RAS Chlorination

When filamentous bulking problems reach the extent that large amounts of solids are washing out of the secondary clarifier, it is time for drastic measures to be taken to regain control of the situation. One way of doing this is through the introduction of a toxic substance, such as chlorine, into the mixed liquor with the intent of killing the filamentous bacteria. Typically, chlorine is applied to the RAS pumping stream and so this method is known as “RAS Chlorination”.  Although it is difficult to control, RAS chlorination can be used quite effectively as a means of reducing bulking caused it is a method of last resort and should only be used when other control methods have failed.

Furthermore, it is very easy to make the situation far worse if RAS chlorination is performed improperly. Excessive RAS chlorination can result in and total die-off of all of the organisms in the aeration basin. At the very least, the population of nitrifying organisms (which are very sensitive to environmental changes) will almost always be killed off. This leads to an increase in the effluent ammonia concentration following RAS chlorination. It should always be understood that RAS chlorination is a method to be used as a short-term fix to the problem. Eliminating the conditions that cause excessive growth of filamentous bacteria offers a much more effective way of controlling bulking over the long-term.

The theory of applying chlorine to the RAS relies upon the idea that because the filamentous bacteria stick out into the bulk solution, they are more prone to be damaged by the chlorine than the round and rod shaped bacteria that form the floc particles themselves. This is true if the filament in question is causing inter-floc bridging type bulking. However, RAS chlorination can be significantly less effective on filaments that occur in the floc itself and create an open floc structure.

To be effective, RAS chlorination must be applied at the proper dosing rate over a period of time that spans only a couple of days or a week at the most. In general, it is not a technique that should be used over long periods. If chlorine is to be applied to the RAS, several important pieces of information must be accurately known. One of these is the total number of pounds of MLVSS in the system. This is because the chlorine dose is based upon this number.

For RAS chlorination, chlorine is applied at a rate of 1 – 10 pounds of chlorine per 1000 pounds of MLVSS in the system per day. It is best to start at the low end of the scale, apply the chlorine for 24 hours and observe the effects. The importance of spreading the chlorine application out over the full 24 hours cannot be overstressed. The same result cannot be obtained by applying 10 kilograms of chlorine over 3 – 4 hours as those that can be obtained by applying 10 kilograms of chlorine over 24 hours. Also of great importance is the need to mix the chlorine rapidly and thoroughly throughout the RAS flow stream. In order to be effective, the chlorine must contact as many of the filamentous bacteria as possible. This is best accomplished by injecting the chlorine into the RAS line ahead of an elbow or pump, where the natural turbulence will mix the solution into the RAS. Solutions made up of chlorine gas are much more effective that solutions of bleach or HTH. The reason for this is not entirely clear, but experience has proven it true.

As RAS chlorination is begun, it is important to carefully observe the effect that the chlorine is having on the mixed liquor. There are three main items to look for:
• When viewed under a microscope, the filamentous bacteria should be seen to “break” as well as curl back upon themselves. This is caused by the direct action of the chlorine damaging individual cells. When the chlorine dose is correct, the effect will be widespread. The higher life forms (Ciliates, Rotifers, Flagellates) should be observed to still be alive and active.
• The SVI should begin to drop. The change should be evident within the first day if the chlorine dose is correct.
• The sludge blanket in the clarifier should begin to compact better. Within several days, the RAS and WAS concentrations should begin to increase as evidence of the improved compaction. The level of the sludge blanket should begin to drop and solids washout should stop.

As mentioned earlier, even when performed correctly, RAS chlorination results in a degraded effluent quality. The Heterotrophic bacteria that are responsible for BOD removal are also killed off to some extent and the nitrifying bacteria suffer badly. However, if the dose is correct, these organisms will rebound within a matter of weeks and the system will slowly return to normal operation.

If the chlorine is over applied, the result will be a complete kill-off of the mixed liquor. The aeration basin will turn a pale tan color or even white/gray. The effluent will resemble raw wastewater and discharge permit violations will occur. At this point, the approach has failed and the system will need to be reseeded with living mixed liquor. RAS chlorination offers a method of dealing with filaments, but it can be a very heavy handed sword if not used with caution. RAS chlorination is not recommended for continuous use.