Coal gasification is often seen as a key to future energy production. It takes advantage of plentiful coal supplies, particularly in the United States and China, in order to create liquid fuels, as well as to produce the hydrogen gas used in the hydrogen fuel cells that many predict will eventually come to power most of our transportation infrastructure. Unfortunately, the process of coal gasification creates wastewater containing many pollutants harmful to the environment, including ammonia (NH4-n), cyclooctadiene (COD), and phenol. Because of this, the continuing and future viability of coal gasification requires effective processes to purify the wastewater that its creation and refinement produces. Traditionally an activated sludge process (ASP) is used to treat this wastewater. Han et al. (2010) found that if a soft fixed biological medium is added into the treatment tanks about 80% of pollutants can be removed in a manner more stable than traditional ASP treatment. —Steven Erickson
Han, H., Li, H., Du, M., Wang, W., 2010. Treatment of coal gasification wastewater by full scale activated sludge process with fixed media. Bioinformatics and Biomedical Engineering (iCBBE), 2010 4th International Conference on, 1–4
Han et al. studied the effects of submerged biofilm on the ASP. Traditionally ASP does a good job in removing phenol and COD, but has a relatively small effect on NH4-n in the system. The biofilm added contained nitrifying bacteria to breakdown the NH4-n in the wastewater.
The ASP uses a two stage process, with each stage composed of one large tank and one sedimentation tank, with a total volume per stage of 7000 m3 . The effluent from the coal gasification is mixed with residential wastewater and then flows through the first tank and moves on to the tanks of the second stage in a process that takes about 42 hours. Each stage works through the same method, with the sludge adsorbing and biodegrading the phenol and COD’s, and the bacteria on the biofilm nitrifying the NH4-n. The research team measured the levels of COD, NH4-n and phenol in the wastewater before it entered the system, after the first stage, and upon its exit of the system. These measurements continued over the course of an entire year.
The results of the experiment were very promising, generating only a few problematic points in the data. COD removal rates after the two stages averaged out to be around 80%. The concentration of COD ranged from 550 to 1760 mg/L in the untreated wastewater, with concentration rates dropping to about 200 mg/L after completing the second stage.
Similarly, phenol rates were greatly reduced following the ASP. The influent total phenol concentrations ranged from 95 to 345 mg/L, and the effluent total concentrations were around 25 mg/L. The average total rate of phenol removal was 90%.
The study points out one main drop in filtration capacity by the first stage of the ADP in the 5 weeks between weeks 35 and 40. Normally the first stage of the process filters out 70% of COD and 80% of phenol, but during this period that rate dropped to 60% for both pollutants. Han et al. suggest that the problem might lie in the process by which the sludge removes these compounds. They state that it is possible that the sludge became saturated with pollutants through adsorption and that the process of biodegradation did not work quickly enough to break down the compounds, allowing unadsorbed contaminants to continue on to the second stage. This did not affect the concentration levels after the second stage, which was able to make up for the deficiency in the first stage tanks.
The rate of removal of NH4-n from the wastewater began fairly low, but ultimately reached 80% after 15 weeks. The influent NH4-n concentrations ranged from 60 to 110 mg/L, and after 15 weeks the concentration in the effluent was down to around 15 mg/L. Thanks to the biofilm inserted into the tanks, nitrifying bacteria were able to adhere onto them and grow successfully. This was not possible during the traditional ASP process, but the bacteria took time to multiply, leading to the low rates of nitrogen filtration and the beginning of the experiment.
The first stage of the ADP had problems removing NH4-n even after the 15 week period in which removal rates finally rose to 80%. The authors hypothesize that this was due to a competitive mechanism in which heterotrophic bacteria were preying upon the nitrifying bacteria. This was particularly a problem in the first stage, in which the wastewater still had a high concentration of organic matter due to mixing of the coal gasification wastewater with sewage from the surrounding residential area. Predation was not a problem in the second stage, as concentration of organic matter was much lower.
Given these results, Han et al. conclude that ADP with a soft biological medium inserted could remove most NH4-n and organic pollutants from coal gasification wastewater. Additionally, they concluded that this process was more stable than traditional ADP.