GB2259699A - Process and plant for the treatment of high-strength ammoniacal liquors - Google Patents

Abstract

A process and plant for purifying high-strength ammoniacal liquor, comprising aerating the liquor in a vessel (1) to effect biological nitrification, providing a source (6) of additional ammonia and dispensing same at a controlled rate into the liquor to maintain a substantially constant ammonia load on the body of liquor in the vessel, and subsequently discharging the nitrified liquor into a sedimentation chamber (9) from which settled sludge is redirected to the vessel (1) and product liquor is withdrawn (at 14). Temperature, pH and oxygen content within the vessel (1) are monitored and automatically controlled. The process achieves biological purification of high-strength ammoniacal liquors by nitrification in the activated sludge process, substantially or entirely without prior dilution of the liquor and by use of a compact plant, and with a substitute, synthetic source of ammonia to ensure that the plant continues to operate in a stable condition when the liquor supply is reduced or discontinued. <IMAGE>

Description

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- 1 PROCESS AND PLANT FOR THE TREATMENT OF HIGH-STRENGTH AMMONIACAL LIQUORS THIS INVENTION concerns a process for treatment of high-strength ammoniacal liquors.

It is known to treat domestic sewage and other waste waters such as those arising from industrial processes, by microbiological oxidation. This process makes use of a bacterial sludge which oxidises the contaminants in the water, in the presence of dissolved oxygen. The effluent from the process is discharged to a sedimentation tank or pond where the sludge is allowed to settle out. Much of this sludge may be recycled to the treatment vessel, but an excess quantity is always generated which must be disposed of elsewhere, often in admixture with so-called primary sludge which is removed prior to the microbiological oxidation process.

These excess sludges may usefully be spread on agricultural land as an organic fertiliser. However, in the condition in which it leaves the sedimentation tank, the sludge is not suitable for such use, as it is very wet, evil-smelling and otherwise contaminated. A method widely used for purifying the sludge and rendering it practically odourless, is anaerobic digestion, in which the readily fermentable material is broken down by microorganisms in the absence of oxygen. However, the product thus stabilised still has a high water content which must be reduced before final disposal of the 2 - sludge, if only to minimise costs of storage and transport.

The water is generally removed by filtering or centrifuging the wet sludge, but the resulting filtrate or centrate is contaminated with substances which represent a high pollution load. In particular, it has a high ammonia content, typically around 300 to 500 milligrams per litre. (mg/L). Although this liquor can be recycled to the microbiolo gical oxidation system at the sewage works, it increases the load on this system, in terms of both volume and pollutants. Consequently, the works may fail to meet its legal requirements for purity of the effluent discharged to a watercourse, or the capacity of the sewage works must be enlarged, thus increasing the cost of sewage treatment. Furthermore, depending on the works schedule, the output of this liquor may be intermittent, necessitating the use of large-capacity storage tanks to ensure a regular feed to the sewage works.

Similar high-strength, predominantly ammonlacal, liquors may also arise from the dewatering of primary sludges in the treatment of municipal or domestic sewage, or as waste waters in the food, chemical, pharmaceutical, fuel and other industries, and pose similar problems of disposal to those just mentioned. Technology does exist for converting the ammonia or ammonium compounds in waste waters to nitrates by biological nitrification, but this is limited to the treatment of liquors with rather low ammonlacal-nitrogen contents, typically around 20 mg/L or lower for municipal sewage works, for 1 example. Hence a strong liquor would have to be diluted many times over, in order to be treatable by this technology. Moreover, the nitrification process as thus practised is sensitive to shocks, such as by sudden changes in volumetric or pollutant loading, and to loss of ammonia in the influent, taking a long time to recover its treatment efficiency after the ammonia content is restored. It also requires plant of large size.

From the foregoing, it can be seen that it is desirable to have available a method of purifying high-strength ammoniacal waste waters or other liquors efficiently and at the same time more cheaply and with less deleterious influence on associated operations.

