DK169737B1 - Process for treating wastewater with an activated sludge system - Google Patents

Description

in DK 169737 B1

The invention relates to a method for treating wastewater with an activated sludge system. It generally relates to improvements in the treatment of urban and / or industrial wastewater by the activated sludge process, and in particular to the control of operating conditions to increase selective production and maintenance of active biomass substantially free of filamentous growth, such that the sludge obtained has favorable settling properties, with the ability to substantially remove phosphate values from the incoming wastewater. Under the selected operating conditions of the present system, the stated targets are achieved at lower oxygen consumption per hour. BOD unit removal than so far required. (BOD denotes "biological oxygen demand").

The closest known technique is thoroughly described in U.S. Patent No. 4,056,465, the method of the invention being directed to improvements over the method of said patent.

The selective production of biomass species capable of removing phosphate values and producing a non-voluminous sludge with rapid precipitation properties is achieved according to the aforementioned patent by strictly maintaining anaerobic conditions during a first stage of operation in which incoming wastewater and recycled sludge from secondary clearance is mixed. Under these conditions, propagation of undesirable large surface area microorganisms is avoided, while significant amounts of BOD are sorbed from the inflowing effluent of organisms with their ability under anaerobic conditions. The first anaerobic zone, as described in the patent, can then be followed by an oxygenated aerobic zone where the food first sorbed in the anaerobic zone is odixed and all remaining BOD is sorbed and oxidized. During this aerobic step, the energy previously lost by the hydrolysis of polyphosphates is recovered and polyphosphates are recovered and stored in the aerated biomass, thereby removing phosphate from the mixed liquid.

If denitrification of the wastewater is also desired, the patent suggests that an anoxic zone may be located between the anaerobic and the oxygenated aerobic zone.

The term "anaerobic" in the aforementioned patent is defined as "the state that exists in a wastewater treatment zone substantially free of Ν0χ" (i.e., less than 0.3 ppm and preferably less than 0.2 ppm, expressed as elemental nitrogen) wherein conditions are maintained such that the dissolved oxygen (DO) concentration is less than 0.7 ppm and preferably less than 0.4 ppm ".

The term "anoxic" in the aforementioned patent is defined as "the condition existing in a wastewater treatment zone in which BOD is metabolized by nitrates and / or nitrites at initial total concentrations greater than about 0.5 ppm, expressed as nitrogen, and dissolved. Oxygen is less than 0.7 ppm, preferably less than 0.4 ppm ".

As further described in the patent, the dissolved oxygen (DO) content of the aerobic zone must be maintained above 1 ppm and preferably above 2 ppm to ensure sufficient oxygen presence in the aerobic oxygenated zone to carry out the desired metabolism of BOD and the desired phosphate uptake. In the several embodiments of the patent, the average DO used in the total aerobic zone is close to or above 6 25 ppm.

Systems of the type shown in FIG. 1 of U.S. Patent No. 4,056,465, with an initial anaerobic zone followed by an oxidation zone is sometimes referred to as "A / O systems". Systems of the type shown in FIG. 2 of said patent, with an anoxic zone between the anaerobic and oxidation zones are referred to as "A / A / O" or "A2 / 0" systems.

It has now been found that with the present system and method, significant savings can be achieved in operating costs with efficient BOD removal, while dense sludge is obtained with good settling properties and a desired high and up to sufficient removal of phosphate from the influx. sewage. These cost savings result substantially in lower oxygen consumption and reduced power requirements for oxygen mass transfer than in both (1) the conventional fully aerobically activated sludge systems of known type using air or 5 more concentrated oxygen gas, and (2) other disclosed systems employing one or more anaerobic steps in conjunction with one or more aerobic steps as in U.S. Patent No. 4,056,465.

The operation of the present system differs from the prior art in that the system according to the invention is not limited by a minimal concentration of dissolved oxygen in the oxidation zone, whereas in the prior art a concentration of dissolved oxygen or NOx equivalent of at least 1 is required. ppm and preferably substantially greater than 2 ppm.

