Nocardia Foam in Activated Sludge Systems

Nocardioforms are filamentous, Gram positive actinomycete bacteria that can cause persistent and excessive foaming in activated sludge plants during the summertime. There are nine main genus of nocardioforms. Two of these genera are involved in activated sludge foaming, Rhodococcus and Nocardia with the latter being the better known troublemaker. How to best control Nocardia foam is a highly debated topic.

Nocardioforms are known for their branch-like hyphae that extend from the cell wall similar to the hyphae found in fungi. These branches link together with other filaments and floc. Simple and complex organic material make up their diet which includes fats, oils, and grease (FOG). Nocardioforms are slow growing and utilize the aerobic conditions established by an aeration tank. These actinomycetes generally have difficulty out-competing other wastewater microorganisms, but once established they're a handful to remove.

Present in lower concentrations, Nocardia help to stabilize floc structure. The bacteria can rapidly breakdown biochemical oxygen demand (BOD) which can be beneficial to high strength wastewater. In higher concentrations, Nocardia can rip the floc apart and swiftly breakdown BOD starving out floc forming bacteria. The dense, brown foam that accompanies an outbreak forms when filaments float to the surface as a result of their low-density fatty acid membrane and the waxy, hydrophobic biosurfactant that coats their bodies. Bubbles from the aeration system can also help the filaments to float. Unlike Microthrix, nocardioforms are not often associated with sludge bulking.

Unfortunately, the conditions required for a nocardioform outbreak are still debated. In general, any change in temperature, pH, dissolved oxygen (DO), solids concentration, or nutrients might spur an outbreak. It’s believed that nocardioforms will be most favored under warm temperatures with a high concentration of FOG, low food to mass (F/M) ratio, and/or a high mean cell residence time (MCRT). Since nocardioforms grow slowly, they need ample time to proliferate, and under low F/M their larger surface area helps to secure nutrients easily. Some people theorize that anaerobic conditions in parts of the aeration tank or surfactants can encourage Nocardia growth as well.

Before deciding on a treatment solution, it helps to confirm that you are dealing with nocardioforms and not some other filament. Just because your foam is brown, doesn’t ensure that Nocardia is the culprit. Toni Glymph has developed a manual that describes how to identify filaments under the microscope. Nocardia is both Gram positive and Neisser positive, but after reading his guide you’ll find that only a Gram stain is really required for identification.

Treatment solutions for nocardioform foam are also highly debated. Using a high volume water spray will temporarily break down the foam, but be prepared for its return. A better solution is to skim off excess foam so the bacteria is not recycled back into the system. Chlorination is not highly recommended. The branching Nocardia filaments prevent sufficient disinfectant contact while healthy floc bacteria are killed. Many companies promote defoaming products, but the interlocking filaments are often too stable for these chemicals as well. Most resources recommend reducing your MCRT to under 8 days while increasing (F/M). Wastewater technician, Jeff Crowther, lists three of his own treatment recommendations on page 10 of the H2Oregon Springs 2016 Newsletter. Solids wasting may be the most common control method. Operators should learn about the life cycle of Nocardia to maintain a system that avoids future foaming incidents.