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Deep injection wells (DIW) mean different things in different parts of the country. In the midwest DIWs have been used for decades to dispose of industrial wastewaters, mining effluent, and produced water from oil and gas production activities and are from 3,500 feet to more than 10,000 feet deep. In Florida, deep injection wells have been used since the 1960s; however, they are used to dispose of treated municipal wastewater, unrecyclable farm effluent, and in some cases landfill leachate. DIWs in Florida range from 1,000 feet to around 4,500 feet deep.
This is a two-part blog, the first part discussing what constitutes a DIW, their general features, their cost relative to other wastewater management alternatives, and the range of industrial wastewaters suitable and safe for disposal. The second part, covered in the next SCS Environment issue, will be on the challenges for deep well developers created by public and environmental organizations, and strategies to counter misinformation and means to obtain consensus from stakeholders.
A DIW construction is a series of casings set in the ground where the initial casing starts out large and subsequent casings become smaller in diameter, progressively telescoping downward. Casing materials are typically steel alloys or fiberglass for better chemical resistance. As a casing is set and rock is drilled out, the next casing is set and cemented with a chemically resistant grout. The process continues with each progressively deeper casing. These redundant “seals” are what keep the injected liquid from escaping into the protected aquifers.
A DIW typically has three upper casings to protect the aquifers and isolate the wastewater to the desired disposal zone. The inner casing, called the injection tube, extends to the injection zone. Mechanical packers seal the space between the injection tube and the last casing with the annular. The resulting annular space is filled with a non-corrosive fluid. This fluid is put under pressure to demonstrate the continuous mechanical integrity of the well. The annulus is monitored for potential leaks, which would register as a loss in pressure and promptly stop the injection. Figure 1 is a simplified view of a DIW casing system used in south Florida.
Vertical turbine pumps working in conjunction with a holding tank, which is a used to smooth out the fluctuating flow of the wastewater feed pumps, propel the liquid down the well. As part of the permitting efforts, a chemical compatibility study is conducted to determine the level of pre-treatment if any to protect the well components and minimize downhole plugging. Municipal wastewater effluent is regulated differently and must receive at least secondary treatment before injection.
In the Midwest, DIWs are constructed to the same EPA criteria with a wide range of operating conditions. Some wells take fluid under gravity with no pumping, while others require higher pressure pumps that exceed 2,500 psi for injection. This blog focuses on wells used in Florida and typical fluid types and operational parameters.
In central and south Florida the injection zone lies below the underground sources of drinking water (USDW) which is the depth at which water with a total dissolved solids (TDS) concentration exceeds 10,000 parts per million (ppm); or the “10,000 ppm line”. This water is considered to be unusable in the future as a drinking water source. In parts of Florida, the injection zone is dolomite overlain by a series of confining units up to 1,000 feet thick made up principally of limestone with permeability several orders of magnitude less than the injection zone. (1)
In central and south Florida the target injection interval is the “Boulder Zone,” reportedly named because drilling into the formation often broke off pieces of the formation and made drilling difficult. The Boulder Zone is also known as the lower portion of the Floridan Aquifer. Later down-hole imaging technologies revealed this zone to be characterized by highly fractured bedrock and large karstic caverns, and the ability to inject relatively high flow rates with relatively little backpressure. It is not uncommon for Florida DIWs to have well flow rates exceeding 15 million gallons per day (MGD) and backpressures ranging from 30 up to 100 pounds per square inch (psi).
The versatility of the DIW in Florida to accommodate numerous different types of wastewater is an advantage. DIWs are being used on a large variety of waste streams that continues to expand, including:
Any wastewater considered for disposal must be compatible with the target formation and the final casing material. Therefore, depending on the wastewater, it may be straightforward to use existing industry references to confirm compatibility. In some cases, laboratory bench tests may be necessary to confirm compatibility.
Compatibility also includes the potential for creating unwanted microbial growth and scale formation within the injection interval. Growth and scale can happen with effluent containing sulfur or ammonia, two food sources for microorganisms or wastewaters supersaturated with minerals. Unless planned for and evaluated properly, both of these items have the potential to grow and clog the formation around the well, significantly reducing flow and increasing back pressure. This can result in higher energy costs, regulatory action and significant, unplanned costs to rehabilitate the well.
