GB2344815A

GB2344815A - Landfill leachate treatment

Info

Publication number: GB2344815A
Application number: GB9915570A
Authority: GB
Inventors: Lionel George Sear; Matthew Coleman
Original assignee: COUNTY ENVIRONMENTAL SERVICES
Current assignee: COUNTY ENVIRONMENTAL SERVICES
Priority date: 1998-12-14
Filing date: 1999-07-02
Publication date: 2000-06-21
Anticipated expiration: 2019-07-02
Also published as: GB9827518D0; GB9915570D0; GB2344815B

Abstract

A method of treating landfill site leachate which comprises subjecting the leachate sequentially to a nitrification step and a denitrification step, in a reactor in the presence of a biomass including aerobic bacteria. The nitrification step comprises aerating the leachate, thereby converting ammonia present to a nitrate. The denitrification step comprises terminating the aeration of the leachate, allowing the leachate to become anoxic, and adding to the nitrified leachate ethyl acetate as a supplementary carbon source, thereby converting nitrate present to nitrogen. The denitrification step may be followed by further aeration.

Description

Landfill Leachate Treatment The present invention relates tc the treatment of leachate from landfill sites.

Domestic, commercial and industrial waste can be disposed of in several ways, one of which entails the use of a landfill site. A landfill site is a natural or excavated pit into which the waste is dumped. It is lined in order to prevent seepage of noxious material from the site into the surrounding ground. A site will have a collection system for leachate from the waste. The collected leachate is treated and then disposed of for example in a sewage treatment facility. Where there is no convenient sewage treatment facility, the leachate may be sprayed onto woodland and/or grassland, etc.

Leachate contains ammonia, which is a pollutant. It must therefore be treated to reduce the ammonia content and one method used is to oxidize the ammonia to nitrate.

This is often carried out in an aeration step on a batch basis in an aeration reactor. The nitrification is facilitated by aerobic bacteria as air is supplied to the contents of the reactor.

Unfortunately, there may be constraints on the total amount of nitrogen that can be discharged into the environment as nitrate and it is therefore generally desirable to provide some means for reducing the amount of nitrate present.

According to the invention there is provided a method of treating landfill site leachate which comprises subjecting the leachate sequentially to a nitrification step and a denitrification step, in a reactor in the presence of a biomass including aerobic bacteria, in which: the nitrification step comprises aerating the leachate, thereby converting ammonia present to a nitrate; and the denitrification step comprises terminating the aeration of the leachate, allowing the leachate to become anoxic, and adding ethyl acetate to the nitrified leachate as a supplementary carbon source, thereby converting nitrate present to nitrogen.

In this way, the ammonia present in the leachate can be disposed of as nitrogen gas as opposed to nitrate and the cleaned leachate can be discharged either into rivers or by spraying over woodland and grassland without exceeding nitrogen loading constraints for ammoniacal nitrogen, nitrate (and nitrite). The system operates as a sequential batch process, using a single reactor and a retained biomass.

Preferably, the nitrification step is continued until substantially all the ammonia present in the leachate has been converted to nitrate. The supplementary carbon source may be added 0 to 24 hours after the aeration is terminated. Preferably it is added from 12 to 16 hours after aeration is terminated. Leachate feed can be used to achieve some denitrification prior to the addition of the supplementary carbon source.

Preferably, the contents of the reactor are stirred during the denitrification step. Thus, the reactor is preferably mixed gently until the denitrification is complete or the required nitrogen level is reached.

During this part of the process the incoming leachate feed can be left on or switched off as required. Preferably, no further leachate is added after the supplementary carbon source is added.

The denitrification step is preferably continued until at least 75% of the nitrate has been converted, though it may be continued until substantially all the nitrate has been converted. If total denitrification is required care must be taken not to allow the reactor to become anaerobic (i. e. septic).

The time taken for denitrification is highly dependent upon the ambient temperature and the state of the biomass in the reactor, but can take from a few hours to several days. Ethyl acetate has been found to be a highly satisfactory carbon source and is particularly suitable if ambient temperatures are low (from 5 to 10 C).

The amount of ethyl acetate added would typically be from 0.5 to 3.0 kg per kg of nitrogen to be removed, preferably 1.2 to 1.4 kg/kg.

