Trophic mode conversion and nitrogen deprivation of microalgae for high ammonium removal from synthetic wastewater

A fast microbial nitrogen-assimilation technology enhances nitrogen migration and single-cell-protein production in high-ammonia piggery wastewater

Conventional methods, such as freshwater dilution and ammonia stripping, have been widely employed for microalgae-based piggery wastewater (PW) treatment, but they cause high freshwater consumption and intensive ammonia loss, respectively. This present work developed a novel fast microbial nitrogen-assimilation technology by integrating nitrogen starvation, zeolite-based adsorption, pH control, and co-culture of microalgae-yeast for the PW treatment. Among them, the nitrogen starvation accelerated the nitrogen removal and shortened the treatment period, but it could not improve the tolerance level of microalgal cells to ammonia toxicity based on oxidative stress. Therefore, zeolite was added to reduce the initial total ammonia-nitrogen concentration to around 300 mg/L by ammonia adsorption. Slowly releasing ammonia at the later phase maintained the total ammonia-nitrogen concentration in the PW. However, the pH increase could cause lots of ammonia loss air and pollution and inhibit the desorption of ammonia from zeolite and the growth and metabolism of microalgae during the microalgae cultivation. Thus, the highest biomass yield (3.25 g/L) and nitrogen recovery ratio (40.31%) were achieved when the pH of PW was controlled at 6.0. After combining the co-cultivation of microalgae-yeast, the carbon-nitrogen co-assimilation and the alleviation of pH fluctuation further enhanced the nutrient removal and nitrogen migration to high-protein biomass. Consequently, the fast microbial nitrogen-assimilation technology can help update the industrial system for high-ammonia wastewater treatment by improving the treatment and nitrogen recovery rates.

A new microalgal negative carbon technology for landfill leachate treatment: Simultaneous removal of nitrogen and phosphorus

Replete with ammonia nitrogen and organic pollutants, landfill leachate typically undergoes treatment employing expensive and carbon-intensive integrated techniques. We propose a novel microalgae technology for efficient, low-carbon simultaneous treatment of carbon, nitrogen, and phosphorus in landfill leachate (LL). The microbial composition comprises a mixed microalgae culture with Chlorella accounting for 82.58%. After seven days, the process with an N/P ratio of approximately 14:1 removed 98.81% of NH4+-N, 88.62 % of TN, and 99.55% of TP. Notably, the concentrations of NH4+-N and TP met the discharge standards, while the removal rate of NH4+-N was nearly three times higher than previously reported in relevant studies. The microalgae achieved a removal efficiency of 64.27% for Total Organic Carbon (TOC) and 99.26% for Inorganic Carbon (IC) under mixotrophic cultivation, yielding a biomass of 1.18 g/L. The treatment process employed in this study results in a carbon emissions equivalent of -8.25 kgCO2/kgNremoved, representing a reduction of 33.56 kgCO2 compared to the 2AO + MBR process. In addition, shake flask experiments were conducted to evaluate the biodegradability of leachate after microalgae treatment. After microalgae treatment, the TOCB (Biodegradable Total Organic Carbon)/TOC ratio decreased from 56.54% to 27.71%, with no significant improvement in biodegradability. It establishes a fundamental foundation for further applied research in microalgae treatment of leachate.

Insight into the mechanism of nitrogen sufficiency conversion strategy for microalgae-based ammonium-rich wastewater treatment

