ABSTRACT
Chemicals are used extensively in both wastewater and water treatment operations. Plant operators add chemicals to various unit processes for slime growth control, corrosion control, odor control, grease removal, BOD reduction, pH control, biosolids bulking control, ammonia oxidation, and bacterial reduction, among other reasons. To apply any chemical dose correctly, it is important to carry out certain dosage calculations. One of the most frequently used of these calculations in wastewater/water mathematics is the dosage or loading. The general types of mg/L to lb/day or lb calculations are used to determine chemical dosage, biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (SS) loading/removal, pounds of solids under aeration, and the waste activated sludge (WAS) pumping rate. These calculations are usually made using either Equation 8.1 or Equation 8.2:
Dosage (lb/day) = Concentration (mg/L) × Flow (MGD) × 8.34 lb/gal (8.1)
Dosage (lb) = Concentration (mg/L) × Volume (MG) × 8.34 lb/gal (8.2)
In chemical dosing, a measured amount of chemical is added to the wastewater (or water). The amount of chemical required depends on the type of chemical used, the reason for dosing, and the flow rate being treated. The two expressions most often used to describe the amount of chemical added or required are
• Milligrams per liter (mg/L) • Pounds per day (lb/day)
A milligram per liter is a measure of concentration; for example, consider Figure 8.1. In this figure, it is apparent that the mg/L concentration expresses a ratio of the milligrams chemical in each liter of water. As shown below, if a concentration of 5 mg/L is desired, then a total of 15 mg chemical would be required to treat 3 L:
5 mg 3 L 3
15 mg 3 L
×
× =
The amount of chemical required therefore depends on two factors:
• The desired concentration (mg/L) • The amount of wastewater to be treated (normally expressed as MGD).
