GB2027420A

GB2027420A - Biological Sewage Treatment

Info

Publication number: GB2027420A
Application number: GB7832881A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-08-10
Filing date: 1978-08-10
Publication date: 1980-02-20
Also published as: GB2027420B

Abstract

In a method of treating sewage in the presence of ammonia and/or urea, activated sludge is added to the sewage, and the dissolved oxygen concentration is maintained at from 0.1 to 4.0 mg/l. Phosphates may also be added for the benefit of the bacteria.

Description

SPECIFICATION Sewage Treatment The present invention relates to a biological method of purifying sewage by removal of nitrates and/or nitrites and organic compounds. The method is useful in industry, especially in the chemical industry.

Thus, the present invention consists in a method of purifying sewage, which comprises treating sewage at a dissolved oxygen content of from 0.1 to 4.1 mg/l with an activated sludge and in the presence of ammonia and/or urea such that polluting organic compounds and nitrates and/or nitrites are removed.

Where the sewage to be treated lacks ammonium ions, it is necessary to add ammonia and/or urea, and an amount of from 10 to 1,000 mg/l is suitable. The ammonia and/or urea may be added to the sewage, to the sludge, to both the sewage and the sludge before mixing or to a mixture of sewage and sludge.

Also, it may be desirable to add a biogenic phosphorus compound since this intensifies the redox process. Superphosphate and trisodium phosphate are preferred, and an amount of up to 200 mg/l will generally be suitable.

The presence of dissolved oxygen in the sewage during treatment allows nitrifying and denitrifying bacteria to operate simultaneously and this significantly simplifies the purification process.

When the concentration of dissolved oxygen is less than 0.1 mg/l, nitrification of ammonium nitrogen is retarded although denitrification proceeds actively. Conversely, when the concentration of dissolved oxygen is higher than 4 mg/l, denitrification is retarded and nitrification prevails. Under this latter condition ammonium nitrogen is converted into nitrate nitrogen. The specified range of dissolved oxygen concentration allows both nitrification and denitrification to proceed simultaneously. This is confirmed by the efficient removal of ammonium nitrogen and nitrate nitrogen and by the presence in the activated sludge of viable stage I and stage II nitrifying and denitrifying bacteria.

Microorganisms in the treated sewage use dissolved oxygen and nitrate and nitrite oxygen to oxidize organic compounds. This produces energy which is not used immediately by the microorganisms but is stored in the high energy phosphate bonds of adenosine triphosphoric acid (ATP). The energy of these bonds is used by the cells in various metabolic processes and is transformed into other types of energy, such as thermal, electric and mechanical, or is used in the synthesis of new molecules.

The rate of respiration of the microorganisms and, therefore, the rate of oxidation of organic compounds can be enhanced by adding biogenic phosphorus compounds to the treated sewage. As the rate of respiration increases, a decrease in the concentration of biogenic phosphorus compounds is observed. This decrease is caused by the synthesis of adhenosine triphosphoric acid. When biogenic phosphorus compounds are added to the treated sewage consumption of oxygen by the microorganisms increases.

A wide pH range of the treated sewage in which nitrification and denitrification take place is possible, and we prefer a range of from 4 to 12. During denitrification the reaction medium becomes more alkaline. A typical stoichiometric equation of the process shows this to be due to the formation of alkali metal hydroxides (for example potassium, sodium and calcium hydroxides) or their carbonates.

5C6H 1206+24KN03=24KHC02+6C02+ I 2N2+ 1 8H20+energy (1) The degree to which the solution becomes alkaline depends on the concentration of nitrates to be reduced: the higher the concentration of nitrates, the lower may be the pH of any water added to the sewage.

Nitrification causes the medium to become more acidic, and this is due to the formation of nitrous and nitric acids. The higher the concentration of ammonia in the sewage, the higher may be the original pH.

2NH3+302=2HN02+2H20+energy (2) 2HNO2+02=2HNO3+energy (3) This increase in pH during denitrification and decrease in pH during nitrification means that the method may be carried out over a wide pH range mentioned above.

A typical way of carrying out the method of the invention is as follows. Sewage with a pH from 4 to 1 2 and containing nitrates and/or nitrites together with biologically oxidizable organic compounds is fed into an aerotank reactor along with activated sludge. Ammonium and/or urea ions are then added and the mixture is aerated with air to give a dissolved oxygen concentration of from 0.1 to 4 mg/l.

