Influence of phytoplankton on the water quality of surface water sources and drinking water

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Introduction / Subhead More than 40% of the world’s population faces water scarcity. Provision of drinking water of normative quality is one of the goals of sustainable development. In regions that are quantitatively supplied with water there are problems with its quality. Drinking water quality is determined by the quality of water in water bodies - sources of drinking water supply and depends on natural and anthropogenic factors affecting the content of mineral and organic impurities of natural and anthropogenic origin [1]. Problems of odors in drinking water have been recorded in the world and the Russian Federation: Moscow, Izhevsk, Yekaterinburg, Kachkanar, Novokuznetsk, Khabarovsk, Perm. The peculiarity of the occurrence of unpleasant odors in water is their episodic nature associated with periods of mass development of cyanobacteria [2]. The development of some species of diatoms, blue-green, green, and flagellated algae is the cause of deterioration of organoleptic properties of drinking water. Application of traditional technological scheme of water treatment (coagulation - sedimentation - filtration - disinfection) does not completely remove odorants from drinking water. Subject area analysis The most common odors in drinking water are soil and musty odors that are characteristic of such substances in water as geosmin and 2-methylisoborneol (MIB), which are products of actinomycetes, cyanobacteria, and many species of algae [1; 3]. The total number of species that are sources of odorants is unknown. In natural waters, geosmin and 2-methylisoborneol can occur together and separately. Odors can be manifested in water source, appear in the process of water treatment and water distribution. It is known that in laboratory cultivation of cyanobacteria odorants are released at all stages of culture growth, but in natural water bodies the intensity of water body “blooming” is not always related to the intensity of odorants manifestation [2]. Most often odor in drinking water appears in spring and at the end of summer - beginning of autumn [3; 4]. Under normal conditions of cyanobacteria growth, the number of odorants produced by them is insignificant and does not represent a problem. When they are exposed to stress factors (deviation from the optimal degree of illumination, temperature, pH of the medium, flow rate, content of bio-genic elements, etc.), the number of odorants produced increases [3; 5] as a result of their natural death and their destruction by heterotrophic organisms (fungi, actinomycetes and small crustaceans). In modern conditions, due to global changes in the Earth’s climate, we can expect an increase in the degree of cyanobacteria distribution in water bodies: · increase of water temperature in water bodies, expansion of cyanobacteria habitat; · increase in the content of nutrients in water bodies; · increase in the frequency of droughts leading to lowering of water levels in water bodies, increase in the degree of illumination of bottom layers, formation of conditions favorable for cyanobacteria. Observations of phytoplankton development in the Volchikhinskoe water reservoir (Sverdlovsk region) allowed us to establish [4] that odor in water appears at a sharp change of dominant species Aphanizomenon flosaquae and Oscillatoria agardhii. Aphanizomenon flosaquae and Oscillatoria agardhii are geosmin producers, and Oscillatoria agardhii is also a MIB. The co-occurrence of these species, even when one of them was dominant, did not result in odors. The odor of water from the Izhevsk pond was characterized as the odor of “dust”. During the identification of phytoplankton composition more than 250 species and intraspecific taxa were identified. Green algae were represented mainly by the genus Scenedesmus, Pediastrum, Oocystis, Tetraedron, Monoraphidium. Of diatom algae, representatives of genera Aulacoseira, Asterionella, Diatoma, Stephanodiscus, Navicula, Fragilaria, Synedra, Nitzschia. Species of blue-green algae were predominantly represented by Aphanizomenon flosaquae, Microcystis aeruginosa, Microcystis pulverea, Merismopedia tenuissima, Oscillatoria agardhii, Anabaena flosaquae, Woronichinia compacta, Woronichinia naegeliana. Observations have established that the “bloom” of water coincides with the mass development of the Aphanizomenon flosaquae [4; 5], at the same time the water contained geosmin. Appearance of odour in water of Izhevsk pond coinciding with mass development of the species was noted Oscillatoria agardhii, which has since become the dominant one. Analysis of scientific and technical information [2-6] allowed us to identify the main types of planktonic organisms that are sources of odorants, priority chemical substances produced by them, and the nature of odors (Table 1). Table 1. Odorants produced by algae and cyanobacteria [6] № Chemical substance Odour character TOC *, mg/dm3 Species - sources of odorant 1. Dimethyl sulfide C2H6S Cabbage, hydrogen sulphide 0.1 Asterionella formosa; Nitzschia actinastroides; Diatoma elongate; Ochromonas danica; Ochromonas malhamensis; Chlamydomonas globose - 1 Anacystis nidulans; Synechococcus cedrorum; Oscillatoria chalybea; Oscillatoria tenuis; Phormidium autumnale; Plectonema boryanum - 2 2. Dimethyl disulfide C2H6S2 Septic, garlic, rot < 0.4 Microcystis aeruginosa; Microcystis wesenbergii - 2 3. Dimethyl trisulfide C2H6S3 Septic, garlic, rot, swamp 0.001 Microcystis aeruginosa; Microcystis wesenbergii - 2 4. Isopropyl disulfide [(CH3)2CH]2S2 Onions, meat, Hydrogen sulphide - Microcystis flos-aquae - 2 5. 6-methyl-5-hepten-2-one (CH3)2C=CHCH2 CH2COCH3 Cabbage, fruit, ether 5.04 Aulacoseira granulata; Cyanidium caldarium; Scenedesmus subspicatus; Syncrypta sp.; Synura sp. - 1 Anabaena cylindrica; Microcystis aeruginosa; Synechococcus sp. - 2 6. Β-cyclocitral C10H16O Tobacco smoke, mould 1.93 Scenedesmus subspicatus; Dinobryon cylindricum; Uroglena sp.; Ulothrix fimbriata - 1 Microcystis aeruginosa; Microcystis flos-aquae; Microcystis botrys; Microcystis viridis; Microcystis wesenbergii - 2 7. 2-methyl-isoborneol C11H20O Earthy, musty, Camphor 0.0015 Hyella sp.; Jaaginema geminatum (syn. Oscillatoria geminate); Leibleinia aestuarii; Oscillatoria curviceps; Oscillatoria limosa; Oscillatoria tenuis; Oscillatoria variabilis; Phormidium breve (syn. Oscillatoria brevis); Phormidium favosum; Phormidium tenue (syn. Oscillatoria tenuis); Phormidium LM689, Phormidium sp.; Planktothrix agardhii (syn. Oscillatoria agardhii); Planktothrix cryptovaginata (syn. Lyngbya cryptovaginata); Planktothrix perornata f. attenuate; Porphyrosiphon martensianus (syn. Lyngbya martensiana), Pseudanabaena articulate; Pseudanabaena catenata; Pseudanabaena limnetica (syn. Oscillatoria lill1netica); Tychonema granulatum (syn. Oscillatoria f. granulata) - 2 8. Geosmin C12H22O Earthy, musty 0.0004 Anabaena circinalis; Anabaena crassa, Anabaena lemmermannii; Anabaena macrospora; Anabaena planctonica; Anabaena solitaria; Anabaena viguieri; Anabaena millerii; Aphanizomenon flos-aquae; Aphanizomenon gracile, Geitlerinema splendidum (syn. Oscillatoria splendida); Leibleinia subtilis (syn. Lyngbya subtilis); cf. Microcoleus sp.; Phormidium allorgei (syn. Lyngbya allorgei); Phormidium amoenum (syn. Oscillatoria amoena); Phormidium breve (syn. Oscillatoria brevis); Phormidium cortianum (syn. Oscillatoria cortiana); Phormidium formosum (syn. Oscillatoria formosa); Phormidium simplicissimum (syn. Oscillatoria simplicissima); Phormidium uncinatum; Phormidium viscosum; Phormidium sp.; Planktothrix agardhii (syn. Oscillatoria agardhii); Planktothrix prolifica (syn. Oscillatoria prolifica); Pseudanabaena catenata; Schizothrix muelleri; Symploca muscorum; Tychonema bornetii (syn. Oscillatoria bornetii); Tychonema granulatum (syn. Oscillatoria f. granulata) - 2 9. Β -ionone C13H20O Violets, fruit 0.0007 Cyanidium caldarium; Scenedesmus subspicatus; Synura sp. - 1 Anabaena cylindrica; Aphanizomenon gracile; Synechococcus 6911 - 2 End of the Table 1 № Chemical substance Odour character TOC *, mg/dm3 Species - sources of odorant 10. 1,2-dihydro-1,1,6-trimethyl-naphthalene C13h16 Licorice - Cyanidium caldarium - 1 11. Geraniol C10H18O Sweet, flowers, fruit, roses, wax, citrus 7.71 Synechococcus 6911 - 2 12. Geranyl-acetone C13H22O Freshness, greenery, fruit, wax, roses, trees, magnolia 0 Cyanidium caldarium; Scenedesmus subspicatus - 1 13. Nerol C10H18O Sweet, citrus, magnolia 29.3 Synechococcus 6911 - 2 14. 