GB1591186A - Process for the aerobic treatment of nitrogen containing waste water - Google Patents

Description

(54) PROCESS FOR THE AEROBIC TREATMENT OF NITROGEN CONTAINING WASTE WATER (71) We, MITSUI TOATSU CHEMICALS, INCORPORATED, a Japanese Body Corporate of No. 2-5, Kasumigaseki 3-chome, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a process for the aerobic treatment of nitrogencontaining waste water.

We explain below how, in the treatment of waste water, microbes are allowed to grow using organic substances in the waste water as substrate and nitrogen and phosphorus in the waste water are fixed as components of the microbial cells by assimilation whereby BOD substance as well as the nitrogen and phosphorus in the waste water can be removed.

In recent years, an increase in the amount of waste water or industrial effluents discharged into rivers, lakes and sea has resulted in ecological damage, in serious cases manifested by e.g. the generation of a red tide or an abnormal proliferation of algae. One primary source of this damage is the presence of nitrogen, phosphorus and organic substances contained in the waste water, the latter being known also as BOD (Biological oxygen demand) substances.

Heretofore, the removal of nitrogen in waste water by biological means required a complicated troublesome procedure comprising first removing BOD substances by the activated sludge method; converting nitrogen, mostly in the form of ammonia, into nitrates, or nitrites by the aerobic action of nitrifying bacteria present in the activated sludge; reducing the nitrate or nitrite into nitrogen with organic substances such as methanol by the anaerobic action of reducing bacteria present in the activated sludge; and then discharging the nitrogen into the atmosphere. Another procedure for removing nitrogen from waste water comprises making the waste water left after the treatment with the activated sludge strongly alkaline with slaked lime, and introducing air vigorously into the waste water whereby nitrogen in the form of ammonia in the water is discharged into the atmosphere as free ammonia gas entrained in the current of air. However, this method has the disadvantage of creating secondary air pollution as a result of the discharge of ammonia to atmosphere and other disadvantages, particularly that arising from the low vapor pressure of ammonia which in colder weather results in a reduction of efficiency, and precipitation of calcium salts which causes clogging of the aeration tower.

The absorption of nitrogen in the form of ammonia with zeolite or an ion exchange resin is also known as yet another method of removing nitrogen but is not practical due to its low efficiency. As far as we are aware, all of the known conventional methods require a preliminary treatment with activated sludge, the removal of nitrogen being conducted as a subse quent treatment stage.

With regard to phosphorous methods for its removal are known in which phosphorus is separated by precipitation as phosphates by the addition of slaked lime, ferric sulfate, aluminum sulfate and polyaluminium chloride to waste water. All these known methods require a preliminary treatment stage with activated sludge and an additional separate step in addition to the nitrogen removal step, except for the method which uses slaked lime.

Furthermore, these known methods produce a large quantity of inorganic sludge as byproduct, disposal of which may be difficult if environmental pollution is to be avoided. In the case of the waste water treatment method using slaked lime, one known procedure comprises baking the precipitated phosphate together with calcium carbonate separated from the waste water by neutralizing the treated waste water with carbon dioxide, and separating the thus formed calcium apatite and quick lime by means of air elutriation to recover phosphorus and to recycle slaked lime. However, this process involves complicated process operations and requires a complicated large-scale plant.

As the conventional activated sludge method is intended merely for removal of BOD substances, attention has exclusively been paid to oxidizing BOD substances to carbon dioxide as far as possible using microbes and to make the amount of waste activated sludge small by maintaining the yield of the microbial cells at a minimum. The extreme cases are seen in Lagoon process or in a process wherein the formed sludge is treated in an anaerobic digestion tank where carbon in the sludge is converted into methane and water is returned to the active sludge tank. According to such processes, carbon in BOD substances is converted into carbon dioxide or methane and the amount of waste activated sludge which requires further treatment is reduced but removal of nitrogen and phosphorus by fixation cannot be expected.

