CHAPTER 2 Literature Review
Current State of Landfills
As the world’s population has increased, we have a greater need to properly treat and dispose of municipal solid waste. Classical methods of tomb land filling have become obsolete because engineered landfill bioreactors have been developed. Typical landfills take too much time to reach a state of closure, eliminating the possibility to use the land for other purposes and causing many problems with post closure site monitoring and pollution of surrounding lands.
Landfills often require large plots of land, which have limited value for future development. This can affect the land for 20-30 years after the landfill is closed and monitored. Because of the positive relationship between moisture content and biodegradation of MSW, the dry tomb approach to land filling actually extends the time it takes to degrade the waste (Wall & Zeiss, 1995).
Landfills are still being designed with dry tomb methodology. It has been shown in the past, however, that this convention provides a risk of uncontrolled leachate and biogas leaks and surrounding contamination. Moisture is important to the initial steps of biodegradation, and continual moisture content provides the proper environment for microbes to receive nutrients and degrade wastes (Yuen et al., 2001).
Municipalities around the world have encountered problems with their management of wastes. The city of Istanbul is expected to see an increase in solid wastes generated per capita of as much as 25% (Šan & Onay, 2001). In the United States, almost half of the MSW produced has been sent to traditional landfills, where degradation takes place under less than optimum conditions (Mehta et al., 2002).
In Florida, it has been discovered that from waste such as batteries, electrical switches, fluorescent light bulbs, and others, methylated mercury compounds can be formed under these methanogenic conditions. These monomethyl and dimethyl mercury compounds are very toxic to humans and other species (Lindberg et al., 2001).
It has been determined that almost half of the greenhouse gasses produced from paper in Australia came from paper that had been landfilled. This paper makes up about 10% of their total MSW volume. Reducing the amount of paper that makes it to the landfills by way of recycling or waste-to-energy recovery has proven to be effective method to reduce these emissions (Pickin et al., 2001).
The classical method of using a landfill as a storage site typically means that the leachate infiltration and migration is reduced. With this comes the problem of decreased rates of degradation. Leachate recirculation allows the landfill to operate as a landfill bioreactor, providing an environment suitable to increased degradation rates as well as providing control of side effects (Townsend et al., 1996).
Because research has told us much about the behavior of landfills, there has been an increased interest in the development of landfill bioreactors. They have been engineered to reduce leachate migration into the subsurface, increase degradation rates, and increase landfill gas production. Another goal of the bioreactor landfill is increased subsidence during the active operating period to provide more space for land filling. Waste shredding, moisture and temperature control, and addition of nutrient rich leachate are a few of the successful methods already used to achieve these goals (Warith, 2001).
By adding moisture, buffers, and microbe sources such as wastewater treatment plant sludge, a degradation rich environment is achieved. Leachate recirculation not only provides the moisture and nutrient transport required for microbe development, it provides the microbes a way to rid themselves of fermentative products that are detrimental to their development (Nopharatana et al., 1998).
To consider a landfill for closure methane production should be minimal, maximum settlement of the MSW should be observed, as well as the absence of adjacent contamination from leachate. Using a bioreactor landfill approach, reduced cost of post-closure monitoring and land reclamation are among the biggest advantages (Lee et al., 2001).
One of the most effective ways to reduce the side effects of land filling and increase the rate of degradation of MSW is the engineered landfill bioreactor. Landfill bioreactors are the alternative to dry-tomb land filling. A landfill bioreactor can provide up to a 10-fold decrease in closure time for a landfill site, when compared to the dry-tomb approach. There is also a greater range of control in a landfill bioreactor, which allows engineers to minimize the environmental hazard from the landfill.
A basic description of a landfill bioreactor is a landfill where additional air and liquids are introduced to the waste mass to enhance the microbial activity and increase the rate of degradation and stabilization of the waste mass. There are three main types of bioreactor landfills, aerobic, anaerobic, and hybrid designs. In an aerobic landfill bioreactor, air is injected into wells throughout the MSW to enhance the aerobic processes, while the addition of recirculated leachate from the bottom of the landfill is added in a controlled manner to enhance the nutrient availability of the aerobes. In an anaerobic landfill bioreactor, degradation takes place in the absence of oxygen, allowing methanogenic anaerobes to break down the wastes while producing methane or natural gas. Optimal moisture levels are obtained by again recirculating leachate and nutrients in a controlled manner. In a hybrid designed landfill bioreactor, both aerobic and anaerobic processes take place. In this design, the upper sections are operated as aerobic systems, allowing for faster degradation and faster onset of methanogenesis in the lower levels. As the methanogens are cultured, they produce methane, which is collected from these lower levels. In the latter two designs, the methane or natural gas that is produced can be collected for onsite energy needs or sold offsite for other uses, adding to the efficiency of the landfill (U.S. EPA, 2003).
