Urban wastewater photobiotreatment with microalgae in a continuously operated photobioreactor: growth, nutrient removal kinetics and biomass coagulation-flocculation

Literature review on the potential of urban waste for the fertilization of urban agriculture: A closer look at the metropolitan area of Barcelona

Urban agriculture (UA) activities are increasing in popularity and importance due to greater food demands and reductions in agricultural land, also advocating for greater local food supply and security as well as the social and community cohesion perspective. This activity also has the potential to enhance the circularity of urban flows, repurposing nutrients from waste sources, increasing their self-sufficiency, reducing nutrient loss into the environment, and avoiding environmental cost of nutrient extraction and synthetization. The present work is aimed at defining recovery technologies outlined in the literature to obtain relevant nutrients such as N and P from waste sources in urban areas. Through literature research tools, the waste sources were defined, differentiating two main groups: (1) food, organic, biowaste and (2) wastewater. Up to 7 recovery strategies were identified for food, organic, and biowaste sources, while 11 strategies were defined for wastewater, mainly focusing on the recovery of N and P, which are applicable in UA in different forms. The potential of the recovered nutrients to cover existing and prospective UA sites was further assessed for the metropolitan area of Barcelona. Nutrient recovery from current composting and anaerobic digestion of urban sourced organic matter obtained each year in the area as well as the composting of wastewater sludge, struvite precipitation and ion exchange in wastewater effluent generated yearly in existing WWTPs were assessed. The results show that the requirements for the current and prospective UA in the area can be met 2.7 to 380.2 times for P and 1.7 to 117.5 times for N depending on the recovery strategy. While the present results are promising, current perceptions, legislation and the implementation and production costs compared to existing markets do not facilitate the application of nutrient recovery strategies, although a change is expected in the near future.

Influence of pH and dissolved oxygen control strategies on the performance of pilot-scale microalgae raceways using fertilizer or wastewater as the nutrient source

Dissolved oxygen concentration and pH are controllable and cost-effective variables that determine the success of microalgae-related processes. The present study compares different control strategies for pH and dissolved oxygen in pilot-scale microalgae production systems. Two 80 m2 raceway reactors were used, one operated with freshwater plus fertilizer and the other with wastewater as the nutrient source. Both were in semi-continuous mode at a fixed dilution rate of 0.2 day-1. A comparison between the classical On-Off and more advanced pH control strategies, such as PI and Event-based control, was performed, focusing on biomass productivity and the influence of all the process parameters on microalgae growth; "No control" of pH was also assayed. The results show that Event-based control was the best algorithm when using freshwater plus fertilizer. In contrast, no significant differences were observed using the different control strategies when wastewater was the nutrient source. These experiments were performed through selective control strategy, prioritizing pH over dissolved oxygen; however, it was demonstrated that they did not allow to achieve satisfactory dissolved oxygen removal results, especially for the fertilizer system. After modifying the gas diffuser configuration and improving the mass transfer, independent on-off strategies have been developed, permitting effective control of both variables and increasing productivity by up to 20% in both systems. Concluding, a detailed analysis of the energy demand for each strategy implemented in terms of gas consumption and gas flow to biomass ratio is provided.

Wastewater substrate disinfection for cyanobacteria cultivation as tertiary treatment

Cultivation of microalgae or/and cyanobacteria in nutrient-rich wastewaters offers an opportunity for enhancing sustainability of tertiary wastewater treatment processes via resources/energy recovery/production, mitigation of emitted GHGs and provision of added value products. However, maintaining a monoculture in wastewater-media constitutes a significant challenge to be addressed. In this regard, the present work assesses the efficiency of the low-cost wastewater substrate disinfection techniques of filtration, use of NaClO, H₂O₂ or Fe(VI), as a preliminary treatment stage upstream a cyanobacteria cultivation photobioreactor. The growth rate of cyanobacterium Synechococcus elongatus PCC 7942, and nitrate and phosphate removal rates, were experimentally assessed in cultivation setups with biologically treated dairy wastewater that had been subjected to a single or a synergetic couple of disinfection techniques. The results showed that filter thickness has a greater effect on disinfection efficiency than filter pore size. Furthermore, the disinfection efficiency of Fe(VI), which was produced on-site by electrosynthesis via a Fe⁰/Fe⁰ cell, was greater than that of NaClO and H₂O₂. Filtration at ≤ 1.2-μm pore size coupled with chemical disinfection led to unhindered Synechococcus elongatus PCC 7942 growth and efficient nitrate and phosphate removal rates, at dosages, in terms of Concentreation-Time (CT) product, of CT ≥ 270 mg min L^-1 for NaClO and CT ≥ 157 mg min L^-1 for Fe(VI). The coagulation action of Fe(III) species that result from Fe(VI) reduction and the oxidation action of Fe(VI) can assist in turbidity, organic compounds and phosphorous removal from wastewater media. Moreover, the residual iron species can assist in Synechococcus elongatus PCC 7942 harvesting and may enhance photosynthesis rate by increasing light transfer efficiency. Thus, a filtration configuration coupled with chemical disinfection, preferably using ferrates, downstream of sedimentation tank of a secondary biological wastewater treatment stage is proposed as a necessary, efficient and low-cost disinfection technique for full-scale scale implementation of cyanobacteria cultivation as tertiary wastewater processes.

