Photosynthetic oxygen-supported algal-bacterial aerobic granular sludge can facilitate carbon, nitrogen and phosphorus removal from wastewater: Focus on light intensity selection.

Photosynthetic O2 is a promising alternative for mechanical aeration, the major energy-intensive unit in wastewater treatment plants. This study aimed to investigate the effects of light intensity varied from 190 to 1400 µmol·s-1·m-2 on photosynthetic O2-supported algal-bacterial aerobic granular sludge (AGS) system. Results indicate photosynthetic O2 can implement aerobic phosphorus (P) uptake and ammonia oxidation under the test illumination range even at dissolved oxygen concentration < 0.5 mg/L. An obvious O2 accumulation occurred after 60-90% nutrients being removed under 330-1400 µmol·s-1·m-2, and highly efficient ammonia removal, P uptake, and dissolved inorganic carbon removal were achieved under 670-1400 µmol·s-1·m-2. On the other hand, photosynthesis as O2 supplier showed little effect on major ions except for K+. This study provides a better understanding of the roles of light intensity on photosynthetic O2-supported algal-bacterial AGS system, targeting a sustainable wastewater industry.

Highly efficient carbon assimilation and nitrogen/phosphorus removal facilitated by photosynthetic O2 from algal-bacterial aerobic granular sludge under controlled DO/pH operation.

Reducing CO2 emission and energy consumption is crucial for the sustainable management of wastewater treatment plants (WWTPs). In this study, an algal-bacterial aerobic granular sludge (AGS) system was developed for efficient carbon (C) assimilation and nitrogen (N)/phosphorus (P) removal without the need for mechanical aeration. The photosynthetic O2 production by phototrophic organisms maintained the dissolved oxygen (DO) level at 3-4 mg/L in the bulk liquid, and an LED light control system reduced 10-30% of light energy consumption. Results showed that the biomass assimilated 52% of input dissolved total carbon (DTC), and the produced O2 simultaneously facilitated aerobic nitrification and P uptake with the coexisting phototrophs serving as a C fixer and O2 supplier. This resulted in a stably high total N removal of 81 ± 7% and an N assimilation rate of 7.55 mg/(g-MLVSS∙d) with enhanced microbial assimilation and simultaneous nitrification/denitrification. Good P removal of 92-98% was maintained during the test period at a molar ∆P/∆C ratio of 0.36 ± 0.03 and high P release and uptake rates of 10.84 ± 0.41 and 7.18 ± 0.24 mg/(g- MLVSS∙h), respectively. Photosynthetic O2 was more advantageous for N and P removal than mechanical aeration. This proposed system can contribute to a better design and sustainable operation of WWTPs using algal-bacterial AGS.

Revealing calcium ion behavior during anaerobic phosphorus release process in aerobic granular sludge system.

Calcium ions (Ca2+) are important for biological phosphorus (P) removal from wastewater, but its behavior has not been well documented during the anaerobic P release process. This study is aimed to explore the mechanisms of Ca2+ release in bacterial aerobic granular sludge (AGS) system. During the non-aeration (anaerobic) phase, nearly 40 % increase in Ca2+ concentration was detected at the bottom of AGS reactor where decrease in pH and increase in Mg2+ concentration occurred. The pH decrease due to anaerobic P release caused CaCO3 dissolution inside the granules, leading to Ca2+ release. In addition, the increased Mg2+ ions from hydrolysis of polyphosphates were detected to reversibly exchange with Ca2+ in granules at a molar ΔCa/ΔMg ratio of 0.51-0.65. Results from this work revealed that dissolution of CaCO3 and ions exchange between Ca2+ and Mg2+ were the two major contributors to Ca2+ release during anaerobic P release process.

Energy saving and rapid establishment of granular microalgae system from tiny microalgae cells: Effect of decrease in upflow air velocity under intermittent aeration condition.

