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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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Landfill leachate composition - image for page

Landfill Leachate Composition

Landfill leachate composition for United Kingdom Landfills was first published in the Waste Management Papers published by the UK Department of Environment. Waste Management Paper 26 contains the most recent table of Landfill Leachate Composition before the WMP series was superseded by later documents, notably the DoE’s Leachate Report of 1995. However, the original table, […]

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How Do You Treat Leachate?

First of all when asked the question, “How Do You Treat Leachate?”, we would like to make a distinction between true treatment which involves converting environmentally damaging substances to less, or non-toxic, ones, and simply concentrating the contaminants from the water in leachate to give a volume reduction. In the water industry both are known as […]

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Wastewater Treatment in Wetlands: Contaminant Removal Processes

An Introduction to Wastewater Treatment in Wetlands

Originally published by U.S. Department of Agriculture, Cooperative Extension Service, University of Florida

Wastewater treatment in wetlands can be highly effective, [including for leachates]. Wetlands are commonly known as biological filters, providing protection for water resources such as lakes, estuaries and ground water. Although wetlands have always served this purpose, research and development of wetland treatment technology is a relatively recent phenomenon. Studies of the feasibility of using wetlands for wastewater treatment were initiated during the early 1950s in Germany. In the United States, wastewater-to-wetlands research began in the late 1960s, and increased dramatically in scope during the 1970s. As a result, the use of wetlands for water and wastewater treatment in wetlands has gained considerable popularity worldwide. Currently, an estimated one thousand wetland treatment systems, both natural and constructed, are in use in North America.

The goal of wastewater treatment is the removal of contaminants from the water in order to decrease the possibility of detrimental impacts on humans and the rest of the ecosystem. The term “contaminant” is used here to refer to an undesirable constituent in the water or wastewater that may directly or indirectly affect human or environmental health. Many contaminants, including a wide variety of organic compounds and metals, are toxic to humans and other organisms. Other types of contaminants are not toxic, but nevertheless pose an indirect threat to our well-being. For example, loading of nutrients (e.g., nitrogen and phosphorus) to waterways can result in excessive growth of algae and unwanted vegetation, diminishing the recreational, economic and aesthetic values of lakes, bays and streams.

Wetlands have proved to be well-suited for treating municipal wastewater (sewage), agricultural wastewater and runoff, industrial wastewater, and stormwater runoff from urban, suburban and rural areas. Municipal wastewater originates primarily from residential and commercial sources. Wetland treatment systems for municipal wastewater vary greatly in size and scope, from single-residence backyard wetlands to regional-scale systems such as the 1200- acre (480-ha) Iron Bridge treatment wetland in central Florida. Agricultural wastewater may include runoff from crop lands and pastures, milking or washing barns and feedlots. Among the types of industrial wastewater that are amenable to treatment in wetlands are those associated with pulp and paper manufacturing, food processing, slaughtering and rendering, chemical manufacturing, petroleum refining, and landfill leachates.

A number of physical, chemical and biological processes operate concurrently in constructed and natural wetlands to provide contaminant removal ( Figure 1 ). Knowledge of the basic concepts of these processes is extremely helpful for assessing the potential applications, benefits and limitations of wetland treatment systems.

contaminant removal in wetlandsFigure 1. Summary of the major physical, chemical and biological processes controlling contaminant removal in wetlands.

Contaminant Removal Processes

Physical Removal Processes

Wetlands are capable of providing highly efficient physical removal of contaminants associated with particulate matter in the water or waste stream when there is wastewater treatment in wetlands. Surface water typically moves very slowly through wetlands due to the characteristic broad sheet flow and the resistance provided by rooted and floating plants. Sedimentation of suspended solids is promoted by the low flow velocity and by the fact that the flow is often laminar (not turbulent) in wetlands. Mats of floating plants in wetlands may serve, to a limited extent, as sediment traps, but their primary role in suspended solids removal is to limit resuspension of settled particulate matter.

Efficiency of suspended solids removal is proportional to the particle settling velocity and the length of the wetland. For practical purposes, sedimentation is usually considered an irreversible process, resulting in accumulation of solids and associated contaminants on the wetland soil surface. However, resuspension of sediment may result in the export of suspended solids and yield a somewhat lower removal efficiency. Some resuspension may occur during periods of high flow velocity in the wetland. More commonly, resuspension results from wind-driven turbulence, bioturbation (disturbance by animals and humans) and gas lift. Gas lift results from production of gases such as oxygen, from photosynthesis in the water, and methane and carbon dioxide, produced by microorganisms in the sediment during decomposition of organic matter. Problems with eventual buildup of sediment to detrimental levels may need to be addressed over the long term.