Moisture content is the number one environmental condition for success in a landfill bioreactor, though there are many other factors involved. Through the rapid stabilization of landfill wastes, less risk of future environmental contamination and post closure costs are sooner achieved, as well as reclamation of the landfill site property (Townsend et al., 1996).
Leachate recirculation rates must be chosen wisely, where too much or too little can both be detrimental to the effectiveness of a landfill bioreactor setup. With too much moisture, problems such as highly acidic conditions as well as ponding and saturation exist. If flow rates are too high, removal of the methanogens and buffering problems will disable the methanogenic processes, where too little flow will allow a buildup of inhibitory products (Šan & Onay, 2001).
With the proper residence time, methanogens are able to maximize the conversion of MSW to methane, including the conversion of recalcitrant compounds. Increased conversion of complex compounds along with increase in initial biomass growth is found with the increase in leachate recirculation (Chen et al., 2000).
Benefits found in bioreactor landfills are present other than just faster degradation. Damaging effects of settlement on the final cover of a landfill are reduced by the increased settlement found in bioreactor landfills, where this increase in waste density also adds to the overall volume available for filling. In addition, the costs of offsite treatment of leachate are reduced because the leachate is partially stabilized during the process of continuous recirculation. Natural gas recovery also becomes a more realistic task when there is an increased rate of gas production (Mehta et al., 2002).
Landfill Bioreactor Chemistry
Landfill bioreactors are noted to go through different phases. There are five main phases of degradation, which are the initial adjustment phase, the transition phase, the acid formation phase, the fermentation phase, and the maturation phase. Each of these phases is characterized by different chemical changes and properties. The initial adjustment phase is where land filling begins, and there is ample moisture to support aerobic decomposition. The second or transition phase is where the moisture content begins to reach field capacity and anaerobic conditions prevail. In the acid formation phase, the volatile organic acids formed by hydrolysis of organics and other wastes cause a drop in pH. Following this is the fermentation phase, where the available acids are then converted into methane by the anaerobic microbes. In the last maturation phase, activity slows down and stability is found in the landfill and its leachate (Kelly, 2002).
During the first three phases, the pH can drop to around 6.5-6.0, and it is during these phases that the methanogens begin to become more active. During the fermentation phase, the pH can increase from 6.8-8.0 due to the degradation of volatile fatty acids.
Leachate is found to contain chloride concentrations anywhere from 1000mg/L and above. Over the course of landfill life, the BOD/COD ratio decreases, indicating an increase in microbial activity and a decrease in biodegradable organic compounds (Warith, 2001). In young leachate, high amounts of volatile fatty acids account for most of the COD, and this causes the BOD/COD ratio to be high.
Biogas produced in the landfills typically contains about 50-65% methane, and the remaining fraction is mostly carbon dioxide. Methane production is also normally related directly to the reduction of COD. For these reasons the COD removal rate and methane production rates are some of the important operating parameters. A larger loading rate of COD also causes a larger volumetric production rate of methane. For approximately 2.86g of COD decomposition, there is 11g of methane production in a typical landfill bioreactor setting. In addition, the production of methane increases linearly with COD loading at a slope of 0.57g CH4 –COD/g COD loaded, indicating that about 57% of the COD is converted to methane where the rest is converted to biomass (Özturk, I., and Timur, H., 1999).
As opposed to control areas of experimental setups, leachate recirculation areas were found to have decreasing methane productions over time where the controls remained mostly constant. Also it has been noted that the methane yield of samples in control areas did not correlate well with age, where the opposite was found in leachate recycle areas (Townsend et al., 1996).
CHAPTER 1 INTRODUCTION
CHAPTER 2 LITERATURE REVIEW
Current State of Landfills
Landfill Bioreactor Chemistry
CHAPTER 3 TESTING MSW AND RELATIONSHIPS WHICH PREDICT LANDFILL STABILITY
Methods and Materials.
Results and Discussion
Summary and Conclusion
CHAPTER 4 TESTING LANDFILL LEACHATE AND PREDICTING LANDFILL STABILITY
Methods and Materials.
Results and Discussion
Investigation into Measurement Errors
Summary and Conclusions
CHAPTER 5 APPENDIX
GET THE COMPLETE PROJECT
Analytical Methods of Testing Solid Waste and Leachate to Determine Landfill Stability and Landfill Biodegradation Enhancement