Municipal solid waste (MSW) management consumes the third largest portion of municipal budgets. To prepare for the estimated 250 million tons of wastes expected in the United States by 2010, the EPA introduced Subtitle D regulations in 1991. In particular, today's 5,000 active U.S. landfills must comply with federal regulations for landfill gas (LFG), leachate and condensate.
The Makings Of A Modern Landfill
A landfill's bacterial decomposition process consumes most of the available oxygen before it produces landfill gas, a mix of carbon dioxide and methane. Methane, which is a natural gas and an energy source, also is flammable and explosive in certain concentrations.
Modern landfills are designed to passively or actively vent LFG, then destroy or convert the gas to fuel.
Federal regulations re-quire:
* Monitoring LFG to de-termine if it's migrating through surrounding soils away from the landfill boundaries. Under RCRA, methane gas cannot exceed 5 percent at landfill boundaries;
* Repairing cracks in the landfill cover, if LFG is emerging; and
* Installing gas control/collection systems to prevent surface emissions from migrating.
Leachate and condensate must be restricted and captured to keep them from seeping outside of the landfill boundaries or into the groundwater. Often, captured leachate can be recirculated on the landfill so that it never leaves its boundaries. It also can be treated on-site and discharged or piped to a wastewater treatment facility for purification.
Federal regulations require:
* Leachate and LFG condensate which result in odors, toxic air contaminants or reactive organic compounds (ROC) must be controlled so that they don't release into the atmosphere.
* Leachate collection systems must be installed in conjunction with landfill liners. The collected leachate must be properly handled or disposed.
* Monitoring groundwater to detect leachate contamination and migration.
* Collection, treatment and disposal of condensate.
Owners or operators must retain inspection records as well as records which state the gas monitoring results, the placement of leachate or gas condensate, groundwater monitoring results and closure and post closure plans including financial assurance documentation.
Financial assurance may be demonstrated through a:
* Trust fund;
* Surety bond guaranteeing payment of performance;
* Letter of credit;
* State approval mechanisms; and
* State assumption of responsibility.
Equipment needs vary for each site. Although each is unique, most include common landfill gas collection systems such as a series of vertical and/or horizontal gas collection wells. The wells are tied into a gas collection header that transports the gas to a flare burner station, electric generation plant or a processing center.
Condensate collection sumps or knockouts are located along the gas collection header. These systems collect and transport the condensate through the collection header to a central collection facility where the liquid is either destroyed or treated and disposed.
With a leachate collection system, horizontal collectors are placed on top of the bottom liner or vertical collectors extend to the bottom of the landfill. Leachate is collected by extraction and transfer pumps and transported to a central collection facility for treatment and disposal.
To improve collection efficiency and reliability while reducing installation and maintenance costs, wellheads and prefabricated condensate and leachate recovery systems can be installed.
When choosing a wellhead, look for equipment that in-cludes an accurate and reliable flow meter package such as an orifice plate flow meter; a temperature probe for precise temperature readings; a control valve for throttling the gas; multiple sampling ports; and connection fittings for your specific application. Also, look for wellheads with corrosion-resistant material. Expect to spend approximately $315 to $345 for wellheads and between $175 to $250 for an orifice plate flow meter.
Typical problems with wellheads and flow metering equipment in-clude:
Low Flow Rates. Low flow rates are difficult to measure because many flow meters, such as pitot tubes, re-quire high flow rates to produce a minimum readable differential pressure. LFG wells often don't generate high enough flow rates for most flow meters to measure. To produce an accurate flow meter reading, choose a flow measurement device that can be customized based on the particular flow rate anticipated, even as low as .5 scfm. Accuracy of this equipment should reach Degrees 1 to 2 percent.
Condensate And Debris In The Gas System. Condensation and debris can corrode and plug a flow meter rendering it ineffective. To prevent clogging, use flow meters manufactured as a single piece, solid state-unit without moving parts. The location of the flow meter and its sensing parts also can eliminate clogging.