Biological Removal Processes

Biological removal is perhaps the most important pathway for contaminant removal in wetlands. Probably the most widely recognized biological process for contaminant removal in wetlands is plant uptake. Contaminants that are also forms of essential plant nutrients, such as nitrate, ammonium and phosphate, are readily taken up by wetland plants during wastewater treatment in wetlands. However, many wetland plant species are also capable of uptake, and even significant accumulation of, certain toxic metals such as cadmium and lead. The rate of contaminant removal by plants varies widely, depending on plant growth rate and concentration of the contaminant in plant tissue.

Woody plants, i.e., trees and shrubs, provide relatively long-term storage of contaminants, compared with herbaceous plants. However, contaminant uptake rate per unit area of land is often much higher for herbaceous plants, or macrophytes, such as cattail. Algae may also provide a significant amount of nutrient uptake, but are more susceptible to the toxic effects of heavy metals. Storage of nutrients in algae is relatively short-term, due to the rapid turnover (short life cycle) of algae. Bacteria and other microorganisms in the soil also provide uptake and short-term storage of nutrients, and some other contaminants.

In wetlands, as in many terrestrial ecosystems, dead plant material, known as detritus or litter, accumulates at the soil surface. Some of the nutrients, metals or other elements previously removed from the water by plant uptake are lost from the plant detritus by leaching and decomposition, and recycled back into the water and soil. Leaching of water-soluble contaminants may occur rapidly upon the death of the plant or plant tissue, while a more gradual loss of contaminants occurs during decomposition of detritus by bacteria and other organisms. Recycled contaminants may be flushed from the wetland in the surface water, or may be removed again from the water by biological uptake or other means.

In most wetlands, there is a significant accumulation of plant detritus, because the rate of decomposition is substantially decreased under the anaerobic (oxygen-depleted) conditions that generally prevail in wetland soil. If, over an extended period of time, the rate of organic matter decomposition is lower than the rate of organic matter deposition on the soil, formation of peat occurs in the wetland. In this manner, some of the contaminants originally taken up by plants can be trapped and stored as peat. Peat may accumulate to great depths in wetlands, and can provide long-term storage for contaminants. However, peat is also susceptible to decomposition if the wetland is drained or otherwise dries up. When that happens, the contaminants incorporated in the peat may be released and either recycled or flushed from the wetland.

Although microorganisms may provide a measurable amount of contaminant uptake and storage, it is their metabolic processes that play the most significant role in removal of organic compounds. Microbial decomposers, primarily soil bacteria, utilize the carbon (C) in organic matter as a source of energy, converting it to carbon dioxide (CO2) or methane (CH4) gases. This provides an important biological mechanism for removal of a wide variety of organic compounds, including those found in municipal wastewater, food processing wastewater, pesticides and petroleum products. The efficiency and rate of organic C degradation by microorganisms is highly variable for different types of organic compounds.

Microbial metabolism also affords removal of inorganic nitrogen, i.e., nitrate and ammonium, in wetlands. Specialized bacteria (Pseudomonas spp.) metabolically transform nitrate into nitrogen gas (N2), a process known as denitrification. The N2 is subsequently lost to the atmosphere, thus denitrification represents a means for permanent removal, rather than storage, of nitrogen by the wetland. Removal of ammonium in wetlands can occur as a result of the sequential processes of nitrification and denitrification. Nitrification, the microbial (Nitrosomonas and Nitrobacter spp.) transformation of ammonium to nitrate, takes place in aerobic (oxygen-rich) regions of the soil and surface water. The newly-formed nitrate can then undergo denitrification when it diffuses into the deeper, anaerobic regions of the soil. The coupled processes of nitrification and denitrification are universally important in the cycling and bioavailability of nitrogen in wetland and upland soils.

Chemical Removal Processes

In addition to physical and biological processes, a wide range of chemical processes are involved in the removal of contaminants in wetlands. The most important chemical removal process in wetland soils is sorption, which results in short-term retention or long-term immobilization of several classes of contaminants. Sorption is a broadly defined term for the transfer of ions (molecules with positive or negative charges) from the solution phase (water) to the solid phase (soil). Sorption actually describes a group of processes, which includes adsorption and precipitation reactions.
Adsorption refers to the attachment of ions to soil particles, by either cation exchange or chemisorption. Cation exchange involves the physical attachment of cations (positively charged ions) to the surfaces of clay and organic matter particles in the soil. This a much weaker attachment than chemical bonding, therefore the cations are not permanently immobilized in the soil. Many constituents of wastewater and runoff exist as cations, including ammonium (NH4+) and most trace metals, such as copper (Cu2+). The capacity of soils for retention of cations, expressed as cation exchange capacity (CEC), generally increases with increasing clay and organic matter content. Chemisorption represents a stronger and more permanent form of bonding than cation exchange. A number of metals and organic compounds can be immobilized in the soil via chemisorption with clays, iron (Fe) and aluminum (Al) oxides, and organic matter. Phosphate can also bind with clays and Fe and Al oxides through chemisorption.

Phosphate can also precipitate with iron and aluminum oxides to form new mineral compounds (Fe- and Al-phosphates), which are potentially very stable in the soil, affording long- term storage of phosphorus. In the Everglades, and other wetlands that contain high concentrations of calcium (Ca), phosphate can precipitate to form Ca-phosphate minerals, which are also stable over a long period of time. Another important precipitation reaction that occurs in wetland soils is the formation of metal sulfides. Such compounds are highly insoluble and represent an effective means for immobilizing many toxic metals in wetlands.

Volatilization, which involves diffusion of a dissolved compound from the water into the atmosphere, is another potential means of contaminant removal in wetlands. Ammonia (NH3) volatilization can result in significant removal of nitrogen, if the pH of the water is high (greater than about 8.5). However, at a pH lower than about 8.5, ammonia nitrogen exists almost exclusively in the ionized form (ammonium, NH4+), which is not volatile. Many types of organic compounds are volatile, and are readily lost to the atmosphere from wetlands and other surface waters. Although volatilization can effectively remove certain contaminants from the water, it may prove to be undesirable in some instances, due to the potential for polluting the air with the same contaminants.

Conclusions

A wide range of physical, chemical and biological processes contribute to removal of contaminants from water in wetlands. These processes include sedimentation, plant uptake, chemical adsorption and precipitation, and volatilization. Removal of contaminants may be accomplished through storage in the wetland soil and vegetation, or through losses to the atmosphere.
An understanding of the basic physical, chemical and biological processes controlling contaminant removal in wetlands will substantially increase the probability of success of treatment wetland applications. Furthermore, a working knowledge of biogeochemical cycling, the movement and transformation of nutrients, metals and organic compounds among the biotic (living) and abiotic (non-living) components of the ecosystem, can provide valuable insight into overall wetland function and structure. This level of understanding is useful for evaluating the contaminant-removal performance of constructed wetlands and for assessing the functional integrity of human-impacted, restored and mitigation wetlands. More detailed discussions of wetland biogeochemistry and contaminant removal in treatment wetlands can be found in the references listed below.

References

Kadlec, R.H., and R.L. Knight. 1996. Treatment wetlands. Lewis Publishers, Boca Raton, FL.
Mitsch, W.J., and J.G. Gosselink. 1993. Wetlands. Van Nostrand Reinhold, New York.

Reddy, K. R., and E. M. D’Angelo. 1994. Soil processes regulating water quality in wetlands. p. 309-324. In Mitsch, W. J. (ed.) Global wetlands: old world and new. Elsevier Science, Amsterdam.

Footnotes

1. This document is a reproduction of SL155, a fact sheet of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Published: May 1999. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.
2. About the author : William F. DeBusk, former assistant professor and extension specialist, Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611-0510.

Originally published by U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean, and reproduced here under licence for educational purposes (see copyright information below).

Copyright Information

This document is copyrighted by the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS) for the people of the State of Florida. UF/IFAS retains all rights under all conventions, but permits free reproduction by all agents and offices of the Cooperative Extension Service and the people of the State of Florida. Permission is granted to others to use these materials in part or in full for educational purposes, provided that full credit is given to the UF/IFAS, citing the publication, its source, and date of publication.

Original url no longer available, but also available at https:// web.archive.org/web/20070217062459/http:// edis.ifas.ufl.edu/SS293

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