Regulatory and siting restrictions are such that many solid waste operators prefer to expand their existing landfill footprint as much as possible instead of finding a new disposal footprint at a different location. As landfills are getting larger in height and greater in footprint area, the location of leachate tanks, leachate ponds, or discharge points to an on-site or off-site leachate treatment plant usually does not change. A larger footprint means leachate force mains are getting longer forcing the existing pumps to work harder to push leachate through the system to a target point. Some operators carry on with the same pumps for decades and do not monitor the performance of the pumps after expanding the landfill footprint, which could be more costly in the long-term.
Hydraulic Evaluations for Lateral Expansion
The longer leachate force main with possibly additional bends in the line increases friction in the line and causes flow rates to reduce to unexpected levels. We recommended that landfill operators evaluate the performance of the existing pumps along with new pumps when designing a lateral expansion. Such an evaluation may require hydraulic analysis of the entire network of pipes along with pumps, or only the segment of the network affected by the expansion. However, the effort is minimal in comparison to the operating costs of inefficient flow and overtaxing the equipment.
Sometimes the results of a hydraulic evaluation may require up-sizing all or specific pumps in leachate sumps because not enough flow can go through the force main due to high friction loss in the expanded leachate force main. Up-sizing pumps may be achievable depending on the type of the leachate sump, i.e., riser system or vertical manholes. If the up-sized pump in a riser system is too long to fit inside a riser system, or so long that it makes routine maintenance too cumbersome, your engineer may consider enhancing the functionality of the design.
Inline and Offline Pumps
Booster pumps located along an expanded leachate force main can certainly be an option. Booster pumps can be the inline or offline type. Install the inline pumps on the actual force main, and position the offline type on the side so that liquids go through bends and elbows to reach the pump, and again through bends and elbows to get back in the force main. In either case, the booster pump adds hydraulic energy to the flow inside the force main to push the liquids at a compensated pressure through the remainder of the force main and to the target point.
Operators need to be aware of the dynamic nature of the leachate piping network and the role of booster pumps in the dynamic environment. Changes to the flow in the force main may change following a landfill expansion when the new cells are coming online increasing leachate generation. Alternatively, after closing portions of the landfill slopes, that decreases leachate generation over time. Sometimes booster pumps have to be up-sized or downsized depending on the flow and pressure in the system.
Optimizing Performance, Reduce O&M Costs
The cost of replacing pumps, up-sizing, or downsizing, is insignificant compared to the revenue that landfills generate. Proper adjustment of the pumping system keeps the entire network operating at the appropriate range of pressure, and velocity in the line; increasing the life of the pumping system. Less wear and tear on the system produces a reduction in maintenance costs along with less equipment downtime.
Lower maintenance requirements may also reduce the number of personnel required to keep the system in operational condition. Landfills with a large pumping system employing a second technician because of the high maintenance of multiple pumps may find a single technician sufficient for the upkeep of the system. Proper sizing of pumps and operating the pumping system as designed within the evaluation parameters can significantly reduce the cost and frequency of pump maintenance.
About the Author: Ali Khatami, PhD, PE, LEP, CGC, is a Project Director and a Vice President of SCS Engineers. He is also our National Expert for Landfill Design and Construction Quality Assurance. He has nearly 40 years of research and professional experience in mechanical, structural, and civil engineering.
It is a general misconception that leachate seeps stop or disappear when slopes receive the final cover. In fact, it is only true if the source of leachate is located directly below the cover, but in most cases, the leachate originates from another location. Continuing seeps eventually reach the bottom of the slope, where two scenarios can happen depending on how the final cover geomembrane is secured at the landfill’s perimeter.
In the first scenario, where the geomembrane is anchored in an anchor trench, liquids will gradually flow underneath the cover geomembrane in the anchor trench and enter the perimeter berm structure. Leachate entering the berm structure softens the berm’s structural fill adversely impacting its shear strength. Additionally, leachate gradually seeps through the berm structure and enters natural formations below the berm and possibly into the groundwater. The operator is alerted when monitoring shows a localized structural failure or a groundwater impact in a nearby groundwater monitoring well.
In the second scenario, where the final cover geomembrane is welded to the bottom lining system geomembrane, leachate seeping out of the slope reaching the toe of the slope accumulates at the toe because it has nowhere to go. Accumulation of leachate behind the final cover geomembrane forces water to gradually move laterally along the landfill perimeter berm behind the final cover geomembrane damaging a larger area behind the final cover. Vertically, more of the area above the toe of the slope becomes engaged by the accumulating leachate. The two obvious consequences are the softening of the soil layer below the final cover geomembrane at the toe of the slope and the water-bedding effect of the area near the toe of the slope.
