MSW Management | Monitoring Your Landfillís Vital Signs

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An automated system can provide significant cost savings to the landfill’s operating budget.

By Daniel P. Duffy

It’s a basic truism of economics that any investment in capital equipment will bring higher rates of return than any equivalent investment in labor. This is true of the mechanization of farming, the automation of industry, and the computerization of data collection. Remote monitoring of landfill fluids can greatly reduce (but not eliminate) the need for onsite manual measurement of these fluids.

Direct sampling of leachate, groundwater, surface water run-off, and landfill gas will always be required to ascertain the chemical constituents and the concentrations of the contaminants transported by these fluids. For now, the flow rates of the primary fluids of concern can be effectively monitored from remote locations. With this flow data, the mechanisms used to regulate these flows and the facilities used to store or treat the extracted fluid can be adjusted in real time to improve the efficiency of the landfill’s control systems.

What Needs to Be Monitored?
So why does a landfill qualify as an industrial process that can be subject to remote monitoring and control? It would be a mistake to view landfills as simple, static piles of waste and dirt enclosed by plastic membranes. They generate fluids, both gaseous and liquid, that have to be extracted and properly managed. These fluids are carried via pipelines by mechanical pumps and blowers whose operations can be adjusted to conform to changes in fluid production rates. And these rates change continuously with variations in precipitation, organic content of the waste, etc. The production rates are monitored by flow meters, probes, and sensors. There is even an “electronic nose” that can detect odor concentrations. So a landfill is in principle no different than any other industrial process.

The gaseous fluid produced by landfills is landfill gas (LFG). It is generated by the anaerobic decomposition of the organic components of the deposited waste. LFG is about 50% to 55% methane, 45% to 50% carbon dioxide, and 0% to 5% trace gases. Gas production involves the extraction of generated gas from the waste pile via a series of extraction wells connected to branch pipes, which are further connected to main header pipe lines. The entire system is connected to an industrial blower sized to apply enough negative pressure to the wellheads so that LFG is drawn to the wells and subsequently extracted. The flow rate of the blowers is then recorded.

LFG production is also monitored by permanent probes and temporary bar holes. Bar holes are screened drive points manually set in the ground just outside the limits of waste disposal. Other temporary monitoring devices are installed in areas that may accumulate dangerous, explosive levels of LFG, such as basements, closets, crawl spaces and manholes. Though useful for spot evaluations of gas migration and accumulation, temporary LFG monitors are not suitable for remote, continuous monitoring applications due to their temporary nature.

Unlike bar holes, which are temporary, LFG probes are permanent. Constructed of polyvinyl chloride (PVC), the pipe is slotted and screened at a design depth that is derived from the landfill configuration, probe locations, and local hydrogeology. The borehole that receives the PVC pipe is made with a hollow stem augur that can reach the necessary depths. The pipe is smaller than the borehole, allowing for the backfilling of the space between the pipe exterior and the borehole walls with sand, which permits LFG inflow to the screened segment of the PVC pipe. A sampling tube is epoxied to the sealed end of the pipe. This type of permanent probe has the potential to be connected to a remote monitoring system.

Of more practical use are remote monitoring systems directly linked to the LFG system’s blower and flare station. Such remote monitors can provide real-time data capture and alarm notification of critical operational characteristics that could lead to a premature shutdown of the flare. Operators can be instantly notified if a condition threatens the system with an unforeseen shutdown. All this accumulated data can be stored in historical files that can be analyzed for trends and cross-referenced for causes.

The other primary fluid created by a landfill is liquid leachate. Leachate is created by precipitation (either rainfall or snowmelt) percolating through the cover of the landfill and coming into contact with the deposited waste on its way down. As the water passes through the waste, it picks up contaminants until it reaches the bottom of the landfill and accumulates. It is this accumulated leachate that has to be removed to prevent dangerous buildups that could cause migration of the leachate into the local hydrogeology or even cause structural instability. The bottom floor of a typical landfill is sloped so that leachate will flow to a series of collection pipes, which in turn will transport the leachate to collection sumps for extraction via submerged pumps. The pumps discharge the leachate through hoses that connect to force mains that carry the leachate to either temporary storage facilities (underground tanks, aboveground tanks, or open lagoons) or onsite pretreatment facilities. Extracted leachate is finally removed from the site for permanent treatment and disposal via hauling by tanker trucks or direct discharge into an adjacent sanitary sewer system.