To overcome the limitations of the existing state of the art, a process for purifying high-strength ammonlacal liquors is now proposed which combines a number of features. In this process, such liquors are treated, substantially or entirely without prior dilution, by a biological nitrification system using activated sludge technology in compact plant of low cost, with specified automatic control features, and the deleterious effects of interruptions in the supply of waste water to the plant, or of fluctuations in the rate of supply or in the ammoniacal nitrogen concentration, are overcome by make-up with a substitute source of ammonia in a synthetic form, so that the ammonia load on the process is kept substantially constant.

According to the present invention a process for purifying high-strength ammoniacal liquor comprises the steps of aerating the liquor in a vessel to effect biological nitrification, providing a source 4 of additional ammonia, and dispensing same into the liquor as required to maintain a substantially constant ammonia load on the process.

Also according to the invention a process plant for operating the aforesaid process includes a treatment vessel, aeration means therein, means for monitoring the ammonia load on the process, and means responsive to said monitoring means for dispensing additional ammonia into the treatment vessel.

It has been established by experiment that the nitrification process works equally well on a synthetic solution of an ammonia compound, such as ammonium sulphate, as on the filtrate or centrate from sludge dewatering. The addition of the substitute can be effected very cheaply, using automatic dosing based on measurement of the flow rate of the incoming waste water. This use of a substitute ammonia source has at least three advantages, namely it avoids the need to construct storage tanks (buffer capacity) to overcome interruptions in liquor supply caused by shutdowns of the sludge treatment plant (e.g. at weekends or for maintenance); it prevents the operation of the nitrification plant from becoming unstable; and it offers a means of "pre-commissioning" the nitrification plant before the supply of sludge filtrate or centrate (or other liquor to be treated) actually becomes available, i.e. the plant can be made ready and in stable operation, in advance.

- 5 In the proposed process, automatic control is applied not only to the addition of the substitute ammonia source but also, primarily, to the temperature and to the pH of the mixture of liquor and sludge in the nitrification vessel and, secondarily, to the dissolved-oxygen concentration in the liquid phase in the vessel. The temperature may be controlled by addition of cooling water, the pH by addition of alkali, and the oxygen content by varying the rate of air supply. Another advantage of the invention is the capability to determine the optimum treatment conditions and plant design for any particular liquor from the results of tests using a laboratory-scale activated sludge unit.

The combination of all the above features ensures that the high-strength influent is consistently purified to zero or very low (a few mg/L) ammonlacal nitrogen content.

An embodiment of the. invention will now be described, by way of example only, with reference to the accompanying drawing which schematically illustrates a plant capable of carrying out the process.

The process includes a tank 1 in which a sludge of nitrifying bacteria is maintained in a completely mixed state with the liquor being treated. The dimensions of the tank are selected in accordance with the rate of supply of liquor and the data (e.g. hydraulic retention time) established from laboratoryscale tests. The contents of tank 1 are supplied with air and kept agitated by one or 6 - more diffusers 2. Alternatively, surface or other mechanical aerators, or pure-oxygen injection, may be used. A supply of strong ammonlacal liquor, for example, sludge filtrate or centrate, is fed to the tank 1 through a pipe 3, normally at a steady rate.

Cooling water, when required, is admitted to the tank through line 4, alkali (e.g. caustic soda solution) for pH control through line 5, a substitute ammonia source (e.g. ammonium sulphate solution with an ammonlacal nitrogen content of c.300 mg/L), when required through line 6, and air to the diffuser(s) through line 7.

Treated liquor, mixed with bacterial sludge, leaves the tank 1 through an overflow pipe 8 and passes to a sedimentation chamber 9 which may be of the radial flow type with a rotary scraper as shown, or of any other appropriate type. Most of the settled sludge is recycled through a pump 10 and line ll-to the tank 1, whilst excess sludge is removed by a pump 12 and is disposed of via a line 13. The purified and clarified effluent is finally discharged through a line 14. Tank 1 may be fitted with a cover 15 which assists in maintaining the contents at a constant temperature, and in confining odours. A vent 16 may be provided in the cover, leading to an odour control unit if necessary.

