[Effect on Ammonia Inhibition Mitigation in the Anaerobic Digestion Process with Zero-Valent Iron].

Ammonia inhibition is an important factor impacting methane production efficiency during the anaerobic digestion of high-solid organic wastes. This study investigated the effect of micro-sized zero-valent iron (m-ZVI) on the anaerobic digestion of excess sewage sludge and thermal hydrolyzed sludge using batch mode experiments. The effect of m-ZVI on ammonia inhibition mitigation was also studied. Results showed that the kinetic characteristics of the methane production rate, lag phase, and methane production potential of the anaerobic digestion of excess sludge and thermal hydrolyzed sludge were not impacted by the addition of m-ZVI at a dosage of 4 g·L-1 and 10 g·L-1. However, during the inhibited anaerobic digestion process with a high ammonia concentration, the addition of 4 g·L-1 and 10 g·L-1 of m-ZVI was able to shorten the lag phase from 18.61 d (the control) to 17.22 d and 16.18 d, respectively. Moreover, the maximum methane production rate (based on the VS) increased from 6.34 mL·(d·g)-1(the control) to 7.84 mL·(d·g)-1 (4 g·L-1 m-ZVI) and 7.39 mL·(d·g)-1 (10 g·L-1 m-ZVI). The pH buffer system was not influenced by the chemical reaction of m-ZVI in the anaerobic digestion, although the relative abundance of the dominant methanogenic archaea (Methanosarcina) improved greatly from 30.71% (the control) to 53.50% (4 g·L-1 m-ZVI) and 60.30% (10 g·L-1 m-ZVI) at 27 d. This study proved that m-ZVI was incapable of improving the methane production potential of sewage sludge, while the mitigation of ammonia inhibition during anaerobic digestion was enhanced by the stimulating effect on methanogenic archaea.

[Effect of Sulfate on the Migration and Transformation of Methylmercury in Advanced Anaerobic Digestion of Sludge].

To study the migration and transformation of methylmercury during advanced anaerobic digestion of sludge and the role of sulfate, this study investigated the migration and transformation of methylmercury during different stages of sludge anaerobic digestion (AD) with thermal hydrolysis pretreatment and under different dosages of sulfate addition. The results showed that mercury methylation occurred in the initial stage of AD (Day 1-3), the ratio of methylmercury to total mercury increased from 0.024% (range of 0.019%-0.033%) to 0.038% (range of 0.030%-0.048%), and the net increment of methylmercury increased by 3.97, 6.09, 0.17, 3.71, and 1.66 times, respectively. In the following Day 3-5, the demethylation process occurred with the net yield of methylmercury decreased by 71.25% (ranging from 67.42% to 75.10%). Sulfate inhibited the methylation of mercury in the initial stage of AD, but had little effect on it in the late stage. This was related to the reduction of the bioavailability of neutral mercury complexes by charged groups of HgHS22- and HgS22-, as well as the immobilization of iron sulfide and mercury sulfide on S2- and bioavailable mercury. Redundancy analysis (RDA) showed that mercury methylation was affected by several factors:organic substances such as propionic acid, isobutyric acid, isovaleric acid, and Fe may promote mercury methylation, whereas protein and higher pH may be inhibitors of mercury methylation in AD of sludge.

[Enhancement for Anaerobic Digestion of Waste Activated Sludge Based on Microwave Pretreatment Combined with Zero Valent Iron].

In this study, we investigated the enhanced performance after addition of zero valent iron (ZVI) under different dosages (low range of 5.19-41.51 g·kg-1 TS and high range of 83.35-853.46 g·kg-1 TS), combined with microwave (MW) pretreatment for anaerobic digestion (AD) of waste activated sludge (WAS). The results demonstrated that the methane production potential of WAS could be increased by 17%-24% with the addition of ZVI combined with MW pretreatment, and especially the methane production rate was enhanced in the initial days (1-4d). ZVI addition could further improve the enhanced performance of AD under MW pretreatment. Compared with the performance of AD with only MW pretreatment, the methane production potential was increased by 7.42%, and methane production flow rate at 2 d was increased by 11.02% with 31.13 g·kg-1 TS of ZVI addition. However, the higher dosage of added ZVI did not show further enhanced performance. It was concluded that ZVI addition promoted the release of dissolved organics at the initial stage of AD. For instance, soluble proteins were increased by 21.16% with the ZVI addition of 31.13 g·kg-1 TS compared with pretreated WAS without ZVI addition. Furthermore, ZVI addition accelerated the degradation of acetic acid, iso-butyric acid, and iso-valeric acid, and led to a significant reduction of orthophosphate and sulfate in the supernatant of the digested sludge. The concentration of iron in the supernatant decreased even with a high dosage of ZVI. Thus, the formation of precipitate that occurred due to reactions between iron and orthophosphate or sulfate, may be the main reason for the lack of enhanced performance even with high dosage of ZVI addition.