Since leachate is relatively slow-moving through soils compared with LFG (and of less immediate danger), groundwater monitoring lacks the immediacy of monitoring for LFG migration. Also, leachate detection requires laboratory analysis of groundwater samples, unlike LFG whose flows can be detected by its unique chemical presence. The addition of leachate to surrounding groundwater can be detected indirectly by increases in groundwater surface elevations (mounding) under and around the landfill. However, these elevation changes are difficult to detect in the short term and tell the landfill operator nothing about the landfill’s leachate production rates. Therefore, most remote leachate monitoring is performed to record leachate pump data, force-main flow rates, and storage facility levels.

SCADA Basics
The primary method for remote monitoring of industrial processes is called SCADA. SCADA is an acronym that stands for “Supervisory Control and Data Acquisition.” SCADA gathers real-time data on the operations of mechanical and hydraulic systems and allows for remote control of these systems from a central remote operator (though some SCADA systems merely monitor without allowing for control). The result is an information feedback loop where data from current operations guides the control changes needed to optimize or modify these operations, which in turn generates a new data set describing these operations, which can then lead to further control changes. SCADA systems tend to be large-scale, with widely distributed measurement and multiple controls.

SCADA itself is not a direct control system. As the name suggests, SCADA performs its function at the supervisory level. As such, it is systematically located on top of the control systems that do the actual work. These control systems actually manage of the processes being monitored. They do so via their own integrated real-time controls, which can perform modifications within the time constraints of the process being managed. The processes can be related to industry, infrastructure, or facilities. Industrial processes are monitored for their manufacturing rates and electrical power usage. These manufacturing processes can be batch, continuous, repetitive, or customized. In addition to monitoring potentially variable manufacturing rates, SCADA can monitor and adjust more predictable, day-to-day utility usage. This includes the flows carried by oil and gas pipelines, water-supply pipelines, sanitary and stormwater-runoff sewers. It also includes non-physical transmissions, such as electrical power transmissions and wired communications.

So where did SCADA start? Way back in the 1960s, the age of go-go boots and mainframe computers, SCADA was born as a simple input/output signal transmission between a master monitoring station and a remote terminal unit (RTU). The master monitoring station simply recorded the transmissions from the RTU and stored the data for later review. A decade later, SCADA capabilities were expanded to include distributed control systems (DCS). Though still analog, DCS allowed for the functional integration of physically separate subsystems. By the eighties, the RTU were replaced by programmable logic controllers (PLC) whose integral software programming allowed them to control site processes without having to await instructions from the master station. Before the end of the century, SCADA and DCS became indistinguishable. Now replace the old telemetry signals with the Internet as a communications channel, along with the necessary automated software packages that allow interconnectivity of the Internet to a SCADA system, and you have real-time, near simultaneous information download and process control.

So why use SCADA and what advantage does it give the landfill operator? Put simply, it saves the operator money by reducing labor costs and providing more accurate and timely information, which allows for better operational decision-making. With SCADA there is no need for dispatching service personnel and sampling technicians out to a site to perform sampling, water-level measurements, or data collection or to adjust the systems managing landfill fluids. Instead of waiting days or even weeks for detailed information and data summaries, the landfill operator can perform system adjustments and troubleshooting in real time. The overall efficiencies positively affect site equipment, reducing wear and tear and extending its operational life. Personnel not used for sampling and monitoring can be usefully employed elsewhere on the landfill. Furthermore, the complete and ready databases produced by SCADA systems provide proof to state regulators that the landfill is in compliance with its operational permit.

