Two ratios, one word, two jobs
In an activated sludge plant the word recycle covers more than one pipe. The two that carry most of the flow are the external recycle — settled sludge drawn from the bottom of the final clarifier and returned to the head of the biological train — and the internal recycle, also called the mixed liquor recycle or the nitrate recycle, which lifts nitrified liquor from the aerobic zone back to the anoxic zone. Both are quoted as a ratio to the incoming flow, and both are called the recycle ratio in conversation, which is where most of the confusion starts. The sludge recycle ratio is the single term that covers the two of them together, so the first question on any plant is which of the two a number refers to.
The two do opposite jobs. The external recycle returns the culture: it keeps the population of organisms in the system instead of letting it leave with the effluent. The internal recycle returns an oxidised form of nitrogen so that it can be turned into gas and leave the water. A pump that does the second cannot do the first, and adjusting one when the other was meant moves the plant in the wrong direction.
The external recycle ratio: how the settled sludge return is set
The external recycle ratio is the half of the sludge recycle ratio that is fixed by two concentrations: the solids concentration the aeration tank is to run at, and the concentration of the sludge being returned. Where the return sludge is thicker, less of it is needed to hold the same tank concentration, so the ratio is a consequence of that pair of numbers rather than a number chosen first. In practice the return sludge ratio in activated sludge is never a free design choice for that reason, and because the tank concentration the process needs is set by the loading and the settleability together, the ratio moves whenever either of them moves.
The concentration of the return sludge follows from how well the sludge settles. A sludge that settles to a thick underflow can be returned at a lower ratio; a sludge that settles poorly arrives back at the tank thin, and the ratio has to rise to move the same mass of solids — which pushes more water round the circuit at the same time. This is why a change in the settling behaviour of the sludge shows up as a change in the hydraulic load on the clarifier, even when nobody has touched a valve.
Ratios of the order of 0.5 to 1.0 times the influent flow are commonly quoted for a conventional plant, and figures of that kind are the ones generally given in the literature and in operating experience. They should be read as a starting method rather than as a specification, because the two concentrations behind the ratio belong to the sludge and the water of a particular plant.
Run the external ratio too low and the solids blanket in the clarifier rises, because sludge is not being taken off the bottom as fast as it arrives; the blanket eventually reaches the outlet and solids leave with the effluent. Run it too high and the clarifier is hydraulically surcharged with its own returned sludge — every litre returned passes through the clarifier again — so the blanket can be pushed out by the flow rate rather than by the solids. Both ends of the range produce solids in the effluent, which is why the symptom alone does not say which end you are at.
The internal recycle ratio: how the nitrate return is set
The second half of the sludge recycle ratio is the internal line, and it is the one more often written as the mixed liquor recycle ratio for denitrification. It carries mixed liquor rather than settled sludge, and it carries it from the aerated zone to the anoxic zone, where nitrogen is actually removed. Nitrogen leaves a plant only when a nitrate ion is reduced to nitrogen gas and escapes, and that can happen only where the nitrate meets anoxic conditions. The nitrate is made in the aerated zone, so the fraction of it that reaches the anoxic zone is bounded by the ratio of the recycle flow to the influent flow.
That makes the internal recycle ratio in wastewater treatment a function of the nitrogen target rather than of the sludge. A plant that has to reach a low total nitrogen figure needs a large share of the nitrate carried back; a plant working to a loose nitrogen figure, or to none, does not need the line to run hard at all. The figure to work from is the mass of nitrogen to be removed per day, and the fraction of the nitrate that has to be returned to remove it.
Ratios of the order of one to four times the influent flow are commonly quoted for the internal recycle on a plant that both nitrifies and denitrifies. The same caution applies: those are the figures generally given, and they should be read as a starting method rather than as a specification, because the nitrogen load, the water temperature and the carbon available in the influent all move the answer.
There is a ceiling on this ratio that is easy to miss. The internal recycle carries water as well as nitrate, and that water has to pass through the anoxic zone and then the aerated zone, so the recycle flow adds to the hydraulic load on both and to the volume they need. Turning the pump up past the point where the anoxic zone is receiving all the nitrate it can reduce adds circulation without adding removal.
Dissolved oxygen is the second ceiling. Mixed liquor leaving an aerated zone carries oxygen with it, and oxygen arriving in the anoxic zone is used in preference to nitrate, which stops the reduction the zone exists to perform. The larger the internal recycle, the more of that oxygen is carried back with it, so the ratio and the dissolved oxygen in the anoxic zone have to be read together.
Working each ratio out before the pump is chosen
A sludge recycle ratio calculation is two calculations, not one, and the second does not use the answer to the first. Both start from measurements the plant can take rather than from a table.
- Write down the tank solids concentration the process needs, and the concentration at which the sludge actually settles. The external ratio follows from those two and from nothing else.
- Take the settling test as an input, not as a check afterwards. A change in how the sludge settles changes the external ratio even when the tank concentration is unchanged.
- State the nitrogen target as a mass per day, and work out the fraction of the nitrate that has to reach the anoxic zone to meet it. The internal ratio follows from that fraction.
- Check what the anoxic zone can actually use. An internal ratio larger than the zone can consume circulates water and oxygen for no removal.
- Add both recycle flows to the hydraulic load on the tanks and the clarifier before their volumes are fixed, because every recycled litre passes through them again.
- Write both ratios into the mass balance at design stage, so that the setting on site is a value to be set to rather than a number to be guessed at.
What an over-run ratio looks like on the plant
The two ratios fail differently, and separating the symptoms is what makes the pump setting useful as a control rather than as a guess.
- External recycle too low: a rising sludge blanket, solids leaving in the effluent and a falling tank concentration. Suspended solids in the effluent climb before the organic result does.
- External recycle too high: a thin return sludge, a clarifier carrying more water than it was sized for, and a blanket pushed out by flow rate rather than by solids. Pump energy rises with no gain in tank concentration.
- Internal recycle too low: nitrate left in the effluent and a total nitrogen result that will not come down however well the rest of the process performs, with the anoxic zone short of nitrate rather than short of carbon.
- Internal recycle too high: dissolved oxygen in the anoxic zone and a loss of nitrogen removal while the recycle flow is being increased, together with a higher hydraulic load on both zones and more aeration energy to make up the oxygen that was carried back.
- Both wrong at once: solids and nitrogen in the effluent at the same time, which reads like a process failure and is more often a pump setting meeting a change in the sludge.
What to record so a ratio can be re-set
- The tank solids concentration and the return sludge concentration, measured on the same day and at the same time.
- The settling test result and the date it was taken, because the ratio that suits one settling behaviour does not suit another.
- Dissolved oxygen in the anoxic zone, which is the reading that says whether the internal recycle is carrying oxygen back.
- The effluent nitrate or total nitrogen, so that the internal ratio is being set from the result it is supposed to change.
- The running hours or the flow of each recycle pump, because a ratio is a flow, and a setting that ignores what the pump actually moves is a design on paper.
From two ratios to a pump duty
The two ratios are the link between the biology a plant is designed to hold and the pumps that are bought for it. The tank volume and HRT chart shows how the residence times the zones are given turn into volumes, and the note on the zones of an A²/O plant covers what each zone removes and why the anoxic zone exists at all. On a membrane plant the same external recycle keeps the culture in front of the membrane, which is described on the integrated MBR equipment page. The recycle flows that suit your tank geometry, your sludge and your nitrogen target are confirmed with the factory for your tank and your sludge rather than taken from a table.