Re: Bioreactor Design And Operation Pdf Free
Melva Simons
The bioreactor provides a central link between the starting feedstock and the product. The reaction yield and selectivity are determined by the biocatalyst, but productivity is often determined by the process technology; as a consequence, biochemical reaction engineering becomes the interface for the biologist and engineer. Developments in bioreactor design, including whole cell immobilization, immobilized enzymes, continuous reaction, and process control, will increasingly reflect the need for cross-disciplinary interaction in the biochemical process industry.
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.
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.
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.
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 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.
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.
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 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.
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.
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.
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:
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.
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.
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.
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.
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.
The availability of large numbers of units of artificial arteries would offer significant benefits to the clinical management of bypass surgery. Tissue engineering offers the potential of providing vessels that can mimic the morphology, function, and physiological environment of native vessels. Ideally this would involve culturing stem cells in vitro within a biodegradable tubular scaffold so as to construct tissue for implantation. Essential to establishing a robust process for the production of tissue-engineered arteries is the understanding of the impact of changes in the operating conditions and bioreactor design on the construct formation. In this article, models of transport phenomena were developed to predict the critical flow rates and mass transfer requirements of a prototype bioreactor for the formation of tissue-engineered arteries. The impact of the cell concentration, tube geometry, oxygen effective diffusivity in alginate, substrate and metabolite concentration levels, feed rate, and recycle rate on the design of the bioreactor was visualized using windows of operation and contour plots. The result of this analysis determined the best configuration of the bioreactor that meets the cellular transport requirements as well as being reliable in performance while seeking to reduce the amount of nutrients to be used.
A bioreactor landfill operates to rapidly transform and degrade organic waste. The increase in waste degradation and stabilization is accomplished through the addition of liquid and air to enhance microbial processes. This bioreactor concept differs from the traditional dry tomb municipal landfill approach.
The Solid Waste Association of North America (SWANA) has defined a bioreactor landfill as "any permitted Subtitle D landfill or landfill cell where liquid or air is injected in a controlled fashion into the waste mass in order to accelerate or enhance biostabilization of the waste." The United States Environmental Protection Agency (EPA) is currently collecting information on the advantages and disadvantages of bioreactor landfills through case studies of existing landfills and additional data so that EPA can identify specific bioreactor standards or recommend operating parameters. The information on this web page is organized to give you information about:
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