Most often, landfill gas system design is added to an existing landfill cell, but the co-authors of this article explain the benefits of considering the LFG system during the landfill bottom liner design process. Doing so during the early stages of the landfill’s life, make it possible to improve collection efficiencies, lower operating costs, and save time in the future.
Continue to the full article, Planning Ahead for the Bottom Liners, published in MSW Magazine’s November 2017 issue and learn about the co-authors from Sarasota County, Florida Public Utilities and SCS Engineers.
Temporary Landfill Caps
Temporarily capping landfill slopes is becoming a common measure for landfill operators. There are many benefits to closing landfill slopes with geomembrane on a temporary basis. One of the benefits is delaying construction of the final cover. Following is a discussion of the steps that should be taken to determine whether temporarily capping the slope with geomembrane and postponing the final cover construction is a better financial/operational decision.
Cost Burden
Constructing the final cover is costly, and it is considered an unavoidable expense that has no return on the money spent. Therefore, some operators perform a financial evaluation to determine whether the final cover construction costs can be delayed (provided, of course, that such delays are acceptable to the regulating agency). When evaluating whether to delay the final cover, the cost of maintaining the slopes during the postponement period should be considered. The operator must look at the financial aspects of either closing the slopes with a temporary geomembrane or of leaving the slopes open during the postponement period.
Temporary Landfill Capping Option
The benefits of temporarily capping the slopes during the postponement period may include:
The other side of the coin is the expense associated with the temporary cap. There may be repair costs associated with the geomembrane every few years in order to ensure that the temporary cap remains intact.
Leaving Slopes Open Option
The option of leaving the slopes open during the postponement period involves maintenance expenses such as:
The benefits of leaving the slopes open are twofold: first, the operator will save the costs of constructing the temporary cap; and second, the operator will gain additional airspace as waste settles during the postponement period.
Experience with the Temporary Capping Option
As discussed above, both options provide the benefit of gaining additional airspace during the postponement period. Constructing a temporary cap involves the costs of materials and installation, including the geomembrane and the ballasting system that keeps the geomembrane in place. Generally, the financial and non-tangible benefits of a temporary cap that remains in place five years or longer are more attractive than leaving the slopes open; therefore, most operators choose to install a temporary cap. The next step in the financial evaluation should be comparing the costs of the temporary cap to permanently closing the slopes without postponement.
Final Step in the Financial Evaluation
The next question is whether it makes financial sense to postpone the construction of the final cover.
Waste settlement during the postponement period and the resulting airspace are considered the determining financial factor in choosing the right option. If the present worth value of the airspace generated from waste settlement during the postponement period is greater than the cost to construct the temporary cap at the present time, then the temporary cap option would make financial sense; otherwise, the final cover should be constructed without postponement.
It should be noted that the length of the postponement period plays a very important role in this financial equation. Longer postponement periods have the potential for a greater gain in airspace. Another incentive that should be factored into the financial evaluation is the potential return on the money set aside for the final cover construction during the postponement period.
To assist with this financial evaluation, landfill operators are encouraged to discuss these options with their landfill engineers. Settlement models can be performed to calculate the amount of airspace that may be generated during the postponement period as well as the present worth value of the generated airspace. The returns on the final cover construction costs during the postponement will just be “icing on the cake.”
Read the related Advice From the Field blogs from the landfill and LFG experts at SCS Engineers:
Contact the author: Ali Khatami or your local SCS Engineers’ office.
The past few decades of advancements in developing new drainage media have led to the use of geocomposites as the primary drainage layer above the bottom lining system geomembrane. However, you need to be watchful for the free flow of leachate through the thin layer of geocomposite under high gas pressures near the bottom lining system.
Short of investigations and clear guidelines for addressing high gas pressure near the bottom lining system, you can use a gas pressure relief system near the bottom in future new disposal cells. The pressure relief system can simply include a few perforated high-density polyethylene pipes laid in parallel directly above the soil layer placed above the bottom lining system drainage layer, as shown in the schematic.
About the author: Dr. Ali Khatami
Landfill Leachate Management Services
The primary role of gas collection system laterals is conveying landfill gas to the final destination in the system; however, lateral pipes are also used to convey condensate in the system to a collection point such as a condensate sump. Between the gas collection laterals and the condensate sump, there are gas headers that provide vacuum to the laterals. Condensate sumps are primarily connected to gas headers for effective management of condensate in the system.
On many occasions, gas headers are installed over the landfill surface, where condensate sumps have to be constructed as well. This type design could potentially create issues during construction of the final cover system in the area. On numerous occasions, the condensate sump sticking up above the surface is too short causing the sump to be extended during the final cover construction.
The gas system in the area must be taken out of service to extend the condensate sump. Such interruptions are never welcome because odors or lack of gas productivity can cause serious issues for operators. Also, waste settlements at or around the condensate sump can cause other issues that must then be addressed during construction of the final cover. Furthermore, if the gas header connected to the condensate sump ends up in the middle of the waste column with tens of feet of waste above the gas header, there is the risk of the gas header collapsing under the waste surcharge load and causing issues in the flow of gas and condensate in the system.
Avoid problems by allowing for constructing the gas header and condensate sumps in the landfill perimeter berm.
There are major benefits when the gas headers and condensate sumps are located outside of the waste. First, settlement issues are avoided and secondly potential gas header collapse is averted because they are not situated below many tons of waste. Construction of the condensate sumps to the correct and final height is accomplished while avoiding any final cover construction delays because of issues with the condensate sump locations. In addition to these benefits, the condensate sumps are readily accessible for maintenance.
Under certain conditions construction of the perimeter, berm needs to be carried out in advance of construction of certain disposal cells to position the gas header and condensate sumps in the berm. The design adds to the planning time and requires close coordination with a landfill engineer, but the return is worth every penny.
