Nutrient Removal · Denitrification
What Is Internal Mixed-Liquor Recycle?
An internal mixed-liquor recycle moves nitrate-rich mixed liquor from an aerobic zone back to an upstream anoxic zone. This gives denitrifiers access to nitrate before the water reaches final treatment. It is separate from return activated sludge. Return activated sludge brings settled biomass back to the process.
Too little internal recycle may leave nitrate in the forward flow. Too much can increase pumping, shorten useful contact, move dissolved oxygen into the anoxic zone, and dilute available carbon. The best setting depends on the plant layout and current load.
Verify actual flow instead of relying only on a speed command. Check pump status, valve position, meter condition, and changes in basin level. Compare recycle changes with anoxic DO, nitrate entering and leaving the zone, and effluent total nitrogen.
A recycle-rate change also changes hydraulic loading. Watch basin behavior and downstream response rather than judging only the pump command.
Key point: Internal recycle must deliver nitrate without overwhelming the anoxic zone with flow or oxygen.
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Nutrient Removal: Nitrogen and Phosphorus for Wastewater Operators
This page comes from the course. It covers nitrification, denitrification, biological and chemical phosphorus removal, monitoring, return loads and permit limits, one step at a time, with short knowledge checks and a certificate of completion.
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