The novel type of microalgae granules (MGs) derived from tiny microalgae cells has received extensive attention due to its great potential for nutrient remediation and resource recovery in wastewater treatment whereas the long start-up time with increased labor expenses remains a bottleneck. In this study, an operation strategy at reduced upflow air velocity (UAV = 0.49 cm/s in RA) under intermittent aeration mode was proposed and compared with RB at a higher UAV (0.98 cm/s) in terms of MGs formation, maintenance, and energy consumption. Although the formation of MGs in RA was delayed for 12 days compared to RB, 40.78 % increase in chlorophyll-a content was detected in MGs in RA along with more cost-effective carbon, nitrogen, and phosphorus removals due to efficient microalgae assimilation and energy reduction. Results from this study provide new insight into minimizing energy input for rapid establishment and stable operation of MG systems towards environmentally sustainable wastewater management.

Insight into enhanced acetic acid production from food waste in anaerobic hydrolysis/acidification with Fe3O4 supplementation.

Fe3O4 supplementation has been reported as a high-efficient approach to enhance biogas production in anaerobic digestion (AD). Volatile fatty acids (VFAs), especially acetic acid (HAc), are considered as important products in acidification process of AD. However, the possible mechanisms involved in promotion effect of Fe3O4 on HAc production in hydrolysis and acidification processes of AD have not been comprehensively studied. This study first investigated the promotion effect of Fe3O4 on hydrolysis, acidogenesis and acetogenesis stages of AD and proposed the underlying mechanisms, using food waste (FW) as the feedstock, which is considered as the most suitable substrate for VFAs production. Results indicated that the HAc production (77.38 g-C/kg-VS) was enhanced by 79 % in AD of FW with addition of 10 g/L Fe3O4. The duration to reach the maximum HAc production was also shortened from 14 days to 10 days. The AD tests using model substrates revealed that Fe3O4 enhanced hydrolysis, acidogenesis, and degradation of propionic acid, thus resulting in enhanced HAc production. The enhanced activities of hydrolytic and acid-forming enzymes, and electron transport system (ETS) with Fe3O4 addition further demonstrated its function as an electron acceptor to stimulate electron transfer and accelerate microbial metabolisms in AD, which contributed to the enhanced HAc production from FW.

A comparative study on simultaneous recovery of phosphorus and alginate-like exopolymers from bacterial and algal-bacterial aerobic granular sludges: Effects of organic loading rate.

The effects of organic loading rate (OLR) on simultaneous phosphorus (P) and alginate-like exopolymers (ALE) recovery from bacterial aerobic granular sludge (AGS) and algal-bacterial AGS were examined and compared during 70 days' operation. With the increase of OLR (0.6-1.2 g COD/(L·day)), both AGS showed good settleability and granular strength with P bioavailability > 92% (Stage III). The moderate increase in OLR had a positive influence on simultaneous recovery of P and ALE. On day 60, the contents of ALE and guluronic acid/guluronic acid (GG) blocks reached the highest in algal-bacterial AGS, about 13.37 and 2.13 mg/g-volatile suspended solids (VSS), respectively. Meanwhile, about daily 0.55 kg of P is estimated to be recovered from the wastewater treatment plant with a treatment capacity of 10,000 m3/day. P mass balance analysis during ALE extraction from both AGS was conducive to further evaluation of P removal pathway and its application potentials.

Insight into aerobic phosphorus removal from wastewater in algal-bacterial aerobic granular sludge system.

This study aimed to figure out the main contributors to aerobic phosphorus (P) removal in the algal-bacterial aerobic granular sludge (AGS)-based wastewater treatment system. Kinetics study showed that aerobic P removal was controlled by macropore (contributing to 64-75% P removal) and micropore diffusion, and the different light intensity (0, 4.0, 12.3, and 24.4 klux) didn't exert significant (p > 0.05) influence on P removal. On the other hand, the increasing light intensity did promote microalgae metabolism, leading to the elevated wastewater pH (8.0-9.8). The resultant pH increase had a strongly negative relationship (R2 = 0.9723) with P uptake by polyphosphate-accumulating organisms, while promoted chemical Ca-P precipitation at a molar Ca/P ratio of 1.05. Results from this work could provide an in-depth understanding of microalgae-bacteria symbiotic interaction, which is helpful to better design and operate the algal-bacterial AGS systems.