The present invention is predicated on the observation that microbes require nitrogen and phosphorus for their growth and fix these elements in their cells in the form of protein and nucleic acid. The present inventors have aimed at devising a process in which the fixation yields of nitrogen and phosphorus with respect to organic carbon, i.e. the value of (the yield of microbial cells) x (the contents of nitrogen and phosphorus in microbial cells), can be maximised for the removal of nitrogen and phosphorus by fixation in contrast to the prior art activated sludge method.

According to a first aspect of this invention, there is provided a process for the treatment of a nitrogen containing waste water, comprising passing nitrogen containing waste water through one or more aeration tanks maintained under aerobic conditions such that the residence time of microbes in said tank or tanks is sufficiently short to wash out slowly propagating microbes thereby achieving a population of rapidl propagating microbes in said tank or tanks; and further regulating the conditions in said tank or tanks such that the microbial concentration in one or more aeration tanks is kept at 20-200 weight % based on the total organic carbon in said waste water and the ratio of said weight of total organic carbon treated per day/microbial weight in said one or more tanks is adjusted to 3-25, whereby in growing, the microbes therein assimulate nitrogen and phosphorus contained in the waste in addition to B.O.D. substances.

As we shall explain below, in specific examples we have achieved removal of nitrogen and phosphorus as well as B.O.D. substances from waste water by making the fixation yields of nitrogen and phosphorus per organic carbon in the waste water, that is, the value of (the yield of microbial cells) x (the contents of nitrogen and phosphorus in microbial cells) maximum, utilizing phenomena that the microbes require nitrogen and phosphorus for their growth and fix these elements in their cells by converting these elements into protein and nucleic acids.

To attain this in our Examples, we maintain the microbial activity in the aeration tank at the highest level and select certain kinds of microbes capable of fixing therein nitrogen and phosphorus in a high content. For this, we (a) eliminate the oxygen-deficient state completely, which state causes a reduction in the microbial activity (or in other words, we maintain the microbes at all times under aerobic condition) and (b) shorten the average residence time of the microbes in the aeration tank whereby natural selection is allowed to occur so as to wash out microbes other than the highly propagating ones from the aeration tank so that the highly propagating microbes alone may occupy the aeration tank.

For satisfying the above requirements, the specific means we adopt differ from the prior art active sludge method inter alia in the following respects: First, a usual procedure in the prior art active sludge method wherein microbes overflowing out of the aeration tank are separated outside the tank and returned thereto is not adopted in the present invention except in a special method which will be referred to hereinafter. In a treatment usually adopted in the prior art active sludge method wherein the activated sludge is separated by precipitation outside the aeration tank and the precipitated sludge is then returned to the tank, microbes are kept under anaerobic conditions for a long period of time during the above separation step and lose their activity. Accordingly such a procedure, involving as it does an an aerobic state, must not be adopted in the practice of the present invention.

Secondly, the residence time of waste water in the aeration tank and the rate of-returning the microbial cells to the tank (including the case wherein the microbial cells are not returned at all) desirably should be controlled so that the microbial concentration in the aeration tank is 20-200 weight % of the total organic carbon concentration (referred to hereinafter simply as TOC) of the waste water entering the'- aeration tank, which concentration is mdch lower than-in the prior art activated sludge method. We consider it important to- shorten the average residence time of microbes in the aeration tank (including a system for returning the microbial cells to the tank if it is used according to the special process as will be referred to hereinafter). Microbes other than those having a high activity and a high rate of propagation cannot propagate and reside in the system while the microbial age becomes younger, because the rate of overflowing out of the tank becomes greater than the rate of propagation, and the microbes are removed from the tank by the sd-called "wash out" phenomenon unless they are high enough in activity and in the rate of propagation.