A typical flow profile shows the ve-locity of gas to be at the minimum point at the outside (the inside wall surface) of the pipe. Low velocity produces less migration of particulates and condensate that may cause plugging in the sensing lines. Avoid flow meters that are located in the center of the pipe where flow is at its greatest; these units obstruct the flow condensate and debris, maximizing the possibility of plugging.
When choosing a condensate and leachate recovery system (see photo on page 26), make sure it includes a collection reservoir, a durable pneumatic pump, a pneumatic level control, built-in air conditioners and a containment vault.
Each system also should feature corrosion-resistant material, including stainless steel, fiberglass, reinforced plastic, PVC, high density polyethylene, polypropylene and aluminum, to withstand the harsh landfill environment. Other important features include automated operation and variable pump capacity. These systems typically cost between $4,450 and $4,700.
To eliminate constant monitoring of liquid levels and pumping of sumps, choose a system that is fully automated and uses automatic level controls and switches.
Also, debris and particulates in the wastewater can affect the operation of the level controls and pump. Look for a system that uses pneumatic level controls and check the pump's solids handling capacity.
Safety in the design is accomplished through the use of air versus electricity. The complete pneumatic system prevents critical components from causing explosions and uses air normally existing at the flair burner station or electrical generation points. It also uses air from the LFG recovery system. Also, a system with a built-in secondary containment vault will protect equipment from outside dangers as well as prevent wastewater from leaking if the system begins to leak. Systems should incorporate a containment vault that is durable and minimally rated for incidental traffic loading.
Although the federal regulations bring new obstacles for landfill owners and operators, efforts to protect the air and ground from LFG, lea-chate and condensate will assist in the pursuit to prolong the well-being of the environment.
Like many of the 5,000 active landfills in the United States, one site in Habersham County, Ga., al-so is revamping to meet federal Subtitle D regulations. "[We would need a] four-acre bathtub to collect leachate," former County Adminis-trator Canup said while planning for the 150-acre site's liner and leachate collection system.
"Everybody wanted to avoid the stuff like the plague," said Consul-tant Steve Harbin of Hodges, Har-bin, Newberry & Tribble, Macon, Ga. The Georgia EPD suggested sending the leachate to a muni-cipal treatment plant. However, many wastewater treatment plants are leery of taking variable waste-water from a landfill. For the time being, the county agreed to haul the wastewater to a commercial treatment facility 100 miles away.
In Harbin's search for the most effective way of dealing with lea-chate, he asked Auburn University to survey the current methods used by landfill operators. As a re-sult, Harbin investigated activated carbon, biological processing, chemical coagulation, reverse os-mosis and various combinations of these methods. Habersham County wanted a treatment system which requires minimal time and labor.
The county elected to conduct a 30-day pilot project with reverse osmosis, a process which removes the clean water from the leachate. A Rochem system that uses a chamber of discs interweaved with flat membrane cushions was tested. Contaminated water is fed into the pressure vessel, where the flow is controlled as it passes through the discs and over the membranes.
A total of 300,000 gallons of lea-chate were processed. Most contaminants were reduced by 98 percent. During the pilot, the final permeate was passed through a small carbon canister to eliminate any remaining quantities of trace, volatile compounds. The concentrate is re-circulated back into the landfill and no materials are discharged off-site.
After the pilot was complete, the county proposed to operate a closed system. Effluent would be treated on-site and used to control dust, irrigate buffer zones and wa-ter grass. EPD required the system to treat the water in accordance to drinking standards.
The cost to operate the reverse osmosis treatment system was es-timated at 1 cent per gallon, not in-cluding capital costs. The current system reportedly is producing 3,000 to 3,500 gallons of clean water (permeate) per day.
Today, the reverse osmosis system has been in operation for six months. According to Landfill Op-erations Manager Stanley Duckett, membranes must be cleaned once a week and an attendant requires approximately two hours a day to take readings and to record data.
The treated effluent is currently being carried to a wastewater treatment plant until EPD grants on-site use. Habersham was paying approximately 10 cents per gallon to haul and treat the leachate at a commercial facility. Today, the county pays 3 to 5 cents per gallon, including capitalized costs.