As any landfill operator knows all too well, many MSW challenges do not emerge until the waste is compacted and set to rest in a landfill.

By Abdul Mulla-Saleh

At that point, deposited materials—like food and other solid waste—break down, creating a variety of chemical and bacterial components that interact in an assortment of ways detrimental to the leachate collection and drainage systems.

The magnitude, degree, and extent of clogging appear to depend on the variety of wastes found in a landfill, as well as on its operational procedures and the performance of the leachate collection system. In some instances, frequent jetting and cleaning of leachate collection piping will maintain the system. In other cases, no matter how frequently an operator flushes a landfill leachate collection system with acid washes or other cleansers, precipitates continue to collect, creating a persistent problem.

Regions that experience dry weather often have few encounters with clogged pipes, but communities in a warm, high-precipitation state like Florida can spend in excess of $100,000 a year maintaining their drainage systems. Some landfill operators work a continuous remediation cycle; by the time they finish flushing the leachate collection system of each cell, it is time to begin the process all over again.

Prompted by widespread clogging phenomena in landfills, especially in Palm Beach County, FL, a team of researchers and industry professionals from the University of South Florida and Cambridge, MA-based CDM analyzed the deposits that clogged the drainage pipes at the landfill site. They also studied laboratory-induced clogging to understand the mechanisms that lead to clogging so they could develop strategies to mitigate or control this all-too-common problem.

Documented cases of clogged leachate collection systems date back to the early 1970s, when the USEPA raised concerns about the clogging of landfill leachate collection systems. This prompted the EPA to conduct research on test cells at the Boone County landfill in Kentucky. By the 1990s, landfills in Germany, England, Canada, and the United States were experiencing clogging problems.

Environment and Health
Aside from the cost and amount of labor involved in flushing leachate pipes, there are a variety of genuine environmental risks associated with clogged pipes. As long as a landfill remains free of such fluid build-up and continues to drain properly, there is no problem.

When the components of these draining systems clog—either with biogeochemical deposits, silt and biofilms, or precipitating minerals—the leachate can build up within a landfill, creating the potential to leak or seep into the groundwater or cause landfill slope instability, which occurs when liquid accumulates in a landfill, exerting pressure on its side.

When rainfall percolates through a landfill’s waste layers, leachate develops, causing the liquid’s constituent elements to clog drainage pipes. Heavy nutrient loading—associated with the movement of leachate initially through the waste matrix and then through the leachate collection system—inevitably causes microbial activity. In turn, the growth of these microorganisms fosters an environment where the biologically induced precipitation of minerals thrives. These minerals, which are present in leachate and form the clogging material, can potentially affect the drainage system’s performance.

After the waste is saturated to capacity, it releases its excess moisture. This is a continuous exchange, since decomposing waste is in a constant flux of absorption and degradation. Once released, leachate percolates through the landfill until it reaches the liner. Leachate derived from MSW is typically unpleasant, but not toxic.

Study Methodology
The waste materials, which were obtained from the North County resource recovery facility in Palm Beach County, included MSW bottom ash (the heavier ash from incineration), fly ash (a finer ash), and residue from water and wastewater treatment facilities.

The study was designed to accomplish two objectives. The first goal was to analyze leachate and precipitate clog material from the landfill site, simulate clog formation in a laboratory setting using lysimeters with different waste matrices, and analyze leachate and clog material formed. The second goal was to evaluate the impact of saturation and submerged conditions on clog formation, and document the steps that led to clogging by analyzing the collected data using geochemical or other appropriate models. Five lysimeters are currently being operated to address the second phase of the study.

Over eight months, eight lysimeters were operated, each simulating a specific landfill condition. Two lysimeters represented ash monofills with 80% bottom ash and 20% fly ash; two represented MSW monofills; and four were MSW reactors containing 60% MSW, 30% combustion residue, and 10% wastewater and water treatment residues.

After field capacity of the waste was reached, 4 liters of distilled water were applied to each lysimeter. The leachate was applied in a flood and drain pattern using a leachate application system comparable to the actual process. Every 24 hours, the leachate was recirculated using pumps. The application system was monitored weekly. This practice was meant to simulate rainfall and provide alternating cycles of flooding and rainfall. These activities accelerated the leaching reactions and provided adequate moisture for biological activity.

In Florida, landfills experience limited recirculation, but the state experiences enough rainfall that waste degrades fairly quickly. Factors that may affect clogging, including pH, alkalinity, calcium, and total dissolved solids, were monitored and analyzed on a regular basis.

Halfway through the eight-month study, leachate deposits began to form in the MSW, clogging the collection system. No deposits developed in the ash lysimeters. By investigating relationships between waste characteristics, leachate composition, and the potential for clogging, the team determined that clogging occurs when the balance of calcium species is disrupted by microbial activity, the additional leaching of materials, or a change in oxidation conditions.

