CH634537A5 - PURIFICATION OF WASTE WATER. - Google Patents

Description

This invention relates to a process for treating waste water to remove impurities and perform nitrification with unprecedented efficiency, using a biological aeration system and an aqueous suspension of oxidized biomass and activated carbon. .

The wastewater created by modern, highly populated industrial societies demands methods of treatment of ever increasing complexity. Particularly demanding are the requirements for removing traces of toxic substances and nutrients from aquatic plants. Recently, methods have been discovered using a combination of biological organisms and a finely divided solid sorbent, methods which provide effective treatment.

The benefit of the solid particulate sorbent is generally attributed to the sorption of substances toxic to microorganisms, to the improved sedimentation of the suspended solids in the clarification steps, and to the maintenance of a high solids content in the reactor. It was generally thought that the sorbent should have a large specific surface, for example from 100 to 2000 m2 / g, and that new sorbent should be applied to the wastewater continuously in a fixed ratio. Commercial sorbents such as activated charcoal, diatomaceous earth, etc., which meet the assumed requirements, increase the cost prices. Methods for recovering spent sorbents have therefore been the goal of a large number of studies.

Numerous processes for the regeneration of activated carbons have been described, which use methods similar to those used in the initial manufacture, that is to say the treatment at high temperature of the solid materials in a gaseous atmosphere of steam, carbon dioxide, carbon monoxide or oxygen. Such methods are designed to render the initial characteristics of the sorbent.

Another recovery process is wet oxidation as described in U.S. patents. Nos 3386922 and 3442798. This process has the advantage of being applicable to an aqueous suspension of the spent sorbent, thereby avoiding expensive filtration and drying. In these patents, the wet oxidation is determined by the temperature, the residence time and the ratio of oxygen to the adsorbed solid matter, so that the organic matter sorbed is practically completely oxidized, a minimum of sorbent is lost , and the sorbent recovered has characteristics close to those of the initial product.

The use of powdered activated carbon in the biological treatment of wastewater to facilitate the removal of organic impurities and / or nitrification is described in U.S. patents. Nos. 3957632,3904518,3980556,3803029 and 3876536, as well as in Canadian Patent No. 1006993.

The present invention provides a method for treating raw wastewater, which consists of:

a) putting said wastewater in contact with a mass of diversified aerobic microorganisms and with powdered activated carbon in the presence of dissolved oxygen maintained at more than

1 part per million (ppm) until substantial reductions in chemical oxygen demand (COD) and biological oxygen demand (BOD) occur by carbon adsorption and biological oxidation;

b) separating the mixture of coal and biomass produced in step a;

c) recycling a portion, preferably between 50 and 98%, of the mixture of coal and biomass obtained in step b to treat other portions of raw wastewater;

d) subjecting the rest of the mixture of coal and biomass obtained in step b to partial wet oxidation at a temperature between 200 and 260 ° C and for a residence time of 30 to 60 min, while limiting the entry of oxygen-containing gases so that the concentration of oxygen in the exhaust gases is less than 7% by volume;

e) recycling the suspension of wet oxidized coal and biomass obtained in step d, for the treatment of additional raw wastewater while adding virgin activated carbon only in the amount necessary to compensate for the process losses;

f) continuously repeating steps a through e;

g) removing the resulting effluent having a chemical oxygen demand, a biological oxygen demand and a substantially reduced ammoniacal nitrogen content.

It has now been found that less than complete wet air oxidation of the sorbed and associated organic material gives a suspended product which, when returned to an aerobic biophysical treatment process, improves the process characteristics over to those that can be obtained with virgin coal or with coal that has been regenerated to regain its original characteristics. We have found that we can obtain a markedly accelerated rate of transformation of ammoniacal nitrogen into nitrates and nitrites, an oxidation process commonly called nitrification, when we refer to the biophysical system a suspension of recycled carbon which is less rigorously oxidized and which contains a certain quantity of non-adsorbed organic substances soluble and non-oxidized and a charcoal whose characteristics are different from those of virgin charcoal.

