Design of Activated Sludge Process
Effluent from the primary clarifier is introduced to an aeration tank and mixed with a mass of micro-organisms comprised bacteria, fungi, rotifers and protozoa. This mixture of liquid, solid waste and micro-organisms is called the mixed liquor suspended solids (MLSS, mg/l). The organisms absorb dissolved organics and break them down into carbon dioxide, water and some stable compounds. Bacteria are primarily responsible for assimilating the organic matter in wastewater, and the rotifers and protozoa are helpful in removing the dispersed bacteria which otherwise would not settle out. The energy derived from the decomposition process is used for cell maintenance and to produce more micro-organisms. Once most of the dissolved organics have been used up, the MLSS is routed to the secondary (or final) clarifier for separation. As along with basic settling, two streams are produced: a clarified effluent that is sent to the next stage of treatment and a liquid sludge comprised largely of micro-organisms. Lying at the bottom of the final clarifier, without a food source, those organisms become nutrient-starved or "activated". A portion of the sludge is then pumped to the head of the tank (return activated sludge) where the process starts all over again. The remainder of the sludge is processed for disposal (waste activated sludge). It is essential to continuously add waste sludge to balance the gain through microbial growth.
The activated sludge system is a continuous process involving the introduction, uptake and breakdown of BOD and the growth and decay of micro-organisms. Equilibrium is reached where the rate of food introduction and the size of the microbial population are in balance leading to a constant BOD concentration in the effluent. The rate of food introduction (BOD loading) is largely fixed by the sewage inflow rate (Qo) and BOD (So) of the influent. A size of the microbial population is equivalent to the product of the MLSS concentration within the reactor (X) and the reactor volume (V). Operating experience in waste treatment plants suggests that MLSS concentrations in the reactor should be maintained at levels ranging from 1000-4000 mg/l. too low concentrations (< 1000 mg/l) lead to poor settling and too high concentrations (> 4000 mg/l) result in solids loss in the clarifier overflow and excessive oxygen requirements. The key process design parameter, used to estimate the needs tank volume, is the food to micro-organism (F/M) ratio. Essentially a feeding rate, the F/M ratio is equivalent to the BOD loading rate divided by the mass of MLSS in the reactor:
F/M = Q S0/ (X. V)
where, So = Influent BOD concentration (kg/m3),
Q = Wastewater Inflow (m3/d),
X = MLSS concentration (kg/m3), and
V = Reactor volume (m3).
Hydraulic retention time (θ) = V/Q .
Because So, Qo, and X are largely fixed, a particular reactor volume is selected to achieve the desired F/M ratio. It is clear from the equation above that the F/M ratio is really a feeding rate. The lower the F/M ratio, the lower the feeding rate, the hungrier the micro-organisms and the more efficient the removal.
At low F/M ratios, the micro-organisms are maintained in the death or endogenous growth phase, i.e. they are starved and, thus, very efficient at BOD removal. Because So is relatively constant for domestic wastes and because there are limits on the levels of X which a reactor can support, maintenance of a low F/M ratio requires either a very small flow or a very large tank volume. In either case, this leads to a long hydraulic residence (aeration) time. Activated sludge operated at low F/M ratios is termed extended aeration. The cost of operation and maintenance is high for large tank volumes and thus extended aeration is largely limited to systems with small organic loads, e.g. mobile home parks and recreational facilities. At high F/M ratios, the micro-organisms are maintained in the accelerating or exponential growth phase.
These organisms are more food-saturated, i.e. there is an excess of substrate, and, thus, BOD removal is less efficient. This approach is termed high-rate activated sludge. Within this approach, higher MLSS concentrations are employed and, therefore, a shorter hydraulic residence time is achieved and smaller aeration tank volumes are required.
Operation of the activated sludge process at mid-range F/M ratios, with micro-organisms in the declining growth phase, is termed conventional activated sludge. That option offers a balance among removal efficiency and cost of operation. Further to influencing BOD removal efficiency, the selection of an F/M ratio impacts the settleability of the sludge flocs and, thus, the efficiency of SS removal. In general, as the F/M ratio decreases the settleability of the sludge increases. Starving micro-organisms flocculate and, therefore, settle well, although those maintained at high F/M ratios form buoyant filamentous growths that settle poorly, a condition words sludge bulking.
In order to keep the F/M stable, some MLSS must be continuously wasted to balance micro-organism biomass produced through growth. The design and operation parameter for determining rates of MLSS wastage is the solids retention time or sludge age (θc, days), defined as the mass of solids present in the reactor over the mass of solids wasted per unit time:
θc = [ X × V / Xw × Qw ]
where Qw is the waste sludge flow from the reactor (m3/d). Values for θc ranging from 3-15 days result in the production of a stable, high-quality effluent with excellent settling characteristics. Values of X and V are dictated by F/M design considerations. Thus, recommended values for θc can be used to calculate the required waste sludge flow.
We can also write,
1/ θc = [ Xw × Qw/X × V ]
A rational loading parameter, which has found wide acceptance, is the specific substrate utilization rate, U, per day, that is described as
U = Q(S0-S)/VX
where, S is the effluent substrate concentration or effluent soluble BOD. Under steady state operation the mass of waste activated sludge is given by
Qw X s = Y Q (So - S ) - kd X V
Where y = Maximum yields coefficient (microbial mass synthesized/mass of substrate utilized),
kd = Micro-organism decay coefficient,
Xs = MLSS concentration in waste activated sludge from secondary settling tank underflow (g/m3), and
Qw = Waste activated sludge rate (m3/day).