Sustainable wastewater treatment and reuse in space.

Exploring the vast extraterrestrial space is an inevitable trend with continuous human development. Water treatment and reuse are crucial in the limited and closed space that is available in spaceships or long-term use space bases that will be established in the foreseeable future. Dedicated water treatment technologies have experienced iterative development for more than 60 years since the first manned spaceflight was successfully launched. Herein, we briefly review the related wastewater characteristics and the history of water treatment in space stations, and we focus on future challenges and perspectives, aiming at providing insights for optimizing wastewater treatment technologies and closing the water cycle in future.

Space aquatic chemistry: A roadmap for drinking water treatment in microgravity.

Rapid advancement in aerospace technology has successfully enabled long-term life and economic activities in space, particularly in Low Earth Orbit (LEO), extending up to 2000 km from the mean sea level. However, the sustainance of the LEO Economy and its Environmental Control and Life Support System (ECLSS) still relies on a regular cargo supply of essential commodities (e.g., water, food) from Earth, for which there still is a lack of adequate and sustainable technologies. One key challenge in this context is developing water treatment technologies and standards that can perform effectively under microgravity conditions. Solving this technical challenge will be a milestone in providing a scientific basis and the necessary support mechanisms for establishing permanent bases in outer space and beyond. To identify clues towards solving this challenge, we looked back at relevant scientific research exploring novel technologies and standards for deep space exploration, also considering feedback for enhancing these technologies on land. Synthesizing our findings, we share our outlook for the future of drinking water treatment in microgravity. We also bring up a new concept for space aquatic chemistry, considering the closed environment of engineered systems operating in microgravity.

Instant water toxicity detection based on magnetically-constructed electrochemically active biofilm.

Water toxicity determination with electrochemically active bacteria (EAB) is promising in the early warning of water pollution. However, limited by tedious biofilm formation, natural EAB biofilms are uncapable of the instant detection of water toxicity, resulting in the failure for the emergency monitoring of water pollution. To solve this problem, a novel method for the rapid construction of EAB biofilms using magnetic adsorption was established, and the performance of instant water toxicity detection with magnetically-constructed EAB biofilm was investigated. The results demonstrate that EAB biofilms were magnetically constructed in less than 30 min, and magnetically-constructed EAB biofilm generated stable currents even under continuous flow conditions. Magnetically-constructed EAB biofilms realized instant water toxicity detection, and the sensitivity increased with the decrease of magnetic field intensity. Low magnetic field intensity resulted in a loose biofilm structure, which is conducive to toxic pollutant penetration. The detection limit for Cu2+, phenol, and Cd2+ achieved 0.07 mg/L with optimal magnetic field intensity, and the detection time was less than 30 min. This study broadens the application of water toxicity determination with EAB, and establishes a foundation for the instant and continuous detection of water toxicity with EAB.

Mechanism and applications of bidirectional extracellular electron transfer of Shewanella.

Electrochemically active microorganisms (EAMs) play an important role in the fields of environment and energy. Shewanella is the most common EAM. Research into Shewanella contributes to a deeper comprehension of EAMs and expands practical applications. In this review, the outward and inward extracellular electron transfer (EET) mechanisms of Shewanella are summarized and the roles of riboflavin in outward and inward EET are compared. Then, four methods for the enhancement of EET performance are discussed, focusing on riboflavin, intracellular reducing force, biofilm formation and substrate spectrum, respectively. Finally, the applications of Shewanella in the environment are classified, and the restrictions are discussed. Potential solutions and promising prospects for Shewanella are also provided.

On-site determination of water toxicity based on freeze-dried electrochemically active bacteria.

Our previous studies have reported water toxicity determination with a fresh electrochemically active bacteria (EAB) suspension as the sensing element, which exhibits high sensitivity and has great prospects in providing early warning about water pollution. However, because the preparation of fresh EAB suspensions is time-consuming, these studies are not suitable for the on-site determination of water toxicity. To solve this problem, this study investigated the rapid preparation of an EAB suspension by the rehydration of freeze-dried EABs and established a novel method for the on-site determination of water toxicity based on the freeze-dried EAB model strain Shewanella oneidensis MR-1. The results demonstrate that the optimal cryoprotectant for S. oneidensis MR-1 freeze drying is 7.5 % (w/v) skimmed milk powder. Compared with fresh S. oneidensis MR-1, freeze-dried S. oneidensis MR-1 exhibits similar extracellular electron transfer (EET) performance (74.7 % ± 0.3 %) and slightly lower sensitivity for water toxicity determination (65.8 % ± 2.2 %) with the optimal cryoprotectant. On-site determination of water toxicity was realized by using freeze-dried S. oneidensis MR-1, and the detection limits of five common toxic pollutants (Cd2+, Pb2+, Cu2+, phenol and dichlorophenol) reached 0.5 mg/L. Water toxicity determination is capable of resisting common interferences, e.g., glucose, lactate, nitrate and nitrite, and shows high accuracy in practical applications.