2,4-decadienal C10H16O Rancid, Fish 1.98 Dinobryon divergens; Dinobryon cylindricum; Mallomonas papillosa; Synura petersenii; cf. Syncrypta sp.; Uroglena americana; Uroglena sp.; Fragilaria sp.; Cryptomonas rostratiformis; Peridinium willei - 1 15. 2,4,7-decatrienal C10H14O Rancid, Fish 1.95 Dinobryon divergens; Dinobryon cylindricum; Synura petersenii; Uroglena americana; Uroglena sp. (UTCC276) - 1 Microcystis papillosa; Microcystis varians - 2 16. Ectocarpene C11H16 Tomato greens - Amphora veneta; Gomphonema parvulum; Phaeodactylum tricornutum; Skeletonema costatum; Lithodesmium undulatum; Ectocarpus spp. - 1 TOC* - Threshold odour concentration; 1 - Eukaryotic algae; 2 - Cyanobacteria. The problem of reducing the content of odorants in drinking water should be solved comprehensively and include the solution of the following tasks [5]: 1. Monitoring of the number of cyanobacteria and the content of cyanotoxins in the water source. 2. Reducing the number of cyanobacteria, content of odourants and cyanotoxins in the water source. 3. Removal of odorants and cyanotoxins from drinking water. Most countries do not have mandatory monitoring requirements for cyanobacteria and cyanotoxins in drinking water sources. Based on the existing experience, the Global water research coalition developed the principles of monitoring the number of cyanobacteria in water bodies [7]: visual assessment of the water source condition, water sampling to study the species composition of cyanobacteria and determine their abundance, and determination of the cyanotoxin content. To assess the danger of water source pollution by cyanobacteria [8; 9], hazard levels were identified, and their monitoring programs were proposed (Table 2). Indirect methods of determining the level of water pollution by cyanobacteria are determination of chlorophyll α concentration in water [9] and determination of vegetation index [10]. These methods are realized, including remotely. In the European Union countries, the concentration of chlorophyll α is a regularly monitored parameter within the framework of the Water Framework Directive. Table 2. Hazard levels for water pollution by cyanobacteria [8; 9] Hazard Level Characterisation Recommendation Low 500-2000 cyanobacteria cells in 1 ml of water Regular monitoring, identification of dominant species. Water sampling once a week to determine the number of cyanobacteria cells. Visual inspection of the water body to identify signs of its «blooming» Medium 2000-6500 cyanobacteria cells in 1 ml of water Water sampling twice a week. Assessment of population growth and species diversity. Assessment of the need for water toxicity control and toxin monitoring High More 6500 cyanobacteria cells in 1 ml of water Assessment of possible risk for human health based on the data of toxin content monitoring. Development of recommendations for consumers using untreated water. Water sampling on a daily basis. Monitoring the content of cyanotoxins in drinking water Very high More 65 000 cyanobacteria cells in 1 ml of water Informing supervisory authorities. Recommendations for consumers using untreated water. Assessment of toxicity or cyanotoxin content in the water supply source and in drinking water. Water body monitoring. Use of alternative sources of drinking water supply if there is a high public health risk Decrease in the number of cyanobacteria in water bodies Currently, physical, chemical, and biological methods have been proposed to reduce the abundance of cyanobacteria in wo-domes [5; 7; 10-16] (Table 3). Table 3. Methods to reduce the number of cyanobacteria in water bodies Method Actions Physical Artificial destratification, aeration, agitation. Bottom cleaning to remove benthic algae and nutrients. Ultrasonic treatment to slow cyanobacteria growth and kill them Chemical Nutrient control: hypolimnetic oxygenation, phosphorus precipitation and capturing. Control of cyanobacteria abundance: application of coagulants and substances with algicidal and algistatic action Biological Application of viruses and infectious bacteria. Regulation of trophic structure of aquatic ecosystem with predominant number of heterotrophs feeding on cyanobacteria or competing with them for nutrition Elimination of cyanobacteria and cyanotoxins from drinking water The following methods are known for reducing cyanobacteria and cyanotoxins from drinking water supplied to consumers: 1. Using of alternative sources of water supply. 