Thus, the invention relates to a method as mentioned above, wherein: a) preparing a mixed liquid by first mixing activated biomass with BOD-containing wastewater in a BOD sorption zone; b) in a subsequent oxidation zone, the BOD present in the mixed liquid oxidizes, including at least part of the BOD sorbed in the biomass; c) allowing the thus oxidized mixed liquid to separate the above liquid from the more dense sludge which includes biomass; d) recycle at least a portion of this more dense sludge to obtain activated biomass in the first BOD sorption zone; and e) optionally interposing an anoxic step between step a) and step b); the process is characterized by performing the mixing in step a) under such selected conditions that less than 5% of the total BOD5 is oxidized by oxygen or other oxygenating agents and that at least 25% of the soluble BOD5 is sorted by the biomass in this sorption zone, since the conditions in this BOD sorption zone are controlled so as to achieve in the zone an F / Mg ratio of 4 DK 169737 B1 less than 10, where F is the weight of the total BOD5 supplied with the effluent flow per day. per day, and M2 is the weight of biomass volatile suspended solids found in this BOD sorption zone and conducting the oxidation in 5 step b) under controlled aeration conditions so that at least 30% of the total influx of BOD5 is oxidized, which conditions include a dissolved oxygen content in the oxidation zone of less than 1 ppm, and controlling oxygen consumption by controlling the total F / M1 ratio 10 (over the entire plant), where M is biomass volatile suspended solids in all the zones of the system, including a possibly anoxic zone present.

Preferred embodiments of the method according to the invention are described in the subclaims.

15 In the first sorption zone, conditions are regulated to avoid excessive oxidation of BOD in this zone, especially during the time from onset, and to achieve essentially "steady state" operation. Once the system has reached "steady state", a higher degree of oxidation can be allowed in the sorption zone without rapid washing of the desired species in the biomass formed.

The biological pressure resulting in the choice of preferred biomass occurs in the BOD sorption zone. In the subsequent BOD oxidation zone, energy is generated by the metabolization of BOD as a result of the oxidation, which energy is used in the growth of the biomass and the removal of phosphate values from the outer liquid to the interior of the biomass. The preferred species of biomass selectively formed and propagated under the initial conditions maintained in the BOD sorption zone of the invention has a characteristic not usually found in most conventional activated sludge systems. It has been observed that the oxygen uptake rates are relatively slow in the BOD oxidation zone by the present process, as compared to conventional activated sludge systems. Due to the slow oxygen uptake, the process of the invention can be practiced as a high speed method with minimal consumption per minute. removed BOD device.

DK 169737 Pg 5

The savings in power consumption and oxygen use are achieved by incorporating oxidant into the oxidation zone at a rate that does not substantially exceed the oxidation rate of BOD5.

5 The attached drawing shows a schematic and diagrammatic side view of a simplified system for practicing the invention.

In the drawing, a modified activated sludge treatment plant is shown which in many respects corresponds to that depicted in FIG. 1 of U.S. Patent No. 4,056,465. The wastewater to be treated, generally, but not necessarily, in the form of clarified wastewater from a primary sedimentation tank or clarifier (not shown), first enters BOD sorption zone A through an inlet line 15 11. In the sorption zone A, the inflow is mixed. wastewater with recycled sludge precipitated in a sedimentation tank or secondary clarifier 12 and recycled to zone A through conduit 13. A minor portion of the precipitated sludge is removed by conduit 14. The purified supernatant is sent through a conduit. 15 for receiving streams or reservoirs with or without further processing as required.

As shown, zone A is preferably subdivided so that two or more fluid treatment sections are obtained to obtain so-called "plug flow" of the liquid through the BOD sorption zone A.

It has been found that providing physically divided sections or the hydraulic equivalent thereof best ensures that the desired freedom of filamentous growth and thereby obtaining good sludge properties is achieved, even under adverse conditions. Such adverse conditions include, e.g. operation with low BOD concentrations in which large surface area biomass would have an advantage in competition for low concentration BOD sorption. Passage of untreated BOD through the BOD sorption zone is minimized.