After this contact with activated sludge the whole mixture may be transferred to a settler.

When the mixture has settled, activated sludge is re-cycled and purified water is discharged from the system. The water that is discharged may be further purified. The time during which the sewage is in contact with the activated sludge will depend on the concentration of impurities in the sewage, and from 1 to 25 hours has been found to be suitable. The concentration of nitrates, nitrites and organic compounds in the sewage to be treated will depend on the concentration of ammonium and urea and on the concentration of dissolved oxygen.

During nitrification, ammonia or ammonium ions are oxidized to nitrates in two stages according to reactions 2 and 3 above. The first stage of this reaction is carried out by the bacterium Nitrosomonas and the second stage by the bacterium Nitrobacter.

Denitrification is a biochemical process where nitrate is reduced to elemental nitrogen by means of denitrifying bacteria. These bacteria are amphimicrobians and are heterotrophic and they derive their energy from the reduction of nitrate. This reduction of nitrate is accompanied by the oxidation of organic compounds which releases energy necessary for the metabolism of the bacterium. Bacterial reduction of nitrate is associated with substrate oxidation and resides in that oxygen of the nitrates and not atmospheric oxygen is used as hydrogen acceptor. Part of this complex redox process may be represented by the following equation.

4NO;+24H+=1 2H2O+N2 (4) The overall process is marked by intensive liberation of gaseous carbon dioxide and nitrogen and by an increase in pH of the medium due to the formation of carbonates.

When the medium contains no nitrates, the bacterium will generally be able to use dissolved oxygen. It can do this because it has two enzyme systems in its cells. One of these systems is an ordinary respiration system employing oxidases and dehydrases. The second system is more specialised and it employs dehydrases and special enzymes, nitratereductases, which activate the oxygen of nitrates. Both of these enzyme systems can function simultaneously, and which prevails will be determined by the chemical nature of the hydrogen donor and the partiai pressure of dissolved oxygen.

Operation over a period of 1 year of an installation for treating sewage containing ammonium and urea ions, nitrates, nitrites and organic compounds has shown high efficiency and stability. The following table gives an approximate composition of sewage purified after a contact time with activated sludge of up to 8 hours.

BOC20 4000-8000 mg/l NO3 2600--4400 mg/l NH+4 200-160 mg/l Degree of purification with respect to: biochemical oxygen consumption. 9598% NO3 99% NH+4 98100%.

It can be seen from the above description that the present invention has certain advantages. It is possible to remove simultaneously ammonium ions, urea, nitrates, nitrites and organic compounds, and the degree of purification can be very high. Also, the biological process of sewage treatment can be intensified and it can be carried out over a very wide pH range. A further advantage of the invention is that it is possible to treat concentrated sewage. This means that a smaller amount of weakly contaminated sewage is needed for dilution of concentrated sewage and the area of land required for purifying installations is thereby reduced. The present invention also makes possible the prevention of contamination of water bodies with nitrogen-containing compounds.

The invention is further illustrated by the following example.

Example 1 Sewage containing 200 mg/l of nitrate nitrogen and organic impurities and having a BOC20 (biochemical oxygen consumption) of 2,500 mg/l was fed continuously into a reactor along with activated sludge and 400 mg/l of urea. The contact time of the activated sludge with the sewage was from 4.5 to 9 hours, after which the activated sludge and the now purified liquid was transferred to a settler from which the sludge was re-cycled to the reactor, maintaining the required concentrations.

The purified water was then discharged from the system. The concentration of dissolved oxygen was from 0.1 to 2.5 mg/l. A pH value before purification of from 7 to 9 gives satisfactory results using this procedure. The degree of purification with respect to nitrate nitrogen and urea was from 98 to 100% and with respect to organic impurities it was 95%.

Example 2 Here the procedure of example 1 was followed. The nitrate concentration was 700 mg/l and the BOC20 value for organic impurities was 6,000 mg/l. Ammonium nitrogen was added to a concentration of 200 mg/l. Using these values, the pH of the sewage may vary from 4 to 8. The concentration of dissolved oxygen was from 0.1 to 2.5 mg/l. The sewage was contacted with the activated sludge for 12 hours. The degree of purification with respect to nitrate nitrogen was from 98 to 100% and with respect to organic impurities it was 98%.

