Q1. Concentration of solids from liquid waste streams involves a combination of a settling tank and a centrifuge. The settling tank generates relatively clean effluent at a solids concentration of 10 gm³ and a sludge at a concentration of 1000 gm³. This sludge waste stream then enters a centrifuge which concentrates the sludge to a solids concentration of 10,000 g/m³ (1 % solids by weight) and generates a concentrate fluid stream at 200 gm³. 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 100 g/m³:

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

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

Q2. The reservoir has been constructed to supply irrigation water to adjacent farm land. Flow rate to the reservoir is 20,000 m³/yr with a dissolved salt concentration of 500 g/m3. The farm land comprises of a water requirement of 104 m³ per year lost by evaporation and transpiration plus extra water to flush out salt carried in by the irrigation water. This salty water is termed as irrigation return flow and can’t exceed 3000 gm³, 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? (Q_out = 10,000 m³/yr)

b) Determine the salt concentration in the reservoir (that is the salt concentration leaving the reservoir)?

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

Q3. Sludge feed to an anaerobic digester is 75 % volatile solids by weight. The sludge withdrawn after digestion is 45 % volatile solids. What fraction of the volatile solids put to the digester has been destroyed?