In the drawing, control valves fitted in lines 4, 5, 6 and 7 are denoted by numerals 17, 18, 19 and 20, respectively. These automatically regulate the flows of cooling water, alkali, substitute ammonia source and air respectively, on the basis of signals (represented by the dashed lines in the drawing) received from the appropriate flow (FIC), temperature (TIC) and concentration (QIC) measuring instruments (indicator-controllers) as shown in the drawing, and in accordance with measurement and control principles well known to those skilled in the art. By these means, the ammonlacal-nitrogen loading on the system is maintained at least substantially constant; the temperature and pH of the tank contents are maintained at, e.g. 22-25'C, and 7.0 respectively; and the dissolved-oxygen concentration is kept at an adequate level.

In the following example, a laboratory-scale activated sludge unit, incorporating an aeration tank of 5-litres capacity, was used to treat the centrate from dewatering of a digested sewage sludge, diluted in the ratio 4 parts of centrate to 1 part of mains water, by volume. Treatment was continued over several days, after which the supply of centrate was completely replaced with a synthetic feed of ammonium sulphate solution of similar ammonlacal-nitrogen concentration. After a further 4 days, the centrate supply was resumed. Throughout the test, nitrification proceeded in a stable and efficient manner, yielding an effluent with zero or very low ammoniacal-nitrogen content, as may be seen from the data in Table 1. The temperature in the aeration tank was kept at 22-25'C, and the pH at 7.

Table 1

Day Feed Hydraulic NH4-N conc'n retention (mg/L) time (hours) Inlet Outlet NH -N load Mixed liquor Effluent kg N1(m 3 /day) suspended suspended solids (mg/L) solids (mg/L) 1 19 370 0 0.42 1712 61 2 3 0.48 - 60 3 centrate 0 0.42 1712 61 4 + water, - 0.42 - 60 4:1 vIv 3 - - 6 1 87 7 0 115 8 245 0 0.32 99 9 17 278 6 0.36 - 63 (NH4)2504 29 0 0.36 2319 74 11 solution 0 0.36 - 74 12 7 0.36 - 68 13 399 3 0.55 - 47 14 Centrate 459 7 0.64 - 80 + water, 0 0.64 - 83 16 4:1 vlv 30 8 - - 101 17 23 0 0.48 2948 The process achieves biological purification of high- strength ammoniacal liquors by nitrification in the activated sludge process, efficiently and consistently, substantially or entirely without prior dilution of the liquor, by the use of a compact plant with specified automatic control features, in combination. with a substitute, synthetic source of ammonia to ensure that the plant continues to operate in a stable condition when the liquor supply is reduced or cut off completely.

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Claims (14)

1. A process for purifying highstrength ammoniacal liquor comprising the steps of aerating the liquor in a vessel to effect biological nitrification, providing a source of additional ammonia, and dispensing same into the liquor as required to maintain a substantially constant ammonia load on the process.

2. A process according to Claim 1, wherein automatic control is applied to the rate of addition of the additional ammonia, to the temperature and the pH of the liquor in the vessel, and to the dissolved oxygen concentration in the liquor in the vessel.

3. A process according to Claim 2, wherein said temperature control is by the addition to cooling water to vessel, the pH control is by the addition of alkali thereto, and the oxygen content is controlled by a variable air supply thereto.

4. A process according to any preceding claim, wherein the high-strength ammoniacal liquor is fed at a controlled rate into said vessel, mixed therein with bacterial sludge and after biological nitrification is transferred from the vessel to a sedimentation chamber from which a substantial proportion of the settled sludge is recycled to the nitrification vessel and excess sludge is removed for disposal, purified and clarified effluent liquor being discharged from the sedimentation chamber, at least substantially free of ammonlacal nitrogen content.

5. A process according to any preceding claim, wherein said source of additional ammonia is provided by an ammonium sulphate solution with an ammoniacal nitrogen content in the region of 300 milligrams per litre.

6. A process according to any preceding claim, wherein the liquor to be treated has an ammonia content of between 300 and 500 milligrams per litre.

7. A process according to any preceding claim, wherein the temperature of the liquor in the nitrification vessel is maintained at between 22 and 25'C.

