Improved Circulation

Prentiss A. Shaw and Maura Dougherty

THE CONCEPT OF RECIRCULATING leachate in a bioreactor landfill has been gaining momentum because bioreactors help to maximize landfill gas (LFG) capture for environmental recovery projects; increase landfill capacity; provide options for leachate treatment and storage; reduce post-closure activities; and abate greenhouse gases. However, bioreactors require additional management and monitoring activities — so operations should be modified to enhance and track decomposition.

Preparing an operations and monitoring (O&M) plan is an effective way to keep a bioreactor running efficiently with minimal impact to the landfill or its surroundings. This allows the owner and operator to see how their system is performing. For example, the Salem County Landfill's O&M plan allowed the authority to determine where to improve bioreactor operations to reduce leachate treatment cost, accelerate waste degradation and encourage gas production.

System Background

The Salem County Landfill in Alloway, N.J., is operated by the Salem County Utilities Authority and accepts nonhazardous solid waste within an approximately 25-acre liner footprint. Another approximately 15-acre liner area is approved in the solid waste facility permit. The landfill currently operates its bioreactor technology in a 5-acre portion of the additional liner area.

The authority began leachate recirculation within the landfill's waste mass in 2000. Less than a year later, the operators wanted to optimize the biostabilization process. Consequently, the authority hired EMCON/OWT Inc., a subsidiary of Shaw E&I, Mahwah, N.J., to prepare an O&M plan. Up to that point, the facility manager had been evenly distributing leachate volumes to each of the recirculation trenches. But the plan would address the facility's existing recirculation system, then offer system enhancements.

Plan Components

A bioreactor O&M plan builds upon the facility's existing practices. Therefore, the first step involved examining existing operations (waste placement, record keeping, operational sequencing) and control systems (leachate and landfill gas monitoring). This included:

Most landfill O&M plans include these items, but it is even more important to evaluate facility operations when transitioning to a bioreactor. At a minimum, the plan for an anaerobic bioreactor operation also should address:

Looking at Leachate

Primarily, it was important to understand the capabilities and limitations of the Salem County Landfill's existing leachate collection and recirculation system, and identify where modifications were necessary. The leachate recirculation system was installed in the existing active landfill and consisted of a pump station, 4-inch diameter high-density polyethylene (HDPE) solid walled force main piping and 4-inch diameter HDPE perforated distribution piping.

Leachate is pumped from an onsite 200,000-gallon storage tank to the leachate recirculation pump station and then to the piping distribution system within the landfill. The lateral distribution piping system consists of perforated pipes installed in trenches within a single layer of the landfill waste mass at around elevation 80. An additional 3 or 4 lifts of waste lie above the recirculation trench system. The trenches are approximately 580 feet long, spaced about 80 feet on center, and backfilled with tire chips. The last 100 feet of pipe are solid walled on each end closest to the side slopes. A valve at the connection of the leachate force main to each distribution pipe provides leachate distribution flexibility.

Leachate application rates were established by breaking down the total application rate to individual application rates for each trench. Typically, the rate is determined during the design phase. However, the landfill had not calculated the measurement, so the O&M plan now addresses application rates for trenches and various operating conditions.

Because calculations for the leachate application rates are theoretical, the suggested maximum rates in the O&M plan were a starting point. Actual field conditions, as determined by the monitoring and inspection activities recommended in the O&M plan, could show that the duration and amount of liquid addition should be adjusted.

Calculations showed that the maximum allowable daily recirculation rate, including leachate and other applied liquids, is limited by the amount that will cause one foot of head buildup over the liner — approximately 2,000 gallons per acre per day at the Salem County site.

Pump analysis identified the need for leachate recirculation on a rotational basis in the lateral distribution pipes. Depending on which two pipes are operating together, the total allowable flow from the pump will vary but be in the range of 4,400 to 6,200 gallons. If surface application is used, liquid application should occur in a separate area from leachate recirculation.

The landfill permit requires inspecting the leachate collection system. Additional inspections are recommended for the pump station, force main, and distribution piping because of the bioreactor operations. [See suggested inspections on page 73.] Measuring leachate collection rates, injection rates and quality aids in assessing the saturation level within the waste mass, areas of “short circuits,” distribution system uniformity and the stage of the bioreactor process.

Currently, leachate samples are collected in accordance with the requirements set forth by the New Jersey Pollution Discharge Elimination System Permit Discharge to Ground Water Permit prior to trucking leachate offsite for treatment. Samples are tested for primary and secondary compounds. Volatile organic compounds (VOCs) testing is conducted semi-annually, and testing for acid and base extractables is completed annually.

