Protecting groundwater quality is a principal focus of MSW regulations.

By Fred Doran

Protecting groundwater quality is one of the principal focuses of MSW regulations. Rules that came into effect more than a decade ago require engineering and operation controls to minimize the amount of moisture entering the waste and to collect generated leachate at the landfill base. Impermeable liners with leachate collection systems intercept percolating leachate before it impacts groundwater resources. This method of landfilling, along with the postclosure care requirements, comprises the "dry tomb" approach.

Just Add Water

But times are changing. Discussions on leachate recirculation and bioreactor technology now dominate landfill symposia. Although recognized as beneficial by academia and a few pioneering operators for decades, the enthusiasm for adding liquid to waste has skyrocketed in the past five years. The United States Environmental Protection Agency (EPA) is currently sponsoring bioreactor projects and drafting new rules more versed in recirculation.

Research indicates that moisture is the most important variable in waste degradation (Chian and DeWalle, 1979). The higher the water content, the greater the rate of decomposition. In the landfill, moisture acts to fill the waste pore space (i.e., the field capacity), promoting anaerobic conditions and facilitating nutrient transport through the waste. The lined landfill serves as a treatment vessel or, dare we say, an MSW composter. Collected leachate is recycled back into the waste. If allowed by research permit, additional liquid (e.g., septage, surface water) can be incorporated into the waste to elevate the moisture level to field capacity. A landfill that operates with moisture levels near field capacity is considered bioreactive (SWANA, 2000). A few projects add air to further enhance the process. These facilities are considered aerobic bioreactors.

Advocates of recirculation equate the greater decomposition rate to reduced long-term pollution potential through:

• additional leachate treatment in the waste;
• leachate volume storage, reducing generation;
• onsite leachate management responsibility;
• accelerated landfill gas (LFG) generation, allowing earlier collection and greater potential for beneficial use due to larger LFG generation over a shorter time frame;
• accelerated settlement before final closure;
• waste stability.

Elevated concentrations of contaminants in leachate, LFG generation, and settlement occur, not well into the postclosure period, but while the landfill is operating, personnel are on-site, and engineered systems for leachate collection and LFG collection are relatively new. Perhaps postclosure will not consume our financial assurance funds as we once thought. From a groundwater perspective, waste stability means that the leachate quality approaches drinking-water standards. John Baker, director of new technology for Waste Management Inc., defines leachate stability as being the point at which biodegradable fraction of the leachate is low. Or, in more technical terms, it is when the ratio of biological oxygen demand (BOD₅) to the chemical oxygen demand (COD) is below 0.2—0.1, according to Baker.

Let's look at data, with regard to leachate stability and a reduced risk for groundwater contamination, from two landfills that have taken the recirculation path.

A Tale of Two Landfills

Both the Delaware Solid Waste Authority (DSWA) and Crow Wing County, MN, believe that leachate recirculation is environmentally prudent.

DSWA is one of the pioneers of full-scale leachate recirculation, beginning with its Central Solid Waste Management Center (CSWMC) near Sandtown, DE, in 1985. This was the first landfill opened by DSWA and was initially constructed in 1980. CSWMC comprises five cells (Areas A—E). Area C is the focus of the following discussion.

Landfilling occurred within this 20-ac. cell from October 1988 through December 1993. During operation, Area C received about 120,000 tpy of MSW. The liner is composed of a 30-mil PVC liner overlain with 2 ft. of washed sand for leachate collection. Recirculation into Area C was accomplished using primarily vertical recharge wells, although spray irrigation and surface application were also used. Recirculation took place between 1990 and 1996, with volumes ranging between 34,200 and 1.4 million gal./yr. A summary of tonnage and leachate-recirculation volume for Area C of the CSWMC is presented in Table 1.

Table 1. Comparison of Tonnage and Recirculation Values

Located near Brainerd, MN, the Crow Wing County Landfill (CWCLF) began operation in 1991. The landfill has a permitted footprint of 22.5 ac. and receives about 34,000 tpy of MSW. Design includes a composite liner consisting of 24 in. of compacted clay overlain by a 60-mil, high-density polyethylene geomembrane and 18 in. of sand for leachate collection. The county uses permeable daily/intermediate cover soils or a spray-on alternate daily cover to promote leachate distribution through the waste.

Cell 2, the focus of our discussion, first received waste in April 1996 and was expected to reach capacity in early 2002. In 1997, the county decided to pursue the advantages of accelerated decomposition, developing and implementing a recirculation demonstration program. The county has installed perforated horizontal laterals at various elevations within Cell 2 over the life of the cell. The laterals are fed via a forcemain from the leachate pump station. The recirculated volumes have ranged from 434,839 gal. during the first full year of operation in 1998 to an estimated 1 million gal. in 2001. A tonnage and leachate recirculation volume summary for Cell 2 of the CWCLF is also presented in Table 1.

Rule of Thumb

According to John Baker, the current rule of thumb for the volume of water addition necessary to bring waste to field capacity is 40-60 gal./ton. This range is due to the variability of moisture contained within the incoming waste, precipitation, and the in-place waste density. Degradation is considered optimal when the water content approaches field capacity. In Table 1, data from both the CSWMC and the CWCLF were compared to this rule of thumb. For Area C, the gallons of leachate recirculated per ton of waste ranged from 3.0 to 11.2 annually. The overall value for Area C is 6.8 gal. recirculated per ton of waste disposed during the entire cell life. This is significantly below the ideal rule of thumb. Accelerated decomposition will occur at this water content, but not at an optimal rate.

