This is the sixth lesson in the independent learning correspondence course on municipal solid waste (MSW) landfills. One lesson in this 12-part series will be published in Waste Age magazine each month throughout the year.
If you are interested in taking the course for two continuing education credits (CEUs), send a check (payable to the University of Wisconsin) for $149 to Phil O'Leary, Department of Engineering Professional Development, University of Wisconsin, 432 N. Lake Street, Madison, WI 53706. Phone (608) 262-0493. E-mail:[email protected]. Website:www.wasteage.com.
Course registration can occur at any time until December 2006. Previous lessons will be sent to you.
To better understand the science of bioreactors.
To know the design elements of a bioreactor.
To understand how the bioreactor operation is different from a conventional landfill.
The development of the bioreactor is an interesting innovation being studied for landfills. As described in Lesson 2, a conventional landfill slowly and naturally decomposes. But decomposition can be controlled to accelerate the process.
Acceleration will generate landfill gas earlier in the life cycle of the landfill, and this will increase the waste consolidation so that it occupies a smaller volume. This potentially could reduce the time that a landfill requires long-term care.
A bioreactor landfill operates similar to a wastewater treatment plant. Waste materials enter the process, then additional materials are added to accelerate decomposition. The byproducts then are recovered, and the residual waste is disposed of in a controlled manner.
At a landfill, the water content of waste primarily controls the decomposition rate. Research has shown that if water is added to a landfill, the waste decomposition process can be greatly accelerated. This water can be leachate that has been recovered from the landfill''''''''s base, water supplied from another source or a waste material, such as liquid municipal sludge.
Leachate recirculation experiments were conducted in the early days of landfill construction. At that time, the objective was to reduce the leachate''''''''s pollution strength by using the landfill in a manner similar to the trickling filter sewage treatment process. However, leachate recirculation often resulted in surface or groundwater contamination, as leachate escaped from the facility. Consequently, landfill managers were discouraged, both from a technical and regulatory compliance viewpoint, from operating their landfills as treatment facilities.
Over time, it became apparent that a conventional landfill with a nearly impervious cover would slowly degrade. During this decomposition process, gas and liquids would be slowly released, creating a need for long-term care and management. These long-term care requirements have renewed interest in looking at landfills as a treatment process.
Many changes have taken place since the previous experiments were conducted. In particular, landfills now have much better liner systems, leachate collection systems and other advanced systems that are available for landfill gas recovery.
As a result of extensive research projects conducted at the University of Wisconsin and at other locations, it has become clear that the most critical factor in determining a landfill's decomposition rate is waste moisture content. Conventional landfills with a tightly sealed cover impede water from entering the waste. Consequently, decomposition is very slow and, in some cases, expected to take up to 1,000 years before waste stabilization is completed. Complete stabilization is when the waste material no longer breaks down into byproducts that are released into the environment.
Based on the original experiments with leachate recirculation and laboratory tests, researchers have found that decomposition time can be reduced to a matter of months under laboratory conditions. Solid waste digesters studied at the University of Illinois and developed commercially at several locations operate similarly to sewage sludge digesters and can stabilize waste in weeks. These systems, however, require high construction costs and are quite challenging, given the heterogeneous nature of municipal solid waste.
It seems more practical to modify an existing landfill operation to accelerate decomposition while maintaining the critical, physical elements of the landfill. Therefore, the current approach to bioreactors is to devise a system in which water is introduced into the waste to wet the material as uniformly as possible. The added moisture then accelerates decomposition, which generates large landfill gas quantities.
The gas is roughly 50 percent methane and 50 percent carbon dioxide, and is an excellent energy source. Methane generally is recovered with conventional gas wells and directed to either a gas turbine or internal combustion engine that is used to power an electrical generator. Thus, the energy content of the landfill waste is converted to electricity.
In a conventional landfill with a gas recovery system, a significant amount of landfill gas is lost to the atmosphere. This is because landfill gas generation begins slowly, and it is not economical to install a gas recovery system immediately after placing the waste. After a landfill gas system is installed, gas generation rates are such that the period in which gas may be economically recovered is shorter than the time necessary for complete waste decomposition.
With a bioreactor landfill, gas generation begins much faster, and generation rates are much higher. In theory, generation is short enough to recover a higher proportion of the waste's energy value as methane. This methane, when used to generate electricity, replaces other energy sources such as coal.
The amount of carbon dioxide released into the atmosphere also is reduced because the energy is recovered from the waste, and it is not necessary to burn another fuel to generate an equal amount of energy. Methane has 25 times more effect as a greenhouse gas than carbon dioxide. Consequently, burning methane and converting it to carbon dioxide reduces the potency of landfill gas as a greenhouse gas.
The principal design element that is added to a bioreactor landfill is the leachate recirculation system. In a conventional landfill, leachate is directed to a treatment facility after being pumped from the landfill. With a bioreactor, various methods are used to inject the leachate into the waste''''''''s top layers.
There are several methods of reintroducing leachate into the landfill environment, depending on specific site conditions. Spray irrigation, surface application, vertical well injection and horizontal well injection are used. Factors such as ease and cost of installation, waste quantity, and climate will affect which method is chosen.
Several factors will influence the recirculation system's effectiveness. When solid waste is placed in the landfill, there will be wide variations in the capability of the waste to transmit water. This will result in a portion of the waste being too wet, and another portion being too dry. The optimal moisture content will depend on the specific nature of the waste. Waste that is saturated with water is undesirable.
As bioreactor operation continues, leachate recirculation adjustments can be made based on operating experience. The amount of gas generated, methane portion, pH of the leachate reaching the landfill base and other measures that describe the biochemical reactions taking place within the landfill can be used to analyze operating results.
Auxiliary Moisture Addition
In some regions of the country, it is not possible to recirculate a sufficient volume of leachate to reach the optimum waste moisture content. Current federal regulations do not allow liquids to be dumped into landfills. Consequently, landfills in drier climates that are converted to bioreactors need special approval to add water from other sources. Clean water, sewage sludge, sewage effluent and other liquid wastes have been proposed as additional moisture sources.
One reason it has been possible to consider bioreactor landfills is because liners systems that did not exist when the early leachate recirculation experiments were conducted have evolved.
One argument in favor of bioreactors is that the accelerated decomposition phase should occur during the early years of the landfill, when the liner system is most effective. It is difficult to predict whether conventional liners under conventional landfills will be intact for the long period of time that it takes for the waste to decompose. With a bioreactor, the high moisture content of the waste materials and the associated decomposition take place when the landfill liner is in the best condition.
Current federal standards require a conventional liner system to include a composite liner, which consists of a 2-foot-thick layer of relatively impermeable soil, over which the membrane is placed. (Details regarding liner regulations can be found in Lesson 4.) These liner systems are much more effective than the liner systems that existed several years ago. Nevertheless, state agencies may require additional liner protection if leachate is being recirculated.
For example, in New York, the double composite liner is standard. Having two liner systems, one above the other, allows the effectiveness of the upper liner to be monitored. Any leachate that leaks through the first liner can be collected from the lower liner. Similar liner systems may be required in other states.
For bioreactor landfills, the leachate collection system must be designed to accommodate the higher volumes of water that will be moving through the landfill. This may mean increasing the pipe size at some locations, adding pumping capacity and specifying a more permeable drainage layer above the landfill liner.
The drainage layer allows leachate seeping from above to move sideways to the collection pipes. If the drainage layer cannot transmit the recirculating leachate, there is a risk that the leachate will leak out of the landfill at another location [See “Bioreactor Landfill” below].
Landfill Gas Recovery
One of the objectives for building a bioreactor is to recover landfill gas and to extract the resulting energy value. (Landfill gas systems were described in Lesson 3.) Some modifications to the conventional landfill gas recovery system will be necessary when building a bioreactor. The most challenging aspect is installing the gas recovery system so that it is operational when gas generation begins.
Generally, gas recovery systems are installed a few years after a landfill cell is completed. With bioreactors, however, gas wells need to be installed immediately after cell completion and, in some cases, it may be necessary to install the gas wells while the cell continues to be filled.
Wells constructed in areas where additional filling may take place must be designed so that they can be later modified to accommodate more waste.
Various alternative configurations to the conventional gas well have been proposed for bioreactors. A horizontal gas collection can be built as the landfill increases in height.
The recovered bioreactor gas generally will be of high quality, meaning that it contains a high proportion of methane relative to the theoretical maximum of approximately 55 percent. This landfill gas can be easily converted to electricity using a gas turbine or internal combustion engine. The gas also can be used to fire boilers or as a vehicle fuel. Experimental systems where landfill gas is converted into electricity by a fuel cell are being studied.
