Calculating the flow rate of sludge and thickened sludge, water flow rate, salt concentration and pumping rate of a reservoir.

1. Concentration of solids from liquid waste streams involves a combination of a settling tank and a centrifuge. The settling tank produces relatively clean effluent at a solids concentration of 10 gm-3and a sludge at a concentration of 1000gm-3. This sludge waste stream then enters a centrifuge that concentrates the sludge to a solids concentration of 10,000g/m3 (1 percent solids by weight) and produces a concentrate fluid stream at 200gm-3. The concentrate is blended with influent to the settler as indicated to the right. For an influent flow rate of 1 m3S-1 at a solids concentration of 100g/m3:

(a) Compute the flow rate of sludge produced if only a settling tank is used (no centrifuge).

(b) Compute the flow rate of thickened sludge leaving the centrifuge and the flow rate to the centrifuge, Os'

2. A reservoir has been constructed to supply irrigation water to adjacent farm land. Flow rate into the reservoir is 20,000 m3/yr with a dissolved salt concentration of 500 g/m3. The farm land has a water requirement of 104m3 per year lost by evaporation and transpiration plus additional water to flush out salt carried in by the irrigation water. This salty water is called irrigation return flow and cannot exceed 3000gm-3, or else plants are killed. The agricultural return flow enters the reservoir as indicated to the right. Suppose steady state conditions, no rainfall, no reservoir evaporation, and no salt lost to the atmosphere.

(a) What is the water flow rate out of the reservoir? (Qout = 10,000 m3/yr)

(b ) b)What is the salt concentration in the reservoir (which is the salt concentration leaving the reservoir)?

(c) What pumping rate is required to irrigate the farm land and flush salts back into the reservoir?

3. Sludge feed to an anaerobic digester is 75 percent volatile solids by weight. The sludge withdrawn after digestion is 45 percent volatile solids. What fraction of the volatile solids put into the digester has been destroyed? (Berthouex and Rudd, 1977).