Wastewater Calculations - Mathematics and Statistics for the Environment - page 753

The biosolids density index is also a method of measuring the settling quality of activated bio-
solids, 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. BDI is the concen-
tration in percent solids that the activated biosolids will assume after settling for 30 min. BDI will
range from 2.00 to 1.33, and biosolids with values of one 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 24.153
Problem: The MLSS concentration in the aeration tank is 2500 mg/L. If the activated biosolids
settleability test indicates 225 mL settled in the 1-L graduated cylinder, what is the biosolids density
index?

Solution:

Density
Volume

2500 mg
225 mL

g
225 mL

2.5

BDI = × 100 = × 100 = ×= 100 1.11 mL

REFERENCES AND RECOMMENDED READING
Camp, T.R. (1946). Grit chamber design. Sewage Works Journal , 14, 368-389.
Lawrence, A.W. and McCarty, P.L. (1970). Unified basis for biological treatment design and operation. Journal
of the Sanitary Engineering Division , 96(3), 757-778.
Mancini, J.L. and Barnhart, E.L. (1968). Industrial waste treatment in aerated lagoons. In: Gloyna, E.R. and
Eckenfelder, Jr., W.W., Eds., Advances in Water Quality Improvement . University of Texas Press, Austin.
Metcalf & Eddy. (1991). Wastewater Engineering Treatment, Disposal, and Reuse . McGraw-Hill, New York.
Russo, R. (2001). Empire Falls. Knopf, New York.
Spellman, F.R. (2010). Spellman's Standard Handbook for Wastewater Operators , Vol 1. CRC Press, Boca
Raton, FL.
Spengel, D.B. and Dzombak, D.A. (1992). Biokinetic modeling and scale-up considerations for biological
contractors. Water Environment Research , 64(3), 223-234.
WPCF. (1985). Sludge Stabilization , Manual of Practice FD-9. Water Pollution Control Federation, Alexandria,
VA .