In the particular embodiment shown, zone A is designated as divided into two sections or chambers 16 and 17, each equipped with stirring means 19. The liquid passes in approximately "plug flow" through the several sections of zone A and is discharged to a BOD-6 DK 169737 B1. oxidation zone B.

Although zone A is represented by two divided sections 16 and 17, it is noted that three or more such sections may be used. Zone A and B may be separate connected vessels provided with appropriate means for conducting a substantially unidirectional flow of fluid from Zone A to Zone B with minimal back mixing.

Ventilation of the liquid is carried out in a known manner in zone B. Thus, as shown, compressed air can be supplied in the bottom of the oxidation zone with rotary cross 20. If desired, mechanical ventilators can be placed in the oxygenated zone instead of or in addition to rotary cross. Further, instead of air, oxygen of any desired purity may be conveyed to zone B, in which case it may be necessary with appropriate means to cover all or part of the zone. In practice, some oxidation may occur, preferably up to no more than ca. 1% of the total BOD5 in the inflowing effluent in zone A, but usually substantially all of the oxidation occurs in zone B.

20 As shown in the figure, zone B is divided into two liquid treatment sections 26 and 27, although it is noted that a greater number of such sections may be used if desired. One of the reasons for division in zone B is that, in experience, phosphate uptake appears to be a first order reaction in relation to the concentration of soluble phosphate. Thus, the low phosphate value in the effluent is best achieved with "plug flow" configuration.

Zone A is here designated as a BOD sorption zone in a wastewater treatment plant. The term "BOD sorption zone" with respect to the disclosed system described refers to and is defined as the zone of the wastewater treatment plant in which the effluent effluent and recycled sludge are first mixed and wherein at least 25% and preferably at least 50% of the soluble BOD5 content in the effluent effluent is transferred from the aqueous phase of the mixed liquid to the solid sludge. The term "soluble B0D5" refers to biological oxygen demand passing through a 1.25 μιη glass fiber filter, in addition to oxygen, required for the oxidation of nitrogen values.

In order to obtain the indicated rate of transfer of the soluble BOD5 from the aqueous phase to the solid sludge, it is important that the following conditions are observed.

1. The F / M2 ratio in the sorption zone, as described below, is maintained at less than 10 and preferably less than 5. F is the weight of the total BOD5 introduced by the effluent effluent per unit area. per day, and M2 is the weight of biomass, measured as volatile suspended solids in the mixed liquid, i.e. MLVSS in the BOD sorption zone.

2. During the initial operation from the start and until a substantially "steady state" operation is achieved, it is essential that less than 2% and 15 preferably less than 1% of the inflowing total BoDs be oxidized, whether of oxygen or other oxidizing agents present in the BOD sorption zone.

3. After steady state operation is achieved, a good effect can be obtained with a somewhat higher degree of oxidation in the BOD sorption zone. Even in such a case, it is best that less than 5% and preferably less than 3% of the influxing total BOD5 be oxidized with oxygen and / or other oxidizing agents in the sorption zone.

As noted above, the significant oxidation of BOD present in the effluent effluent takes place in the oxidation zone B. The term "oxidation zone" as used with reference to the present system is defined as the zone of wastewater treatment plant wherein agents for oxygen mass transfer are used and the mixed liquid from the BOD sorption zone is contacted with oxygen and / or oxidizing agents under conditions and for a period sufficient to oxidize at least 30% of the total BOD 5 present in the original inflowing effluent.

