Insights into the Superior Bioavailability of Biogenic Sulfur from the View of Its Unique Properties: The Key Role of Trace Organic Substances.

Elemental sulfur (S0) is widely utilized in environmental pollution control, while its low bioavailability has become a bottleneck for S0-based biotechnologies. Biogenic sulfur (bio-S0) has been demonstrated to have superior bioavailability, while little is known about its mechanisms thus far. This study investigated the bioavailability and relevant properties of bio-S0 based on the denitrifying activity of Thiobacillus denitrificans with chemical sulfur (chem-S0) as the control. It was found that the conversion rate and removal efficiency of nitrate in the bio-S0 system were 2.23 and 2.04 times those of the chem-S0 system. Bio-S0 was not pure orthorhombic sulfur [S: 96.88 ± 0.25% (w/w)]. Trace organic substances detected on the bio-S0 surface were revealed to contribute to its hydrophilicity, resulting in better dispersibility in the aqueous liquid. In addition, the adhesion force of T. denitrificans on bio-S0 was 1.54 times that of chem-S0, endowing a higher bacterial adhesion efficiency on the sulfur particle. The weaker intermolecular binding force due to the low crystallinity of bio-S0 led to enhanced cellular uptake by attached bacteria. The mechanisms for the superior bioavailability of bio-S0 were further proposed. This study provides a comprehensive view of the superior bioavailability of bio-S0 and is beneficial to developing high-quality sulfur resources.

The removal of veterinary antibiotics in the high-rate anaerobic bioreactor: continuous and batch studies.

Veterinary antibiotics in swine wastewater has drawn great public attention. The removal processes of sulfamethizole (SMZ), enrofloxacin (ENR) and chlortetracycline (CTC) were investigated in the high-rate anaerobic process. The continuous experiments demonstrated that in 3 L working volume and with the organic loading rate 5 kg/(m3·d) rised to 20 kg/(m3·d), the average removal efficiencies of the high-rate anaerobic bioreactor for SMZ, ENR and CTC were 0, 54 and 100%, respectively. By using fixed-bed adsorption models, the saturation time of SMZ, ENR and CTC were 4 hydraulic retention time (HRT) (24 h), 8 HRT (48 h) and 372 HRT (2,232 h). In the batch experiments, the adsorption and biodegradation characteristics of anaerobic granular sludge were determined. In the high-rate anaerobic bioreactor, SMZ removal process mainly relied on the adsorption but it was very weak; ENR removal process was based on the adsorption and biodegradation; CTC removal process was based to a large extent on the adsorption because of the big capacity of AnGS. These results were helpful to create a rational basis for designing more suitable treatment systems as feasible barriers to the release of antibiotics into the environment.

The evolution of calcified anaerobic granular sludge bed informs the deep insight into its agglomeration process.

Calcium-induced agglomeration of anaerobic granular sludge bed (AGSB) has become a critical factor in performance decline of calcified anaerobic reactors. However, the agglomeration process of AGSB and the underlying mechanisms remain unclear and elusive. This study delved into the evolution of calcified AGSB, and four typical states of normal AGSB (Nor-AGSB), calcified dispersed AGSB (Dis-AGSB), calcified dimeric AGSB (Dim-AGSB), and calcified polymeric AGSB (Pol-AGSB) were characterized. It was found that the minimum transport velocity of Dis-AGSB was 3.14-3.79 times higher than that of Nor-AGSB, and surpassed both the superficial velocity and the bubble-induced wake velocity. This led to the sedimentation of AGS at the bottom of reactor, resulting in stable contacts with each other. Solid fillers between AGS, namely cement, were observed within Dim-AGSB and Pol-AGSB, and could be classified as tightly- and loosely- bonded cement (T- and L-cement). Further analysis revealed that T-cement was rich in extracellular polymeric substances and intertwining pili/flagella, serving as the primary driving force for robust inter-AGS adhesion. While the L-cement was primarily in the form of calcite precipitation, and blocked the convective mass transfer pathways in Pol-AGSB, leading to the decreased convective mass transfer capacity. The critical distance between calcite and AGS was further revealed as 5.33 nm to form stable initial adhesion. Consequently, the agglomeration mechanism involving the evolution of AGSB was proposed as calcium-induced sedimentation, calcium-induced adhesion, and calcium-induced stasis in order. This study is expected to offer deep insight into the calcium-induced agglomeration especially from the overlooked perspective of AGSB, and provides feasible control strategies to manage the pressing calcification issues in engineering applications.

The culprit for the declining performance of anaerobic reactors caused by calcification: Bioavailability deterioration.

