Bridging is when the filaments are external to the normal floc structure and cause a bridge between the floc structures.

Many plants think there is only a problem when they have external filaments that are causing bulking problems.  In reality there are three ways to cause bulking at a plant, internal filaments, external filaments or non-filamentous bulking- which can be caused by zooglea, fungi and in many municipalities or papermills, fiber or tissue.

What is the difference and how do they impact the plant? Let's take a closer look at all of these scenarios.

Filaments can be internal, external or free floating. Internal filaments are like a sponge. They can be hard to settle and dewater. External filaments keep the floc structures from coming together and compacting. Free-floating filaments can cause TSS problems. Zooglea and fungi can cause foaming, sliming or bulking. Let's first look at some samples from a few plants that have external bulking and what most operators think is the only case for filamentous problems. External filaments are those filaments that extend into the bulk solution and cause difficulty in settling. They form a bridge between the floc structures and make compaction difficult. They can take up large amounts of space in a clarifier or digestor or even aeration basin.

This plant on the left has abundant levels of filaments and this one on the right has excessive levels of filaments that will make it hard to settle in the clarifier, hard to dewater and increase polymer consumption and increase solids hauling costs.

This is typically what people think of when they talk about filamentous bulking. While you may think you have a large amount of bacteria when doing a settlometer, in reality, the filaments are just taking up space.

Most operators are well aware of bridging and this is usually what is thought of for filamentous problems. But you need to check again.
Sometimes you need to go to a much higher level on the microscope objective.
Staining the sample helps, but some skill is required. A quick and dirty way to help bring out some of the fine details on the sample without having to go to all the trouble of drying a slide and going through all the difficulties of staining is to use LactoPhenol Cotton Blue stain. It can be purchased at VWR, USA Bluebook, Fisher, Scientific or any of your favorite supply chains. Use the stain along with one drop of wastewater on the slide. The stain will help bring out any fine details, internal filaments, many characteristics such as sheath, cell walls or shape and size.

 
***Note- if you use this stain, all the higher life forms will slow down and die. Do not worry, that is supposed to happen. Just keep it in mind if doing a wastewater biomass analyses and make sure to use a normal sample when checking higher life forms for count and identification. This stain will make it easier to spot higher life forms though and to photograph them since it does slow them down.

Internal bulking is when the filaments are inside the floc structure and make up a backbone for the floc. In a small amount, internal filaments are a great plus. They help keep the floc together, help avoid shear and give stability to the floc. In large amounts, they can create a floc that is more like a sponge. It is very hard to dewater and compress. It can take large amounts of polymer to get the floc to settle in the clarifier. Solids can build up. Carryover or washout can occur quite easily. Polymer consumption on a belt press can increase. Solids dewatering is harder to achieve. Dryer cake solids are harder to achieve. With solids handling costs on the rise and a big part of a budget at any wastewater treatment plant, filaments can add a significant chuck to any budget.

Internal filaments are very common, often overlooked and ignored and yet many operators or supervisors of wastewater plants cannot figure out why they have too many solids, overloading or problems and high costs with solids handling.