Wastewater Calculations - Mathematics and Statistics for the Environment - page 692

Return biosolidsrate(gpd)
1

RASrate(GPM) = (24.46)

440 min/day

where it is assumed that

• %SBV 60 is representative.
• Return rate in percent equals %SBV 60 .
• Actual return rate is normally set slightly higher to ensure that organisms are returned to
the aeration tank as quickly as possible.

The rate of return must be adequately controlled to prevent the following:

• Aeration and settling hydraulic overloads
• Low MLSS levels in the aerator
• Organic overloading of aeration
• Septic return activated biosolids
• Solids loss due to excessive biosolids blanket depth

■ EXAMPLE 24.51
Problem: The influent flow rate is 5.0 MGD, and the current return activated sludge flow rate is 1.8
MGD. The %SBV 60 is 37%. Based on this information, what should be the return biosolids rate in
million gallons per day?

Solution:

Return (MGD) = (5.0 MGD + 1.8 MGD) × 0.37 = 2.5 MGD

24.7.1.8 Biosolids Volume Index (BVI)
The biosolids volume index (BVI) is a measure (an indicator) of the settling quality (a quality
indicator) of the activated biosolids. As the BVI increases, the biosolids settle more slowly, do not
compact as well, and are likely to result in an increase in effluent suspended solids. As the BVI
decreases, the biosolids become denser, settling is more rapid, and the biosolids age. BVI is the
volume in milliliters occupied by 1 g of activated biosolids. For the settled biosolids volume (mL/L)
and the mixed liquor suspended solids (MLSS) calculation, mg/L are required. The proper BVI
range for any plant must be determined by comparing BVI values with plant effluent quality:

SBV(mL) ×

LSS(mg/L)

1000

BiosolidsVolumeIndex(SBI) = (24.47)

M

■ EXAMPLE 24.52
Problem: The SBV 30 is 250 mL/L, and the MLSS is 2425 mg/L. What is the BVI?
Solution:

350 mL ×

55mg/L

1000

BiosolidsVolumeIndex(SBI) = = 144

242

The biosolids volume index equals 144. What does this mean? It means that the system is
operating normally with good settling and low effluent turbidity. How do we know this? We know
this because we compare our result (144) with the following parameters to obtain the expected
condition (the result):