SCADA Monitoring Applications for Landfills
Managing fluid flows at a landfill can be rather complicated. For example, take leachate-extraction-pump operations and the resultant system heads, required pipe diameters, check valves, and pressure controls. All of these require different operational methods depending on their configuration. Even though we often assume (when designing a leachate-extraction pump and force main with associated flow rates and resulting operating heads) that no more than one leachate-extraction pump will be operating at any one time, we often have situations where more than one pump will be operating. This is especially true where a landfill is being developed in phases that include multiple cells and extraction sump pumps being constructed and operated at the same time. This potential for the simultaneous operation of multiple leachate-extraction sump pumps presents several operating issues that can be better managed with proper monitoring.

First, what assumptions should be made when designing the diameter of the leachate force main’s carrier pipe? Too big a diameter could result in insufficient flow velocities when only one pump is operating. Too small a diameter could result in excessive head losses and/or clogging when multiple pumps are operating. The standard minimum flow velocity for a typical sanitary force main is 2 feet per second. (Note: This standard is applied to sanitary flows that can carry particles as large as 3 inches in diameter. The suspended particles in landfill leachate are often silt-sized, so this flow standard may not be strictly applicable to all leachate force mains, though it may still be required by permit or regulation.) A typical range of force-main diameters runs from 4 inches to 6 inches, though it can be as small as 2 inches and as large as 10 inches. How the on and off cycles of the various landfill pumps are managed will determine the total combined flow in the force main and affect the required force-main size. Monitoring flow velocities within each force-main segment can ensure that standards are met.

How much and how long a pump operates is an important concern. While it may seem to be a good idea to increase the pump’s flow capacity in an effort to increase flow velocity for a given pipe diameter, this approach could result in short operating cycles and lead to pump burnout or other failure. Furthermore—HELP models or other leachate water-balance models not withstanding—day-to-day leachate production and flow rates are inherently unpredictable. However, if proper monitoring is performed, a substantial database describing a pump’s operational history can provide valuable information for optimizing pump cycles. This data would include total flow per time period, records of leachate-head buildup as recorded by the pump’s pressure transducer, and measurements of the pump’s on- and off-cycle times.

Also, the potential for multiple simultaneous pump operations requires an examination of how these flows are to be coordinated. Typically, check valves prevent backflow down a discharge pipe and back into a sump, and there are check valves that ensure one-way flow in a circular force main ringing a landfill. But depending on operating conditions, the amount of pressure acting on a check valve can vary considerably. The same check valve that prevents backflow down into a neighboring sump may have too much pressure in the force main behind it to allow the sump’s extraction pump to discharge into the force main. The result is a pump that tries to operate outside of its maximum pump head. This can lead to pump damage and burnout. Automated controls can ensure that the pumps that are allowed to operate do so only when the current system heads are within their operating range. Concurrently, the monitoring system can ensure that leachate-head buildup in sumps whose pumps are waiting to operate does not exceed the maximum depth allowed by the regulations.

Then there is the question of onsite leachate storage (and possibly onsite leachate treatment). Usually, leachate is carried by the force main to a central location. This could either be the storage facility itself (aboveground storage tank, underground storage tank, or open-air lagoon) or a surge tank from where the accumulated flows from several force mains can be pumped or even gravity-drained to their final destination. Once accumulated in the on-site storage facility, leachate volumes can be measured with changes in storage depths. This information can be used by a SCADA system to signal tanker trucks to come out to the site and remove the leachate, to manage and measure the flows from the storage unit to nearby sanitary sewers, or to control the leachate feed into the site’s pretreatment facility. Whether batch-process or continuous-feed, leachate pretreatment systems require carefully managed inflow rates to optimize performance.

