Environmental Engineering Reference
In-Depth Information
Example 16.19
Problem: The settleability test indicates that, after 30 min, 220 mL of biosolids
settle in the 1-L graduated cylinder. If the mixed-liquor suspended solids (MLSS)
concentration in the aeration tank is 2400 mg/L, what is the biosolids volume index?
Solution:
Volume
Density
220 mL/L
2400 mg/L
220 mL
2
220 mL
2.4 g
BVI
=
=
=
=
= 92
400 mg
The biosolids density index is also a method of measuring the settling quality
of activated biosolids, yet it, like the BVI parameter, may or may not provide a true
picture of the quality of the biosolids in question unless compared with other relevant
process parameters. It differs from the BVI in that the higher the BDI value, the
better the settling quality of the aerated mixed liquor. Similarly, the lower the BDI,
the poorer the settling quality of the mixed liquor. The BDI is the concentration in
percent solids that the activated biosolids will assume after settling for 30 minutes.
The BDI will range from 2.00 to 1.33, and biosolids with values of 1 or more are
generally considered to have good settling characteristics. To calculate the BDI, we
simply invert the numerator and denominator and multiply by 100.
Example 16.20
Problem: The MLSS concentration in the aeration tank is 2500 mg/L. If the acti-
vated biosolids settleability test indicates 225 mL settled in the 1-L graduated cylin-
der, what is the biosolids density index?
Solution:
Density(determined by MLSS concentrati
on)
Volume (determinedbysettleability test)
2500 mg
225 mL
BDI
=
×
100
2.5 g
225 mL
=
×
100
=
×
100
=
1.11
CHEMICAL DOSAGE CALCULATIONS
Chemicals are used extensively in wastewater treatment plant operations. Wastewater
treatment plant operators add chemicals to various unit processes for slime-growth
control, corrosion control, odor control, grease removal, BOD reduction, pH control,
sludge-bulking control, ammonia oxidation, and bacterial reduction, among other
reasons. To apply any chemical dose correctly it is important to be able to make
certain dosage calculations. Chemical dosages are measured in ppm (parts per mil-
lion) or mg/L (milligrams per liter). Parts per million (ppm) is always a comparison
of weight (pound per million pounds). One pound of chemical added to 1 million
pounds of water would be a dosage of 1 ppm. Because each gallon of water weighs
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