Another significant aspect of municipal wastewater is that they are primarily composed of freshwater and thus when injected into the highly saline Boulder Zone or similar saline zones, will tend to have a vertical migration component because of the density difference and greater buoyancy than the target zone. A few wells have been taken out of service because the seals designed to prevent this migration failed and allowed wastewater to seep upwards into the USDW.
The US EPA Underground Injection Control (UIC) program is designed with one goal: protect the nation’s aquifers and the USDW. There are several protective measures in a DIW that are intended to meet this objective;
The U.S. EPA conducted a study in 1989-1991 of health risks comparing other common and proven disposal technologies to deep wells injecting hazardous waste. The U.S. EPA concluded that the current practice of deep well injection is both safe and effective, and poses an acceptably low risk to the environment. In 2000 and 2001 other studies by the University of Miami and U.S. EPA, respectively, suggested that injection wells had the least potential for impact on human health when compared to ocean outfalls and surface discharges(3). William R. Rish examined seven potential well failure scenarios to calculate the probabilistic risk of such events.These scenarios included four types of mechanical failures, two breaches of the confining units, and the accidental withdrawal of wastes. The overall risk was quantified by Rish as from 1 in 1 million (10-6) to 1 in 100 million (10-8) (4), which is no greater than the current EPA risk criteria for determining carcinogen risk. As a comparison, 10-6 is the same risk level used by EPA for contaminants in soil or groundwater that are a known carcinogen.
There are several studies in Florida conducted by researchers and practitioners in the deep injection well field to assess the actual potential for municipal wells to contaminate the USDW. The maximum identified risk associated with injection well disposal of wastewater in south Florida is the potential migration of wastewater to aquifer storage and recovery (ASR) wells in the vicinity of injection wells (Bloetscher and Englehardt 2003; Bloetscher et al. 2005).
In a 2007 study, 17 deep wells in south Florida, used for municipal waste disposal, that had known upward migration into the USDW, were evaluated to develop a computer model to simulate these phenomena and extrapolate vertical migration over longer time periods. The results indicated that the measured vertical hydraulic conductivities of the rock matrix would allow for only minimal vertical migration. Even where vertical migration was rapid, the documented transit times are likely long enough for the inactivation of pathogenic microorganisms (5).
In a 2005 study of 90 South Florida deep injection wells, the authors took actual field data and constructed a computer model calibrated to actual operating conditions. The intent was to model performance of two injection wells in the City of Hollywood, Florida that the authors were familiar with and to determine the likelihood of migration, and what might stop that migration. Density differential and diffusion were likely causes of any migration. No migration was noted in Hollywood’s wells. The preliminary results indicate that Class I wells can be modeled and that migration of injectate upward would be noticed relatively quickly (3).
A deep injection well lifecycle cost compares favorably with other traditional waste treatment and disposal techniques. A life-cycle cost includes the capital cost and operating and maintenance costs for the useful life of the system. On a recent project, the wastewater for disposal was groundwater contaminated with ammonia nitrogen from a former landfill. The estimated groundwater recovery rate and the deep injection well disposal rate was calculated to be 1.2 million gallons per day (MGD). The proposed deep well was designed to have a final casing of 12-inch diameter, an 8-inch diameter injection tubing to a depth of 2,950 feet, and below that approximately 550 feet of open bore hole. The lifecycle cost estimate comparison to other viable technologies is shown in the Table below.
In this case, there were no projected revenues, so the alternative with the lowest net present value (NPV) would technically be the preferred alternative. Even though the aerated lagoon had the lowest NPV, it was ultimately judged too risky with a long break-in treatment period and significantly more space for treatment ponds needed.
The increasingly stringent surface water discharge standards are an ongoing challenge for industries generating a wastewater stream. DIW’s should be considered as a potentially viable option for long-term, cost-effective wastewater disposal, where a viable receiving geologic strata exists and when wastewater management alternatives are evaluated. In Florida, they currently provide an environmentally sound disposal option for many regions.
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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.
Landfill operators are seeking new means to dispose of leachate generated at their facilities more economically. Rising costs of leachate treatment at publicly owned treatment works (POTWs) obliges landfill operators to look for alternative disposal means at a lower price. These situations encourage landfill operators and consultants to do more with less when designing preventative solutions to reduce leachate generation in the first place.
Upstream measures to reduce leachate generation range from standard operating procedures to innovative ideas with significantly high benefit to cost ratios. SCS compiled a list of our Top 10 measures to consider, including:
1. Grading – Creating landfill plateaus to maximize rainwater runoff toward perimeter storm water ditches, using low permeable soil to seal landfill slopes that will not receive waste for an extended time.