Physiological and transcription level responses of microalgae Auxenochlorella protothecoides to cold and heat induced oxidative stress

Temperature is a crucial factor affecting microalgae CO₂ capture and utilization. However, an in-depth understanding of how microalgae respond to temperature stress is still unclear. In particular, the regulation mechanism under opposite temperature (heat and cold) stress had not yet been reported. In this study, the physicochemical properties and transcription level of related genes of microalgae Auxenochlorella protothecoides UTEX 2341 under heat and cold stress were investigated. Heat stress (Hs) caused a drastic increase of reactive oxygen species (ROS) in UTEX 2341. As key elements responded to Hs, superoxide dismutase (SOD) enzyme increased by 150%, 70%, and 30% in activity, and nitric oxide (NO) grew by 409.6%, 212.5%, and 990.4% in content compared with the control at 48 h, 96 h, 168 h. Under cold stress (Cs), ROS increased in the early stage and decreased in the later stage. As key factors responded to Cs, proline (Pro) increased respectively by 285%, 383%, and 81% in content, and heat shock transcriptional factor HSFA1d increased respectively by 161%, 71%, and 204% in transcript level compared with the control at 48 h, 96 h, 168 h. Furthermore, the transcript level of antioxidant enzymes or antioxidant coding genes was consistent with the changing trend of enzymes activity or antioxidant content. Notably, both glutathione (GSH) and heat shock protein 97 (hsp 97) were up-regulated in response to Hs and Cs. In conclusion, GSH and hsp 97 were the core elements of UTEX 2341 in response to both Hs and Cs. SOD and NO were the key elements that responded to Hs, while proline and HSFA1d were the key elements that responded to Cs. This study provided a basis for the understanding of the response mechanism of microalgae under temperature stress and the improvement of the microalgae tolerance to temperature stress.

Respirometric assessment of bacterial kinetics in algae-bacteria and activated sludge processes

Algae-bacteria (AB) consortia can be exploited for effective wastewater treatment, based on photosynthetic oxygenation to reduce energy requirements for aeration. While algal kinetics have been extensively evaluated, bacterial kinetics in AB systems are still based on parameters taken from the activated sludge models, lacking an experimental validation for AB consortia. A respirometric procedure was therefore proposed, to estimate bacterial kinetics in both activated sludge and AB, under different conditions of temperature, pH, dissolved oxygen, and substrate availability. Bacterial activities were differently influenced by operational/environmental conditions, suggesting that the adoption of typical activated sludge parameters could be inadequate for AB modelling. Indeed, respirometric results show that bacteria in AB consortia were adapted to a wider range of conditions, compared to activated sludge, confirming that a dedicated calibration of bacterial kinetics is essential for effectively modelling AB systems, and respirometry was proven to be a powerful and reliable tool to this purpose.

Mass cultivation and harvesting of microalgal biomass: Current trends and future perspectives

Microalgae are unicellular photosynthetic organisms capable of producing high-value metabolites like carbohydrates, lipids, proteins, polyunsaturated fatty acids, vitamins, pigments, and other high-value metabolites. Microalgal biomass gained more interest for the production of nutraceuticals, pharmaceuticals, therapeutics, food supplements, feed, biofuel, bio-fertilizers, etc. due to its high lipid and other high-value metabolite content. Microalgal biomass has the potential to convert trapped solar energy to organic materials and potential metabolites of nutraceutical and industrial interest. They have higher efficiency to fix carbon dioxide (CO₂) and subsequently convert it into biomass and compounds of potential interest. However, to make microalgae a potential industrial candidate, cost-effective cultivation systems and harvesting methods for increasing biomass yield and reducing the cost of downstream processing have become extremely urgent and important. In this review, the current development in different microalgal cultivation systems and harvesting methods has been discussed.

