SAMPLE PROBLEM 4 | A settling tank has 2m³/min inflow and 2100mg/L solids concentration. The effluent from I this settling tank goes to sand filters. The concentration of sludge coming out of the bottom of the settling tank is 18000mg/L & the flow to the filters is 1.8 m³/ min. a) What is the underflow flowrate? b) What is the solids concentration in the effluent (flow to the filters)? c) How large must the sand filters be (m²) if the filter loading is 4 gal/ ft².min? Given: b. Component Material Balance around settling tank 2m³/min VINCIN = VUFCUF + VorCoF J(2m³/min) (2100mg/L) 21000rbg/L solids, 1.8m³/min. (0.2m³/min)(18000mg/L) + (1.8m/min)(Cor) Cor = 333.3333 mg/L solids Skudge Req'd: Vur, Cor, Armer c. Now we focus on the filter filter inflow filter surface area Soln: filter loading = For convenience, let's denote underflow as UF and overflow as OF. Since the water inflow in the rapid sand is the overflow of the settling tank, filter inflow is our CoF. a. Total Material Balance around settling tank VN = Vor + Vur gal 1m 2m/min = 1.8 m/min+ Vur ft-.min \7.481 gal) filter area Vur = 0.2000 m³/min AFILTER = 11.0420 m² Redo Sample Problem 4 if the water inflow to the sedimentation tank is 2.1m³/min and the sludge concentration coming out at the underflow is 12000mg/L. consider also the filter loading rate of the sand filter to be 2.1 gal/d.ft².