Why Dissolved Oxygen Should Not Be Too High in an Aeration Tank
An aerobic treatment system works by using the metabolism of aerobic micro-organisms to convert organic pollutants in the wastewater into harmless carbon dioxide and water, and into the energy the organisms need to live. Oxygen is essential to that metabolism. So does more dissolved oxygen always mean better treatment?
Start with the Food-to-Microorganism Ratio
To answer that question you first have to understand the food-to-microorganism ratio (F/M). Take the conventional activated sludge system: the ratio of the total BOD supplied to the aeration tank each day to the total quantity of activated sludge in that tank is the F/M ratio, where the BOD supplied is the food available to the micro-organisms.
F/M = Q × BOD5 / (MLVSS × Va)
F, for food, is the quantity of food (BOD) entering the system; M, for micro-organism, is the quantity of active biomass (sludge); 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.
The suitable range for F/M is normally 0.1 to 0.25 kg BOD5 per kg MLSS per day. An F/M above that range means the micro-organisms have food in excess and the aeration tank is running at high load; below it, the tank is running at low load.
What the Ratio Does to the Sludge
1. Within the suitable F/M range, the activated sludge forms good flocs with excellent settling properties, and the effluent is clear and transparent.
2. 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.
3. At a low F/M the shortage of food makes the activated 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.
How Dissolved Oxygen Affects Performance
1. 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 the organic matter is degraded.
2. 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 eventually causes deflocculation — the condition usually called over-aeration. High dissolved oxygen accelerates microbial metabolism, 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.
What Dissolved Oxygen Should Be Set By
The main basis for control is the raw water quality (organics, nitrogen, phosphorus), the activated sludge concentration, the sludge settling ratio, pH, temperature and the F/M ratio. The textbook values are 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 — but 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. Watch both the change in influent flow and the organic content of the influent when setting dissolved oxygen.
Activated sludge concentration. Provided the pollutant removal and discharge limits are met, keep the sludge concentration as low as possible: that reduces the aeration demand and the power bill. At low sludge concentration, be careful not to over-aerate, or the sludge will bulk and the effluent turn turbid. A high sludge concentration, conversely, needs a higher dissolved oxygen level; otherwise anoxic conditions develop and treatment performance is suppressed.
Sludge settling ratio. Excessive aeration makes fine bubbles adhere to the zoogloea, so the sludge floats to the surface and its settling performance deteriorates. Watch for this in practice, particularly when filamentous bulking is already present: fine aeration 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 micro-organisms. Besides understanding what the equalisation tank can do, keep in contact with the discharging units so that you know the character of the incoming wastewater and can dose the right reagent to 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 treatment 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 and so raise efficiency.
F/M ratio. The higher or lower the F/M, the higher the relative oxygen demand. This is also the lever for saving energy: raise the F/M as far as treatment performance allows, so that unnecessary aeration is avoided.
In an aerobic system, then, 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 dissolved oxygen in the aeration tank is controlled through the blower frequency, the blower running time or the opening of the bleed valve.