As already stated above, it is important that the oxidation is restricted in the BOD sorption zone, even during steady state operation and that during the initial phase of the process according to the invention, conditions are maintained so that no more than 2% and preferably less than 1% of the total BOD5 is oxidized by reaction with either 5 oxygen or other oxidizing agents (such as nitrite and / or nitrate-NOX) in the BOD sorption zone. To ensure that the degree of oxidation in the BOD sorption zone is below or not greater than the specified maxima, one or more of the following precautions may be used: 10 A: The vessel (or vessels) constituting zone A may be provided with a blanket of nitrogen or other inert gas at the liquid surface to prevent access to atmospheric air; or a loosely fitting cover may be disposed at or above the liquid surface, or a rigid cover may be disposed above the liquid surface with or without an inert gas blanket. Instead of or in addition to these stated ways of limiting the degree of possible oxidation that could occur in the BOD sorption zone, nitrogen flow gas can be added in zone A.

B. Gas mass transfer means are excluded from the BOD sorption zone. The zone is equipped with agitators such as. shown at 19 in the drawing, as opposed to rotary crosses, surface vents, or other gas-liquid mass transfer apparatus.

C. Excess amounts of any oxidizing agent such as nitrate and / or nitrite (NOX ') must be carefully avoided to be added to the BOD sorption zone. The latter results in the control of not only NOx, which may be present in the effluent, but also NOx, which could be recirculated to this zone from a source located below the zone in the direction of flow. Wastewater usually contains little or no NOx due to the reduction of nitrates and / or nitrites by BOD in the presence of microorganisms in the sewage pipes leading to the treatment plant. A potential source of NOX 'is recycled mixed liquid from the BOD BOD oxidation zone in nitrifying biological systems, i.e. those designed to effect oxidation of ammonia BOD to ΝΟχ '. In such systems, a portion of the mixed liquid from the aerobic zone is recycled as shown in FIG. 2 of U.S. Patent No. 4,056,465, wherein an anoxic zone is disposed between the first anaerobic treatment zone and the oxygenated aerobic zone, to the intermediate anoxic zone to effect a reduction of NOX therein.

The present invention can be applied to systems that include an anoxic zone located between the BOD sorption zone (A) and the oxidation zone (B). To calculate the degree of BOD oxidation of the invention, BOD oxidation in the anoxic zone must be counted as it was in the oxidation zone. However, in such nitrifying systems, the amount of NOX "introduced into the BOD sorption zone must be controlled to avoid recirculation of mixed fluid from the aerobic zone in such a system to the BOD sorption zone, and also by controlling NOx. -NOX * -20 The concentration of the recycled sludge can be controlled by providing a sufficient residence time in the clarification tank and sludge recirculation line to allow for sufficient removal of NOx by reduction to elemental nitrogen upon reactive contact with it. biomass present for 25 in the recycled sludge mixture.

Conventional biological wastewater systems have been unable to generate dense, active filament-free sludge and also remove significant phosphorus values without the addition of chemicals. This suggests that the biology of the present process does not occur naturally, but is a result of the pressure applied to produce a "steady-state" biology having the desired properties.

Starting the system according to the invention requires that a strict biological pressure be applied so that the preferred organisms can outperform the conventional ones. This is achieved by operating the BOD sorption zone with minimal presence of oxidizing agents to maximize the applied pressure. In this case, maximum pressure is carried out at maximum outside oxygen and other oxidizing agents from the first BOD sorption zone. Thus, they have organisms that can use a non-oxidative energy source, i.e. hydrolysis of polyphosphates, an advantage that only they have the necessary energy to carry out active transport of BOD from the liquid itself across the cell wall of the microorganism to the interior of the organism. To the extent that the energy for the BOD transport is obtained in this way, polyphosphate-containing microorganisms have an advantage in sorbing BOD (or food supply), which allows these organisms to dominate the population after successive circulations through the system. This is not the case in conventional activated sludge systems.

In the initial operation of carrying out the process in question from the start and until steady state operation is achieved, as indicated above, care must be taken to minimize the degree of BOD oxidation in the BOD sorption zone. In practice, the maximum permissible content of oxygen and / or other oxidizing agents such as nitrate and / or nitrite, expressed as oxygen equivalent, should be such that less than 2% and preferably less than 1% of the total B0D5 in the effluent effluent is oxidized in this zone. If these precautions are adhered to during the initial and initial operation of the system, both good sludge properties and increased phosphate removal will occur within two to six weeks. Achieving "steady state" operation with these desired properties can be accelerated by the addition of "graft sludge" containing polyphosphate values.