When submitting a suspension of spent carbon sorbent and excess biological material, which has been removed from a treatment of

5

10

15

20

25

30

35

40

45

50

55

60

65

3

634,537

combined aerobic physico-biological sorption of waste water, with wet air oxidation at temperatures between 200 and 260 ° C for a residence time of 30 to 60 min and the air flow introduced into the oxidation process is limited so that the oxygen concentration of the outlet gases does not exceed 7% by volume, the product suspension acquires properties which are unexpectedly beneficial to the biophysical treatment of wastewater.

Wet oxidation under the previous conditions The rest of the material is mineral. 20

Another characteristic of the wet oxidized suspension of spent carbon sorbent obtained by the process of the present invention is the presence of low molecular weight oxygenated organic compounds in solution. Alcohols, aldehydes, ketones, acids and C1-C4 esters have been identified. The concentrations are such that they provide 3000 to 15,000 mg / 1 of COD and 2,000 to 10,000 mg / 1 of BOD. It has been found that the soluble organic materials constitute an exceptional biological substrate. Extensive experiments have shown conclusively that the use of this wet oxidized slurry in place of industrial activated charcoal results in significant improvements over prior art procedures, particularly in the degree and the nitrification speed.

It has been found that the formation of the desired wet oxidized suspension can be effectively achieved by introducing 35

first a certain quantity of commercial activated carbon in the reactor of a biological system, then by continuously operating the biological treatment system for waste water and by starting the wet, periodic and controlled oxidation of the suspensions withdrawn from the system. After a few days of operation, a wet oxidized suspension is obtained having the optimum properties. Then the desired wastewater treatment operation is maintained by limiting the addition of virgin activated carbon to the system to the amount necessary to compensate for normal losses in the process. 45

The raw wastewater which is treated by the process of the invention can come from household, municipal or industrial sources, including sewers and industrial effluents or their mixtures. The process is particularly effective on wastewater which contains substantial amounts of ammoniacal nitrogen.

The operating mode is further illustrated with reference to the accompanying drawings. Wastewater 1 continuously flowing enters a mixing chamber 2 where mixing with the biomass and the sorbent takes place. Virgin activated carbon is introduced at 9. The mixture flows to reactor 3 where dissolved oxygen is maintained at more than 1 ppm by any known device for contacting a liquid with a gas containing oxygen, such as sprinklers, stationary film reactors, and side-stream pressure vessels. Alternate aerobic-anaerobic regions are also possible in the reactor. From the reactor, the mixture passes through a clarifier 4 where the suspended solids settle, the dissolved oxygen rate drops to 0-2 ppm and the biological transformation of nitrates and nitrites into elemental nitrogen (denitrification)

may occur. The purified and clarified water 5 leaves the device 4 in the form of an overflow. A suspension of sorbent and biomass 6 is withdrawn from the clarifier and, subsequently, a portion of the suspension, preferably from 50 to 98%, is returned to mixer 2 and the rest is treated in the oxidation reactor 10 by wet.

defines certain characteristics of the suspension. The carbonaceous sorbent in suspension has a pore diameter / specific surface area distribution that is totally different from that of typical activated activated carbon, the wet oxidized product having significantly less pore surface area of less than 30 Å in diameter and significantly more surface area in pores from 30 to 600 Å in diameter.

Wet oxidized solids also differ from commercial activated carbon in their elemental composition. These differences are given below in the table:

Depending on the needs of the treatment, the reactor 10 can be used intermittently. The excess of inert mineral ash 8 is withdrawn from the wet oxidation reactor and the wet oxidized suspension 7 is returned to the mixer 2.

The process of this invention can be started in several ways. For example, you can start a wastewater flow and start the wastewater oxygenation device. The wastewater can contain sufficient populations of microorganisms for the biological action to begin, or else reactor 3 can be seeded with sewage, or the like. In either case, the population of microorganisms will grow and eventually the mixture in the reactor will contain a mass of microorganisms, typically in the range of 1000 to 5000 mg / 1, measured by the material volatile insoluble. A sufficient quantity of activated carbon is then added so that there is from 500 to 50,000 mg / l, preferably approximately 3000 to 15,000 mg / l, of volatile activated carbon. The term volatile refers to the content of carbon derived from organic sources; commercially available activated carbons contain for the most part 5 to 30% of ash (mineral matter). Or you can introduce activated carbon at the start and then let the biomass accumulate. In the presence of activated carbon, a higher concentration of biomass can be obtained, for example from 5,000 to 10,000 mg / l.