As landfills have increased in size and height, the number of slope stability failures has increased. The addition of water into the landfill profile can increase the possibility that a landslide may occur within a landfill. This is because the recirculating leachate adds weight and hydraulic pressure to the waste. This also could reduce the waste's structural characteristics. Consequently, when planning a bioreactor, it is important to carefully evaluate slope stability issues.
A bioreactor should be operated as a waste processing facility. This is in contrast to some landfill operations, where leachate recirculation alone was implemented as a way to enhance gas production and to reduce leachate treatment costs. With a bioreactor, cells need to be carefully configured so that:
Waste is placed in the cell so that it can be quickly closed. Daily cover should not be allowed to impede liquid movement within the waste.
Leachate or another source of moisture is introduced into the waste at the appropriate time.
The wet cell is carefully observed to ensure that no leachate escapes into either surface waters or groundwaters.
Gas collection wells are installed in time to recover the first significant quantities of landfill gas.
Leachate is collected from the base of a cell and either reintroduced into the landfill or removed for treatment.
The waste cell is monitored to determine the decomposition stage. Additional monitoring points are established to detect abnormal releases of leachate into the groundwater. Procedures also should be implemented to detect leachate discharge through the landfill sidewalls.
Monitoring points are established to measure waste consolidation and settlement. Additionally, the landfill's cover is observed to ensure that no slope stability problems develop.
The quality of landfill gas and leachate is monitored to determine when bioreactor cell shutdown is appropriate.
Procedures are agreed to in advance as to when, and if, it is appropriate to add more waste into a closed bioreactor cell.
Because a bioreactor will contain a higher proportion of water than conventional landfills, the operator must be prepared to act quickly if a leachate management problem emerges. For example, it may be necessary during wet weather to remove leachate from the collection system and direct it to the treatment system. This may not be the case in a conventional landfill. The operator also must be prepared to repair any damage to the landfill's cap and cover that may result from leachate escaping through the side of the bioreactor.
Odor control can be more challenging when waste is wet. Consequently, the operator must be prepared to take appropriate action if problems arise. This could include quickly covering an area with earth or introducing a fresh waste layer over a bioreactor cell. The operator also must be prepared to discontinue leachate recirculation if any of these issues emerges.
To discontinue leachate recirculation, it may be necessary to have auxiliary leachate storage facilities, or to quickly move the leachate from the landfill to the treatment system.
Plans for installing gas recovery equipment will need to be implemented on an ongoing basis during the bioreactor's operation. Landfill managers must primarily consider that they are dealing with a frequently changing landfill cell layout that is subject to settling. The shifting waste, as it rapidly decomposes, may break some of the collection equipment. So the operator needs to be prepared to quickly fix any damage that occurs to prevent odor problems and energy loss.
Regulatory Controls and Variances
Specific regulations require attention when implementing the bioreactor.
Current regulations prohibit the addition of liquids into a landfill. Consequently, to add additional water besides leachate, a variance is necessary.
Existing regulations specify that no more than 1 foot of leachate be allowed to accumulate on top of the landfill liner. Bioreactor operators will find it difficult to consistently meet this requirement.
Landfill operators may find that state regulatory agencies require more extensive and frequent groundwater monitoring. This is because bioreactors contain more water than conventional landfills, and the pressure from the leachate is higher.
Landfill operators may receive special approval from state regulatory agencies to refill the space that becomes available as the waste consolidates.
Regulatory authorities may consider reducing the landfill's long-term care requirements, given that bioreactors stabilize waste much faster than conventional landfills.
Bioreactor developers must take into account the public's concerns, given their limited amount of bioreactor experience. Developers should carefully plan and organize the bioreactor's operation, but also be forthright regarding plans for refilling the consolidated bioreactor cells.
Facility neighbors will be particularly concerned about potential groundwater and odor problems. The long-term disposition of the bioreactor site also should be specified.
In the long run, a bioreactor landfill may create benefits not available with a conventional landfill. The bioreactor will reach a stabilized state much more quickly than a conventional facility. Consequently, the landfill is less likely to contaminate the environment over the long-term.
Phil O'Leary and Patrick Walsh are solid waste specialists at the University of Wisconsin-Madison. Lesson 7 will discuss preparing landfill design plans and specifications. For more information, visitwww.wasteage.com.