The performance of the present process depends on the initial presence of an effective BOD sorption zone. However, even in the case of excessive oxidation in the sorption zone, the biology of the present system works until the desired active sludge species is replaced by conventional sludge. It has been observed that such replacement occurs relatively slowly, e.g. within a period of approx. one to two months. Once the system has reached "steady state", a slightly higher percentage of oxidation can be allowed in the BOD sorption zone without a rapid leaching of polyphosphate-containing sludge. Leaching is detected by the degradation of the sludge precipitation properties and a reduced ability to remove phosphate. If a disturbance 5 occurs, normal operation can be restored in time by strictly limiting the permissible degree of oxidation in the BOD sorption zone in the same way as during the start.

Although, as explained above, the biological pressure resulting in the selection of a preferred biomass occurs in the BOD sorption zone, the function of the BOD oxidation zone (B) is to generate energy by oxidation of BOD. This energy is used for the growth of biomass and the transfer of phosphate values from the actual liquid to the biomass interior. Phosphate removal and biomass storage as 15 polyphosphates are estimated to require from ca. 1 to 5% of the energy generated by BOD oxidation. Therefore, it is believed that the preferred biomass of the invention does not occur in most conventional activated sludge systems, since those species which have no energy requirement for phosphate sorption and storage are able to use more energy for growth and therefore will dominate unless BOD is applied in a sorption zone as defined by the present invention.

It has also been observed that the oxygen uptake rates are relatively slow in the BOD oxidation zone by the process of the present invention compared to conventional activated sludge systems. For example, an oxygen uptake rate of 30 milligrams of oxygen per day is exceeded. grams of WS pr. per hour at 20 ° C, rarely in the practice of the present invention, whereas the rate of oxygen uptake by conventional aeration may be more than twice as described in e.g. U.S. Patent No. 3,864,246. This suggests that the BOD sorbed in the sorption zone is stored in a form that is only slowly available for oxidation. The result is that the initial oxygen uptake rate is slow and further decreases with time.

Because of the foregoing considerations, the method of the process of use in a high-speed system requires minimal oxygen per day. removed BOD device. Unoxidized BOD is naturally removed with the sludge. However, sufficient oxidation must take place to oxidize at least 30% 5 of the total lead B0D5. If BOD is not sufficiently oxidized, the sorption of fresh BOD is inhibited by recirculation of the clearance tank sub-stream to the first sorption zone. This has a detrimental effect in that progressively smaller amounts of influxing BOD are sorbed in the first zone after repeated circulations, and progressively larger amounts of non-sorbed BOD are transferred to the oxidation zone where it is sorbed and metabolized by conventional microorganisms. Under such conditions, the polyphosphate accumulating microorganisms are washed out over time.

It has been found that the minimum oxidation required in the system is 30% of the total BODs in the inflowing effluent. In a high-speed system, oxygen consumption ranges from 30 to 100% of B0D5 in the influent 20 wastewater. A high speed system is defined by the total F / M1 as such with a total F / M. Ratio greater than or equal to approx. 0.3. F is the weight of the total of the waste water added to BOD5 per liter. per day, and M1 is the weight of MLVSS found in all zones of the system concerned, 25 incl. The BOD sorption zone, the BOD oxidation zone and the anoxic zone, if present. With a total F / M1 of less than approx. 0.3 and down to approx. 0.08, the oxygen consumption ranges from approx. 80 to 150% of the total BOD5 content in the incoming wastewater. Oxygen demand of over 100% is attributed to the fact that B0D5 only expresses approx. 2/3 of BOD (infinite corpse).