The strategy of nitrogen sufficiency conversion can improve ammonium nitrogen (NH4+-N) removal with microalgal cells from ammonium-rich wastewater. We selected and identified one promising isolated algal strain, NCU-7, Chlorella sorokiniana, which showed a high algal yield and tolerance to ammonium in wastewater, as well as strong adaptability to N deprivation. The transition from N deprivation through mixotrophy (DN, M) to N sufficiency through autotrophy (SN, P) achieved the highest algal yields (optical density = 1.18 and 1.59) and NH4+-N removal rates (2.5 and 4.2 mg L-1 d-1) from synthetic wastewaters at two NH4+-N concentrations (160 and 320 mg L-1, respectively). Algal cells in DN, M culture obtained the lowest protein content (20.6%) but the highest lipid content (34.0%) among all cultures at the end of the stage 2. After transferring to stage 3, the lowest protein content gradually recovered to almost the same level as SN, P culture on the final day. Transmission electron microscopy and proteomics analysis demonstrated that algal cells had reduced intracellular protein content but accumulated lipids under N deprivation by regulating the reduction in synthesis of protein, carbohydrate, and chloroplast, while enhancing lipid synthesis. After transferring to N sufficiency, algal cells accelerated their growth by recovering protein synthesis, leading to excessive uptake of NH4+-N from wastewater. This study provides specific insights into a nitrogen sufficiency conversion strategy to enhance algal growth and NH4+-N removal/uptake during microalgae-based ammonium-rich wastewater treatment.

Microalgal-bacterial biofilms for wastewater treatment: Operations, performances, mechanisms, and uncertainties

Microalgal-bacterial biofilms have been increasingly considered of great potential in wastewater treatment due to the advantages of microalgal-bacterial synergistic pollutants removal/recovery, CO2 sequestration, and cost-effective biomass-water separation. However, such advantages may vary widely among different types of microalgal-bacterial biofilms, as the biofilms could be formed on different shapes and structures of attachment substratum, generating "false hope" for certain systems in large-scale wastewater treatment if the operating conditions and pollutants removal properties are evaluated based on the general term "microalgal-bacterial biofilm". This study, therefore, classified microalgal-bacterial biofilms into biofilms formed on 2D substratum, biofilms formed on 3D substratum, and biofilms formed without substratum (i.e. microalgal-bacterial granular sludge, MBGS). Biofilms formed on 2D substratum display higher microalgae fractions and nutrients removal efficiencies, while the adopted long hydraulic retention times were unacceptable for large-scale wastewater treatment. MBGS are featured with much lower microalgae fractions, most efficient pollutants removal, and acceptable retention times for realistic application, yet the feasibility of using natural sunlight should be further explored. 3D substratum systems display wide variations in operating conditions and pollutants removal properties because of diversified substratum shapes and structures. 2D and 3D substratum biofilms share more common in eukaryotic and prokaryotic microbial community structures, while MGBS biofilms are more enriched with microorganisms favoring EPS production, biofilm formation, and denitrification. The specific roles of stratified extracellular polymeric substances (EPS) in nutrients adsorption and condensation still require in-depth exploration. Nutrients removal uncertainties caused by microalgal-bacterial synergy decoupling under insufficient illumination, limited microbial community control, and possible greenhouse gas emission exacerbation arising from microalgal N2O generation were also indicated. This review is helpful for revealing the true potential of applying various microalgal-bacterial biofilms in large-scale wastewater treatment, and will provoke some insights on the challenges to the ideal state of synergistic pollutants reclamation and carbon neutrality via microalgal-bacterial interactions.

Potential disruptive effects of zoosporic parasites on peatland-based organic freshwater aquaculture: Case study from the Republic of Ireland

Irish freshwater aquaculture holds great potential for aiding food security. However, its necessary expansion has been hampered by the adoption of important environmental EU directives. A novel peatland-based recirculating aquaculture multi-trophic pond system (RAMPS) was developed to assess its potential to assist in the sustainable development of industry whilst remaining aligned with environmental protection by adhering to organic aquaculture practices. Microalgae play a pivotal role in the farms' wastewater bioremediation. However, a collapse of the algal population within the system towards the end of the pilot study was observed. No relationship between physicochemical fluctuations and the collapse were indicated. Further investigations into the potential presence of biological agents were then conducted and fourteen species of zoosporic parasites from five different genera (Labyrinthula, Vampyrella, Amoeboaphelidium, Paraphelidium and Aphelidium) were identified after conducting next-generation sequencing (MinION). The presence of these species indicated the potential cause of algal collapse. Additionally, changes in weather conditions may have also contributed to the issue. Given the lack of data available on zoosporic parasites and their potential impact on organic aquaculture practices, additional research needs to be conducted. Developing a means to monitor and mitigate against these complex zoosporic parasites will inform food security, it will particularly help safeguard "organic" freshwater aquaculture where there is a reliance on using natural-based approaches to address disease mitigation. This information will in turn inform the replication of this RAMPs system in peatlands internationally creating local employment in green technologies, as communities' transition away from burning peat as fossil fuel. Also, zoosporic parasites may reduce important microalgae in peatland-based culture ponds that serve as exceptional sequesters of carbon. Findings of this study will inform related research that focus on the emergence of microbial pathogens in local aquatic ecosystems brought on by variances in climate.

