When National Source Performance Standards (NSPS) surface emissions monitoring (SEM) began in 1999, many landfill operators worried how the guidelines would affect them. However, a year-long study by EMCON/OWT Solid Waste Services, Bolingbrook, Ill., shows that SEM has presented few problems.
NSPS requires landfill gas (LFG) collection systems to maintain less than a 500 parts per million by volume (ppmv) methane concentration on the landfill's surface. SEM must be conducted quarterly. If emissions limits are exceeded, methane levels must be corrected, and the landfills must be retested at 10- and 30-day intervals to avoid violations. Also, NSPS requires visual observations to detect elevated concentrations of methane, including cover system cracks or distressed vegetation.
EMCON/OWT's nationwide survey in 1999 of 45 municipal landfills included 180 monitorings, with additional monitoring resulting from required 10- and 30-day retesting. During the first four quarters of SEM, the majority of the facilities tested complied with NSPS requirements. Facilities were monitored according to site-specific gas collection and control system (GCCS) designs using a flame ionization detector (FID).
Through the monitoring program, EMCON/OWT discovered:
- Approximately 80 percent of the LFG monitorings resulted in no SEM readings above the 500 ppmv regulatory threshold.
- When emissions exceeded threshold levels, simple cover system maintenance or minor adjustments to the GCCS corrected the problem. This was especially true at landfills using soil cap systems over the majority of the disposal area.
- Only one facility required installing additional GCCS components including extraction wells; constructing additional header piping or increasing an existing header piping size and capacity.
- Surface monitoring equipment does not operate well when temperatures fall below 40 degrees Fahrenheit. Low temperatures in northern states required late first quarter monitoring at several facilities for the SEM program.
- Increasing vacuum, flow and aggressive extraction in problem areas may be detrimental to long-term operations and wellfield productions, and may contribute to landfill fires. This may be due to stress on systems that weren't designed to function under NSPS operating requirements. Supplemental extraction components should be considered in these cases, and owners should consider operating them separately from the primary recovery system.
- Dry and hot summer conditions in southern landfills and freezing winter conditions in northern landfills did not cause problems as originally expected.
- Sixty percent of monitored areas already are under final cover. As sites continue to develop new disposal areas and expand their GCCS under interim fill conditions, future monitoring may require additional collection devices to maintain surface emissions limits.
The engineers also found that approximately 95 percent of the landfills exceeding the threshold passed the 10- or 30-day retesting after minor wellfield adjustments. Eighty percent of the retested facilities required cover system modifications.
Corrective actions primarily involved local cap system maintenance, including the addition of low-permeability soil or bentonite, or repairs to the flexible membrane liner (FML). Modifying or tuning the existing wellfield components, especially in areas adjacent to where overages occurred, was the secondary corrective action. Often, a redistribution of vacuum application and local extraction rates corrected minor surface emission problems.
Adjustments to existing wellfields to provide greater vacuum application and increased flow in problem areas may have been composting the waste and resulting in subsurface fires. Fires dramatically increased in the past 18 months as landfill operators prepared for and executed the NSPS monitoring protocols. This is because most systems were designed to operate in the center of the disposal area and provide high-quality gas recovery for beneficial-use projects. These systems do not lend themselves to efficient perimeter or surface control of landfill gas emissions.
Beyond that, anticipated surface cracking in northern and southern states was not a problem. It was suspected that landfills in the south would be more susceptible to surface emissions during the summer months due to a relatively arid climate and the drying effect on soil cap components. Conversely, it was suspected that surface cracks from repeated freezing and thawing at northern landfills would occur during winter months. However, while some surface cracks were found in both northern and southern landfills, none represented a major SEM impact.
Overall, EMCON/OWT found that SEM has not presented many problems to landfill operators. The majority of the facilities efficiently operated its LFG collection and control systems according to the GCCS plans. And, generally, where exceedances were discovered, the majority of facilities were able to address the overages and eliminate any violation of the NSPS surface emissions monitoring requirements.