KRAAN MARTINUS WILHELMUS (NL)
DE KREUK MERLE KRISTA (NL)
DE BRUIN LAMBERTUS MARTINUS MA (NL)
KRAAN MARTINUS WILHELMUS (NL)
DE KREUK MERLE KRISTA (NL)
| WO1998037027A1 | 1998-08-27 |
| US20050000873A1 | 2005-01-06 | |||
| NL9301792A | 1995-05-01 | |||
| US5057221A | 1991-10-15 | |||
| US20030159991A1 | 2003-08-28 | |||
| US5985150A | 1999-11-16 | |||
| NL9301792A | 1995-05-01 |
LIU Y ET AL: "State of the art of biogranulation technology for wastewater treatment", BIOTECHNOLOGY ADVANCES, ELSEVIER PUBLISHING, BARKING, GB, vol. 22, no. 7, September 2004 (2004-09-01), pages 533 - 563, XP004522072, ISSN: 0734-9750
ARROJO B ET AL: "Aerobic granulation with industrial wastewater in sequencing batch reactors", WATER RESEARCH, ELSEVIER, AMSTERDAM, NL, vol. 38, no. 14-15, August 2004 (2004-08-01), pages 3389 - 3399, XP004522403, ISSN: 0043-1354
| CLAIMS
1. A process for the purification of waste water comprising organic matter and nutrients in a reactor, wherein the waste water is contacted with granular sludge particles that contain microorganisms, oxygen-containing gas is sup- plied to the sludge particles, the sludge particles are allowed to settle and the organic matter- and nutrient- depleted waste water is discharged, characterised in that both suspended material and granules having poor settling ability are withdrawn from the reactor via a tap point lo- cated at a height between the top rim of the reactor and the layer of granules having good settling ability.
2. A process according to claim 1, characterised in that the tap point is provided at 50 to 98% of the height of the reactor. 3. A process according to claim 1 or 2, characterised in that the tap point is embodied in the form of an annular conduit.
4. A process according to one of the claims 1 to 3, characterised in that withdrawal of suspended material and the granules having poor settling ability takes place at the end of the settling phase.
5. A process according to one of the claims 1 to 3, characterised in that withdrawal of suspended material and the granules having poor settling ability takes place during, or immediately after, the filling phase
6. A process according to one of the claims 1 to 5, characterised in that between the filling phase and the reaction phase, an amount of medium is withdrawn from the reactor to compensate for the expansion of the medium in the following reaction phase.
7. A process according to one of the preceding claims, characterised in that medium containing sludge granules having poor settling ability and which is washed out of the reactor is collected separately and returned to the following settling phase.
8. A process according to one of the preceding claims, characterised in that the largest sludge granules with good settling ability are withdraw from the bottom of the reactor.
9. A process according to claim 1, characterised in that the oxygen-containing gas is not supplied in the phases where no feed is supplied, no settling occurs and/or no medium is discharged.
10. A process according to one of the preceding claims, characterised in that the filling speed ranges from between 1.5 m 3
/m 2
/hour and 4.0 m 3
/m 2
/hour. 11. A process according to claim 10, characterised in that the filling speed ranges from between 2.0 m 3
/m 2
/hour
12. An apparatus for treating waste water, to be used in the process according to the preceding claims, ehar- actθrisβd by a reactor (1) having at the bottom side an inlet opening (2A) for waste water, an inlet opening (2B) for an oxygen-containing gas, an outlet opening (3) for settled granular sludge (4), an outlet point (5) for granules having poor settling ability and suspended material, and an outlet opening (6) for purified effluent. |
Process and apparatus for the purification of waste water
The invention relates to a process for the treatment of waste water comprising organic matter and nutrients, wherein the waste water is contacted with sludge particles that contain microorganisms, oxygen-containing gas is sup- plied to the sludge particles, the sludge particles are allowed to settle and the organic matter- and nutrient- depleted waste water is discharged. The invention further relates to an apparatus for treating waste water, to be used in the process according to the invention. Such a process is known from the international patent application PCT/NL98/00100. This document describes a process for the purification of waste water with granular sludge under aerobic conditions, in a reactor. Organic matter and nutrients are removed in three successive steps, namely a filling phase excluding oxygen, or a reaction phase supplying oxygen, and simultaneously fluidising the granular sludge. According to the prior art process, the organic matter- and nutrient-depleted waste water is either discharged during the filling phase, or after at least part of the granular sludge is settled. In the third step, subsequently, sludge granules that settle more slowly are discharged from the reactor, while sludge granules settling more quickly remain in the reactor. In the prior art process furthermore, part of the material suspended in the raw waste water is collected re- suiting in granular sludge, and part is discharged with the purified waste water withdrawn from the reactor. An extra amount of suspended material or "fines" is formed during the aeration and fluidisation phase, wherein considerable shearing forces occur, and also as a result of the disintegration of sludge granules during growth.
Despite the growth of biomass (surplus sludge) being kept under control by discharging part of the suspended material in the effluent of the reactor and/or by discharging granular sludge that settles less readily, it is a drawback of the known process that the presence of suspended material
and of granular sludge settling less readily in the effluent of the reactor, generally makes it necessary for this already substantially purified water to undergo a "polishing" or post-treatment. This involves extra costs and takes up space. A further drawback of the known process is the fact that there is a limited selection for readily settling aerobic granular sludge, because only sludge granules having poor settling ability are discharged and not at the same time a large amount of suspended material. As opposed to granules having poor settling ability, this suspended material remaining in the reactor and thus not being discharged with the effluent, interferes with the formation of granules having good settling ability. It is an object of the invention to provide a process and apparatus, wherein the above-described drawbacks of the prior art process and apparatus are effectively removed. In particular the amount of suspended material and granules having poor settling ability is drastically reduced in the effluent. Finally, the process according to the invention saves space and costs, because post-treatment may be dispensed with.
DESCRIPTION OF THE INVENTION
The invention provides a process for the purifica- tion of waste water comprising organic matter and nutrients, wherein the waste water is contacted with granular sludge particles that contain microorganisms, oxygen-containing gas is supplied to the sludge particles, the sludge particles are allowed to settle and the organic matter- and nutrient- depleted waste water is discharged, withdrawing both suspended material and granules having poor settling ability from the reactor via a tap point located at a height between the top rim of the reactor and the layer of granules with good settling ability. . It has been shown that during the prior art process a stratified layer of slowly settling suspended material is formed above the layer of granular sludge with good settling ability, and that this occurs not only during the settling phase but also during the filling phase. By selectively removing this layer from the reactor this suspended material
is, to a large extent, prevented from being discharged via the effluent stream, and the amount of suspended material in the effluent is drastically reduced. In addition, this removal of suspended material contributes to the necessary removal of surplus sludge from the reactor.
According to the invention, the suspended material and the granules having poor settling ability are usually withdrawn from the reactor at a tap point located at 50-98% of the reactor's height. According to another preferred embodiment, the suspended material is withdrawn during or immediately after the filling phase above the bed of settled granular sludge. Depending on the filling speed of the reactor and the volume of granular sludge, the tap point for suspended material and granules having poor settling ability is located at 50 to 98% of the height of the reactor.
The waste water is supplied evenly and slowly, passing through the reactor as a sluggish flow from the bottom side of the bed of settled granules upward. A sluggish flow is necessary in order to avoid turbulence and as a consequence mixing of influent and effluent, and to thus maintain a high-quality effluent. It has been shown, that if the chosen filling speed is not too high (typical is a maximum of 4.0 m 3 /m 2 /hour) , no short-circuit flow will occur so that the components from the raw waste water will not find their way into the tap point for the suspended material located higher up, or into the effluent that is discharged from the reactor. The filling speed ranges preferably from between 1.5 rcι 3 /m 2 /hour and 4.0 m 3 /m 2 /hour, more preferably it is from between 2.0 m 3 /m 2 /hour and 3.0 mVmVhour.
The suspended material and the granules having poor settling ability are preferably discharged by using a tap point in the form of an annular conduit, so as to allow the medium containing the suspended material and granules having poor settling ability to be discharged evenly from the whole reactor at the same height. The frequent withdrawal of granules having poor settling ability as well as suspended material from the reactor has a favourable effect on the formation of and selection for granular sludge with good settling ability. By thus withdrawing sludge, the need to withdraw
surplus sludge {growth of sludge) from the system is at least partly satisfied. In addition, the removal of the suspended material and the smallest granules makes it possible to effectively remove from the system the phosphate contained in biomass. The selective removal of sludge from the reactor contributes to a drastic reduction of sludge particles in the effluent. The result of the above-mentioned effects is that extra purification is not, or only to a very limited extent, necessary. This saves a considerable amount of space and time as well as costs for the equipment required for this.
The discharge of the suspended material and the granules having poor settling ability via the tap point according to the present process could be combined with a withdrawal of effluent, wherein both processes are performed at different times.
According to the invention it is preferred for the withdrawal of suspended material and the granules having poor settling ability to take place at the end of the settling phase. It is further advantageous to introduce an extra effluent withdrawal between the filling phase and the reaction phase, at which withdrawal an amount of medium can be withdrawn via the above described tap point, in order to avoid that as a consequence of fluidisation and/or aeration, and the ensuing expansion of the medium containing the sludge granules, sludge and suspended material are unintentionally lost at the top of the reactor. If no extra effluent withdrawal is introduced into the cycle, medium containing sludge granules will wash out of the reactor, and during the reac- tion phase they will have to be collected separately and returned during the following settling phase.
In contrast to the prior art process, the new process does not require the supply of oxygen during the reaction phase with simultaneous fluidisation of the bed of sludge granules to be continuous. According to general knowledge, it may be advantageous when treating waste water and removing nutrients, to alternate the oxygen-rich or aerobic phase once or several times with one or several anoxic phases, in which phase no oxygen is supplied. This promotes the microbial processes, which are able to take place more effectively or
even exclusively without oxygen. Regarding the above described invention, this means that during the reaction phase, the bed of sludge granules settles temporarily completely or partly at those moments when no oxygen is supplied. With the new process it may be advantageous to selectively withdraw the largest sludge granules with good settling ability from the bottom of the reactor. This is the case when the granules become too large and too solid and do not fall apart spontaneously, as a consequence of which the specific surface area (surface area-volume ratio) of the sludge granules becomes too small for a good conversion of compounds that are to be degraded in the waste water. Such a process may also serve for withdrawing surplus sludge from the system. In addition, the extra tap thus provided in this embodiment, facilitates periodic removal of the inert material (sand and the like) collecting in the bottom of the reactor.
The invention further relates to an apparatus for the treatment of waste water that can be used with the proc- ess according to the invention as described above. This apparatus is represented in Figure 1. The apparatus comprises a reactor (1) having at the bottom side an inlet opening (2A) for the waste water to be purified and an inlet opening (2B) for an oxygen-containing gas. At the beginning of the filling phase, the reactor is 25-50% filled with well-settled sludge granules (4) . The well-settled granular sludge is periodically discharged via an outlet opening (3) . The apparatus further comprises an outlet point (5) for granules having poor settling ability as well as for suspended material. This outlet point is located at approximately 50-98% of the height of the reactor. The purified water leaves the apparatus as effluent via an outlet opening (6) .