Ultra-High-Pressure Reverse Osmosis for Landfill Leachate
1 November 2019
- Reverse osmosis (RO) is the best available technology to treat landfill leachate for surface discharge. Possible trace volatile organic compounds (VOCs) and ammonia emerging in the RO permeate can be removed with a polishing step to meet the highest discharge standards.
- Membrane fouling and RO leachate brine volume management need to be considered up front, and will influence treatment costs.
- A robust and fouling resistant ceramic ultrafiltration module can pretreat the foulants in landfill leachate to make existing spiral wound RO more reliable.
- RO leachate brine volume can be minimized through a membrane brine concentrator: ultra-high-pressure reverse osmosis in conjunction with advanced scaling-removal chemical softening. It can concentrate leachate total dissolved solids (TDS) up to 140,000 mg/L – around 10X volume reduction for the raw leachate depending on its water chemistry.
Current State of Reverse Osmosis for Landfill Leachate
Landfill leachate is a foul-smelling wastewater generated when water trickles down through buried solid wastes. Landfill leachate contains a wide range of pollutants, such as ammonia, heavy metals, organic matter, personal care and pharmaceutical chemicals, pesticides, per- and polyfluorinated substances (PFAS), total dissolved solids (TDS), and many more. In many jurisdictions, landfill leachate is prohibited from discharge into a public sewer without treatment. However, landfill leachate can be treated to meet regulation guidelines for safe discharge into an aquatic environment.
Existing landfill leachate treatment generally integrates the following options:
- physical-chemical (phys/chem) treatment using adsorption, air-stripping, flotation, coagulation/flocculation, chemical precipitation, membrane separation (ultrafiltration, nanofiltration, and reverse osmosis), and advanced oxidation;
- biological treatment method using anaerobic, aerobic and anammox treatments; and
- evaporation using mechanical vapor recompression and submerged combustion evaporation.
When landfill leachate must be treated to meet surface water discharge quality, reverse osmosis (RO) is the best available technology. RO offers an absolute separation barrier for all pollutants, more compact footprint and greater automation over other options. A final polishing step after RO can remove any trace volatile organic compounds (VOCs) and ammonia that may slip through the RO membrane into the permeate.
RO however also has two disadvantages for landfill leachate: membrane fouling and high disposal cost for a large volume of RO leachate brine. RO with unique modules such as disc-tube RO (DTRO) has claimed to address the membrane fouling issue to a certain degree. DTRO equips larger feed channels than existing spiral wound RO. During operation, landfill leachate is recirculated in DTRO module under high turbulence and crossflow to clean membrane surface. However, active membrane area of DTRO is much smaller than that of a spiral wound RO. DTRO requires a big footprint and high energy consumption with excessive brine recirculation. Operators also allege that DTRO require frequent cleaning maintenance and internal rebuilds. DTRO has not addressed the RO brine issue. The RO brine is usually about 20 – 40% by volume of the original leachate depending on the leachate chemistry. RO leachate brine is oftentimes disposed by evaporation, incineration, advanced oxidation, and/or
solidification/stabilization, which are energy intensive and with high cost.
Recent technology and process breakthroughs in desalination can help address both RO disadvantages while treating landfill leachate. There are several emerging commercial versions of ceramic membrane modules (for example, XtremeUF) that enable foulant removal pretreatment and the use of commoditized spiral wound RO for wastewaters with heavy foulants (for example, produced water from oil/gas production). Ceramic membranes are long-lasting, fouling-tolerant, and easy to clean. After many competitive suppliers entered the ceramic membrane market, membrane costs lowered tremendously over the years. As for RO brine volume reduction, a membrane concentrator, which uses a process of ultra-high-pressure RO (for example, XtremeRO) in conjunction with scaling-removal chemical softening (for example, BrineRefine), can now further concentrate RO brine up to a TDS of 140,000 mg/L, around double the upper TDS limit of existing spiral wound RO.
Ultra-High-Pressure RO for Landfill Leachate
Saltworks developed a more cost-effective solution for landfill leachate. The solution integrates three recent breakthrough water treatment technologies with four existing options: phys/chem treatment, XtremeUF, membrane bioreactor (MBR), Seawater RO (SWRO), BrineRefine, XtremeRO, and evaporation. Each step addresses a specific pollutant in landfill leachate to deliver an optimized treatment train. The process flow diagram present in Figure 1 shows all steps to provide a comprehensive view to the reader. Some steps are optional, depending on specific project requirements.
Saltworks engineers can review specific projects to deliver the balance of a simple but cost-effective treatment train.
The phys/chem treatment, if required, precipitates heavy metals that are toxic or inhibitory to microorganisms in the downstream MBR. Some fouling organics (for example, natural organic matters (NOM)) and scaling inorganics (for example, calcium carbonate) are also partially precipitated out.
XtremeUF is an ultra-robust ceramic based ultrafiltration module. It removes the solid slurries from the phys/chem precipitation and most organic foulants (NOM, oil and grease) that will otherwise foul downstream MBR and RO. XtremeUF is built with a well-engineered and intelligently automated package that cleans itself during operation. XtremeUF provides a pretreated “clean” landfill leachate for downstream treatment.
Membrane bioreactor (MBR) biologically degrades ammonia and biochemical oxygen demand (BOD) based pollutants. The MBR is divided into two reaction zones in series: a smaller anoxic zone for nitrogen removal, and a larger aerobic zone where the pretreated leachate is oxygenated for BOD removal. If ammonia removal is not required, the MBR is optional. As the reader can see, every case is unique and may only need a simple treatment train.
Seawater RO (SWRO) operates at less than 1200 psi to first remove dissolved solids, and non-volatile and non-degradable organics in the leachate after MBR. SWRO produces a brine about 20-40% by volume of the raw leachate as well as a treated water. The treated water may require an additional polishing step for trace ammonia and/or VOCs. This added polishing step is nominal in cost; Saltworks has successfully applied it in several projects to meet the highest discharge standards, including offering onsite pilots that result in successful full-scale projects.
BrineRefine removes potential scaling components in the SWRO brine for the downstream ultra-high-pressure RO. BrineRefine is an improvement to existing chemical softening: it helps avoid using coagulants that foul downstream RO systems, and incorporates a simple solid management system that reduces the sludge volume, and improves sludge settling and dewaterability.
XtremeRO is an ultra high pressure RO that operates at up to 1800 psi and is designed with widely available ultra-high-pressure RO spiral wound membrane modules. XtremeRO concentrates the BrineRefine treated brine to approximately 140,000 mg/L TDS. Compared to any existing thermal evaporation method, XtremeRO is about 3 times more energy efficient. The volume of landfill leachate brine after the Xtreme Reverse Osmosis is about 10% of the raw leachate volume, enabling a significant downsizing of final brine treatment which could include recirculation back to the landfill, solidification with fly ash or cement, or a final leachate evaporation by an evaporator/crystallizer.
Saltworks has successfully installed SaltMaker evaporator-crystallizers at several US landfill sites to evaporate RO leachate brine for minimum liquid discharge (MLD) or zero liquid discharge (ZLD).
Please feel free to contact Saltworks for a detailed review of your landfill leachate project, including risks and opportunities, as well as options to manage the resulting brine residuals.