A Review about Microalgae Wastewater Treatment for Bioremediation and Biomass Production—A New Challenge for Europe

Microalgae have received much attention in the last few years. Their use is being extended to different fields of application and technologies, such as food, animal feed, and production of valuable polymers. Additionally, there is interest in using microalgae for removal of nutrients from wastewater. Wastewater treatment with microalgae allows for a reduction in the main chemicals responsible for eutrophication (nitrogen and phosphate), the reduction of organic substrates (by decreasing parameters such as BOD and COD) and the removal of other substances such as heavy metals and pharmaceuticals. By selecting and reviewing 202 articles published in Scopus between 1992 and 2020, some aspects such as the feasibility of microalgae cultivation on wastewater and potential bioremediation have been investigated and evaluated. In this review, particular emphasis was placed on the different types of wastewaters on which the growth of microalgae is possible, the achievable bioremediation and the factors that make large-scale microalgae treatment feasible. The results indicated that the microalgae are able to grow on wastewater and carry out effective bioremediation. Furthermore, single-step treatment with mixotrophic microalgae could represent a valid alternative to conventional processes. The main bottlenecks are the large-scale feasibility and costs associated with biomass harvesting.

Microalgal Systems for Wastewater Treatment: Technological Trends and Challenges towards Waste Recovery

Wastewater (WW) treatment using microalgae has become a growing trend due the economic and environmental benefits of the process. As microalgae need CO2, nitrogen, and phosphorus to grow, they remove these potential pollutants from wastewaters, making them able to replace energetically expensive treatment steps in conventional WW treatment. Unlike traditional sludge, biomass can be used to produce biofuels, biofertilizers, high value chemicals, and even next-generation growth media for “organically” grown microalgal biomass targeting zero-waste policies and contributing to a more sustainable circular bioeconomy. The main challenge in this technology is the techno-economic feasibility of the system. Alternatives such as the isolation of novel strains, the use of native consortia, and the design of new bioreactors have been studied to overcome this and aid the scale-up of microalgal systems. This review focuses on the treatment of urban, industrial, and agricultural wastewaters by microalgae and their ability to not only remove, but also promote the reuse, of those pollutants. Opportunities and future prospects are discussed, including the upgrading of the produced biomass into valuable compounds, mainly biofuels.

Use of ultrasound for aiding lipid extraction and biodiesel production of microalgae harvested by chitosan

In this work, chitosan, a biodegradable flocculant, was investigated to determine its utility in flocculating microalgae, its effect on cell integrity, and its impact on lipid extraction and the conversion to fatty acid methyl ester (FAME). Results showed that chitosan adequately performed flocculation on Chlorella vulgaris microalgae and achieved a high harvesting efficiency of 96.35 ± 1.96% when implemented under the following conditions: chitosan dose = 120 mg/L^-1, pH = 5, mixing speed = 150 rpm for 20 min, followed by 10 min of settling time. Moreover, scanning electron microscope (SEM) combined with transmission electron microscope (TEM) demonstrated that chitosan protected the cells' structure from morphological damage. Finally, the highest lipid extraction yield and biodiesel production was obtained from the chitosan-harvested biomass when the microalgae were pretreated with ultrasound.

Algae-mediated processes for the treatment of antiretroviral drugs in wastewater: Prospects and challenges

The prevalence of pharmaceuticals (PCs), especially antiretroviral (ARV) drugs in various aquatic ecosystems has been expansively reported, wherein wastewater treatment plants (WWTPs) are identified as the primary point source. Consequently, the occurrence, ecotoxicity and treatment of ARV drugs in WWTPs have drawn much attention in recent years. Numerous studies have shown that the widely employed activated sludge-based WWTPs are incapable of removing ARV drugs efficiently from wastewater. Recently, algae-based wastewater treatment processes have shown promising results in PCs removal from wastewater, either completely or partially, through different processes such as biosorption, bioaccumulation, and intra-/inter-cellular degradation. Algal species have also shown to tolerate high concentrations of ARV drugs than the reported concentrations in the environmental matrices. In this review, emphasis has been given on discussing the current status of the occurrence of ARV drugs in the aquatic environment and WWTPs. Besides, the current trends and future perspectives of PCs removal by algae are critically reviewed and discussed. The potential pathways and mechanisms of ARV drugs removal by algae have also been discussed.

