The present invention relates to a method and a device for purifying waste water.
Such a method and such a device are known from EP 0 170 332 B1. The known device operates in an anaerobic way, preferably according to the known UASB method (upflow anaerobic sludge blanket), in which a container is used into the lower portion of which the waste water to be purified is passed and from the upper portion of which the purified waste water is discharged. Anaerobic microorganisms are active in the container. Disposed between the waste water inlet and the overflow pipe for the purified waste water are gas collectors which are arranged in superimposed fashion in the container in the form of hoods whose upper portion is connected via a pipe to a gas-sludge separating means. Gas is produced by the action of microorganisms and gets attached to the sludge in such a way that said sludge floats upwards as so-called buoyant sludge. Said buoyant sludge is trapped by the hood, thereby gradually releasing its gas, so that it becomes heavier again and settles again on the bottom as so-called settled sludge. The gas released from the pellets further rises upwards in the pipes together with the free gas bubbles trapped by the hoods, thereby entraining buoyant sludge particles and liquid which are separated in the gas-sludge separating chamber. The gas is expediently discharged while the entrained liquid, which may also contain sludge particles, passes into a downpipe leading back to the bottom of the container. The sludge which has settled on the bottom is thereby to be whirled, which is to loosen the sludge zone in the bottom portion and effect an improved thorough mixing of the microorganisms with the inflowing waste water. However, since water is relatively heavy, the amount of the waste water which can be transported by the buoyant gas and thus the whirling efficiency of the returned waste water are limited. Furthermore, it is known that waste water reactors of this type must have reactor heights of at least 11 m before the said effect can be observed.
It is therefore the object of the present invention to provide a method and a device for purifying waste water, whereby an effective whirling and thorough mixing of the settled sludge. can be achieved in a constructionally simple way.
According to the inventive design, and in contrast to the prior art, the settled sludge is no longer indirectly moved by impinging, returned water, but directly by being sucked into a rising pipe. This results in a considerably more effective flow which in particular extends over substantially the whole settled sludge zone and which ensures that there are nowhere any dead volumes where the microorganisms cannot find nutrition, or that none of the so-called short-circuit flows is formed in the case of which waste water is moved too rapidly through the sludge bed and insufficiently purified. As a result, the waste water reactors may be of a considerably smaller structural size having the same efficiency. Nevertheless, the turbulence created inside the settled sludge zone cannot become so great that sludge pellets are destroyed by excessively great shear forces because a whirling effect is still achieved by the lifting action of the gas produced in the waste water itself. Furthermore, in the case of high reactors the upper reactor chamber may show a higher density of pelletized sludge, whereby the efficiency is enhanced once more. Furthermore, in particular in the case of protein- or starch-containing waste waters forming so-called buoyant coagulates, the latter can also be returned into the bottom portion.
The efficiency is further enhanced because the lifting capacity of the gas can exclusively be exploited with this measure for lifting the settled sludge.
Expediently, the settled sludge is returned into an area below the gas collector back into the container.
Additional sludge can be entrained. For instance, in the case of protein- or starch-containing waste waters forming the so-called buoyant coagulates, the latter can also be returned into the bottom portion.
The rising capacity can be improved and the lifting of the settled sludge can immediately be started upon activation of the container.
The design according to the invention is suited for a combined anaerobic and aerobic operation.
Normally, anaerobic decomposition processes show optimum conditions between 25xc2x0 C. and 37xc2x0 C. With the increased flows according to the present invention, it is possible to reduce the reaction temperature. The optimum temperature for the inventive method is thus between 10xc2x0 C. and 37xc2x0 C. Energy can thereby be saved considerably because the Water need no longer be heated.