Two polarity reversal modes lead to different nitrate reduction pathways in bioelectrochemical systems.

Polarity reversal is one of the effective strategies to rapidly start up denitrifying BESs，but the long-term performances of the denitrifying BESs operated under polarity reversal receive little attention. This study investigated the effects of periodic polarity reversal (PPR) and polarity reversal once only (PRO) on the long-term performances of denitrifying BESs. Repeatable oxidative and reductive currents were observed in the BESs obtained by PPR (PPR-BESs). The peak reductive currents of the PPR-BESs reached 0.95 A/m2, and nitrate was mainly removed by dissimilatory nitrate reduction to ammonium pathway with removal rates higher than 95 %. In contrast, the peak reductive currents of the BESs obtained by PRO (PRO-BESs) progressively decreased from 1.01 A/m2 to 0.12 A/m2. The nitrate removal rates of the PRO-BESs were <50 %, and the product of nitrate reduction turned to N2 instead of ammonium. 16S rDNA sequencing and metatranscriptomic analysis revealed that Geobacter capable of bidirectional extracellular electron transfer (EET) and Afipia capable of autotrophic growth were the dominant genera in the two types of BESs. Outer membrane cytochrome c and formate dehydrogenase were potentially involved in the cathodic electron uptake. These findings contribute to a better understanding of the EET mechanisms of electroautotrophic denitrifiers.

High-fat and high-protein diets from different sources induce different intestinal malodorous gases and inflammation.

Long-term unbalanced diet might lead to intestinal malodorous gas generation and inflammation, which would cause health problems, especially in confined environment. In this study, the effects of high-fat and high-protein diets from different nutrient sources, including casein lard, soy protein and soybean oil, and pork protein and lard, respectively, on the differences of malodorous gas compounds emission and inflammation in rats were explored as well as the correlation with gut microbiota. The results showed that all the high-fat and high-protein diets could induce organ damage, abnormal serum biochemical indexes and inflammation. However, the malodorous gas compounds in the three experimental groups were totally different, which might be because the production of malodorous gas compounds was jointly regulated by the intestinal microbiota and liver cytochrome P450.

Enhancing the sensitivity of water toxicity detection based on suspended Shewanella oneidensis MR-1 by reversing extracellular electron transfer direction.

Water toxicity detection is of great significance to ensure the safety of water supply. With suspended electrochemically active bacteria (EAB) as the sensing element, a novel microbial electrochemical sensor (MES) has recently been reported for the real-time detection of water toxicity, but its practical applications need to further improve the sensitivity. Extracellular electron transfer (EET) is an important factor affecting MES performance. In the study, the EET of suspended EAB-based MES was optimized to further enhance the sensitivity. Firstly, by using a model EAB stain Shewanella oneidensis MR-1, it was revealed that the sensitivity was increased at most 2.7 times with inward EET (i.e., cathodic polarization). Then, a novel conjecture based on electron transfer and energy fluxes was proposed and testified to explain this phenomenon. Finally, three key operating parameters of inward EET were orthogonally optimized. The optimized parameters of inward EET included a potential of - 0.5 V, a cell density of 1.8 × 108 CFU/mL, and an electron acceptor concentration of 15 mM.

Water recycle system in an artificial closed ecosystem - Lunar Palace 1: Treatment performance and microbial evolution.