Bio-products from algae-based biorefinery on wastewater: A review

Increasing resource demand, predicted fossil resources shortage in the near future, and environmental concerns due to the production of greenhouse gas carbon dioxide have motivated the search for alternative 'circular' pathways. Among many options, microalgae have been recently 'revised' as one of the most promising due to their high growth rate (with low land use and without competing with food crops), high tolerance to nutrients and salts stresses and their variability in biochemical composition, in so allowing the supply of a plethora of possible bio-based products such as animal feeds, chemicals and biofuels. The recent raising popularity of Circular Bio-Economy (CBE) further prompted investment in microalgae, especially in combination with wastewater treatment, under the twofold aim of allowing the production of a wide range of bio-based products while bioremediating wastewater. With the aim of discussing the potential bio-products that may be gained from microalgae grown on urban wastewater, this paper presents an overview on microalgae production with particular emphasis on the main microalgae species suitable for growth on wastewater and the obtainable bio-based products from them. By selecting and reviewing 76 articles published in Scopus between 1992 and 2020, a number of interesting aspects, including the selection of algal species suitable for growing on urban wastewater, wastewater pretreatment and algal-bacterial cooperation, were carefully reviewed and discussed in this work. In this review, particular emphasis is placed on understanding of the main mechanisms driving formation of microalgal products (such as biofuels, biogas, etc.) and how they are affected by different environmental factors in selected species. Lastly, the quantitative information gathered from the articles were used to estimate the potential benefits gained from microalgae grown on urban wastewater in Campania Region, a region sometimes criticized for poor wastewater management.

Analysis of the Status and Improvement of Microalgal Phosphorus Removal from Municipal Wastewater

Phosphorus, as one of the main pollutants in municipal sewage, has received increasing attention recently. Phosphorus recovery also increases the sustainable development of municipal wastewater. Since algae have the ability to effectively redirect nutrients, including phosphorus, from municipal sewage to algae biomass, municipal sewage treatments involving microalgae have piqued the interest of many researchers. The phosphorus removal depends on the potential of the microalgae to absorb, preserve, or degrade phosphorus in municipal wastewater. It is, therefore, of great interest to study the mechanisms underlying the absorption, storage, and degradation of phosphorus by microalgae to ensure the viability of this phosphorus removal process in wastewater. The objectives of this review were to summarize phosphorus metabolism in microalgae, examine key external and internal factors impacting phosphorous removal by microalgae from wastewater, and examine the status of phosphorous-metabolism-related research to improve our understanding of microalgae-based municipal wastewater treatments. In addition, the methods of recovery of microalgae after phosphorous removal were summarized to ensure the sustainability of municipal wastewater treatment. Finally, a potential approach using nanomaterials was proposed to enhance the overall phosphorous removal performance in municipal wastewater through the addition of nanoparticles such as magnesium and iron.

Microalgae as promising source for integrated wastewater treatment and biodiesel production

Microalgae have been studied for their potential of wastewater treatment as well as a promising source for biodiesel production. This study investigates the potential of microalgae to remove nutrients from domestic wastewater (DWW) while producing lipids-rich biomass for biodiesel production. Eight microalgae were cultivated in (DWW) to evaluate their nutrients removal capacity and biomass production. Total phosphorus (TP) of DWW reduced from 2 mg L^-1 to 0.02 mg L^-1 with the treatment efficiency of 99.15% and the highest performance was noted in Chlamydomonas reinhardtii (C. reinhardtii). For total nitrogen (TN), treatment efficiency climbed to 99.07%. It is reduced from 18.35 to 0.17 mg L^-1 recorded in C. reinhardtii and Chlorella pyrenoidosa (C. pyrenoidosa). On the other hand, all microalgae showed a high lipids-rich biomass in wastewater compared to BG11. The highest lipid content was 36.93% noted in Chlorella sorokiniana (C. sorokiniana). Fatty acids methyl ester (FAME) profiles showed a high content of palmitic C16:0, oleic C18:1 and stearic acids C18:0 in studied microalgae strains. In summary, microalgae envisage its potential application in integrated wastewater treatment and biodiesel production. In perspective, the authors focus on the validation of this bioprocess in pilot scale. Furthermore, the use of microalgae for other applications such CO₂ biosequestration and added value products. Novelty statement: The present study investigates the potential of Moroccan microalgae as candidates to wastewater remediation and high biomass production with high lipid rate for biodiesel production.

