Membranes are now commonly used in purification and treatment of drinking water and wastewater. The tightening of regulations elsewhere on landfill leachate disposal is now requiring more complicated filtration techniques. Philippe Moulin addresses how ultrafiltration modules are being applied to the area, as well as reverse osmosis developments.
The release from a sanitary landfill consists mainly of leachate, which has become the subject of recent interest as a heavily polluted wastewater and biogas that can be used as a resource for energy production.
Leachates are defined as the aqueous effluent generated as a consequence of rainwater percolation through wastes, biochemical processes in waste cells and the inherent water content of the wastes themselves. Leachates may contain large amounts of organic matter (biodegradable, but also refractory to biodegradation), where humic-type constituents make up an important group, as well as ammonia-nitrogen, heavy metals, chlorinated organic and inorganic salts.
The removal of organic material based on Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD) and ammonium from leachate is the usual prerequisite before discharging the leachates into natural waters. Toxicity analysis carried out using various test organisms (Vibrio fisheri, Daphnia similes, Artemia salina, Brachydanio rerio…) have confirmed the potential dangers of landfill leachates and the necessity to treat it so as to meet the standards for discharge in receiving waters.
Year after year, growing recognition of the impact that landfill leachate has on the environment has forced authorities around the world to implement increasingly stringent requirements for pollution control. Today, the use of membrane technologies, more especially Reverse Osmosis (RO), either as a main step in a landfill leachate treatment chain or as single post-treatment step has shown to be an indispensable means of achieving purification.
Leachate characteristics
The two factors characterising a liquid effluent are the volumetric flow rate and the composition - which in the case of leachate are related. There are many factors effecting the quality of such leachates, i.e., age, precipitation, seasonal weather variation, waste type and composition.
In particular, the composition of landfill leachates varies greatly depending on the age of the landfill. Data that summarise the ranges of leachate composition show that the age of the landfill and thus the degree of solid waste stabilisation has a significant effect on water characteristics. Values of COD vary from 70,900 mg.L-1 for a leachate sample obtained from the Thessaloniki Greater Area in Greece to 100 mg.L-1 with a sample from a landfill near Marseille, France that was over 10 years old. With few exceptions, the pH of leachates lie in the range 5.8 to 8.5, which is due to the biological activity inside the landfill. It is also important to notice that the majority of Total Kejeldahl Nitrogen (TKN) is ammonia, which can range from 0.2 to 13,000 mg.L-1 of N.
In addition, the ratio of BOD/COD decrease rapidly with the aging of the landfills from 0.7 to 0.04. This is due to the release of the large recalcitrant organic molecules from the solid wastes. Consequently, old landfill leachate is characterised by its low ratio of BOD/COD and fairly high NH3-N. The existing relation between the age of the landfill and the composition of the organic matter, may provide a useful criteria to choose a suited treatment process.
Conventional treatments
Conventional landfill leachate treatments can be classified into three major groups:
• Leachate transfer: recycling and combined treatment with domestic sewage
• Biodegradation: aerobic and anaerobic processes
• Chemical and physical methods: chemical oxidation, adsorption, chemical precipitation, coagulation/flocculation, sedimentation/flotation and air stripping.
For many years conventional biological treatments and classical physico-chemical methods have been considered the most appropriate technologies for the manipulation and management of high strength effluents such as landfill leachates. When, treating young leachate, biological techniques can yield a reasonable treatment performance with respect to COD, NH3-N and heavy metals. When treating stabilised (less biodegradable) leachate, physico-chemical treatments have been found to be suitable as a refining step for biologically treated leachate, in order to remove organic refractory substances.
With the continuous hardening of the discharge standards in most countries and the ageing of landfill sites with more and more stabilised leachates, conventional treatments (biological or physico-chemical) are no longer sufficient to reach the level of purification needed to fully reduce the negative impact of landfill leachates on the environment.
This implies that new treatment alternatives must be proposed. Therefore, in the last 20 years, more effective treatments based on membrane technology have emerged as a viable treatment alternative to comply and pending water quality regulations in most countries.
MEMBRANE TREATMENT METHODS
Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) are the main membrane processes applied in landfill leachates treatment. MF remains interesting each time that an effective method is required to eliminate colloids and suspended matter. For instance, in pre-treatment for another membrane process (UF, NF or RO) or in partnership with chemical treatments, but it cannot be used alone.
UF is effective to eliminate the macromolecules and the particles, but it is strongly dependant on the type of material constituting the membrane. Past studies have suggested that UF might prove to be effective as a pre-treatment process for reverse osmosis, but the elimination of polluting substances is never complete (COD between 10% and 75%).
More recently, UF has been applied to biological post-treatment of landfill leachate. Several hybrid processes such as activated sludge-ultrafiltration-chemical oxidation and activated sludge-ultrafiltration-reverse osmosis have been tested. Studies have demonstrated that 50% of the organic matter could be separated by the UF step alone.
Finally, UF membranes have been successfully used in full scale membrane bioreactor plants and high treatment levels have been achieved for landfill leachate in such a process.
The combination of membrane separation technology and bioreactors has led to a new focus on wastewater treatment. It contributes to very compact systems working with a high biomass concentration and achieving a low sludge production with an excellent effluent quality. However, few research studies are related to landfill leachate purification by membrane bioreactors.
NF technology offers a versatile approach to meet multiple water quality objectives, such as control of organic, inorganic, and microbial contaminants. Few studies mention the use of NF to treat landfill leachates. Nearly 60% to 70% COD and 50% ammonia were removed by NF, whatever the membrane material and geometry (flat, tubular, or spiral wounded), with an average velocity of 3 m.s-1 and a transmembrane pressure between 6 and 30 bars.
However, successful application of membrane technology requires efficient control of membrane fouling. A wide spectrum of constituents may contribute to membrane fouling in leachates nanofiltration: dissolved organic and inorganic substances, colloidal and suspended particles. In particular, natural organic matter fouling has recently gained interest.
RO seems to be one of the most promising and efficient methods among the new processes for landfill leachate treatment. In the past, several studies, performed both at lab and industrial scale, have demonstrated RO performance on the separation of pollutants from landfill leachate. Values of the rejection coefficient referred to COD parameters and heavy metal concentrations higher than 98% and 99%, respectively.
Back in 1998 one study reported that more than 80% of the total installed capacity of leachate purification by RO used a disc tube (DT) module. However, two issues have been identified, and remain today major drawbacks for the implementation of pressure-driven membrane processes, and particularly RO, to landfill leachate treatment.
Firstly membrane fouling, which requires extensive pre-treatment or chemical cleaning of the membranes, results in a short lifetime of the membranes and decreases process productivity. Secondly the generation of a large volume of concentrate, which is unusable and has to be discharged or further treated.
Discussion
Developing a suitable treatment strategy for any given landfill site depends on major criteria, such as the initial leachate quality. The knowledge of these specific parameters may help with the selection of suitable treatment processes for the lowering of organic matter present in leachate to meet the final requirements given by local discharge standards.
Today, the hardening of landfill regulations, controls and management hamper an efficient conventional treatment, which appears under-dimensioned or does not allow the specifications required by the legislator to be reached. Now membrane processes, and most particularly RO, offer the best solution and have been proven to be a more efficient, adaptable and indispensable means of (a) achieving full purification (rejection rates of 98-99% for RO) and (b) solving the growing problem of water pollution.
However, landfill leachate RO feasibility is highly conditioned by the control of concentrate treatment costs and the choice of the feed pre-treatment mode in order to reduce membrane fouling.
Residue production, which constitutes a capital environmental concern, still remains a major hurdle since it is usually unusable and has to be discharged, further treated or landfilled. Transport to an incineration plant equipped for the burning of liquid hazardous waste remains the preferred option (in spite of many controversies) but leads to high treatment costs and other possibilities are slowly gaining importance.
One such option is the solidification of residues with different materials, such as fly ash or sludges from wastewater treatment plants for disposal on the landfill itself. Another is the controlled reinjection of the concentrate into changing areas of the landfill.
Methods to reduce the cost of treating residues must be developed or improved with respect to ecological and economical requirements. Moreover, techniques to prevent or control membrane fouling need to be further investigated - such as suitable pre-treatment choice, modifications affecting surface membrane roughness or hydrophilicity/hydrophobicity, cleaning of membrane surface, etc.
While some research has shown that biological pre-treatments are often ineffective as RO pre-treatment, lime precipitation does appear a promising option for the pretreatment of RO membranes and the removal of colloidal particles and organic macromolecules that are the principal RO foulants of landfill leachates.
Although lime precipitation is traditionally used to eliminate the temporary hardness of the water by decarbonation, it has been shown by a number of studies – focusing mainly on underground or surface water treatment – to be able of removing by co-precipitation certain high molecular weight organic molecules such as humic and fulvic acids. As these are responsible for irreversible membrane fouling, the may be a solution for landfill leachate treatment using RO.
Year on year authorities have imposed more and more stringent requirements for pollution control. Even by combining biological and physico/chemical processes, only partial destruction of contaminants will be achieved. Due to the so called "hard COD", new regulations will not be reached. In recent years, membrane filtration has emerged as a viable treatment alternative to comply with existing and pending water quality regulations.
Author's note: Philippe Moulin is from the Laboratory of Mechanics and Acoustics at Aix Marseille Université. Co-authors of the article include E. Carretier, J.P. Bonnet, Y. Wyart B. Marrot. For more information, please email: [email protected]
Credit: TECAM
The system – studied from TECAM's Technical Sales Engineers on the base of leachate's parameters of young landfills of Campania evaluating the relation between costs and benefits of investment – is able to reduce until to 10% the leachate volume, producing an amount of dischargeable water until to 90% of the matter introduced.
