Draw

The use of a floating decanter, or a decanter that moves downward during draw, offers several advantages. Draw is initiated earlier because the effluent is removed from near the surface while the biomass continues to settle at lower depths. The effluent is removed from a selected depth below the surface by maintaining outlet orifices or slots at a fixed depth below the variable water surface. This avoids removal of floating materials and results in effluent removal from high above the settled biomass. Floating decanters allow maximum flexibility, because fill and draw volumes can be varied from time to time, or even from cycle to cycle. However, lower-cost fixed-level decanters can be used if the settle period is extended, to assure that the biomass has settled below the decanter orifices. Fixed-level decanters can be made somewhat more flexible, if they are designed to allow operators to occasionally lower or raise the location of the decanter.

Rapid draw rates allow use of smaller reactors, but cause high surges of flow in downstream units and in receiving waters. Effluent flow equalization tanks or reduced draw rates will reduce peak flow discharges.

Figure 11.18.2 illustrates the hydraulic conditions over two days in a three-tank SBR system under design flow conditions. The illustration shows influent flow into Tank 1 beginning at 6:00 a.m. The treatment strategy provides a static fill (Fs) for 1.67 hr followed by an aerated fill for 1.0 hr. The influent is diverted to Tank 2 at 8:40 a.m., and to Tank 3 at 11:20 a.m. In Tank 1, a 2.33 hr react is followed by a 1.0 hr settle and a 1.0hr draw. Tank 1 idle occurs from 1:00 p.m. to 2:00 p.m. At 2:00 p.m. the influent is again diverted to Tank 1, and the cycle is repeated. As shown in Figure 11.18.2, each tank cycle is the same, with an 8-hr cycle divided as follows: fill 2.67 hr (i.e., static fill, 1.67 hr and aerated fill, 1.00 hr), react 2.33 hr, settle 1.0 hr, draw 1.0 hr, and idle 1.0 hr. Total aeration time is 3.33hr in both aerated fill and react.

The shaded areas of the illustration show that influent flows continuously into one of the three tanks. Under design conditions, flow occurs at a constant rate and every cycle is identical. Effluent is not continuous, and is illustrated by the cross-hatched areas. In this design flow example, each cycle fill time is 2.67 hr and draw time is 1.0

hr, resulting in an effluent flow rate equal to 2.67 times the influent flow rate.

Under actual flow conditions, diurnal flow rates vary. Typically, flow rates increase throughout the morning, peak in the early afternoon, and decrease later in the day with minimum flow in the early morning hours. Figure 11.18.3 illustrates a three-tank SBR system with a typical diurnal flow variation. Note the short fill and idle periods during high flow rate times, and the long fill and idle periods during low flow rate times. During peak flow, no idle period exists (e.g., about 2:00 p.m. of Day 2 in Tank 1).

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