Markel

HOME : UV LIGHT AND OZONE FOR WATER PURIFICATION

UVLight and Ozone for Water Purification

May 28, 2004

Ultraviolet Light and Ozone are very good treatment methods for treating water with bacterial issues. Both of these treatment methods are not new, but they are becoming more widely accepted and proven around the industry. Not only are they good for bacterial reduction they both offer several other benefits. UV light can help reduce TOC's (total organic compounds) and also be used for ozone destruction. Ozone can oxidize and flocculate iron, manganese, and hydrogen sulfide so these contaminants can be filtered, plus destroy carbon-based contaminants. They are both used widely in clean water remediation and also in wastewater processing plants. Large scale systems, once thought of as cost prohibitive are now becoming, cost effective and returns on investment are short term and these systems are less maintenance intensive. Newer technologies of manufacturing both types of systems offer the consumer, whether they are residential, commercial, industrial, or agricultural, better alternatives than chemical dosing. Parameters that determine sizing and limitations of the treatment must be taken into account when using these methods, but overall, they offer treatment professionals a venue that is much more accepted in an every growing environmentally conscious society.

Let's first touch upon UV light and explain some chemistry and dosage calculations and then do a brief overview of ozone. The disinfection qualities of UV have been known for centuries, but now in an industrialized sense this medium of disinfection when applied properly and correctly can remedy some simple bacterial issues. The dose calculation is very easy to understand.

* Dose ((W-sec/cm²) = Intensity ((W-sec/cm²) x Time (sec) x Transmission rate.

Computation Example:

A UV lamp with an intensity of 4,000 (W-sec/cm² multiplied by a 10 second contact time would give a theoretical dosage rate of 40,000 (W-sec/cm². Manufacturers usually rate this dosage against distilled water, which has a transmission rate of 100 percent. A transmission rate of 90 percent is more practical in the real world, so this dosage would translate into 36,000 (W-sec/cm². Most waterborne organisms can be destroyed with a 15,000 (W-sec/cm² or less dose of UV. However, some organisms do require higher rates, so be sure to check or measure the effects on any organism that is to be controlled or destroyed.

This dose rate may also be expressed in mJ-sec/cm². It is the same measurement, but expressed differently and is becoming the new standard expression, 36,000 (W-sec/cm² = 36 mJ-sec/cm². The UV lamp power determines the intensity. Time is calculated by the duration of exposure of the process fluid to the UV. The dose rate is affected by factors, such as flow rate, water temperature, water quality, lamp life, and bulb protection materials. Flow rates and contaminants are the principle variables in any water treatment application. Typical disinfection systems are rated at the maximum flow that will provide a 30,000 (W-sec/cm² dose at the end of the lamp's usable life. The water temperature is only a factor in low-pressure lamp systems, because the operating temperature is lower. Low pressure UV output drops drastically if process water rises above or drops below the optimum temperature. Medium pressure lamps have a higher operating temperature so the output is not affected. Turbidity and other contaminants absorb UV light, plus organic and inorganic materials increase the dose required. Other factors are color, metals, and/or suspended and dissolved solids. UV light systems have many parameters that need to be considered for a system to operate effectively.

Quartz sleeves have been the standard method of bulb protection for many years, but new, clearer and heat resistant fluropolymer tubes have emerged as good alternative choices. These fluropolymer tubes are coiled to provide longer contact times and tests have proven them to be effective, not only for cost, but easier maintenance. The coiled tubes are safer and easier to replace, because they don't break and smaller systems can be designed for POU/POE applications. The transmission rates of the fluropolymers are not as high, but the contact times make up for this variance in the dosage formula. Different materials are being developed that are clearer, UV resistant, durable, and reliable, plus there are up and coming uses in ozone contact chambers and bubblers for these coiled tubes and fluropolymers.