The O&M plan recommends sampling occur semi-annually. The consultants also suggested the authority test leachate for dissolved oxygen and conductivity levels to assist in estimating the waste decomposition phase. And, the plan recommends the authority calculate an annual average value for each of the reported parameters prior to leachate recirculation as a basis for comparison.

Capturing the Gas

To improve bioreactor operations, it was important to determine whether the existing passive gas collection system was adequate. The LFG collection system consisted of six, 4-inch horizontal collectors, each of which discharges to a candle flare, and four additional 8-inch diameter horizontal collection pipes installed within subsequent lifts of waste. The six wells were located adjacent to the active landfill section and onsite scalehouse and maintenance buildings. An additional 10-inch Hyex horizontal collection pipe was installed adjacent to the 8-inch line to accommodate both LFG collection and leachate recirculation. Gas collection piping (four-inch diameter perforated HDPE pipe) is located in the leachate recirculation trenches approximately 15 to 18 feet above the recirculation pipes. Extraction wellheads are at the end of the pipes, connected to an 8-inch header pipe.

Again, the gas system typically would be evaluated during the bioreactor's design phase, but because the authority's system was already in place, a gas model was developed in the O&M plan. Based on the LFG generation estimates with bioreactor operations, gas generation is projected to peak in 2009 between 746 standard cubic feet per minute (SCFM) and 498 SCFM (average of 622 SCFM).

The peak annual recovery rate under leachate recirculation is determined by the waste composition, because of the relative effect of recirculation on the food waste and paper waste fractions in the waste stream. The amount of LFG generated is related to the quantity of degradable organic waste in the landfill. So, LFG generation estimates would increase if the proportion of municipal solid waste (MSW) increased above Salem County's projected 45.3 percent.

Following the O&M plan, wells are measured quarterly to determine whether LFG has migrated offsite. A methane gas survey is performed annually in the soil around the landfill's perimeter in the buffer zone. Measurements are conducted at least every 300 feet around the facility perimeter.

The existing gas migration monitoring program ensures that any gases from refuse decomposition found in the soils surrounding the landfill do not create an explosion or adversely affect personnel or the environment. Leachate recirculation will enhance biodegradation, thereby accelerating and increasing the LFG generation peak rate during the landfill's operational rate. Enhanced monitoring will help define the system's behavior and provide a comparison of predicted and actual performance.

Gas flow measurements at the candle flares and wellheads on the landfill help to indicate the biological activity level. Trends will show the rate of waste stabilization. However, measuring gas flow through passive venting systems is difficult, given the typically low velocities seen in these systems. Therefore, the consultants recommended the use of a thermal anemometer to measure the gas.

Other weekly or quarterly gas system sampling requirements were recommended at each of the wellheads in the leachate recirculation area. This helps to determine the biologic activity rate within the waste mass. Particularly, the methane concentration indicates waste decomposition because of the relationship between gas production and solids loss.

Because the Salem County Landfill is using a passive gas collection system, surface monitoring of the leachate recirculation area should occur quarterly (excluding active landfilling areas) to confirm that the system is not harming the environment.

Additional monitoring would help to determine whether an active system is required. Modifications to the passive system would include adding a collection header piping system, a blower to apply a vacuum to the extraction wells or trenches, and a flaring or energy-recovery flaring system.

Waste Handling Procedures

The procedures for the receipt and monitoring of incoming waste, as well as waste placement and daily cover use, also were reviewed for the O&M plan. The incoming weight, quantity, in-place unit weight and waste type help to determine leachate recirculation's effects.

After evaluating the Salem County landfill's waste receipts, the consultants concluded that the incoming waste did not need to be amended for bioreactor operations. However, they recommended the landfill use a permeable daily cover material to prevent the leachate flow from being impeded.

Even the best-run facility must be ready to handle unexpected events. So the final section of the O&M plan discusses contingency measures typically associated with leachate recirculation, including:

Monitoring and inspection during the transition to a bioreactor operation, as well as throughout the system's life, helps to determine whether the landfill is operating appropriately and can be improved. Using information on the leachate and gas quality and quantity, the authority can better estimate the waste decomposition phase, which can be used to assess how to optimize the bioreactor.

Following the Plan

Based on the O&M plan, the authority now collects data on leachate generation and recirculation through instrumentation and visual observation of the leachate manholes, pump stations, storage tank, associated valves, piping and meters. Total estimated leachate generation can be calculated as the sum of leachate recirculated and transported.

