Biogeochemical Clogging of Landfill Leachate Collection Systems: Why It Is Occurring, How It Can Be Cleaned and What Can Be Done to Prevent It Part II

Nathan P. Mayer, Manuel J. Hernandez, Abdul Mulla Saleh, S. Steven Carl and Ralph Calistri

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In Part 1 of this article (国产麻豆, July 2013), the reasons behind precipitate formation in landfill leachate collection systems (LCS) were discussed. Leachate analyses and precipitate mineralogy show that supersaturated conditions with respect to calcium, chloride, sodium, sulfate, etc., as well as the presence of sulfate and iron reducing bacteria (SRB/IRB), play a major role in forming biologically induced calcium carbonate precipitation in a reduced landfill environment. Part 1 of this article also discussed cleaning technologies available to remove precipitate formation from the LCS, and the results of the cleaning demonstrations used to evaluate both the effectiveness and cost of the cleaning technologies discussed. Based on the results of the demonstration and an evaluation of cost, chemical cleaning was determined to be the best technology for removing heavy precipitate buildup, especially in the lower lengths of the LCS, pipe sections with multiple access points, and non-perforated piping.

Part 2 of this Article discusses the full-scale chemical cleaning projects employed, including methods used and results.

Full-Scale Chemical Cleaning

During the cleaning demonstrations, chemical cleaning was successful at removing most of the precipitate formation, especially in the lower lengths of the LCS. Video inspections confirmed that this technology was successful at removing heavy buildup and restoring flow in areas that were clogged and/or appeared stagnant at the beginning of the cleaning demonstration. Chemical cleaning is capable of cleaning over 2,000 feet of pipe per week, which equates to approximately $20 to $40 per foot of cleaned pipe.

The chemical cleaning technology was provided by Progressive Environmental Services (PES) (Norfolk, VA). Prior to the LCS cleaning demonstrations, PES has had extensive experience in cleaning piping system for commercial customers and the Navy, including Collection, Holding, and Transfer (CHT), Auxiliary Salt Water (ASW), fire mains, gray water systems, clean water systems, and combat related systems including Counter Measure Washdown and Combat System Cooling Loops. Scale and/or debris creates blockages within these systems, which typically consist of both 90-10 copper nickel and stainless steel piping ranging from 1.5 to 10 inches. PES鈥檚 proprietary products are designed to penetrate, disperse, dissolve, and remove scaling and corrosion by-products, biofilm and all other existing microbial activity. Metal surfaces, seals and gasket materials are unaffected by contact through the use of an inhibitor. The byproducts of the reaction between precipitates and the chemical cleaning solution are calcium, sodium chloride, carbon dioxide and water, all of which are non-toxic and non-hazardous. Once the acid solution has reacted and has neutralized, no corrosive ingredients remain and the solution is approved for disposal in sanitary sewer systems.

Using a proprietary, certified biodegradable acid solution consisting of hydrochloric acid, carboxylic acid, alkanolamine and sodium alkylsufonate with the addition of catalysts, dispersants and inhibitors to protect any associated base metallurgy from acid attack, this technology was shown to be a very effective mechanism in removing heavy precipitate formations and restoring flow in the landfill LCS. During the cleaning demonstrations, chemical cleaning was conducted by surging the system with large volumes of solution and allowing the chemical to soak and react with the precipitate. Formulas were modified to create foaming from the cleaning solution to enhance cleaning activity farther into the system and into the aggregate envelope surrounding the perforated LCS pipes. In locations with non-perforated piping and perforated LCS pipe with multiple access points, chemical cleaning was conducted by setting up a 鈥渓oop鈥� and allowing the solution to pass through and penetrate the precipitate. Chemical cleaning was successful at removing nearly all of the precipitate in non-perforated LCS piping and perforated LCS pipe with multiple access points. In single access LCS piping with perforations, the surging method was shown to be successful at removing nearly all of the precipitate from the first 100 to 150 feet. Some precipitate remained in the perforated laterals beyond this distance.

A combination of surging, recirculation and foaming was used to clean the laterals, headers and gravity mains within the LCS during the full-scale chemical cleaning projects. Additionally, the team developed a technique to enable cleaning single access LCS piping with perforations at lengths greater than 150 feet by combining the chemical cleaning and high-pressure water jetting technologies. To do this, a wye-connection was attached to the end of each lateral and a high-pressure water jet was inserted into the top of the wye. The thrust of the jet was used to transport a Kevlar jet hose to the end of each lateral. Florida Jetclean (Jetclean America) created a 鈥渙ne-of-a-kind鈥� jet nozzle with a relief disk welded to the top. Once the jet was set in place at the end of the lateral, the pressure of the jet was increased to 鈥渂low out鈥� the relief disk; this was done to reduce the pressure required to pump through the nozzle. The Kevlar hose was then disconnected from the jetting equipment and connected to a pneumatic diaphragm chemical feed pump in order to introduce the chemical solution at the end (top) of each lateral. As designed, the LCS laterals conveyed the chemical solution back to the manhole, cleaning the perforated lateral and aggregate filter as it traveled downstream. The chemical solution was then collected at the wye-connection in the manhole and pumped back through the jet hose to the end of the lateral. This chemical feed 鈥渓oop鈥� enabled the team to clean precipitate along the full length of the lateral (see Figure 1, Figure 2, Figure 3).

