Glossary term
Internal Recycle Flow
Mixed-liquor recycle flow used in biological nutrient removal to return nitrate-rich mixed liquor to anoxic zones for denitrification control.
Definition
metricInternal recycle flow is the mixed-liquor flow returned within a biological treatment process, commonly from an aerobic nitrate-rich zone to an anoxic zone.
In biological nutrient removal, internal recycle flow is used to move nitrate-rich mixed liquor to a zone where denitrification can occur. It is different from return activated sludge because it recirculates mixed liquor within the biological reactor rather than settled sludge from a clarifier. Interpretation depends on recycle ratio, nitrate concentration, dissolved oxygen carryover, anoxic volume, carbon availability, pump calibration, hydraulic short-circuiting, EBPR nitrate intrusion and the nutrient objective being controlled.
Internal recycle flow is the mixed-liquor flow returned within a biological treatment process. In nutrient-removal plants, it commonly moves nitrate-rich mixed liquor from an aerobic zone back to an anoxic zone for denitrification.
Internal recycle matters because nitrate cannot be removed if it never reaches the anoxic biomass with enough usable carbon. Too little recycle can leave nitrate in the effluent. Too much recycle can waste pumping energy, carry dissolved oxygen into the anoxic zone, disturb hydraulics or move nitrate into zones where EBPR needs anaerobic conditions.
Engineering Meaning
Internal recycle is not the same as return activated sludge. RAS returns settled biomass from a clarifier. Internal recycle moves mixed liquor inside the biological process, often before final clarification.
The stream is commonly represented as:
or:
where MLR means mixed-liquor recycle. The value should state whether it is measured, pump-rated, inferred from speed or estimated from a hydraulic balance.
Recycle Ratio
A basic recycle ratio is:
where Q is influent or process feed flow on the chosen basis.
If:
then:
or 200\% of influent flow. The right ratio depends on nitrate load, anoxic volume, carbon, DO carryover, hydraulics and process objective.
Nitrate Load Returned
The nitrate nitrogen returned to an anoxic zone can be screened as:
For:
the returned nitrate load is:
This load can exceed the plant influent nitrate load because the same water may circulate internally more than once.
Carbon Demand Screen
The returned nitrate load needs electron donor for denitrification. A common COD screen is:
For:
the screened COD demand is:
If available rbCOD or VFA is low, raising internal recycle may move more nitrate without increasing actual removal.
DO Carryover
Internal recycle can carry oxygen into an anoxic zone:
For:
the oxygen carryover is:
This oxygen must be consumed before truly anoxic conditions develop, so a high recycle can suppress the denitrification it was meant to improve.
EBPR Interaction
In EBPR systems, nitrate intrusion into an anaerobic zone can weaken phosphorus release and PAO selection. The same internal recycle setting that helps total nitrogen can damage biological phosphorus removal if flow splitting, baffles, recycle routing or control logic sends nitrate to the wrong zone.
Validation Evidence
Useful evidence includes calibrated recycle flow, pump curve, valve position, nitrate at recycle source and anoxic outlet, DO at recycle source, ORP trend, anoxic volume, influent flow, rbCOD or VFA, total nitrogen, orthophosphate release where relevant, MLSS, SRT, RAS flow, mixer status, tracer or hydraulic evidence and trend after setpoint changes.
Common mistakes include treating pump speed as flow, increasing recycle without nitrate-load calculation, ignoring DO carryover, confusing internal recycle with RAS, using one recycle ratio across seasons, and optimizing TN while damaging EBPR. A strong review states the flow basis, nitrate load, oxygen carryover, carbon basis, hydraulic boundary and validation evidence.