 1997). Settling Tank Depth The ATV (2000) approach for determining settling tank depth (H2/3 = h1 + h2 + h3 + h4) is as follows: h1 = clean water zone and safety factor which serves to balance out undesirable influences from wind, density currents and uneven flow distribution within the tank. As a minimum, h1 is set at 0.5m. h2 = separation zone/return flow zone where flocculation and sludge settling occurs. It is dimensioned based on inflow (including return sludge flow) such that the free water volume has a detention time of 0.5 hours at peak wet weather flow. Horizontal flow settling tanks should be designed for RS = 0.75. h3 = density flow and sludge storage zone. This is the volume provided to store sludge under wet weather conditions when the sludge removal capacity is exceeded. An allowance should be made to store 30% of the solids entering the settling tank during 1.5 hours of peak flow conditions. h4 = thickening and sludge removal zone. Tank geometry plays an important role in determining the hydraulic efficiency of secondary settling tanks. ATV (2000) recommends that the depth (H2/3) should not be less than 3m for horizontal flow settling tanks (note that the volume of the conical base is equal to the depth of the cone at two-thirds radius multiplied by the plan area), while the side water depth for circular secondary settling tanks with sloping floors should not be less than 2.5m. Where the effluent-suspended solids requirements are particularly demanding, or where the loading rates are high, consideration should be given to hydrodynamic modelling, alternative inlet designs, baffling, inboard weirs and different sludge removal methods as ways of optimising hydraulic efficiency (Ekama et al., 1997; McCorquodale et al., 2005; Merlo et al., 2006). Comparison of ATV (2000) and Solids Flux Theory Predictions In the 1970s and 1980s, researchers in the United Kingdom and South Africa developed relationships between SSVI and the parameters describing solids flux theory behaviour of secondary settling tanks (White, 1976, Pitman, 1984, Ekama and Marais, 1986). These relationships enable the maximum permissible solids loading rate from solids flux theory (SLRmax) to be predicted from SSVI. Testing of full-scale rectangular and circular settling tanks has shown that solids flux theory may over-predict the maximum permissible solids loading rate for secondary settling tanks by up to 25% in some situations (White, 1976; Ekama and Marias, 1986; Göhle et al., 1996; Ekama et al., 1997). A capacity factor of 0.8 is, therefore, commonly used to reduce the predicted maximum permissible solids loading rate from solids flux theory. This approach has been used successfully as the basis for sizing secondary settling tanks in the United Kingdom and elsewhere for many years. It is uncertain, however, if this capacity factor needs to be applied to all secondary settling tank designs (Ekama et al., 1997). To compare the ATV (2000) sizing criteria with the predictions from solids flux theory, a relationship between SSVI and DSVI must be established. It is important to recognise that despite their apparent similarities, SSVI and DSVI are measures of quite different sludge behaviour. The DSVI test procedure ensures that the settled sludge progresses to the compression phase during the test, and is therefore a measure of the compaction properties of the sludge. For values of DSVI ≤ 70mL/g, both tests are essentially the same except for the impact of stirring. In this paper, SSVI is assumed to be 0.9 DSVI, for values of DSVI ≤ 70mL/g. At higher values of SSVI the conditions during the settling test are governed more by zone settling behaviour and good correlations have been observed between SSVI and zone settling parameters (Pitman, 1984). At very high values of SSVI the settling column depth becomes more important and impacts on the value of SSVI obtained, resulting in an under- estimation of zone settling parameters (White, 1976). Where both DSVI and SSVI tests have been conducted on samples of the same sludge, the values of the ratio SSVI/DSVI generally fall in the range 0.5--1.0 (Ekama and Marias, 1986). When the results from many tests are averaged, the following relationship is commonly observed for values of DSVI greater than 70mL/g (Ekama et al., 1997). SSVI = 0.67 DSVI (5) Caution is recommended when using relationships between SSVI and DSVI. However, where the predicted values of Table 1: Comparison between ATV (2000) recommendations (5) and maximum permissible solids loading rates from solids flux-based relationships. DSVI (mL/g) SLRapplied (1) SLRmax (2)(3) DSV (L/m3) 300(4) 400 500 600 60 0.91 80 0.70 (0.81) [0.80] 100 0.69 (0.75) [0.76] (0.87) [0.89] 120 0.71 (0.73) [0.73] (0.84) [0.85] (0.94) [0.96] 150 0.76 0.78 (0.82) [0.81] (0.92) [0.91] 180 0.84 0.85 0.88 (0.93) [0.88] 200 0.90 0.91 0.93 0.98 (1) qsv = 500 L/m2h, RS = 0.75, qA ≤ 1.6m/h, SSAT ≤ 5.0g/L. (2) From White (1976), Pitman (1984) and Equation (5). (3)u=R/A. (4) Where qA is capped at 1.6 m/h, qsv reduced accordingly. (5) No allowance for inlet disturbance zone. (6) Conditions above the broken line u ≤ uc, conditions below broken line u > uc.
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