The formation of foam in aeration tanks, anaerobic digesters or secondary clarifiers can cause problems if it gets out of control. If foam builds up it can quickly fill the tank meaning there is less space for the process or operate. The foam could also potentially overspill the tank causing contamination issues.
Foam is formed by the action of filamentous bacteria on fats oils and greases (FOG) present in the water being treated. Without these bacteria the FOGs will not be broken down and so they are a vital part of the treatment process. If, however, the content of the FOGs is too high then the foam caused by the bacterial action may get out of hand and need treating.
There are various methods for treating foams caused in this way. Many involve adding anti-foaming agents to the water or by altering the balance of the bacterial eco-system. Another line of attack is to try to lower the FOG’s content before treatment. Sometimes, however, these other methods are impractical and so water sprays are used to knock down the foam as it forms, thus keeping it under control.
Foam suppressing sprays work by the liquid bursting the foam bubbles that have been formed. If the spray droplets are too small then they will be ineffective in bursting foam bubbles, but if they are too large then there will not be enough of them to work well. For this reason a medium level of spray atomisation is required for most foam suppression systems. Spiral and swirl chamber full cone nozzles sprayed at low pressures will generally give the correct spray properties.
Often the fluid being used in foam suppression systems is not clean or well filtered. As such clog-resistant nozzles are normally required. Spiral nozzles make a good choice as do wide free passage swirl nozzles like the MP range.
Often the area needing treatment is quite large and sometimes there are restrictions on the distance between the nozzle and the foam. As such wide angle spray nozzles have clear advantages. With wide spray angles fewer nozzles can be used to distribute the foam suppressing fluid over a wider are. This saves on pipework and so costs.
Amount of liquid
When treating foams in water treatment plants generally the liquid in the tank is recirculated through the spray system. This means that over-spraying will not generally cause problems but, of course, there is a cost associated with pumping and moving liquid, so thought needs to be given to how much fluid is to be pumped though the nozzles.
General rules of thumb for foam suppression are :
Very light foams require 4-10 litres per minute per m2
Light foams require 11-20 litres per minute per m2
Medium foams require 21-50 litres per minute per m2
Heavy foams require 50+ litres per minute per m2
Normally foams formed in water treatment applications will be light-medium in nature, although in some environments they can be heavy.