In the first scenario, the operator has to handle a non-compliance issue, either a failure in the slope or impacts to groundwater. In the second case, the leachate remains contained, but the operator has to address the issue by opening the final cover and removing leachate accumulated behind the final cover geomembrane. The geomembrane opening is closed, and final cover soils are restored after liquids are removed. Both are costly and complicated solutions. Moreover, the problem does not end after completion of the repair because the source of leachate seep is not eliminated.
Landfill operators can require their engineers to design a leachate toe drain system located at the toe of the slope and connected to the leachate collection system at the bottom of the landfill before the final cover geomembrane is installed. The leachate toe drain system is the only way to collect and route leachate to a location at the bottom of the landfill constructed for removal of leachate.
If you are closing a portion of your landfill slope and you find no leachate toe drain system in the construction plans, you can ask for a system to be added to the design plans before the commencement of the construction project.
SCS has significant experience with various types of leachate toe drain system constructed at different locations under various conditions. If you like to know more about the design of leachate toe drain systems or if you are looking for an experienced engineer for the design of your next final cover contact SCS.
Author: Dr. Ali Khatami
Landfill base grades not only make leachate collection and removal possible but also have a significant impact on the amount of landfill airspace. For landfill operators, airspace is the primary asset, because it represents the level of revenue the operator can expect. Airspace is a commodity to be maximized.
Operators expect to get the most airspace from their landfill designer and depend on the engineer to design the grades to maximize it. Placing your trust in an engineer is a noble matter, but as the operator, you check, verify, and confirm that what the designer has engineered is what is needed to provide you with the expected value. An experienced landfill designer looks for ways to provide airspace above and beyond the operator’s expectations.
SCS has been in the business of designing landfills for nearly half a century. We have significant experience in optimizing landfill designs and maximizing airspace. SCS is often retained to design a new expansion to an existing landfill. Upon starting work we analyze the entire facility holistically to see all of the potential ways to maximize airspace around and above the existing landfill. Every cubic yard of additional airspace is a big achievement for our clients and in turn for us.
SCS often evaluates permitted, yet to be developed, base grades for operators. The intent is to determine whether additional airspace can be achieved by applying a different design to the base of the landfill. SCS has turned the science of geometry into mathematical models utilized to quickly evaluate base grades. Specific parameters of the currently permitted base grades are plugged in the mathematical model along with those of the alternative and the model provides quantitative values (cubic yards) of the difference between the permitted grades and the alternative. The values are quickly returned. After modeling, the operator may decide to modify the design to gain the additional airspace based on the alternative design. Contact us to work with our landfill design experts to assist you with an evaluation.
Contact Dr. Ali Khatami with questions about the model.
It’s been 10 years since the first Research, Development, and Demonstration (RD&D) Plans were approved allowing liquids to be applied to municipal solid waste landfills in Wisconsin. What have we learned?
Under an approved RD&D Plan, landfill operators can apply liquids other than recirculated leachate to the waste at municipal solid waste landfills. The RD&D Rule was published by US EPA in 2004, and states had the option of adopting the rule and issuing RD&D approvals. Wisconsin was an early adopter, and 13 of the approximately 30 landfill sites in the US with RD&D approvals are in Wisconsin.
This presentation will look at data from the Wisconsin landfills with RD&D Plans. Each site is required to report annually on a very detailed basis. For this presentation we will zoom out and look at the data on an aggregated basis to address big-picture questions. What are the trends in volumes applied for leachate recirculation versus RD&D Liquids? How do these volumes compare with precipitation? What liquid waste streams have been accepted and how have they been applied? How has RD&D liquid application affected landfill gas generation?
We will also provide an update on the regulatory status of the RD&D rule. On May 10, 2016, a final federal rule was published that revised the maximum permit term from 12 years to 21 years; however, WDNR will have to adopt this change in order for it to be available to Wisconsin landfills.
High-density polyethylene pipes have been used for landfill leachate collection and conveyance lines for several decades because of the chemical compatibility of HDPE material with many different types of liquids and chemicals. Designing a leachate collection system for a landfill disposal cell involves numerous engineering analyses of different components involved in collecting and conveying leachate. One of the important engineering evaluations is a determination of structural stability of HDPE leachate collection pipes at the bottom of the landfill.
