WASTEWATER TREATMENT USING MICROALGAE: А MINI REVIEW

ABSTRACT

The use of microalgae 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 (biochemical oxygen demand) and COD (chemical oxygen demand) and the removal of other substances such as heavy metals and pharmaceuticals. In this review, special attention is paid to the post-treatment of wastewater from nutrients: nitrogen and phosphorus with the participation of microalgae.

АННОТАЦИЯ

Использование микроводорослей распространяется на различные области применения и технологий, такие как продукты питания, корма для животных и производство ценных полимеров. Кроме того, существует интерес к использованию микроводорослей для удаления питательных веществ из сточных вод. Очистка сточных вод микроводорослями позволяет снизить содержание основных химических веществ, ответственных за эвтрофикацию (азота и фосфатов), уменьшить количество органических субстратов, путем снижения таких параметров, как БПК (биохимическая потребность в кислороде) и ХПК (химическое потребление кислорода) и удалить другие вещества, такие как тяжелые металлы и фармацевтические препараты. В этом обзоре особое внимание уделяется доочистке сточных вод от биогенных элементов: азота и фосфора с участием микроводорослей.

Keywords: microalgae, wastewater, biosorption, biological treatment, nitrogen, phosphorus, elements

Ключевые слова: микроводоросли, сточные воды, биосорбция, биологическая очистка, азот, фосфор, элементы

Introduction

Water is one of the most important natural resources on our planet. However, in addition to an inadequate clean water supply in many developing countries, water quality in industrialized nations has reached a worrying state [1, 2]. Currently, the demand for fresh water is increasing sharply due to the growth of world population, food production and industry, hence this is leading to an increase in wastewater produced from both domestic and industrial sources. Wastewater is known to contain a wide range of organic and inorganic compounds that can cause huge problems for the environment. In this context, microalgae are promising organisms for wastewater treatment, as they are able to reduce the amount of nitrogen and phosphates, as well as other toxic compounds, including pesticides, heavy metals or pharmaceuticals [3]. Wastewater constitutes various contaminants and pollutants, involving nutrients such as phosphorus and nitrogen, and heavy metals such as lead and zinc, which are of emerging concern; furthermore, it has been reported that in 2020, emissions of total phosphorus and total nitrogen reached 336,700 tons and 3,223,400 tons, respectively. In addition, emissions of chemical oxygen demand (COD) were five times greater in 2020 than in 2019, extending to 25.6476 MT, and the overall discharge of heavy metals reached around 26,680 kg [3, 4]. These pollutants can have dire consequences for the environment and for ecosystems into which they are discharged. Some pollutants, mainly those of organic nature, are generally degradable (either naturally or with the help of microorganisms) and therefore do not cause major problems for the environment [5]. Most of the ECs lack any regulatory standards since many hypotheses regarding ECs appear to be unfounded [6]. The various conventional methods for waste water treatment are present since the ancient time but they are very costly and not economical. So, the new green technical methods are being introduced to overcome the conventional methods of waste water treatment. The present study is related with new green technical methods which are proving them to be superior over the conventional methods; out of them low-cost waste water treatment using microalgae is the potential one. Photosynthetic microalgae require less energy as they use sunlight as an energy source and at the same time reduce the carbon footprint of the overall purification process [7].

literature survey was done to find out the new methods of waste water treatment using microalgae and their development till now and to find out its application in management of natural water resources.

Nutrient removal capability of Microalgae

A major requirement in waste water treatment is the removal of nutrients and toxic metals to acceptable limits prior to discharge and reuse [8]. Algae are autotrophs, i.e. they can synthesize organic molecules themselves from inorganic nutrients. A stoichiometric formula for the most common elements in an average algal cell is C₁₀₆H₁₈₁O₄₅N₁₆P, and the element should be present in these proportions in the medium for optimal growth [9]. The rate at which an algal cell takes up a specific nutrient depends on the difference between the concentration inside and outside the cell, and also on the diffusion rates through the cell wall [10]. Microalgae have unique characteristics of CO₂ sequestration for the production of biofuels which allow them to be potentially utilized in broad and versatile ways in climate change technologies [11]. Microalgae have been proven to be efficient in removing nitrogen, phosphorus, and toxic metals from a wide variety of waste waters [12]. There are extensive studies of algae growth in municipal [13] agricultural and industrial waste waters [14, 15]. Substantial amounts of nutrient removal and algae biomass production were obtained in these studies. Hence, controlled microalgae cultivation shows promise as a potential biological treatment method for waste water [16]. This integrated waste water treatment and biofuel production system can thus benefit the community as well as the environment [17].

