Two views of one balance: the F/M ratio and sludge age in wastewater treatment
The food to microorganism ratio in wastewater treatment is the mass of food arriving each day divided by the mass of organisms kept to consume it, written in kilograms of BOD per kilogram of mixed liquor suspended solids per day. COD is used in place of BOD on some industrial plants, and the choice matters: an F/M ratio quoted against COD is not comparable with one quoted against BOD.
The sludge age, also called the solids retention time and the mean cell residence time, is the mass of solids held in the system divided by the mass removed from it each day, in days. The system means the aeration tank and the clarifier together, because the blanket in the clarifier is part of the inventory.
The two describe the same mass balance from opposite ends. Hold more sludge against the same load and the ratio falls while the age rises; waste more sludge and the ratio rises as the age falls. Quantitatively they are related through the observed yield of the culture, which says how much of the food becomes new organisms, and through its decay rate, which says how much of the population dies off each day. That relationship is why the pair travels together on a design sheet, and why a plant cannot hold a low ratio at a short sludge age or a high ratio at a long one, however it is operated.
How each dial is actually turned
The wasting rate in activated sludge is the dial, and it is the only one. The return sludge pump sets where the solids are, not how many there are: it moves the same population between the clarifier and the tank. The only continuous removal from the system is the waste sludge, together with whatever solids leave in the effluent. A plant that adjusts its return rate to change its sludge age is adjusting the wrong pump.
The ratio can be moved by two different actions, and confusing them is a common error. Wasting changes the total mass of solids and therefore the age and the ratio together. Raising the return rate at the same wasting rate moves solids out of the clarifier and into the tank, which raises the concentration the tank runs at and lowers a ratio computed on the tank alone, while the age of the system is unchanged. Both effects are real, and they act on different numbers.
The food term is the part the operator does not control. It varies by hour, by day and by season, so a ratio is always an average of a moving quantity. A design ratio is calculated on a design load, and the operating ratio is calculated on the load that actually arrived, which is why the two are recorded rather than interchanged.
Sludge retention time in wastewater treatment and the F/M ratio side by side
Commonly quoted starting figures put conventional activated sludge at a food to microorganism ratio of roughly 0.2 to 0.4 kg BOD per kg MLSS per day and a sludge age of roughly 5 to 15 days, with extended aeration lower in ratio, of the order of 0.05 to 0.15, and longer in age, of the order of 20 to 30 days. Read them as a starting method rather than as a specification. What they illustrate is the pairing: the low ratio and the long age are the same statement, and a design sheet quoting a very low ratio beside a very short age is describing a plant that cannot be operated as drawn.
The pairing is also a check on the load. Two otherwise identical plants, one taking a municipal sewage and one taking a partly biodegradable industrial discharge, will hold different ratios at the same age, because the biodegradable fraction of the load differs. The ratio follows the water, and the age follows the effluent requirement.
Turning the dial up: a young sludge
Shortening the sludge age, which raises the ratio, is done by wasting more. The first symptom is rarely organic. Nitrifying organisms grow more slowly than the organisms that remove organic matter, so they are washed out first, and the effluent ammonia rises while the BOD is still acceptable. A plant that loses nitrification after a change in wasting is showing the change in the parameter with the longest generation time, not the weakest one.
Further in the same direction the symptoms arrive in a recognisable order. Oxidation becomes incomplete and the effluent BOD follows the ammonia up. The culture shifts towards organisms that grow fast but do not flocculate, so the effluent turns turbid with suspended solids that pass through the clarifier rather than settling in it. More sludge is produced per kilogram of BOD removed, because more of the food is converted into new cells instead of being oxidised. The oxygen demand per kilogram removed rises as well, and the aeration system that was adequate at the design load runs short. A very short age also leaves the plant with no buffer against a shock load.
Turning the dial down: an old sludge
Lengthening the sludge age, which lowers the ratio, is done by wasting less. The population is held longer, the culture becomes more diverse and more stable, and nitrification becomes easier to hold. Effluent quality improves to a point, and then the account has to be paid on two other lines.
The first is settling. A sludge kept short of food produces a floc that is small, dense and slow to settle, and the light turbidity of pin-point floc that carries over the weir is the classic sign of a plant run too far in this direction. Certain filamentous organisms are also favoured when food is scarce, so a low ratio is one of the conditions under which a bulking sludge develops. The clarifier, not the aeration tank, is where an over-long sludge age is usually detected.
The second is mass. A long sludge age holds more solids in the system, and inert matter accumulates in them because it leaves only with the waste sludge. The volatile fraction of the solids falls, the mass of sludge to thicken and dewater is larger, and the clarifier has to carry a heavier blanket. Oxygen consumption per kilogram of BOD removed rises as well, since a larger share of the food is spent on the maintenance of a larger population, even though less excess sludge is produced per kilogram removed. Both effects are the same mass balance seen from two places.
Between the two extremes there is no universal setting. The age a plant needs follows from the slowest organism it has to retain at the coldest water it will see, and the ratio follows the age and the load. What the operating record adds is the direction of travel: a plant whose ratio has been drifting down for months with a steady load is accumulating something.
Common mistakes
- Changing the return sludge rate to change the sludge age, when the wasting rate in activated sludge is the dial that does it.
- Computing the sludge age from the aeration tank alone and leaving the solids in the clarifier blanket out of the inventory.
- Reading a low ratio as stable operation without checking how the sludge settles, since the same condition that suits nitrification can favour filaments.
- Judging the ratio from the design load rather than from the load that actually arrived.
- Correcting a nitrification failure by reducing the waste rate without confirming that the problem was washout rather than a temperature effect, a low dissolved oxygen level or an alkalinity shortage.
- Holding a sludge age copied from another plant on different water and with a different clarifier.
- Watching the ratio and the age as two independent indicators, when neither can be interpreted without the other.
Self-check before the dials are set
- Is the sludge retention time in wastewater treatment computed on the whole system, aeration tank and clarifier blanket together?
- Is the mass of solids wasted each day recorded, rather than only the concentration it left at?
- Is the food term measured on the same basis, BOD or COD, as the design calculation?
- Is the design temperature the coldest the plant will operate at, since the age that holds nitrifiers rises as the water cools?
- Is the settling behaviour of the sludge being watched alongside the ratio, through the sludge volume index or a settling test?
- Is the oxygen supply checked against the load actually being oxidised, in both its carbonaceous and its nitrogenous parts?
- Are the ratio and the age recorded on the same worksheet as the load they were computed from?
- Has the excess sludge production been estimated at the chosen age, so that the wasting and dewatering duty is known?
From two dials to a plant that holds them
Two dials, one balance, and a tank whose volume, aeration and clarifier have to be able to hold whatever the dials are set to. The integrated MBR equipment page describes a plant where the solids are separated by a membrane rather than by a clarifier, which changes the settling constraint without changing the balance, and the article on the zones of an A²/O plant follows the same solids through the anoxic and anaerobic stages that remove nitrogen and phosphorus. What sludge age and mixed liquor concentration a packaged unit can hold on your water is confirmed with the factory for your influent and your discharge standard rather than read from a general table.