At co-disposal sites, where substances like ash and MSW are landfilled together, the team observed an accelerated degree of calcium-carbonate (also known as calcite) precipitation in the leachate collection systems. In terms of pH, total dissolved solids, and concentrations of microorganisms, the leachates from the ash monofills were significantly different from the lysimeters containing MSW. The ash monofill lysimeter leachates were relatively free of particles of biomass.

These results, paired with the field study, indicate that landfills that contain a combination of combustion residues (ash) and MSW are more susceptible to clogging. The combustion residues provide the minerals, while the MSW provides the biomass and carbonate. The problem intensifies when treatment plant residuals are added
to the mix.The ash leachates had higher concentrations of total dissolved solids, calcium, sodium, potassium, and chloride than the MSW leachates. These were characterized by a greater presence of microorganisms, alkalinity, volatile solids, total organic carbon, turbidity, nitrogen, phosphorus, and silica.

Analyzing Constituents
In these studies, the clog material was formed in MSW and mixed-waste lysimeters (two mixed and two layered). Analysis of the clog materials revealed that the dominant elements of the precipitate included calcium, silica, phosphorus, sulfur, and iron.

Over eight months, a series of lysimeters was operated, each lysimeter simulating a specific landfill condition.

Although the precipitate composition varied among the samples, calcium levels were found to be between 50% and 80%. Calcium levels tended to be higher in the deposits formed in mixed-waste lysimeters containing MSW, ash, and water and wastewater sludges. Evidence of bacterial presence on all of the deposits suggests that anaerobic microorganisms played a role in the deposition process.

The process of such clogging appears to go through a number of microbially mediated stages. Anaerobic degradation of organic substrate, formation of surface biofilms, generation of slimes, and the growth of interconnected minerals and biomass gradually become denser. Entrapped within these formations are particles (such as silt, sand, or fines derived from the waste) that may accelerate clogging.

Leachate Variables
The makeup of a landfill’s waste varies, along with its chemical composition, depending on the types of waste deposited in the landfill. The relative amounts of MSW and process residues will affect the short- and long-term composition of leachates and biogeochemical reactions that result from waste degradation. Likewise, the chemical composition of landfill leachates depends on such characteristics as the age and components of the waste, the landfill age, and the amount of liquid that percolates through the landfill. Other factors include available moisture and decomposition.

Schematic of lysimeters used in this study: a) Group 1 lysimeters (1-4) with gravel over the leachate collection pipe; and (b.) Group 2 lysimeters (5-8) with sand above the leachate collection pipe.

The most important factor is the type of waste being disposed of, since this correlates directly with levels of calcium and alkalinity. Though various combinations of waste will exist in each landfill, Class I landfills are allowed by federal law to receive a combination of MSW, bottom and fly ash from combustion processes, and sludges from water and wastewater treatment processes.

Calcite: The No. 1 Culprit
Based on field studies and lysimeter tests, calcium was determined to be the dominant element responsible for the precipitate clogging. Carbonates, sulfates, iron, and phosphorous were also found to cause precipitate formation. These findings are consistent with other reported landfill clogging investigations.
By assessing both leachate from existing landfills and leachate created under laboratory conditions, the team determined that clogging occurs when the equilibrium among calcium species is disrupted by microbial activity, the presence of additional minerals, and a change in oxidation conditions.

At this stage of research, the rate of clog formation, the likelihood that the phenomenon will peak, and the extent of the formation over the life of the landfill are still unknown. Anaerobic bacterial activities in landfills will be present as long as the source of carbon and substrates exists. The team is hopeful that these questions will be answered through additional analysis and modeling efforts.

A New Era of Monofills
While this study focused on preventing clogging of leachate drainage pipes, many of its conclusions can be applied to today’s leachate management practices. Clog-reduction techniques include monofills for ash and MSW wastes, identifying effective acids or chemical agents that may control precipitate formation.

Leveraging Conditions
At this point, clogged pipes are usually fixed by flushing or power washing. But by routinely monitoring microbial activity and chemical-oxygen demand in landfill leachates, landfill owners and operators can develop a better understanding of exactly what materials interact to create the pesky biogeochemical deposits that clog collection systems.

With accepted landfilling practices unlikely to change in the near future, solid waste professionals should address these issues now. Waste will continue to be collected and its elements will continue to react to create clogging. Monofilling and co-disposal practices should be assessed further to prevent the formation of precipitates.
Although landfill operators can flush a drainage system with biocides and cleaning agents to control mineral formation and biofilm, this technique is unlikely to prevent future clogs.

Abdul Mulla-Saleh is an associate and solid waste specialist in CDM’s Tampa, FL, office.

MSW - July/August 2006