Example 3 Again, the procedure of example 1 was followed. Ammonium nitrogen was added to a concentration of 70 mg/l to a sewage initially containing 700 mg/l of nitrate nitrogen and a BOC20 value for organic impurities of 6,000 mg/l. The concentration of dissolved oxygen in the reactor was from 0.1 to 1.5 mg/l, the pH was from 4 to 6 and the contact time was 12 hours. The degree of purification with respect to nitrate nitrogen and ammonium nitrogen was from 98 to 100% and with respect to organic impurities it was 96%.

Example 4 The procedure of example 1 was repeated. Ammonium nitrogen was introduced to a concentration of 100 mg/l into sewage containing 900 mg/l of nitrate nitrogen and having a BOC20 value for organic impurities of 6,500 mg/l. The concentration of dissolved oxygen was varied within the range of 0.7 to 3 mg/l, the pH of the sewage to be treated was from 4 to 7 and the contact time was 12 hours. The degree of purification with respect to nitrate nitrogen and ammonium nitrogen was from 98 to 100% and with respect to organic impurities it was 95%.

Example 5 The procedure of example 1 was repeated. Ammonium nitrate was introduced to a concentration of 300 mg/l into sewage containing 700 mg/l of nitrate nitrogen and having a BOC20 value for organic impurities of 7,000 mg/l. The pH of the sewage was from 5 to 8. The concentration of dissolved oxygen was varied from 1.2 to 4 mg/l and the contact time was 1 2 hours. The degree of purification with respect to nitrate nitrogen and ammonium nitrogen was from 99 to 100% and with respect to organic impurities it was 95%.

Example 6 The procedure of example 1 was repeated. Ammonium nitrogen was added to a concentration of 1,000 mg/l into sewage containing 500 mg/l of nitrate nitrogen and having a BOC20 value for organic impurities of 1 500 mg/l. Superphosphate was introduced to a concentration of 200 mg/l. The concentration of dissolved oxygen was from 2 to 4 mg/l., the pH of the sewage was from 9 to 12 and the contact time was 25 hours. The degree of purification with respect to nitrate nitrogen and ammonium nitrogen was from 98 to 100% and with respect to organic impurities it was 95%.

Example 7 The procedure of example 1 was repeated. Trisodium phosphate and ammonium nitrogen were introduced to concentrations of 200 mg/l and 400 mg/l respectively into sewage containing 2,000 mg/l of nitrate nitrogen and nitrite nitrogen and having a BOC20 value for organic impurities of 1 800 mg/l. The concentration of dissolved oxygen was from 0.1 to 2 mg/l, the pH of the sewage was from 4 to 5 and the contact time was up to 25 hours. The degree of purification with respect to ammonium and nitrite nitrogen was from 98 to 99% and with respect to organic impurities it was 96%.

Example 8 The procedure of example 1 was repeated. Ammonium nitrogen was introduced to a concentration of 10 mg/l into sewage containing 100 mg/l of nitrite nitrogen and having a BOC20 value for organic impurities of 700 mg/l. The concentration of the dissolved oxygen was from 1 to 2 mg/l, the pH of the sewage was 8 and the contact time was 1 hour. The degree of purification with respect to nitrite nitrogen and ammonium nitrogen was from 99 to 100% and with respect to organic impurities it was 98%.

Claims

1. A method of purifying sewage, which comprises treating sewage at a dissolved oxygen contact of from 0.1 to 4.0 mg/l with an activated sludge and in the presence of ammonia and/or urea such that polluting organic compounds and nitrates and/or nitrites are removed.

2. A method according to claim 1, which additionally comprises adding ammonia and/or urea to the sewage and/or sludge or to a mixture of the sewage and the sludge.

3. A method according to claim 2, in which the amount of ammonia and/or urea added is from 10 to 1 to 1,000 mg/l.

4. A method according to any one of the preceding claims, which additionally comprises adding a biogenic phosphorus compound to the sewage and/or sludge or to a mixture of the sewage and the sludge.

5. A method according to claim 4, in which the biogenic phosphorus compound is superphosphate or trisodium phosphate.

6. A method according to claim 4 or 5, in which the biogenic phosphorus compound is added in an amount of up to 200 mg/l.

7. A method according to any one of the preceding claims, in which the sewage during treatment has a pH of from 4 to 12.

8. A method according to claim 1, substantially as herein described with reference to any one of the foregoing example.

9. Purified material when obtained by a process according to any one of the preceding claims.