The search is on for a solution to a problem caused by a growing global population: more garbage. And it’s not just the volume of trash, but the fact that it can contain increasing volumes of materials with contaminants of emerging concern.
Constituents from these materials often make their way into landfill leachate, which requires treatment before being released into the environment. Landfill owners and operators are challenged with finding a cost-effective and reliable method of managing contaminated leachate.
The problem becomes more acute as science discovers the potentially harmful effects of some leachate constituents, some of them from new sources and some coming from products used for decades. One example of constituents found in many leachates, which have the potential for adverse human health impacts, is Per-and Poly-Fluoroalkyl Substances, collectively known as PFAS. PFAS are commonly used in products ranging from breathable outerwear to non-stick cooking pans. When these products are disposed of, some of their PFAS content can be released and enter the landfill’s leachate. Other sources of PFAS can include bio-solids from wastewater treatment plants (WWTP), industrial wastes, or contaminated soils that may be disposed of in the landfill.
PFAS pose problems for biological treatment systems which are typically used for leachate (by onsite facilities or direct discharge to a WWTP) as these systems are often unable to remove some constituents including PFAS. Moreover, the treatment of leachate containing PFAS at a centralized WWTP can generate PFAS concentrated sludge which may be land applied or disposed of at another landfill, thereby spreading PFAS contamination. Other treatment processes frequently used in drinking water or groundwater applications for PFAS (such as granular activated carbon and ion exchange media) are often not suitable for complex leachate matrices.
With the limitations of biological and media-based treatment, some landfills are looking to alternative treatment methods such as reverse osmosis and evaporation, as well as other developing technologies such as electro-coagulation (EC) and electro-oxidation (EO). Technologies developed for other applications such as macro porous polymer extraction (MPPE) are also being tested for potential PFAS removal. The variability between different leachates and other site constraints means that the preferred treatment process may be different from site to site and may require site-specific treatability testing before full scale application.
Electrocoagulation and electro-oxidation may be promising for bulk removal of PFAS where the concentrations of these compounds are significantly elevated. These technologies may not always reduce concentrations to sufficiently low levels and may require additional polishing. Because EC/EO are promising from the standpoint of simplicity of operation and avoidance of the use of chemical additions, these technologies continue to be studied for ways to fine-tune the process to be capable of treating PFAS to lower levels.
In evaporation, the leachate is heated so that the water converts to a vapor leaving the leachate constituents within a concentrated slurry stream. Some landfills use the methane produced by decomposing waste to fuel the evaporator; however, evaporation has its own problems. One is that constituents such as PFAS may escape into the atmosphere along with the water vapor. Another is that using landfill gas this way means it is not then available to produce electricity, an income source for some landfills.
Accordingly, one of the more promising leachate-treatment technologies is reverse osmosis (RO). It uses a pump and high pressures to push leachate against a series of semi-permeable membranes. Larger molecules and particles stay behind, while the smaller water molecules pass through the membrane to the other side. Today, RO is used in drinking water, wastewater re-use, and industrial applications, as well as landfill leachate treatment.
One of the main advantages of RO is that it can deal with a wide range of contaminants, including PFAS and also has a good chance of being able to treat contaminants that may become of concern in the future.
RO can, in many cases, produce effluent pure enough that it can be discharged directly to streams and rivers or could be used for onsite dust control. The concentrated leachate left behind by the RO process, generally about ten to twenty percent of the original volume, may be recirculated or used to optimize waste compaction within the landfill depending on regulations. Recirculation retains the PFAS in the landfill and minimizes further spreading of contamination to the environment although also risks generation of higher strength leachate.
Further management of the concentrated brine would be required in cases where leachate recirculation is not permitted or there are concerns with contaminant cycle-up. These management steps could include evaporation, thermal destruction, and/or solidification/stabilization with placement back in the landfill.
Paste technology, long used to manage waste materials in mining, could be incorporated into solidification/stabilization processes for brine or treatment residual management. Golder is currently testing the combinations of concentrated leachate with coal ash to produce an inert physical mass called “paste,” which hardens to lock the harmful constituents away long-term.
Many factors including sustainability, site-specific constraints, capital and operating costs, and an understanding of the latest advancements are critical to developing the best overall solutions. Leachate treatment for contaminants of emerging concern such as PFAS can represent a significant challenge, however new developments and technology applications are helping to overcome these challenges.