The use of aerobic reactors for the treatment of heavily burdened waste water involves the production of large amounts of sludge by the microorganisms that have to remove pollution from the waste water. Thus, 50% on average of the pollution that is eliminated in an aerobic reactor is transformed into sludge, which is another form of pollution that should in turn be treated. The aerobic processes are therefore used first for the effluents that are not very heavily polluted. Furthermore, the aerobic reactors require a frequent and extended, energy-consuming aeration.
It is therefore known to use an anaerobic reactor for certain biological treatments of waste water, whose concentration of pollution is high. The pollution that is treated in this type of reactor is primarily transformed into fuel gas, which can subsequently be used for other applications.
Among the advantages of this method for removing pollution from waste water, it is possible to cite the fact that the production of excess sludge is low because the biomass yield, i.e., the kilograms of microorganisms formed per kilogram of COD that is eliminated or supplied is low. Among the drawbacks of this method for removing pollution from waste water, it is possible to cite the fact that the anaerobic microorganisms have a lower growth rate than the aerobic microorganisms and the fact that in a reactor with microorganisms in suspension, it generally is not possible to exceed a microorganism concentration of 15 to 20 g/L. The COD is the chemical oxygen demand, i.e., the amount of oxygen that is necessary to oxidize the entire polluting material chemically, in particular the organic compounds that may or may not be biodegradable.
Also, to obtain a larger amount of microorganisms in the reactor, it is known to use particular supports that are immersed and immobile in the liquid phase and to which the microorganisms are attached. There are two types of supports, the so-called “bulk” supports and the so-called “ordered” supports.
The use of such stationary supports in the reactor makes it possible to keep the biomass in the reactor in the form of biofilm and thus to increase the amount of microorganisms in the reactor, which makes it possible to increase the performance levels of the reactor. Biomass is defined as all the microorganisms that are used to degrade the pollution contained in the effluent. Biofilm is defined as the highly structured cellular formations within which microbial cells that are used to degrade the pollution that is contained in the effluent are encompassed in a complex matrix.
The effluent to be treated circulates through the support from bottom to top (upflow) or from top to bottom (downflow). The effluent is thus in contact with the biomass that initiates the removal of pollution.
The two primary drawbacks of such an anaerobic biological reactor are, on the one hand, as for any anaerobic process, the length of the start-up phase of the reactor, which most often lasts between 3 and 6 months, and, on the other hand, the risk of clogging of the reactor that can bring about, at the end of a more or less long time, a total blocking of the reactor. Actually, although the production of sludge by the microorganisms is less significant in this type of reactor than in aerobic reactors, over time the sludge accumulates on the walls of the supports and in the openings between the supports and cannot be evacuated. The accumulation of the sludge can result in a shutting-off of the reactor by plugging it completely.
In the field of anaerobic treatment of waste water, for example, a biological reactor that uses ordered stationary supports on whose walls the microorganisms are attached is known. The supports are formed by hollow tubes that are attached by one end and in an ordered manner in the tank of the reactor. The tubes have a diameter of 102.5 mm and are divided in their diameter into fourteen channels in which the microorganisms are attached and can thus be organized into biofilm.
Experience shows, however, that with this type of reactor, a gradual clogging unavoidably occurs at the end of several months of operation. Actually, over time, the sludge accumulates on the walls of the columns and cannot be evacuated. This clogging greatly disrupts the operation of the reactor and, at the end, the accumulation of the sludge can result in a shutting-off of the reactor by plugging it completely.
Alternatively, the use of an anaerobic reactor that operates in a fluidized bed, i.e., with permanent movable supports, is known. The microorganisms are attached to supports of very small size that are kept in suspension in the reactor. The supports are movable in the reactor and rub against one another and/or with the inside walls of the tank of the reactor, tearing the biofilm that is formed by the microorganisms as soon as it exceeds a certain thickness. The sludge falls into the bottom of the tank where it can be purged or exists with the treated effluent. There is thus no risk of the reactor clogging since the accumulation of the sludge at the supports is rendered almost impossible. The permanent or quasi-permanent mobility of the supports is obtained by the creation of permanent or quasi-permanent turbulence in the reactor.
As for drawbacks, however, such a reactor, on the one hand, consumes an enormous amount of energy to keep the supports permanently in suspension and, on the other hand, requires an intensive support of the installation.