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When to Install Reed Beds or Biological SBR Treatment

“Making a good choice whether to Install Reed Beds or Biological SBR Treatment Processes for Treating Landfill Leachate”

The question often arises for landfill operators, of whether to use a “SBR” Leachate Treatment Process, or if the cheaper alternative of a reed bed can be used.

Reed Beds or Biological SBRThe answer is that it depends entirely on the strength of the contaminants in landfill leachate.  The first choice, when enough land is available to site a reed bed, is the use of low running cost, low energy consumption, reed beds.

However, simple reed beds designed to be fed with leachate as horizontal flow type, engineered wetlands have a limited application for landfill leachate, because these traditionally laid-out reed beds can only be used for very dilute leachate from the very oldest landfill sites. These are sites which were built before the adoption of sanitary (lined) landfill practise, where the leachate is at its weakest.

An Example of reed beds used to treat a weak leachateThese landfills where reed beds alone can be used for leachate treatment are usually those landfills which are not lined nor capped, and located in temperate and wet places. This means that their leachate is substantial diluted, and old, and has been weakened by the addition of groundwater and rainwater entering the landfill. These would have been called tips or dumps in their day.

Modern “sanitary” (lined and capped) Municipal Solid Waste (MSW) Landfills invariably have a much more heavily contaminated and “fresher” (more acetogenic) leachate, and a more high-tech treatment system then becomes essential.

For many landfill operators the most successful method a leachate treatment for the past 30 years has been the biological aeration of a microbiologically active sludge, and this is followed by a anoxic phase reaction known as denitrification, where removal of total nitrogen is required.

“If a reed bed won’t do the job, an SBR, or nitrification followed by denitrification type system, is the next option to look at in most cases”, Leachate Expert, Steve Last of IPPTS Associates said.

A common feature of such process designs is that they are “sequencing reactors”, which simply means that they are run by computer controls, on a batch process system. The computer is used to automatically control the periodic feeding of leachate, aeration, chemical dosing, and discharge of each batch, etc.

Plants like this have become known as simply “SBRs” (Sequencing Batch Reactor Plants), in the leachate treatment industry.

An SBR Plant for leachate treatmentSBR Plants, usually with a denitrification stage, can be configured to:

  • discharge it directly to a river or stream, usually via a reed bed which is used as a final polishing process stage. In which case the plant will need to treat the leachate to a high quality, or
  • they may alternatively only pre-treat the leachate to remove a proportion of the contaminants, before discharging it for further treatment into a public sewer. In such cases the receiving sewage works provides the necessary additional treatment.

When to Install Reed Beds or Biological SBR Treatment – SBR Leachate Treatment Process Description

Reed Bed vs SBR Leachate treatment plant - image

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The simplest SBR landfill leachate treatment plants use the biological aeration (nitrification) process in a single tank or lagoon.

These plants work automatically to run each batch filling of leachate through its treatment cycle.

Once every predetermined time interval of from 6 hours to a day or more depending on the raw leachate strength and the discharge effluent quality needed, to satisfy the requirements of the local environmental regulating body.

More than 50 SBR landfill leachate treatment plants have been constructed to designs by Last, Robinson and Olufsen, in many countries worldwide, and original plants have been in operation for in excess of 20 years. SBR landfill leachate treatment plant technology is now thoroughly tried and tested, and a wide range of operational data on such plants while in operation, has been reported in published papers.

The design of this type of SBR is similar in principle to the activated sludge process, which until recently was the process adopted for almost all wastewater treatment works (sewage works). In an SBR, similar aerobic reactions occur. Similar microorganism do the work of “treatment” in the tanks of both systems.

But, there are important differences between sewage and leachate, and the most important difference is the far higher ammonia (ammoniacal-N) in leachate, so that is where the similarity ends.

In fact, the activated sludge process as used by designers of sewage treatment works, suffers from the problem that it becomes unstable when treating the high ammoniacal-nitrogen concentrations in full strength sanitary landfill leachates.

Perfectly well experienced sewage treatment process experts have many times come unstuck, when making attempts to modify sewage works type activated sludge treatment systems to adapt sewage works designs for use in leachate treatment.

Activated sludge wastewater processes which have been used to treat leachate cannot match the robustness of a well-designed SBR/ nitrification/ denitrification process, provided by a leachate treatment expert experienced in leachate treatment plant design.

Most people in the landfill industry say that leachate is hard to treat. But, that sentiment comes from the large number of failed leachate plants which have been designed as if leachate was somehow comparable with sewage, and in that the mis-judged use of reverse osmosis (RO) Plants can also be considered a factor.

However, use of the SBR nitrification/ denitrification process, when applied correctly, often in combination with RO and/or ultrafiltration offers a robust and as stated previously proven technique for the treatment of strong leachate from all modern sanitary landfills.