8. A process according to any preceding claim, wherein the pH of the liquor in the nitrification vessel is held at a value in the region of 7.0.

9. A plant for carrying out a process as defined in any preceding claim, for purifying high-strength ammonlacal liquor the, process plant comprising a treatment vessel, aeration means therein, means for monitoring the ammonia load on the process, and means responsive to said monitoring means for dispensing additional ammonia into the treatment vessel.

10. A process plant according to Claim 9, wherein said aeration means comprises one or more air diffusers which serve to keep the liquor aerated and agitated within the vessel.

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11 - 11. A process plant according to Claim 9 or Claim 10, including a plurality of control valves adapted automatically to regulate supplies into the vessel of cooling water, alkali, substitute ammonia source and air on the basis of signals received respectively from a plurality of indicator-controllers sensing the instantaneous conditions within the vessel.

12. A process plant according to any one of Claims 9 to 11, including a sedimentation chamber connected to the vessel and adapted to receive nitrified liquor therefrom, means being provided to extract settled sludge from said chamber and to recycle same predominantly to the vessel, and partly for disposal.

13. A process plant for purifying high-strength ammonlacal liquor, substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawing.

14. A process for purifying high-strength ammonlacal liquor, substantially as hereinbefore described.

14. A process for purifying high-strength ammonlacal liquor, substantially as hereinbefore described.

Amencknents to the claims have been filed as follows 1. A process for purifying high-strength ammoniacal liquor comprising the steps of aerating the liquor in a vessel to effect biological nitrification, providing a source of additional ammonia, and dispensing same into the liquor as required to maintain a substantially constant ammonia load on the process.

2. A process according to Claim 1, wherein automatic control is applied to the rate of addition of the additional ammonia, to the temperature and the pH of the liquor in the vessel, and to the dissolved oxygen concentration in the liquor in the vessel.

3. A process according to Claim 2, wherein said temperature control is by the addition to cooling water to vessel, the pH control is by the addition of alkali thereto, and the oxygen content is controlled by a variable air supply thereto.

4. A process according to any preceding claim, wherein the high-strength ammonlacal liquor is fed at a controlled rate into said vessel, mixed therein with bacterial sludge and after biological nitrification is transferred from the vessel to a sedimentation chamber from which a substantial proportion of the settled sludge is recycled to the nitrification vessel and excess sludge is removed for disposal, purified and clarified effluent liquor being discharged from the sedimentation chamber, at least substantially free of ammonlacal nitrogen content.

13.5 5. A process according to any preceding claim, wherein said source of additional ammonia is provided by an ammonium sulphate solution.

f 6 A process according to any preceding claim, wherein the liquor to be treated has an ammonia content in excess of 300 milligrams per litre.

7. A process according to any preceding claim, wherein the temperature of the liquor in the nitrification vessel is maintained at between 22 and 25'C.

8. A process according to any preceding claim, wherein the pH of the liquor in the nitrification vessel is held at a value in the region of 7.0.

9. A plant for carrying out a process as defined in any preceding claim, for purifying high-strength ammoniacal liquor the, process plant comprising a treatment vessel, aeration means therein, means for monitoring the ammonia load on the process, and means responsive to said monitoring means for dispensing additional ammonia into the treatment vessel.

10. A process plant according to Claim 9, wherein said aeration means comprises one or more air diffusers which serve to keep the liquor aerated and agitated within the vessel.

11+ i 11. A process plant according to Claim 9 or Claim 10, including a plurality of control valves adapted automatically to regulate supplies into the vessel of cooling water, alkali, substitute ammonia source- and air on the basis of signals received respectively from a plurality of indicator-controllers sensing the instantaneous conditions within the vessel.

12. A process plant according to any one of Claims 9 to 11, including a sedimentation chamber connected to the vessel and adapted to receive nitrified liquor therefrom, means being provided to extract settled sludge from said chamber and to recycle same predominantly to the vessel, and partly for disposal.

13. A process plant for purifying high-strength ammonlacal liquor, substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawing.