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Technical Renovation of the Heavy Oil to Gas Wastewater Treatment System (A/O) Abstract: This paper describes the process of improving the heavy oil to gas wastewater treatment system at the Guangzhou Oil to Gas Plant. The in-depth renovation of the wastewater treatment system involved the introduction of bioenhancement techniques ； It changed the water inlet method in the anoxic tank of the traditional A/O process from bottom to top to top to bottom ； New types of fillers were used to replace the commonly used plastic soft and semi-soft fillers. The sewage treatment capacity has been significantly enhanced through upgrades. 1 Introduction: The Guangzhou Oil-to-Gas Plant produces pipeline gas through the catalytic cracking of heavy oil. Analysis of the wastewater generated in this process using gas chromatography coupled with mass spectrometry (GC-MS) revealed the presence of 97 different chemical compounds, among which aromatic compounds account for more than half of the organic substances in the wastewater. As many as 21 organic compounds are listed among the 58 pollutants in China that are given priority for environmental control, as well as those on the list of pollutants of priority control by the U.S. Environmental Protection Agency (EPA). During the plant’s infrastructure construction phase, over 27 million yuan was invested in building a wastewater treatment system. The original design underestimated the concentration of pollutants in the production wastewater; it predicted a COD value of 200,400 mg/l and an NH3–N value of 6,070 mg/l. However, the actual values were much higher than these predictions, making wastewater treatment more difficult. 2 Brief introduction to the existing treatment process: The anoxic-aerobic treatment process, abbreviated as A/O process, is currently the most widely used wastewater treatment method in China. This process involves aerobic surface aeration following primary facultative anaerobic treatment. The advantage of this process is that it can be used for the treatment of high-concentration industrial wastewater. It can handle a large volume of water, and its operation is relatively simple. The wastewater treatment system of Guangzhou Oil-to-Gas Plant employs an A/O biological treatment process after oil separation and flotation (Figure 1). The anoxic tank uses a bottom-up water inlet method ； Furthermore, unlike conventional push-flow A/O processes, a combined aeration tank developed in the 1970s is used; this type of tank integrates aeration and sedimentation functions, with surface aerators being employed to enhance aeration. Taiwan-style aeration tanks have proven to be inefficient in terms of aeration, and they were gradually phased out in the late 1980s. Furthermore, due to the design specifying a high wastewater treatment volume, the system cannot operate continuously ； Lack of experience, imperfect operating parameters, and insufficient understanding of the various factors that affect system operation ； The insufficient aeration capacity of the aeration tank results in a low level of decomposing bacteria required for the biochemical treatment process in the system. 3 Preliminary renovation: Due to inherent deficiencies in the design and infrastructure of the wastewater treatment system, it caused significant difficulties in wastewater treatment operations once it was put into use. Since 1992, the wastewater treatment system has been gradually improved and upgraded in terms of processes, equipment, and infrastructure. 3.1 Modification of the oil removal process: In the pretreatment stage, the original tar recycling tank with three sedimentation tanks was replaced by one with five sedimentation tanks, doubling the sedimentation time. This improved the efficiency of tar sedimentation, reducing the oil content in the tar-containing water that flows into the wastewater treatment system by more than half, thereby lowering the processing load on the oil separation unit in that wastewater system ； An oil discharge pipe is connected to the bottom opening of the oil-water separator, along with a steam heating device; the oil at the bottom layer is regularly discharged into a newly built oily waste tank for treatment, which improves the oil removal efficiency of the oil-water separator. The petroleum content in the wastewater after the oil separation process is significantly reduced, dropping from 800 mg/L before the modification to around 350 mg/L, with a marked improvement in the removal rate of various pollutants. 3.2 Improvement of the flotation process: The original design did not take into account ways to handle the oil and foam water generated during flotation; after operation, this waste had to be transported outside for treatment, which was costly. In 1995, a set of filtration equipment was installed to recycle oil-foam water ； The flotation process was changed from the original configuration of two parallel tanks, one in operation and one as a backup, to one that can operate in series or parallel; an additional dosing and air dissolution system was added, thereby improving the flotation efficiency ； An additional return pipe has been added to allow the flotation effluent that does not meet the biochemical inlet requirements, as well as the wastewater from the anaerobic suction wells, to be returned to the flotation tank for further treatment ； Regular cleaning of the flotation tanks and air