Water recycle systems have important implications to realize material circulation in biological regeneration life support systems, which is of significance for long-term space missions and future planetary base. Based on membrane biological activated carbon reactor (MBAR) technologies, the 'Lunar Palace 365' experiment established various treatment processes for condensate wastewater, domestic wastewater, urine, and used nutrient solutions. The 370-day operation data showed the CODMn index of purified condensate wastewater decreased to 0.74 ± 0.15 mg/L, which met the standards for drinking water quality. The average removal rate of organic contaminants in domestic wastewater by the MBAR was 85.7% ± 10.2%, and this MBAR also had a stable nitrification performance with effluent NO3--N concentrations fluctuating from 145.57 mg/L to 328.59 mg/L. Moreover, the purification of urine achieved the conversion of urea-N to NH4+-N and thus the partial recovery of nitrogen. 16S rDNA sequencing results revealed the evolution of microbial diversity and composition during the long-term operation. Meiothermus, Rhodanobacter, and Ochrobactrum were the dominant microorganisms in various MBARs.

Effect of solid waste fermentation substrate on wheat (Triticum aestivum L.) growth in closed artificial ecosystem.

Bioregenerative Life Support System (BLSS) is a closed artificial ecosystem and could provide oxygen, food, water and other substrates for long-term deep space survival. The treatment and recycle of the solid waste are crucial and rate-limiting steps in BLSS, and it's reported that the solid waste such as the inedible plants and human feces could be fermented aerobically and then reused as fertilizer for growing plants in BLSS, which may be an effective way to improve the solid waste recycling rate. However, the recycling performance and the effect on the system need to be evaluated. In this study, the fermented and decomposed solid waste product from the 365d BLSS experiment with human involved in Lunar Palace 1 was utilized, and was added to the Hoagland nutrient solution as a supplementary fertilizer in the weight proportion of 5% and 10%, respectively, for the cultivation of wheat (Group-5% and Group-10%). Then, the effects on wheat germination, morphology, photosynthesis, biomass, the conductivity of the cultured substrates and microorganisms were detected and compared with those of the CK group cultured using only Hoagland nutrient solution. The results showed that this planting method had no inhibitory effect on the wheat germination, root length and yield, and might even promote the vegetative growth of wheat in terms of Vigor index, plant height, leaf area and net photosynthesis rate to some extent. The added solid waste fermentation substrate as well as the planting environment in Lunar Palace 1 both had significant influences on the rhizosphere microorganisms of wheat. The bacteria diversity was more abundant than fungi at phylum level, and the relative abundance varied along with the wheat growth period. The relative abundance of the cellulose degrading microorganisms including Actinobacteria and Ascomycota increased in Group-5% and Group-10% compared with CK group along with the growth of wheat. Moreover, the proper reuse of the fermentation substrate could reduce the use of inorganic salts by 9.8%-11.9% and save 40L•m - 2 of water for wheat cultivation. This research has considerable application significance in future deep space exploration.

Semi-continuous fermentation of solid waste in closed artificial ecosystem: Microbial diversity, function genes evaluation.

Bioregenerative Life Support System (BLSS) is a closed artificial ecosystem and could provide oxygen, food, water and other substances for space survival. Solid waste treatment is a key rate-limiting step in BLSS. In this study, solid wastes including wheat straw, human and yellow mealworm feces were disposed in a semi-continuous bio-convertor for 105 days in a ground-based experimental BLSS platform (Lunar Palace 1). Solid wastes at different periods were sampled and the microbial community variation, functional genes and metabolic pathways were analyzed. The results showed phyla Firmicutes, Bacteroidetes and Proteobacteria predominated in all samples. While microbial community structures at genus level were significantly different, indicating selective enrichment during the 105-day process. The abundance of functional gene related to carbohydrate transport and metabolism was predicted higher on 45-day and 70-day. The metabolic pathway analysis revealed the degradation mechanisms and provided evidence for metabolic regulation.

Dual detection of biochemical oxygen demand and nitrate in water based on bidirectional Shewanella loihica electron transfer.

This study for the first time proposed a method for simultaneously measuring BOD and nitrate in water using electrochemically active bacteria. Firstly, the bidirectional extracellular electron transfer (EET) capability of a model electricigen Shewanella loihica PV-4 was revealed. Then, based on the respective outward and inward EET, S. loihica PV-4 was utilized to detection BOD and nitrate. The results demonstrated a positive correlation between the outward EET and BOD (from 0 mg/L to 435 mg/L) while a negative correlation between the inward EET and nitrate (from 0 mg/L to 7 mg/L); both the relationships were well fitted by the combination of traditional linear model and Michaelis-Menten model (R2>0.96). Finally, a dual detection method for BOD and nitrate measurements was established based on the ano-cathodophilic capability of S. loihica PV-4 biofilm, and exhibited the characteristics of high accuracy (>80%) and fast analysis (<1h), suggesting a promising prospect in water monitoring.

