The production of foam in a biological reactor is a common wastewater problem experienced by plants around the world. Though common, excessive foaming is a difficult problem to correct because foam can be caused by many factors including:
Severe foam prevents access to equipment.
Surfactants can increase both the amount and the stability of foam. As a result, foams generated from wastewater containing surfactants will last longer than those produced in the absence of surfactants. A rapid increase the quantity of foam is believed to be related to an increase in the surfactant concentration entering the biological treatment system. Slug discharges of poorly degradable surfactants from cleaning operations can result in a rapid increase in foam levels in the biological reactor.
Foam production can be so great that it overflows the biological reactor, as you can see in the photograph below, making access to the process impossible for operations and maintenance personnel.
There is another component that contributes to excess foam, once foam development starts, that’s often overlooked. It has to do with the design of the biological reactor and it’s called “foam trapping.” Foam trapping occurs when the discharge from a biological system is from a location beneath the surface rather than overflowing from the surface. Foam spilling over a weir will “collapse” to some degree on its way to the secondary clarifier. When the biological reactor does not have this type of arrangement for its outlet or overflow to the clarifier, foam developing on the surface becomes trapped which significantly contributes to an increasing accumulation. The illustration below portrays the difference between a surface discharge from the biological reactor and a sub-surface discharge.
Our discussion here is going to focus on foam caused by an excessive growth of filamentous bacteria and the use of chlorination to reduce the foam. When the cause of the foaming has been determined to be filamentous bacteria, which requires a microscopic analysis of the foam, no the MLSS, an often used approach to reducing the foam is chlorination, usually by sodium hypochlorite injection into the return activated sludge (RAS).
The bleach (sodium hypochlorite @ 12.5% strength) will need to be pumped into the suction or discharge side of a return sludge pump. The suction side is preferable. The goal is to provide maximum contact and mixing between the bleach and the microorganisms and to do so continuously in a controlled manner. Effective chlorination of the return sludge requires, as a rule-of-thumb, contact between the return sludge and the bleach ≥ 3 times per day.
Some wastewater plants have feed sodium hypochlorite directly to the aeration basin. When they have done this has a “huge” slug dose, I have seen it help reduce the filamentous foam. But adding bleach this way is also much more destructive to the “good” bacteria. Feeding the bleach at a controlled, continuous rate, into the RAS, results in a level of mixing between the bleach and the RAS that optimizes the impact on the filamentous while minimizing the attack on beneficial bacteria as portrayed in the illustration below.
The typical chlorine dosage rate to control filamentous bacteria is 1 to 10 pounds of chlorine for every 1,000 pounds of mixed liquor volatile solids (MLVSS). The usual starting point is 5 lbs chlorine/1,000 lbs MLVSS. Keep in mind that dosage rates as high as 20 lbs chlorine/1,000 lbs MLVSS have been required. Also note that when using the term “chlorine” we are referring to 100% chlorine. Our use of sodium hypochlorite means we are using a product with about 12.5% chlorine. Here are the calculations for determining the feed rate of bleach starting at 5 lbs chlorine/1,000 lbs MLVSS.
The first step is to calculate the pounds of MLVSS in the biological reactor. For example, let’s say that we’ve calculated the volume of the aeration tank found the total volume to be 660,578 gallons or 0.66 million gallons (MG). If we have an MLVSS concentration of 2,500 mg/L the pounds of MLVSS in the aeration tank are:
If we are going to feed 5 lbs of bleach at 12.5% strength (10.8 lbs/gal) to 13,761 lbs of MLVSS our feed rate will be:
Some final comments on using sodium hypochlorite to reduce biological foam concern one other chemical application point. When you have a surface discharge from the biological reactor the MLSS often flows down a channel to the clarifier such as the one shown in the photograph below. When you have this type of configuration to your wastewater another excellent chemical application point is through a spray bar directly on to the MLSS. In this picture the foam has been brought under control and is no longer a problem. But this type of application point is excellent because the bleach is spraying directly onto the foam that will be on the surface as the MLSS flows down the channel.