Ozone/Advanced Oxidation of Methane Condensate at Landfill Co-generation Utility
The degradation of waste in a landfill is accompanied by production of gas by anaerobic bacteria that digest buried refuse.
The degradation of waste in a landfill is accompanied by production of gas by anaerobic bacteria that digest buried refuse. Under the right conditions, a municipal solid waste (MSW) landfill will produce up to 1.4 cubic feet of gas per lb of waste, 50% of which is methane gas (see Figure 1).
The environmental, health and safety hazards posed by the discharge of this explosive, greenhouse gas are significant. Thus, local, state and federal agencies tightly regulate all landfill gas emissions. Small landfill operations frequently dispose of the gas by burning it off with gas flares designed for methane gas combustion. Larger landfill operations, though, often have sufficient volume to utilize the methane as a fuel source to produce power. It’s during processing of the gas that some landfill co-generation facilities produce a waste stream too toxic for direct discharge to the environment.
In October 2002, Mazzei Injector Corp., a specialist in ozone use for water and wastewater disinfection and advanced oxidation, was contacted by a co-generation utility that was extracting methane gas from a 260-acre landfill in Salem Township, Washtenaw County, MI. The utility utilized the landfill methane as combustible fuel for three 30-megawatt gas turbines.
The plant was designed to operate as a closed system, with the exception of a small water stream generated as a by-product of the dehumidification process used to prepare the methane for combustion. The 10 gpm gas condensate generated by the drying process had a dark, oily appearance. Laboratory analysis of the waste condensate showed it contained a range of organic contaminants:
- Trichloroethylene (PCE)
- Tetrachloroethylene (TCE)
- 2,4 Dichlorobenzene
- 2,5 Dichlorobenzene
Initially, the utility set up a system to collect the waste condensate for weekly transportation to an offsite processing site. In recent years, however, the number of regional offsite processing facilities became limited, resulting in a significant increase in processing costs. This prompted the utility to investigate methods for on-site remediation of its waste stream.
Evaluating Effective Options
Prior to contacting Mazzei, the utility looked at two non-ozone methods for onsite remediation: a biological process and activated carbon.
Biological processing of the waste stream was attractive because of its wide acceptance by state and federal environmental agencies. The proposed process, though, had a capital cost exceeding $400,000 and required excessive land space as well as a licensed operator.
Activated carbon was initially viewed as a cost effective alternative to a biological wastewater plant. It was later determined adsorption of organics onto the carbon would result in the spent carbon drums being classified as a toxic waste. The cost to dispose of 1,000 lbs of expended carbon per week would far exceed the utility’s current expenditures for offsite wastewater processing. An extensive Internet search by the facility’s environmental engineer eventually led the utility to consider ozone oxidation as a possible method to detoxify the plant’s waste stream.
Initial conversations with the utility’s engineering staff provided little information on the exact nature of the waste stream. Rather than provide Mazzei’s technical staff with a water analysis, it asked for a general overview of the type of compounds ozone would remediate. Several weeks of conversation, and e-mails were devoted to discussing the oxidation of organic compounds by type. For example, ozone will readily oxidize aromatic compounds (see Figure 2).
Ozone isn’t an effective oxidizer for chlorinated hydrocarbons or for the degradation of alcohols. To rapidly degrade these types of organics, an advanced oxidation process (AOP) is more effective. In an AOP, a chemical species called a hydroxyl radical is generated. Unlike molecular ozone, a hydroxyl radical is non-specific with regard to the type of chemical bonds it will oxidize. Methanol, for example, is rapidly oxidized to carboxylic acid and carbon dioxide by hydroxyl radicals (see Figure 3.).
Hydroxyl radicals can be formed using a variety of methods. One method requires the injection of hydrogen peroxide into a water stream followed by radiation of the stream with UV light. The photolysis of a single hydrogen peroxide molecule generates two hydroxyl radicals and occurs rapidly (<1 second) at a UV wavelength of 254 nanometers (see Figure 4).
Though UV irradiation of hydrogen peroxide dosed water is an effective means to produce an AOP, success requires light rays to penetrate through the water stream; consequently, in this case, the waste condensate’s poor water clarity immediately eliminated any consideration of UV driven radical formation.
A review of the chemical properties of hydrogen peroxide also prevented its use. The utility’s preference was to work with a stable chemical additive whose concentration could easily be quantified by its plant chemist. The utility eventually settled on an AOP that generated hydroxyl radicals through dissolution of ozone into water held at highly alkaline conditions. This process is typically established through pre-ozone injection of caustic soda, using a chemical feed controller set to maintain a pH of 10. The reaction of molecular ozone with the resulting excess hydroxyl ions is shown in Figure 5.
To confirm efficacy of an ozone-driven, high pH AOP and determine the required ozone dosage, the utility leased a packaged ozone system that would allow it to pilot the process. The pilot study was implemented on Feb. 26, 2003, and the equipment consisted of the following major components:
- 25 gram per hour ozone generator
- Stainless steel centrifugal pump
- Kynar® ozone gas injector
- Kynar Flash ReactorTM
- Degas SeparatorTM
- Degas Relief Valve
- Ozone off-gas P-trap drain and demister
- Thermal-catalytic ozone destruct
• Ambient air ozone safety monitor (gas leak detector).
A key feature of the ozone pilot skid was inclusion of a recirculation line that by-passed part of the oxidized water back into the gas injector pump for repeated ozone injection. This batch operation of the ozone skid allowed the utility to apply high ozone dosages to the raw wastewater at gas to liquid (Vg/Vl) ratios favorable for high ozone mass transfer efficiency.
The recirculation feature of the skid also provided two methods of controlling the applied ozone dosage - changing the ozone generator power setting to increase/decrease ozone gas concentration, and adjusting the pilot skid’s effluent/influent wastewater flow rate to change the flow volume of raw water oxidized. Using the two variables of gas concentration and wastewater flow rate, the applied ozone dosage becomes the ratio of the quantity of ozone delivered via the gas injector to the skid’s wastewater influent or processing flow (see Figure 6 & Table 1).
Another key design feature of the ozone skid was inclusion of a high velocity gas mixing device, to replace the standard pressurized ozone reaction vessel. The utilization of the gas mixing device reduced the skid footprint and weight, making it easier to set up and - when necessary - move to accommodate daily plant activity.
Based on pilot data, a 160 gram per hour ozone system was selected to treat the 10 gpm waste stream as part of a high pH AOP remediation system. Today, the waste condensate is sent to a storm sewer that discharges into a local river. Toxic contaminants, phenol and benzene, are at the non-detect level and oxidation by-products remaining are non-toxic organic acids, aldehydes and ketones. The total installed cost of the system, including the pipe run to the local storm sewer, was just under $100,000, making the return on this capital investment less than 10 months.
About the Author: Jim Jackson is national sales manager for water and wastewater systems at Mazzei Injector Corp., a Bakersfield, CA-based manufacturer of venturi injectors, specialized mass transfer nozzles and degassing separators. An active member of the International Ozone Association, he has been a water treatment professional for 26 years, working in the ozone industry since 1994. Contact: firstname.lastname@example.org