The effect of anode hydrodynamics on the sensitivity of microbial fuel cell based biosensors and the biological mechanism.

Fluid dynamics in the anodic chamber of a microbial fuel cell (MFC) is a key factor affecting the distribution of substrates and the efficiency of mass transport. However, the effect of hydrodynamics on MFC based biosensor (MFC-Biosensor) sensitivity has not been established. In this study, the three-dimension anode flow field of a two chamber MFC was visualized, and anodic configuration optimized by a reasonable serpentine flow field and inlet/outlet settings. Through optimization, the proportion of the dead zone in the anodic configuration decreased by 14.1%, and the velocity at the anode surface increased by 334.6% with better homogeneity of distribution. Moreover, electricity production and the sensitivity of MFC-Biosensors was improved by 42.0%, 46.1% and 52.3% for the detection of CTC, AVM and Hg, respectively. Biofilm viability analysis further proved that the enhanced surface velocity was of benefit for the permeation of toxicants into anodic biofilms, thus improving the sensor performance.

Effect of control mode on the sensitivity of a microbial fuel cell biosensor with Shewanella loihica PV-4 and the underlying bioelectrochemical mechanism.

A relatively poor sensitivity is a critical challenge for the application of microbial fuel cell biosensors (MFC-biosensors). This study investigated the effects of two control modes on sensor sensitivity and revealed the underlying bioelectrochemical mechanism. The results demonstrated that the sensitivity of an S. loihica PV-4 MFC-biosensor increased by 6.1 times when the anode was controlled at a constant potential (CP) instead of being operated with a fixed external resistance (ER). This obvious difference in sensor sensitivity was partly attributed to the masking effect of the observable offset current under ER mode and the lower electricity production capacity under CP mode. Moreover, the analysis of metabolic structure showed that under CP mode the anodic biofilm presented lower viability after toxic shock, due to a poorer ability to synthesize and secrete extracellular polymeric substances. Electrochemical measurements further revealed a lower capacitance under CP mode, which favored the permeation of Cd2+ into the biofilm.

Sequential flowing membrane-less microbial fuel cell using bioanode and biocathode as sensing elements for toxicity monitoring.

Traditional microbial fuel cell based biosensor (MFC-Biosensor) utilizes bioanode as sensing element and delivers high sensitivity for single toxic shock but it fails to alert the combined shock of organic matter (OM)/toxic agent (TA). To address this limitation, this study developed a sequential flowing membrane-less MFC based biosensor (SMFC-Biosensor) using both bioanode and biocathode for toxicity monitoring. Results demonstrated the shocks of 1.5 mg/L Hg2+, 1.0 mg/L avermectin and 1.0 mg/L chlortetracycline hydrochloride to SMFC-Biosensor led to inhibition ratios of 36%, 15% and 9%, which were over twice higher than those of bioanode-based and biocathode-based MFC-Biosensors. The viabilities of anodic and cathodic biofilms were both inhibited by the toxic shock. Besides, the excessive organic matters caused a decay in the SMFC-Biosensor current and consequently the OM/TA combined shock could be successfully monitored. This study for the first time testified the feasibility of simultaneously using bioanode and biocathode as sensing elements for toxicity monitoring.

Effect of external resistance on the sensitivity of microbial fuel cell biosensor for detection of different types of pollutants.

The relatively poor sensitivity is the main bottleneck restricting the application of microbial fuel cell biosensor (MFC-biosensor) for toxicity monitoring. Previous studies have shown that external resistance (Rext) had an obvious effect on sensor sensitivity. However, these studies reported different results and the reason of this discrepancy was not clear. The objective of this research was to observe the effect of Rext on sensor sensitivity when detecting different types of pollutants and reveal its microbiological mechanism. Results demonstrated that the optimal Rext of MFC-biosensor varied with the type of pollutants. The optimal values for detecting avermectins, tetracyclines and heavy metals were 100 Ω, 330â€¯Ω and 680 Ω, respectively. This discrepancy was mainly due to the visible differences in anodic microbial communities at different Rext settings. Both Azospirillum and Acinetobacter were susceptible to Cd and Pb, occuping 19.20% of the anodic microbial population in 680â€¯Ω MFC-biosensor. Pseudomonas accounted for 10.73% in 330â€¯Ω MFC-biosensor and possessed the sensitivity to tetracyclines. As for 100â€¯Ω MFC-biosensor, the avermectin-intolerant Ocillibacter made up 2.55% of the anodic microbial community. This study indicated that the Rext of MFC-biosensor should be optimized according to the potential pollutants.

