Saline wastewater treatment with purple phototrophic bacteria

Establishing an efficient membrane bioreactor for simultaneous pollutant removal and purple bacteria production under salinity stress

Recovering resources from wastewater to alleviate the energy crisis has become the prevailing trend of technological development. Purple phototrophic bacteria (PPB), a group of fast-growing microbes, have been widely noticed for their potential in producing value-added products from waste streams. However, saline contents in these waste streams, such as food processing wastewater pose a big challenge, which not only restrain the pollutant removal efficiency, but also hinder the growth of functional microbes. To overcome this, a photo anaerobic membrane bioreactor cultivating PPB (PPB-MBR) was constructed and its performance upon long-term salinity stress was investigated. PPB-MBR achieved desirable pollutants removal performance with the average COD and NH4+ removal efficiency being 87% (±8%, n = 87) and 89% (±10%, n = 87), respectively during long-term exposure to salinity stress of 1-80 g NaCl L-1. PPB were predominant during the entire operation period of 87 days (60%-80%), obtaining maximum biomass yield of 0.67 g biomass g-1 CODremoved and protein productivity of 0.18 g L-1 d-1 at the salinity level of 20 g NaCl L-1 and 60 g NaCl L-1, respectively. The sum of value-added products in proportion to the biomass reached 58% at maximum at the salinity level of 60 g NaCl L-1 with protein, pigments and trehalose contributing to 44%, 8.7%, and 5%, respectively. Based on economic analysis, the most cost-saving scenario treating food processing wastewater was revealed at salinity level of around 20 g NaCl L-1. However, more optimization tools are needed to boost the production efficiency so that the profit from value-added products can outweigh the additional cost by excess salinity in the future implication.

Polyhydroxyalkanoates production in purple phototrophic bacteria ponds: A breakthrough in outdoor pilot-scale operation

The versatile capacity of purple phototrophic bacteria (PPB) for producing valuable bioproducts has gathered renewed interest in the field of resource recovery and waste valorisation. However, greater knowledge regarding the viability of applying PPB technologies in outdoor, large-scale systems is required. This study assessed, for the first time, the upscaling of the phototrophic polyhydroxyalkanoate (PHA) production technology in a pilot-scale system operated in outdoor conditions. An integrated system composed of two up-flow anaerobic sludge blanket (UASB) reactors (for fermentation of wastewater with molasses), and two high-rate algal ponds retrofitted into PPB ponds, was operated in a wastewater treatment plant under outdoor conditions. UASB's adaptation to the outdoor temperatures involved testing different operational settings, namely hydraulic retention times (HRT) of 48 and 72 h, and molasses fermentation in one or two UASBs. Results have shown that the fermentation of molasses in both UASBs with an increased HRT of 72 h was able to ensure a suitable operation during colder conditions, achieving 3.83 ± 0.63 g CODFermentative Products/L, compared to the 3.73 ± 0.85 g CODFermentative Products/L achieved during warmer conditions (molasses fermentation in one UASB; HRT 48 h). Furthermore, the PPB ponds were operated under a light-feast/dark-aerated-famine strategy and fed with the fermented wastewater and molasses from the two UASBs. The best PHA production was obtained during the summer of 2018 and spring of 2019, attaining 34.7 % gPHA/gVSS with a productivity of 0.11 gPHA L-1 day-1 and 36 % gPHA/gVSS with a productivity of 0.14 gPHA L-1 day-1, respectively. Overall, this study showcases the first translation of phototrophic PHA production technology from an artificially illuminated laboratory scale system into a naturally illuminated, outdoor, pilot-scale system. It also addresses relevant process integration aspects with UASBs for pre-fermenting wastewater with molasses, providing a novel operational strategy to achieve photosynthetic PHA production in outdoor full-scale systems.

Biological treatment processes for saline organic wastewater and related inhibition mechanisms and facilitation techniques: A comprehensive review

Owing to its profound pollution-inducing properties and resistance to biodegradation, saline organic wastewater (SOW) has unavoidably emerged as a predominant focal point within the wastewater treatment domain. Substantial quantities of SOW are discharged by diverse industries encompassing food processing, pharmaceuticals, leather manufacturing, petrochemicals, and textiles. Within this review, the inhibitory repercussions of elevated salinity upon biological water treatment systems are subject to methodical scrutiny spanning from sludge characteristics, microbial consortia to the physiological functionality of microorganisms have been investigated. This exposition elucidates the application of both anaerobic and aerobic biological technologies for SOW treatment, which noting that conventional bioreactors can effectually treat SOW through microbial adaptation, and elaborating that cultivation of salt-tolerant bacteria and the design of advanced bioreactors represents a promising avenue for SOW treatment. Furthermore, the mechanisms underpinning microbial acclimatization to hypersaline milieus and the methodologies aimed at amplifying the efficacy of biological SOW treatment are delved into, which point out that microorganism exhibit salt tolerance via extracellular polymeric substance accumulation or by facilitating the influx of osmolarity-regulating agents into the bacterial matrix. Finally, the projections for future inquiry are proffered, encompassing the proliferation and deployment of high salt-tolerant strains, as well as the development of techniques enhancing the salt tolerance of microflora engaged in wastewater treatment.

Evolution of aniline degradation and nitrogen removal performance in electro-enhanced sequence batch reactor under salinity stress: Sludge characteristics and microbial diversity

To explore the influence mechanism of different concentrations of salinity on the electro-enhanced aniline biodegradation system, a control group and experimental groups (0%-NaCl, 0.5%-NaCl, 1.5%-NaCl, 2.5%-NaCl, 3.5%-NaCl) were established. The experimental results showed that the electric field strengthened the denitrification performance, while salinity had little effect on the degradation efficiency of aniline and chemical oxygen demand (COD). The removal rate of TN reached 79.6% and 74.9% in 0.5%-NaCl and 1.5%-NaCl, respectively, which were superior than 0%-NaCl. As salinity increased, the nitrogen removal effect was negatively affected. Microbial diversity analysis indicated that the microbial community structure was uniform in the control group, 0%-NaCl, and 0.5%-NaCl, with the dominant genus OLB8 ensuring the nitrogen removal performance. In contrast, in the 2.5%-NaCl and 3.5%-NaCl experimental groups, the organic degrading bacteria were still active, while nitrifiers and denitrifiers were severely damaged. In conclusion, this study suggested that low concentrations of salinity can improve the decontamination performance of the electro-enhanced aniline biodegradation system, while high concentrations of salinity could lead to the collapse of the decontamination mechanism.