Contact Dr. Khatami to learn more about the specific conditions that can increase your ROI.
Landfill operators are making significant investments developing comprehensive lining systems to protect human health and the environment. These lining systems are normally equipped with drainage layers to convey leachate reaching the lining system to collection pipes and sumps for removal from the landfill. Landfill operators are also heavily investing in collecting and removing landfill gas for disposal or conversion to renewable energy.
After three decades of experience with these systems, landfill liquids may still accumulate in some gas wells adversely impacting gas removal efficiencies. In these situations installing a pneumatic submersible pump in the gas well to lower the liquid head in the well, restoring gas removal efficiencies is standard practice. However, this remediation technique requires additional capital investment.
An Alternative Solution Exists
A more recent alternative, constructing vertical drains from the bottom up where gas wells are located, may be a better solution. Construction of a vertical drain/gas well begins by constructing a gravel pad at the bottom of the landfill after completion of the lining system and placement of the protective soil cover over the lining system. The gravel pad may be 15 ft. by 15 ft. in lateral dimensions by 10 ft. in height. Gravel pads are normally constructed where future vertical wells are planned to be drilled. The center of each pad is surveyed, and the information is used to locate future gas wells at some vertical distance above waste that is placed in the new cell over time. The gas well drilling continues until it reaches the gravel pad at the bottom of the landfill. The connection between the gas well gravel pack and the gravel pad at the bottom makes it possible for landfill liquids to flow down and drain directly into the leachate collection system below the gas well.
Vertical drains help landfill liquids reaching the gas well gravel pack to flow to the leachate collection system at the bottom of the landfill; thus preventing watering out the gas wells. This sustainable alternative keeps gas production efficient and is environmentally sound, requiring less capital investment.
About the Author: Dr. Ali Khatami
More information about Liquids Management and Landfill Engineering
Related Advice, Whitepapers, and Articles
Thanks to you, our clients, SCS Engineers has received many awards and industry recognitions for research achievements and technology innovations. Engineering News-Record (ENR) recently released the Top 500 Design List, ranking SCS Engineers in the top 100 for the 9th year in a row. In the same publication, SCS is ranked in the Top 10 Sewerage/ Wastewater Firms.
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A typical SCS landfill expansion project contains an engineering evaluation and analyses addressing important technical considerations which include the existing hydrogeologic conditions, global slope stability, Landfill base settlement, geomembrane compression and strain, leachate pipe strength, useful life of the existing infrastructure and utility lines, stormwater management, leachate and landfill gas system expansion.
This case study site, it shows that the vertical or piggyback expansion of a landfill is a unique way of solving landfill airspace shortage problem. Its feasibility is always site specific and depending on the existing waste types, slopes, liners, design capacity of leachate and gas collection, and stormwater management systems. In addition, the landfill design needs to be thoroughly investigated, engineered, and operated.
From the results of the global final slope stability and the landfill base settlement analyses, it concluded that a vertical expansion at the case study landfill will not increase the risk to human health or the environment over the existing regulatory approved conditions. A vertical expansion provides the landfill owner with an opportunity to increase the landfill volume and provide the residents with the maximum service life within the existing footprint of the permitted Landfill. This maximization of available resources does not expand the environmental footprint of the site and provides better environmental protection and at the same time creates a sustainable landfill site.
A vertical expansion provides the landfill owner with an opportunity to increase the landfill volume and provide the residents with the maximum service life within the existing footprint of the permitted Landfill. This maximization of available resources does not expand the environmental footprint of the site and provides better environmental protection and at the same time creates a sustainable landfill site.
This case study was presented at ISWA 2016.
Read and share the complete case study here.
SCS periodically prepares technical bulletins to highlight items of interest to our clients and friends. These are published on our website. This SCS Technical Bulletin addresses:
Read and share the SCS Technical Bulletin here.
SCS Coal Combustion Residual Services
Survivability of leachate collection pipes depends upon the gravel placed on all sides of the pipe. Proper placement of gravel around the pipe and the granular soil material over the completed pipe/gravel/geotextile burrito is of significant importance in the protection of the leachate collection pipe.
Read the article by Dr. Ali Khatami here.
SCS Advice from the Field is a collection of blogs, articles, and white papers written by SCS professionals like Dr. Khatami. Search “advice from the field” to browse all of the topics.
Jeff Marshall, PE, SCS Engineers will be presenting the topic of Hydrogen Sulfide Issues at CCR and MSW Co-Disposal Sites during the EREF and NWRA sponsored Coal Ash Management Forum in July.
The co-disposal of municipal solid waste and coal combustion residuals – particularly flue gas desulfurization (FGD) material – poses a significant concern regarding the generation of hydrogen sulfide gas. Hydrogen sulfide has an exceptionally low odor threshold, and can pose serious health concerns at higher concentrations. This presentation will identify the biological, chemical and physical conditions necessary for FGD decomposition and hydrogen sulfide generation. Recommendations for reducing the potential for FGD decomposition at co-disposal facilities will be presented. Technologies for the removal and treatment of hydrogen sulfide from landfill gas will also be addressed.
Jeff Marshall, PE, is a Vice President of SCS Engineers and the Practice Leader for Environmental Services in the Mid-Atlantic region. He also serves as the SCS National Expert for Innovative Technologies. He has a diversified background in environmental engineering and management, with emphasis on the chemical and human health aspects of hazardous materials and wastes. Mr. Marshall’s experience with hydrogen sulfide, odors, sulfate decomposition in landfills, and ash issues includes scores of projects dating back to the 1980s.
SCS Coal Combustion Residual Services
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