Multiple choice

An analysis for determination of solids in the return sludge of activated sludge process was done as follows: (1) A crucible was dried to a constant mass of 62.485 g. (2) 75 ml of a well mixed sample was taken in the crucible. (3) The crucible with the sample was dried to a constant mass of 65.020 g in a drying oven at 104o C. The crucible with the dried sample was placed in a muffle furnace at 600o C for an hour. After cooling, the mass of the crucible with residues was 63:145 g.

The concentration of organic fraction of solids present in the return sludge sample is

  1. 8800 mg/l

  2. 25000 mg/l

  3. 33800 mg/l

  4. 42600 mg/l
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B Correct answer
Explanation

The volatile suspended solids, representing the organic fraction, are determined by the loss on ignition between 104 degrees C and 600 degrees C. The initial dry mass of residue at 104 C is 65.020 g - 62.485 g = 2.535 g (or 2535 mg). After ignition at 600 C, the fixed residue is 63.145 g - 62.485 g = 0.660 g (or 660 mg). The volatile solids are 2535 mg - 660 mg = 1875 mg in 75 ml, which translates to 1875 / 0.075 = 25000 mg/l.

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