Achieving Discharge Limits in Single-Stage Domestic Wastewater Treatment by Combining Urban Waste Sources and Phototrophic Mixed Cultures

This work shows the potential of a new way of co-treatment of domestic wastewater (DWW) and a liquid stream coming from the thermal hydrolysis of the organic fraction of municipal solid waste (OFMSW) mediated by a mixed culture of purple phototrophic bacteria (PPB) capable of assimilating carbon and nutrients from the medium. The biological system is an open single-step process operated under microaerophilic conditions at an oxidative reduction potential (ORP) < 0 mV with a photoperiod of 12/24 h and fed during the light stage only so the results can be extrapolated to outdoor open pond operations by monitoring the ORP. The effluent mostly complies with the discharge values of the Spanish legislation in COD and p-values (<125 mg/L; <2 mg/L), respectively, and punctually on values in N (<15 mg/L). Applying an HRT of 3 d and a ratio of 100:7 (COD:N), the presence of PPB in the mixed culture surpassed 50% of 16S rRNA gene copies, removing 78% of COD, 53% of N, and 66% of P. Furthermore, by increasing the HRT to 5 d, removal efficiencies of 83% of COD, 65% of N, and 91% of P were achieved. In addition, the reactors were further operated in a membrane bioreactor, thus separating the HRT from the SRT to increase the specific loading rate. Very satisfactory removal efficiencies were achieved by applying an HRT and SRT of 2.3 and 3 d, respectively: 84% of COD, 49% of N, and 93% of P despite the low presence of PPB due to more oxidative conditions, which step-by-step re-colonized the mixed culture until reaching >20% of 16S rRNA gene copies after 49 d of operation. These results open the door to scaling up the process in open photobioreactors capable of treating urban wastewater and municipal solid waste in a single stage and under microaerophilic conditions by controlling the ORP of the system.

Biofilm reactors filled with Stick-bed Biofix and Swim-bed Biofringe biomass carriers in treating chitin production wastewater containing high salinity

The production of chitin generates wastewater containing high content of organic compounds, nutrients, and salinity, thus a biofilm system including anaerobic-anoxic-aerobic bioreactors was employed. This study aims to evaluate the performance of Stick-bed Biofix and Swim-bed Biofringe added to bioreactors as a biomass carrier in order to enhance biomass concentration. The results indicated that the organic removal has been insignificantly affected by high salinity, the removal efficiency was obtained at 95 ± 2% corresponding to a rate of 5.78 ± 1.10 kg COD/m3/d. Otherwise, the nitrogen removal rate was achieved at around 0.45 ± 0.17 kg N/m3/d and strongly decreased to 0.24 ± 0.10 kg N/m3/d under high salinity of 18,000 mg/L and a high loading rate of 1.03 ± 0.10 kg N/m3/d. Phosphorus removal was obtained at 0.032-0.057 kg P/m3/d and decreased by 1.5 times when the salinity is over 10,000 mg/L although the influent load was strongly reduced by pre-treatment. Besides, the biofilm system can also remove around 50% of calcium ions which causes high salinity in chitin production wastewater.

Expanding mechanistic models to represent purple phototrophic bacteria enriched cultures growing outdoors

The economic feasibility of purple phototrophic bacteria (PPB) for resource recovery relies on using enriched-mixed cultures and sunlight. This work presents an extended Photo-Anaerobic Model (ePAnM), considering: (i) the diverse metabolic capabilities of PPB, (ii) microbial clades interacting with PPB, and (iii) varying environmental conditions. Key kinetic and stoichiometric parameters were either determined experimentally (with dedicated tests), calculated, or gathered from literature. The model was calibrated and validated using different datasets from an outdoors demonstration-scale reactor, as well as results from aerobic and anaerobic batch tests. The ePAnM was able to predict the concentrations of key compounds/components (e.g., COD, volatile fatty acids, and nutrients), as well as microbial communities (with anaerobic systems dominated by fermenters and PPB). The results underlined the importance of considering other microbial clades and varying environmental conditions. The model predicted a minimum hydraulic retention time of 0.5 d^-1. A maximum width of 10 cm in flat plate reactors should not be exceeded. Simulations showed the potential of a combined day-anaerobic/night-aerobic operational strategy to allow efficient continuous operation.

Efficient biodegradation of acetoacetanilide in hypersaline wastewater with a synthetic halotolerant bacterial consortium

The high concentrations of salt and refractory toxic organics in industrial wastewater seriously restrict biological treatment efficiency and functional stability. However, how to construct a salt-tolerant biocatalytic community and realize the decarbonization coupled with detoxification toward green bio-enhanced treatment, has yet to be well elucidated. Here, acetoacetanilide (AAA), an important intermediate for many dyes and medicine synthesis, was used as the model amide pollutant to elucidate the directional enrichment of halotolerant degradative communities and the corresponding bacterial interaction mechanism. Combining microbial community composition and molecular ecological network analyses as well as the biodegradation efficiencies of AAA and its hydrolysis product aniline (AN) of pure strains, the core degradative bacteria were identified during the hypersaline AAA degradation process. A synthetic bacterial consortium composed of Paenarthrobacter, Rhizobium, Rhodococcus, Delftia and Nitratireductor was constructed based on the top-down strategy to treat AAA wastewater with different water quality characteristics. The synthetic halotolerant consortium showed promising treatment ability toward the simulated AAA wastewater (AAA 100-500 mg/L, 1-5% salinity) and actual AAA mother liquor. Additionally, the comprehensive toxicity of AAA mother liquor significantly reduced after biological treatment. This study provides a green biological approach for the treatment of hypersaline and high concentration of organics wastewater.