When the denitrification step is complete, the incoming leachate feed (if any) should be switched off and, preferably, full aeration resumed in order to use up any unreacted ethyl acetate. The length of this period of aeration should be kept as short as possible. Preferably, the second aeration step is continued for less than four hours.

Preferably, a flocculant is added during or after the second aeration step. Preferably, after the nitrification and denitrification steps and any further aeration, all aeration and mixing are terminated and the contents of the reactor are allowed to settle, prior to discharge. Up to two-thirds of the content of the reactor may be discharged after the treatment is complete.

The invention may be carried into practice in various ways, and one embodiment will now be described by way of example.

In this embodiment, a cycle begins with an aeration reactor, having a capacity of 1700 m3 containing about 700 m3 of treated leachate and biomass. Up to lOOOm3 of untreated leachate is added to the reactor and the content of the reactor is aerated until substantially all the ammonia present has been nitrified to nitrate. Typically this might take about 24 hours at an aeration rate of up to 50 kg per hour of 2 at an ambient temperature of from 0 to 25 C. Aeration is then stopped and a supplementary source of carbon, in the form of ethyl acetate is added to the reactor in an amount of 2.0 kg ethyl acetate per kg of nitrogen to be removed. The ethyl acetate can be added by sucking it in through the air intake of the venturi aerators used in the equipment.

The contents of the reactor is mixed gently to maintain anoxic conditions until denitrification is complete or the required level of conversion of nitrate to nitrogen gas has been attained. In order to mix the contents of the reactor, in this case, the air intakes of the venturi aerators are mechanically blocked. The aerators then effect mixing without adding air.

When the required denitrification level has been attained, full aeration at a rate of 50 kg O2/hr is restarted in order to use up any unconsumed ethyl acetate.

This is continued for as short a period as possible, in practice, normally about 4 hours. Towards the end of this period, % kg of a flocculent (in this case Allied Colloids Magnafloc 351) is added.

Finally, after removal of the residual ethyl acetate, the material in the reactor is allowed to settle. This would normally take about 5 hours. The treated leachate, containing the required reduced amount of nitrogen, is discharged and disposed of in the normal way. Up to twothirds of the reactor contents can be discharged.

Claims

Claims: 1. A method of treating landfill site leachate which comprises subjecting the leachate sequentially to a nitrification step and a denitrification step, in a reactor in the presence of a biomass including aerobic bacteria, in which: the nitrification step comprises aerating the leachate, thereby converting ammonia present to a nitrate; and the denitrification step comprises terminating the aeration of the leachate, allowing the leachate to become anoxic, and adding ethyl acetate to the nitrified leachate as a supplementary carbon source, thereby converting nitrate present to nitrogen.
2. A method as claimed in Claim 1, in which the nitrification step is continued until substantially all the ammonia present in the leachate has been converted to nitrate.
3. A method as claimed in Claim 1 or Claim 2, in which the supplementary carbon source is added 0 to 24 hours after the aeration is terminated.
4. A method as claimed in any preceding Claim, in which further leachate is added after the aeration is terminated but before the supplementary carbon source is added.
5. A method as claimed in any preceding Claim, in which the contents of the reactor are stirred during the denitrification step.
6. A method as claimed in any preceding Claim, in which no further leachate is added after the supplementary carbon source is added.
7. A method as claimed in any preceding Claim, in which the denitrification step is continued until at least 75% of the nitrate has been converted.
8. A method as claimed in Claim 7, in which the denitrification step is continued until substantially all the nitrate has been converted.
9. A method as claimed in any preceding Claim, in which the amount of ethyl acetate added is 0.5 to 3.0 kg per kg of nitrogen to be removed.
10. A method as claimed in any preceding Claim including a second aeration step, after the denitrification step has been completed to the desired level of conversion.
11. A method as claimed in Claim 10, in which the second aeration step is continued for less than four hours.
12. A method as claimed in Claim 10 or Claim 11, in which a flocculent is added during or after the second aeration step.
13. A method as claimed in any preceding Claim, in which, after the nitrification and denitrification steps and any further aeration, all aeration and mixing are terminated and the contents of the reactor are allowed to settle, prior to discharge.
14. A method as claimed in any preceding Claim in which up to two-thirds of the content of the reactor is discharged after the treatment is complete.
15. A method of treating landfill site leachate, substantially as herein specifically described.