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WaterOperator.org Blog

Articles in support of small community water and wastewater operators.

Nocardia Foam in Activated Sludge Systems

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.

Preventing a Bloodworm Invasion

Preventing a Bloodworm Invasion

Midge fly infestations can pose considerable challenges for activated sludge systems and lagoons. Also known as Chironomids or bloodworms in their larvae stage, these insects resemble mosquitos without the blood sucking proboscis. Adult males can be distinguished from females based on their feather like antennae. After dormancy in the winter, midge flies emerge in the summer ready to lay between 100 and 3,000 eggs per female.

Though midge flies do not suck blood like mosquitos, they disrupt communities in other ways. Swarms annoy both local residents and operators by flying into unsuspecting mouths and flooding outdoor lighting. A study by Selden et al. (2013) found that wastewater operators can develop allergic reactions from midge fly exposure. Chironomids can also cause quite a startle to the public when bright red larvae make their way into drinking water systems.

When it comes to maintaining treatment systems, wastewater operators may be most concerned with the larvae stage of midge flies. Their sticky red bodies cling to suspended solids encasing them in a cocoon of decaying organic matter. Under the protection of these cocoons, they can consume considerable amounts of sludge, bacteria flocc, and nitrifying bacteria. An infestation will cause sludge clumping, rising solids, or foaming issues. In one small town a bloodworm invasion wreaked havoc on an activated sludge plant over a single weekend. The wastewater operator found sticky clumps of eggs had congested the system’s pumps while larvae had eaten away at his mixed liquor suspended solids (MLSS).

Facultative lagoons and secondary clarifiers are a favored breeding ground for these pests. Midge flies prefer to lay their eggs in still, high-nutrient water with fixed media, floating scum, or algae. Once the eggs hatch, larvae will likely sink to the bottom to feed on organic matter and sludge. The hemoglobin that gives bloodworms their red color also allows them to live in low dissolved oxygen (DO) conditions.

To avoid bloodworm infestations, operators should focus on encouraging circulation and limiting food sources. Systems can start midge fly control with mixing, limiting surface scum and algae, installing bug zappers, attracting bats and swallows, or turning off lights at night. Introducing a predatory fish can also help. Lagoon operators can encourage circulation by cutting back overgrown vegetation. Any dead spots in circulation should be addressed. When these methods don’t work, some systems will use larvicides and chemical agents as a last resort. Operators should check that the control methods they’ve selected are approved by their local regulatory authorities before use.

When summer starts make sure your treatment system is kept clean and free of obstructions to circulation. With good preventative maintenance, you can spare yourself the nightmares of a bloodworm invasion.