Filamentous Bacteria - Problems and Solutions

Filamentation is the abnormal growth of some bacteria in which cells do not separate but elongate and have multiple chromosomal copies. They grow in long thread-like strands.

The causes of bacterial filamentation are:

• A response to DNA damage
• Nutritional changes.

The presence of some filamentous bacteria is benefitial in floc formation to a biomass.

The filaments connect to each other and form a mesh that is crucial for floc formation.

Filamentous bacteria serve as a base of the structure on which other bacteria can attach and form flocs which are important in flocculation and settling.

However, excessive amount of the filamentous bacteria in wastewater may cause problems with sludge settling (bulking and foaming) and have impact on wastewater treatment efficiency.

Excessive filamentous microorganisms will appear due to:

• Little sludge load - filamentous microorganisms attain high growth rates despite low substrate and oxygen levels,

• Lopsided composition of wastewater – in industrial firms and variations in effluent,

• Variations in the wastewater - varying flow or temperatures, changes in nutrients…

Foaming and sludge bulking negatively affect the water treatment plant cleaning process and the first step in getting rid of them, e.g. controlling filamentous bacteria growth is to identify the filaments.

Depending on the nutrients, different filamentous bacteria will dominate in wastewater. The most common are the Gram Stain and the Neisser Stain.

Identifying filamentous bacteria is time-consuming and not easy. The Document Filamentous Bacteria Identification & Process Control may help you simplify the process.

Bulking sludge happens due to bad settling and thickening of the solids. Uncontrolled growth of filamentous bacteria influences settling of activated sludge. Bulking sludge has a sludge volume index (SVI) above 150 ml/g (normal SVI=100 ml/g)

The reason behind the unstoppable filamentous growth is usually the hydrophobic surface of bacteria, which leads to flotation of the sludge.

Since a certain amount of filamentous bacteria is useful in the activated sludge process, lack of it may cause flocs with good settling abilities but which leave behind a turbid effluent. This is because filaments are a backbone of a floc structure and a base for larger flocs and some filaments also “catch” small particles during settling, therefore, lowering the effluent turbidity. Too many filaments will, on the other hand, cause bulking.

Depending on the type of filaments, two forms in sludge settling can be present:

• interfloc-bridging – filaments extend from the floc surface and physically hold the floc particles apart

• open-floc structure - where the filaments grow mostly within the floc and the floc grows around and attached to the filaments.

There are 6 environment conditions and causes of filament overgrowth in activated sludge and 4 of these are present in municipal wastewater systems while all of them occur in industrial wastewater systems and low nutrients and low pH are specific only for industrial wastewater systems.

Table source Activated Sludge Microbiology Problems and Their Control

The foam floats and accumulates on the surface and decreases the effluent quality and control of sludge retention time (SRT).

There are 3 filamentous organisms which cause activated sludge foaming.

Table source Activated Sludge Microbiology Problems and Their Control
 

Solutions to sludge bulking:

Sludge Juggling

Sludge juggling is a short-term solution to bulking which involves return sludge (RAS) flow rate and waste feed points manipulation.

Solids must be removed from the final clarifier faster than they are added so the RAS flow rate must be increased. This increase is limited because the increased return flow to the system hydraulically pushes more sludge from the clarifier, making effluent TSS losses worse.

Polymer and Coagulant Addition

There are different types of chemicals which can be added to enhance activated sludge settling. They are usually added to the MLSS as it leaves the aeration basin or to the secondary clarifier center well.

Chlorination

Chlorine and hydrogen peroxide successful temporary control method for filamentous growth.

Chlorine is cheap and available on-site at most plants so it is widely used. Sufficient chlorine damages the filaments extending from the floc surface while leaving organisms within the floc largely untouched. (If the bulking problem is not caused by filamentous bacteria, adding chlorine can worsen the bulking – for slime bulking or poor floc development)

It is crucial to properly dose the chlorine and add it frequently to activated sludge. To find the right dose, one should start with smaller doses and increase them gradually. When the correct dose is added, the sludge settleability improves within 3 days.

Long term measures include activities such as: control of influent waste septicity (organic acids and H2S); nutrient additions (industrial waste systems only); changes in aeration; and changes in biomass concentration or changes in waste feeding pattern.

 

Sources:

• https://wtp-operators.thewaternetwork.com/_/sludge-recovery-and-treatment/document-XAs/activated-sludge-microbiology-problems-and-their-control-1-fieuj6kCVXY3sECm4Dm8kA

• https://en.wikipedia.org/wiki/Filamentation

• http://www.lenntech.com/library/sludge/filamentous/filamentous-bacteria.htm

Media

Taxonomy

• Bacteria
• Industrial Wastewater Treatment
• Sludge Treatment
• Sludge Management
• Waste Water Treatments
• Wastewater Treatment
• Sludge Treatment & Management