Such younger microbes furnished with a high activity and a high rate of propagation have extremely high nitrogen and phosphorus contents. In many city sewage disposal works for example, the contents of nitrogen and phosphorus in excess activated sludge are 5-8% and 0.5 - 1.5%, respectively, in the prior art active sludge method, whereas the contents of nitrogen and phosphorus may be 10-14% and 4-6%, respectively, in examples of the process of this invention, thus making a great difference between both. To mention a more specific example, -TOC in city sewage is generally 100-200 ppm,- and cnsequently, the microbial concentration in the aeration tank is preferably adjusted t-50-400 ppm by maintaining the residence time of the sewage in the aeration tank within the period of 1.5-6 hours when the microbial cells are not at all returned or within the period of 1-4 hours when the microbial cells are returned according to the special process to be described below.

Thirdly, the operation of our process is desirably carried out at a value of 3-25 in terms of the ratio of the weight of the treated TOC per day to microbial:weight in-the aeration tank by utilizing microbes possessing a high activity and a high rate of propagation, the value being much greater than in the prior art active sludge method. The prior art active sludge method is carried out at a high concentration of microbes; the concentration in this case is determined almost regardless of the BOD or TOG concentration in the waste water to be treated and is usually 1500-3000 ppm and at most 400-800 ppm-even in the modified aeration method which is the method operated at the lowest microbial concentration. Thus, a large amount of the microbial cells must be returned to the 'tank. The microbial cells to be returned are in the form of a sludge separated by precipitation in the separation tank and contains a large amount (95-99%) of water. It follows that the sludge composed predominantly of water is -returned in an amount as large as 50-200 % to the original waste water. The returning of the sludge in the prior art active sludge method means, in addition to loses in microbial activity as described previously, the return of a large amount of water to the aeration tank thereby diluting the' BOD and TOC concentrations in the aeration tank and slowing down the reaction rate. At the same time, the volume of the aeration tank cannot but be made greater in order to keep the residence time of the original waste water constant. According to the prior art active sludge method, therefore, the ratio of the weight of treated BOD to microbial weight in the aeration tank per day is within 'the range of 0.04 -4, depending on the process. The ratio between BOD and TOC is about 1.2 - 1.8 in terms of BOD/TOC, depending on the waste water, indicating that the weight of the treated TOC to microbial weight in-the aeration tank per day in the prior art active sludge method is much lower than may be achieved in the process of this invention (for example: 3-25).

In the accompanying drawings: Fig. 1 is a graph showing the experimental conditions and the results of Example 2; and Figs. 2, 3, and 4 are flow sheets of the treatments shown in Examples.

In our process, the liquid inthe aeration tank may be nearly completely mixed without uneven distribution of the microbial concentration and hence -the microbial concentration in the treated water overflowing from the aeration tank is generally equal to that in the aeration tank. It is also possible to carry out the process in such a manner thatin a part of the aeration tank, aeration conditions suitable for flotation concentration are created, for example, by forming fine bubbles of air and/or oxygen with - a diameter not greater than 1 mm, preferably not greater than 0.5 mm at a rate of 1-500 cm3/min per 1 cm2 of the sectional area of said part (such bubbles can be formed by aeration through a porous filter with a mesh or opening size of 100 ) or by introducing water or the treated water into which air and/or oxygen has been dissolved under pressure to the lower portion of said part of the tank, and a floating concentrated phase of the microbes is formed in the upper portion of said part while a diluted phase of the microbes is present in the lower portion of said part of the tank. By drawing the diluted phase out of the system, the microbial concentration in the aeration tank can be maintained higher than the case where such aerating operation is not applied.

In the case of multiple serial aeration tanks, it is another preferred arrangement to quickly transfer the floating concentrated phase in the upper portion of said part of the tanks to the first aeration tank by some proper means. Thus, it is a preferred embodiment for keeping the high activity of the microbial cells to avoid separation of the microbial cells outside the aeration tanks and subsequent returning of the cells thereto.

As a means for separating microbes from the treated water overflowing from the aeration tank, the flotation separation method wherein air and/or oxygen is used is preferred, particularly in that the residence time of the microbes is extremely short and the separation tank is kept under aerobic conditions unlike the sedimentation separation method commonly adopted in the prior art activated sludge method. Any release of nitrogen and phosphorus from the microbial cells due to self-oxidation of the microbes under anaerobic conditions and/or at a low substrate concentration can be avoided in this manner.

