What is interesting is that the causes of both filaments and viscous bulking are similar - it is just the mix of factors that determines if the system experiences which problem and to what extent.
The most common factors are:
- Influent Composition - influents with high levels of soluble organic acids, sulfides, and other highly soluble organics often cause a oxygen depression near the influent and also favor organisms having the ability to quickly degrade soluble organics and metabolize sulfide. Other factors include oil and grease that in long MCRT systems can raise chances of Nocardia (thick greasy foam on aeration basins).
- Dissolved Oxygen - persistent low dissolved oxygen (but no true designed anoxic zones) favors organisms with high surface area - filaments. This can occur with systems that have anoxic/aerobic zones for nutrient removal if flows increase changing the designed retention times which are predicated on the delicate balance required to prevent conditions favoring filaments. We often see the low DO filaments where we have prolonged zones with DO between 0.1 - 0.5 mg/L.
- Macro Nutrients (N,P) - per 100 g biomass the cells contain 12 gram of nitrogen and 2.3 grams of phosphorus. This is where operators derive the 100:5:1 ratio for influent BOD:N:P. When you have long term significantly low N or P relative to carbon, the conditions favor organisms with more efficient nutrient uptake. Often we see filaments but a strategy of many non-filamentous bacteria under low phosphate stress is to excrete excess polysaccharides. Key here is a long run nutrient deficiency and the pressures for a change from good floc formation to either filamentous or viscous (non-filamentous) bulking.