2. Caps & Covers – Using temporary geomembrane caps over areas that will not receive waste for a long time. Constructing final cover over landfill final slopes to eliminate rainwater percolation into landfill.
3. Swales – Constructing temporary and strategic swales on landfill outside slopes to capture rainwater runoff before causing soil erosion on the slope and to convey runoff water to perimeter ditches.
4. Berms & Downchutes – Constructing temporary berms and swales on landfill slopes that drain toward new disposal cells to capture water before reaching waste in the new cell and directing water to perimeter ditches. Also, constructing a berm at the crest of slopes to minimize the flow of rainwater runoff over slopes that are causing soil erosion. Constructing engineered temporary and sturdy downchutes for rainwater runoff from top areas of the landfill to the perimeter ditches.
5. Tarps – Installing rain tarp over a portion of a new cell that will not be in service for some time.
6. Exposure – Minimizing exposure of the active face while the remaining areas are properly graded to shed rainwater runoff to perimeter ditches.
7. Shedding – Grading the top area of each lift to shed rainwater runoff to outside slopes and the perimeter ditches.
8. Plantings – Installing sod or seeding on exterior slopes and those interior slopes that will not receive waste for an extended time to reduce soil erosion.
9. Roads – Constructing ditches adjacent to access roads to safely convey runoff water to the bottom of the slope – pitching access roads toward the ditch adjacent to the road, and building a proper road surface to minimize erosion during severe storms while lasting long under traffic loading.
10. Maintenance – Establishing routine maintenance protocol for the aforementioned measures because regular maintenance sustains long life and performance.
For facilities outside the landfill area, special measures, such as using floating covers on leachate ponds or canopies over operations that could potentially generate leachate without the canopy, also help reduce leachate generation.
Upstream measures are not necessarily limited to our Top 10 list but depend on the type and extent of operations at a facility. The will of the landfill operator and the expertise of the solid waste engineer can go a long way to reducing leachate generation at landfill facilities, and we all strive for that.
More about Liquids Management including case studies.
Many landfill operators and owners are now spending more than 10 or 20 cents per gallon for leachate management, which can become quite costly. One of the primary reasons that leachate management has become an expensive challenge in the United States is more stringent regulatory policies regarding the discharge of liquids into public waters. The regulations affect publicly owned treatment works (POTW), which has led some POTWs to require that leachate entering their plants have adequate pre-treatment to remove contaminants. Under these circumstances, some landfills are forced to collect and haul their leachate to a different POTW or to consider installation of pre-treatment equipment themselves.
Each landfill needs a solution to its leachate management issues that depends on applicable regulatory restrictions, the capability of the local POTW, and the leachate composition. Due to chemical reactions and biological activity inside the landfill, the leachate’s temperature is frequently warmer than area groundwater. Also, leachate is odorous, and generally brown in color, with colloidal suspended solids. The composition of leachates depends on the composition of a landfill’s waste, the landfill’s decomposition stage, and weather conditions. Many factors are evaluated to arrive at an efficient and economical treatment method for disposal of landfill leachate.
There are some treatment alternatives available to reduce the high organic and nitrogen loads in leachate. For some leachate applications, the treatment methods are sufficient to allow the POTW to process the leachate safely. If treatment is not possible, or cost prohibitive another alternative is to pre-treat the leachate, lowering the contaminant load to prevent subsurface precipitation, and then dispose of it using deep well injection. Some states require little or no pre-treatment before discharging leachate to deep injection wells.
The following technologies are available for the pre-treatment of landfill leachate: biological processes for wastewater treatment such as membrane bioreactors, sequencing batch reactors, activated sludge processes plus reverse osmosis. Wet oxidation processes, activated carbon adsorption, as well as precipitation, coagulation, and flocculation techniques are also used, depending on the contaminants and their concentrations. These two counties are using a combination of treatment technologies for their leachate management strategies.
Hillsborough County’s 60,000 Gallon Per Day Leachate Treatment Facility at Southeast County Landfill
New Hanover County’s Landfill Leachate Treatment Plant Using Reverse Osmosis
Effective leachate management applies unique combinations of technologies which most adequately address the previously mentioned factors. To provide a truly sustainable solution to leachate management, SCS suggests another approach, which is to consider the landfill design and operations as part of the solution. By using the existing landfill design and operations, SCS develops an integrated approach to leachate management that is preventative. The benefits of using an integrated approach are that they are often a more cost-effective solution in the long-term, sustainable, working with the existing landfill’s infrastructure.