Supplementation of KOH to improve salt tolerance of methanogenesis in the two-stage anaerobic digestion of food waste using pre-acclimated anaerobically digested sludge by air-nanobubble water.

Air-nanobubble water (NBW) was applied to pre-acclimate anaerobically digested sludge that was then used as the inoculum in the two-stage anaerobic digestion (AD) of high saline (20 g NaCl/L) food waste (FW) to optimize NBW application in the AD of high saline FW. K+ was simultaneously supplemented during the methanogenic stage to resist the inhibition of salt on methanogens. Results showed that after the second pre-acclimation cycle, the inoculum activity was increased 27% in the Air-NBW supplemented reactor in comparison to the deionized water (DW) supplemented one. In the first-stage AD, H2 yield was enhanced by 46% in the Air-NBW pre-acclimated sludge reactor compared with the DW pre-acclimated sludge reactor. Besides, supplementation of KOH in the methanogenic stage could enhance methane production by 17-25% in the DW reactors at initial pH 7.5, 8.0, and 9.0 when compared to the control reactor (using NaOH adjusted initial pH to 7.5), respectively.

Simultaneous recovery of phosphorus and alginate-like exopolysaccharides from two types of aerobic granular sludge.

Wastewater treatment plants are expected to realize not only pollutants removal from wastewater but also resources recovery such as phosphorus (P) and alginate-like exopolysaccharides (ALE) from the produced sludge. In this study, ALE extraction and fractionation from the same activated sludge-derived bacterial aerobic granular sludge (AGS) and algal-bacterial AGS were performed in addition to P fate examination during ALE recovery. Results showed that the ALE content recovered from algal-bacterial AGS was 8.81 ± 0.02 mg/g-volatile suspended solids (VSS), about 2.8 times higher than that from bacterial AGS when fed with the same synthetic wastewater. Moreover, the mannuronic acid to guluronic acid (MG) blocks accounted for the largest proportion of ALE from the two granular sludges. In particular, the extracellular polymeric substances (EPS) extracted from bacterial and algal-bacterial AGS contained about 25.10 ± 1.85 and 19.53 ± 0.04 mg-P/g-SS, respectively, and both granular sludges possessed high P bioavailability of 97-99%.

Achieving stably enhanced biological phosphorus removal from aerobic granular sludge system via phosphorus rich liquid extraction during anaerobic period.

In order to sustainably manage wastewater treatment plants and the environment, enhanced biological phosphorus (P) removal (EBPR) was proposed to achieve P recovery through extracting P-rich liquid (i.e., Phostrip) from the bottom of aerobic granular sludge (AGS)-based sequencing batch reactors (SBRs) under no mixing during the anaerobic phase. Results showed both tested bacterial AGS (BAGS) and algal-bacterial AGS (A-BAGS) systems stably produced low effluent P (<0.05 mg-P/L) with little impact on their organics and NH4+-N removals (>99%). The collected P-rich liquids (55-83 mg-P/L) from both systems showed great potential for P recovery of about 83.85 ± 0.57 % (BAGS) or 83.99 ± 0.77% (A-BAGS), which were contributed by the influent P (>95%) and P reserves in granules based on P balance analysis. This study suggests that the AGS-based SBRs coupling the Phostrip holds great potentials for P recovery profit and further reduction in energy consumption.

Alleviation of ammonia inhibition via nano-bubble water supplementation during anaerobic digestion of ammonia-rich swine manure: Buffering capacity promotion and methane production enhancement.

