| Some UV manufacturers use cagey math to rate their units. If the manufacturer says a unit is rated for a certain flow, check what the UV dosage is at that flow. Many will base recommended flow on a dose of 15 microwatt-seconds per square centimeter because that dose will kill many bacteria. However, it usually takes at least 22,000 microwatt-seconds per square centimeter to kill algae. Some say that the dose for algae should be 40,000 mw-s/cm2 to accommodate less-than-clear water and the degradation of the UV bulb over time.
That 22,000 microwatt-seconds per square centimeter dosage means 22 watts (= 22,000 microwatts) of UV radiation bombards a square centimeter for one second. You could also use 11 watts of radiation bombarding a square centimeter for 2 seconds and get the same dosage. Therefore, if you slow down the flow, you increase the UV dosage. You can also increase the dosage by increasing the wattage of the bulb.
The unit of measure is square centimeter (a surface area) not cubic centimeters (a volume). The effective depth depends on the amount of turbidity (such as algae and other solids) which will absorb the radiation. Dissolved iron and magnesium seem to absorb a lot of UV radiation, as do many other compounds which contribute to general hardness. This effective depth is usually a couple of centimeters. Recommended flow rates usually assume the water is very clear and low in dissolved solids.
The dosage to kill big stuff like parasites is about 10 times higher than what is needed to kill algae. This assumes the parasite happens have a free-living form which would pass through the UV unit.
If you know the wattage and length of the bulb, the diameter of the quartz sleeve the bulb sits in, and the flow rate of the water, you can calculate the dosage. This gets a little cumbersome though. An easier approach is to take the manufacturer’s recommended flow rate and correct it, if needed, for killing algae instead of bacteria. If the manufacturer says the unit is rated for 20 gallons per minute, then look for information on the UV dosage that will be delivered at that 20 gpm flow rate. If a dosage is not given by the manufacturer, then they are probably using a dosage of 15,000 mw-s/cm2. If you want a dosage which is twice as high, say 30,000 mw-s/cm2, then the flow you use through the unit should be half as much, only 10 gallons per minute.
The common bulb sizes are 8, 15, 25 and 40 watts. For larger units, they generally just add more 40-watt bulbs.
The UV output of a bulb degrades over time. They usually recommend that you replace the bulb after 8 to 12 months of continuous use. If the unit is designed for a dosage higher than the minimum dosage required to kill algae (>22,000 mw-s/cm2), then the unit will put out an adequate amount of radiation to kill algae over a longer period of time.
The glass sleeve is also an important consideration. There are some inexpensive UV units which do not have a quartz sleeve between the bulb and the water. A problem with these is that the water in contact with the bulb keeps the bulb cool which reduces it’s UV output. The quality of the glass is important. A good UV unit will have a sleeve of quartz glass since quarts absorbs less UV radiation than silica glass. I am told that there are various grades of quarts glass with the better grades absorbing less UV than the poorer grades. Quartz glass is not cheap.
Finally, cleaning the quartz sleeve is important. When the surface of the sleeve in contact with the water becomes dirty, it absorbs some of the UV radiation. Some UV units used in industrial aquaculture have a little gizmo which slides back and forth to clean the surface of the sleeve without having to dismantle the unit. For most units used in koi ponds, the sleeve has to be removed from the unit to clean it. This is not only time consuming, it can be the root of other problems such as wearing out the O-ring seal and causing the unit to leak. More often than not, when bulbs and sleeves are accidentally broken it happens when the unit is being taken apart for cleaning.
Those darn bulbs are a real pain in the rear. The end caps fall off, the connections become corroded, the little wire dangling precariously alongside the bulb breaks, tap the bulb once on a hard object and the glass breaks. Yet, to get the maximum effect, you have to handle and remove the bulb and sleeve for cleaning on a regular basis.
One more little consideration. When calculating the flow needed to get the dosage you require, you have to be careful not to slow down the flow too much. If you use only half of the rated flow, its probably OK. However, if you slow down the flow to 10% of the rated flow, then you need to make sure the water is not being heated up too much by the prolonged contact. If the water gets too hot, the plastic will warp and the thing will start to leak at the seals.
-steveho pkins | 
12-12-2005
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