Wastewater Calculations - Mathematics and Statistics for the Environment - page 750

■ EXAMPLE 24.149
Problem: Calculate the percent removal of settleable solids if the settleable solids of the sedimenta-
tion tank influent are 13 mL/L and the settleable solids of the effluent are 0.5 mL/L.

Solution: First determine removed settleable solids:

13 mL/L - 0.5 mL/L = 12.5 mL/L

Next, insert the parameters into Equation 24.180:

12.5 mL/L
13.0 m

%Settleable solids removed = ×= 100 96%

L/L

24.15.7 b iosolids t otal s olids , F ixed s olids and v olatile s olids C alCulations
Wastewater consists of both water and solids. The total solids may be further classified as either
volatile solids (organics) or ixed solids (inorganics). Normally, total solids and volatile solids are
expressed as percents, whereas suspended solids are generally expressed as mg/L. To calculate
either percents or mg/L concentrations, certain concepts must be understood:

• Total solids —The residue left in the vessel after evaporation of liquid from a sample and
subsequent drying in an oven at 103 to 105°C
• Fixed solids —The residue left in the vessel after a sample is ignited (heated to dryness at
550°C)
• Volatile solids —The weight loss after a sample is ignited (heated to dryness at 550°C)

Key Point: When the term biosolids is used, it may be understood to mean a semiliquid mass com-
posed of solids and water. The term solids , however, is used to mean dry solids after the evaporation
of water.

The percent total solids and percent volatile solids are calculated as follows:

% Total solids = (Total solids weight/biosolids sample weight) × 10 0 (2 4.181)

% Volatile solids = (Volatile solids weight/total solids weight) × 10 0 (24.182)

■ EXAMPLE 24.150
Problem: Given the information below, determine the percent solids in the sample and the percent
volatile solids in the biosolids sample:

Biosolids Sample After Drying After Burning (Ash)

Weight of sample and dish 73.43 g 24.88 g 22.98 g

Weight of dish (tare weight) 22.28 g 22.28 g 22.28 g

Solution: To calculate percent total solids, the grams total solids (solids after drying) and grams
biosolids sample must be determined: