The Reverse Osmosis (RO) technique aims to extract clean water from the aqueous solution of organic and
inorganic contaminants that constitute the landfill leachate.
The process exploits the natural phenomenon of osmosis where by, if two aqueous solutions, with different degree
of concentration, are separated by a semi-permeable membrane, water from the weakest solution will pass through the
membrane to dilute the higher concentration solution on the other side.
The process will continue until the solutions on both side of the membrane display the same degree of
With reverse osmosis the process is reversed. Pressure is applied to a water solution, (leachate), against a
semi permeable membrane forcing the water molecules to pass through the membrane, thus forming the clean
The majority of the solutes or contaminants will be left behind forming the concentrate.
Reverse Osmosis is the finest/smallest physical separation method known.
Whereas in normal filtration solids are eliminated from a liquid, Reverse Osmosis holds the distinction that it
will remove solutes from a solvent.
As a technology, RO is well established in wastewater treatment applications.
Advances in membrane technology, in particular in the last 15 to 20 years, have allowed the development of RO
systems designed specifically for the treatment of leachate.
The retention efficiency is primarily depended upon the molecular weight and polarity of contaminants.
Reverse Osmosis membranes can result in the retention of more than 98% of large molecules dissolved in
Ions of valance 1 such as Na+, Cl- can also be retained.
Most commercially available plants are constructed as two stage plants with contaminant removal rates better
than 99.6%. Where unusually high strength leachate is treated or very stringent discharge consents apply, three
stage plants can be employed and achieve contaminant removal rates better than 99.98%.
Reverse Osmosis leachate treatment plants are widely used on landfill sites throughout Europe including Germany,
France, Holland, Belgium, Italy, Switzerland, Spain, Portugal and Greece.
More than 100 plants are currently operational some of them for longer than ten years.
A variety of membrane module systems are available including; proprietary tubular modules, spiral wound modules,
hollow fibre modules and disc tube modules. Standard spiral wound modules, hollow fibre modules and disc tube
modules are sensitive to the presence of solids in the leachate.
For this reason RO plants commonly incorporate a pre-filtration stage by sand-filters and other fine filters, such as ultrafiltration (UF) membranes.
Extremely fine filtration which due to the small size of this "molecular filter" can have a chemical
Can be used on leachates which:
- contain substances which would prevent biological treatment by inhibition of the growth of biomass to
provide suitable biologically active sludge
- are chemically contaminated and do not contain enough biological nutrients to support a biological sludge
for biological treatment
In common with all physical and filtration techniques the process separates and concentrates the contaminants
but does not render the biological contaminants and the ammonia present in almost all leachates harmless.
Quoted rates of 75% - 83% of separation of leachate by volume into a high quality water stream, difficult to
sustain over a long period.
Flux rates gradually reduce during periods between cleaning of membranes, and over the life of membrane
(Also, the RO industry routinely quotes for plant flow rates and %age separation as the plant size based upon
the flow into the process, and also for the same projects recommends recirculation of the concentrate back into the
landfill waste. Thus, in those circumstances the real net disposal rate and true %age separation rate is less by
the amount recirculated back into the landfill.)
High initial investment and running cost of the plant.
Immediately after backwashing initial flush of the discharge may show high contaminant levels, depending on RO
Concentrate expensive to dispose of off-site and may require costly further treatment such as encapsulation or
Where best used:
Industrial and chemical waste leachates if biological treatment is not feasible.
Only when biological treatment is unable to fully reduce COD, Total Nitrogen or salts to comply with a discharge
As a final stage after biological treatment to avoid colloidal biological etc, fouling.
High initial and running cost, including spent membrane replacement.
The least sustainable of the potential leachate treatment processes, due to:
- separation technique does not "treat"
- concentrate must be disposed responsibly
- disposal back into the landfill is unsustainable in the long-term, and contrary to EU policy regarding
- disposal to landfill eventually results in a recirculation effect within a lined landfill resulting in a
continually rising salinity and contaminant strength in the leachate. Logically after some years the RO Plant
will become less and less efficient due to rising contaminant levels in the leachate
- recirculation back into the landfill raises ammonia levels with some opinions being the landfill gas
producing methanogenic organisms may be inhibited and landfill gas yield reduced or poisoned.
Energy usage comments:
High energy use, as high or higher than any other process. Treatment of the concentrate requires further
intensive energy use.
Chemical usage/by-product production:
Chemicals used to reduce or prevent fouling may be hazardous.
Concentrate is toxic and requires special disposal.