Use of next generation sequencing and bioinformatics for profiling freshwater eukaryotic microalgae in a novel peatland integrated multi-trophic aquaculture (IMTA) system: Case study from the Republic of Ireland

Development of integrated multi-trophic aquaculture (IMTA) systems constitutes a step change in the sustainable production of freshwater fish to meet emerging needs for high-protein foods globally. Recently, there has been a paradigm shift away from harvesting peat as a fuel towards the development of wettable peatland innovation (termed 'paludiculture'), such as aquaculture. Such eco-innovations support carbon sequestration and align with a balanced environmental approach to protecting biodiversity. This novel peatland-based IMTA process in the Irish midlands relies upon natural microalgae for waste treatment, recirculation and water quality where there is no use of pesticides or antibiotics. This novel IMTA system is powered with a wind turbine and the process has 'organic status'; moreover, it does not discharge aquaculture effluent to receiving water. However, there is a significant lack of understanding as to diversity of microalgae in this 'paludiculture'-based IMTA processes. This constitutes the first case study to use conventional microscopy combined with next-generation sequencing and bioinformatics to profile microalgae occurring in this novel IMTA system from pooled samples over a 12 month period in 2020. Conventional microscopy combined with classic identification revealed twenty genera of algae; with Chlorophyta and Charophyta being the most common present. However, algal DNA isolation, 16 s sequencing and bioinformatics revealed a combined total of 982 species from 341 genera across nine phyla from the same IMTA system, which emphasized a significant underestimation in the number and diversity of beneficial or potentially harmful algae in the IMTA-microbiome. These new methods also yield rich data that can be used by digital technologies to transform future monitoring and performance of the IMTA system for sustainability. The findings of this study align with many sustainability development goals of the United Nations including no poverty, zero hunger, good health and well-being, responsible consumption and production, climate change, and life below water.

Enhancing Algal Yield and Nutrient Removal from Anaerobic Digestion Piggery Effluent by an Integrated Process-Optimization Strategy of Fungal Decolorization and Microalgae Cultivation

The dark brown anaerobic digestion piggery effluent (ADPE) with a large amount of ammonium generally needs high dilution before microalgae cultivation due to its inhibiting effects on algal growth. Due to the strong decolorization of fungi by degrading organic compounds in wastewater, the process-optimization integrated strategy of fungal decolorization of ADPE and subsequent microalgae cultivation with ammonium-tolerant strain may be a more reliable procedure to reduce the dilution ratio and enhance algal biomass production, and nutrient removal from ADPE. This study determined a suitable fungal strain for ADPE decolorization, which was isolated and screened from a local biogas plant, and identified using 26s rRNA gene sequence analysis. Subsequently, ADPE was pretreated by fungal decolorization to make low-diluted ADPE suitable for the algal growth, and conditions of microalgae cultivation were optimized to achieve maximum algal yield and nutrient removal from the pretreated ADPE. The results showed one promising locally isolated fungal strain, Nanchang University-27, which was selected out of three candidates and identified as Lichtheimia ornata, presenting a high decolorization to ADPE through fungal pretreatment. Five-fold low-diluted ADPE pretreated by L. ornata was the most suitable medium for the algal growth at an initial concentration of ammonium nitrogen of 380 mg L−1 in all dilution treatments. Initial optical density of 0.3 and pH of 9.0 were optimal culture conditions for the algal strain to provide the maximum algal yield (optical density = 2.1) and nutrient removal (88%, 58%, 65%, and 77% for the removal rates of ammonium nitrogen, total nitrogen, total phosphorus, and chemical oxygen demand, respectively) from the pretreated ADPE. This study demonstrated that fungal decolorization and subsequent microalgae cultivation could be a promising approach to algal biomass production and nutrient removal from ADPE.

