CH633238A5 - WASTEWATER TREATMENT. - Google Patents

Description

This invention relates to a method of treating wastewater with an adsorbing agent and a rotary biological contactor.

Several methods are available for the biological treatment of municipal and industrial wastewater. The commonly used methods are the activated sludge process using air and pure oxygen and the trickle filter (bacterial filter). More recently, the rotary biological contactor, for example like that described in the US patent. N ° 3557954, has found an application in which a series of discs carried by a common shaft rotate in the mass of waste water, alternately by immersing in the liquid a biological film formed on the disc and exposing the film to air at above the liquid to obtain the oxygen intended to maintain a biological film on the surface of the disc to carry out the biological oxidation of the waste water. Reduced energy consumption for oxygen transfer is obtained in such a system which can be designed for various applications ranging from the elimination of ordinary carbon-based BODs (BODs = biological oxygen demand at 5 d) to biological nitrification .

The general concept of passing wastewater against the flow of continuously moving activated carbon to remove organic matter from the water is described in the U.S. Patent. No 3763040.

More recently, it has been suggested to add powdered activated carbon to the process using biological sludge to improve the characteristics of the activated sludge system, as described in the U.S. Patent. N ° 3904518. The claimed benefits are improved stability, better separation of liquid and solids and a higher degree of treatment. The process described in the U.S. patent. No. 3904518 is a single-stage suspension slurry contact system in which the carbon is brought into contact only with the equilibrium concentration of adsorbable organic matter in the wastewater in the presence of activated sludge. For more effective contact of carbon with adsorbable substances, it is advisable to make a counter-current contact with water to expose the carbon in portions or continuously to a higher concentration of adsorbable substances in the wastewater.

Japanese Patent Publication No. 139169/76 describes a method for the biological treatment of wastewater, comprising contacting the wastewater with activated carbon attached to the surface of a fixed bed or disc.

As more and more wastewater treatment requirements have developed, it has become necessary to consume larger amounts of energy. For example, when the oxidation of reduced nitrogenous materials is desired, the oxygen requirements are markedly increased and require greater energy consumption. In addition, the longer residence times of the cells lead to a greater transformation of the cell mass into CO 2 and H 2 O, requiring an even greater transfer of oxygen and energy consumption. In addition, more stringent treatment requirements may make it necessary to add a tertiary treatment to remove residual organic substances as well as any toxic organic substances present in the wastewater.

The invention relates to a method for treating wastewater comprising the following steps which consist of:

a) putting the wastewater forming a current in a treatment zone in contact with a series of surfaces alternately immersed in the liquid and in the gas, generally the air, which is above the liquid, for a period d 'at least' A h by adding to the contactor an adsorbent capable of adsorbing impurities in the liquid, and b) removing the suspended solids in the contact zone at a speed equivalent to the speed at which the solids accumulate in the contactor .

The preferred adsorbent is powdered activated carbon, although the use of grain activated carbon or the like is contemplated. adsorbents such as fuller's earth, fly ash, etc. A preferred range of adsorbent concentration is 25 to 5000 mg / l.

The invention will now be described with reference to the accompanying drawings in which:

fig. 1 and 2 show the basic process, and Figs. 3,4 and 5 illustrate detailed embodiments making it possible to effect countercurrent contact of the suspension of biomass and of coal with the liquid phase.

Consider fig. 1; the raw sewage entering 10 undergoes a preliminary treatment consisting of sieving and elimination of the waste in a suitable device 12 and passes to a possible primary treatment stage normally consisting of a simple sedimentation at 14 where the floating solids in suspension and the decanted solids can be removed, as via line 16, and discharged by conventional means 18. The decanted wastewater then passes through a line 20 to a tank 22 in which a series of discs 24 are mounted on a common shaft 25 and are rotated by suitable means to be alternately in the liquid of the basin and in the gas space above the liquid. Normally, the flow in the basin is in a direction parallel to the shaft and perpendicular to the plane of the rotating discs. In some cases, the flow is perpendicular to the shaft and parallel to the rotating discs.

Activated charcoal powder is added to the mixture in the tank as in 26 for example, to allow the adsorption of biodegradable and non-biodegradable substances contained in the wastewater. The rate of addition of carbon varies according to the degree of treatment required as well as according to the composition and the characteristics of the adsorbable substance contained in the wastewater. Normally 10 to 500 mg / l of treated wastewater is required, but for some industrial wastewater, as large as 5000 mg / l of wastewater may be required.

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The film formed on the surface of the disc consists of a mixture of biomass and adsorbent (activated carbon). It has surprisingly been observed that the film has the unusual property of accumulating the adsorbent in its structure, thereby entraining the adsorbent on the surface of the disc with the biomass. The mass ratio of biological organisms to activated carbon will essentially depend on the residence time of soluble materials in the system and the composition of the wastewater that is introduced into the system. It will be seen from the following examples that the adsorbent not only helps to remove the adsorbable matter from the waste water, but also has the effect of substantially increasing the residence time of the adsorbable and slowly biodegradable substances, and also has the effect of significantly increasing the overall residence time of the solids in the system.

This effect is interesting when you want nitrification (biological oxidation of ammoniacal nitrogen), because it allows the accumulation of slow-growing nitrifying bacteria.

Another unexpected benefit from the addition of activated carbon to the contact area of the rotary biological contactor is the tendency of the carbon to reduce foaming. In many applications, the rotational speed of the contactor is limited, among other things, by the foaming tendency that occurs in the contactor. Foaming may occur due to the presence in the wastewater of surface-active substances or due to the presence of exogenous surface-active organic materials produced by the organisms themselves. Like most, if not all, of these materials are adsorbed on activated charcoal, foaming is reduced or eliminated, allowing higher rotational speeds and higher concomitant oxygen transfer.

Another surprising result of the addition of activated carbon to the rotary biological contactor is the effect of the addition of carbon on the overall mass transfer rate of oxygen. Measurements of the overall oxygen mass transfer rate (KLa) in the contactor show that it is 25% higher than that obtained in the normal biological contactor. The improved mass transfer rate has a double effect on the characteristics of the contactor. Firstly, a higher transfer rate allows slower rotation speeds and therefore lower energy costs; second, the oxygen profile through the film on the disc is improved by providing deeper penetration into the film with a greater resulting active aerobic biological mass. This effect is particularly important in nitrifying systems where nitrifying organisms must be exposed to dissolved oxygen concentrations greater than 0.5 mg / 1 to be effective.

Although charcoal can be added at various points in the basin, there are advantages to adding charcoal at various points along the length of the basin. For example, as shown in fig. 3, if the carbon is added uniformly along the length of the basin and the solids are removed at 28.28-1.28-2 and 28-N along the length, the contact by the carbon consists essentially of a series of single-stage contactors. The rate of addition at each point can vary to control the coal load so as to meet the processing requirements of each successive stage.

As is known, in adsorption systems, it is indicated to charge the adsorbent more effectively with the adsorbed product by moving the adsorbent against the current of the adsorbed product. In suspended systems, this is generally done by providing for a separation step after each contact stage. The solids separated on each stage are moved upstream, against the flow of the liquid stream. Fig. 4 shows how this can be done in a rotary biological contactor, by introducing the adsorbent (activated carbon) at the downstream end and by providing a means making it possible to move the carbon upstream, so that the particles separating at the bottom of the contactor tend to move against the current along the bottom of the pool providing any number of equivalent counter-current stages, thereby providing maximum carbon adsorption efficiency. Various means can be used to effect the counter-current movement of the solids. For example, the discs can be mounted in a chamber having a sloping bottom, so that the suspended particles tend to move upstream as soon as they are moved to the lower part of the chamber.

For systems in which the flow is introduced perpendicular to the shaft and parallel to the discs, a series of parallel shafts, rotating so that the direction of rotation of the disc in the liquid is against the flow , will produce a counter-current movement of solids in the system.

Or the periphery of each disc 24 may be provided with blades 30, directing the net flow of solids against the current, as illustrated in FIG. 5. A possible relationship exists between the angle of attack of the blades, the effective speed of the counter-current movement and the energy consumption necessary to rotate the disc.

In another embodiment, the disc can be designed so that its periphery takes the form of a screw with a small slope angle so that, when the disc rotates, the solids are pushed upstream. The downstream face can remain flat or preferably take the corresponding shape of the upstream face.

Other methods of obtaining counter-current flow will be evident by studying the following examples. The basic principle implied in the above is that the added adsorbing agent moves against the flow of most of the liquid medium, here carrying out higher adsorption charges.

