Sequential Ozonation/Biodegradation to Treat Nitrophenolic Wastewater

• Main Authors: Yu Chang Ping, 于昌平
• Other Authors: Yu Hwa Yu
• Format: Others
• Language: zh-TW
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/12187417617057385222

博士 === 國立臺灣大學 === 環境工程學研究所 === 89 === Among the processes developed for wastewater treatment, each has its limitations in applicability, effectiveness and cost. Integration of chemical and biological processes for recalcitrant or inhibitory contaminant treatment is an advantageous concept. The aim of chemical oxidation is to partially oxidize the recalcitrant contaminants to more easily biodegradable intermediates since a total mineralization may be not cost effective. Preoxidation of phenolic wastewasters by using ozone as the chemical oxidant has been found previously to be quite effective in enhancing their biodegradability and reducing their toxicity. Ozone is a powerful oxidant among the commercially available oxidants used in water and wastewater treatment plants. In some cases organic compounds can be oxidized completely to CO2 and H2O with ozone, but such occurrences require relatively long ozone contact periods and relatively high ozone dosages. In most cases, organic compounds are oxidized to other materials which, as more and more oxygen atoms are incorporated during ozonation, become more refractory to continued ozonation. Such phenomena can be observed by following changes in TOC levels during ozonation. In such cases, ozone oxidation simply converts one organic compound into other organic compounds. However, the fact that ozone oxidation introduces oxygen into the original organic compound implies that the ozonated compounds now should be more biodegradable than the starting compounds. Indeed, preoxidation of wastewasters by using ozone as the chemical oxidant has been found previously to be quite effective in enhancing their biodegradability and reducing their toxicity. However, preozonated wastewaters may show some adverse effect on biological processes because some byproducts formed during oxidation appear to be more toxic than the original compound. Therefore, ozonation products play a key role in the total process, and careful analysis of the products should be carried out in advance. "Biodegradability" is considered to be an index that can be used to evaluate the optimum conditions. As a result, to find the optimum condition of chemical pretreatment, methods for measuring biodegradability in these systems have been proposed by a number of authors as reviewed in the text. However, we found out some insufficiency and proposed several recommendations for future studies, two of which are to use bioreactors instead of biodegradability and to study reaction intermediates. To predict the behavior in actual combined processes, there should be more efforts done to reveal if these short-term biodegradability tests could represent the results of those long-term biological processes. In addition, those gross parameters of biodegradability and toxicity, such as BOD5 or EC50 provide little information about the ultimate fates of the reaction intermediates. In previous literatures, although a few researches ran biological processes, which mimicked actual processes, none of them study the conversion of reaction intermediates that the bioreactor received from the chemical pretreatment. In order to minimize complexity, ozonation and a single compound p-nitrophenol were chosen as the model chemical pretreatment and the target compound in our study. Ozonation of organic compounds doesn’t generate sludge and usually produced oxygenated organic materials instead of introducing any other atom into it. These make us more easily to operate and predict the reaction intermediates. p-Nitrophenol, with the annual production of 20 million kg ,has been listed as "priority pollutants" by the U.S. Environmental Protection Agency and recommended restricting their concentrations in natural waters to < 10 ng/l. First, this study was undertaken to examine the reaction between ozone and p-nitrophenol. The appearance and disappearance of various reaction intermediates and products will be analyzed quantitatively over the course of time using HPLC. Several intermediates formed before and after ring-opening of p-niitrophenol were quantitatively determined by HPLC in our work. GC/MS and electrospray ionization mass spectrometry (ESI-MS) were also used to help establish the reaction pathway. The purpose of this part is to develop an understanding of the ozonation process for nitrophenolic compounds. This research not only finds out the ozonation products but also focuses on the biological process. Sequencing Batch Reactors (SBRs) were chosen to be the subsequent biological system. The SBR, because of its batch nature, can let us observe substrate degradation by following changes of the concentrations of all compounds in the bioreactor. In conclusion, this study was undertaken to examine the ultimate fates of reaction intermediates in integrated ozonation-biotreatment process. The appearance and disappearance of various reaction intermediates and products will be analyzed quantitatively over the course of time using HPLC. Microtox toxicity test and spectrometric analysis were also used to help establish the degradation scheme. Different from any previous researches concerning combined processes, a detailed community-level characterization of microbial biomass from SBRs, was carried out using BIOLOG redox-based carbon source utilization assay. The purpose of this paper was undertaken to examine if these short-term biodegradability tests could represent the results of those long-term biological processes. Ozonation of p-nitrophenol followed by lab-scale SBRs, rather than biodegradability tests, was conducted in the research. By observing the degradation of intermediates, the elimination of DOC, color and toxicity, and the variation of microbial ecologies, several conclusions can be drawn as followed: First, ozonation of p-nitrophenol generates nonbiodegradable compounds which might include polymerized intermediates, ring-opening products with nitro groups, and metabolic dead-end products. Second, there was no Microtox toxicity detected from any biological effluents, which means that either those nonbiodegradable compounds comprising the appreciable amount of residual DOC are nontoxic materials, or their concentration is not high enough to show toxicity. Third, the microbial ecology analysis showed that different ozonation times generate distinct amounts of intermediates and lead to dissimilar microbial communities. Finally, from the results of the community metabolism pattern and the overall system efficiency, we found that adverse effects will be brought about by improper ozonation. Instead of finding out an optimum ozonation time, a least efficient condition was observed. This implies that the result of the biodegradability test, which reflect the initial situation when a group of microorganisms were first exposed to some new substances, will not always guarantee the same result in a biological treatment system.