Concentrate management from reverse osmosis leachate treatment – challenges and possibilities
As discharge limits tighten, many landfill owners have begun considering advanced leachate treatment processes, in many cases membrane-based technologies. These processes use physical separation to remove pollutants. I wanted to share my thoughts on the waste stream produced by reverse osmosis (RO) treatment and give my take on the challenges it presents.
Reverse osmosis is the gold standard
The ‘gold standard’ membrane process for landfill leachate is reverse osmosis (RO), which relies on physical separation based on the molecular size of constituents. RO produces a highly concentrated brine that includes organic compounds, microorganisms, and ions. Table 1 shows typical concentrations for key raw leachate and concentrate parameters.
On-site RO facilities can produce effluent that readily meets discharge criteria, specifically for ammonia (NH3), 60 mg/l. However, on average, the concentrate is about 2.6 times more ‘enriched’ than the raw leachate. RO concentrate is also up to 250 times more concentrated than typical sanitary wastewater, as you can see in Table 2. Based on total dissolved solids (TDS) and conductivity measures, the concentrate is equivalent to typical seawater. These characteristics make concentrate management a vital component of RO leachate management.
Proactive management of the concentrate stream is often postponed initially due to cost considerations – and the brine is reintroduced into the landfill. Due to its high contaminant load, concentrate recirculation does not produce the benefits that are usually associated with leachate recirculation. Instead, the highly concentrated constituents may even inhibit biological activity.
Depending on the site’s hydraulic properties, concentrate volume, and leachate collection system design, concentrate breakthrough will occur, contributing to an increase in leachate contaminant loads and the same molecules will be removed from the raw leachate again … and again …and (you get the picture).
Treatment options currently available
Several treatment options are available for treating concentrate, but no one technology has risen to the top of the heap yet. Site-specific and cost factors play the major role in determining which solution is selected. In general, distillation processes can achieve high volume reductions through phase transitions of the liquid portion into steam (for example, evaporation and compression). Electro-dialysis reversal (EDR) is an advanced separation process that combines the effects of selective membranes with the application of electric fields, similar to dialysis. EDR has been applied at a limited number of pilot plants with promising results.
The energy demand for distillation is low compared to evaporation. It has minimal impact on co-located landfill gas-to-energy projects. Unless waste heat from existing energy projects is available, the opportunity cost for using LFG in enhanced evaporation needs to be considered as part of a total cost of ownership evaluation.
Enhanced evaporation, shown in the following graphic, essentially boils the water, releasing water vapor (aka steam) into the atmosphere with any volatiles present in the concentrate. While thermodynamically preferable to evaporation, eutectic freezing has only successfully been demonstrated for neat, (single salt) brines. Because of the energetic advantage (freezing requires about 1/7 of the energy of evaporation), future developments of this technology may provide more comprehensive solutions for landfill leachate RO concentrate.
All processes produce a highly concentrated slurry. Chemical stabilization creates low solubility minerals or can selectively extract potentially reusable products, such as nitrogen compounds for use as fertilizers. Stabilized contaminants are less mobile and can be landfilled, in most cases even into sanitary solid waste landfills.
While RO can produce high quality effluent, the concentrate stream needs to be considered from the very beginning of a project. Concentrate recirculation is an interim solution and can provide some relief, but in the long run we will require a more comprehensive and sustainable solution for concentrate management. There are many challenges with using some of the available technologies due to the presence of multiple salt species and other contaminants.
If your site is using reverse osmosis for leachate treatment, I would like to hear about your experience managing the concentrate.
Dr. Kremen is a Client Manager based in Cornerstone’s New Jersey office.
Categories: Environmental Planning & Compliance, Landfill Engineering and Design, Operations and Maintenance, Solid Waste
Posted By Arie Kremen, PhD at 11:30 AM | No Comments on Concentrate management from reverse osmosis leachate treatment – challenges and possibilities
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