Comparative analysis of microbial fuel cell based biosensors developed with a mixed culture and Shewanella loihica PV-4 and underlying biological mechanism.

Microbial fuel cell based biosensors (MFC-biosensors) utilize anode biofilms as biological recognition elements to monitor biochemical oxygen demand (BOD) and biotoxicity. However, the relatively poor sensitivity constrains the application of MFC-biosensors. To address this limitation, this study provided a systematic comparison of sensitivity between the MFC-biosensors constructed with two inocula. Higher biomass density and viability were both observed in the anode biofilm of the mixed culture MFC, which resulted in better sensitivity for BOD assessment. Compared with using mixed culture as inoculum, the anode biofilm developed with Shewanella loihica PV-4 presented lower content of extracellular polymeric substances and poorer ability to secrete protein under toxic shocks. Moreover, the looser structure in the S. loihica PV-4 biofilm further facilitated its susceptibilities to toxic agents. Therefore, the MFC-biosensor with a pure culture of S. loihica PV-4 delivered higher sensitivity for biotoxicity monitoring. This study proposed a new perspective to enhance sensor performance.

Urea removal coupled with enhanced electricity generation in single-chambered microbial fuel cells.

High concentration of total ammonia nitrogen (TAN) in the form of urea is known to inhibit the performance of many biological wastewater treatment processes. Microbial fuel cells (MFCs) have great potential for TAN removal due to its unique oxic/anoxic environment. In this study, we demonstrated that increased urea (TAN) concentration up to 3940 mg/L did not inhibit power output of single-chambered MFCs, but enhanced power generation by 67% and improved coulombic efficiency by 78% compared to those obtained at 80 mg/L of TAN. Over 80% of nitrogen removal was achieved at TAN concentration of 2630 mg/L. The increased nitrogen removal coupled with significantly enhanced coulombic efficiency, which was observed for the first time, indicates the possibility of a new electricity generation mechanism in MFCs: direct oxidation of ammonia for power generation. This study also demonstrates the great potential of using one MFC reactor to achieve simultaneous electricity generation and urea removal from wastewater.

Brush-like polyaniline nanoarray modified anode for improvement of power output in microbial fuel cell.

Carbon cloth with brush-like polyaniline (BL-PANI) nanowire arrays generated on the surface was utilized as anode material in this study to improve the power output of MFCs. A novel pulsed voltage method was applied to fabricate BL-PANI with PANI nanowires of ∼230nm of length. By using BL-PANI modified carbon cloth as anode, the power output was improved by 58.1% and 36.1% compared to that of plain carbon cloth and PANI modified carbon cloth with ordinary structure, respectively. Electrochemical tests revealed that both electron transfer resistance and charge transfer resistance were decreased owing to high specific area for microbes' growth and diffusion of charged species.

Element Cycling and Energy Flux Responses in Ecosystem Simulations Conducted at the Chinese Lunar Palace-1.

Bioregenerative life-support systems (BLSS) address interactions between organisms and their environment as an integrated system through the study of factors that regulate the pools and fluxes of materials and energy through ecological systems. As a simple model, using BLSS is very important in the investigation of element cycling and energy flux for sustainable development on Earth. A 105-day experiment with a high degree of closure was carried out in this system from February to May, 2014, with three volunteers. The results indicate that 247 g·d-1 carbon was imported into the system from stored food. Most hydrogen is circulated as water, and more than 99% H2O can be lost through leaf transpiration into the atmosphere. A total of 1.8 g·d-1 "unknown oxygen" emerged between the input and output of the plant growth module. For the urine processing module, 20.5% nitrogen was reused and 5.35 g·d-1 was put into the nutrient solution. Key Words: Bioregenerative life-support systems (BLSS)-Lunar Palace-1-Artificial ecosystem-Element cycling-Energy flux. Astrobiology 17, 78-86.