In practice, it is never possible to completely separate the suspended solids in the chamber in which the discs rotate due to the turbulence created by the rotating disc. To obtain an improved separation, there is optionally provided a rest chamber at the downstream end of the contactor as in 32, fig. 2. The rest chamber has a strongly sloping bottom 34 which directs the solids upstream in the region where the discs can direct them even further upstream. Alternatively, a conventional collecting mechanism can be provided to transport the solids materially to the region of the discs for upstream transfer against the current.

Similarly, a second clarifier 36 can be provided to better clarify the liquid stream. The material decanted in this clarifier as at 38, can be directed to the inlet end of the contact chamber as via line 39, fig. 1, or it can be mixed with the underflow of the contactor at 42 for another treatment, for example regeneration of the adsorbent carbon or rejection of the solids.

Virgin coal can be added to the system to finish the effluent in the event that spent coal is regenerated.

The spent coal at 42, as well as the associated biomass, can optionally be thickened and rejected by conventional means 44, or it can be regenerated as at 46 and returned to the contactor for reintroduction into the treatment step at various points in the process. contactor, via line 48.

The treated wastewater 40 can then be further treated by a conventional sand filter 50 and disinfected and discharged into the receiving stream. The suspended solids captured in the sand filter can be washed back and returned upstream to any of the upstream points of the system shown in fig. 1. Other configurations are possible. For example, biological nitrification and biological denitrification can be obtained by operating a first stage under aerobic conditions to carry out the oxidation of ammonia and using a nitrogen elimination stage under anaerobic conditions denitrification of nitrogen from nitrites and nitrates to elemental nitrogen.

The following examples are given to illustrate the process.

Example 1:

The laboratory biological rotary switch consists of 52 polyethylene discs 25 cm in diameter and 6.3 mm

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633238

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thick suspended from a horizontal shaft and placed in a 201 hemispherical tank equipped with a conical decanter near its outlet end. When the discs spin, about 40% of their area is immersed in the liquid. A primary effluent from domestic sewer is metered at one end of the tank. The mixed liquid comes out at the opposite end in a clarifier equipped with a slow-moving blade. Two recycling lines are used to return the thickened decanted products from the clarifier and the conical decanter to the inlet end of the tank at speeds of 40 ml / min and 12 ml / min respectively.

The system is initially operated on the primary effluent for 19 d to accumulate layers of biomass on the discs. During the next 8 d, data on samples continuously taken are obtained in the system. On the 28th day of operation, 300 g of activated charcoal are added near the entrance to the tank. 11 of mixed liquor are withdrawn from the cone daily for the next 38 d and 15.0 g of dry activated charcoal are added to the tank. The mixed liquor containing biomass and charcoal is filtered. We obtain dry filter cake weights between 0.3 and 26.4 g. A feed rate of approximately 100 ml / min is maintained throughout the experiment. Rotation speeds of 6 and 10 rpm are used to maintain the dissolved oxygen range of 0.5 to 5.0 and 0.6 to 6.0 mg / l in the charcoal and charcoal systems. The average characteristics of the currents are given in Table I.

It will be noted that the system operating with carbon exhibits a better reduction in COD and BODs as well as better nitrification than the system operating without carbon.

Table I

Without coal

With charcoal

Charge

Effluent

Charge

Effluent

COD (mg / 1)

160

72

186

36

% COD reduction

-

54

-

81

BOD (mg / 1)

64

16

34

2

% BOD reduction

-

75

-

94

Total nitrogen Kjeldahl

(mg / 1)

43.3

31.9

50.5

14.4

% total reduction

nitrogen Kjeldahl

-

26

-

71

Suspended solids

(mg / 1)

32

24

38

9

Hanging ash

(mg / 1)

9

3

17

2

Ammonia nitrogen (mg / 1)

-

-

39.8

10.5

Nitrite in nitrogen (mg / 1)

-

1

0.1

1.5

Nitrates in nitrogen (mg / 1)

-

1

1.5

27.8

Total phosphorus (mg / 1)

9.4

8.7

12.5

11.5

pH

7.3

7.3

7.2

6.6

Example 2

The apparatus described in Example 1 is used with a feed rate of approximately 200 ml / min for 30 d. No comparative data are available for the coal-free system. The values for the chemical oxygen demand, the biological oxygen demand and the reductions in the Kjeldahl nitrogen content of these carbon-free systems with a faster feed rate would not exceed the figures obtained for the flow rate of 100 ml / min, indicated in Example 1. The weights of daily dry filter cakes vary from 4.08 to 26.14 g and are on average 11.50 g / d. The average data for the experiment at 200 ml / min are given in Table II.