An integrated process for struvite recovery and nutrient removal from ship domestic sewage

Marine pollution caused by the untreated and substandard discharge of ship domestic sewage has received widespread attention. A novel integrated process for struvite recovery and nutrient removal from ship domestic sewage (SRNR-SDS) based on seawater magnesium source was developed in this study. Removal efficiencies of the total nitrogen (TN) and total phosphorus (TP) for the activated sludge unit in SRNR-SDS process were approximately 67.61% and 41.35%, respectively, under the salinity of 7.85 g/L. The coupling-induced struvite crystallization unit significantly improved the removal efficiency of TN and TP, and the scanning electron microscopy and X-ray diffraction demonstrated that magnesium ammonium phosphate (MAP) crystals were successfully formed on the surface of zeolite. The SRNR-SDS process had an ideal performance for pollutant removal and MAP recovery under the optimal hydraulic retention time of 20 h. The effluent concentrations of COD, NH₄⁺-N, TN and TP in SRNR-SDS process were approximately 34.73 mg/L, 4.31 mg/L, 10.07 mg/L and 0.23 mg/L, respectively, which meet the Chinese and international ship sewage discharge standards. SRNR-SDS process has obvious environmental, social and economic benefits, which could save 6.20%∼57.14% of the operation cost of ship domestic sewage treatment via MAP recovery. The results could provide theoretical and technical support for the development and application of ship sewage treatment process with the functions of pollutant removal and resource recovery.

Nitrogen recovery from wastewater by microbial assimilation - A review

The increased nitrogen (N) input with low utilization rate in artificial N management has led to massive reactive N (Nr) flows, putting the Earth in a high-risk state. It is essential to recover and recycle Nr during or after Nr removal from wastewater to reduce N input while simultaneously mitigate Nr pollution in addressing the N stress. However, mechanisms for efficient Nr recovery during or after Nr removal remain unclear. Here, the occurrence of N risk and progress in wastewater treatment in recent years as well as challenges of the current technologies for N recovery from wastewater were reviewed. Through analyzing N conversion fluxes in biogeochemical N-cycling networks, microbial N assimilation through photosynthetic and heterotrophic microorganisms was highlighted as promising alternative for synergistic N removal and recovery in wastewater treatment. In addition, the prospects and gaps of Nr recovery from wastewater through microbial assimilation are discussed.

Hydrobiogechemical interactions in the hyporheic zone of a sulfate-impacted, freshwater stream and riparian wetland ecosystem

Coupled abiotic and biotic processes in the hyporheic zone, where surface water and groundwater mix, play a critical role in the biogeochemical cycling of carbon, nutrients, and trace elements in streams and wetlands. Dynamic hydrologic conditions and anthropogenic pollution can impact redox gradients and biogeochemical response, although few studies examine the resulting hydrobiogeochemical interactions generated within the hyporheic zone. This study examines the effect of hyporheic flux dynamics and anthropogenic sulfate loading on the biogeochemistry of a riparian wetland and stream system. The hydrologic gradient as well as sediment, surface water, and porewater geochemistry chemistry was characterized at multiple points throughout the 2017 spring-summer-fall season at a sulfate-impacted stream flanked by wetlands in northern Minnesota. Results show that organic-rich sediments largely buffer the geochemical responses to brief or low magnitude changes in hydrologic gradient, but sustained or higher magnitude fluxes may variably alter the redox regime and, ultimately, the environmental geochemistry. This has implications for a changing climate that is expected to dramatically alter the hydrological cycle. Further, increased sulfate loading and dissolved or adsorbed ferric iron complexes in the hyporheic zone may induce a cryptic sulfur cycle linked to iron and carbon cycling, as indicated by the abundance of intermediate valence sulfur compounds (e.g., polysulfide, elemental sulfur, thiosulfate) throughout the anoxic wetland and stream-channel sediment column. The observed deviation from a classical redox tower coupled with potential changes in hydraulic gradient in these organic-rich wetland and stream hyporheic zones has implications for nutrient, trace element, and greenhouse gas fluxes into surface water and groundwater, ultimately influencing water quality and global climate.

Sulfide removal characteristics, pathways and potential application of a novel chemolithotrophic sulfide-oxidizing strain, Marinobacter sp. SDSWS8

Sulfide generally exists in wastewater, black and odor river, as well as aquaculture water, and give rise to adverse effect on ecological stability and biological safety, due to the toxicity, corrosivity and malodor of sulfide. In the present study, a chemolithotrophic sulfide-oxidizing bacteria (SOB) was isolated and identified as Marinobacter maroccanus strain SDSWS8. And it produced no hemolysin and was susceptible to most antibiotics. There were no accumulation of sulfide, sulfate and thiosulfate during the sulfide removal process. The optimum conditions of sulfide removal were temperature 15-40 °C, initial pH value 4.5-9.5, salinity 10-40‰, C/N ratio 0-20 and sulfide concentration 25-150 mg/L. The key genes of sulfide oxidation, Sox system (soxB, soxX, soxA, soxZ, soxY, soxD, soxC), dissimilatory sulfur oxidation (dsrA, aprA and sat) and sqr, were successfully amplified and expressed, indicating the three pathways coordinated to complete the sulfide oxidation. Besides, strain SDSWS8 had inhibitory effect on four pathogen Vibrio (V. harveyi, V. parahaemolyticus, V. anguillarum and V. splendidus). Furthermore, efficient removal of sulfide from real aquaculture water and sludge mixture could be accomplished by strain SDSWS8. This study may provide a promising candidate strain for sulfide-rich water treatment.

Outdoor demonstration-scale flat plate photobioreactor for resource recovery with purple phototrophic bacteria

To make purple phototrophic bacteria (PPB)-based technologies a reality for resource recovery, research must be demonstrated outdoors, using scaled reactors. In this study, a 10 m long PPB-enriched flat plate photobioreactor (FPPBR) with a volume of 0.95 m³ was operated for 253 days, fed with poultry processing wastewater. Different operational strategies were tested, including varying influent types, retention times, feeding strategies, and anaerobic/aerobic conditions in a novel mixed metabolic mode concept. The overall results show that regardless of the fermented wastewater fed (raw or after solid removal via dissolved air flotation) and the varying environmental conditions (e.g., light exposure and temperatures), the FPPBR provided effective volatile fatty acids (VFAs), N, and P removals (average efficiencies of >90%, 34-77%, and 28-45%, respectively). The removal of N and P was limited by the availability of biodegradable COD. Biomass (C, N and P) could be harvested at ∼90% VS/TS ratio, 58% crude protein content and a suitable amino acid profile for potential feed applications. During fully anaerobic operation with semicontinuous/day-only feeding, the FPPBR showed biomass productivities between 25 and 84 g VS m^-2 d^-1 (high due to solid influx; the productivities estimated from COD removal rates were 6.0-24 g VS•m^-2•d^-1 (conservative values)), and soluble COD removal rates of up to 1.0 g•L^-1•d^-1 (overall average of 0.34 ± 0.16 g•L^-1•d^-1). Under these conditions, the relative abundance of PPB in the harvested biomass was up to 56%. A minimum overall HRT of 2-2.4 d (1.0-1.2 d when only fed during the day) is recommended to avoid PPB washout, assuming no biomass retention. A combined daily-illuminated-anaerobic/night-aerobic operation (supplying air during night-time) exploiting photoheterotrophy during the day and aerobic chemoheterotrophy of the same bacteria at night improved the overall removal performance, avoiding VFA accumulation during the night. However, while enabling enhanced treatment, this resulted in a lower relative abundance of PPB and reduced biomass productivities, highlighting the need to balance resource recovery and treatment goals.

Application of a Spiral Symmetric Stream Anaerobic Bioreactor for treating saline heparin sodium pharmaceutical wastewater: Reactor operating characteristics, organics degradation pathway and salt tolerance mechanism

In this study, a Spiral Symmetry Stream Anaerobic Bioreactor (SSSAB) was adopted for treating actual saline heparin sodium pharmaceutical wastewater (HSPW). After adaptation, under the influent COD of 8731 mg/L, OLR of 6.98 kg COD/(m³•d) and salinity of 3.57 wt%, the COD removal reached up to 82%. This value is much higher than the reported for the other reactors at similar salinity. Benzenes are the major organic compounds in HSPW. The main rate-limiting steps are the degradations of phenol and p-cresol. In addition, the degradation pathways of typical benzenes in HSPW were analyzed. After adaptation, the soluble salt content in the granular sludge increased, and the bacterial extracellular polymers (EPS), especially tightly-bound EPS also significantly increased. 16S rRNA analysis revealed that the microbial community in the anaerobic granular sludge (AGS) had become adapted to the HSPW treatment since Mesotoga (12.4%), Anaerophaga (9.0%), Oceanotoga (6.1%) and Aminobacterium (4.1%) increased from previously below 1.0% values. The relative abundance of Methanosarcina in the upper layer of the reactor (68.7%) is significantly higher than that at the bottom (3.8%). This proves the superiority of the SSSAB structure. Finally, a model for salt-tolerant microorganisms is given, which proposes a mechanism for this study and provides reference for other anaerobic biological treatments of high-salt containing wastewater.

Enhancement of mariculture wastewater treatment using moving bed biofilm reactors filled with modified biocarriers: Characterisation, process performance and microbial community evaluation

This research investigated two proposed modified biofilm carriers' performances in treating recirculating aquaculture systems (RAS) wastewater under different salinities (12‰, 26‰, and 35‰) for about 92 days. Three moving bed biofilm reactors (MBBRs; R1, R2, and R3) were filled with unmodified novel sponge biocarriers (SB) served as a control, modified novel SB with ferrous oxalate (C₂FeO₄@SB), and modified novel SB with combined ferrous oxalate and activated carbon (C₂FeO₄-AC@SB), respectively. Under the highest saline condition, a significantly higher ammonia removal efficiency of 98.86 ± 0.7% (p ˃ 0.05) was obtained in R3, whereas R2 and R1 yielded 95.18 ± 2.8% and 91.66 ± 1.5%, respectively. Microbial analysis showed that Vibrio, Ruegeria, Formosa, Thalassospira, and Denitromonas were predominant genera, strictly halophilic heterotrophic nitrifying bacteria involved in nitrogen removal. In conclusion, the synergistic effects of novel sponge, C₂FeO₄ and AC accelerated biofilm formations and stability, subsequently enhanced the removal of ammonia from the mariculture RAS wastewater by the C₂FeO₄-AC@SB carriers in R3.

The application of Marinobacter hydrocarbonoclasticus as a bioaugmentation agent for the enhanced treatment of non-sterile fish wastewater

Wastewaters generated by fish processing are characterised by salt concentrations similar to or greater than that of seawater together with high nutrient concentrations (e.g. organic carbon and total nitrogen) due to the presence of blood, oil, and fish tissues. Fish processing wastewater entering rivers and oceans have become a key factor leading to the pollution of receiving waters; the adequate treatment of this wastewater is, therefore, crucial to a sustainable fish industry. The present study aimed to determine whether augmentation of fish wastewater with either Marinirhabdus sp., Marinobacter hydrocarbonoclasticus or a consortium of the two halobacteria, could successfully enhance the removal of both chemical oxygen demand (COD) and total nitrogen (TN) from fish wastewater. Following 9 days of incubation, the bioaugmentation treatment resulted in a significant reduction in COD, 88%, 91%, and 92% in fish wastewater augmented with either Marinirhabdus sp., Marinobacter hydrocarbonoclasticus respectively, or a consortium of the two halobacteria compared with the control (non-bioaugmented) treatment (77% removal). In tall bioaugmentation treatments (79-88%) TN removal was also significantly greater than the control treatment (57%). After 9 days of incubation, the COD and TN in bioaugmentation reached the European Union's (EU) wastewater discharge standard (Level B, COD < 120 mg L^-1, TN < 70 mg L^-1). The addition of monoculture was effective in enhancing the removal of COD, while co-culture significantly improved TN removal. Results of 16S rDNA sequence analysis investigating the survival of these introduced bacteria showed that only Marinobacter hydrocarbonoclasticus was detected at the end of the treatment, constituting 36% of the total bacterial population when added alone to the wastewater. This study confirms the effectiveness of bioaugmentation in removing COD and TN in saline fish wastewater. The ability of Marinobacter hydrocarbonclasticus to enhance the treatment and dominate the bacterial community suggests the commercial potential of this organism for bioaugmentation of aquaculture wastewater without the need for further bioaugmentation.