During the full scale chemical cleaning projects, the team was able to remove most of the precipitate from more than 10,000 feet of perforated laterals, 4,000 feet of perforated headers, and 4,000 feet of LCS gravity mains. Post-cleaning video inspections confirmed that chemical cleaning was successful at removing heavy buildup and restoring flow in areas that were clogged and/or appeared stagnant at the beginning of the cleaning projects. Although unquantifiable, it is believed that chemical cleaning was also successful in removing precipitate from the aggregate envelope surrounding perforated leachate collection pipes. The figure below shows the volume of leachate collected in the leak detection system before and after the cleaning demonstration. As shown, the leakage rate through the primary liner was significantly reduced through the techniques employed (see Figure 4).

Conclusions

Clogging in any portion of the LCS can lead to higher hydraulic heads within the landfill, increasing the potential for leachate outbreaks over containment berms and/or leakage through the liner. Leachate analyses and precipitate mineralogy showed that precipitate formation can be attributed to supersaturated conditions with respect to calcium, chloride, sodium, sulfate, etc., as well as the presence of SRB/IRB, which play a major role in forming biologically induced calcium carbonate precipitation in a reduced landfill environment.

Cleaning of the LCS was slow and costly due to the magnitude of precipitate buildup in the systems. Chemical cleaning was determined to be the best technology for removing heavy precipitate buildup, especially in the lower lengths of the LCS, pipe sections with multiple access points, and non-perforated piping. Both chemical and mechanical cleaning technologies have distinct advantages and disadvantages, but when combined, the effectiveness and efficiency of cleaning is improved.

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Design and Operational Considerations

Research has shown that there is a correlation between precipitate formation and leachate flow patterns. Minimal flow and stagnant conditions create saturated zones within the LCS, allowing the formation and growth of precipitate over time, especially where nucleation has already occurred. It is important to keep leachate levels within the LCS as low as possible and controlled by gravity to minimize the inflow and outflow response time within the cells. Maintaining continuous flow within the LCS reduces both sediment buildup and the formation of precipitate within the pipe. Landfill operators should be encouraged to reduce, to the greatest extent possible, storage of leachate within the LCS and within the landfill cells. Additionally, design engineers should consider flow velocity when designing the lateral drainage slopes of the LCS.

Research has shown that leachate from ash monofills are dominated by high concentrations of dissolved calcium whereas leachates from MSW landfills have higher levels of bicarbonate and microbial activity, but contain lower levels of calcium species. Batch testing has also been conducted to evaluate the leaching potential of combustion residues from different sources. To summarize, fly ash tended to yield a supersaturated solution for both calcite and gypsum, but leachate derived from bottom ash was unsaturated for both calcite and gypsum. One reason for the higher level of calcium in fly ashes is the use of hydrated lime, usually in excess, in Spray Dry Absorbers (SDAs) to scrub sulfur dioxide gasses generated from waste-to-energy facilities. Acid gases react with lime to form solid salts, which are removed in particulate control devices such as baghouses. Even though bottom ash typically comprises 70 to 90 percent of the mass of combustion residues, fly ash yields a higher degree of calcium and other constituents that contribute to the formation of deposits. These results suggest that optimizing the lime slurry flow rate, further stabilization of the fly ash, and/or development of alternative ash management and disposal practices may help to reduce the extent of precipitate formation at facilities that landfill ash.

Similar studies have also shown that landfills containing both combustion residues and MSW appear to be more susceptible to clogging due to the relative contributions of each waste stream. Combustion residues provide the minerals (calcium) while the MSW provides biomass, carbonate, and electron acceptors. Furthermore, landfilling of treatment plant residuals can introduce more minerals (water treatment) and more biomass sources (wastewater treatment) into the landfill. Separating disposal of ash and MSW may help prevent or reduce the formation of precipitate at facilities that accept both ash and MSW.

In addition to continuous monitoring and maintenance of the LCS, the design and operational considerations discussed above may be used to help reduce and/or prevent the formation of precipitate. If precipitate formation has clogged the LCS and leachate head and leakage begin to increase, the combination of chemical cleaning and high-pressure water jetting can be used. The project team (CDM Smith, Progressive Environmental Services, and Jetclean America) continues to develop and improve the combined chemical/mechanical methods to further increase the efficiency of LCS cleaning.

Nathan P. Mayer, P.E., is a Project Manager for CDM Smith (West Palm Beach, FL). He can be reached at (561) 689-3336 or via e-mail at [email protected].

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Manuel J. Hernandez, P.E., BCEE, is a Senior Project Manager for CDM Smith (West Palm Beach, FL). He can be reached at (561) 689-3336 or via e-mail at [email protected].

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Abdul Mulla Saleh, Ph.D., P.E., BCEE, is the Solid Waste Practice Leader for CDM Smith (Tampa, FL). He can be reached at (813) 281-2900 or via e-mail at [email protected].

S. Steven Carl is President of Progressive Environmental Services (Portsmouth, VA). He can be reached at (757) 606-1840, via e-mail at[email protected]or visit the Web site at www.progressive.us.com.

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Ralph Calistri is an Officer for Florida Jetclean (Tampa, FL). He can be reached at (800) 226-8013 or via e-mail at [email protected].

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