Wastewater Calculations - Mathematics and Statistics for the Environment - page 752

24.6268 g (weight of dish and suspended solids)
-24.6222 g (weight of dish)

00.0046 g (weight of suspended solids)

Next, we calculate mg/L suspended solids (using a multiplication factor of 20, a number that will
vary with sample volume) to make the denominator equal to 1 L (1000 mL):

0.0046gSS
50 mL

1000 mg
1g

20
20

92 mg
1000

× × = = 92 mg/L SS

mL

To calculate percent volatile suspended solids, we must know the weight of both total suspended
solids (calculated above) and volatile suspended solids:

24.6268 g (weight of dish and SS before burning)
-24.6232 g (weight of dish and SS after burning )

0.0036 g (weight of solids lost in burning)

Volatilesolidsweight
Tot

0.0036 g
0.0046 g

%Volatile solids = ×= × 78% 100 100

al solidsweight

24.15.9 b iosolids v olume i ndex and b iosolids d ensity i ndex C alCulations
Two variables are used to measure the settling characteristics of activated biosolids and to deter-
mine what the return biosolids pumping rate should be. These are the biosolids volume index (BVI)
and the biosolids density index (BDI):

%MLSS volumeafter 30 min
%MLSS(mg/L

BVI = = Settledbiosolids (mL) × 1000 (24.183)

MLSS)

%MLSS
%MLSS volumeafter 30 min

BDI = × 100 (24.184)

These indices relate the weight of biosolids to the volume the biosolids occupies. They show how
well the liquid-solids separation part of the activated biosolids system is performing its function on
the biological floc that has been produced and is to be settled out and returned to the aeration tanks
or wasted. The better the liquid-solids separation is, the smaller will be the volume occupied by the
settled biosolids and the lower the pumping rate required to keep the solids in circulation.

■ EXAMPLE 24.152
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?

Solution:

Volume
Density

220 mL/L
2400 mg/L

220 mL
2

220 mL
2.4 g

BVI = = = = = 92

400 mg