Metagenomic insights into mixotrophic denitrification facilitated nitrogen removal in a full-scale A2/O wastewater treatment plant

Wastewater treatment plants (WWTPs) are important for pollutant removal from wastewater, elimination of point discharges of nutrients into the environment and water resource protection. The anaerobic/anoxic/oxic (A2/O) process is widely used in WWTPs for nitrogen removal, but the requirement for additional organics to ensure a suitable nitrogen removal efficiency makes this process costly and energy consuming. In this study, we report mixotrophic denitrification at a low COD (chemical oxygen demand)/TN (total nitrogen) ratio in a full-scale A2/O WWTP with relatively high sulfate in the inlet. Nitrogen and sulfur species analysis in different units of this A2/O WWTP showed that the internal sulfur cycle of sulfate reduction and reoxidation occurred and that the reduced sulfur species might contribute to denitrification. Microbial community analysis revealed that Thiobacillus, an autotrophic sulfur-oxidizing denitrifier, dominated the activated sludge bacterial community. Metagenomics data also supported the potential of sulfur-based denitrification when high levels of denitrification occurred, and sulfur oxidation and sulfate reduction genes coexisted in the activated sludge. Although most of the denitrification genes were affiliated with heterotrophic denitrifiers with high abundance, the narG and napA genes were mainly associated with autotrophic sulfur-oxidizing denitrifiers. The functional genes related to nitrogen removal were actively expressed even in the unit containing relatively highly reduced sulfur species, indicating that the mixotrophic denitrification process in A2/O could overcome not only a shortage of carbon sources but also the inhibition by reduced sulfur of nitrification and denitrification. Our results indicate that a mixotrophic denitrification process could be developed in full-scale WWTPs and reduce the requirement for additional carbon sources, which could endow WWTPs with more flexible and adaptable nitrogen removal.

Purple phototrophic bacteria are outcompeted by aerobic heterotrophs in the presence of oxygen

There is an ongoing debate around the effect of microaerobic/aerobic conditions on the wastewater treatment performance and stability of enriched purple phototrophic bacteria (PPB) cultures. It is well known that oxygen-induced oxidative conditions inhibit the synthesis of light harvesting complexes, required for photoheterotrophy. However, in applied research, several publications have reported efficient wastewater treatment at high dissolved oxygen (DO) levels. This study evaluated the impact of different DO concentrations (0-0.25 mg·L^-1, 0-0.5 mg·L^-1 and 0-4.5 mg·L^-1) on the COD, nitrogen and phosphorus removal performances, the biomass yields, and the final microbial communities of PPB-enriched cultures, treating real wastewaters (domestic and poultry processing wastewater). The results show that the presence of oxygen suppressed photoheterotrophic growth, which led to a complete pigment and colour loss in a matter of 20-30 h after starting the batch. Under aerobic conditions, chemoheterotrophy was the dominant catabolic pathway, with wastewater treatment performances similar to those achieved in common aerobic reactors, rather than those corresponding to phototrophic systems (i.e. considerable total COD decrease (45-57% aerobically vs. ± 10% anaerobically). This includes faster consumption of COD and nutrients, lower nutrient removal efficiencies (50-58% vs. 72-99% for NH₄⁺-N), lower COD:N:P substrate ratios (100:4.5-5.0:0.4-0.8 vs. 100:6.7-12:0.9-1.2), and lower apparent biomass yields (0.15-0.31 vs. 0.8-1.2 g COD_biomass·g COD_removed^-1)). The suppression of photoheterotrophy inevitably resulted in a reduction of the relative PPB abundances in all the aerated tests (below 20% at the end of the tests), as PPB lost their main competitive advantage against competing aerobic heterotrophic microbes. This was explained by the lower aerobic PPB growth rates (2.4 d^-1 at 35 °C) when compared to common growth rates for aerobic heterotrophs (6.0 d^-1 at 20 °C). Therefore, PPB effectively outcompete other microbes under illuminated-anaerobic conditions, but not under aerobic or even micro-aerobic conditions, as shown by continuously aerated tests controlled at undetectable DO levels. While their aerobic heterotrophic capabilities provide some resilience, at non-sterile conditions PPB cannot dominate when growing chemoheterotrophically, and will be outcompeted.

Experimental Study on a Closed-Cycle Humidification and Dehumidification System for Treating Wastewater Containing High Concentrations of Inorganic Salts and Organic Matter

Industrial wastewater contains high concentrations of inorganic salts and organic matter. This experiment studied a system for treating wastewater containing high concentrations of inorganic salts and organic matter. The setup consists of a closed-cycle humidification and dehumidification system and a filter press. Chemical wastewater was used as the treatment solution, and the treatment performance of the system was tested and analyzed. The system effectively reduced the chemical oxygen demand (COD), electric conductivity (EC), total nitrogen (TN), and ammonia nitrogen (NH4-N) in the wastewater and, at the same time, dehydrated sludge was obtained through a filter press. The system maintains a stable removal rate of each index (COD, EC, TN, and NH4-N) in wastewater and can remove inorganic salts and organic matter from wastewater. The system can successfully treat industrial wastewater containing high concentrations of inorganic salts and organic matter.

Biological nutrient removal by halophilic aerobic granular sludge under hypersaline seawater conditions

Biological nutrient removal and physical properties of halophilic aerobic granular sludge (hAGS) cultivated from autochthonous seawater-born microbes were investigated under hypersaline seawater conditions. hAGS achieved stable total nitrogen (TN) and total phosphorus (TP) removals of 96 ± 3% and 95 ± 4%, respectively, from seawater-based wastewater at 3.4% salt. At 4 to 12% salt concentrations, stable TN and TP removals of 82-99% and 95-96%, respectively, were maintained over 4 months under seawater conditions. Ammonium and phosphorus were mainly removed by nitritation-denitritation and enhanced biological phosphorus removal pathways, respectively. Stappiaceae (45%) and Rhodobacteraceae (21%) were the dominant genera in hAGS performing nutrient removal at 12% salt. hAGS contained acid-soluble extracellular polymeric substance as the major structural polymer which increased from 0.43 ± 0.02 g/gTS at 3.4% salt to 0.93 ± 0.03 g/gTS at 12% salt. Cultivation of hAGS from autochthonous wastewater-microbes can be a promising approach for achieving biological nitrogen and phosphorus removals from hypersaline seawater-based wastewaters.