Anaerobic digestion (AD) of ammonia-rich swine manure (SM) with nano-bubble water (NBW) supplementation was studied in this work with the expectation of ammonia inhibition alleviation, buffering capacity promotion, and methane production enhancement. Results indicated that cumulative methane yield was elevated by 12.3-38.7% in NBW groups. Besides, the reduced methane production rate and elongated lag phase under ammonia inhibition were increased and shortened by NBW supplementation, respectively. The rapid increase of total alkalinity (TA) and partial alkalinity (PA) could be observed with NBW supplementation, as well as the rapid decline of VFA/TA, thus improved buffering capacity and alleviated ammonia inhibition. Moreover, higher level of extracellular hydrolases and coenzyme F420 could be detected in NBW groups. In conclusion, NBW with higher mobility and zeta potential (absolute value) could be a promising strategy for the alleviation of ammonia suppression during the AD of SM.

Novel insight into enhanced recoverability of acidic inhibition to anaerobic digestion with nano-bubble water supplementation.

Nano-bubble water (NBW) has been proven to be effective in promoting organics utilization and CH4 production during anaerobic digestion (AD) process, suggesting its potential in improving the stability of the AD process and thereby alleviating acidic inhibition. In this work, the effect of NBW on digestion stability and CH4 production was investigated to evaluate the ability of NBW on AD recovery from acidic inhibition. Results showed that NBW supplementation increased the total alkalinity (TA) and partial alkalinity (PA), and reduced the ratio of VFA/TA, thus maintained the stability of the AD process. Generation/consumption of VFAs was also enhanced with NBW supplementation under acidic inhibition with pH values of 5.5, 6.0 and 6.5. The cumulative CH4 production was 246-257 mL/g-VS in NBW groups, which was 12.1-17.2% higher than the control. Moreover, with NBW supplementation, the maximum CH4 production rate was raised according to the modeling results.

Enhanced energy recovery via separate hydrogen and methane production from two-stage anaerobic digestion of food waste with nanobubble water supplementation.

This study investigated the enhancement effect of N2- and Air-nanobubble water (NBW) supplementation on two-stage anaerobic digestion (AD) of food waste (FW) for separate production of hydrogen and methane. In the first stage for hydrogen production, the highest cumulative H2 yield (27.31 ± 1.21 mL/g-VSadded) was obtained from FW + Air-NBW, increasing by 38% compared to the control (FW + deionized water (DW)). In the second stage for methane production, the cumulative CH4 yield followed a descending order of FW + Air-NBW (373.63 ± 3.58 mL/g-VSadded) > FW + N2-NBW (347.63 ± 7.05 mL/g-VSadded) > FW + DW (300.93 ± 3.24 mL/g-VSadded, control), increasing by 24% in FW + Air-NBW and 16% in FW + N2-NBW compared to the control, respectively. Further investigations indicate that different gas-NBW may positively impact the different stages of AD process. Addition of N2-NBW only enhanced the hydrolysis/acidification of FW with no significant effect on methanogenesis. By comparison, addition of Air-NBW promoted both hydrolysis/acidification stage and methanogenesis stage, reflecting by the enhanced activities of four extracellular hydrolases at the end of hydrolysis/acidification and coenzyme F420 at the end of methanogenesis, respectively. Results from this work suggest the potential application of Air-NBW in the two-stage AD for efficient renewable energy recovery from FW.

Ionic response of algal-bacterial granular sludge system during biological phosphorus removal from wastewater.

