When active sludge is kept in an aerobic environment without feed, in time a reduction of the volatile solids concentration is observed, with a concurrent consumption of oxygen. These phenomena characterise aerobic sludge digestion and are attributed to the oxidation of microbial protoplasm, which releases the energy required to maintain vital cell functions. The oxidation of cellular matter is called endogenous respiration, in order to distinguish it from the oxidation of extra-cellular organic material, which is called exogenous respiration.
The following items will be discussed in this section:
- Kinetics of aerobic sludge digestion
- Aerobic digestion in the main activated sludge process (endogenous respiration)
- Design of aerobic sludge digesters
- Optimisation of aerobic sludge digestion
- Operational parameters of the aerobic digestion process
(1) Kinetics of aerobic sludge digestion
Endogenous respiration and aerobic digestion can be considered as first order processes with respect to the active volatile solids concentration. The difficulty with using the active sludge concentration as a parameter is that it cannot be measured directly. Thus, in order to evaluate the kinetics of aerobic sludge digestion, parameters must be identified that will change during aerobic sludge digestion, that can be measured easily, and that can be linked to the active sludge concentration. The following parameters can be used:
- Volatile sludge concentration
- Mixed liquor alkalinity
- Nitrate concentration
- BOD of the digesting sludge
- Oxygen uptake rate
The use of these parameters to determine the decay rate of the active sludge is amply discussed in Example 8.1. To download this section, click here
(2) Aerobic digestion in the main activated sludge process (endogenous respiration)
Having established a consistent model for aerobic digestion, the question emerges if this model is also applicable in the activated sludge process itself. This is not a priori evident, as aerobic digestion was assumed to be a process by which cellular material is oxidised to obtain the necessary energy to maintain the vital functions of the micro-organism. In an aerobic digester, the only source of organic matter for oxidation is the protoplasm of the active sludge. In contrast, in the activated sludge process there is also extra-cellular organic material present. It might be assumed that the bacteria would rather "save" their protoplasm and use preferentially the extra-cellular material, so that endogenous respiration would be substituted by exogenous respiration. In this section it will now be shown that experimental results indicate that the endogenous respiration rate is indeed independent of the exogenous respiration rate. To download this section, click here.
(3) Design of aerobic digesters
Aerobic digesters are usually constructed as completely mixed reactors. The reactor may be fed continuously or intermittently with excess sludge. The objective of the digestion is to reduce the fraction of biodegradable organic material to such a level (in practice between 10 - 20% of the volatile solids) that the digested sludge can be disposed of without problems. For the design of the aerobic digester the following factors are important:
- Flow and composition of the sludge to be digested
- Maximum allowable fraction of active sludge remaining after digestion
- Digestion temperature
- Configuration of the aerobic sludge digester: i.e. the number of digesters in series or parallel.
These items will now be discussed. To download this section, click here.
(4) Optimisation of aerobic sludge digestion
In general when aerobic sludge digestion is applied, a sludge thickener precedes the digester in order to increase the sludge concentration and to reduce the flow of excess sludge to the digester. The retention time in the digester, needed to effect the required reduction of the active sludge fraction, is dependent on the excess sludge composition. Therefore, for a given excess sludge composition, the volume of the digester will be inversely proportional to the volume of the thickened sludge flow. The objective of the optimisation of the activated sludge system with aerobic digestion is to minimise the total construction cost of the biological reactor, thickener and digester while producing digested sludge with an active sludge fraction below a certain specified maximum value, for example 0.1 - 0.2. Two different situations might be applicable:
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The sludge age in the activated sludge process is defined by factors unrelated to sludge stabilisation: for instance by the requirements for nutrient removal
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The sludge age may be defined by the optimal value for sludge stabilisation
In the first case the optimisation procedure is limited to a calculation of the minimum costs for construction of the thickener and the digester. In the second case, the construction costs of the aeration tank and the final settler must also be taken into consideration. Both cases will be evaluated. To download this section, click here.
(5) Operational parameters of the aerobic sludge digestion process
Once the optimal configuration of the aerobic sludge digester has been determined, it is a relatively simple matter to calculate the main operational parameters. These are:
- Reduction of the volatile sludge concentration
- Oxygen uptake rate
- Increase of the nitrate concentration
- Alkalinity demand
The above parameters are directly related to the oxidation of active sludge mass in the digester. Refer to Example 8.2 and Example 8.3. To download this section, click here.