Leachate Treatment by Biological SBR Processes for Full Strength MSW Landfill Leachate

Application of the "SBR" Leachate Treatment Process

Depending on the strength of the contaminants in landfill leachate the use of low running cost, low energy consumption, reed beds is the first choice. However, simple reed beds configured as horizontal flow type engineered wetlands have a limited application for landfill leachate from all but the oldest landfill sites where leachate is weakest.

Old landfills which are not lined nor capped, located in temperate and wet regions, and which have substantial dilution from groundwater entering the landfill are therefore the most likely candidates for reed bed treatment alone.

For virtually all other Municipal Solid Waste (MSW) Landfills which have a strong leachate, a more intensive treatment system is needed, and the most successful method is that of biological aeration of a microbiologically active sludge.

A common feature of such process designs is that they are run on a batch flow basis, under the control of a computer which controls the feed of leachate, aeration, chemical dosing, and discharge of each batch, etc.

The simplest of these have become known as simply "SBRs" (Sequencing Batch Reactor Plants).

The purpose of SBR Plants can be either to:

• treat the leachate to a high quality and discharge it directly to a river or stream, usually via a reed bed

or they may alternatively pre-treat leachate before discharging it:

• into a public sewer, for further treatment at the receiving sewage works.

SBR Leachate Treatment Process Description

The most common design for SBR landfill leachate treatment plants is the biological aeration (nitrification) process which is most often carried out in a single tank or lagoon, and which runs through its treatment cycle on one batch of leachate, once every 6 to 24 (or more) hours depending the initial strength and effluent quality required.

More than 50 have been constructed to designs by Last, Robinson  and Olufsen, in temperate, semi-tropical, and tropical locations, and original plants have been in operation for in excess of 20 years. The technology is well proven and has been extensively reported in peer reviewed published papers.^¥

A Sequencing Batch Reactor design SBR is similar in principle to the activated sludge, biological nutrient removal (nitrification) process, commonly used throughout the world for sewage treatment. Similar aerobic reactions occur and similar microbiology is present in the reactors of both systems. However, that is where the similarity ends.

The activated sludge process as applied to sewage treatment suffers from the disadvantage that it becomes unstable when treating high ammoniacal-nitrogen concentrations. Many attempts have been made by designers of activated sludge treatment systems to adapt sewage works type designs to leachate treatment, which when modified cannot match the robustness of the SBR process. This is one of several reasons leachate is seen as a difficult effluent to treat.

However, use of the SBR process when applied correctly offers a robust and proven technique for the treatment of strong leachate from sanitary MSW landfills.

It may be necessary to carry out additional purification of the leachate after the completion of the nitrification process described above, to remove additional COD and/ or Total Organic Nitrogen among other requirements that the local environmental regulatory authority may stipulate. If so, additional treatment by a further stage of treatment known as denitrification, is likely to be proposed.

Denitrification, and/ or the removal of Total Nitrogen would be provided as a refinement of the SBR process.

Other processes may be required depending on the local environmental regulator’s requirements for such higher quality water at the discharge point as local conditions, and national regulations, may dictate.

These processes may comprise other water treatment technologies, and may include:

• Membrane Processes
• Dissolved Air Flotation (DAF)
• Activated Carbon.

In many case these are utilized in combination with a small reed bed for polishing of the effluent to a very high level of purity before it enters the natural environment.

The SBR Treatment Cycle

Important features of the SBR technology include:-

• Minimisation of energy use through control of aeration rate and duration;
  
• Elimination of the need for secondary clarifiers in many cases, and therefore a compact footprint;
  
• Removal of contaminants with minimum chemical addition through pH control and oxygen demand/supply management;
  
• Highly robust due to the large water volume provided, which resides throughout; at, or close to final effluent quality, absorbing shock loadings;
  
• Economic operation under normal low dry weather flow/ loading conditions but readily adjustable to operation at a higher flow per cycle of operation during extended wet weather periods;
  
• Operates with minimal sludge generation for most leachates.

Referenced Published Conference Paper:

^¥ Robinson, H.D. Olufsen, J.S. and Last, S.D. (2005). Design and operation of cost-effective leachate treatment schemes at UK landfills: Recent case studies. Published in the CIWM Scientific and Technical Review, April 2005, pp 14-24 © 2005 IWM Business Services Ltd.

Other Published Conference Papers:

CIWM Scientific and Technical Review, April 2005, 14-24.
ROBINSON H D*, FARROW S**, CARVILLE M S*, GIBBS L**, Operation of the UK’s Largest Leachate Treatment Plant 6 Years of Experience at Arpley Landfill, Roberts S J** and Jones D**. Paper presented to XII International Landfill Symposium Sardinia, October 2009, 10pp

Robinson H.D., Farrow, S., Last, S.D. and Jones, D. (2003). Remediation of leachate problems at Arpley Landfill Site, Warrington, Cheshire, UK. Paper presented to XII International Landfill Symposium Sardinia, October 2003, 10pp, 10pp

by Steve Last - 6 February 2011

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