On the other hand, Cell 2 at the CWCLF had annual values ranging from 13.1 to 36.1 gal. of recirculated leachate per ton of MSW disposed. The overall value is 18.0 gal. recirculated per ton of waste disposed during the life of Cell 2. Although these annual and total saturation values are not in the optimum range, one would expect evidence of accelerated degradation in the CWCLF leachate data, relative to the CSWMC data. Note that the CWCLF has a waste saturation level of this magnitude due to a waste disposal rate about 30% of DSWA's, while both landfills have recirculated significant leachate volumes.

From a bioreactor perspective, however, both sites would need to add significant quantities of liquid to reach a water content level of 40 gal./ton of in-place waste, the lower end of the optimum decomposition range. For CSWMC (Area C), an additional 20 million gal. would be needed; for Crow Wing (Cell 2), almost 5 million gal. This is the reason why bioreactors require additional sources of liquid. Some sites cannot generate the necessary leachate volumes to satisfy the optimal degradation demand.

Are We Stable?

Although the CWCLF needs a couple of more years for confirmation, both Area C at the CSWMC and Cell 2 at the CWCLF are stable. This is based on an evaluation of the leachate biodegradable fraction and quality relative to groundwater standards.

Figures 1 and 2 provide a comparison of BOD₅ and COD concentrations for the two sites. The time axis on each plot has been normalized so the two data sets can be temporally compared. For Area C at the CSWMC, both BOD₅ and COD values reach an apparent steady state near the end of Year 5. At the CWCLF, although more confirmatory data are needed, BOD₅ is nearing steady state during Year 4, almost one year earlier than CSWMC. Note that during Year 4, CWCLF did not have a BOD₅ or COD spike, as in previous years, indicating that the biodegradable waste fraction has been flushed or is held in the biomass. Similar spikes are not present after Year 3 in the CSWMC data set as well.

Figure 1. Comparison of BOD Concentration

Figure 2. Comparison of COD Concentration

Figure 3 provides a normalized comparison of the BOD₅/COD ratios using three-month moving averages to smooth the data. Although more data are required, it appears that leachate stability is approaching at the CWCLF. The BOD₅/COD ratios were less than 0.2 the last four months in 2001, or Year 4. Counter that with the ratios from CSWMC, which were primarily below 0.2 only after Year 8.

Figure 3. 3- Month Moving Average BOD/COD Ratio

Another key to leachate stability is quality that meets drinking-water standards. The premise is that groundwater quality would not be impacted by a leachate release under these conditions. For environmental risk evaluation, it should be pointed out that the rules (EPA and most states) require proof of attainment of groundwater standards at a point of exposure (e.g., compliance boundary) outside the landfill footprint. In the event of a leachate release, attenuation, dispersion, and dilution should further reduce concentrations prior to this point.

At our two sites, the results of a recent leachate sampling event were compared to the EPA primary drinking-water standards, also known as the maximum contaminant levels, or MCLs. This comparison is provided in Table 2.

Table 2. Comparison of Leachate Quality to EPA MCLS

In Area C at CSWMC, none of the parameters analyzed exceeded the MCLs in October 2001 (Year 12). October 2000 (Year 11) was the last round that an MCL was exceeded, for both lead and thallium. In Cell 2 at the CWCLF, only arsenic exceeded the MCL during the October 2001 (Year 4) event. Otherwise, the last parameter to exceed an MCL was cadmium in October 2000 (Year 3). Arsenic has persistently been above the MCL since 1998 (Year 1) and may be present due to the disposal of treated lumber. Minnesota has not adopted the federal hazardous waste exemption for chromated copper arsenate (CCA) treated wood. Therefore, disposal of such material in MSW landfills is required, rather than in construction/demolition landfills (Tom, 2001).

Conclusions

The focus of landfill design and operation is changing from the dry-tomb approach to liquid addition through leachate recirculation and bioreactor methods.

The amount of leachate recirculated per ton of waste is a good tool for predicting an increase in leachate stabilization. As evidenced through comparison of leachate-quality data from DSWA and the CWCLF, leachate stability occurs sooner and leachate quality improves more quickly with increased liquid added per ton.

Even though both sites have added significant liquid volumes, both would have to add significantly more leachate to reach the lower rule of thumb for optimum degradation.

By recirculating more than twice as much leachate per ton of MSW as DSWA, the CWCLF was able to reduce the time required to attain a steady-state BOD₅ concentration by 20% and a BOD₅/COD ratio consistently less than 0.2 by 50%.

Leachate recirculation allowed leachate quality to meet primary drinking-water standards within 11 years at the CSWMC and, with the exception of arsenic, within three years at the CWCLF.

The data verify that the higher the water content in waste, even if below the optimum rule of thumb, the faster the rate of degradation and, thus, leachate stabilization.

References

Chian, E.S.K. and F.B. DeWalle. "Effect of Moisture Regimes and Temperature on MSW Stabilization." Proceedings of the Fifth Annual Research Symposium, Municipal Solid Waste: Land Disposal. EPA-600/9-79-023, 32-40. 1979.

SWANA. "The Bioreactor Landfill — An Innovation in Solid Waste Management." Bioreactor Committee White Paper. Solid Waste Association of North America, Baltimore, MD. 2000.

Tom, Patricia-Anne. "Good Wood Gone Bad." Waste Age, Volume 32, Number 8. August 2001.

Fred Doran, P.E., is a senior environmental engineer at R.W. Beck in Minneapolis, MN. The author would like to thank John Baker, director of new technology for Waste Management Inc.; Dan Fluman of the Delaware Solid Waste Authority; Anne Germain of the Delaware Solid Waste Authority; and Doug Morris of Crow Wing County for their input into this article.