Evaluation of the k-nearest neighbor method for forecasting the influent characteristics of wastewater treatment plant

The k-nearest neighbor （k-NN） method was evaluated to predict the influent flow rate and four water qualities, namely chemical oxygen demand （COD）, suspended solid （SS）, total nitrogen （T-N） and total phosphorus （T-P） at a wastewater treatment plant （WWTP）. The search range and approach for determining the number of nearest neighbors （NNs） under dry and wet weather conditions were initially optimized based on the root mean square error （RMSE）. The optimum search range for considering data size was one year. The square root-based （SR） approach was superior to the distance factor-based （DF） approach in determining the appropriate number of NNs. However, the results for both approaches varied slightly depending on the water quality and the weather conditions. The influent flow rate was accurately predicted within one standard deviation of measured values. Influent water qualities were well predicted with the mean absolute percentage error （MAPE） under both wet and dry weather conditions. For the seven-day prediction, the difference in predictive accuracy was less than 5% in dry weather conditions and slightly worse in wet weather conditions. Overall, the k-NN method was verified to be useful for predicting WWTP influent characteristics.

Natural continuous influent nitrifier immigration effects on nitrification and the microbial community of activated sludge systems

Two sequencing batch reactors（SBRs） were operated for 100 days under aerobic conditions,with one being fed with unsterilized municipal wastewater（USBR）, and the other fed with sterilized municipal wastewater（SSBR）. Respirometric assays and fluorescence in situ hybridization（FISH） results show that active nitrifiers were present in the unsterilized influent municipal wastewater. The maximum ammonia utilization rate（AUR） and nitrite utilization rate（NUR） of the unsterilized influent were 0.32 ± 0.12 mg NH4＋-N/（L·hr） and0.71 ± 0.18 mg NO2--N/（L·hr）. Based on the maximum utilization rates, the estimated seeding intensity for the ammonia oxidizing bacteria（AOB） and nitrite oxidizing bacteria（NOB） of the USBR was 0.08 g AOB/（g AOB·day） and 0.20 g NOB/（g NOB·day） respectively. The fraction of nitrifiers/total bacteria in the influent was 5.35% ± 2.1%, the dominant AOB was Nitrosomonas spp., Nitrosococcus mobilis hybridizated with Nsm156, and the dominant NOB was Nitrospira hybridizated with Ntspa662. The influent nitrifiers potentially seeded the activated sludge of the bioreactor and hence demonstrated a mitigation of the acclimatization times and instability during start-up and early operation. The AUR and NUR in the USBR was 15% and 13% higher than the SSBR respectively during the stable stage, FISH results showed that nitrifiers population especially the Nitrospira in the USBR was higher than that in the SSBR. These results indicate that the natural continuous immigration of nitrifiers from municipal influent streams may have some repercussions on the modeling and design of bioreactors.

Selection of microalgae for high CO_2 fixation efficiency and lipid accumulation from ten Chlorella strains using municipal wastewater

As significant differences in cellular physiology, metabolic potential and genetics occur among strains with morphological similarity, the screening of appropriate microalgae species for effective CO2 fixation and biodiesel production is extremely critical. In this study, ten strains of Chlorella were cultivated in municipal wastewater influent（MWI） and their tolerance for MWI, CO2 fixation efficiency and lipid productivity were assessed. The results showed that the biomass concentrations of four strains（Chlorella vulgaris, Chlorella64.01, Chlorella regularis var. minima and Chlorella sp.） were significantly higher than other strains. When the cultivation systems were aerated with 10% CO2, Chlorella sp. showed the highest CO2 fixation efficiency（35.51%）, while the highest lipid accumulation（58.48%） was observed with C. vulgaris. Scanning electron microscopy images revealed that the cells of both Chlorella sp. and C. vulgaris kept their normal morphologies after 15 day batch culture.These findings indicated that Chlorella sp. and C. vulgaris have fairly good tolerance for MWI,and moreover, Chlorella sp. was appropriate for CO2 fixation while C. vulgaris represented the highest potential for producing biodiesel.