Design Example

Activated Sludge Process
Activated Sludge Process Variables
Mixing Regime
Loading Rate
Flow Scheme
Conventional System and its Modifications

Activated Sludge Process

The most common suspended growth process used for municipal wastewater treatment is the activated sludge process as shown in figure:

Activated sludge plant involves:

  1. wastewater aeration in the presence of a microbial suspension,
  2. solid-liquid separation following aeration,
  3. discharge of clarified effluent,
  4. wasting of excess biomass, and
  5. return of remaining biomass to the aeration tank.

In activated sludge process wastewater containing organic matter is aerated in an aeration basin in which micro-organisms metabolize the suspended and soluble organic matter. Part of organic matter is synthesized into new cells and part is oxidized to CO2 and water to derive energy. In activated sludge systems the new cells formed in the reaction are removed from the liquid stream in the form of a flocculent sludge in settling tanks. A part of this settled biomass, described as activated sludge is returned to the aeration tank and the remaining forms waste or excess sludge.

Activated Sludge Process Variables

The main variables of activated sludge process are the mixing regime, loading rate, and the flow scheme.

Mixing Regime

Generally two types of mixing regimes are of major interest in activated sludge process: plug flow and complete mixing. In the first one, the regime is characterized by orderly flow of mixed liquor through the aeration tank with no element of mixed liquor overtaking or mixing with any other element. There may be lateral mixing of mixed liquor but there must be no mixing along the path of flow.

In complete mixing, the contents of aeration tank are well stirred and uniform throughout. Thus, at steady state, the effluent from the aeration tank has the same composition as the aeration tank contents.

The type of mixing regime is very important as it affects (1) oxygen transfer requirements in the aeration tank, (2) susceptibility of biomass to shock loads, (3) local environmental conditions in the aeration tank, and (4) the kinetics governing the treatment process.

Loading Rate

A loading parameter that has been developed over the years is the hydraulic retention time (HRT), q, d

q = V         

V= volume of aeration tank, m3, and Q= sewage inflow, m3/d

Another empirical loading parameter is volumetric organic loading which is defined as the BOD applied per unit volume of aeration tank, per day.

A rational loading parameter which has found wider acceptance and is preferred is specific substrate utilization rate, q, per day.

q= Q (SO - Se)
         V  X

A similar loading parameter is mean cell residence time or sludge retention time (SRT), qc, d

qc =          V X         
      QwXr + (Q-QwXe)

where SO and Se are influent and effluent organic matter concentration respectively, measured as BOD5 (g/m3), X, Xe and Xr are MLSS concentration in aeration tank, effluent and return sludge respectively, and Qw= waste activated sludge rate.

Under steady state operation the mass of waste activated sludge is given by

QwXr = YQ (SO - Se) - kd XV

where Y= maximum yield coefficient (microbial mass synthesized / mass of substrate utilized) and kd = endogenous decay rate (d-1) .

From the above equation it is seen that 1/qc = Yq - kd

If the value of Se is small as compared SO, q may also be expressed as Food to Microorganism ratio, F/M

F/M = Q(SO- Se) / XV = QSO / XV

The qc value adopted for design controls the effluent quality, and settleability and drainability of biomass, oxygen requirement and quantity of waste activated sludge.

Flow Scheme

The flow scheme involves:

  • the pattern of sewage addition
  • the pattern of sludge return to the aeration tank and
  • the pattern of aeration.

Sewage addition may be at a single point at the inlet end or it may be at several points along the aeration tank. The sludge return may be directly from the settling tank to the aeration tank or through a sludge reaeration tank. Aeration may be at a uniform rate or it may be varied from the head of the aeration tank to its end.

Conventional System and its Modifications

The conventional system maintains a plug flow hydraulic regime. Over the years, several modifications to the conventional system have been developed to meet specific treatment objectives. In step aeration settled sewage is introduced at several points along the tank length which produces more uniform oxygen demand throughout. Tapered aeration attempts to supply air to match oxygen demand along the length of the tank. Contact stabilization provides for reaeration of return activated sludge from from the final clarifier, which allows a smaller aeration or contact tank. Completely mixed process aims at instantaneous mixing of the influent waste and return sludge with the entire contents of the aeration tank. Extended aeration process operates at a low organic load producing lesser quantity of well stabilized sludge.