What’s sludge buildup?

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Sludge buildup is a problem that can occur in a wastewater treatment method known as an activated sludge process. This treatment involves introducing a combination of air and microorganisms, such as aerobic bacteria and protozoa, into wastewater and industrial organic waste to encourage growth and multiplication. The microorganisms gather around the suspended organic material they feed on, causing it to settle out of the suspension and leaving a relatively clean effluent for further treatment. Sludge build-up is a problem that occurs due to the overgrowth of certain types of bacteria and hinders the sedimentation process, making the water cloudy and leading to the release of effluents that do not meet environmental standards.

The activated sludge process works by flocculation, the aggregation of suspended particles to form larger masses which settle out of the water. Typically, wastewater containing large amounts of small suspended particles of organic matter enters a tank where suitable microorganisms are introduced, along with sufficient air to promote the growth and multiplication of the right types of bacteria and protozoa, enabling flocculation. Microorganisms tend to attach themselves to organic material and to each other, creating relatively large particles known as flakes. The water is continuously or periodically drawn into a clarification tank, where the flakes settle to the bottom. Part of the sedimented material, still rich in the microorganisms necessary for flocculation, is then reintroduced into the flocculation tank to allow the process to continue.

Three main types of sludge buildup can affect the settling process. Bulking caused by filamentous bacteria is the most common form. These bacteria normally aid the flocculation process, as the filaments they produce help trap suspended particles and generate larger, more stable flocs. Excessive amounts of filamentous bacteria, however, lead to bulking as the filaments can extend from small particles to flocs and keep them apart from each other, preventing the formation of larger particles which will easily settle. There are thought to be about 25 different types of filamentous bacteria in activated sludge, any of which can cause problems.

Polysaccharide buildup, also known as “slime buildup,” occurs when activated sludge bacteria produce too many polysaccharides. This can occur when the mix is ​​low in nutrients or oxygen, or when the food to microorganism (F/M) ratio is too high. Polysaccharides, complex sugars that include starch, are produced outside bacterial cells and enable flocculation by helping the bacteria stick together. If too much is produced, however, it can prevent the material from settling.

Zoogleal bulking is caused by an overabundance of the bacterium Zoogloea ramigera, a flocculent microorganism with a dendritic or tree-like growth pattern. This can happen when the F/M ratio is too high or under low oxygen conditions. The bacterium is a major contributor to floc formation under normal conditions, but an overabundance of this species can hamper the process and lead to a type of lump similar to that caused by filamentous bacteria.

Numerous methods are used to treat or prevent sludge buildup. Sewage samples can be examined under a microscope at regular intervals to monitor the abundance of microorganisms that can cause problems. Where a potential problem is identified, early action can be taken, such as careful regulation of the rate of return of the activated sludge to the flocculation tank. If bulking is already taking place, it can be treated by adding coagulants or certain polymers to the water to promote flocculation and settling. Another approach is chlorination to a level that inhibits the bacterial filaments on the outside of the flakes, but does not penetrate the flakes, leaving the beneficial internal microorganisms unharmed.

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