Microalgae based wastewater treatment: a shifting paradigm for the developing nations

Decreased water quality in freshwater resources due to untreated or partially treated wastewater disposal resulting in eutrophication has led to water scarcity. Hence, the present work was aimed to determine the effectiveness of Chlorella vulgaris for municipal wastewater treatment in terms of various physico-chemical parameters and nutrient removal. Primary treated effluent was collected from a sewage treatment plant as an influent for the study. Parameters analyzed during the lab-scale batch study of 7 hours of detention time were pH, EC, TDS, TSS, TS, COD, phosphate, ammonia, nitrate and DO. Removal efficiency reached 98.32, 97.26 and 84.71% for phosphate, ammonia and COD, respectively, for non-filtered effluents. However, filtered effluent removal efficiency reached 98.53, 98.63 and 89.41% for phosphate, ammonia and COD, respectively. The study revealed that microalgal treatment, if incorporated in conventional wasteater treatment, can be a solution to the limitations of the activated sludge process. It could be a promising technique for low income and developing countries, which could efficiently reduce the effluent concentration to much lesser than the desirable limits in an eco-friendly and cost-effective way. Statement of novelty Municipal wastewater treatment in most developing countries is confined to aerobic secondary treatments, which are costly and are not efficient in removing nutrients from the treated effluents before discharging and leading to the imbalance and eutrophication in the receiving bodies. Hence in this study, an attempt was made to study the effectiveness of Chlorella vulgaris for wastewater treatment at a detention time of 7 hours without any external aeration. The present study revealed that microalgae have efficiently removed organics and nutrients to much lesser than the desirable limit. Thus, if the Chlorella vulgaris is introduced in the wastewater treatment system can reduce the nutrients and organics concentrations without the need for aeration, which can be an energy-saving and cost-effective approach.

Optimization of microalgae harvesting by sedimentation induced by high pH

Microalgae harvesting is a major hurdle for the production of high-value microalgal bioproducts on a large scale. Among harvesting techniques, pH-induced sedimentation stands out as an inexpensive and technically viable method. Nevertheless, there is little information available on the application of this method for microalgae cultivated in wastewater. In this context, the present study investigated the optimization of sedimentation parameters for Chlorella sorokiniana harvesting from wastewater. Parameter optimization was statistically determined by the response surface methodology. The optimal values included a velocity gradient of 250 s^-1, mixing time of 10 seconds, and pH of 12, which enabled microalgae harvesting efficiencies of more than 97.8%. These optimal parameters also showed resilience through the physico-chemical variation of the photobioreactor effluent. Furthermore, wastewater quality improved significantly after microalgae harvesting. High removal was found for turbidity (97.9-98.3%), apparent color (92.2-97.2%), total Kjeldhal nitrogen (91.0-94.4%), and total phosphorus (92.8-98.6%). Centrifugation, as the dewatering method, and its operational parameters were also evaluated. Sedimentation followed by centrifugation increased the initial microalgae concentration by about 123 times. This study shows the importance of operational optimization and the results can be used as practical guidelines for microalgae harvesting on a large scale.

Microalgae harvesting from wastewater by pH modulation and flotation: Assessing and optimizing operational parameters

Microalgae harvesting is one of the major bottlenecks for the production of high-value microalgal products on a large scale, which encourages investigations of harvesting methods with better cost-benefits. Among these harvesting techniques, flotation stands out as a promising method, however it is still minimally explored when compared to the sedimentation method. In this study, the pH modulation followed by dissolved air flotation (DAF) was tested as a harvesting method for Chlorella sorokiniana cultivated in wastewater. The main aims of this study were to optimize the operational parameters of coagulation (pH, velocity gradient, and mixing time) and flotation (recirculation rate), check their reproducibility and resilience with the variability of wastewater characteristics, and evaluate the final wastewater quality after treatment using an optimized harvesting method. Parameter optimization was carried out using the one-factor-at-a-time method. The optimal parameters were a velocity gradient of 500 s^-1, mixing time of 30 s, pH 12, and 20% of recirculation rate. High efficiencies were obtained for C. sorokiniana removal (96.5-97.9%), making it a successful process. Moreover, the photobioreactor effluent quality was also improved significantly after microalgae harvesting, with high nutrient removal (88.6-95.1% of total Kjeldahl nitrogen and 91.8-98.3% of total phosphorus) and organic matter removal (80.5-86.8% of chemical oxygen demand). The results showed the pH modulation and DAF as an effective process for wastewater treatment and biomass harvesting. This study also indicated the importance of operational optimization, not studied until now, in which the achieved results could be potentially applied as practical guidelines for microalgae harvesting on a large scale.