Structural Stability of HDPE Pipe
Modern landfills are gradually becoming larger and deeper; deeper landfills will naturally impose a higher surcharge loading on the HDPE leachate collection pipes below the waste column. Engineering methodologies for the structural stability evaluation of HDPE pipes with significant surcharge loading have been around as long as HDPE pipes have been in production.
There are three criteria used when evaluating the structural stability of HDPE pipes; wall crushing, wall buckling, and ring deflection. Wall crushing can occur when the stress in the pipe wall, due to external vertical pressure, exceeds the compressive strength of the pipe material. Wall buckling, a longitudinal wrinkling in the pipe wall, can occur when the external vertical pressure exceeds the critical buckling pressure of the pipe. Ring deflection is the change in vertical diameter of the pipe as the pipe deforms under the external pressure. Empirical formulas by HDPE pipe manufacturers or researchers are available to check each criterion.
SDR 11 vs. SDR 17 HDPE Pipe
When a structural stability evaluation involves high surcharge loading on the pipe, an engineer may automatically select SDR 11 HDPE pipe without going through an evaluation process. The engineer’s reasoning is that the higher wall thickness of SDR 11 pipe, as compared to SDR 17 pipe, is the logical choice because it provides a higher level of structural stability to the pipe. In the case of wall bucking and wall crushing, where the pipe strength in these two criteria is inversely proportional to the SDR value, the engineer is making the right choice. The strength is greater for the lower SDR value that represents thicker pipe wall thickness; making SDR 11 stronger than SDR 17.
However, in the case of ring deflection, the pipe strength is not a function of SDR, but a function of another parameter called allowable ring deflection. The allowable ring deflection value varies from one SDR to another and is generally reported by pipe manufacturers. The allowable ring deflection for SDR 17 pipe is greater than all other SDR pipes, which makes SDR 17 pipe stronger when considering ring deflection. SDR 17 pipe is also the most commonly used HDPE pipe in the landfill industry, being lighter in weight per unit length of the pipe than SDR 11, thus making it less expensive than SDR 11 pipe.
Which Is Best For My Landfill?
SCS Engineers recommends that landfill engineers consider SDR 17 pipe as the first choice for use as a leachate collection pipe below the waste column, and then other SDRs if SDR 17 does not pass the three structural stability criteria mentioned above.
Read more blogs by Ali Khatami, click here and type “Advice from the Field” in the search box.
We continue SCS’s Advice from the Field blog series with guidance from an article in MSW Magazine by Daniel R. Cooper, Jason Timmons, and Stephanie Liptak.
The authors of a recent article in MSW Management Magazine present engineering ideas that provide for more efficient construction of a GCCS. Gas system operators will benefit by having fewer pumps to operate and maintain and shallower headers that are more easily accessible. Odor management will be easier along with other benefits.
Read the full article here to learn about the design elements for maximizing long-term benefits, impacting: bottom liners, location of the blower/flare station, leachate risers, extraction well targets, and external header piping.
The Pennsylvania Keystone Chapter of the Solid Waste Association of North America (SWANA) is an organization of public and private sector professionals committed to advancing solid waste and resource management through education, advocacy, and research.
Keystone SWANA announced the election of Denise Wessels of SCS Engineers to the Board of Directors at the Association’s annual business meeting held in conjunction with the 19th Annual Pennsylvania Waste Industries Association (PWIA) and Keystone SWANA Fall Conference in Harrisburg on September 7, 2017. Ms. Wessels will serve for two years starting on October 1, 2017.
Directors are elected by majority vote of the members at the annual business meeting. Ms. Wessels, a long-standing member of the Association, had agreed to step in to fill a vacant director’s seat earlier in the year; this election serves to confirm that members are more than happy with her service. As a board member, she is responsible for the activities, property, and affairs of the Chapter.
Ms. Wessels is a Project Manager with SCS’s Landfill Engineering Group in Pennsylvania. She has more than 20 years of experience in the waste management industry and environmental consulting, including landfill design, project management, regulatory compliance, permitting, environmental monitoring and studies, auditing, and budgeting.
“The PWIA and SWANA both make important contributions to Pennsylvania’s economy and the environment by supporting those who manage the nearly nine million tons of municipal solid waste that citizens generate each year,” stated Paul Mandeville, SCS Senior Vice President, and Regional Director. “Denise is sharp and always brings great value to clients, association members, and her co-workers.”
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
SCS Engineers will provide landfill and landfill gas engineering and construction management services to the Authority. Consulting services will include technical and financial advisory services related to the operation, expansion, and development of the Authority’s landfills, and work with utilities and regulatory agencies.