dissolution units has been intensified, thereby maximizing the effectiveness of the flotation process; see Table 2: 3.3 Preliminary improvement of the biological wastewater treatment process. The path by which domestic wastewater enters the wastewater treatment system has been changed from the anaerobic tank to the flotation process. When domestic wastewater contains high levels of oil, it can first be treated by flotation to remove the oil before being sent to the anaerobic tank, thus avoiding any potential negative effects on the biological treatment process ； Install a variable frequency speed control device on the surface aerator to ensure the stable operation of the aeration tank. Through years of effort, significant changes have taken place in wastewater treatment; the rate of water quality meeting standards at discharge points has been increasing year by year, as shown in Table 3. 4 In-depth renovation of the wastewater treatment system: After years of upgrades, the potential of the existing wastewater treatment system has been largely realized, but NH3-N and COD levels still fail to meet the required standards. Therefore, it is classified as a province-managed project under the “one control and two compliance standards” framework. Through extensive research and in-depth analysis, Guangzhou Oil-to-Gas Plant realized that only by carrying out thorough modifications to the system and adopting practical technologies could it ultimately ensure that the levels of N and COD at the NH plant met the required standards. In collaboration with the Guangdong Institute of Microbiology, the following actions were taken. 4.1 Experimental Setup In line with the principle of minimizing environmental protection-related investments, an anoxic- aerobic experimental setup was constructed on a scaled-down basis according to the dimensions of the on-site biochemical treatment system, with the aim of applying the experimental results to the renovation of the original system. The wastewater to be treated is mixed in a regulating tank and then pumped into an anoxic biological filter; after aeration with aerobic activated sludge and clarification filtration, it is discharged. The process flow is shown in Figure 2. 4.2 Experiments The experiments were carried out in three stages. The first stage focused on the selection of microorganisms capable of degradation and denitrification, as well as their introduction and acclimatization into the experimental reactor ； In the second phase, operating parameters are adjusted to ensure that the effluent meets the standards ； In the third phase, data accumulation under various conditions is carried out. 4.2.1 Selection and acclimatization of degrading bacteria: Due to the poor biodegradability of wastewater, an imbalance in the C/N ratio, and the pressure to remove high concentrations of ammonia nitrogen, if the activated sludge in the aeration tank system does not receive sufficient nutrients, heterotrophic microorganisms will gradually exhaust their own resources, leading to sludge mineralization and a decrease in sludge concentration. Under these special circumstances, to ensure an appropriate microbial content, conventional microbial fermentation methods cannot be used. The highly efficient degrading bacteria developed in the laboratory must be gradually introduced into the mixed culture and added to the treatment system through repeated acclimatization processes. If degrading bacteria are cultured in large quantities using conventional rich media and then added to wastewater, their degradation activity will decline, or they may even fail to grow at all. 4.2.2 Factors affecting the efficiency of treatment systems The key to biological treatment methods is microorganisms. The pH level, concentration of toxic substances, and treatment temperature in wastewater treatment systems have a strong impact on microorganisms, leading to significant changes in the efficiency of treatment. In literature or certain wastewater treatment processes, the required pH value is 6, whereas experiments have shown that wastewater with a pH of 8.5 has a significant negative impact on wastewater treatment systems ； In cases of poor water quality, the separate anoxic tanks can be operated in parallel, series, or alternately, allowing the biofilm structure to fully exert its buffering effect in adsorbing, degrading, and reducing toxic substances in wastewater ； After being poisoned by exposure to high concentrations of toxic substances, the packing on the surface of the anoxic tank can be replaced to reduce the harmful effects of these toxic substances on the treatment system ； The same temperature also has a significant impact on the removal of ammonia nitrogen. When the treatment temperature drops to 2–15°C, the activity of nitrifying bacteria decreases significantly (Table 4). 