Managing LFG flow is inherently simpler but has potentially graver consequences when mismanaged. Aside from the obvious problem of LFG migration offsite (which an active gas-extraction system is supposed to prevent), there is the constant need to balance the pressures within the gas-extraction system. Municipal solid waste is highly heterogeneous—not just the waste itself, but as it gets deposited in the landfill. One landfill cell may have little in the way of organics, while another cell may have received large deposits of organic sludges. A simple operating change like switching to alternate daily cover instead of volume-utilizing soil cover can change the amount of organics found in different parts of the landfill. The gaseous flows in the pipe are not so much a problem as is the need to maintain adequate extraction pressures (and resultant flow velocities) across a highly variable environment. Failure to provide adequate pressure can result in LFG buildup that can trigger offsite migration, the killing of nearby vegetation, or the formation of a bubble under the final cover’s geomembrane. This last possibility is very dangerous due to both the accumulation of explosive gas and the resultant slope instabilities caused by the sloughing off of cover soils and other materials as the bubble gets bigger and bigger.

Finally, LFG has a rather nasty byproduct. This is the condensate that forms out of the gas when it cools. Though smaller in amount than leachate, condensate can be very concentrated with dangerously high levels of contaminants and impurities. Often, condensate is discharged back into the waste mass via drip legs with the goal of diffusion and dilution as it flows to the bottom of the landfill and joins the leachate. Other landfills require separate discharge and treatment of condensate. Like leachate this has to be stored and possibly treated prior to final disposition. Again, SCADA monitoring can effectively manage this material and optimize its management process.

SCADA Landfill Examples
All these liquids bubbling, churning, and flowing through the landfill and its pipe, pumps, and blowers can be better managed through the use of SCADA systems. More and more landfills, both operational and closed sites, are installing SCADA systems to help manage their leachate and LFG. The following are a few examples:

The Onyx Emerald Park Landfill in Muskego, WI, utilizes a SCADA system to manage its LFG flare. The system transmits data via the Internet, e-mail, or fax. Monitoring is continuous and data output is performed every 15 minutes. Overall monitoring at the site is extensive, with 145 environmental systems keeping tabs on air monitors, gas probes, the underground water supply, private wells, and leachate generation.

Cedar Hills Landfill in Kings County, WA, has developed a SCADA system in cooperation with ECS Engineering Inc. The system monitors the landfill along with regional transfer stations and closed landfills serviced by LFG and leachate-extraction systems. Accessible through a local area network, this system monitors and transmits data concerning the operations of pumps, blowers, and sensors at multiple locations.

Located in Ontario, Canada, the Glenridge Quarry Landfill operates a SCADA system that remotely monitors the site’s LFG management system. While monitoring the overall performance of the gas system, the SCADA performs observations of well-field and manhole systems. If necessary, these systems are adjusted on a biweekly basis. In addition to SCADA monitoring, the site performs regular maintenance of the system that includes field monitoring which checks all mechanical parameters on a weekly basis. Follow-up spot monitoring ensures smooth operations of the system.

Across the Atlantic in Ireland, the Arthurstown Landfill Facility utilizes a centralized SCADA monitoring system to assist site operations. This extensive system controls and monitors most of the landfill’s functions. For liquid flows (leachate, groundwater, and surface-water runoff) the SCADA system monitors all levels, pumps, and pump alarms. Additionally, the system records precipitation and other meteorological data, monitors traffic flows and scale operations, and ensures site safety by monitoring fire alarms across the site.

All this information goes into a flexible database that can be expanded and reconfigured as the site increases in size.

Probably the best example of the extensive utilization of a SCADA system by a landfill anywhere can be found at the Yolo County Landfill bioreactor test project located in California (EPA Project XL). As a bioreactor landfill, Yolo County makes extensive use of leachate recirculation and expanded LFG extraction systems. Its SCADA system controls the leachate-injection well points and feeder pipes, provides real-time monitoring, and allows for data export to a centralized database.

Data collection from the demonstration cells (each with a 9,000 ton disposal capacity) is fully automated. Despite early teething problems concerning the breakdown of some sensors, the SCADA system has exceeded expectations. The system provides detailed and timely feedback on the performance of the bioreactor cells. This allows the operator to fine-tune the system and optimize its performance.

Writer Daniel P. Duffy, P.E., is employed by the URS Corporation in Akron, OH.

MSW - November/December 2007