Food-to-Mass Ratio and Dissolved Oxygen: Why Extra Aeration Can Worsen Effluent

The ratio that sets everything downstream

An aerobic treatment system works by using the metabolism of aerobic micro-organisms to convert organic pollutants into carbon dioxide, water and the energy the organisms need to live. Oxygen is essential to that metabolism. The tempting conclusion, that more dissolved oxygen must therefore mean better treatment, is wrong often enough to be worth examining properly.

Start with the food-to-microorganism ratio, usually written F/M. Take a conventional activated sludge system: the ratio of the total BOD supplied to the aeration tank each day to the total quantity of activated sludge held in that tank is the F/M ratio, where the BOD supplied is the food available to the organisms. That is expressed as F/M = Q x BOD5 / (MLVSS x Va). F is the quantity of food entering the system as BOD; M is the quantity of active biomass; Q is the flow; BOD5 is the influent BOD5; MLVSS is the mixed liquor volatile suspended solids concentration; and Va is the aeration tank volume. Every term is measurable, which is what makes the ratio useful rather than academic.

The suitable range for F/M is normally 0.1 to 0.25 kg BOD5 per kg MLSS per day. Above that range the organisms have food in excess and the tank is running at high load. Below it, the tank is running at low load, and the consequences are just as real.

What the ratio does to the sludge

Within the suitable range, activated sludge forms good flocs with excellent settling properties, and the effluent is clear. That is the definition of a healthy plant, and it is a narrow band rather than a broad plateau.

At a high F/M, at or beyond overload, the excess of food degrades the settling properties of the sludge. The effluent turns turbid and the BOD in the wastewater cannot be fully degraded, because the biomass simply cannot consume substrate as fast as it arrives. The plant is not short of organisms; it is short of time and of the enzymatic capacity to use the food.

At a low F/M the shortage of food makes the sludge age. Prolonged operation at low F/M can cause the sludge to deflocculate and can even induce filamentous bulking. Once ageing sets in and deflocculation follows, the floc structure becomes loose and the effluent carries large numbers of fine sludge fragments, so clarity falls and water quality deteriorates even though the influent load was low enough to look easy.

Dissolved oxygen is a consequence, not a target

Oxygen has to be matched to the demand the biomass is generating at that moment. Two rules follow, and they point in opposite directions.

When the aeration tank is running at high F/M, holding a relatively high dissolved oxygen concentration is an advantage: it accelerates the rate at which organic matter is degraded, because the terminal electron acceptor is not limiting. The organisms are working hard, and the oxygen supply has to keep up.

When the tank is running at low F/M, holding a high dissolved oxygen concentration is harmful. With food already short, high dissolved oxygen speeds up endogenous metabolism, and endogenous metabolism eventually causes deflocculation. The condition has a name in plant practice, over-aeration, and the analogy is simple: a person who is not getting enough to eat, and is then driven to work harder, will only lose condition faster until they fail.

This is why dissolved oxygen cannot be commissioned to a fixed setpoint and left alone. A setpoint that is correct at design load is excessive at half load, and a plant that spends most of its life at half load spends most of its life over-aerating.

Six variables that decide the setpoint

The main basis for control is raw water quality (organics, nitrogen, phosphorus), activated sludge concentration, sludge settling ratio, pH, temperature and the F/M ratio. Textbook values exist: dissolved oxygen of 2.0 mg/L or more under aerobic conditions, 0.2 mg/L or less under anaerobic conditions, and 0.2 to 0.5 mg/L under anoxic conditions. Those numbers describe zones, not a control target, and the actual figure has to be set from site conditions.

• Raw water quality. The more organic matter in the raw water, the greater the oxygen demand of microbial decomposition, and nitrification adds its own demand on top. Watch both the change in influent flow and the organic content of the influent when setting dissolved oxygen.
• Sludge concentration. Provided the pollutant removal and discharge limits are met, keep the sludge concentration as low as possible; that reduces aeration demand and the power bill. At low concentration, do not over-aerate or the sludge will bulk and the effluent turn turbid. A high concentration, conversely, needs a higher dissolved oxygen level, or anoxic conditions develop and treatment performance is suppressed.
• Sludge settling ratio. Excessive aeration makes fine bubbles adhere to the zoogloea, so the sludge floats and its settling performance deteriorates. Watch for this particularly where filamentous bulking is already present, because fine bubbles then attach to the flocs more readily and a heavy scum appears on the surface.
• pH. pH acts indirectly, through its effect on sludge concentration and on the organisms themselves. Beyond understanding what the equalisation tank can absorb, keep in contact with the discharging units so that the character of the incoming wastewater is known in advance and the right reagent can neutralise an abnormal pH.
• Temperature. The saturation concentration of dissolved oxygen varies with temperature, and both low and high temperatures affect sludge concentration and microbial activity, so efficiency falls at either extreme. In northern China the usual answer to low temperature is to build the plant underground, semi-underground or indoors; in hot weather, use the equalisation tank to moderate the temperature.
• F/M ratio. The higher or lower the F/M, the higher the relative oxygen demand, which makes the ratio a lever for saving energy as well as for protecting the sludge. Raise the F/M as far as treatment performance allows, and unnecessary aeration disappears with it.

Turning the setpoint into blower action

In an aerobic system, dissolved oxygen control and F/M control are inseparable. High F/M justifies a higher dissolved oxygen concentration, which drives effective degradation of the organic pollutants. When food is short, keep dissolved oxygen correspondingly low: that slows endogenous metabolism, prevents sludge ageing and deflocculation, and cuts both power consumption and operating cost.

In practice the dissolved oxygen in the aeration tank is controlled through the blower frequency, the blower running time or the opening of the bleed valve. Frequency control gives the finest adjustment and the best specific power, running time suits small plants with limited instrumentation, and valve throttling is the crudest option but the one that can be retrofitted to almost any blower set. Whichever is used, the control loop needs a signal that represents the whole tank rather than one probe near the outlet, because a single point in a long basin can read clean while the inlet end is starved.

One further check is worth building into the routine. If dissolved oxygen holds steady at setpoint while the blower runs at rising output, the demand has grown without a load increase, which points to an oxygen sink other than carbonaceous oxidation: nitrification, sulphide, or a failing diffuser. The probe is reporting correctly; the interpretation is what needs attention.