Browsing by Author "Andrews, John F."
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Item Anaerobic digestion process control using a knowledge-based system(1989) Barnett, Michael William; Andrews, John F.Much attention has been devoted to the design and construction of wastewater treatment plants. However, relatively little attention has been devoted to plant operations. Knowledge based or expert systems are oriented toward improving operational procedures and thus have significant potential for solving control problems. This paper presents the results of a study conducted to examine the use of expert systems technology for computer assisted operation of the anaerobic digestion process. Knowledge was divided into mechanistic understanding of the process and that which comes from experience (heuristic). Effective operation requires both types of knowledge. A process task analysis identified four major tasks. These are monitoring, state assessment, control decision and execution tasks. The analysis suggested an expert system configuration consisting of software modules corresponding to these tasks. Development of the state assessment and control action modules was stressed. The capture of mechanistic knowledge was emphasized. Knowledge was represented using if-then rules. Diagnostic rule development was accomplished using computer simulations of hydraulic, organic, toxic and ammonia upsets which could cause process failure. Rules relating to control included decision rules which specified appropriate control actions and fuzzy rules which specified the amount of control. Additional simulations of process upsets were used to examine the behavior of the system. Several different upset states were properly identified and corrected by the system. The control actions used were influent flow control, dilution, organism recycle and acid or base addition. The rule-based approach was found to be flexible and transparent. Theoretical knowledge from the mechanistic model could be converted into a language understood by operations personnel. However, the expressive capabilities of the rule language and the inability to ensure consistency and completeness in the rule base limited the system. An expert system is knowledge intensive and knowledge transfer from the expert to the computer and from the computer to the operator are important factors. Field testing will be needed to validate the rules developed for the expert system. Future research should emphasize the development of a body of knowledge about process operation. Alternative knowledge representations and hybrid systems integrating expert, fuzzy and stochastic control techniques should be studied. In combination these provide powerful tools for solving the problems of control in wastewater treatment.Item Dynamics and control of temperature in the aerobic digestion of waste activated sludge(1984) Sparks, John P.; Andrews, John F.; Bedient, Philip B.; Tomson, Mason B.Pilot plant data and computer simulation show significant process benefits from the separation of aeration and mixing in open tank aerobic digestion. Open tank operation results in surface heat losses which cancel the heat gain of the exothermic organism decay reaction and drive digestion temperatures toward ambient atmospheric conditions. Diffused aeration produces a further evaporative heat loss through vapor enrichment of the diffused air. Mechanical mixers, however, elevate digestion temperatures through heat input from mixing and lower required air flow rates. Solids destruction rates are increased by higher digestion temperatures and increased oxygen transfer efficiency. For this reason, mechanical mixers should be included in the design of aerobic digesters and may relieve overload conditions at existing wastewater facilities. Also, the correlation between temperature history and solids destruction indicates that time-temperature data may be a valid indicator of sludge stability.Item Dynamics, stability and control of anaerobic packed bed processes(1988) Ide, Shinji; Andrews, John F.Anaerobic packed bed processes are used to treat industrial wastes containing high concentrations of readily biodegradable organics. Dynamic mathematical models for anaerobic up-flow and down-flow packed bed processes have been developed and verified semi-quantitatively by using experimental data and information from the literature. The dynamic models are then used to study the dynamics, stability and control of the process. The biological kinetic models involve four different groups of bacteria: acid producing bacteria (X$\sb{\rm G}$), hydrogen utilizing methanogens (X$\sb{\rm H}$), acetoclastic methanogens (X$\sb{\rm A}$) and propionic acid utilizing acetogens (X$\sb{\rm P}$). The effect of hydrogen on the acid formation stage and propionic acid degradation has been incorporated in the models using complementary Monod type equations. Modified inhibition functions are proposed for the growth rates of X$\sb{\rm A}$ and X$\sb{\rm P}$. In these functions, high concentrations of total unionized volatile fatty acids and unionized propionic acid are assumed to be toxic to X$\sb{\rm A}$ and X$\sb{\rm P}$, respectively. The hydraulic model of the up-flow anaerobic packed bed process is represented by a