We all know that a dual alternating biocide program is better than a single biocide program. So why do you still have problems maintaining a kill in your cooling tower? The most common problem associated with biocide feed is adequate biocide dosage and contact time.
Biocide feed instructions are always listed on Momar’s Aquatrol product label and the label states that if the system is badly fouled, it must first be cleaned. If you have mineral or iron deposits, use the appropriate descaler. If the system is fouled, a strong biocide feed with a biodispersant is recommended. Most biocides can only kill surface layers with marginal penetrating abilities for biofilms and none for mineral and iron deposits. Just remember, a biocide can only kill what it comes in contact with.
For clean to moderately clean systems, you can go directly to the initial and subsequent dosage rates. Remember that the dosage rates are typically listed as ___ gallons or ___ pounds per 1,000 gallons volume in the system. One of the more common problems is that not enough biocide was fed to achieve the maximum kill. That is why it is important to know the volume of the system you are treating. Too little biocide will achieve a partial kill, but a quick re-growth will occur. Within a short amount of time, the biocide will become ineffective and fouling will occur. Remember that for slug or intermittent feeds, your dosage rates will be higher (a larger volume of biocide) and that continuous feed is maintained at a lower rate.
Even though the biocide is being fed at the dosage listed on the label, problems may occur. The most common problem is insufficient contact time. Contact time is the number of hours that the biocide is present in the cooling tower (at the recommended concentration or dosage rate). Oxidizers fed at a rate of 1 to 2 parts per million quickly kill microorganisms, usually within 1 hour. Therefore, the contact time for most oxidizers is one hour. This means that the oxidizer is fed at an amount to maintain 1 to 2 parts per million in the system for a duration of one hour.
For non-oxidizing biocides the contact time is longer — from three hours to 36 hours depending on the type of non-oxidizing biocide. Cooling towers receive continuous blow down to maintain proper cycles of concentration and the biocide is being removed with the blow down water. For non-oxidizing biocides there are two methods of maintaining the recommended dosage in the system for the contact time required. First, if you are manually feeding the biocide, feed a high enough dosage so that, after the required blow down, the biocide is maintained at the recommended rate for the duration of the required contact time. A second method is to utilize a feature available on many controllers; pre-bleed the system to a low conductivity and then lock out subsequent bleed for a period of two to four hours. This will allow you to feed a lower amount of the biocide and still maintain the desired dosage rate for the recommended contact time.