Biological phosphorus removal (BPR) from wastewater can be generally realized through alternative non-aeration and aeration operation to create anaerobic and aerobic conditions respectively for P release and uptake/accumulation by polyphosphate accumulating organisms (PAOs), with P removal finally achieved by controlled discharge of P-rich sludge. In this study, the response of algal-bacterial aerobic granular sludge (AB-AGS) during BPR to main ions including Ac- (acetate), Cl-, SO42-, NH4+, K+, Mg2+, Ca2+ and Na+ in wastewater was investigated with conventional bacterial AGS (B-AGS) as control and acetate as the sole carbon source. Results show that BPR process mainly involved the changes of Ac-, K+, Mg2+, and Ca2+ rather than Cl-, SO42-, NH4+ and Na+. The mole ratio of ΔP/ΔAc kept almost unchanged during the non-aeration (P release) phase in both B-AGS and AB-AGS systems (ΔPB-AGS/ΔAcB-AGS > ΔPAB-AGS/ΔAcAB-AGS), and it was negatively influenced by the light in AB-AGS systems, in which 62% of acetate was not utilized for P release at the high illuminance of 81 k lux. During the entire non-aeration/aeration period, both ΔK/ΔP and ΔMg/ΔP remained constant, while ΔKAB-AGS/ΔPAB-AGS > ΔKB-AGS/ΔPB-AGS and ΔMgAB-AGS/ΔPAB-AGS ≈ ΔMgB-AGS/ΔPB-AGS. The presence of algae seemed not beneficial for PAOs to remove P, while more K+ and P uptake by algae in AB-AGS suggest its great potential for manufacturing biofertilizer.

Toxic cyanobacteria and microcystin dynamics in a tropical reservoir: assessing the influence of environmental variables.

Toxic cyanobacterial blooms (TCBs) have become a growing concern worldwide. The present study investigated the dynamic of toxic cyanobacteria and microcystin (MC) concentrations in the Tri An Reservoir (TAR), a tropical system in Vietnam, with quantitative real-time polymerase chain reaction (qPCR) and high-performance liquid chromatography (HPLC), respectively. The results of the qPCR quantification revealed that Microcystis was the dominant group and the primary MC producer in the TAR. Potentially toxigenic cyanobacteria varied from 1.2 × 104 to 1.58 × 107 cells/mL, and the mean proportion of toxic Microcystis to that of the total toxic cyanobacteria varied from 21 to 88%. Microcystin concentrations in raw water and sediment samples often peaked during June to October as blooms occurred and varied from 0.27 to 6.59 µg/L and from 1.79 to 544.9 ng/g in wet weight, respectively. The results of this study indicated that conditions favoring Microcystis proliferation lead to the selection of more toxic genotypes. Water temperature and light availability were not driving factor in the formation of TCBs in the TAR. However, the high loads of total nitrogen (TN), phosphate, and total phosphorus (TP) into the water via rainfall runoff in combination with a high total suspended solid (TSS) and decreased water level during the early months of the rainy seasons did lead to a shift in Microcystis blooms and higher proportions of toxic genotypes of Microcystis in the TAR. This research may provide more insight into the occurrence mechanism of TCBs in tropical waters. The strategy to control TCB problems in tropical regions should be focused on these limnological and hydrological parameters, in addition to a reduction in nitrogen and phosphorus loading.

Behavior of algal-bacterial granular sludge in a novel closed photo-sequencing batch reactor under no external O2 supply.

Algal-bacterial aerobic granular sludge (AB-AGS) as a symbiosis system possesses high potential for being operated without external O2 supply. In this study, a novel lab-scale closed photo-sequencing batch reactor (PSBR) was developed for AB-AGS operation under successively open (Phase Ⅰ) and closed (Phase Ⅱ) conditions. Results show that AB-AGS maintained almost 100% of organics removal, exhibiting higher removals of phosphate (63 ± 20%), K+ (19 ± 12%) and Mg2+ (26 ± 12%), and higher chlorophylls content during Phase II. Meanwhile, only O2 besides N2 was detectable in the headspace of PSBR. The change of granular structure and faster algae growth during Phase Ⅱ may contribute to the increase of microbial activity and phosphorus bioavailability, in which lower extracellular polymeric substances content may account for low biomass retention. Results from this closed PSBR imply that AB-AGS has the potential to reduce some greenhouse gases like CO2 and CH4 emission.

Coupling biogas recirculation with FeCl3 addition in anaerobic digestion system for simultaneous biogas upgrading, phosphorus conservation and sludge conditioning.

