Recirculation in fixed-film reactors serves to?

• A. Equalize out flows
• B. Reseed media
• C. Reduce organic strength
• D. Increase DO

Answer: (A)

Recirculation in fixed-film reactors is used to smooth the flow and distribute the load more evenly across the media. By returning a portion of the treated or mixed flow back through the bed or across different paths, the hydraulic and organic loading become more uniform, reducing short-circuiting and dead zones and helping the biofilm on all media receive consistent contact with the wastewater. This improves overall treatment performance and stability.

That’s why equalizing out flows is the best fit. Reseeding the media would require introducing new biomass rather than relying on flow paths; reducing organic strength isn’t accomplished by recirculation alone, and increasing dissolved oxygen mainly depends on aeration rather than recirculation.