When the separated microbes are returned to the aeration tank, we find that it is important to adopt the above-mentioned flotation separation method wherein air and/or oxygen is used as a separating means. In this case, the use of a gas which is higher in oxygen concentration than air (including the use of pure oxygen) as a gas for flotation separation and/or a confined flotation separation tank which may be operated under superatmospheric pressure so as to ensure the efficient replacement of oxygen rapidly consumed by the microbial cells in the water of the separation tank as well as the use of a short connecting tube between the aeration tank and the separation. tank so as to make the residence time of the microbes outside the aeration tank short as far as possible is desirable for preventing the release of nitrogen and phosphorus from the microbial cells and any loss of the microbial activity. A process wherein microbes inactivated in anaerobic atmosphere during the sedimentation separation step are re-exposed to air or, in some cases, oxygen for reactivation and the reactivated microbes are returned to the aeration tank is also reported. However, nitrogen and phosphorus once released from the microbial cells by self-oxidation of the microbes are not recovered by such reactivation, nor is such reactivation satisfactory for adoption in the practice of the present invention.

If the amounts of nitrogen and phosphorus to be removed are too great in relation to the amount of TOC in waste water, removal of these elements may not be satisfactory even if the yield of microbial cells utilizing the TOC is at a maximum. In such a case, an organic substance free from nitrogen and phosphorus such as methanol is preferably added to the original waste water. Such organic substance may be added as a secondary treatment, not to the original waste water but to the water which has been aerated once and is substantially free from TOC.

When waste water, for example, ordinary city sewage is treated according to examples of our process, removal of 70-90% of nitrogen and removal of 50-70% of phosphorus can be achieved and 90-96% of BOD substances can also be removed simultaneously. Furthermore, if the secondary treatment is carried out by adding an organic carbon source free from nitrogen such as methanol to the water having been subjected to the primary treatment in such proportion that 20-60g of the carbon source as carbon is added to every 1 m3 of the water, removal of 90-99% of nitrogen, 70-99% of phosphorus and 97-99% of BOD substances can be achieved in particular examples throughout the primary and secondary treatments. In other words: by only a little modification of the facilities for the activated sludge treatment utilized as the secondary treatment in the conventional method, an advanced treatment of the waste water accompanying simultaneous removal of nitrogen and phosphorus becomes also possible, thus bringing about such advantages that complicated steps and various reagents required hitherto in the conventional method for attaining the effects equivalent to those in our treatment are no longer necessary, In addition, only a small process plant area is required for the facilities of such advanced treatment. Consequently, the technical and economical profits achieved by processes in accordance with the present invention may be significant.

Example 1 The waste water shown in Table 1 was treated under the conditions given in Table 2 whereby the results shown in Table 2 were obtained. The aeration tank used was a so-called jar fermenter having a capacity of 10t and equipped with a stirrer.

Table 1 (Unit: mg/e) Waste Origin BOD COD Mn * TOC Total Phosphorus Water No. nitrogen (as PO,-") 1 City sewage 270 247 193 21.1 26.0 2 Effluents from 2,020 1,840 1,550 202.5 155 a food factory 3 " 4,830 4,460 3,050 315 94.5 *(COD Mn) is a COD value determined by measuring the quantity of oxygen consumed when the waste water sample is oxidised using potassium permanganate.

Table 2 Waste Species Tempera- Residence time Rate of return- Microbial** Microbial A* water of ture in the aeration ing the concentra- concent No. microbe ( C) tank (Hrs) microbial cells tion in the ration x 100 (%) * aeration TOC tank (mg/l) (%) 1 Bacteria 30 6.7 0 130 67.3 4.68 2 " 25 5.0 0 1,570 101.3 4.62 3 " 35 6.2 0 3,000 98.4 3.84 1 " 30 5.0 10 145 75.1 5.76 1 Mold 30 7.0 0 135 79.2 3.72 A * amount of TOC treated per unit weight of microbes per day (litres/day) ** Microbial concentration is determined as follows: A sample is carefully collected from the solution or culture without affecting the concentration of the cells therein. The sample is centrifuged at 4000 G (gravity), the supernatant removed and the sample filtered using a membrane filter of pore size 0.45 m. The filter and solid are dried and weighed.