The high costs involved in sewage sludge treatment and disposal in wastewater treatment plants (WWTPs) not only bring about improper sludge disposal and thus environmental pollutions, but also limit the investment on construction of WWTPs, especially in rural areas or low-income regions. This comparative study examined the effect of biogas recirculation coupled with chemical addition in a semi-continuous anaerobic digester for sludge treatment, which was proven to achieve biogas upgrading, phosphorus conservation and sludge conditioning simultaneously, largely reducing the sludge treatment cost. Results show that FeCl3 addition coupling biogas recirculation can improve sludge dewaterability by 94% in comparison to 75% by equivalent MgCl2 addition, and 97% phosphorus in digestate can be conserved in solid with formation of vivianite-like crystals. Biogas recirculation can enhance CH4 yield and content by 13% and 11%, respectively, likely attributable to the increased relative abundances of both hydrogenotrophic Methanomicrobiales and acetoclastic Methanosarcinales.

Enhanced hydrolysis and acidification of cellulose at high loading for methane production via anaerobic digestion supplemented with high mobility nanobubble water.

In this study, CH4 production from anaerobic digestion (AD) of refractory cellulose was investigated at a high loading of 3.5 (VScellulose/VSinoculum) under nanobubble water (NBW) addition. A longer proton spin-spin relaxation time (2611-2906 ms) of NBW during 35 days' storage reflected its high mobility and diffusion of water molecules. Higher volatile fatty acids were yielded at the hydrolysis-acidification stage under NBW addition. Methanogenesis tests showed that Air-NBW and CO2-NBW supplementation accelerated the utilization of crystalline cellulose, achieving methane yields of 264 and 246 mL CH4/g-VSreduced, increasing by 18% and 10% compared to deionized water addition (the control), respectively. In addition, under NBW addition the cellulose crystallinity reduction was enhanced by 14-20% with microbial community being enriched with hydrolytic and methanogenic bacteria. Results from this work suggest that NBW environment with no chemical addition and relatively low energy consumption is advantageous for enhanced AD process of cellulosic biomass.

Stability and performance of algal-bacterial granular sludge in shaking photo-sequencing batch reactors with special focus on phosphorus accumulation.

The granular stability, nutrients removal and phosphorus (P) accumulation of algal-bacterial aerobic granular sludge (AGS) was examined by using shaking photoreactors (at a fixed light/dark cycle of 12 h/12 h). During the 25 days' operation, algal-bacterial AGS possessed good granular integrity (8.4 ±â€¯0.6%), and excellent removals of dissolved organic carbon (94.8 ±â€¯1.6%) and total nitrogen (71.1 ±â€¯3.3%). More extracellular proteins (153.7 ±â€¯2.3 mg/g) were excreted from the granules with a high proteins/polysaccharides ratio (∼7.4) on day 25, especially the tightly bound proteins mainly responsible for granular stability. Decrease in P content, especially non-apatite inorganic P relating to Fe-PO4 precipitates, was detected in the granules to some extent, although 54.8 ±â€¯17.1% of total P removal was achieved during the light-on cycles. Still, high P bioavailability (92.0%) was kept in the algal-bacterial AGS throughout the test period. Further optimization of light-on/light-off cycle and hydraulic/sludge retention time is demanding for better and stable P accumulation in the algal-bacterial granules with high bioavailability.

Fertilizer potential of liquid product from hydrothermal treatment of swine manure.

Compared with composting, hydrothermal treatment (HTT) technology can dramatically shorten the duration for manure waste treatment. This study firstly investigated the effect of HTT on solubilization of N, P and organics from swine manure, and then evaluated the phytotoxicity of liquid product from hydrothermally treated manure by seed germination test. Results show that 98% of N in manure could be converted into soluble form after HTT at 200 °C for 60 min. Soluble P in hydrothermally treated manure (at 150 °C for 60 min) was 2.7 times that in raw manure. The germination indices (GI) were all greater than 100% when the liquid product (from HTT at 150 °C for 60 min) or its diluted samples being used. Results from this study suggest that HTT could be a promising technology for producing safe and value-added liquid fertilizers from swine manure.