Which influent scheme is described as long, conventional, high length and low width?

• A. Complete mix: MLSS same throughout, short and wide, low F/M.
• B. Plug flow: long length, low width; high MLSS at beginning; nitrification; endogenous respiration; aeration tapered.
• C. Step feed: fed at multiple points to equalize F/M.
• D. Batch feed: single point injection.

Answer: (B)

The flow pattern and geometry being tested here is how a reactor can maintain a gradient along the flow path. A plug flow scheme is characterized by a long, narrow channel where the liquid moves as a cohesive plug with very little mixing in the flow direction. That means the influent moves through the reactor from the inlet to the outlet, creating a high length-to-width ratio.

Because the flow is not fully mixed along its path, the substrate concentration is highest at the entrance and decreases downstream. This arrangement often uses a high mixed liquor suspended solids (MLSS) at the beginning to handle the incoming load and initiate treatment as the wastewater travels. Nitrification tends to occur in the early portion where oxygen is available, and as the substrate is depleted downstream, microorganisms may enter endogenous respiration, consuming stored or residual biomass. Aeration can be tapered along the length to match the decreasing substrate and maintain appropriate oxygen levels without wasting energy.

In contrast, complete-mix configurations are short and wide, producing a uniform environment with evenMLSS throughout. Step feed distributes wastewater at multiple points to smooth out the food-to-microorganism ratio, and batch feed injects all influent at a single time, operating differently from a continuous plug flow pattern.