Air washers are used in commercial buildings to humidify and dehumidify large volumes of air circulating throughout occupied areas. The same principle of air handling is used in mills and manufacturing plants to remove lint/fibers, dust and vapors from the building air.

Air washers are also used to remove volatile organics from manufacturing processes through the exhaust fumes.

Large air-handling washers can move in excess of 100,000 cubic feet of air per minute (cfm). An elaborate spray system inside of the air-handling room creates a wall of water in which all the air must pass through.

As the name implies, air washers pick up organic and inorganic materials that eventually dissolve in the water. In almost all cases the spray water is re-circulated from a central tank. The reservoir must be treated with chemical biocides — both oxidizing and non-oxidizing — to maintain a healthy working environment, reduce biofilm formation and insure proper heat transfer in the cooling/heating equipment. Chlorine/bromine combinations are almost always avoided due to the odor they impart into the air stream.

Effective biocide

Ozone has been used as a stand-alone biocide in air washers for a number of years and with excellent results. Ozone is dissolved in a side-stream loop with the air washer reservoir. Ozone levels are controlled by monitoring the oxidation reduction potential (ORP), which assures low bacteria counts and near complete viral inactivation (650-700 ORP).

Dual Injection Ozone System

Unlike cooling towers (see Water Technology's September 2001 issue), air washers typically have a small volume reservoir with a high circulation rate. When the spray system atomizes the water into the air stream, all dissolved ozone (O3) that is present quickly reverts back to oxygen (O2). The entire contents of the reservoir may be turned over as often as three times per minute. This greatly reduces the contact time between gas and water.

Maintaining an adequate ORP in the washer reservoir becomes the main focus of the water treatment program. The end result is to break the food chain between bacteria and virus and dissolved organic matter. Contact time (CT) is important in exploiting the oxidizing potential of ozone and air washers.

Improving contact time

Two methods to increase CT have been used, both with advantages and disadvantages:

1. Increase the side stream flow rate so a larger volume of water (gallons per minute) is being treated with ozone per minute. No more ozone is required for this method. This increases the energy penalty for operating the system, but you get good dissolving rates in any case.

2. Increase the distance between the gas injection point and the area where the ozonated water enters the washer reservoir. This can be inexpensively accomplished by installing a serpentine piping system made from schedule 80 PVC. In some cases 100 feet of additional piping can be attached to a wall adjacent to the air washer room.

In both cases the following guidelines should be considered:

1. An injector should be used to dissolve the gas into the water stream.

2. A contact tank should be installed downstream of the injector. This will assist in gas/water mixing and increase CT.

3. The primary function of the contact tank is to remove any un-dissolved ozone gas from the water before it is returned to the main washer reservoir. Undissolved gas should never enter the air washer air stream even though the dilution rate of the ozone gas at this point is in excess of 500,000 times.

4. Air washer water is cooled (often down to 42 degrees Fahrenheit) using a centrifical chiller. Most chillers are cooled using a cooling tower.

A single ozone system becomes very economical when used to treat both systems. Two separate side-stream water flows have ozone injected into them on a time basis. Ozone gas injection is still controlled by ORP, but is sent to either of the water streams through a series of solenoid valves based on time clock settings.

Neither system needs constant ozone treatment. If the water reservoir reaches its ORP set point five to six times a day, this is sufficient treatment for excellent biological control.

Gregg Sanders is the water resources manager at PACE Inc. in Charlotte, NC. He can be reached at gsanders@pace-incorporated.com.