Reverse Osmosis

Description:

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 concentration.

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 permeate.

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 leachate.

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.

Process Type:

Extremely fine filtration which due to the small size of this "molecular filter" can have a chemical treatment-like effect.

Advantages:

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

Disadvantages:

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 components.

(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 Plant design.

Concentrate expensive to dispose of off-site and may require costly further treatment such as encapsulation or incineration.

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 consent.

As a final stage after biological treatment to avoid colloidal biological etc, fouling.

Costs comments:

High initial and running cost, including spent membrane replacement.

Sustainability comments:

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 sustainable landfills
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.