Immobilization altering the growth behavior, ammonium uptake and amino acid synthesis of Chlorella vulgaris at different concentrations of carbon and nitrogen

Nitrogen recycling by microalgae has aroused considerable attention. In this study, immobilized Chlorellavulgaris with 5-day mixotrophic cultivation to recover ammonium (NH₄⁺-N) were systematically investigated under various sodium acetate (CH₃COONa) and ammonium chloride (NH₄Cl) concentrations, and evaluated by comparison with suspended cells. The results revealed that, unlike suspended cells, NH₄⁺-N uptake by immobilized cells was not in direct proportion to chemical oxygen demand (COD) concentrations. The immobilized cells to NH₄⁺-N uptake was all inferior to that of suspended cells, presenting the maximum rate of 68.92% in group of 30 mg/L NH₄⁺-N and 200 mg/L COD. Free amino acids in immobilized cells such as glutamate (Glu), arginine (Arg), proline (Pro) and leucine (Leu) were more sensitive to NH₄⁺-N assimilation, as higher values observed by suspended cells. Low carbon-nitrogen (C/N) ratio showed remarkable benefits to amino acid synthesis. These results could provide a reference for manipulating the algal system and biomass accumulation.

Ammonium removal potential and its conversion pathways by free and immobilized Scenedesmus obliquus from wastewater

In this study, the immobilization with sodium alginate (SA) for cultivating microalgae in entrapped matrix gel beads was conducted for separating it from water. Batch experiments with a period of 5 days were carried out for free and immobilized Scenedesmus obliquus simultaneously under two trophic modes, to compare the removal performances of different initial ammonium (NH₄⁺-N) concentrations. In both free and immobilized form, the positive C-dependent effect in mixotrophy and the negative N-dependent effect in heterotrophy were observed. And the performances of immobilized form were all superior to that of free form, which showed greater tolerance to high concentration, maximally representing 96.6 ± 0.1% removal in 50 mg/L of NH₄⁺-N in mixotrophy. Assimilation of NH₄⁺-N was the main removal pathway resulting the protein synthesis with the dominant component including glutamic acid (Glu), cystine (Cys), arginine (Arg) and proline (Pro). The results demonstrated a systematic understanding for NH₄⁺-N removal in microalgae-based system.

A Review on the Use of Microalgae for Sustainable Aquaculture

Traditional aquaculture provides food for humans, but produces a large amount of wastewater, threatening global sustainability. The antibiotics abuse and the water replacement or treatment causes safety problems and increases the aquaculture cost. To overcome environmental and economic problems in the aquaculture industry, a lot of efforts have been devoted into the application of microalgae for wastewater remediation, biomass production, and water quality control. In this review, the systematic description of the technologies required for microalgae-assisted aquaculture and the recent progress were discussed. It deeply reviews the problems caused by the discharge of aquaculture wastewater and introduces the principles of microalgae-assisted aquaculture. Some interesting aspects, including nutrients assimilation mechanisms, algae cultivation systems (raceway pond and revolving algal biofilm), wastewater pretreatment, algal-bacterial cooperation, harvesting technologies (fungi-assisted harvesting and flotation), selection of algal species, and exploitation of value-added microalgae as aquaculture feed, were reviewed in this work. In view of the limitations of recent studies, to further reduce the negative effects of aquaculture wastewater on global sustainability, the future directions of microalgae-assisted aquaculture for industrial applications were suggested.