Knowing the weight of the filter the weight of the solid can be calculated. Microbial concentration is then found by dividing the weight of solid by the weight of the original sample.

Table 2 (cont'd) Waste Quality of the treated water (mg/ e) water No. BOD COD Mn TOC Total Phosphorus nitrogen (as Po4-3) 1 11 9.6 23 5.7 12.0 2 51 40 40 26.8 29.2 3 70 60 75 30.0 20.8 1 9 8.5 19 6.5 11.5 1 15 12 27 4.5 10.3 * The percentage of the microbial cells to be returned to the aeration tank versus the microbial cells at the outlet of the aeration tank. The separation of the microbial cells from the treated water is effected by the floation separation method using air.

Example 2 Using the same bacteria as used in Example 1 without returning the microbial cells, three samples of artificially made sewage containing 190 ppm of TOC with different total nitrogen concentrations were treated in a 10 e jar fermenter as aeration tank whereby the results as shown in Fig. 1 were obtained. In the figure, the solid lines stand for the changes in weight of the total organic carbon treated per day per microbial weight in the aeration tank, the broken lines for the changes in concentration (ppm) of the total organic carbon in the treated water, the chain lines for the changes in concentration (ppm) of the total nitrogen in the treated water and the dotted lines for the microbial concentration (ppm) in the aeration tank in relation to the rate of dilution shown on the abscissa. The rate of dilution can be calculated according to the following equation and is an inverse number of the residence time in the aeration tank.

The amount of waste water flowing into the Rate of dilution = treating tank per hour The capacity of the treating tank Parameter numbers 4, 6 and 9 in the figure stand for the weight ratio of carbon/nitrogen in the water water at the inlet of the tank. In the case or ordinary city sewage, this value is within the range of 6-10. From the results shown in Fig. 1, it is evident that nitrogen is removed satisfactorily from the waste water having a carbon/nitrogen ratio within the above range.

Example 3 Some of the results obtained in a long-term continuous operation of the city sewage treatment are shown in Tables 3 and 4. A flow sheet of the treatment is shown in Fig. 2. An aeration tank 1 with an effective liquid capacity of 400 e and other aeration tank 2 with an effective liquid capacity of 200 e were used in tandem. Before the water treated in the aeration tank 1 entered the aeration tank 2, a certain amount of methanol was added to the treated water. The sludge was not returned to the tanks. The residence times of the treated water in the aeration tanks 1 and 2 were 4 hours and 2 hours, respectively. The data in Table 3 show that both the tank 1 and the tank 2 satisfy our requirements and that nitrogen and phosphorus were removed together with the BOD substances from the waste water by the process.

Table 3 Composition of the Temp. Aeration tank 1 Run original water (mg/l) Microbial Microbial Composition of the treated A * No. ( C) concent- concentration x 100 water (mg/l) (1/D) TOC BOD N P ration (TOC) (%) TOC BOD N P 1 98 157 18.7 3.9 21 127 129 33 30 5.9 1.5 3.1 2 122 195 21.2 4.6 23 138 113 13 12 6.6 1.1 4.7 3 88 141 19.4 3.1 20 138 156 14 13 8.3 1.4 3.2 4 122 190 18.2 4.1 10 108 89 22.5 20 9.5 1.7 5.6 Table 4 Run Aeration tank 2 TOC ** Microbial Microbial *** Composition of the treated water (mg/l) A * No. at inlet concentration concentration x100 (1/D) (mg/l) (mg/l) (TOC) (%) TOC BOD N P 1 75 160 114 25 22.5 1.9 0.3 3.7 2 65 156 90 15 13 2.2 0.1 3.8 3 58 168 127 15 13 4.0 0.2 3.1 4 66 162 98 20 18 4.2 0.2 3.4 Remarks: * A = (Amount of the treated TOC)/(Microbial weight)/(Day) ** The difference between TOC at the inlet of the aeration tank 2 and TOC of the water treated in the aeration tank 1 corresPonds to TOC of the added methanol.

