An Overview of Common Physical Treatment Technologies for Landfill Leachate

Physical treatment is the most direct route for handling landfill leachate and its results are particularly strong. Using methods such as physical adsorption, gas stripping and membrane separation, it effectively separates and removes the harmful substances contained in leachate.

Air stripping

In this technique a gas is introduced into the wastewater and brought into intimate contact with the liquid, so that dissolved ammonia nitrogen crosses the gas-liquid interface and passes into the gas phase, thereby being removed. Air is normally used as the carrier gas. The method is now widely used at municipal landfill sites and can achieve ammonia nitrogen removal of about 60% to 80%, a very significant result.

Adsorption

This technique mainly uses activated carbon as the adsorbent. Because activated carbon has an enormous specific surface area and a well-developed pore structure, as leachate flows through it the organic matter and heavy metals it contains are intercepted and held inside the pores and thus removed. Activated-carbon adsorption is a mature technology in wastewater treatment. Studies show removal rates of about 75% for total organic carbon (TOC), up to 85% for chemical oxygen demand (COD) and as much as 95% for ammonia nitrogen (NH3-N) in landfill leachate.

Membrane separation

Membrane separation is the collective name for a family of methods in which a specific membrane allows selective permeation of different components of a liquid. Polymeric separation membranes are used to separate and concentrate target substances in solution effectively. The family covers microfiltration, ultrafiltration, nanofiltration and reverse osmosis; in current leachate practice, nanofiltration and reverse osmosis are the most widely applied.

• Nanofiltration: the separation performance of nanofiltration membranes lies between that of ultrafiltration and reverse osmosis, with a molecular-weight cut-off of roughly 200 to 1,000 daltons and a pore size of about 1 nm, making it particularly suitable for separating dissolved components of around 1 nm. In leachate treatment trains nanofiltration is often used as a pretreatment step whose permeate can meet discharge limits; where stricter limits on effluent salinity apply, a reverse-osmosis stage is connected downstream of the nanofiltration system for further desalination.

• Reverse osmosis: originally used mainly for seawater desalination, reverse osmosis has in recent years been extended to the concentration of landfill leachate. It delivers excellent performance in reducing COD, NH3-N and salinity in leachate.