In any case, once the desired degree of concentration of activated carbon and biomass has been reached, the process of forming a preferred sorbent suspension can be started. A portion of the thickened suspension withdrawn from the clarifier 4 is diverted to send it to the wet oxidation reactor 10. The reaction temperature, the residence time and the weight ratio of oxygen in the compressed gas to the solids in the suspension will determine the yield of active oxygenated sorbent. The volumetric flow rate of the thickened suspension diverted to wet oxidation, the solids concentration of the thickened suspension, the yield of wet oxidation and the rate of ash removal 8 will determine the speed at which the active oxygen-containing sorbent solids are returned to the wastewater treatment reactor 3. The initial amount of activated carbon introduced into the reactor 3 and any additional activated carbon 9 that is added determines the amount of total activated carbon introduced into the system .

In the aeration system, the aeration time of the mixed liquor varies between 0.5 and 24 h, and the residence time of the solid materials varies from 2 to 50 d. The temperature is room temperature (5-30 ° C).

The following examples will better illustrate the invention.

Example 1:

A pilot plant arranged according to the diagram in the drawing is set up to determine the characteristics of an organic system

% in weight

VS

H

Organic N

O organic

Charcoal

60-90

0.5-1

0.5-1

0.5-1

Wet oxidized materials

20-60

1-2

1-3

5-20

55

60

634537

4

physics in the treatment of municipal wastewater. During a period of 33 d called period A, only virgin activated carbon is added to the mixed liquor in the wastewater treatment reactor. During a subsequent period of 46 d, called period B, the thickened suspension is separated, it is oxidized wet and the suspension of active oxygenated sorbent is returned to the wastewater treatment reactor. The wet oxidation is carried out at 200 ° C. with a residence time of 60 min and an oxygen content of less than 7% by volume in the effluent gas. Compressed air is the source of oxygen. The biomass to sorbent weight ratio is 0.4 to 1.0.

Table 1 gives details of operation and results. Although the initial wastewater is more charged during period B due to an increase in the climatic temperature, the results with respect to effluent BOD and COD are as high in period B as in period A The elimination of the ammonia by biological nitrification could not be induced during the period A. On the contrary, one obtains an elimination of approximately 84% of the ammonia in the period B. In this case, one uses in the bioprocessing system carbon regenerated by wet oxidation; the soluble fraction and / or the regenerated carbon stimulate the biological nitrification agents.

Table 1

Period A

Period B

Bio-treatment reactor temperature (° C)

15.5

17.5

Solid residence time (d)

> 30

30

Aeration time of the mixed liquor (h)

4.5

4.5

Ventilation volume (1)

72

48

Volatile activated carbon in the aeration zone (g): interval

430-790

530-720

average

608

643

Volatile activated carbon addition rate (mg / ml)

124

107

Starting BOD (mg / ml)

58

105

BOD of the effluent (mg / ml)

3

3

Starting COD (mg / ml)

235

314

COD of the effluent (mg / ml)

27

32

N starting ammonia (mg / ml)

22.9

23.1

N ammonia of the effluent (mg / ml)

23.3

3.8

The rate of addition of volatile activated carbon is a measure of the mass of fresh carbon (virgin coal + regenerated carbon) added per unit volume of wastewater entering the system. 35

Example 2:

The possibilities for treating wastewater comprising 60% of household sources and 40% of industrial sources are determined by a pilot plant and a normal installation of the type illustrated in the drawing.

sin. Various wet oxidation temperatures are used to regenerate the active sorbent suspension from the mixtures of spent sorbent and biomass.