Calcification is a critical challenge for achieving anaerobic reactors' high-efficiency. However, the aggregation caused by calcification at both granular sludge and reactor levels remain to be fully understood. Herein, this study investigated the characteristics of calcification in an anaerobic reactor (RH) operated with high calcium-containing wastewater for over 200-day. It was found that the COD removal efficiency in RH dropped from 98.00 ± 2.06% to 41.29 ± 3.79%, which was much lower than that of 95.50 ± 1.55% in the control reactor. Morphological analysis revealed that the high influent calcium caused granular sludge aggregation, which would further led to the rapid deterioration in bioavailability, as confirmed by both mass transfer tests and theoretical simulations. Moving forward, statistical analysis showed that the proportion of bioavailability deterioration zones in RH system (61.68%) was similar to the decreased COD removal efficiency (57.87%), proving that bioavailability deterioration was the culprit for the performance decline of anaerobic reactor.

Ecological distribution of anaerobic granular sludge towards efficient anaerobic reactor.

Anaerobic reactors often underperform compared to expectations. To identify the key factors, an ecological anaerobic reactor (EAR) with vertical partitions was developed and compared to a physical anaerobic reactor (PAR) as the control. It was observed that EAR achieved a much higher organic loading rate (OLR) compared to PAR (>100 vs 45 kg/m3·d). The different vertical distribution characteristics of anaerobic granular sludge could be ascribed to two vertical distribution patterns dominated in EAR and PAR, i.e., ecological and physical distributions. It was revealed that ecological distribution was formed by the habitat selection, resulting in promoted substrate availability and higher OLR. While physical distribution was mainly affected by hydraulic selection via granule settleability, causing declined substrate availability and lower OLR. Consequently, the promoted ecological distribution and weakened hydraulic selection in EAR contributed to its good performance. Overall, these findings could offer novel concepts for the development of reactors towards high performance.

Response of FANIR system to starvation stress: "Dormancy".

Anaerobic ammonium oxidation (Anammox) process has been successfully applied in the nitrogen removal from high-strength wastewaters. However, little information is available for its treatment of low-strength wastewaters. In this study, a Famine Anammox NItrogen Removal (FANIR) system was developed to investigate the effect of long-term substrate starvation at the low nitrogen concentration (the influent total nitrogen was set at ∼1 mg/L). The results showed that the response of FANIR system to the starvation stress took on two phases: the functional decline phase (0-54 day) and the functional stabilization phase (62-116 day). Over the two phases, the Nitrogen Removal Rate (NRR) of anammox reactor firstly dropped sharply; and then came to a constant level. The activity and settleability of Anammox Granular Sludge (AnGS) firstly deteriorated seriously, and then stayed in a stable range. The relative abundance of Anaerobic Ammonium Oxidation Bacteria (AnAOB) firstly decreased markedly, and then approached a steady state with the change of dominant genus from Candidatus Brocadia to Candidatus Kuenenia. The abundance of 16S rRNA gene and hzs gene of AnAOB and their transcription level firstly declined largely as well, and then became stable with the 16S rRNA gene, hzs gene, 16S rRNA and hzs-mRNA of AnAOB at 23.9%, 9.1%, 1.2% and 1.0% of the initial value, respectively. To our delight, the behavior of FANIR system in the functional stabilization phase was proved indeed consistent with the feature for AnAOB to enter the dormancy state. These findings are helpful to understand the physiology of AnAOB over the starvation stress and to promote the extension of anammox process to the treatment of low-strength wastewaters.

Surface convexity of anammox granular sludge: Digital characterization, state indication and formation mechanism.

Anammox granular sludge system is a promising biotechnology for nitrogen removal from wastewaters. The anammox granules possess the distinctive morphological features which can be developed as visible indicators for anammox process monitoring. In this study, the surface convexity of anammox granular sludge (AnGS) was systematically investigated for the first time. The AnGS was withdrawn from four long-term operated anammox bioreactors at different nitrogen loading rates. Firstly, the spherical convexity and gap concavity with a diameter of 50-100 µm were observed to be distributed on the granules surface under the microscopic observation. Then, the surface convexity was determined by image processing technology and the statistical analysis showed that the convexity had a significant difference (p = 0.003) among bioreactors and the average surface convexity decreased from 0.937 ±â€¯0.030 to 0.899 ±â€¯0.034, and then rose to 0.914 ±â€¯0.035 which had a significant correlation with the volumetric gas production rate of bioreactor (r = -0.873, p < 0.05). An optical method was further developed to fast characterize the surface convexity using relative lightness (LSCE⁎/LSCI⁎) as the index. At last, the composition and structure of AnGS were investigated to deduce the formation mechanism of surface convexity. The formation could be attributed to the outward growth of zoogloea led to the surface protrusion (convexity); the periodical extrusion of microbubbles caused the striping of surface zoogloea (concavity) and the gas-driven collision and friction between granules which finally shaped the surface convexity. The produced dinitrogen gas links the metabolic activity with the formation of surface convexity and concavity of AnGS. This finding provided an alternative visible performance indictor of anammox process.