An overview of anoxygenic phototrophic bacteria and their applications in environmental biotechnology for sustainable Resource recovery

Anoxygenic phototrophic bacteria (APB) are a phylogenetically diverse group of organisms that can harness solar energy for their growth and metabolism. These bacteria vary broadly in terms of their metabolism as well as the composition of their photosynthetic apparatus. Unlike oxygenic phototrophic bacteria such as algae and cyanobacteria, APB can use both organic and inorganic electron donors for light-dependent fixation of carbon dioxide without generating oxygen. Their versatile metabolism, ability to adapt in extreme conditions, low maintenance cost and high biomass yield make APB ideal for wastewater treatment, resource recovery and in the production of high value substances. This review highlights the advantages of APB over algae and cyanobacteria, and their applications in photo-bioelectrochemical systems, production of poly-β-hydroxyalkanoates, single-cell protein, biofertilizers and pigments. The ecology of ABP, their distinguishing factors, various physiochemical parameters governing the production of high-value substances and future directions of APB utilization are also discussed.

Development of a treatment train for the remediation of a hazardous industrial waste landfill leachate: A big challenge

This study focuses on the development of a treatment train for a leachate from a hazardous industrial waste landfill (HIWL) previously treated by: (i) catalytic oxidation with hydrogen peroxide (H₂O₂) for sulphide and sulphite conversion into oxidized sulphur species, including sulphate, and (ii) chemical precipitation of sulphate as barite. The complete treatment line counted on four more stages: (iii) 1^st biological oxidation for removal of biodegradable organic compounds and nitrogen species, (iv) coagulation with ferric chloride (coagulant dose of 100 mg Fe L^-1, pH 2.8) for removal of a fraction of recalcitrant organics and suspended solids, (v) photo-Fenton oxidation using ultraviolet A (UVA) radiation (PF-UVA) (pH 2.8, initial total dissolved iron content of 140 mg L^-1, treatment time of ~4 h) for recalcitrant organics degradation and biodegradability improvement, and (vi) 2^nd biological oxidation for removal of the biodegradable organic matter resulting from the PF-UVA process. The use of anodic oxidation or photoelectro-Fenton processes in stage (v) demonstrated to be unfeasible. A chemical oxygen demand (COD) below 1000 mg O₂ L^-1, a common limit imposed by municipal wastewater treatment plants (MWWTPs) to effluents discharged into the municipal sewer, was achieved after a feasible treatment time (~4 h) using the multistep approach. The remediation of the HIWL leachate proved to be a big challenge.

Deciphering pollutants removal mechanisms and genetic responses to ampicillin stress in simultaneous heterotrophic nitrification and aerobic denitrification (SHNAD) process treating seawater-based wastewater

Pollutants removal and genetic responses of simultaneous heterotrophic nitrification and aerobic denitrification (SHNAD) treating seawater-based wastewater were studied under ampicillin stress. Marine SHAND bacteria exhibited good tolerance to 10 mg/L ampicillin with nitrogen removal efficiency and organics removal efficiency of 94.5% and 82.6%, respectively. Besides, the half-inhibitory concentration of ampicillin on marine SHAND bacteria was 50 mg/L. The relative abundances of antibiotic resistance genes (ARGs) first decreased and then increased with ampicillin addition. The blaVIM played an important role to resist 25 mg/L ampicillin, which contributed to the recovery of pollutants removal. BlaSHV and blaTEM dominated ARG subtypes, which accounted for 96.6% of ARGs abundance. At 50 mg/L ampicillin, reactive oxygen species (ROS) production and cell numbers of apoptosis increased by 47.9% and 367.5%, respectively. The overproduction of ROS was stimulated by ampicillin, which caused bacterial cell apoptosis. Marine SHNAD bacteria produced more extracellular polymeric substances to resist ampicillin.

Denitrifying sulfur conversion-EBPR (DS-EBPR) process for treatment of seawater-based highly saline wastewater: Evaluation on performance, kinetics and microbial community structure

DS-EBPR is an alternative to the conventional activated sludge process which face great challenge for treatment of seawater-based highly saline wastewater. This study aims to investigate the impacts of long-term (248 days) 20% and 30% seawater fractions and short-term shock of 30%, 40%, 70% and 100% seawater fractions (corresponding to 1.0, 1.4, 2.5 and 3.5% of salinity) on the DS-EBPR performance, kinetics and microbial community structure. Long-term operation with high fraction (30%) of seawater marginally decreased the sulfur conversion and phosphorus uptake, which correlated well with the microbial dynamics. Temporal salinity shock from 1.0% (30% seawater) to 3.5% (100% seawater) remarkably reduced the phosphorus release/uptake by 36-44%, which was partly due to the decrease in the abundance of functional bacteria and chlorapatite (Ca₅[PO₄]₃Cl) forming as P precipitates with 70-100% seawater addition. The formed chlorapatite contributed to approximately 8-26% of total P removal estimated by X-ray photoelectron spectroscopy analysis.

Application of purple phototrophic bacteria in a biofilm photobioreactor for single cell protein production: Biofilm vs suspended growth

Single cell protein (SCP), has been proposed as alternative to effectively upgrade and recycle organics and nutrients from wastewater. Biomass recovery is a critical issue, and recovery as a biofilm is effective in comparison with sedimentation of suspended biomass. This study aims to determine the applicability of purple phototrophic bacteria (PPB) biofilm on infra-red irradiated, submerged surfaces for the treatment of pre-settled red meat processing wastewater, and SCP generation. PPB removed up to 66% of COD and 42% of TN and TP during batch operation with total areal productivities between 15 and 20 gVS m^-2 d^-1 achieved. More than 60% of the total biomass grew attached (as biofilm) with the remainder being suspended. The biofilm can be harvested at around 160 gTS L^-1 with high protein (>96 g L^-1) and low ash contents (>4.0% compared to >30% in the wastewater). The compositions of attached and suspended biomass differed significantly, where the suspended fraction resembled the wastewater composition (e.g. in terms of inert components). The PPB community was similar in the suspended and biofilm fractions while the biofilm had higher relative abundance of PPB representatives (57% vs 43%). A consistent product composition is highly relevant for the manufacturer and ultimately determines the value as feed, feed additive, or supplement.

