A pre-sedimentation basin has been designed that reduces the river water suspend

Question

A pre-sedimentation basin has been designed that reduces the river water suspended solids concentration from 500 to 200 mg/L. How many pounds of dry solids does this represent per million gallons of treated water? If the specific gravity of these solids is 2.60 (e.g. the solids have a density 2.6 times that of water, or about 2.6 g/mL) and the sludge removed has a solids concentration of 2% (on a volume basis), what volume of sludge must be removed every week at a design flow of 4 million gallons per day (MGD).

Explanation / Answer

part (1).

A pre-sedimentation basin has been designed that reduces the river water suspended solids concentration from 500 to 200 mg/L.

that means, removed concentration of suspended solids = 300 mg/L

from the unit conversion of mg/L to pound/gallon(US), we get;

concetration of dry solids = 2.503621*10-3 lb/gallon

in million of gallons, it will be ;

concentration of dry solids = 2.503621*10-3 *106 = 2.503621*103 lb/million gallons.

hence, 2.503621*103 pounds of dry solids will removed in per million of gallons.

part (2).

specific gravity of suspended solids is given = 2.6

means the solids have a density 2.6 times that of water (density of water = 0.8345 lb/gallon).

therefore, the density of suspended solids = 2.6*0.8345 = 2.1697 lb/gallon

we know that volume = mass of solids/ density of solids

mass of solids per day = concentration of suspended solids removed * flow

therefore, mass of solids per day = (2.503621*103 lb/million gallons) * (4 million gallons/d) = 10014.484 lb

hence, volume of solids per day = 10014.484 lb/2.1697 lb/gallon = 4615.6077 gallons.

but this much of solids are 2% of total concentration of sludge (on a volume basis).

therefore, total volume of sludge per day = 4615.6077 gallons/2% = 4615.6077 gallons/0.02 = 230780.385 gallons.

for one week (or = 7days), total volume of sludge = 230780.385*(7) =1615462.695 gallons.

hence, 1.6155 MG (million gallons) must be removed every week.

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