[Energy Consumption Comparison and Energy Saving Approaches for Different Wastewater Treatment Processes in a Large-scale Reclaimed Water Plant].

Energy consumption is the main performance indicator of reclaimed water plant (RWP) operation. Methods of specific energy consumption analysis, unit energy consumption analysis and redundancy analysis were applied to investigate the composition and spatio-temporal distribution of energy consumption in Qinghe RWP with inverted A2/O, A2/O and A2/O-MBR processes. And the A2/ O-MBR process was mainly analyzed to identify the main nodes and causes for high energy consumption, approaches for energy saving were explored, and the energy consumption before and after upgrading for energy saving was compared. The results showed that aeration was the key factor affecting energy consumption in both conventional and A2/O-MBR processes, accounting for 42.97% and 50.65% of total energy consumption, respectively. A pulsating aeration allowed an increasing membrane flux and remarkably reduced the energy consumption of the A2/O-MBR process while still meeting the effluent standard, e.g., the membrane flux was increased by 20%, and the energy consumptions per kiloton wastewater and kilogram COD(removed) were decreased by 42.39% to 0.53 kW-h-kg-3 and by 54.74% to 1.29 kW x h x kg(-1), respectively. The decrease of backflow ratio in the A2/O-MBR process within a certain range would not deteriorate the effluent quality due to its insignificant correlation with the effluent quality, and therefore may be considered as one of the ways for further energy saving.

[Influencing Factors for Hydrolysis of Sewage Sludge Pretreated by Microwave-H2O2-Alkaline Process].

Pretreatment can improve carbon source utilization of sludge. In this study, influencing factors of hydrolysis including hydrolysis time, ratio of seed sludge and temperature were investigated for sewage sludge pretreated by microwave-H2O2-alkaline process through batch experiments. Meanwhile, effects of hydrolysis and releasing characteristics of organic matters were also investigated under the optimized conditions. The results showed that the optimal hydrolysis time was 12 h and the optimized inoculum to substrate ratio (I/S) was 0.07. Under optimized conditions (12 h, I/S =0.07), SCOD, soluble proteins, soluble sugars and total VFAs content increased with increasing temperature, reaching the maximum at 65 degrees C. Acetic, propionic and iso-valeric acids were the dominant VFAs produced, and the percentage of acetic acid accounting for total VFAs was between 42.7% and 59.7%. In terms of carbon source composition, SCOD accounted for 37.8%-40.8% of total COD, soluble proteins accounted for 38.3%-41.3% of SCOD, soluble sugars accounted for 9.0%-9.3% of SCOD and total VFAs accounted for 3.3%-5.5% of SCOD. The COD/TN watio was between 15.79 and 16.50 in the sludge supernatant. The results of the three-dimensional fluorescence spectra and apparent molecular weight distributions showed that the fluorescence intensity of tyrosine-like substances in the soluble microbial products was the highest and increased with the increasing temperature in the sludge supernatant. After the sewage sludge was pretreated by microwave-H2O2-OH process, a lot of organic matters were released, including small molecule organics (M 100-350), while after hydrolysis, M, 3000-60,000 organics were degraded.

[Performance and Factors Analysis of Sludge Dewatering in Different Wastewater Treatment Processes].

Sludge dewatering is one of the keys for sludge disposal and treatment of municipal wastewater treatment plants. In this study, the sludge dewaterability, flocculant consumption and costs of sludge dewatering for different wastewater treatment processes including A2/O and A2/O-MBR processes were analyzed, as well as the factors of sludge dewatering were analyzed by redundancy analysis (RDA) method, based on the data of one municipal wastewater treatment plant of Beijing in 2013. Results showed that both sludge dewaterability and flocculant consumption presented the seasonal variation, which means sludge dewatering was harder and coupled with higher flocculant consumption in the winter. Although the lower moisture content of dewatered sludge was obtained in the A2/O-MBR process (81.92% ± 1.64% ) compared with that in the A2/O process (82.56% ± 1.35%), the consumptions of flocculant [ (8.70 ± 7.25) kg x t(-1) DS] and electric energy (331.82 kW x h x t(-1) DS) in the A2/O-MBR process were higher than those in the A2/O process [(7.42 ± 2.96) kg x t(-1) DS, 121.57 kW x h x t(-1) DS for flocculant consumption and electric energy respectively], resulting in higher operation costs (RMB 204.76 yuan x t(-1) DS of flocculant consumption and RMB 231.61 yuan x t(-1) DS of energy consumption for the A2/O-MBR, RMB 175.00 yuan x t(-1) DS of flocculant consumption and RMB 84.86 yuan x t(-1) DS of energy consumption for the A2/O, respectively). Results of RDA showed that the seasonal variation of sludge dewaterability mainly depended on the content of organic matter in sludge which was related to the seasonal factors such as temperature, and was also impacted by the operating parameters such as SRT in wastewater treatment.