Waste Management is using an integrated approach at Monarch Hill Landfill in Pompano Beach, Florida. Monarch Hill Landfill is a 385-acre landfill with a waste flow of 5,000 tons per day. SCS helped decrease leachate formation as part of overall landfill design and operation. Waste Management reduced leachate formation using the following methods:
Temporary caps – SCS designed and provided monitoring services during installation of a 10-acre temporary geomembrane cap over a portion of the top intermediate plateau of the landfill. It reduced leachate generation, decreased odors, increased gas collection efficiencies, and addressed leachate seeps on the slope, as well as making surface water runoff over the top of the landfill easier and more efficient.
Final covers – SCS designed, permitted, and provided monitoring services during construction of six partial closure projects. The final covers were equipped with leachate toe drain systems below the final cover geomembrane, enabling leachate seeps to be collected and disposed of efficiently. The design also allowed collection of gas from the lower portion of the slope after completion.
Rainwater Toe Drain Systems – SCS designed a toe drain system above the final cover geomembrane that enabled water to be collected and diverted to the landfill perimeter ditches, preventing pore pressure build up and keeping the system stable.
Tack-on Swales – SCS designed tack-on swales that were implemented to catch runoff and convey water to downchute pipes. The swales could be easily adjusted based on the size of each partial closure and the overall management of stormwater.
Our approach is to develop a robust, tailored liquids management program comprised of the most appropriate technologies and engineering design to tackle the unique set of challenges facing each landfill client. In an era of doing more with less, clients find our programs are more cost-effective because they are more efficient and well designed.
Learn more at SCS Liquids Management
Contact one of our National Experts or :
Darrin Dillah: Landfill Leachate – Upstream
Ron Wilks: Landfill Leachate – Downstream and Evaporators
Monte Markley: Deep Well Injection
Bob Speed: Dewatering
Ali Khatami: Landfill Engineering and Construction Impacts on Liquids Managment
Sam Cooke: Industrial Wastewater Treatment
SCS Engineers met a tight, non-negotiable regulatory deadline to get the new plant on-line while meeting non-toxic effluent standards.
Everyone enjoys before and after pictures; just look at the results New Hanover County’s program is producing. This and other County programs are helping this North Carolina county reduce reliance on landfill disposal while creating a comprehensive and sustainable solid waste management system that is protective of the environment.
In 2016 a new wastewater treatment plant was commissioned at the New Hanover County Landfill. The new facility processes approximately 65,000 gallons per day (GPD) of leachate using state-of-the-art ultrafiltration (UF) and reverse osmosis (RO) technologies to meet or exceed federal and state treatment standards.
The raw leachate is pre-treated in an existing aerobic lagoon followed by a sequencing batch reactor (SBR) to reduce organic constituents. The pretreated effluent then flows into the membrane system. Using state-of-the-art membrane filtration technology, including ultrafiltration (UF) pictured at lower left, and a reverse osmosis (RO) system, pictured lower right, to produce crystal clear, effluent discharged to an upper tributary of the Cape Fear River.
The new facility can process 75,000 GPD and the Wastewater treated through the new system meets state Drinking Water standards for quality.
Tough surface water discharge standards and predictable performance in cold weather drove the design to use UF/RO systems. The results are impressive; metals including arsenic are BDL, ammonia <0.2 ppm, and TSS < 2 ppm. The system produces approximately 13,000 GPD of RO concentrate that is pumped to the working face and safely disposed of in the landfill. The County has certified operators that have played a big role in getting the plant shaken down and running smoothly.
“New Hanover County is an industry leader in adopting proven technologies to better manage solid waste, and protect the environment. This kind of planning and approach can benefit many other public works departments,” stated Bruce Clark, PE, BCES, LEED AP®, and SCS Engineers National Expert on Waste Conversion.
As Joe Suleyman, the County’s Environmental Management Director put it, “Let’s face it – people move to New Hanover County because they love to be in, on, or near the water. Our technical staff is composed of very talented folks who have environmental science and biology backgrounds. They believe in what they’re doing to help protect our delicate coastal environment, and this state-of-the-art system is a huge stride towards meeting our own expectations and those of the citizens we serve.”
See more case studies, services, and professionals on the SCS Engineers – Liquids Management Website
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