SCS Engineers has been awarded a contract to provide professional engineering services to the Solid Waste Authority (Authority) of Palm Beach County, Florida. The Authority selected SCS as its preferred full-service provider based on the firm’s wide range of expertise and local presence. SCS Engineers will provide the Authority with professional engineering and consulting services for landfill, landfill gas (LFG), and other solid waste facility management projects.
SCS will provide the technical, construction quality assurance, regulatory, and financial expertise necessary to provide safe, sustainable solutions. The landfill, solid waste management, and LFG support will vary according to task and can include civil, structural, mechanical, environmental, and electrical engineering. SCS’s general responsibilities will include landfill siting, expansions, closures, end use operations, and other improvements or modifications as needed by the Authority to continually improve its landfill and LFG services keeping its landfills in compliance with Federal and Florida Department of Environmental Protection (FDEP) regulations.
“Landfills are complex ecosystems that require the expertise of many types of technical professionals to ensure that the County’s systems are built to last, continue to meet the needs of the community, and safeguard the community’s general health and welfare,” said Eddy Smith, SCS senior vice president. “The planning, design, construction, and maintenance phases of a landfill require a mix of professional engineers and specialists in geotechnical, geological, hydrogeological, environmental, and civil engineering all working on the same team in collaboration with the Solid Waste Authority of Palm Beach County and the FDEP.”
One general problem that is encountered in traditional designs is the potential for clogging of geotextiles in the vicinity of the leachate collection pipes.
Traditionally, leachate collection pipes are encased in gravel, wrapped in geotextile, and positioned above the leachate collection system geocomposite drainage layer inside a trench or at the trough of the bottom of a cell. In a traditional design, leachate travels through the geonet component of the geocomposite and reaches the leachate trench where the leachate collection pipe is located. Here, leachate must flow out of the geocomposite, through the upper geotextile component, and then through the geotextile wrapped around the gravel, before entering the gravel and eventually flowing through the pipe. The flow through the geotextiles is concentrated in small areas on the two sides of the leachate collection pipe-gravel-geotextile wrap. Considering the large volume of leachate that follows this path over the life of the cell, it is evident why traditional designs are doomed to clog.
The clogging impedes the free flow of leachate from the geocomposite drainage layer to the leachate collection pipe. As the clogging occurs, the leachate must find a new flow path (most likely further back from the collection pipe), and flow out of the geocomposite, through the geotextile wrap at a different location, and eventually enter the gravel and pipe. This new location will eventually clog as well for the same reasons that the initial location was clogged. This process continues until the geotextile within the leachate trench becomes completely clogged and the system loses functionality. Unfortunately, the periodic cleaning of leachate collection pipes (usually every few years) cannot address this issue because the problem is outside the pipe and the high-pressure jets inside the pipes do not reach the clogged locations.
The solution is to eliminate geotextiles from the flow path of the leachate, extending from the geocomposite drainage layer to the leachate collection pipe. Over the past several years, SCS has successfully designed and constructed numerous landfill cells with no geotextile in the flow path of leachate from the geocomposite drainage layer to the leachate collection pipe. The design follows the “Rule of Transmissivities” which dictates that a proper design should provide the free flow of leachate from one medium to another and that only occurs when the transmissivity of the latter medium is equal to or greater than the transmissivity of the former medium. If a design does not satisfy the Rule of Transmissivities, there may be potential for clogging, bottlenecking of flow, and other consequences resulting from impeded flow in the system.
SCS Engineers is a leader in the design of landfill lining systems, and we have experience with issues that may not be familiar to other firms. If you are interested in the design of a leachate collection system at your facility, please contact SCS. Our professional engineers will gladly review your design and make recommendations if needed. We can identify potential issues and improve designs to prevent future problems and maintenance during the life of your facility.
Questions? Contact Ali Khatami, PhD, PE, LEP, CGC, is a Project Director and a Vice President of SCS Engineers. He is also our National Expert for Landfill Design and Construction Quality Assurance. He has nearly 40 years of research and professional experience in mechanical, structural, and civil engineering. Dr. Khatami has acquired extensive experience and knowledge in the areas of geology, hydrogeology, hydrology, hydraulics, construction methods, material science, construction quality assurance (CQA), and stability of earth systems. Dr. Khatami has applied this experience in the siting of numerous landfills and the remediation of hazardous waste contaminated sites.
Read more here. Rule of Transmissivities at Material Interfaces in Landfill Leachate Collection Systems, in Talking Trash