It has been observed that in the operation of high-speed systems, i.e. with a total F / M1 greater than 0.3, the process in question uses substantially less oxygen than conventional activated sludge systems. Furthermore, even at such high throughput rates, such a system continues to remove BOD and significant amounts of phosphate, while maintaining a desirable non-filamentous biomass species producing sludge with excellent precipitation properties.

The sludge obtained has an SVI of generally less than approx. 100 and a clearance tank undercurrent with a solids concentration greater than ca. 1%. In order to maintain these desired properties of the system, the conditions must be adjusted so that at least approx. 30 to 40% of the total BoD5 is oxidized in the system. A failure thereof results in progressive deterioration of the sludge properties and less phosphate removal.

An important consequence of the biological selection taking place in the BOD sorption zone is that there is no longer a requirement for a minimum dissolved oxygen (DO) concentration to be maintained in the oxidation zone to obtain the required minimum 30% oxidation. . This is in contrast to the prior art (as disclosed, for example, in U.S. Patent Nos. 3,864,246 and 4,162,153), where minimum DO levels of at least 1 ppm and generally more than 2 ppm of dissolved oxygen are required. Maintaining the high 20 levels of dissolved oxygen is expressed by high power requirements. For example. by reducing the DO level from 2 ppm to a level of less than 1 ppm in a system using atmospheric air oxidation, the energy requirement to transfer a given amount of oxygen from the gas to the liquid phase of the system is reduced by from approx. 14 to 25%. In carrying out the process of the present invention, the level of dissolved oxygen is not the controlling factor, but rather the percentage of total BOD oxidized in the respective zones.

The only thing required to obtain the necessary oxidation is for the oxidant to be introduced into the oxidation zone at a rate substantially equal to the oxidation rate.

If the oxidant is added at a rate which does not substantially exceed the oxidation rate, great savings in power and oxygen consumption can be achieved. It was found that by adding oxidizing agent at a rate not exceeding the measured oxidation rate by approx. 0.5% and preferably not exceeded the oxidation rate by 0.4%, could achieve a much more efficient system than the previously known systems requiring the presence of a DO level of at least 1 ppm. Addition of oxidizing agents at this rate 5 results in a low DO level in the oxidation zone. Therefore, it is possible to operate this system by monitoring the DO level in the oxidation zone as well as by measuring the oxygen uptake rate. If the system is operated by monitoring the DO level, a DO level of approx. 0.6 ppm is not exceeded 10 to ensure that the oxidizing agent is not added at a rate exceeding the oxidation uptake rate by more than approx. And preferably a DO level of about 0.5%. 0.4 ppm is not exceeded to ensure that the oxidizing agent is not added at a rate exceeding the oxidation rate by more than approx. 0.4%.

Although a system for carrying out the process can be operated effectively if the oxidant is added to the oxidation zone at a rate equal to the oxygen consumption rate, resulting in a DO-20 level of 0 ppm, the system will no longer work effectively and the selected biomass would be washed out over time if the rate of addition of the oxidant falls below this value. To prevent this type of leaching, it is preferred that such a system be operated by adding oxidant in the oxidation zone at a rate in excess of the measured oxygen uptake rate by at least approx. 0.1% or, if the system is operated by monitoring the DO level in the oxidation zone, maintaining a DO level of at least approx. 0.1 ppm.

The initial period of a system for carrying out the process of the present invention is defined as the time required for the system to develop biomass of the desired type, as seen by constant BOD at phosphate removal and sludge precipitation properties. It can also be defined as the time required by the system to recover from interruptions.

The start-up period can be significantly reduced by grafting a sludge system previously worked into a type of operating system of this type.

Example 1 A test plant with a BOD sorption zone A, consisting of three 220-liter sections and a BOD oxidation zone B, consisting of four 557-liter sections, was seeded initially with 28.4 liters of liquid containing approx. 1% phosphate-removing sludge and obtained from "steady-state" operation of a 10 laboratory 0/0 unit.