slurry reactor followed by a fixed film reactor approximated by complete mixing tanks in series. A constant thickness of biofilm and uniform bioactivity are assumed for the down-flow anaerobic packed bed process. A single complete mixing tank with dead space, short circuiting, and complete mixing tanks in series is used for approximating the hydraulics of down-flow process with and without recirculation, respectively. Semi-quantitative agreement between simulation results using the proposed models and data obtained from experiments and the literature was obtained. High resistance to hydraulic overloading and rapid recovery after shock loadings was demonstrated for the anaerobic packed bed processes. The model suggests that the more intense the overload, the greater the accumulation of propionic acid. System failure due to organic loadings could be predicted by incorporating modified inhibition functions into the overall model. The concentrations of hydrogen and propionic acid can be good stability indicators. On-off control of the recycle flow rate using hydrogen and propionic acid concentrations as the measured variable was simulated. This control strategy can prevent process failure for pulses of short duration and delay process failure for step changes. The time delay obtained by this control can provide operators the time needed to take other corrective actions.Item Ground water contaminant modeling applied to plume delineation and aquifer restoration at an industrial site(1985) Freeberg, Kim Melanie; Bedient, Philip B.; Andrews, John F.; Tomson, Mason B.Numerical modeling of contaminant transport is a useful tool applied to ground water investigations at industrial sites, not only in delineating the geographical extent of the contaminant plume, but in evaluating remedial schemes intended to mitigate the contamination problem. Transport processes, mathematical models, and recovery schemes are reviewed in this thesis, as a preface to the main study, the application of the USGS Solute Transport Model to a site contaminated by trichloroethylene and other industrial solvents. The USGS model, which utilizes the finite difference technique and the method of characteristics, was applied in its two-dimensional, steady-state form. At the study site, the model gave a good prediction of the movement of the contaminant plume and on the performance of a recovery system, when the results were compared with field data. It predicted that four withdrawal wells would reduce trichloroethylene concentrations in the ground water by approximately 99 percent, after two years of pumping.Item Hydrologic analysis of urbanization and related parameters on flooding in the Brays Bayou watershed(1985) Juchs, Martha Ferrero; Bedient, Philip B.; Andrews, John F.; Rowe, Peter G.Although floods are natural phenomena, their impact is often aggravated, if not actually caused, by man's activities and occupation of floodplains. Urbanization of a floodplain drastically alters the flood characteristics of a stream by increasing the percentage of rainfall that becomes runoff and moving that runoff into drainage channels more rapidly. Flood flows are therefore quicker to peak and peak higher for the same amount of rainfall in an urban basin. One of the most rapidly urbanizing cities in the United States is the city of Houston, Texas; a city which also has one of the nation's most severe flood problems. In 1979 alone, Houston suffered $4 million in property damages as a direct result of flooding. Consequently, this research addresses the effects of urbanization on flooding by focusing on the Brays Bayou watershed, a basin which has experienced an enormous increase in development in the past ten years and is also one of the most frequently flooded areas in Houston. By modeling the watershed with the U.S. Army Corps of Engineers' HEC1-1 computer model, the following conclusions were reached: 1) Peak discharges are significantly increased as a result of urbanization; 2) The Brays Bayou channel can be expected to carry less than a 5-year design storm uniformly spread over the entire watershed; 3) The HEC-1 kinematic wave technique for modeling discharge is a valid simulation method when applied to a large urban watershed; the effects of storage due to backwater effects during large storms, however, must be simulated using a storage routing method in conjunction with the kinematic wave overland flow simulation.Item Implications of physical and chemical factors to bacteriological water quality of Lake Houston(1984) Solek, Elizabeth A.; Bedient, Philip B.; Andrews, John F.; Scott, David W.Lake Houston serves as the source of drinking water for approximately 4% of the population of the City of Houston, and it also serves as a major recreational resource. Rapid development of the western portion of the lake's watershed has led to deterioration of lake water quality, raising concerns over continued present and future use. In response to concern over observed sporadically high levels of fecal coliform bacteria, research was undertaken to investigate bacteriologic water quality. The objective of this research was to evaluate yearly, seasonal and spatial water quality trends and correlations of bacterial densities to physical and chemical water