Ozone is another method of disinfection and an oxidizer of many contaminants. Ozone can be injected into the water stream using a venturi that is designed to transfer the ozone into water or it can be bubbled into a holding tank with a diffusion stone. Contact tanks need to be sized correctly to provide adequate time to kill the bacteria or oxidize contaminants. Contact tanks can be atmospheric and water can be made up in batches or tanks can be pressure vessels and sized by flow rate. A good rule of thumb would be to design a system that is practical for a given application and the flow rates that can be handled to provide good results. For example a 10 minute contact time on a 10 gpm(37.85 L/minute) system would require a 100 gallon(378.5 L) tank to perform optimally. An atmospheric tank or even a pressure vessel should always use an ORP (oxidation reduction potential) or Dissolved Ozone monitor in order to assure that the correct dose of ozone is being injected or bubbled into the water source. Understanding the load factor on the ozone is one of the biggest factors missed when sizing systems. A general formula for determining ozone can be calculated as follows, but many times years of experience with ozone use is the best method for applying the proper amount without ever over or under ozonating.

The basic calculation will be a starting point.

gr./hr. = L/hr. x O3D, where gr./hr. = Grams of ozone per hour and L/hr. = Liters per hour flow rate to be treated. There are four steps in calculating the gr./hr. of ozone needed to treat water.

1. The first step is to figure the flow rate in L/hr. If the flow rate is in gallons per minute it must be converted to L/hr. Multiply the rate by 60 to get gallons per hour and then multiply that figure by 3.785 to get the equivalent flow in L/hr.

2. Next determine contaminant demand on ozone by multiplying the mg/L of each contaminant found in the source and adding 0.5 mg/L for disinfection and adding it all together. Use the corresponding equivalents below.
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 and add all demands to get total demand.
Sum total = mg/L = mg/L(O3) demand = O3D

3. Multiply the O3D by L/hr. to calculate mg/L of ozone required.

4. Divide by 1,000 to convert to grams per hour needed to treat the water.

A computation example:

The water sample shows 3.0 mg/L Fe, 0.5 mg/L Mn, 1 mg/L Tannin and the flow rate is 10 gpm.

1. 10 gpm x 60 x 3.785 = 2271 L/hr.
2. 3.0 Fe mg/l x 0.43 = 1.29 mg/L + 0.5 mg/L Mn x 0.87 = 0.435 mg/L + 1.0 mg/L Tannin = 0.1 mg/L + 0.5 mg/L = 2.325 mg/L O3D
3. 2271 L/hr. x 2.325 = 5280 mg/hr.
4. 5280 divided by 1,000 = 5.280 gr./hr. total ozone demand.

Whether using UV or Ozone in water treatment, a good analysis of the water to be treated is the most important parameter and then flow rates or water usage must be determined, followed by understanding the application and desired end use of the water. Some water only needs to be cleaned up enough for utility use, such as washing cars or flushing toilets, while other uses, like drinking water would require better filtration and bacterial control. The scope of this article does not allow for a full review of UV or ozone uses, but provides some insights on determining dosage rates and factors in system design. Ozone, more than UV, has compatibility issues with many materials, so be sure to check that piping and other products that will come into contact with the ozone will not result in system failure.
Water reclaim systems are on the rise in many countries and even areas of the United States are reclaiming more water. UV and ozone are great methods of treatment to protect the consumer from bacterial issues and keep storage tanks clean from algal growth. Rainwater can be collected and used for non-potable uses and irrigation applications and both these methods are superb methods to prevent problems with bacteria growth. Wastewater treatment is another growing market for both of these technologies and if designed, applied, and installed correctly will be a water treatment professional's choice of treatment methods.

References
1. Gabe Ergler, "Successful Treatment Using Ozone in Residential Potable Water Systems," Water Conditioning and Purification, May 2003.
2. Jeffrey Roseman CWS-III, "Ultraviolet Technology, Coiled Tubes vs. Quartz Sleeves in UV Applications," Water Quality Products, December 2002

About the Author: Jeffrey Roseman is a CWS-III with the Water Quality Association, with specialty certifications in Ozone and Disinfection. He has a background in chemistry and physics from studies in Electrical Engineering from Purdue University and is the owner of Aqua Ion Plus Technologies in La Porte, Indiana. He can be reached for comment at jeff@aquaionplus.com or 219-362-7279.

Markel Corporation
435 School Lane
Plymouth Meeting, PA 19462
610-272-8960