Three weeks after the addition of the graft sludge, a stable operation for phosphate removal was achieved.

The plant was operated for a week at a low DO level (0.27 ppm). During the low DO operating period, all 15 sections of the oxidation zone (B) were maintained at a level below 1 ppm.

In both periods, no attempt was made to regulate available oxygen and oxygen was abundantly available in the oxidation zone, as indicated by the relatively high oxygen consumption of over 1.5 kg of spent oxygen per day. kg of 20 removed BOD (not filtered inlet minus filtered outlet), i.e. kg 02 / kg B0DR (U-F).

The high DO operation showed better phosphate removal than the low DO operation due to the higher total F / M ratio and higher B0Ds / Ps ratio.

The data obtained in Example 1 are listed in Table 1.

Table 1 IDT (hour) 1.59 T, eC 22.5 30 medium MLVSS, ppm 2683 medium DO, ppm A zone 0.2 B zone 0.27 F / M, total 0.63 F / M2 A zone 2.77 35 B0Ds / Ps 11.2 B0Ds, ppm inlet 55 expiration 2 16 DK 169737 B1

Table 1 (continued)% sorbed in A 57 TSS, ppm inlet 104 5 outlet 21 ΒΟϋγ / ppm inlet 112 outlet 9.5

Ps, ppm 10 inlet 4.9

Output 1.9 Net Spring 3.0 SVI 23 15 SVI = sludge volume index (Mohlmann) BODT = total BOD5 BODs = soluble B0D5

Ps = soluble phosphate, expressed as elemental phosphorus, P

20 TSS = total suspended solids

From the operation of Example 1, it is clear that a significant amount of energy can be saved for oxygen transfer in the oxidation zone. Thus, it is calculated that a power saving of 23% could be achieved by operating the oxidation zone (B) at a DO level of 0.5 ppm, in comparison with e.g. operation at 10 ppm. This calculation is based on the use of relatively pure oxygen in the system, as determined by the solubility of the oxygen in water at atmospheric pressure.

Phosphate removal by the process of the invention is not greatly affected, whether high or low DO in the oxidation zone. Therefore, it appears that satisfactory results can be obtained with an A / O system during a period of stable operation if the DO in the oxidation zone is at a level below 1 ppm, provided that no more than 2% of the total influx of B0D5 is oxidized in the BOD sorption zone and that not less than 30% of the influent BOD5 is oxidized in the total system.

Example 2

The following shows a preferred embodiment 5 of the process according to the invention and shows an on-going operation of an A / O system where the DO level maintained in the oxidation zone is 0.3 ppm, with consequent large savings in the required aeration power.

The test facility consisted of a BOD sorption zone with three 10 sections of 220 liters each and an oxidation zone with four sections of 557 liters each. The operating conditions and results are listed in Table 2.

Table 2 15 Withdrawal Retention Time (IDT) hours 1.6

Sludge recirculation in relation to the supply (volume / volume) 0.20 MLVSS average, ppm 2700

Temperature, ° C 23 20 F / M1 total 0.63 BOD total, ppm inlet 112 outlet 9.5 BOD soluble, ppm 25 inlet 55 outlet 2

Total suspended solids, ppm inlet 104 outlet 21 30 Soluble phosphate, ppm (inlet) 4.9 (outlet) 1.9 BODs / Ps inlet 11% sorption of soluble B0D5 in the 35 BOD sorption zone 57 DO average in the BOD sorption zone , ppm 0.2 DO average in the oxidation zone, ppm 0.3 DK 169737 B1

ICE

From the above Table 2, it is clear that an A / O system can work satisfactorily when working with relatively low DO in the oxidation zone. In such a low DO operation, significant savings can be obtained in the energy requirement 5 for oxygen transfer from the air to the liquid (at the same oxygen consumption). The calculated savings that can be achieved at the lower DO levels compared to operation at DO of 3 will be apparent from the following tabulation, calculated on the basis of use of air, and that the oxygen saturation level in the liquid is at a concentration of 8 ppm.

