Water/Wastewater Laboratory Calculations - Basic Mathematics for Water and Wastewater Operators - page 244

Solution: To calculate the milligrams suspended solids per liter of sample (mg/L),
we must first determine grams suspended solids:

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

0.0046 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
1

20
20

92 mg
1000 mL

Suspendedsolids = × ×= = 92 mg/L

g

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 suspended solids before burning)
-24.6234 g (weight of dish and suspended solids after burning)

0.0034 g (weight of solids lost in burning)

weight
Suspendedsolidsweight

Volatile solids 0.0034

×=

g
0.0046 g

%Volatile suspendedsolids = 100 ×= 100 70%

BIOSOLIDS VOLUME INDEX AND BIOSOLIDS DENSITY INDEX

Two variables are used to measure the settling characteristics of activated biosolids
and to determine 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

BVI = = Settle ddbiosolids (mL) × 1000 (16.16)

%MLSS
%MLSS volumeafter 30 min settli

BDI = × 100 (16.17)

ng

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 sys-
tem 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-sol-
ids 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.