Nitrogen Removal

Nitrogen is a critical nutrient required in the growth of all organisms. Organic nitrogen is found in a variety of biological substances, such as peptides, proteins, enzymes, chlorophylls, energy transfer molecules (ADP, ATP), and genetic materials (RNA, DNA). Organic nitrogen is derived from inorganic sources including nitrate (NO₃), nitrite (NO₂), nitric acid (HNO₃), ammonium (NH₄), ammonia (NH₃), and nitrogen gas (N₂). Micro-algae play a key role in converting inorganic nitrogen to its organic form through a process called assimilation. In addition, cyanobacteria are capable of converting atmospheric nitrogen into ammonia by means of fixation [17, 18].

Microalgae are capable of converting nutrients (nitrogen) from wastewater into biomass and bioproducts, thereby increasing the sustainability of wastewater treatment. In algae ponds, biomass productivity and water treatment efficiency are highly dependent on environmental parameters such as temperature, light intensity and photoperiod. Research by Delgadillo-Mirquez et al., [19] reported that temperature affected the growth rate and biomass production of microalgae, as well as the rate of ammonium removal. At temperatures of 15 and 25°C, the average degree of total nitrogen removal ranged from 72 to 83%. In recent years, microalgae have been in the spotlight as an alternative biological wastewater treatment system with several applications in wastewater treatment. Microalgae provide the ability to remove pollutants (nitrogen, phosphorus and carbon) from wastewater to produce biomass, which can be used to produce valuable chemicals (algal metabolites) and/or biogas through anaerobic digestion [20]. In addition, microalgae can reduce the harmful effects of wastewater and reduce eutrophication of the aquatic environment [21]. Wang et al., [22] reported a decrease in nitrogen content (83% N as NH₄⁺) in municipal wastewater caused by Chlorella sp. They suggested that the rate of nutrient removal is independent of the optimal ratio of contaminants, but that the concentrations of these nutrients are important for algal growth systems. The net removal of pollutants in these systems is mainly the additive effect of assimilation by algae [23], biological processes (nitrification/denitrification), and removal phenomena such as ammonia volatilization. Thus, the efficiency of nutrient uptake by an algal consortium depends not only on the bioavailability of nutrients, but also on complex interactions between physicochemical factors such as pH, light intensity, photoperiod, temperature and biological factors. In studies by Larsdotter [24], nitrogen removal was achieved primarily through nitrate assimilation into algal biomass and removal efficiencies of about 40% (nitrate) could be achieved for most of the year, although nitrogen removal efficiencies were highly variable. However, up to 60% - 80% could be achieved in summer in shallow water crops. Final total nitrogen removal of up to 40% was observed in shallow water crops in summer, which was most likely a consequence of zooplankton grazing and subsequent excretion of urea and ammonia volatilization resulting from high pH values.

Phosphorous Removal

Phosphorus is also a key factor in the energy metabolism of algae and is found in nucleic acids, lipids, proteins, and the intermediates of carbohydrate metabolism. Inorganic phosphates play a significant role in algae cell growth and metabolism. Phosphates are transferred by energized transport across the plasma membrane of the algal cell. Not only are inorganic forms of phosphorus utilized by microalgae, but some varieties of algae are able to use the phosphorus found in organic esters for growth [25].

If the wastewaters are directly released into the environment without any effective treatment, such toxic pollutants will not only harm aquatic life, but will also risk human health [26]. It is estimated that by 2030 the world will be faced with a 40% water shortage in existing water resources such as rivers, lakes and glaciers [27]. In light of the above-mentioned problems, development of an efficient and sustainable wastewater treatment system is of the utmost importance. Presently, the major wastewater treatment methods include physical methods (filtration screening, sedimentation, membrane filtration), chemical methods (ion exchange, chemical precipitation, electrochemical treatment, adsorption) and biological methods (bio precipitation, biosorption, biological activated sludge) [29]. The positives of these methods for treating wastewater are very well known, but the methods are also associated with certain limitations, such as irregular removal efficacy, and the uneconomical and increased cost of installation, which can increase the problem of successive treatment, resulting in secondary pollution. Nutrient pollution is a great environmental issue and challenge. Microalgal cells absorb N and P from wastewater and use these nutrients to produce biomass and the removal of one nutrient depends on the availability of the other. At high N supply, the concentration of P in the microalgal biomass was a function of the supply of P [30]. It should be noted that phosphorus (P) emissions from anthropogenic activities contribute to the eutrophication of aquatic organisms in the ecosystem. For example, in the UK, the main sources of phosphorus entering rivers are sewage and agricultural run-off [31], with up to 70% coming from wastewater discharges [32].