Die. in the BOD oxidation zone, ppm 0.3 1.0 2.0 3.0

Energy saving,% 65 29 17 0

Although the invention has been described in connection with systems wherein the first BOD sorption zone is followed by an oxidation zone (A / O system), it is equally applicable to systems designed for NOx removal in which an anoxic zone between the BOD sorption zone and the oxidation zone.

20

Claims (11)

A process for treating wastewater with an activated sludge system, wherein: a) preparing a mixed liquid by first mixing activated biomass with BOD-containing wastewater in a BOD sorption zone; b) in a subsequent oxidation zone, the BOD present in the mixed liquid oxidizes, including at least part of the BOD sorbed in the biomass; C) allowing the thus oxidized mixed liquid to separate the above liquid from the more dense sludge which includes the biomass; d) recycle at least part of this more dense sludge to obtain activated biomass 15. the first BOD sorption zone; and e) optionally interposing an anoxic step between step a) and step b); characterized in that the mixing is carried out in step a) under such selected conditions that 20 less than 5% of the total BOD5 is oxidized by oxygen or other oxygenating agents and that at least 25% of the soluble BOD5 is sorbed by the biomass of this sorption. zone, the conditions of this BOD sorption zone being controlled so as to achieve in the zone an F / M2 ratio of 25 less than 10, where F is the liquid of the total BOD5 supplied with the effluent flow per day. per day, and M2 is the weight of biomass volatile suspended solids found in this BOD sorption zone and conducting the oxidation in step b) under controlled aeration conditions such that at least 30% of the total influxing B0D5 is oxidized, which conditions include dissolved oxygen content in the oxidation zone of less than 1 ppm and controlling oxygen consumption by controlling the total F / M1 ratio (over the entire plant), where M1 is the weight of biomass volatile 35 suspended solids in all zones of the system, including an anoxic zone possibly present. Process according to claim 1, characterized in that at least 50% of the soluble BOD in the effluent in the DK 169737 B1 is sorbed by the biomass in the sorption zone. A method according to claim 1, characterized in that the BOD sorption zone consists of a series of at least two hydraulically separated consecutive sections. Method according to claim 1, characterized in that the oxidation zone consists of a series of at least two hydraulically separated consecutive 10 sections. 5. A method according to claim 1, characterized in that the first BOD sorption zone is maintained under anaerobic conditions so that less than 2% of the influxing total BOD5 is oxidized in the zone. Process according to claim 1, characterized in that the effluent feed contains phosphate values and that a substantial part of the phosphate is removed from solution in the oxidation zone liquid and stored in the biomass as polyphosphate. Process according to claim 1, characterized in that the feed rate of the effluent is connected to the total biomass in the sorption zone and the oxidation zone in such a way that a total F / M ratio of more than 0.3 is obtained, at which F is the weight of the 25 total of the effluent added B0D5 per per day, and M1 is the weight of biomass volatile suspended solids in the BOD sorption zone and the oxidation zone. Process according to claim 1, characterized in that an anoxic zone is used between the BOD sorption zone and the oxidation zone and that the feed rate of the wastewater supply is associated with the total biomass in the BOD sorption zone, the anoxic zone and the oxidation zone in such a way. , that a total F / M. ratio of more than 0.3 is obtained? whereby F is the weight of the total 35 of the effluent added BOD5 per per day, and M1 is the weight of biomass volatile suspended solids found in the BOD sorption zone, the anoxic zone and the oxidation zone. DK 169737 B1 Process according to claim 1, characterized in that the F / M2 ratio in the BOD sorption zone is maintained at less than 5. A process according to claim 1, characterized in that the BOD sorption zone is operated under conditions which are such that less than 1% of the total BOD5 supplied is oxidized by oxygen or other oxidizing agents in this zone. A method according to claim 1, characterized in that at least a portion of the biomass in this system is obtained by grafting with biomass from a system which has achieved "steady state" operation under conditions as defined in claim 1.