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236 STORMWATER MANAGEMENT ALTERNATVES FOR WATER OUALITY IMPROVEMpR <br />Thus the surface area as a function of design settling velocity, which is equ� <br />to the overflow rate, is calculated from <br />Q x 3600 <br />A= <br />uo <br />7,6 REMOVALOFDISSOLVEDG <br />7�e flew rate through a pond is rare <br />�graph form. Also, the size/weight a <br />,.,,,;on time, stortn efficiencies, and ove <br />(7.9) +� <br />;n ,R pEMOVAL OF DISSOWED CONTi <br />The outflow rate depends on a storage—discharge relationship for the contrd <br />device. The change in storage depends on the difference between the inflar <br />and outflow rates, and the inflow rate is based on a specific :ainfall volumq <br />rainfall distribution, and watershed characteristia. If average precipitation <br />volume and intensity are used, the overflow rate and detention time will be <br />available over 50% of the time (because the mean is greater than the mediau <br />for these type of distributions). <br />Ezample Pro6km 7.2 Environmental impact on fishing bed areas requira <br />that particles greater than 8 µm in diameter be removed from stortnwater <br />90% of the time. The watershed area is 240 acres with a runoff coefficient ot <br />0.3. The settling pond volume is 4.0 ft deep. The rainfall intensity associated <br />with the 90%a or less occurrence is 0.4 in./h. (Data for intensiry are given in <br />Figure 2.11, Chapter 2 for large geographic areas in the Unitcd States.) <br />SOLUTION: From Table 7.4, the settling velociry is 0.00019 ft/s. The <br />design runoff rate is calculated as <br />Q s(0.4 in./h)(0.3)(240 acres)(43,560 ft2/acres)(1 ft/12 in.)(1 h/3600 s) <br />= 29 ft'/s <br />Using equation 7.9 and assuming outflow rate equals runoff rate: <br />29 X 3600 a 152,632ft� or 3.5 acres <br />A (0.00019 ft/s)(3600 s/h) <br />Since depth is 4.d ft, volume = 14.0 acre•ft. With this volume, check the <br />design settling velocity using equations 7.6 and 7.7. <br />and <br />Vp 609,540 ft� e 5.84 h <br />�d @ 3600Q 6 3600(29) <br />ho 40 <br />�o �— n = 0.68 ft/h = 0.00019 ft/s <br />tQ 5.84 <br />Thus the design will settle particles with faster rates (0.00019 ft/s) 9090 of <br />the time. <br />�e orBanics and metals are dissolvec' i <br />�rption. Adsorption is the process whe <br />od metals) moves trom the solution pha <br />fiere it is held by attractive forces. T. <br />��Sical, chemical, electrical, or a combin <br />�rbon are excellent for adsorption. Soil: <br />pm� �mpurities. Heavy metals from highv <br />y�ha soils and most heavy metals in sron <br />�he bottom sediments (1'ousef, 1984). <br />aggested, which leads to a design usi <br />matcrial, or activated carbon. Other al <br />chemicals in stormwater are the addition c <br />md biological uptake. <br />,6,1 Activeted Carbon <br />Commercial carbons can be prepared <br />'mcluding W�d� ��Bnite, coal, bone, petrol <br />tion, the carbons are termed activated c <br />qlled granular activated carbon. Activate <br />als that are subjected to selective oxid <br />structure. The high porosiry and suda� <br />sdsorptive properties. <br />In water treatment activated carbon '. <br />cause objectionable taste, odo*, or color. <br />been partly removed. In industrial wastev <br />mainly used to adsorb toxic, organic com <br />Three of the major considerations in <br />F are the fonn of the activated carbon, tt <br />� adsorption. The capacity and rate of adsc <br />and concentration of the activated carbo <br />i the impurities present, the nature of the <br />'r operating conditions. <br />` The equilibrium adsorptive capaciry <br />y isothermal batch tests. Various dosage< <br />t different containers of the stormwater <br />i: organics removed calculated by the expr� <br />, X= (Co' <br />