4.3 Renovation of the wastewater treatment system: Based on experimental results and the actual conditions of the system, a thorough renovation of it was carried out. 4.3.1 Application of biological enhancement technology: In conventional wastewater treatment systems, the numbers of microorganisms capable of efficient degradation and those capable of nitration are limited. To improve such systems, 117 strains belonging to 7 genera of microorganisms were selected through methods such as starvation breeding and selective pressure; these microorganisms can effectively break down refractory organic compounds in wastewater, including some strains that can degrade heterocyclic compounds, for which there is limited prior reporting ； Nitrifying bacterial strains capable of denitrification were also enriched and screened. Due to the difficulty in degradation, the numbers and growth rates of microorganisms in systems for treating toxic industrial wastewater are much lower than those in systems for treating non-toxic, high-concentration organic wastewater. By introducing and acclimating highly efficient degrading bacteria and nitrifying bacteria, the numbers of microorganisms cultivated in the fillers of the anoxic tanks and in the activated sludge in such systems reach a relatively high level; during cold seasons, the number of microorganisms is only one order of magnitude lower than in summer. The processing efficiency of the system has improved significantly. 4.3.2 Further improvement of the anoxic-oxic treatment process: Currently, in the anoxic-oxic processes used in China, the anoxic tank mostly operates with water being introduced from the bottom. This hydrolysis-acidification treatment process has distinct advantages for high-concentration organic wastewater. However, when treating petrochemical wastewater containing a high amount of reduced compounds in biological treatment, aerobic reactions should be the primary approach. In the anoxic tank with water inflow at the bottom, the packing is submerged in water, resulting in a high degree of biological oxygen deficiency ； The system modification utilizes an upper water inlet method; a three-dimensional biofilm is formed on the surface of the packing in the anoxic tank. The metabolic type of the microorganisms on this biofilm is primarily aerobic when wastewater flows through it, enabling more effective treatment of the wastewater. 4.3.3 Replacing the fillers in the anoxic tank: Microorganisms possess strong adsorption capabilities. By using porous fillers in the anoxic tank and adding bacteria capable of degradation as well as nitrifying bacteria, it is possible to develop a biofilm on the surface of these porous fillers that contains a large number of microorganisms. Even under conditions of low temperatures in winter, along with high concentrations of COD and NH3—N, the anoxic tank can still maintain a bacterial count of 1×106 to 1.7×107 bacteria per gram of filler. This effectively enhances the anoxic tank’s resistance to stress and its ability to reduce toxicity. When the influent flow is normal, the role of the anoxic tank does not appear significant; however, when the influent flow is abnormal, the detoxifying function of the anoxic tank can greatly reduce the severe impact of toxic substances on the aerobic activated sludge. 5 Technical indicators achieved through the renovation and existing problems Domestic investigations show that, due to factors such as funding shortages, design flaws, and poor management, the treatment facilities in some oil-to-gas plants have stopped functioning; wastewater is even discharged directly without any treatment. During the investigation, the wastewater treatment system of a gas plant was operating normally; however, due to the large volume of wastewater generated, the system was overwhelmed and could not handle the load effectively, resulting in severe excess levels of COD and NH3—N. Only one factory in Shanghai and one factory in Beijing achieved slightly better treatment results (Table 8), but neither of them was able to fully meet the **first-class discharge standards, namely COD ≤ 150 mg/L and NH3-N < 20 mg/L. Through system modification and tuning as well as the acclimatization of microorganisms, the wastewater from oil-to-gas production can meet the local discharge standards in Guangzhou when its COD is at 480–640 mg/L and its NH3–N level is between 58–182 mg/L; these standards require COD ≤ 110 mg/L and NH3–N ≤ 10 mg/L (Figure 3). There are no reports in China on the achievement of compliance standards using the conventional anoxic-aerobic treatment process (A/O) without the addition of external carbon sources. Guangzhou Oil-to-Gas Plant has unique production characteristics: during the colder months of winter, the demand for gas is high. At this time, microbial activity is low, resulting in a large volume of wastewater with high levels of NH3-N and COD, making wastewater treatment quite difficult. Continued research is needed to address this situation. 6 Conclusion Applying bioenhancement technology to the treatment of industrial wastewater containing high concentrations of ammonia nitrogen and refractory organic pollutants can yield good results. By selecting and breeding degrading bacteria capable of effectively breaking down refractory organic substances in wastewater, as well as biological nitrogen-removing microorganisms ； Improving the packing material of anoxic biological filters ； By organically combining biofilm treatment methods with activated sludge treatment methods, the detoxification capacity and treatment efficiency of the system can be significantly improved. The results show that this microbial treatment method does not rely on special treatment structures, and its operation is relatively simple and cost-effective. No carbon source is required for high concentrations of ammonia nitrogen, offering good application prospects. References: Tangsonben et al., Chemistry of Environmental Organic Pollution, Metallurgical Industry Press, 1995. This post was last edited by icesharp on 2008-1-1 22:00.]