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The emission and fate of methyl mercaptan from the residential complex treatment plant (RCTP) moving bed bioreactor (MBBR) process in the city of Al-Hur in Karbala governorate in Iraq were studied using the TOXCHEM 4.1 model. The release of odorous sulfur compounds from treatment plants harms workers and the surrounding area. Methyl mercaptan, in particular, is responsible for odors similar to rotten cabbage. The sensitivity analysis for the methyl compounds in the MBBR system was conducted based on the following factors: a large thick biofilm layer, the specific surface area of media, media fill fraction, and aeration flowrate. The model was validated via RMSE and R, which showed the model outputs are representatives of real-world observations. Degradation and emission were shown to be the two most important processes in the system. During the summer (32 °C) and winter (12 °C), about 13 and 10%, 2 and 4%, 0.5 and 1%, and 85 and 85% were emitted into the atmosphere, discharged with effluent, sorbed into sludge, and biodegraded, respectively. The overall concentrations of CH4S emitted in summer and winter were 1.78 and 1.38 ppm, respectively. Operating the MBBR system with a thick biofilm layer, a large specific surface area of media, a greater media fill fraction, and a low aeration rate contributed significantly to the decomposition of methyl mercaptan and thus decreased emission into the atmosphere. Finally, the TOXCHEM simulation accurately predicts the fate of CH4S and the emissions inherent to the MBBR system. The manipulation of the operating factors led to the improvement of the system and the reduction of methyl mercaptan gas emissions without the need to add units and chemical additives.

Odors due to the emission of hydrogen sulfide (H2S) have been a concern in the sewage treatment plants over the last decades. H2S fate and emissions from extended aeration activated sludge (EAAS) system in Muharram Aisha-sewage treatment plant (MA-STP) were studied using TOXCHEM model. Sensitivity analysis at different aeration flowrate, H2S loading rate, wastewater pH, wastewater temperature and wind speed were studied. The predicted data were validated against actual results, where all the data were validated within the limits, and the statistical evaluation of normalized mean square error (NMSE), geometric variance (VG), and correlation coefficient (R) were close to the ideal fit. The results showed that the major processes occurring in the system were degradation and emission. During summer (27 °C) and winter (12 °C), about 25 and 23%, 1 and 2%, 2 and 2%, and 72 and 73% were fated as emitted to air, discharged with effluent, sorbed to sludge, and biodegraded, respectively. At summer and winter, the total emitted concentrations of H2S were 6.403 and 5.614 ppm, respectively. The sensitivity results indicated that aeration flowrate, H2S loading rate and wastewater pH highly influenced the emission and degradation of H2S processes compared to wastewater temperature and wind speed. To conclude, TOXCHEM model successfully predicted the H2S fate and emissions in EAAS system.

Phenol is one of a major pollutant existing in the wastewaters from several industrial activities. Biological removal of the phenolic compounds at full scale treatment plant is very complicated processes. This study investigates the fate of phenol biodegradation in moving bed biofilm reactor sewage treatment plant (MBBR-STP) was investigated. A computer-based mechanistic model, TOXCHEM V4.1, and as well as sensitivity analysis were used to predicts the fate of phenol biodegradation in MBBR-STP. The results showed that MBBR-STP has successfully achieved removal efficiency of 90%, 87%, 100%, 89%, 48%, 90%, and 96% for COD and BOD5, PO4, H2S, oil and grease, SO4, NH4, TSS, and phenol, respectively. However, a high nitrate (NO3) concentration of 60 mg/L was produced, indicating the occurrence of nitrification process. The phenol fate analysis demonstrated that 0.3, 0.5, 6, 93.2% went for volatilization, sorption, untreated and biodegradation, respectively. Nevertheless, the sensitivity analysis indicated that increasing the influent flow rate from 200 to 1000 m3/day has reduced biodegradability of phenol from 96 to 82%, respectively, whereas increasing MLSS concentration 500–6000 mg/L incorporated resilient sorption and degradation processes by 0.2–66% and 93–34%, respectively, and MBBR media fill fraction of 88% achieved about 100% phenol removal. The results conclude that MBBR-STP is efficient in removing phenol, and the TOXCHEM V4.1 has successfully predict the fate of phenol.