Leachate collected from the landfill is analyzed annually and semi-annually prior to transport offsite for metals, toxicity characteristic (TC) organics, non-TC organics and miscellaneous compounds to help characterize the stage of the bioreactor process. Comparison between levels of biological oxygen demand (BOD), chemical oxygen demand (COD), total volatile acid (TVA) such as acetic acid, ammonia, pH and conductivity may be used to establish the degree of landfill stabilization.

Information on the leachate's physical characteristics in the storage tank also has helped Salem County to more closely observe the leachate recirculation pump. Typically, solids precipitate out of the leachate and settle over time. Prior to the O&M plan, it was assumed that the recirculated leachate was clean of solids. However, solids in the leachate eroded the pump impellers, and the pump had to be replaced due to damage from sand suspended in the liquid.

To avoid this problem, the landfill operator now consistently monitors the flowmeter on the pump to detect problems. If flow decreases, the operator can clean the buildup before it damages the pump. The authority learned that cleaning the storage tank more frequently will reduce solids buildup in the system.

Leachate seep problems also have been corrected, thanks to the O&M plan. As data was gathered at the landfill, the authority noticed there was no ponding but that there had been one leachate seep due to the high-level pump rate through one recirculation line. When high recirculation rates were pumped through the system, minor breakouts were noted, so the authority limited the rates to 3,000 to 4,000 gallons per day — half the original rate — to avoid future leaks.

The plan has helped the authority to improve the gas collection system, too. The authority now conducts quarterly surveys of the methane gas monitoring wells located adjacent to the active landfill section and onsite structures to determine whether gas has migrated offsite and for methane concentration. Flares are monitored daily and cleaned every two weeks. Periodically, the flare arrester elements and the spark pilots and batteries are replaced. A methane gas survey is performed annually in the soil around the landfill perimeter within the buffer zone. This helps to assess whether the flares are working properly.

Prior to implementing the O&M plan, the effectiveness of bioreactor operations was unknown. Typically, only two of the six candlestick flares installed were operating at any time. But if the flares weren't producing gas, it was unclear whether this was a result of limited LFG production or a mechanical malfunction. Now, even if gas flow volumes aren't available for the candlestick flares or well heads, daily visual observations indicate whether all 10 flares are working continuously.

Consistent gas production because of the bioreactor operations also has encouraged the authority to investigate alternative gas collection and control methods. The Salem County Landfill is below the capacity threshold that would cause it to be regulated by NSPS rules. Nevertheless, the authority is investigating installing an active gas collection system in conjunction with a landfill gas-to-energy system. The proposed system could include three generators and produce nearly 1 megawatt of power.

Finally, because of the O&M plan, the authority can determine the effectiveness of leachate recirculation on the waste mass. Waste delivered to the site is recorded daily in tonnages. The characterization aids in identifying the decomposable percentage of the waste stream. This number is used to estimate both the gas generation and appropriate leachate recirculation rate.

Based on the measurements and topical surveys, the authority has noticed that liquid applications have affected the waste. Even if the workability of the waste has not noticeably changed, it is easier to compact the waste with liquid application. Thus, implementing the O&M plan has provided the authority with a system to operate the bioreactor landfill at a controlled rate and with which to measure the landfill's progress. The Salem County Landfill also has established a consistent leachate recirculation operation.

Lessons Learned

The O&M plan is not a rigid document, but should be modified as more data is received. Consequently, the authority plans to continue reforming its plan and reporting procedures for the bioreactor landfill, which will help to regulate the bioreactor activity and assess necessary changes to improve system performance.

When investigating the bioreactor operation and setting up the O&M plan, the authority initially set goals to reduce leachate treatment cost, accelerate waste degradation in the current cell and further encourage gas production. To date, all three goals are being achieved. The treated leachate volume has been reduced by approximately one-third over one year. Visual estimates approximate there has been more than 5 feet of landfill settlement, an increase over results prior to the plan's implementation. And LFG production has been consistent, powering 10 candlestick flares steadily, compared with only two in previous years. Finally, the landfill was able to implement the changes without a significant increase in workforce.

Prentiss A. Shaw is a project manager and Maura Dougherty is a former project engineer at EMCON/OWT Inc., a subsidiary of Shaw E&I. E-mail: [email protected]

A version of this paper was presented at the 2002 Waste Tech Landfill Conference. Web site: www.envasns.org.

SUGGESTED BIOREACTOR INSPECTIONS

Daily:

Weekly:

After Storm Events:

Annually:

Biennially: