How adding influent step-feed can help control filamentous bulking in a plug-flow system

Narrow channel plug-flow activated sludge unit

Plug-flow systems are normally represented by a pipe with flow going in one direction towards the effluent. Many of our common systems operate in plug-flow or close to plug-flow reactor design. The most prevalent being oxidation ditches and what I call tightly baffled activated sludge where the flow runs through narrow, long channels. 

Let's recap the benefits of plug-flow:

• Good for BOD5 removal followed by ammonia oxidation, denitrification, and even phosphorus removal. Much flexibility in changing aeration, anoxic, anaerobic zone sizes.
• Works well with low BOD5 or degradation of insoluble BOD5 including resistant organics 

Now the problems

• High soluble BOD5 loadings near the inlet can create low D.O., promoting filaments
• Susceptibility to shocks - pH, influent temperature, toxic shock loadings (this is why plug flow is much less common in industrial wastewater)

For most plug flow systems, the biggest problems are caused by high soluble organic loadings near the influent depressing DO and promoting the low DO filaments. If the loadings are consistent and the ability to add more oxygen to the first part of the system is not efficient, another solution is to step-feed the influent. By having multiple injection sites along the length of plug-flow system, you reduce the amount of soluble BOD5 entering any one section per unit of aeration. This prevents excessive anoxic or low DO conditions favoring filaments and odors. It can also be useful in increasing nutrient removal efficiency by altering DO, Redox, nutrient, or soluble organic concentrations in individual sections.