Mechanisms of algal P removal P is an essential nutrient for algal growth and under certain conditions, algae will uptake P in excess of growth requirements [24, 33]. Under such circumstances, P is taken up as orthophosphate and stored as polyphosphate granules for use as a growth reserve for when there is a lack of P in the environment. Where inorganic orthophosphate is unavailable, algae will uptake organic P, converting to orthophosphate at the cell surface via the enzyme phosphatase [24].

Algal treatment solutions are typically either closed or open suspended systems, or biofilm systems, most commonly using flat-bed or tubular orientations [34]. However, much of the work done to understand the metabolism of P by algae has been related to waste stabilization ponds. Within these contexts, only 15–30% P-removal is reported with significant variations as a function of temperature [33], which is unacceptable for targeted P-removal, especially for temperate or cooler climates. For example, high levels (up to 90%) of P-removal has been achieved by the immobilization of these microalgae on synthetic substrate, either sheets or as beads [35, 36]. Microalgal biofilm photobioreactors have also shown effective P-removal (97% Total P-removal) in recent testing [37].

Phosphorus is an essential constituent of all living organisms but it is non-renewable and its natural reserves are fast depleting. Phosphorus discharged in wastewater could be sustainably reused by microalgae. Knowledge about cellular phosphorus dynamics in microalgae has been rapidly advancing and luxury phosphorus (poly-P) uptake phenomenon by microalgae is becoming the focus point for many research studies [38]. Microalgae cells accumulate P from the environment and use this intracellular (stored) P later for N-NH4⁺ consumption and other cellular functions [39]. Microalgae cells take up N and P from wastewater and use these nutrients to produce biomass, and the removal of one nutrient depends on the availability of the other. At high N supply, P concentration in microalgae biomass depended on P supply [40]. Nitrogen and phosphorus are natural components of aquatic ecosystems. They both support the growth of algae and aquatic plants, which provides food and habitat for other organisms. However, in excess quantity in the air or in water they become a nuisance. Nutrient pollution has impacted many streams, rivers lakes and coastal waters for the past years, resulting in serious environmental and human health issues, which also impact the economy. The rapid growth of algae is called algal blooms and it is considered to be more than the ecosystem can handle. It decreases the oxygen availability that fish and other organisms need to survive and therefore leads to declining fisheries. Harmful algal blooms (HAB) are related mainly to blue-green algae [41]. They can be unsightly and smelly. Some algal blooms produce toxins and support bacterial growth, which is harmful to humans if they come in contact with polluted water or consume tainted fish. Excess nitrogen in the atmosphere can produce ammonia and ozone which could inhibit the ability to breath and alter plant growth. The primary sources of excess nitrogen and phosphorus are from human activities such as agriculture, industries, storm water and domestic water as [42]. Microalgae have been at the focus of attention in recent years as an alternative system for biological wastewater treatment with several applications in wastewater treatment.

Conclusion

In recent years, microalgae have been used as an alternative biological wastewater treatment system with several applications in wastewater treatment. Microalgae are photosynthetic microorganisms that can grow rapidly and live in harsh conditions due to their unicellular or simple multicellular structure. They provide the ability to remove pollutants (nitrogen, phosphorus) from wastewater to produce biomass that can be used to produce valuable algal metabolites.  The sustainable development of a wastewater treatment system needs to be technologically feasible, environmentally friendly and economically viable. The current evidence is that integrating micro-algae as an alternative biological wastewater treatment option is technologically and environmentally feasible. Consequently, the use of micro-algae to reduce nitrogenous, phosphorous and carbonaceous material does have the potential to operate at a lower footprint in terms of energy consumption and greenhouse gas generation compared to conventional biological wastewater treatment processes. Thus, microalgae are capable of converting nutrients (nitrogen and phosphorus) from wastewater into biomass and bioproducts, thereby increasing the sustainability of wastewater treatment.

References:

