Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2000-05-04
2001-05-29
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S623000
Reexamination Certificate
active
06238562
ABSTRACT:
The invention relates to a process for biological purification of waste water by the activated sludge method, whereby the waste water is passed continuously through one or more treatment zones and a clarification zone under such conditions that an accumulation of sludge takes place in the treatment zone from which the treated waste water is withdrawn.
It is known that the efficiency of a biological purification of waste water by the activated sludge method increases with increasing amounts of sludge in the treatment zones, and that in order to obtain high sludge concentrations it has been necessary to employ high sludge recirculation ratios, i.e. high ratios of sludge, which is recirculated from the clarification zone to the first treatment zone, to the amount of non-treated waste water.
However, the large amount of sludge causes problems in case of increased supply of water, e.g. in case of heavy showers. In such a case the amount of sludge in the clarification tank will start increasing due to overload, which ultimately entails that the sludge is entrained out into the recipient with resultant pollution hereof.
It has therefore been attempted to find a way in which it is possible, perhaps temporarily in case of peak loads, to reduce the load on the clarification tank, so that the situation described above is avoided.
Bent Tholander has described a method which is based on a basic module consisting of a continuously aerated tank and two side tanks with alternating function as aeration tanks or clarification tanks, where the waste water is always supplied to the centre tank. If two such modules are connected, a system is obtained which in case of particularly heavy showers can be adjusted, so that two clarification tanks are in operation.
PCT-application No. PCT/DK95/00309 (WO 96/02468) discloses a method of obtaining high sludge concentrations without using high sludge recirculation ratios by maintaining for a period such conditions in one or more treatment zones that sludge precipitation occurs herein, and by passing, in a succeeding period, a mixture of non-purified waste water and recirculated sludge to the zone which in the preceding period served as sludge precipitation zone.
By using the above mentioned method it has been possible to increase the sludge concentration in the treatment zone from 4 to about 5.9 kg/m
3
and to reduce the sludge recirculation ratio from 1.0 to 0.5, which has resulted in a reduction of the load on the clarification tank from about 8.000 kg/h to about 3.000 kg/h.
In practice, the method described above presupposes that at least two, and preferably four, separate tanks and a clarification tank be used for treating the waste water.
This entails comparatively high costs for the construction and operation of water purification plants in which the method described above can be used.
DE 3 147 920 discloses a method for biological purification of waste water, where the waste water is periodically passed through two or three physical sub-tanks. The method is based on one of the three sub-tanks being isolated for a period in which non-treated waste water is supplied to the centre tank before the flow is subsequently reversed, and flow again takes place through all three tanks.
U.S. Pat. No. 5 228 996 also discloses a method for biological treatment of waste water. The tank is divided into three elongated sub-tanks, each of which is in turn divided into
4
cells. The method is based on a series of phases in which the water is periodically introduced into each of the three sub-tanks, and where aerobic and anaerobic conditions, respectively, can be maintained in the individual cells.
From these two patents it is consequently known to reverse the direction of flow in the tank. However, in both cases it is a matter of three separate tanks which intercommunicate via bottom flaps and one-way valves, respectively. This physical division into three sub-tanks with restricted possibilities of transport of material between them, and the periodical isolation of one of the tanks entail that it is not possible to fully utilize the purification capacity of the sludge.
The process according to the invention is characterized in that the treatment is carried out in the same undivided tank, and that the non-treated waste water is introduced into one end of the tank for a period which is sufficiently long for an accumulation of sludge to take place at the outlet end of the tank, whereafter the direction of flow is reversed so that the non-treated waste water in a succeeding period is introduced at this end for a period which is sufficiently long for an accumulation of sludge to take place at the opposite end of the tank, from which end waste water is withdrawn.
By making such reversal of the supply of non-treated waste water to the tank in which the waste water is subjected to purification, it is achieved that the treatment is carried out in the presence of large amounts of sludge which, as mentioned above, results in an increased capacity of the purification.
It could be expected that the non-treated waste water, which has been introduced into the tank immediately before performing reversal for introduction of waste water at the end which used to be outlet end, would entail that the waste water withdrawn after the reversal had an unacceptably high content of contaminants, e.g. in the form of unreacted nitrogen compounds.
However, it has surprisingly turned out that this is not so, which presumably is due to the fact that such compounds are immediately adsorbed onto the sludge particles, and are consequently retained in the tank.
Thus, it is quite unexpectedly possible to completely reverse the process in a single step without the treated waste water loosing its quality.
That the treatment at the same time can take place in only one single, undivided tank entails that the initial costs of construction can be significantly reduced.
When implemented in existing plants, the volume load can be significantly increased because of the large amount of sludge which is retained in the tank, as the purification efficiency is increased, and the load on the clarification tank is reduced.
REFERENCES:
patent: 5228996 (1993-07-01), Lansdell
patent: 3147920 (1983-10-01), None
patent: 123814 (1972-08-01), None
patent: 131279 (1975-06-01), None
patent: 454508 (1988-05-01), None
patent: 9602468 (1996-02-01), None
Dahl Claus Poulsen
Nielsen Marinus
Barry Chester T.
Dykema Gossett PLLC
Kruger A/S
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