A systematic optimization of piggery wastewater treatment with purple phototrophic bacteria

The increase in natural water bodies pollution caused by intensive animal farming requires the development of innovative sustainable treatment processes. This study assessed the influence of piggery wastewater (PWW) load, air dosing, CO₂/NaHCO₃^- supplementation and pH control on PWW treatment by mixed cultures of purple phototrophic bacteria (PPB) under infrared radiation in batch photobioreactors. PPB was not able to grow in raw PWW but PWW dilution prevented inhibition and supported an effective light penetration. Despite the fact that PPB were tolerant to O₂, carbon recovery decreased in the presence of air (induced by stripping). CO₂ supplementation was identified as an effective strategy to maximize the removal of carbon during PPB-based PWW treatment with removal efficiencies of 72% and 74% for TOC and VFAs. However, the benefits derived from CO₂ addition were induced by the indirect pH control exerted in the cultivation medium. Thus, PPB supported an optimal pollutant removal performance at pH 7, with removal efficiencies of 75%, 39% and 98% for TOC, TN and VFAs.

Nitrogen removal bacterial strains, MSNA-1 and MSD4, with wide ranges of salinity and pH resistances

Nitrogenous wastewater is difficult to treat using conventional microorganisms in high salinity and acidic/alkaline environments. Two halotolerant bacteria, heterotrophic nitrifying Stenotrophomonas sp. MSNA-1 and aerobic denitrifying Pseudomonas sp. MSD4, were isolated, and the amplification of functional genes provided the evidences of nitrogen removal performance. The results regarding salinity and pH resistance showed that strain MSNA-1 is robust at salinities of 0-15% and pH of 3-10. It can remove 51.2% of NH₄⁺-N (180 mg/L) at salinity of 10% (pH: 7) and 49.2% of NH₄⁺-N under pH 4 (salinity: 3%). For strain MSD4, it is robust at salinities of 0-10% and pH of 5-11. It can remove 62.4% of TN (100 mg/L) at salinity of 7% (pH: 7) and 72.2% of TN under pH 9 (salinity: 3%). Their excellent salinity and pH resistances make them promising candidates for treating nitrogenous wastewaters under extreme conditions with low operational cost.

Effect of low-intensity electric current field and iron anode on biological nitrate removal in wastewater with low COD to nitrogen ratio from coal pyrolysis

Mixotrophic nitrate removal in wastewater from coal pyrolysis was achieved in microbial electrolysis cell with iron anode (iron-MEC). The effect of voltage, iron anode and conductivity were investigated. The effluent TN concentration was 8.35 ± 1.94 mg/L in iron-MEC when the conductivity of the wastewater was adjusted to 3.97 ± 0.08 mS/cm, which was lower than that in no-treated reactor. The increase of current density, which was resulted from the elevation of conductivity, promoted the iron corrosion and Fe2+ ion generation. Therefore, more Fe2+ ion was utilized by nitrate reducing ferrous oxidation bacteria (NRFOB) used to reduce nitrate. The microbial community analysis demonstrated that NRFOB, including Acidovorax and Bradyrhizobium, possessed a higher abundance in iron-MEC. The enrichment of Geobacter in iron-MEC might imply that the part of Fe(III) produced by ferrous oxidation was reduced by Geobacter, which established an iron cycle. Moreover, the production of N2O was decreased by the formation of Fe2+ ion.

Purple phototrophic bacteria for resource recovery: Challenges and opportunities

Sustainable development is driving a rapid focus shift in the wastewater and organic waste treatment sectors, from a "removal and disposal" approach towards the recovery and reuse of water, energy and materials (e.g. carbon or nutrients). Purple phototrophic bacteria (PPB) are receiving increasing attention due to their capability of growing photoheterotrophically under anaerobic conditions. Using light as energy source, PPB can simultaneously assimilate carbon and nutrients at high efficiencies (with biomass yields close to unity (1 g COD_biomass·g COD_removed^-1)), facilitating the maximum recovery of these resources as different value-added products. The effective use of infrared light enables selective PPB enrichment in non-sterile conditions, without competition with other phototrophs such as microalgae if ultraviolet-visible wavelengths are filtered. This review reunites results systematically gathered from over 177 scientific articles, aiming at producing generalized conclusions. The most critical aspects of PPB-based production and valorisation processes are addressed, including: (i) the identification of the main challenges and potentials of different growth strategies, (ii) a critical analysis of the production of value-added compounds, (iii) a comparison of the different value-added products, (iv) insights into the general challenges and opportunities and (v) recommendations for future research and development towards practical implementation. To date, most of the work has not been executed under real-life conditions, relevant for full-scale application. With the savings in wastewater discharge due to removal of organics, nitrogen and phosphorus as an important economic driver, priorities must go to using PPB-enriched cultures and real waste matrices. The costs associated with artificial illumination, followed by centrifugal harvesting/dewatering and drying, are estimated to be 1.9_, 0.3-2.2 and 0.1-0.3 $·kg_{dry biomass}^-1. At present, these costs are likely to exceed revenues. Future research efforts must be carried out outdoors, using sunlight as energy source. The growth of bulk biomass on relatively clean wastewater streams (e.g. from food processing) and its utilization as a protein-rich feed (e.g. to replace fishmeal, 1.5-2.0 $·kg^-1) appears as a promising valorisation route.

