Sizing Ozone Generators

Most ozone generators designed for residential water treatment are sized by grams of ozone produced per hour of operation. To choose the right sized ozone generator for each job, you must determine the potential work ozone can do. This work can also be thought of as the ozone demand of the water.

Determining the ozone demand of the water requires an accurate water analysis and a little math, using the demand each contaminant places on ozone, or the amount of ozone required to oxidize the contaminant. The amount of ozone needed for oxidation is known as the demand or required dosage. The following table shows dosages which have proven to be very reliable.

Basic Calculations:

Demand Formula: gr/hr = L/hr x O₃D where:

gr/hr = Grams of ozone per hour
L/hr = Liters per hour flow rate to be treated
O₃D = Ozone demand in mg/l to treat the contaminants in the water

There are three basic steps to calculate the gr/hr of ozone needed to treat the water:

1. Calculate the flow rate to be treated in L/hr. If your data state the flow rate in gallons per minute (gpm), you must multiply that rate by 60 to determine the equivalent flow rates in gallons per hour. Then, multiply the gallons per hour figure by 3.785 to determine the equivalent flow in L/hr.
   
   
2. Determine the contaminant demand on ozone by multiplying the mg/L of each contaminant found in your water and adding 0.5 mg/L for disinfection to the sum total.

   Fe demand = X x 0.43 = mg/l
   Mn demand = X x 0.87 = mg/l
   H₂S demand = X x 3.0 = mg/l
   Tannins demand = X x 0.1 = mg/l
   For disinfection, add 0.5 mg/l
   Sum Total = mg/l = O₃D

3. Multiply O₃D by L/hr to calculate mg/hr.
   
   
4. Divide by 1,000 to convert to grams per hour needed to treat the water.

Computation Example: Water sample shows 1.7 mg/l Fe and the flow to be treated is 5 gpm: Convert gpm flow rate to liters per hour (L/h): 5 gpm x 60 x 3.785 = 1135.5 L/hr Determine the contaminant demand on ozone: 1.7 mg/l Fe x 0.43 = 0.731 mg/l + 0.5 mg/l (for disinfection) = 1.231 Multiply O3D by rate of flow: 1135.5 L/hr x 1.231 mg/l = 1397.8 mg/hr Divide by 1000 to convert from milligrams to grams: 1397.8 / 1000 = 1.4 gr/hr total ozone demand

Keep in mind that ozone will react first with H₂S, then Fe followed by Mn, and lastly with tannins. There are other things, such as temperature of the water, organically bound compounds or seasonal variations that bear on the exact ozone treatment system design. It would be sensible to figure a slightly higher (perhaps 20 percent) ozone demand and increase contact time to factor in unknowns.