quality parameters. Information gained during these studies may be used in further evaluation and control of sources of bacterial pollution and to delineate periods when bacteriologic water quality may be unsuitable for such uses as contact recreation.Item Modeling and control of pumping stations and equalization basins(1992) Yin, Tingyong; Andrews, John F.Diurnal variations of flow rate and organic load cause difficulties in the operation of wastewater treatment plants. The man-made hydraulic shocks produced by the installation of fixed speed pumps upstream of the treatment processes further amplify the frequency and magnitude of these variations. The major objective of this study is to develop control strategies to minimize these variations. A dynamic model has been developed for the operation of fixed speed pumping stations. The optimum control strategy based on this model can reduce the on-off pump cycles up to 86% for a typical day as compared with ordinary two point control. Optimum models and corresponding control algorithms have also been developed for the operation of in-line equalization basins. These models have shown potential for the reduction of the required equalization volume and can provide smoother outflow to the downstream processes. A reduction of tank volume by 63.5% has been obtained in the simulation using the optimum control strategy.Item Nitrification and denitrification in the activated sludge process(1992) Idury, Radha Modukuri; Andrews, John F.The characteristics of the single-sludge nitrification-denitrification process were investigated using influent organics as the carbon source and nitrate as an electron acceptor to see if any oxygen savings could be obtained. The key research tools used were mathematical modeling, computer simulation, the library, and discussions with experienced operating engineers. A mathematical model was obtained from the literature for the nitrification process. The summary of simulations using the model was that complete nitrification can be achieved at a temperature of 20$\sp\circ$C, with a dissolved oxygen concentration of 2 mg/l and a sludge age of 4-5 days. A mathematical model was developed for the single-sludge nitrification-denitrification process and the influence of denitrification on oxygen requirements was studied by simulation. The conclusion was that, depending on the COD/NH$\sb4\sp+$ ratio, savings in oxygen, by the use of nitrate as an electron acceptor are in the range of 3 to 8 percent.Item On-line hydrologic simulation of Lake Conroe Reservoir releases for downstream flood control(1985) Rifai, Hanadi Said; Bedient, Philip B.; Andrews, John F.; Rowe, Peter G.Lake Conroe was designed for water supply and not for flood control. As a result, a flood control release policy was not established for the reservoir. This study seeks to establish the effect of Lake Conroe releases on downstream flooding and to develop operational policies to release the excess stormwater from the reservoir. A computer model was programmed on a microcomputer (Apple IIe) to simulate the response of the Lake Conroe watershed to a storm event and to provide the operators at Lake Conroe with a viable method of on-line simulation. The model was calibrated with data from four historical events and verified against simulations from HEC-1, a flood hydrograph model. Recommended operational policies on four design storms were developed with the objective of minimizing peak releases from the dam. Guidelines for developing a release scheme for any design storm event were established based on the results of the HEC-1 analysis.Item The characterization of hazardous waste sites with analytical and numerical models(1983) Bouvette, Tracy; Bedient, Philip B.; Andrews, John F.; Anderson, John B.This research presents the use of analytical and numerical modeling techniques to aid in the characterization of a subsurface contaminant plume associated with a hazardous waste site (HWS). This study presents a discussion of various model types and their governing equations, as well as outlines the specific analytical and numerical models used for the research. The U. S. Geological Survey (USGS) Method of Characteristics (MOC) model is investigated in depth, including an overview of the model, with a description of the code's solution technique and input parameters, and a sensitivity analysis to determine the effect of each parameter on the model's simulation of a subsurface contaminant plume. An abandoned hazardous waste site in Conroe, Texas is employed to illustrate the use of modeling techniques to help evaluate the data base, investigate the hydrogeologic parameters which most influence the contaminant plume migration, and determine new objectives for the ground water monitoring program.Item Water quality and sediment dynamics in a detention pond system(1984) King, Thomas L.; Bedient, Philip B.; Anderson, John B.; Andrews, John F.An investigation into the Lake Harrison detention pond system revealed that the design enhanced the removal efficiency of total suspended solids (TSS) and, to a lesser degree, nutrient constituents in the runoff. The design increased TSS flocculation, which ensured particulate removal, and protected settled particles from resuspension by scouring.