Table II

With charcoal

Charge

Effluent

COD (mg / 1)

215

49

% COD reduction

-

77

BODs (mg / 1)

60

7

% BOD reduction

-

88

Total nitrogen Kjeldahl (mg / 1)

48.9

12.7

% total nitrogen reduction Kjeldahl

74

Suspended solids (mg / 1)

45

7

Ash in suspension (mg / 1)

7

2

Ammonia nitrogen (mg / 1)

40.2

5.2

Nitrite in nitrogen (mg / 1)

0.1

1.1

Nitrates in nitrogen (mg / 1)

1

23.2

Total phosphorus (mg / 1)

14.6

13.1

pH

6.9

6.9

It will be noted that, despite the doubling of the feed rate in the system, comparable reductions in BOD5 and COD are obtained and nitrification of the ammoniacal nitrogen comparable to the results of Example 1.

Example 3

The apparatus described in Example 1 is modified to obtain a countercurrent flow of the activated carbon and the primary wastewater effluent. The conical decanter is moved to the inlet of the tank. The recycling suspensions for the clarifier and the conical settling tank plus the virgin coal are introduced near the outlet of the tank. The spent coal suspension is withdrawn daily from the conical settling tank. Average feed rates of 111 ml of primary effluent per minute and 9 mg of activated charcoal per liter of effluent are used. An average of 16.1 g of dry solids are removed daily. The average analytical data are given in Table III.

Table III

Charge

Effluent

COD (mg / 1)

227

43

% COD reduction

-

81

BODs (mg / 1)

60

7

% BOD reduction

-

88

Total nitrogen Kjeldahl (mg / 1)

48.9

12.7

% total nitrogen reduction Kjeldahl

-

74

Suspended solids (mg / 1)

28

6

Ash in suspension (mg / 1)

2

1

Ammonia nitrogen (mg / 1)

47.4

11.8

Nitrite in nitrogen (mg / 1)

0.1

0.5

Nitrates in nitrogen (mg / 1)

1.5

17.0

Total phosphorus (mg / 1)

16.0

14.7

pH

6.5

6.3

Example 4

To illustrate the effect of the addition of activated carbon to the contactor on the overall mass transfer rate of oxygen, tests are carried out according to the methods described by Eckenfelder and Ford ["Experimental Procedures for Process Design", "Water Pollution Control ", Pemberton Press, Jenkins Publishing Co., Austin, Tex. (1970)] to determine a, the ratio of the mass transfer rate of oxygen in the suspension of coal and biomass and in biomass alone. The table below gives the observed values:

K> has wastewater

Alpha (a) =

Tap water

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633238

Waste

System

Biological system

organic plus charcoal

Pharmaceuticals

-

2.43

Domestic

0.82

1.27

Municipal / industrial

0.81

1.05

The addition of carbon has the effect of increasing in relation to the biological system and allows a significant oxygen transfer the rate of transfer of mass of total oxygen (KLa) by 10 equivalent to significantly reduced peripheral speeds.

2 sheets of drawings

Claims (6)

633238 1. Process for treating wastewater, characterized by the following stages consisting of: a) contacting the wastewater forming a current in a biological contactor which comprises a series of surfaces alternately immersed in the liquid and the gas above the liquid, for a period of at least Vi h, by adding to the current an adsorbent capable of adsorbing impurities from the liquid, and b) removing from the contactor the suspended solids accumulated at a speed equivalent to the speed at which the solids accumulate in the contactor. 2. Method according to claim 1, characterized in that the adsorbent is powdered activated carbon. 2 3. Method according to either of claims 1 or 2, characterized in that the adsorbent is added to the downstream end of the biological contactor and that it is displaced by the surfaces of the contactor against the current of the mass of the liquid flow. 4. Method according to one of claims 1,2 or 3, characterized in that the amount of adsorbent added is between 25 and 5000 mg / 1. 5. Method according to one of claims 1 to 4, characterized in that the solids are removed from the wastewater before the contact step. 6. Method according to one of claims 1 to 5, characterized in that the contact surfaces consist of a series of discs mounted on a common shaft which rotate so as to immerse the disc and its associated biological film alternately in the liquid and in the gas above the liquid.