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 may include:

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

A stated earlier, as well, in many examples these are provided in combination with a compact reed bed which provides polishing of the effluent to a very high level of purity, and provides a process performance buffer, before the treated water enters the natural environment.

The SBR Treatment Cycle and features (in a simple nitrifying SBR) are:

Feed and fill the tank/ reactor

  • Minimise energy use through control of aeration rate and aerator run-time duration, throughout the aeration period in each cycle
  • Settle the contents by waiting for the particles to coagulate and fall to the bottom
  • Open a valve and draw-off the clean water off the top of the tank.

Secondary clarifiers for particulates removal are not needed in many cases, and therefore a compact footprint is possible.

The removal of pollutants is achieved by biological action, requiring a very minimum of chemical addition, and this is achieved through

  • Close monitoring and supply adjustment for pH control and oxygen demand adjustment
  • Robustness in operation arising from the large water volume stored in the reactor tank(s)
  • Minimising and absorbing shock leachate flow and high strength 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;

The SBR process, also operates with minimal sludge generation for most leachates, so there are none of the large sludge disposal cost of some other designs.

Conclusion

So, you now should have found out when to Install Reed Beds or Biological SBR Treatment.

References:

  1. Robinson, H.D. Olufsen, J.S. and Last, S.D. (2005). Design and operation of cost-e=ective leachate treatment schemes at UK landfills: Recent case studies. Published in the CIWM Scienti%c and Technical Review, April 2005, pp 14-24 © 2005 IWM Business Services Ltd.
  2. Other Published Conference Papers:
  3. 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
  4. 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

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Leachate

Leachate with a high iron content in a polluted streamLeachate starts as rainfall.

Rain falling on the top of the landfill is the main contributor to the generation of leachate, and is by far the largest contributor for modern sanitary landfills which do not accept liquid waste. In old unlined and un-engineered landfills, some leachate is produced from groundwater entering the waste. Some, additional leachate volume is produced during waste decomposition, and some additional surface water will sometimes run onto waste from its surroundings.

The decomposition of carbonaceous material produces some additional water, and a wide range of other materials including methane, carbon dioxide and a complex mixture of organic acids, aldehydes, alcohols and simple sugars, which dissolve in the leachate cocktail.

The precipitation percolates through the waste and takes in dissolved and suspended components from the biodegrading waste, through physical and chemical reactions.

Leachate history graphic adMost landfills are designed to minimise the amount of leachate they create during their lifetimes. However, there are good scientific reasons to suggest that it would be better to flush all landfills out and to do this, would produce more leachate, faster. Landfills where the latter philosophy is adopted, are called, “bio-reactor” landfills. In Europe, bioreactor landfills are effectively prohibited by EU directives, leading them to be called “dry tombs” by some, due to their rapid capping, and minimised leachate production.

The environmental risks of leachate generation arise from it escaping into the environment around landfills, particularly to watercourses and groundwater. These risks can be mitigated by properly designed and engineered landfill sites. Such sites are those that are constructed on geologically impermeable materials or sites that use impermeable liners made of  geotextiles  or engineered  clay . The use of linings is now mandatory within both the United States and the European Union, except where the waste closely controlled and genuinely inert.

Most toxic and difficult materials are now specifically excluded from landfill. However, despite much stricter statutory controls the leachates from modern sites are currently stronger than ever. They also contain a huge range of contaminants. In fact, anything soluble in the waste disposed will enter the leachate. Within the lists of substaces present in leachate are very low concentrations of “trace contaminants” which can have quite strongly contaminating effects. These are nowadays most often derived  from materials in household and domestic retail products which enter the waste stream perfectly legally.

Unfortunately, the leachate draining from most landfills will continue to reflect the contaminants of past years, when regulatory controls were less.

These substances in include extremely low concentrations of heavy metals (for example from batteries), herbicides and pesticides (as used in gardens), etc. However, leachate is becoming less contaminated with difficult substances as time goes forward, and public awareness, recycling and increased statutory control over these substances, throughout the industrialized world is making leachate less harmful in this respect.

“Leachate has a very high ammoniacal nitrogen concentration”

The concern about environmental damage from waste leachate, largely arises from its high organic contaminant concentrations and much higher ammoniacal nitrogen than commonly found in any other organic effluent.  Pathogenic   microorganisms and toxic substances that might be present in it have in the past been described as the most important. However, pathogenic organism counts reduce rapidly with time in the landfill, so this only applies to the youngest leachate and leachate is seldom removed from the landfill in this condition.

One of the most comprehensive scientific studies yet undertaken worldwide on leachate, was published by the United Kingdom, DOE., in 1995. It is titled: “A review of the composition of leachate from domestic wastes in landfill sites”; Department of Environment Research Report No. CWM 07294, and still provides much essential data on the range of contaminands present in Municipal Solid Waste, and Commercial and Industrial Waste landfill leachate.