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Successful Treatment, Using Ozone in Residential water Systems

Ozone is new to many water treatment professionals and naturally there are a great many questions most first time users have. This article will guide you through basic design considerations for successful residential potable water treatment with ozone.

Any treatment technique has certain steps that when followed are found to be reliable and provide consistently satisfactory results. There are four basic steps to get you on your way to successful treatment using ozone.

The four steps to successful treatment with ozone are;

Gather good information.
Determine ozone demand.
Determine equipment and chemical needs.
Design new ozone based treatment system.

1. Gather good information.

Use a questionnaire to facilitate the information gathering process. A diligently completed questionnaire with good water quality information will serve as the basis of your treatment system design and guide to specifying the correct equipment.

1.1. Fill out the questionnaire.

Make copies of the Project Form in your Professional Residential Dealer Catalog from O3 Water Systems. Use the questionnaire as an opportunity to thoroughly investigate and gather essential information that can make the difference between successful ozone treatment and failure.

If your are not familiar with the application of ozone or have had hit and miss success, you are encouraged to fax a copy of the completed questionnaire to O3 Water Systems. O3 Water Systems will gladly assist with system design and equipment specifications until you are comfortable with the entire process.

1.2. Test the Water.

It is recommended that you have a water analysis performed by a certified laboratory. Be sure to have an analysis done to determine levels of each contaminant you will be treating the in the water. If possible get the results from several analyses performed over a period of time to determine if there are seasonal variations in the water supply and what the variations are. If you suspect high levels of organics there are two other tests that will help with system design. They are the Chemical Oxygen Demand (COD) and Biochemical Oxygen Demand (BOD) of the water you are treating. The COD and BOD results will bear on calculating total ozone demand and treatment system design.

Some water quality information is best gathered "on-site" because the test must be performed immediately upon taking a water sample for the results to be of any value to the treatment design. The information best gathered "on-site" includes the level of hydrogen sulfide (H2S) and water temperature. Less sensitive to time but more relevant if monitored immediately is the Oxidation/Reduction Potential (ORP) of the water to be treated.

For residential potable water treatment systems it is recommended that at minimum you have the ability to test for the items listed in Table 1:

Table 1

Iron	Manganese	Hydrogen Sulfide
Tannins	Temperature	pH
Ozone	Hardness	Alkalinity
Total Dissolved Solids	Nitrates	Nitrites

2. Determining the ozone demand

Most ozone generators designed for residential water treatment are sized by the 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 on ozone or the required dosage. There are as many different demand or dosage figures as there are people calculating ozone treatment. The dosage figures in Table 3 are the figures we use at O3 Water Systems when calculating ozone demand. We have found the dosages in Table 2 to be very successful and reliable in our ozone treatment system designs.

Table 2

Common Contaminants In Well Water
And Suggested Minimum Ozone Dosages

Contaminant & Chemical Symbol

Mg Ozone, per Mg of contaminant

Iron (Fe)

Manganese (Mn)

Hydrogen Sulfide (H2S)

Tannins

For Disinfection

0.43mg

0.87mg

3.0mg

0.1mg

0.5mg

Demand Formula: Gr./hr = L/hr x O3D

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.

Step 1: Calculate the flow rate to be treated in L/hr.

If your data states the flow rate in gallons per minute you must multiply that rate by 60 to determine the equivalent flow rater in gallons per hour. Then multiply the gallons per hour figure by 3.785 to determine the equivalent flow in L/hr.

Step 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 (add demands to get total demand)

Sum Total = mg/l = mg/l(O3) demand = O3D

Step 3: Multiply O3D by L/hr to calculate mg/hr

Step 4: Divide by 1000 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;

Step 1: Convert gpm flow rate to liters per hour (l/h);

5 gpm x 60 x 3.785 = 1135.5 L/hr.

Step 2: 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

Step 3: Multiply O3D by rate of flow;

1,135.5 l/h x 1.231 mg/l = 1,397.8 mg/hr;

Step 4: Divide by 1000 to convert from milligrams to grams;

1,397.8 / 1000 = 1.4 gr./hr Total Ozone Demand.