[Effect of Residual Hydrogen Peroxide on Hydrolysis Acidification of Sludge Pretreated by Microwave -H2O2-Alkaline Process].

Previous studies have found that in the hydrolysis acidification process, sludge after microwave -H2O2-alkaline (MW-H2O2-OH, pH = 10) pretreatment had an acid production lag due to the residual hydrogen peroxide. In this study, effects of residual hydrogen peroxide after MW-H2O2-OH (pH = 10 or pH = 11) pretreatment on the sludge hydrolysis acidification were investigated through batch experiments. Our results showed that catalase had a higher catalytic efficiency than manganese dioxide for hydrogen peroxide, which could completely degraded hydrogen peroxide within 10 min. During the 8 d of hydrolysis acidification time, both SCOD concentrations and the total VFAs concentrations of four groups were firstly increased and then decreased. The optimized hydrolysis times were 0.5 d for four groups, and the optimized hydrolysis acidification times were 3 d for MW-H2O2-OH (pH = 10) group, MW-H2O2-OH (pH = 10) + catalase group and MW-H2O2-OH (pH = 11) + catalase group. The optimized hydrolysis acidification time for MW-H2O2-OH (pH = 11) group was 4 d. Residual hydrogen peroxide inhibited acid production for sludge after MW-H2O2-OH (pH = 10) pretreatment, resulting in a lag in acidification stage. Compared with MW-H2O2-OH ( pH = 10) pretreatment, MW-H2O2-OH (pH = 11 ) pretreatment released more SCOD by 19.29% and more organic matters, which resulted in the increase of total VFAs production significantly by 84.80% at 5 d of hydrolysis acidification time and MW-H2O2-OH (pH = 11) group could shorten the lag time slightly. Dosing catalase (100 mg x -L(-1)) after the MW-H2O2-OH (pH = 10 or pH = 11) pretreatment not only significantly shortened the lag time (0.5 d) in acidification stage, but also produced more total VFAs by 23.61% and 50.12% in the MW-H2O2-OH (pH = 10) + catalase group and MW-H2O2-OH (pH = 11) + catalase group, compared with MW-H2O2-OH (pH = 10) group at 3d of hydrolysis acidification time. For MW-H2O2-OH (pH = 10) group, MW-H2O2-OH (pH = 10) + catalase group and MW-H2O2-OH(pH = 11) + catalase group, the dominant VFAs were acetic, iso-valeric and n-butyric acids. For MW-H2O2-OH (pH = 11) group, the dominant VFAs were acetic, propionic and iso-valeric acids. In the optimized hydrolysis acidification time for each group, percentages of the three main acids accounted for more than 75% of total VFAs, and percentages of acetic acid accounted for more than 41% of total VFAs.

[Enhancement for anaerobic digestion of sewage sludge pretreated by microwave and its combined processes ].

To improve anaerobic digestion and dewatering of sludge, impacts of sludge pretreated by microwave (MW) and its combined processes on sludge anaerobic digestion and dewatering were investigated. The results showed that microwave and its combined processes could effectively enhance anaerobic sludge digestion. Not only the cumulative methane production in the test of the MW-H2O2-alkaline (0. 2) was increased by 13. 34% compared with the control, but also its methane production rate was much higher than that of the control. Compared with the single MW process, the addition of both H2O2 and alkaline enhanced the solubilization of particle COD( >0. 45 micron) , indicating that synergistically generated soluble organics were faster to biodegrade which resulted in the enhancement of anaerobic digestion. The MW-acid process was effective in improving sludge dewaterability, e. g. , Capillary Suction Time (CST) at only 9. 85 s. The improvement of sludge dewatering was significantly correlated with sludge physical properties such as zeta potential, surface charge density and particle size. Under different sludge pretreatment conditions, the sludge dewatering after anaerobic digestion was similar, though the difference of sludge dewatering to some degrees was observed for pretreated sludge.