Research On Treatment Efficiency Of Phenolic Compounds In Coal Gasification Wastewater By Enhanced Anaerobic Process

In China, the development of coal gasification was decided by the energy characteristics of rich in coal, less oil and gas shortage. In the next few decades, the energy transition will make a new generation of coal gasification industries play a special role in the new clean and renewable energy market. However, coal gasification wastewater was typically high concentrations of phenols, refractory organics and toxic pollutants. The treatment technologies of coal gasification wastewater from abroad and domestic usually had several problems, such as unsatisfactory effluents, poor stability and high handling costs. Now, the wastewater treatment has become the restricting bottleneck for the development of coal gasification industries. With the continuous development of anaerobic technology, the researchers had an unprecedented attention to the role of anaerobic biotechnology in the field of coal gasification wastewater treatment. For the characteristics of coal gasification wastewater, the subject aimed to system and in-depth study anaerobic treatment efficiency of phenolic compounds in coal gasification wastewater. The treatment efficiencies and improvement of aerobic biodegradability were investigated in the conventional anaerobic processes of upflow anaerobic sludge blanket (UASB), thermophilic anaerobic process, two-phase anaerobic process, two-stage anaerobic process and the enhanced anaerobic processes of methanol addition of co-substrate and powdered activated carbon, respectively. In the engineering application, the effects of methanol addition and two-stage anaerobic process with step-feed on the treatment efficiencies of phenolic compounds were studied. Anaerobic degradation efficiencies and properties of phenolic compounds were investigated and anaerobic degradation metabolism of phenolic compounds was discussed preliminarily.By the analysis of gas chromatography and mass spectrometry (GC-MS), the phenolic compounds in the coal gasification wastewater were mainly composed of phenol, alkyl phenols and binary phenols, etc., accounting for about 30-50% of the total chemical oxygen demand (COD) of the wastewater; while the wastewater also contained more aromatic hydrocarbons, heterocyclic, long chain aliphatic hydrocarbons, ammonia and cyanide and other pollutants. Studies showed that the treatment efficiency of phenolic compounds was very low in the conventional anaerobic process, and the removal efficiency of total phenols were only about 30-40% in the UASB process, 55-60% in the thermophilic anaerobic process, around 60% in the two-stage anaerobic process and around 40% in the two-phase anaerobic process. Dilution of influent phenolic concentration or extended hydraulic retention time (HRT) had no significant promoting effect on anaerobic treatment efficiency of phenolic compounds.The treatment efficiencies of phenolic compounds in coal gasification wastewater were compared with the enhanced anaerobic processes of methanol co-substrate and powdered activated carbon. The results showed that the addition of powdered activated carbon (1g/L) and methanol co-substrate (500mgCOD/L), enhanced the removal efficiencies of phenolic compounds to 73% and 75%, respectively. The enhanced anaerobic processes could not only greatly improve treatment efficiency of phenolic compounds, but also significantly improved aerobic biodegradability of coal gasification wastewater. In a comprehensive comparison of these anaerobic processes in the improvement of aerobic biodegradability, the order was: methanol co-substrate anaerobic process > powdered activated carbon anaerobic process > two-stage anaerobic process with step-feed > thermophilic anaerobic process > two-phase anaerobic process > UASB process. Co-metabolism of coal gasification wastewater with methanol substrate could reduce the biological toxicity of the wastewater, and improve the metabolic activity of anaerobic bacteria for enhancing the removal efficiency of phenolic compounds in essence.In the engineering application, anaerobic treatment of phenolic compounds in coal gasification wastewater was difficult and ineffective. The removal efficiencies of COD and total phenols were less than 20% and 26%. Dilution or extension of hydraulic retention time was difficult to significantly improve the anaerobic treatment efficiency of phenolic compounds. When the methanol concentration of 200-500mgCOD/L was added in coal gasification wastewater, the removal efficiencies of COD and total phenols were around 40.7% and 35.2%, and the treatment efficiency of phenolic compounds and methane production had been significantly improved. However, the relatively stable wastewater quality was the prerequisite and important guarantee to achieve successful treatment of phenolic compounds in coal gasification wastewater in the anaerobic process.The phenolic wastewater composing of phenol, (m-, p-) cresols, o-cresol, xylenols, catechol, resorcinol and hydroquinone was treated with acetic acid, methanol and phenol as co-substrates, respectively. Results indicated the co-metabolism was helpful to improve anaerobic degradability of phenolic compounds. When the influent concentration of phenols were around 408mg/L, 612mg/L and 816mg/L, the removal efficiencies of phenolic compounds under the condition of methanol co-substrate (500mgCOD/L) were increased by 13.8%, 14.4% and 15.6% with compared to the results under the condition of without co-substrates; and under the condition of acetic acid co-substrate (500mgCOD/L) were increased by 11%, 12.4% and 10.3% with compared to the results under the condition of without co-substrates. Moderate concentration of phenol co-substrate also helped to improve the removal efficiency of phenolic compounds, but the high concentration of phenol co-substrate would inhibit the activity of anaerobic bacteria, resulting in removal efficiency of phenolic compounds dropped significantly. Co-substrates of methanol and acetic acid could significantly enhance the degradation efficiencies of methyl phenols, which played a key role in the improvement of anaerobic biodegradability of phenolic compounds.The experimental results showed that the difficulty of anaerobic degradability of phenolic compounds were as follows: xylenol, o-cresol, m-cresol, hydroquinone, catechol, p-cresol, resorcinol, phenol. Anaerobic degradability of methyl phenols was much lower than the phenol and binary phenols, and the five-day average degradation rates of methyl phenols were only 1.3-4.5mg/d (except p-cresol). Anaerobic degradation ways of phenolic compounds closely linked to its chemical structure, in which the carboxylation stage and benzene acylation stage were the key step in the degradation process. In the process of anaerobic degradation of phenolic compounds, the inhibition caused by high concentrations of phenolic compounds was a temporary inhibition, and the inhibition caused by cyanide and long-chain hydrocarbons belonged to interference inhibition. The inhibition of anaerobic degradation of phenolic compounds was not only affected by the type and concentration of inhibitors and other factors, but was also closely associated with the phenolic composition. Under the inhibiting environment of cyanide (5mg/L) and long chain fatty hydrocarbon (50mg/L), the presence of co-substrate could reduce the inhibitory effect and synergistically improve anaerobic degradation efficiency of phenolic compounds.