To be effective, environmental geotechnologists must not only be armed with the traditional knowledge of fields such as geology and civil engineering, but also be knowledgeable of principles of hydrogeology, chemistry, and biological processes. The purpose of this book is to summarize the current state of practice in the field of environmental geotechnology. Part Two describes in detail the underlying principles for design and construction of new waste disposal facilities.
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Part Three covers techniques for site remediation. Finally, Part Four addresses the methodologies for monitoring. Product Details Table of Contents. Table of Contents Part I: General principles. Contaminant transport. Part II: New disposal facilities. Landfills and impoundments. Leachate and gas generation. Clay liners. Geomembrane liners. Collection and removal systems. Water balance for landfills. Stability of landfills. Mine waste disposal. Part III: Remediation technologies.
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Strategies for remediation. Geophysical techniques for subsurface site characterisation. Soil exploration at contaminated sites. Vertical cutoff walls. Cover systems. Recovery well systems. Perhaps the most comprehensive compilation of leachate quality data applicable to full-scale landfills was provided by Ehrig He obtained, leachate composition data from 15 West German landfills ranging in age from 0 to 12 years. These data are summarized in Tables 6. Wide ranges in concentration data are apparent.
Con- stituents for which the solubility is relatively unaffected by pH, such as chlorides, show no clear shift in going from acidic leachate, character- istic of the anaerobic acid phase, to the methane producing phases. Constituents which are more soluble under acidic conditions, such as iron, show a clear decrease in concentration in progressing from the acid phase to the methane producing phases.
Ehrig also measured the COD in numerous landfill leachates. This occurred at about eight years. Thus it is only possible to make general state- ments on potential effects of hazardous wastes on leachate and gas characteristics. During the aerobic phase, there is an increase in tem- perature due to the waste heat of microbial metabolism. This, com- bined with vigorous CO2 production during the aerobic and initial part of the anaerobic acid phase will enhance gas stripping of volatile chemicals.
Gas stripping does not necessarily mean release of an organic com- pound to the environment. As volatile organic compounds pass through soil separating the refuse from the atmosphere, there is an additional The relationship between laboratory and field-scale data Table 6. Whether this is a significant sink for volatile organics buried in landfills has not been studied. In addition to gas stripping, there are other potential fates of organics in landfills. Refuse in an active state of methane production represents a healthy ecosystem which may be capable of decomposing substrates other than carbohydrates and proteins.
Thus some hazardous organics may be anaerobically degraded in sanitary landfills although there is very little data in this area. The existing data do show that phenol is readily degraded by refuse microorganisms. Watson-Craik and Senior reported on the conversion of phenol to methane in laboratory-scale lysimeters.
Halogenated solvents such as perchloroethylene and trichloroethylene have been shown to undergo a biologically mediated reductive dehalogenation reaction under methanogenic conditions Fathepure et al. One end- product of reductive dehalogenation is vinyl chloride. The presence of vinyl chloride in landfill gas suggests that chlorinated solvents were previously buried in the landfill. Organics may also preferentially sorb to the non-degraded organic fraction of the decomposed refuse, thus reducing or eliminating transport.
However, this does not necessarily mean that metal concentrations will increase in leachate. Once refuse is in the accelerated methane production phase, indicative of active anaerobic metabolism, sulfate is reduced to hydrogen sulfide. Most metal sulfides are extremely insoluble and their formation would reduce metal mobility. As refuse decomposition proceeds to the decelerated methane production phase, humic materials are produced. These materials can be expected to behave as natural chelating agents, enhancing metal mobility.
Carboxylic acids also act as chelating agents Francis and Dodge, Pohland and Gould , in an evaluation of the co-disposal of metal sludges with municipal solid waste, reported an increase in metal concentrations in leachate as the refuse became well decomposed.
Since then, numerous researchers have tried to enhance refuse methanogenesis by manipula- tion of the landfill ecosystem Barlaz et al. Many factors influence the onset and rate of methane production including moisture content, pH, nutrient concentrations, and temperature among others Farquhar and Rovers, It is important to note that all of the microorganisms necessary for the conversion of refuse to methane have been detected in fresh refuse Barlaz et al. As a result of a review of data on both laboratory- and field-scale tests, it appears that the two variables most important to refuse methanogenesis are moisture content and pH.
Wujcik and Jewell studied the effect of moisture content on the batch fermentation of wheat straw and dairy manure - compounds which are analogous in chemical composition to refuse. In many laboratory-scale studies on the effects of moisture on refuse decomposition, methane yields have been too low for quantification of yield as a function of moisture content.
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The broadest data sets where moisture content can be evaluated are those of Emberton and Jenkins and Pettus Emberton Factors limiting the onset of methane production evaluated methane production rate data for landfills across the US and categorized the landfills based on annual precipitation. Jenkins tested the effect of moisture content in refuse sampled from landfills. In both studies, the methane production rate exhibited an upward trend with increasing moisture contents; confounding factors such as density, refuse age and refuse composition notwithstanding.