Keep in mind that ozone will react first with H2S, then with 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%) ozone demand and increase contact time to factor in unknowns.

To treat surface waters some will say that having a residual of ozone after appropriate contact time is all that is needed. We recommend that when treating surface waters with bacteria, virus, or cysts and multiple other oxidizable contaminants that ozonation be a two step process. In step one, ozonation and filtration will purify the water (remove iron, manganese, hydrogen sulfide, etc.). In step two, ozone will again be injected and a ozone residual will be maintained to insure disinfection. A possible alternative to the second point of ozone injection could include the use of Giardia approved filter systems or ultra-violet radiation post filtration.

3. Determine Equipment and Chemical needs

Each component of the ozone treatment system must be properly sized to work together and produce treated water at the required rate of flow.

3.1. Ozone Injection

There is two most common methods of injecting ozone into the water, using a pump to force ozone through a diffuser and venturi injection. We prefer venturi injection because it is very efficient and requires no moving parts. Water pressure at the inlet of the venturi injector must be higher than the outlet pressure during the entire pump cycle. This difference is known as the pressure differential. The pressure differential required for each treatment system is determined by two variables.

Variable 1.

Required rate of injection, stated as liters per minute (l/m) or equivalent Standard Cubic Feet per Hour (SCFH). The required rate for your ozone generator can be obtained from your ozone equipment or manufacturer. Request a chart showing various rates of ozone production at various injection rates. Proper use of this information will enable a certain amount of "customization" for each application.

Variable 2.

The selected venturi injector. Again, use the injector charts to select an injector that meets or exceeds the required injection rate set by your ozone equipment manufacturer.

3.2. Ozone Contact and off-gas.

After injection the ozone must have time to do its work. This time is known as contact time. Contact time is the time ozone has to oxidize and disinfect or the time the water is allowed to hold the disinfectant. For ozone, contact time is measured from the point of injection to the filter. Depending upon the circumstances of each installation, there may still be ozone in the water after filtration but for purposes of calculating contact time, the filter marks the end. Suppose you inject ozone into a 25 gallon contact vessel followed by a filter and the flow rate is 5 gpm, the contact time in this situation would be 25 gallons divided by 5 gallons per minute resulting in 5 minutes contact time.

The contact time needed varies with the matter to be oxidized. Time required for oxidation ranges from almost instantly to 10 minutes or more. A general rule of thumb is no less than 4 minutes contact time with a measurable residual of 0.1-mg/l ozone and more contact time is better. Don’t take a good thing too far, because ozone is so fast reacting and has such a short life once generated it is possible to have too much contact time in which case you wouldn’t have a measurable residual.

There are several reasons more contact time is better, here are two;

1. Contaminants floc or precipitate at varying rates and with varying degrees of density, giving more time will often make a treatment system a success by providing time for material to fully precipitate which in turn enhances filterability.

2. Ozone must contact the contaminant to oxidize or disinfect; more contact time increases the odds that all water will be subjected to oxidation.

If using ozone to disinfect, there are guidelines set by the Environmental Protection Agency (EPA). The EPA has proposed the use of a "CT value" to assure the attainment of primary disinfection at a minimum cost. The CT value is the numerical product of the concentration of residual disinfectant ("C"), in mg/l, multiplied by the time (T), in minutes, during which the residual is present.

C (mg/l) x T (min.) =

CT value (mg/l-min)

Thus, units for CT-values are expressed as mg/l-min.

CT tables show the required CT value at a given temperature and pH. To be more precise and meet regulatory requirements please refer to the approved CT tables used by the regulating authority in your area. Most agencies use the EPA recommended CT values.

All gasses inducted through the venturi must be properly disposed of or "off-gassed". At a bare minimum, this is accomplished by simply venting the contact tank to the outside. It may be required to route the gasses through an ozone destruct unit to insure no ozone gasses are being emitted from the treatment system.

3.3. Filtration of oxidized particulate.

At O3 Water Systems we most often specify multi-media filtration with our ozonation systems. In most cases, a properly sized multi-media depth filter is very well suited to filtering objectionable oxidized contaminants and will give years of operation with little required maintenance. In some cases a series of filters will be needed to accomplish adequate filtration. Circumstances (usually regulatory enforcement of the SWTR) may require additional filtration to physically remove Giardia and/or Cryptosporidium.

It is recommended that activated carbon filtration be used as a final filtration step to remove any dissolved ozone potentially remaining in the water.

4. System Design

There is no one right ozonation system design. With ozone there is a great deal of design flexibility, please refer to manufacturers guidelines for proper design and installation of their ozonation equipment.

There are a few rules that must be followed no matter which ozone manufacturers equipment you are using.

1. Safety precautions must be taken to prevent prolonged exposure to ozone gas by humans or other animals.

2. All materials in contact with ozone must be ozone resistant.

3. Atmospheric conditions where the equipment is installed must be favorable to highly sophisticated electrical equipment. Please consider temperature, humidity, exposure, etc.

Many ozonation systems will be replacing a treatment system using another oxidation/reduction process. It can be (with customer consent) to everyone’s benefit to incorporate as much of the existing system as possible into the new ozone treatment system (be sure any warranty issues are resolved and to note the items for re-use on your completed questionnaire).

The most commonly adaptable components are system pumps, contact tanks, filters and holding tanks. Be sure that all the components are ozone resistant. Beware of using any non-stainless steel metal tanks and pumps where ozone will be in contact. Ozone will oxidize them too!

Because ozone is a stronger oxidant and more reactive than other oxidizers it is likely that any existing ozone compatible contact/storage tanks will be adequately sized. Filter media in existing filters may need to be changed and it may be necessary to update the filter control valve.

4.1. Two Basic Design Models to Choose From – Pressure Systems and Atmospheric System

What is meant by a "pressure system"?

A treatment system that is consistently under greater than atmospheric pressure during the entire treatment process is considered a pressure system. A pressure system is found most often where the water is not too bad and the flow rates are low (usually less than 10 gpm).

What is meant by an "atmospheric system"?

A treatment system that is not constantly under pressure greater than atmospheric. An atmospheric system will most always have 1 our more pumps to re-pressurize after water comes to atmospheric pressure.

What are some of the differences between pressure and atmospheric systems?

Pressure aside, some of the differences between pressure and atmospheric systems are highlighted in Table 3.

Table 3.

	Pressure Systems	Atmospheric Systems
Concept	The treatment system remains under pressure during the entire treatment process.	The treatment system comes to atmospheric pressure at some point during the treatment process.
Cost	May be less expensive (especially where the water to be treated is relatively clean and the volume of water to be treated is low).	May be less expensive where water is very dirty (especially where the volume of water to be treated is over 10 gpm).
Suitability	Most suitable in residential treatment systems treating less than 10 gpm and where there is at least a 80 gallon pressure tank.	Suitable for any size treatment system.
Advantages	Simple design. Requires only one pump Lower system space requirements.	Flexible treatment design. Can incorporate multiple injection points. Easier to control ozone residual levels. Can use smaller ozone generator with recirculation through the contact tank.
Disadvantages	Single injection point May require a larger ozonation system for single point injection. The single pump used to supply the treatment system may need to be larger to insure the pressure differential across the venturi injector is maintained during the entire pump cycle. A larger pump requires more power.	Generally requires larger space for treatment system. Requires more than one

Two common questions;

Will ozone take out hardness?

Ozone will not oxidize hardness minerals such as calcium. Incorporating a softener into your ozone water treatment system to remove hardness requires only two special considerations, whether all components are ozone compatible and where to place it. In a treatment system where ozone is employed, a softener is installed after oxidation and filtration processes.

Using ozone before your softener will likely decrease the work load of the softener as contaminates such as iron are removed prior to the softener and taken out of the softener sizing equation. This leaves only the hardness minerals for the softener to contend with and may reduce the size of softener needed and its operating cost while extending its service life.

Will ozone fix my low pH?

Ozone will not directly effect Low pH. Though ozone is relatively unaffected by normally encountered pH levels, if possible, adjust pH prior to ozonation as some contaminants are more readily removed with ozone where the pH is between 6.0 and 9.0. pH can be adjusted in a treatment system based on ozone just as it would without ozone.