Selective carbon sources and salinities enhance enzymes and extracellular polymeric substances extrusion of Chlorella sp. for potential co-metabolism

This study investigated the extracellular polymeric substance (EPS) and enzyme extrusion of Chlorella sp. using seven carbon sources and two salinities for potential pollutant co-metabolism. Results indicated that the levels of biomass, EPS and enzymes of microalgae cultured with glucose and saccharose outcompeted other carbon sources. For pigment production, glycine received the highest chlorophyll and carotene, up to 10 mg/L. The EPS reached 30 mg/L, having doubled the amount of protein than carbohydrate. For superoxide dismutase and peroxidase enzymes, the highest concentrations were beyond 60 U/ml and 6 nmol/d.ml, respectively. This amount could be potentially used for degrading 40% ciprofloxacin of concentration 2000 µg/L. When increasing salinity from 0.1% to 3.5%, the concentrations of pigment, EPS and enzymes rose 3 to 30 times. These results highlighted that certain carbon sources and salinities could induce Chlorella sp. to produce EPS and enzymes for pollutant co-metabolism and also for revenue-raising potential.

Novel approach for the treatment of the organic fraction of municipal solid waste: Coupling thermal hydrolysis with anaerobic digestion and photo-fermentation

In this study, two different organic fractions of municipal solid waste (OFMSW) served as raw material in a novel treatment process that combines thermal hydrolysis (TH) pretreatment at different times, followed by anaerobic digestion of the solid fraction and photo-fermentation of the liquid fraction. The results indicate that both wastes performed similarly, and no statistically relevant differences stand out on the overall performance regarding TH times. The thermal pretreatment improves the biodegradability of the solid fraction during anaerobic digestion compensating the loss of the organic matter in the liquid fraction. The produced biogas may feed a combined heat and power (CHP) system, making the process energetically positive in all studied scenarios. In addition, the combination of TH and anaerobic digestion decreased the volume of the waste to be disposed by 59-61%, which is 5-11% higher than that obtained with the traditional treatment of anaerobic digestion process. Specific phototrophic activity tests were performed on the liquid phase using a mixed culture of purple phototrophic bacteria (PPB) that consumed up to 80% of the soluble organics. The assays yielded an average 52% efficiency on specific phototrophic activity (k_M) and 62% on biomass yield (Y_X/S), compared to an optimized growth medium. PPB was also capable of producing polyhydroxyalkanoates, bioH₂ and single-cell protein without optimization. Apart from methane, the overall mass balances showed yields up to 150 g of high added-value products per Kg of initial total solids on this proof-of-concept platform.

Photosynthetic bacteria-based technology is a potential alternative to meet sustainable wastewater treatment requirement?

A paradigm shift is underway in wastewater treatment from pollution removal to resource or energy recovery. However, conventional activated sludge (CAS) as the core technology of wastewater treatment is confronted with severe challenges on high energy consumption, sludge disposal and inevitable greenhouse gas emission, which are posing a serious impact on the current wastewater industry. It is urgent to find new alternative methods to remedy these defects. Photosynthetic bacteria (PSB) have flexible metabolic modes and high tolerance, which enhance the removal of nutrients, heavy metals and organic contaminants efficiency in different wastewater. The unique phototrophic growth of PSB breaks the restriction of nutrient metabolism in the CAS system. Recent studies have shown that PSB-based technologies can not only achieve the recovery of nutrient and energy, but also improve the degradation efficiency of refractory substances. If the application parameters can be determined, there will be great prospects and economic effects. This review summarizes the research breakthroughs and application promotion of PSB-based wastewater treatment technology in recent years. Comparing discussed the superiority and inferiority from the perspective of application range, performance differences and recovery possibility. Pathways involved in the nutrient substance and the corresponding influencing parameters are also described in detail. The mode of PSB biodegradation processes presented a promising alternative for new wastewater treatment scheme. In the future, more mechanical and model studies, deterministic operating parameters, revolutionary process design is need for large-scale industrial promotion of PSB-based wastewater treatment.

Effect of alkyl polyglycosides on the performance of thermophilic bacteria pretreatment for saline waste sludge hydrolysis

In this study, alkyl polyglycosides (APG) was used to further accelerate the hydrolysis of saline waste sludge with thermophilic bacteria (TB) pretreatment. In the presence of 0.4 g/g TSS APG, the concentrations of soluble chemical oxygen demand (SCOD), soluble carbohydrate and soluble protein in dissolved organic matters (DOM) were 0.4, 2.4 and 1.3 times of that without APG addition, respectively. Excitation emission matrix (EEM) fluorescence spectroscopy revealed that the addition of APG led to the increase of soluble microbial materials and the decrease of fulvic acid-like substances in DOM, which was beneficial for the subsequent process of anaerobic digestion. Using APG promoted the releasing of enzymes trapped in saline waste sludge and improved the activity of enzymes during hydrolysis. The activities of α-glucosidase and protease increased by 8.8% and 21.3% respectively in the presence of 0.4 g/g TSS APG comparing no APG addition.

Influence of salinity on the biological treatment of domestic ship sewage using an air-lift multilevel circulation membrane reactor

Recently, strict standards for ship domestic sewage discharge have been implemented by the International Maritime Organization (IMO). The high salinity of ship sewage was considered a key factor influencing the removal efficiency of ship sewage treatment systems. In the present study, the salinity effect on the removal of chemical oxygen demand (COD) and ammonia nitrogen (NH₄⁺-N) from ship domestic sewage was investigated by using a novel air-lift multilevel circulation membrane reactor (AMCMBR). Enzyme activity analysis and wavelet neural network (WNN) models were built to determine the mechanisms of the process. The experimental results indicate that high salinity levels (> 21 g/L) had a negative impact on COD and NH₄⁺-N removal efficiencies, and low saline concentrations (≤ 21 g/L) caused a negligible effect. The COD and NH₄-N removal efficiencies were 84% and 97%, respectively, at a salinity of 21 g/L, which were higher than those at low salinities (i.e., 7 g/L and 14 g/L). Invertase and nitrate reductase had a close relationship with removal performance, and they can be considered important indicators reflecting the operation effort under saline environments. With high predictive accuracies, the constructed WNN models simulated the complex COD and NH₄⁺-N removal processes well under different saline concentrations, ensuring the long-term stable operation of the AMCMBR under different salinities.