Internal mixed liquor recycle is the flow that carries nitrate-rich mixed liquor from the aerobic zone of a biological nutrient removal reactor back to the anoxic zone ahead of it, giving the denitrifying bacteria the nitrate they need to work. It is a different recycle from return activated sludge, which comes from the clarifier, and it is the defining feature of the Modified Ludzack-Ettinger process and its relatives. This page explains what the recycle does, how its ratio is chosen, and how it is paced in SCADA against the energy it costs.
Internal Mixed Liquor Recycle in one line: Internal mixed liquor recycle (IMLR) is the pumped return of nitrate-rich mixed liquor from the aerobic zone to an upstream anoxic zone so denitrifying bacteria can convert nitrate to nitrogen gas. The recycle ratio, expressed as a multiple of plant influent, sets how much nitrate is delivered for denitrification, and it is paced in SCADA by controlling the recycle pump speed against a nitrogen removal target.
Biological nitrogen removal happens in two steps. In the aerobic zone, nitrifying bacteria convert ammonia to nitrate under a supply of dissolved oxygen. To finish the job, that nitrate must be converted to harmless nitrogen gas, which is done by denitrifying bacteria that work only in an anoxic environment, without free oxygen, using nitrate as their oxygen source while they consume organic carbon. The problem is that nitrate is produced downstream, in the aerobic zone, while the carbon-rich influent and the anoxic conditions are upstream.
The Modified Ludzack-Ettinger, or MLE, process solves this by placing an anoxic zone first, fed with the raw influent's carbon, and the aerobic zone second. Internal mixed liquor recycle then pumps nitrate-laden mixed liquor from the end of the aerobic zone back to the head of the anoxic zone, delivering the nitrate to where the carbon and the anoxic conditions are. There the denitrifiers strip the oxygen from the nitrate, releasing nitrogen gas and removing nitrogen from the water. This internal loop is what makes the process a nutrient-removal process rather than a nitrification-only one.
This recycle is distinct from return activated sludge. Return sludge comes from the secondary clarifier and carries settled biomass back to keep the population working. Internal mixed liquor recycle stays inside the biological reactor, moving unsettled mixed liquor from one zone to another, and its purpose is to deliver nitrate for denitrification, not to manage biomass inventory. Many nutrient-removal plants run both loops at once, and confusing the two is a common source of operating mistakes.
The recycle is quantified as a ratio, the internal recycle flow divided by the plant influent flow, and it is often a large multiple of the influent because each pass through the anoxic zone only exposes the nitrate to denitrification for a limited time. A higher recycle ratio delivers more nitrate to the anoxic zone and, up to a point, allows more nitrogen to be removed, because the theoretical ceiling on removal is tied to how much of the nitrate-bearing flow is routed back through the anoxic conditions rather than passing straight to the clarifier.
There are diminishing returns, though. Beyond a certain ratio, each additional increment of recycle removes less extra nitrogen while pumping more flow, and it also carries a hidden penalty: the recycle drags dissolved oxygen from the tail of the aerobic zone into the anoxic zone, where that oxygen is consumed preferentially over nitrate and interferes with denitrification. Pushing the recycle too hard can therefore undermine the very anoxic condition it is meant to feed, so operators tune the ratio to a balance rather than a maximum.
In practice the recycle ratio is set against the plant's effluent nitrogen target and adjusted as loading and temperature change. Some plants run a fixed ratio paced off influent flow, so the recycle rises and falls with load. More advanced plants trim the ratio from an effluent or anoxic-zone nitrate measurement, raising recycle when nitrate breakthrough rises and easing it when the anoxic zone shows nitrate to spare. The right setting is the one that meets the nitrogen limit without needlessly pumping flow or importing oxygen.
In a SCADA-controlled plant the internal recycle pump, usually a large low-head axial or mixed-flow pump on a variable frequency drive, is paced by a loop that computes a recycle flow setpoint. The simplest loop multiplies the measured influent flow by the operator-entered ratio and trims the drive until the measured recycle flow matches. A more responsive loop uses an online nitrate analyzer in the anoxic or effluent zone as the feedback, adjusting the recycle to hold nitrate at a target, which lets the plant meet its limit while pumping only as much as it needs.
The energy tradeoff is real and worth trending. The internal recycle can move several times the plant's influent flow, so the recycle pumps are among the larger continuous electrical loads in a nutrient-removal plant, and running the ratio higher than necessary wastes power and, by importing oxygen, can force the aeration system to work harder too. Trending recycle flow, recycle pump energy, dissolved oxygen at the aerobic tail, and anoxic-zone nitrate together lets an operator see whether the ratio is set for genuine nitrogen removal or just moving water.
For plants running lean or across multiple sites, a cloud SCADA platform such as Merobix puts recycle flow, nitrate trends, and pump energy on one screen viewable from any browser, so an operator can tune the ratio for both compliance and cost without standing at the panel. Alarms on a failed recycle pump or a nitrate breakthrough catch a loss of denitrification early, while the energy trend supports the slower work of dialing the ratio down to the lowest value that still meets the nitrogen permit, which at a large plant is a meaningful continuous saving.
Internal mixed liquor recycle stays inside the biological reactor, moving nitrate-rich mixed liquor from the aerobic zone back to the anoxic zone so denitrifiers can remove nitrogen. Return activated sludge comes from the secondary clarifier and carries settled biomass back to the aeration basin to keep the microbial population working. They serve different purposes, and many nutrient-removal plants run both loops simultaneously.
The nitrogen removal achievable by the process is tied to how much of the nitrate-bearing flow is routed back through the anoxic zone rather than passing straight to the clarifier, so a high recycle multiple delivers more nitrate for denitrification. Because each pass only removes part of the nitrate, several times the influent flow is often recycled to reach a low effluent nitrogen. There are diminishing returns and an energy and dissolved-oxygen penalty at very high ratios, so the ratio is tuned rather than maximized.
Yes. A high recycle drags dissolved oxygen from the tail of the aerobic zone into the anoxic zone, where the denitrifiers consume that oxygen preferentially over nitrate, interfering with the anoxic conditions they need. Pushing the recycle harder can therefore undermine the very process it feeds while also wasting pumping energy. Operators watch anoxic-zone dissolved oxygen and nitrate and tune the ratio to a balance.
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