The long-term kinetic rates of solubilization/utilization of proteins, carbohydrates, and lipids in secondary solids from three municipal wastewater treatment plants were measured. Temperature, input sludge retention time (SRT₀), and terminal electron acceptor were used as control variables for two four-month digestion studies employing seven mixed batch reactors. Three types of adenosine triphosphate (ATP) measurements were used to differentiate between viable endogenous biomass, viable anaerobic/facultative biomass and intact non-viable solids. Solids solubilization was significantly affected by all three control variables. Temperature and terminal electron acceptor showed greater effects on solubilization than SRT₀. The effect of temperature was time-dependent and differed for the two types of secondary solids evaluated. Proteins were removed to the greatest extent, lipids to an extent comparable to that of volatile suspended solids (VSS), and carbohydrates were the most resistant component. Direct sample ATP or VSS measurements were poor indicators of the time-dependent potential of mixed microbial population for further waste stabilization. However, the analysis of the ratios of recovery ATP to sample ATP (ATP recovery ratio) provided more insights into the microbial behavior during sludge digestion. Under aerobic conditions, ATP recovery ratios generally increased with time. This result implied that during the initial phase of digestion the volatile solids consisted of a higher fraction of intact non-viable material, but after prolonged digestion the solids included a greater fraction of viable endogenous biomass. Under anaerobic conditions, ATP recovery ratios increased and then decreased with time, implying that the fermentative microbial fraction was highest at some intermediate point in the time frame employed. Four phase degradation patterns were observed in all cases, indicating an inability for any single equation to effectively model the long-term solubilization process.

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Thesis (Ph. D.)--University of Hong Kong, 1994. / Includes bibliographical references (leaves 113-121).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1974. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references (leaves 80-82). Also available in electronic version.

The anaerobic digestion of cheese whey was studied in an upfiow anaerobic sludge blanket reactor for its start-up characteristics, the effects of various process parameters, the effect of sulfate addition and the determination of optimal operating conditions. Start-up of an UASB reactor treating cheese whey was extremely difficult due to its tendency to acidify. Various start-up strategies were tested to facilitate start-up and to ensure stable operation. Among the operating parameters, sludge loading rate was the most critical for proper start-up of the UASB reactor. The initial sludge loading rate during start-up period should not exceed 0.25 g COD/g VSS. The response of whey digestion to several process parameters was investigated. Without pH-control, over 97% COD removal was obtained for influent concentrations from 5 to 28.8 g COD/1 and HRT of 5 days. However, instability was observed when the influent concentration was increased to 38.1 g COD/1. Gas production from whey is affected by organic loading rate (OLR). At an OLR less than 4 g COD/l-d, higher influent strength resulted in a higher methane production rate. When the OLR was greater than 6, higher strength feed or shorter hydraulic retention time (HRT) produced less methane. From the profiles of substrate concentration measured at various levels above the bottom of the reactor, two reaction stages, acidogenesis and methanogenesis were distinguished. It was experimentally illustrated that the rate of acidogenesis is much faster than the rate of methanogenesis in a whey anaerobic digestion system. The accumulation of VFAs in the first stage being faster than its assimilation in the second stage creates a distinct acidogenic phase in the bottom of the reactor. The instability caused by high influent concentration could be attributed to the accumulation of VFAs beyond the assimilative capacity of the methanogenic stage. A set of empirical models for accumulation and degradation of VFAs was developed using linear regression analysis. The requirement for maintaining this system in a dynamic balance was that the degradation capacity for VFA in the second stage be greater than the accumulation of VFA in the first stage. Based on this idea, the optimal influent concentration was given as between 25 to 30 g COD/1 for system stability. A hypothesis was proposed in this study that a proper amount of sulfate may be applied to moderate the detrimental influence of excess hydrogen on a stressed anaerobic reactor. The effect of sulfate was tested to study the biochemical mechanism. The permissible influent COD concentration was increased from 30 g COD/1 to 50 g COD/1 by using sulfate addition. The pH in the reactor was on the average 0.8 units higher and the concentration of butyric acid in the acidogenic phase much lower with added sulfate than without sulfate addition. The significant improvement of process stability and treatment efficiency made by the addition of sulfate clearly illustrated that sulfate acted like a stimulator which helped to maintain conditions favorable to methanogenesis. The mechanism of this stimulation is explained according to thermodynamics and hydrogen regulation which suggested that sulfate is able to promote the β-oxidation of VFAs by consuming hydrogen. A two-stage inhibition mechanism was proposed to explain the inhibition of high VFA concentrations and the stimulation of sulfate. Higher hydrogen pressure is the cause of preliminary inhibition, resulting in the accumulation of VFAs, which subsequently inhibit the activity and growth of methanogens in the second inhibition stage. The mechanism of inhibition of methanogens from VFAs was interpreted as being caused by the acidification of the internal cytoplasm and destruction of the pH gradient by non-ionized acids based on the theory of bacterial membrane transport. A new control strategy for stabilization of an anaerobic system is recommended. Under the optimal operating conditions based on the results in the first three steps, over 97% reduction of COD was achieved when the influent COD was 30 g /l using an HRT of 2 days, an OLR of 16.61 g COD/l-d and sulfate concentration of 0.2 g/1. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Continuous flow, daily fill and draw, and batch aeration digesters were studied on a laboratory scale, to develop low temperature characteristics and design criteria for aerobic digestion of waste activated sludge. These results were compared against full-scale data from three independent sources. Raw sludge used in these studies was obtained from a municipal high rate activated sludge plant. The digestion systems were operated at liquid temperatures of 20, 10, and 5°C, and at six different sludge ages. Measurement of all parameters studied for the continuous feed systems were made under steady-state conditions. Parameters studied were divided into three main groups, namely: (1) Parameters related to aerobic digestion kinetics, such as solids destruction and oxygen uptake rate; (2) Parameters related to aerobic digestion sludge characteristics, such as biochemical oxygen demand, viable bacteria, organic carbon, nitrogen forms, and odour values; (3) Parameters relating to supernatant quality, such as dissolved solids, organic carbon, biochemical oxygen demand, chemical oxygen demand, nitrogen forms, and pH. The results show that the effect of low temperature on aerobic digestion performance was pronounced. The combined effect of sludge age and temperature was shown to be an important design parameter. Kinetic reaction rates and temperature sensitivity coefficients were calculated for the various conditions studied on the basis of volatile suspended solids. It was shown that reaction rates for batch digestion and continuous feed digestion systems were not interchangeable. This is significant, as most pilot plant and bench-scale studies on aerobic digestion are being conducted using batch digestion, the results of which are then being used for design of continuous feed digesters. It appears that, as temperature decreases, the daily fill and draw method of digester operation resulted in significantly higher reaction rates than the continuous feed method of digester operation. At higher temperatures, the two were about equal. Oxygen uptake rate was not considered a reliable indicator of digested sludge stability; instead, mixed liquor BOD₅, was introduced as a possible means of determining such stability. Nitrification and denitrification is shown to be appreciable during aerobic digestion in all systems and at all temperatures studied. Although somewhat tentative, the results show important nitrification and denitrification trends at temperature and pH levels well below optimum values for these processes. The difference between batch and continuous feed digestion is shown in various ways, but probably most clearly through monitoring supernatant characteristics. The continuous feed systems show increased solubilization of organics with increased sludge age, whereas the batch systems do not show any solubilization of organics. Reasonable correlation is demonstrated between the laboratory and full-scale aerobic digestion data. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

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Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2004. / Includes bibliographical references (leaves 188-201). Also available in electronic version. Access restricted to campus users.

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