*** The value of (Microbial concentration)/TOC) in the aeration tank 2 was calcu lated by taking as TOC the sum of TOC at the inlet of the aeration tank 1 and TOC of the added methanol.

Example 4 In Fig. 3, the waste water introduced into a first precipitation tank 100 through a pipe 1 flows through aeration tanks 101,102,103 and 104 in the written order and is aerated with air bubbles formed by the aid of an air-dispersing plate fitted to each tank at the bottom thereof.

In a floating tank 105, microbial cells contained in the waste water are floated with air bubbles formed by the aid of a porcelain sintered plate with an opening size of 10 ym installed over the entire bottom surface of the tank. The microbial cells concentrated by flotation overflow and are immediately returned to the neighboring first aeration tank 101.

The water in the lower part of the flotation tank having a lower microbial concentration is carried through a pipe 3 into a final precipitation tank 106 where the water is finished to a clear treated water which is then discharged through a pipe 4.

An excess sludge separated in the final precipitation tank is discharged from the bottom thereof through a pipe 5.

The original waste water was introduced at a rate of 120 m3/D into the aeration tanks and contained 150 ppm of suspended substances, 150 ppm of TOC, 185 pom of BOD, 120 ppm of COD, 20 ppm of total nitrogen and 5 ppm of phosphorus (as POJ3.

The capacity of the aeration tanks 101, 102 and 103 was 3.5 m3 and that of the tank 104 was 2m3. The flotation tank 105 had a cross sectional area of 0.9m x 0.9m, a height of the liquid of 2.5m and a height of the foamed surface of 2.7m. The amount of the air blown into the tank was 1 m3/hour. Air bubbles having a diameter of 0.1 - 0.5 mm were distributed evenly and fusion of the bubbles into larger ones was not at all observed.

As a result of the treatment, the microbial concentration in the aeration tanks was 280 ppm and BOD at the inlet of the flotation tank was 14 ppm. The treated water at the outlet of the flotation tank contained 125 ppm of suspended substances (chiefly excess waste sludge), 10 ppm of BOD, 10 ppm of COD, 4 ppm of total nitrogen and 2.5 ppm of phosphorus (as Pro4~3).

In order to achieve the same rate of BOD removal by the conventional method wherein the aeration tanks of the same shape and size as described above were used and the microbial concentration was maintained at 2800 ppm, the tanks 101, 102 and 103 each required a liquid volume of 7-8 m3 and the tank 104 4-4.5 m3. Furthermore, a result of investigating the quality of the treated water at the outlet of the final precipitation tank showed that both BOD and COD were 10 ppm as in the foregoing example but total nitrogen and phosphorus (as Pro4~3) contained in an amount of 18 ppm and 4.5 ppm, respectively, were scarcely removed.

(The above-mentioned analytical values of BOD and the like were obtained from the liquid from which suspended substances had been removed by filtration.) Example 5 In Fig. 4, the original waste water introduced into a first precipitation tank 100 through a pipe 1 passed through an ejector 107, a pipe 2 and then branch pipes 21-25 and entered a complete mixing type aeration tank 101 where the waste water was aerated. The aerated water then passed through pipes 61-65 and then a pipe 6 and entered a pressure aeration tank 102 where the water was aerated with oxygen-enriched air having an oxygen concentration of 50% at 5 atm (blowing pressure at the bottom of the tank). The aerated water was then introduced through a pipe 7 into a flotation tank 105 kept at atmospheric pressure where the dissolved gas formed bubbles around the microbial cells as nuclei and floated along with the microbial cells to form a condensed phase of the microbial cells.

The condensed phase was recirculated by suction through a pipe 8 to the original waste water flowing into the aeration tank by means of the ejector 107.