1. Muñoz, I., Gómez-Ramos, M. J., Agüera, A., Fernández-Alba, A. R. et al., Chemical evaluation of contaminants in wastewater effluents and the environmental risk of reusing effluents in agriculture. Trends Anal. Chem. 2009, 28, 676–694.
2. Sousa, J. C. G., Ribeiro, A. R., Barbosa, M. O., Pereira, M. F. R. et al., A review on environmental monitoring of water organic pollutants identified by EU guidelines. J. Hazard. Mater. 2018,344, 146–162.
3. Plöhn M, Spain O, Sirin S, Silva M, Escudero-Oñate C, Ferrando-Climent L, Allahverdiyeva Y, Funk C. Wastewater treatment by microalgae. Physiol Plant. 2021 Oct;173(2):568-578. doi: 10.1111/ppl.13427.
4. D.L. Sutherland, P.J. Ralph, Microalgal bioremediation of emerging contaminants-Opportunities and challenges, Water Res. (2019) 114921.
5. Nandeshwar S. N, Satpute G. D. Green Technical Methods for Treatment of Waste Water Using Microalgae and its Application in the Management of Natural Water Resources –A Review. Curr World Environ 2014; 9(3).
6. Ting Cai 1, Stephen Y.Park 1, Yebo Li, Nutrient recovery from wastewater streams by microalgae: Status and prospects, Renewable and Sustainable Energy Reviews 19 (2013) 360–369.
7. Oswald, W.J. (1988) Micro-algae and waste-water treatment, in Micro-algal biotechnology, M.A. Borowitzka and L.J. Borowitzka, Editors. Cambridge University press: Cambridge. p. 305–328.
8. Zhou W, Min M, Li Y, Hu B, Ma X, Cheng Y, et al. A hetero-photoautotrophic two-stage cultivation process to improve wastewater nutrient removal and enhance algal lipid accumulation. Bioresource Technology 2012; 110:448–55.
9. Boelee NC, Temmink H, Janssen M, Buisman CJN, Wijffels RH. Scenario analysis of nutrient removal from municipal wastewater by microalgal biofilms. Water 2012; 4:460–73.
10. Sturm BSM, Lamer SL. An energy evaluation of coupling nutrient removal from wastewater with algal biomass production. Applied Energy 2011;88: 3499–506.
11. Z.N. Norvill, A. Shilton, B. Guieysse, Emerging contaminant degradation and removal in algal wastewater treatment ponds: identifying the research gaps, J. Hazard Mater. 313 (2016) 291–309.
12. B. Petrie, R. Barden, B. Kasprzyk-Hordern, A review on emerging contaminants in wastewaters and the environment: current knowledge, understudied areas and recommendations for future monitoring, Water Res. 72 (2015) 3–27.
13. M.E. Ali, A.M.A. El-Aty, M.I. Badawy, R.K. Ali, Removal of pharmaceutical pollutants from synthetic wastewater using chemically modified biomass of green alga Scenedesmus obliquus, Ecotoxicol. Environ. Saf. 151 (2018) 144–152.
14. C.O. Lee, K.J. Howe, B.M. Thomson, Ozone and biofiltration as an alternative to reverse osmosis for removing PPCPs and micropollutants from treated wastewater, Water Res. 46 (4) (2012) 1005–1014.
15. G.J. Zhou, G.G. Ying, S. Liu, L.J. Zhou, Z.F. Chen, F.Q. Peng, Simultaneous removal of inorganic and organic compounds in wastewater by freshwater green microalgae, Environ. Sci. J. Integr. Environ. Res.: Processes&Impacts 16 (8) (2014) 2018–2027.
16. J.P. Hoffmann, Wastewater treatment with suspended and nonsuspended algae, J. Phycol. 34 (5) (1998) 757–763.
17. E.J. Olguin, Phycoremediation: key issues for cost-effective nutrient removal processes, Biotechnol. Adv. 22 (1–2) (2003) 81–91.
18. A. Agüera, P. Plaza-Bola~ nos, F.A. Fernandez, Removal of contaminants of emerging concern by microalgae-based wastewater treatments and related analytical techniques, in: Current Developments in Biotechnology and Bioengineering, Elsevier, 2020, pp. 503–525.
19. Liliana Delgadillo-Mirquez, Filipa Lopes, Behnam Taidi, Dominique Pareau, Nitrogen and phosphate removal from wastewater with a mixed microalgae and bacteria culture. Biotechnology Reports, Volume 11, 2016, P. 18-26.
20. R. Muñoz, B. Guieysse Algal–bacterial processes for the treatment of hazardous contaminants: a review Water Res., 40 (2006), pp. 2799-2815.
21. N. Abdel-Raouf, A.A. Al-Homaidan, I.B.M. Ibraheem Microalgae and wastewater treatment Saudi J. Biol. Sci., 19 (2012), pp. 257-275.
22. L. Wang, M. Min, Y. Li, P. Chen, Y. Chen, Y. Liu, Y. Wang, R. Ruan Cultivation of green Algae Chlorella sp. in different wastewaters from municipal wastewater treatment plant Appl. Biochem. Biotechnol., 162 (2010), pp. 1174-1186.
