In a biological wastewater treatment plant, filamentous bacteria form the backbone of floc formation that leads to a healthy sludge. So presence of filamentous bacteria in small amounts is desirable. But high density of filamentous bacteria could indicate that the food to microbe ratio, pH, dissolved oxygen may not be optimal in your wastewater treatment plant. Investigating the root cause is important as continued presence of high amount of filamentous bacteria causes heavy foaming and other issues which could badly affect your treatment efficiency.