Anaerobic reactors differ from aerobic reactors primarily because the former must be closed in order to exclude oxygen from the system, since this could interfere with anaerobic metabolism. A noticeable exception is constituted by anaerobic ponds and the bottom of facultative ponds, in which anaerobic conditions are established as a result of stratification and oxygen depletion in the lower part of the pond.
An additional reason to require closed anaerobic reactors is the odours associated with anaerobic fermentation. An anaerobic reactor must also be provided with an appropriate vent or collection system to remove the gases (mainly methane and carbon dioxide) produced during anaerobiosis.
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Conversely, aerobic reactors containing suspended biomass almost invariably require the use of an air-sparging or bubbling system to provide the microorganisms with oxygen. One of the main drawbacks of oxygen as a key substrate is its low solubility in water (about 10 ppm at room temperature) as opposed to most other substrates, (e.g., glucose, nitrates), which have much higher saturation concentrations.
In addition, because of the low oxygen concentration gradient (equal, under the most favourable circumstances, to the difference between the saturation concentration and the actual concentration in the water), the driving force for the mass transfer of oxygen from the air bubbles to the water is quite small.
Therefore, a large air-water interface must be generated in order to supply enough oxygen to the system. Typically, this is accomplished by the use of one or more impellers which break up large air bubbles and disperse them in the liquid, with significant expenditure of energy.
The vast majority of existing biological treatment plants are aerobic. The reasons for this preference over anaerobic systems are the greater range of wastewaters that can be treated, easier control and greater stability of the process and more significant degree of removal of BOD, nitrogen and phosphorus.
Because of the slower metabolism, anaerobic systems require a longer residence time of the waste in the reactor. This translates into a larger reactor volume to treat the same amount of waste. The slow metabolism also implies that a longer period of time is required for anaerobes to colonise the reactor.
This, in turn, means that start-up time can be significant and that a longer period of time is required to bring the reactor back to full operation in case the bacterial population is lost because of a process upset. Anaerobes also require a more precise control of the operating parameters such as temperature or pH.
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Whereas aerobic degradation is typically carried out by many organisms operating by and large independently and in parallel, anaerobes live in consortia in which different classes of organisms are responsible for carrying out single steps of the degradation process. This makes anaerobic reactors more prone to failures.
The reasons for this can be traced back to hydraulic, organic, or toxic overloading of the reactor. Hydraulic overloading in continuous reactors is produced when the microbial population is washed out of the reactor as a result of too high a flow rate. This occurs especially when the population reproduces slowly, as in the case of anaerobes. This problem can be minimised by the use of immobilisation, as described below in greater detail.
Organic overload is produced when the wastewater contains a higher concentration of organic compounds. This results in the rapid production of a significant amount of volatile acids by one of the intermediate classes of anaerobic organisms in the consortium (the acetogenic bacteria) and in the inhibition of the methanogens (the last organisms to act in a methanogenic consortium), with consequent failure of the reactor. Toxic compounds can also inhibit the activity of the methanogens or cause their washout with resulting reactor failure.
However, anaerobic processes have advantages of their own. They are typically capable of tolerating higher loading rates, do not require high mechanical energy input for air dispersion (as in the aerobic case) and produce less biomass per unit of waste degraded.
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