Leachate Evaporation Using Landfill Gas

By Dennis E. Purchwitz

Evaporative systems typically require an air permit for an enclosed flare and, usually, a modification to the landfill's solid-waste permit to address leachate-management practices.

Evaporation is the only "treatment" technology available today that actually rids the water component from water-based waste streams. It can, for example, reduce the total volume of leachate to less than 5% of original volume.

Landfill-gas-fueled evaporation is a technology that effectively integrates the control of landfill gas and landfill leachate. During recent years several forms of evaporation utilizing LFG as a fuel have emerged. The different types of evaporation fall into the categories of:

• evaporation vessels
• spray-type dryers
• direct injection-devices

This article covers the first of the three.

Evaporation-vessel Basics
Several companies manufacture leachate evaporators. Depending on the manufacturer and the type of system selected, the volume of leachate evaporated by a single unit varies between 1200 GPD (0.83 GPM) and 30,000 GPD (21 GPM). Most installed systems are in the 5 to 10 GPM range.

Evaporation of landfill leachate involves heating the leachate to produce a water vapor. Metals in the leachate concentrate and precipitate, primarily as salts, while the organics volatilize and stripped away by the water vapor.

The organics are transferred from the liquid leachate phase to the exhaust vapor phase by a process analogous to air stripping. Most evaporative systems use a modified commercial enclosed LFG flare for a downstream thermal oxidation stage to destroy the trace organics. Because the operating temperature of the evaporator is low (180°F to 190°F), most of the heavy metals do not vaporize.

Leachate evaporators apply energy developed by burning landfill gas to heat and vaporize leachate. The primary features distinguishing different commercial leachate evaporation systems are their methods for transferring heat to leachate and treating the exhaust vapor.

Direct Transfer
Most commercial systems available use direct-contact evaporative technology, where heat transfers by means of direct contact between the leachate and the hot combustion gas. Depending on the manufacturer of the evaporator, the LFG combustion unit can be located:

• on top of the evaporation vessel—the hot combustion gases from the burner being directed downward through a downcomer pipe and the gases being bubbled through a small pool of leachate in the bottom of the vessel;

• on the side of the evaporator vessel—the hot combustion gases being exhausted through submerged horizontal burner tubes located within the vessel (a process known as "submerged combustion"). The hot gases inside the burner tubes are exhausted into the liquid through orifices located along the bottom of the burner tubes.

Indirect transfer
Alternatively, heat may be transferred indirectly from a landfill-gas burner through the walls of the heat exchanger to the leachate. A major concern in selecting the method used to transfer heat is to minimize harmful effects that precipitated solids may have on process efficiency. With solid heat transfer surfaces such as tubes, scale buildup will gradually reduce heat transfer efficiency. Cleaning then is required to restore performance.

Exhaust Vapor
Due to vapor stripping, the exhaust vapor from the leachate evaporator normally is laden with trace quantities of many different organic compounds. This exhaust vapor exits through a mist eliminator, which condenses large water droplets and returns most of the entrained liquid back into the evaporator. By removing large water droplets, the mist eliminator also removes much of the particulate matter from the evaporator exhaust.

The exhaust water vapor from the evaporators can carry the odor of the stripped organic compounds. To treat this condition, the vapor can be injected directly into a modified LFG enclosed flare. The enclosed flare burns LFG and the water vapor at high temperatures (that is, >1600°F for a minimum of 0.5 seconds) before the exhaust gas is discharged to the atmosphere. This temperature and residence time allow for the destruction of more than 98% of the volatile organic compounds (VOCs) present in the gas stream.

Data from the operations of different leachate evaporation facilities also have shown the emissions from the enclosed flare to:

• reduce the concentration of carbon monoxide (CO)
• very slightly increase in the concentration of nitrogen oxides (NOx)
• little change in the concentration of sulfur oxides (SOx)

Appraisal
Leachate evaporation systems are generally economically feasible only at sites where there is an adequate supply of LFG to evaporate the volume of leachate generated.

A typical landfill leachate requires approximately 22 to 25 standard cubic feet (scf) of LFG to evaporate one gallon of leachate. Additional energy is required in evaporative systems that employ thermal oxidation (landfill gas flare) to treat exhaust gases. This second thermal energy requirement depends on the quantity and quality of vapor generated in the evaporation process. Typically, a flare requires approximately 70 to 75 scf of LFG for each gallon of leachate evaporated. Thus, a reasonable estimate of the amount of LFG required to evaporate one gallon of leachate and treat the resultant exhaust vapor in a downstream enclosed flare system is 100 scf, assuming a methane concentration of 50%.

Unlike conventional treatment systems, evaporative systems are typically insensitive to changes in leachate characteristics including concentrations of BOD, COD, suspended and dissolved solids, and variations in feed temperature. Generally, pH is the only factor to which the evaporative systems are sensitive, and this is due solely to the potential corrosiveness of acidic leachate on alloys used in the evaporators. To ensure against pH being a problem, systems at sites where pH might drop below 7.0 should provide for pH adjustment.

Depending on the evaporative system, the material produced as a by-product is solid and grit-like material or a semi solid. The former can be buried directly in with the solid waste; the latter can be recirculated into the buried waste (if permittable by the respective state regulatory agencies) or buried in with the waste by adding a bulking agent such as sawdust, flyash, or lime.

About the Author: Dennis E. Purchwitz is a professional engineer with PBS&J, Inc., Bowie, MD.

The previous article was adapted from an excerpt of the presentation, "Emerging Technologies for Managing Leachate," delivered at Wastecon 1999, Reno, Nevada, Oct. 18 to 21, 1999.