On the other hand, water in the lower part of the tank 105 having a lower microbial concentration was drawn out through a pipe 3 and introduced into a final precipitation tank 106 from which the separated excess waste sludge was discharged through a pipe 5 and a clear treated water through a pipe 4.

The original waste water was introduced at a rate of 240 m3/D into the aeration tank and contained 180 ppm of suspended substances, 180 ppm of TOC,220 cylindrical tank with a capacity of 2.6 m3, a diameter of 1m and a height of 2.6 m while the flotation tank 105 was a cylindrical tank with a diameter of 2 m and a height of 2.6 m.

A floating scum in the flotation tank was immediately collected and allowed to flow into the pipe 8 by means of a rotary scraper.

The pressure aeration tank 102 had a porous air-dispersing plate at the bottom thereof.

The gas collected at the top of the tank was recycled by a circulating compressor. To make up the consumed gas, 0.6 m3/h of air and 1 m3/h of pure oxygen were supplied thereby maintaining the oxygen concentration in the gas at the inlet of the tank at 50%.

As a result of the treatment, the microbial concentration in the aeration tank was 350 ppm, while BOD of the treated water at the outlet of the aeration tank 101 was 15 ppm and that of the treated water at the outlet of the pressure aeration tank 102 was 11 ppm. The treated water at the outlet of the flotation tank 105 contained 150 ppm of suspended substances, 10 ppm of BOD, 14 ppm of COD, 6 ppm of total nitrogen and 3.5 ppm of phosphorus (as PO4-3).

Example 6 The waste water No. 1 in Example 1 was treated with bacteria without returning the microbial cells to the tank. To the treated water from which the majority of the microbial cells had been removed was added methanol in an amount of 80 g per cubic meter of the waste water. The water was then continuously supplied to other aeration tanks so that its residence time became 5 hours. As a result of this treatment, the microbial concentration of the water was decreased to 30 ppm, TOC to 21 mg/e, total nitrogen to 1.5 mg/e and phosphorus (as Pro4~3) to 6 ppm. In this case, the microbial concentrationn/TOC ratio was 56.6 So and the TOC treated/microbial weight/day was 5.12.

Claims (9)

WHAT WE CLAIM IS:

1. A process for the treatment of a nitrogen containing waste water, comprising passing nitrogen containing waste water through one or more aeration tanks maintained under aerobic conditions such that the residence time of microbes in said tank or tanks is sufficiently short to wash out slowly propagating microbes thereby achieving a population of rapidly propagating microbes in said tank or tanks; and further regulating the conditions in said tank or tanks such that the microbial concentration in one or more aeration tanks is kept at 20-200 weight % based on the total organic carbon in said waste water and the ratio of said weight of total organic carbon treated per day/microbial weight in said one or more tanks is adjusted to 3-25 whereby in growing, the microbes therein assimulate nitrogen and phosphorus contained in the waste in addition to B.O.D. substances.

2. A process according to Claim 1, wherein nitrogen and phosphorus are removed by transferring same from said waste water to said microbial cells as microbial constituents.

3. A process according to Claim 1 or Claim 2, wherein an organic carbon source is previously added to said waste water to be introduced into said one or more aeration tanks to increase the rate of removal of nitrogen and phosphorus.

4. A process according to any preceding claim, wherein a floating separated phase of said microbial cells is formed in a part of said one or more aeration tanks and a diluted phase of said microbial cells in the lower portion of said part of said aeration tanks is taken out of said aeration tanks.

5. A process according to any preceding claim, wherein microbial cells separated from the treated water at the outlet of said one or more aeration tanks are not returned to said aeration tanks.

6. A process according to any one of Claims 1 to 4, wherein said microbial cells in said treated water at the outlet of said one or more aeration tanks are immediately concentrated by flotation and returned to said aeration tanks.

7. A process according to Claim 6, wherein the partial pressure of oxygen in bubbles used for concentrating said microbial cells by flotation is made higher than the partial pressure of oxygen in the air.

8. A process for the treatment of waste water substantially as hereinbefore described with reference to the Examples.

9. Treated water whenever having been subject to a process according to any preceding claim.