Table 2 gives details of the operating parameters and results. This example shows the regular high characteristics which are obtained with the suspension of active oxygenated sorbent regenerated at wet oxidation temperatures of between 200 and 250 ° C.

Table 2

Equipment size

Pilot

Normal

Pilot

Bioprocessing temperature (° C)

17-20

20-26

12-18

Solid residence time (d)

16.9

35

13

Aeration time of the mixed liquor (h)

4.4-8.0

8-12

8

Concentration of volatile activated carbon in the reactor

bioprocessing (mg / ml)

5870

3100

5200

Addition rate of volatile activated carbon (mg / ml)

72

52

97

Oxidation temperature (° Ç)

215

232

250

Starting BOD (mg / ml)

268

200

240

BOD of the effluent (mg / ml)

0

3

1

Starting COD (mg / ml)

680

455

690

Effluent COD (mg / ml)

76

65

60

N starting ammonia (mg / ml)

17.9

17.1

18.6

N ammonia of the effluent (mg / ml)

2.0

1.2

2.0

Example 3:

The sorbent solid materials are collected from the wet oxidized suspensions of Example 2, by filtration, washing and drying. Chemical analysis, surface analysis and analysis of the relative efficiencies of adsorption of conventional test substances are carried out. Table 3 gives the results of these analyzes.

This example shows that the active oxygenated sorbents formed according to this invention have characteristics which differ considerably from the starting active carbon. The previous examples illustrate the superior operating characteristics of these regenerated sorbents.

634537

Table 3

Period

1

2

3

Virgin coal

Oxidation temperature (° C)

215

232

250

-

Composition

% inorganic

60.77

52.0

71.71

31.20

% organic carbon

23.39

37.16

16.70

64.37

% organic hydrogen

1.54

1.56

1.04

0.85

% organic nitrogen

1.83

2.14

2.51

0.50

% organic sulfur

0.47

0.20

1.45

% organic oxygen

12.47

6.67

7.84

1.63

Specific surface (m2 / g)

97

86

236

419

Relative efficacy of sorbed products

% iodine

12

30

34

100

% methylene blue

35

38

117

100

% erythrosine

15

28

69

100

% molasses color

34

39

85

100

1 drawing sheet

Claims (4)

634537 1. Process for treating raw wastewater, characterized in that: a) said wastewater is brought into contact with a mass of diversified aerobic microorganisms and powdered activated carbon, in the presence of dissolved oxygen maintained at more than 1 part per million, until a substantial reduction in the chemical oxygen demand and biological oxygen demand by adsorption by carbon and by biological oxidation; b) separating the mixture of coal and biomass produced in step a; c) a portion of the mixture of coal and biomass obtained in step b is recycled to treat other portions of raw wastewater; d) the remainder of the mixture of coal and biomass obtained in step b is subjected to partial wet oxidation at a temperature between 200 and 260 ° C. and for a residence time of 30 to 60 min, while limiting the entry of oxygen-containing gases so that the oxygen concentration of the outlet gases is less than 7% by volume; e) the suspension of coal and wet oxidized biomass obtained in step d is recycled for the treatment of additional raw wastewater while adding virgin activated carbon only in the amount necessary to compensate for the losses occurring in the process; f) continuously repeating steps a through e, and g) removing the resulting effluent having a chemical oxygen demand, a biological oxygen demand and a substantially reduced ammoniacal nitrogen content. 2. Method according to claim 1, characterized in that the product of step d is a suspension of an active sorbent agent having a composition of 20 to 60% of carbon, from 1 to 2% of hydrogen, from 1 at 3% organic nitrogen, 5 to 20% organic oxygen, the remainder being mineral matter, in an aqueous phase containing a chemical oxygen demand of 3000 to 15,000 mg / 1 and a biological oxygen demand of 2000 to 10 000 mg / 1. 2 3. Method according to one of claims 1 or 2, characterized in that the concentration of volatile activated carbon in step a is between 500 and 50,000 mg / 1. 4. Method according to one of claims 1,2 or 3, characterized in that from 50 to 98% of the mixture are recycled in step c.