23. O. Perez-Garcia, F.M.E. Escalante, L.E. de-Bashan, Y. Bashan Heterotrophic cultures of microalgae: metabolism and potential products Water Res., 45 (2011), pp. 11-36.
24. Karin Larsdotter (2006): Microalgae for phosphorus removal from wastewater in a Nordic climate. A doctoral thesis from the School of Biotechnology, Royal Institute of Technology, Stockholm, Sweden.
25. Apna N. Nandeshwar and Ganesh D. Satpute Green Technical Methods for Treatment of Waste Water Using Microalgae and its Application in the Management of Natural Water Resources: A Review. Current World Environment. Vol. 9(3), (2014), 837-842.
26. Yakamercan, E.; Ari, A.; Aygün, A. Land application of municipal sewage sludge: Human health risk assessment of heavy metals. J. Clean. Prod. 2021, 319, 128568.
27. https://www.theguardian.com/environment/2023/mar/17/global-fresh-water-demand-outstrip-supply-by-2030.
28. Gururani P, Bhatnagar P, Kumar V, Vlaskin MS, Grigorenko AV. Algal Consortiums: A Novel and Integrated Approach for Wastewater Treatment. Water. 2022; 14(22):3784. https://doi.org/10.3390/w14223784
29. Silva JA. Wastewater Treatment and Reuse for Sustainable Water Resources Management: A Systematic Literature Review. Sustainability. 2023; 15(14):10940. https://doi.org/10.3390/su151410940
30. Bunce JT, Ndam E, Ofiteru ID, Moore A and Graham DW (2018) A Review of Phosphorus Removal Technologies and Their Applicability to Small-Scale Domestic Wastewater Treatment Systems. Front. Environ. Sci. 6:8. doi:10.3389/fenvs.2018.00008
31. Вowes, M. J., Jarvie, H. P., Halliday, S. J., Skeffington, R. A., Wade, A. J., Loewenthal, M., et al. (2015). Characterising phosphorus and nitrate inputs to a rural river using high-frequency concentration-flow relationships. Sci. Total Environ. 511, 608–620. doi: 10.1016/j.scitotenv.2014.12.086
32. EA (2015). Water for life and Livelihoods A Consultation on the Draft Update to the River Basin Management Plan Part 2: River Basin Management Planning Overview and Additional Information. Bristol: EA.
33. Powell, N., Shilton, A., Chisti, Y., and Pratt, S. (2009). Towards a luxury uptake process via microalgae - Defining the polyphosphate dynamics. Water Res. 43, 4207–4213. doi: 10.1016/j.watres.2009.06.011
34. Hoh, D., Watson, S., and Kan, E. (2016). Algal biofilm reactors for integrated wastewater treatment and biofuel production: a review. Chem. Eng. J. 287, 466–473. doi: 10.1016/j.cej.2015.11.062
35. Wei, Q., Hu, Z., Li, G., Xiao, B., Sun, H., and Tao, M. (2008). Removing nitrogen and phosphorus from simulated wastewater using algal biofilm technique. Front. Environ. Sci. Eng. China 2, 446–451. doi: 10.1007/s11783-008-0064-2
36. He, S., and Xue, G. (2010). Algal-based immobilization process to treat the effluent from a secondary wastewater treatment plant (WWTP). J. Hazard. Mater. 178, 895–899. doi: 10.1016/j.jhazmat.2010.02.022
37. Sukacova, K., Trtílek, M., and Rataj, T. (2015). Phosphorus removal using a microalgal biofilm in a new biofilm photobioreactor for tertiary wastewater treatment. Water Res. 71, 55–63. doi: 10.1016/j.watres.2014.12.049
38. Zareen T. Khanzada, Phosphorus removal from landfill leachate by microalgae, Biotechnology Reports, Volume 25, 2020, e00419. https://doi.org/10.1016/j.btre.2020.e00419.
39. Y.H. Wu, Y. Yu, H.Y. Hu Potential biomass yield per phosphorus and lipid accumulation property of seven microalgal species Bioresour. Technol., 130 (2013), pp. 599-602, 10.1016/j.biortech.2012.12.116
40. A. Beuckels, E. Smolders, K. Muylaert Nitrogen availability influences phosphorus removal in microalgae-based wastewater treatment Water Res., 77 (2015), pp. 98-106, 10.1016/j.watres.2015.03.018
41. G. Markou, O. Depraetere, D. Vandamme, K. Muylaert Cultivation of Chlorella vulgaris and Arthrospira platensis with recovered phosphorus from wastewater by means of zeolite sorption Int. J. Mol. Sci., 16 (2015), pp. 4250-4264, 10.3390/ijms16024250
42. G. Procházková, I. Brányiková Effect of nutrient supply status on biomass composition of eukaryotic green microalgae J. Appl. Phycol., 26 (2014), pp. 1359-1377, 10.1007/s10811-013-0154-9