Carbon-dependent alleviation of ammonia toxicity for algae cultivation and associated mechanisms exploration

Ammonia toxicity in wastewater is one of the factors that limit the application of algae technology in wastewater treatment. This work explored the correlation between carbon sources and ammonia assimilation and applied a glucose-assisted nitrogen starvation method to alleviate ammonia toxicity. In this study, ammonia toxicity to Chlorella sp. was observed when NH₃-N concentration reached 28.03mM in artificial wastewater. Addition of alpha-ketoglutarate in wastewater promoted ammonia assimilation, but low utilization efficiency and high cost of alpha-ketoglutarate limits its application in wastewater treatment. Comparison of three common carbon sources, glucose, citric acid, and sodium bicarbonate, indicates that in terms of ammonia assimilation, glucose is the best carbon source. Experimental results suggest that organic carbon with good ability of generating energy and hydride donor may be critical to ammonia assimilation. Nitrogen starvation treatment assisted by glucose increased ammonia removal efficiencies and algal viabilities.

Microalgae-based advanced municipal wastewater treatment for reuse in water bodies

Reuse of secondary municipal effluent from wastewater treatment plants in water bodies could effectively alleviate freshwater resource shortage. However, excessive nutrients must be efficiently removed to prevent eutrophication. Compared with other means of advanced wastewater treatment, microalgae-based processes display overwhelming advantages including efficient and simultaneous N and P removal, no requirement of additional chemicals, O₂ generation, CO₂ mitigation, and potential value-added products from harvested biomass. One particular challenge of microalgae-based advanced municipal wastewater treatment compared to treatment of other types of wastewater is that concentrations of nutrients and N:P ratios in secondary municipal effluent are much lower and imbalanced. Therefore, there should be comprehensive considerations on nutrient removal from this specific type of effluent. Removal of nutrients and organic substances, and other environmental benefits of microalgae-based advanced municipal wastewater treatment systems were summarized. Among the existing studies on microalgal advanced nutrient removal, much information on major parameters is absent, rendering performances between studies not really comparable. Mechanisms of microalgae-based nitrogen and phosphorus removal were respectively analyzed to better understand advanced nutrient removal from municipal secondary effluent. Factors influencing microalgae-based nutrient removal were divided into intrinsic, environmental, and operational categories; several factors were identified in each category, and their influences on microalgal nutrient removal were discussed. A multiplicative kinetic model was integrated to estimate microalgal growth-related nutrient removal based majorly on environmental and intrinsic factors. Limitations and prospects of future full-scale microalgae-based advanced municipal wastewater treatment were also suggested. The manuscript could offer much valuable information for future studies on microalgae-based advanced wastewater treatment and water reuse.

Application of nitrogen sufficiency conversion strategy for microalgae-based ammonium-rich wastewater treatment

Ammonium ([Formula: see text]-N)-rich wastewater, a main cause for eutrophication, can serve as a promising medium for fast microalgae cultivation with efficient [Formula: see text]-N removal. To achieve this goal, a well-controlled three-stage treatment process was developed. Two trophic modes (mixotrophy and heterotrophy) in Stage 1 and Stage 2, with two nitrogen availability conditions (N sufficient and N deprived) in Stage 2, and different [Formula: see text]-N concentrations in Stage 3 were compared to investigate the effects of nitrogen sufficiency conversion on indigenous strain UMN266 for [Formula: see text]-N removal. Results showed that mixotrophic cultures in the first two stages with N deprivation in Stage 2 was the optimum treatment strategy, and higher [Formula: see text]-N concentration in Stage 3 facilitated both microalgal growth and [Formula: see text]-N removal, with average and maximum biomass productivity of 55.3 and 161.0 mg L(-1) d(-1), and corresponding removal rates of 4.2 and 15.0 mg L(-1) d(-1), respectively, superior to previously published results. Observations of intracellular compositions confirmed the optimum treatment strategy, discovering excellent starch accumulating property of strain UMN266 as well. Combination of bioethanol production with the proposed three-stage process using various real wastewater streams at corresponding stages was suggested for future application.