A second key factor influencing the rate and onset of methane pro- duction is pH. As discussed in the section on anaerobic microbiology, microorganisms responsible for the conversion of refuse to methane are quite sensitive to pH. Their pH optimum is between 6.
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Theoretically, refuse pH is an excellent indicator of potential methanogenic activity. Unfortunately, as discussed previously, leachate composition data at a field-scale landfill may well not be indicative of the decomposition state of the refuse. Leachate with a pH below 5 has been observed in landfills actively producing methane. Such leachate has most likely been in contact with some refuse in the anaerobic acid phase as well as refuse actively producing methane.
Hence, leachate pH is not always a useful indicator of methanogenic activity. Leachate recycle and neutralization has been shown to enhance the onset and rate of methane production Pohland, ; Buivid et al. Given that moisture and pH are reported to be the two most significant factors limiting methane production, the stimulatory effect of leachate recycle and neutralization is logical. Recycling neutralized leachate back through a landfill increases refuse moisture content, substrate availability, and provides a degree of mixing in what may otherwise be an immobilized batch reactor.
Neutralization of the leachate provides a means of externally raising the pH of the refuse ecosystem. There is limited field experience with leachate recycle systems and more is needed to fully document its value in a field-scale situation. Moisture content is the factor which most often limits methane pro- duction. This is to be expected in dry climates where there is little opportunity for infiltration, and in wet climates where biological activity may be limited because landfills are typically designed to minimize water infiltration.
Prior to the onset of methane production, the rate at which the methanogenic bacteria convert acetate to methane limits the onset of methane production. In laboratory-scale work, seeding fresh refuse with anaerobically decomposed refuse reduced the time to the onset of methane production significantly Barlaz et al.
With the onset of methane production and depletion of the soluble substrates, the rate at which polymers cellulose, hemicellulose and protein are hydrolyzed limits the rate of methane production. Historically, municipal solid waste was dumped in holes and occasionally covered with whatever material was available. This often led to groundwater contamination and the release of contaminated stormwater to surface water.
Subsequently, states began to regulate the burial of municipal solid waste, bringing about the modern sanitary landfill, complete with liners and leachate collection as described in Chapter 1. Sanitary landfill design standards were developed to minimize the amount of moisture which came in contact with refuse, thus minimizing leachate production.
However, at the time that the philosophy of a dry landfill was adopted, methane recovery for energy was in its infancy.
http://tax-marusa.com/order/wizisece/localiser-adresse-ip-tunisie.php The design of landfills to enhance methane production, by allowing the moisture content of fresh refuse to increase by surface water infiltra- tion, was not given serious consideration. More recently, some solid waste regulations recognize that leachate recycle and neutralization for enhancement of methane production may be advantageous. In addition to energy recovery, enhanced methane production and recovery as an energy source offers other advantages. With the onset of methane production, there is a reduction in leachate COD, thus reducing leachate treatment costs and the potential for groundwater contamination.
Refuse settles as it decomposes and the resulting settlement necessitates significant maintenance of the landfill cover to repair cracks. More complete refuse decomposition prior to placement of the final cover would minimize settlement and long term main- tenance costs. Enhanced methane production would make energy recovery projects more economical, thus more would be implemented.
This would reduce emissions of methane to the atmosphere. Methane is a gas which has a greenhouse effect at least twenty times more damaging than carbon dioxide on a volume basis. Enhanced refuse decomposition reduces the impact of future leachate and gas emissions, most likely reducing long-term monitoring and care requirements. Enhanced decomposition means that most of the gaseous and leachate products of decomposition are released during the period when the gas and leachate control systems are most likely to func- tion as designed, and when responsible parties are present to monitor and repair the site as necessary.
Landfills designed and operated in a mode of leachate recycle and neutralization may be recognized as more desirable in the future. It will Mass balance analysis of refuse decomposition then be necessary to design landfills not only with liners and leachate collection systems similar to those used today, but also with systems for distributing leachate over refuse without creating odor or side slope seepage problems. The methane potential of refuse, combined with an estimate of the current rate of methane production as measured in a pump test, is useful for evaluation of the economics of landfill gas recovery projects.
The mass of methane which would be produced if all of a given constituent were converted to carbon dioxide, methane and ammonia may be calculated from Eq. Based on Eq. The methane potential of carboxylic acids can be calculated from the reactions governing the conversion of valerate to propionate and acetate, butyrate and propionate to acetate and hydrogen; hydrogen and carbon dioxide to methane; and acetate to methane and carbon dioxide Mclnerney and Bryant, Calculated methane potentials are Barlaz et al.