2. Preventing the introduction of cyanobacteria and/or cyanotoxins during water intake from a water supply source, including changing the level of the water intake head. 3. Water treatment from cyanobacteria and/or cyanotoxins. Characteristic of methods of water treatment from cyanobacteria and cyanotoxins produced by them is presented in Table 4. At the first stage of water treatment it is recommended to remove cyanobacteria together with cyanotoxins and odorants contained in them by methods: pre-oxidation, coagulation, sedimentation, filtration and flotation. In the process of water treatment it is expedient to apply micro-ultrafiltration by metal or fabric filters with different pore size. The use of nanomembranes, including those made of polymeric biodegradable materials, is known [20]. Extraction of cyanobacteria cells from water by sedimentation and flotation allows to prevent their destruction and odorants entering the water. Sedimentation extracts up to 80% of cyanobacteria, while flotation extracts up to 98% [21]. The efficiency of application of coagulants and flocculants is influenced by the species of cyanobacteria. Studies [21] have shown the advantages of polymeric coagulants in comparison with metal salts: higher efficiency, easy separation of the formed sludge, the possibility of application in a wider range of pH and temperature [19]. Table 4. Characteristics of methods of water treatment from cyanobacteria and cyanotoxins [1; 5; 7; 19-25] Method Application efficiency Removal of intracellular cyanotoxins (intact cells) Pre-oxidation May cause cell lysis and subsequent release of cyanotoxins into the water. If oxidation is used to clean up other contaminants, low doses of oxidising agents that do not cause cell lysis (potassium permanganate) should be used. If high doses are used, they should be sufficient to destroy the toxins Coagulation, sedimentation, filtration Used to remove intracellular toxins in cases where cells are able to aggregate into easily separable precipitates Compartment on membranes Little data. Presumably effective for removal of intracellular toxins. Microfiltration and ultrafiltration are not effective if cells accumulate on the membrane Flotation Effective in removing intracellular cyanotoxins Removal of extracellular cyanotoxins (dissolved) Compartment on membranes Depends on material, membrane pore size and water quality. Nanofiltration is effective for removal of extracellular microcystins. Reverse osmosis is used to remove extracellular microcystin and cylindrospermopsin Potassium permanganate oxidation Effective for the oxidation of microcystins and anatoxins Ozonation Effective for the oxidation of extracellular microcystin, anatoxin-α and cylindrospermopsin Chloroaramines Not effective Chlorination Effective for oxidation of extracellular cyanotoxins at pH = 8 and below. Not effective for anatoxin-α UV irradiation At high doses, effective in breaking down microcystin and cylindrospermopsin Sorption by activated carbons The effectiveness of powdered activated charcoal depends on the type and pore size. Wood activated carbon is effective for adsorption of microcystins (at doses greater than 20 mg/dm3), not effective for removal of saxitoxins, taste and odour. Granular activated carbon is effective for removal of microcystins, less effective for removal of anatoxi-na-α and cylindrospermopsin To reduce the intensity of odors in drinking water it is not enough to remove only cyanobacteria cells, it is necessary to clean it from cyano-toxicants. The use of sorbents for this purpose, in particular activated carbon, is the most effective and technically available method. Powdered activated carbons (PAC) are used together with coagulants or after treatment by them. The effectiveness of their introduction in the form of suspension with a dose of 1-7 mg/dm3 into water at the beginning of the technological process with subsequent removal in the process of purification is shown. The advantage of PAH is the possibility of its short-term use, the disadvantage is the impossibility of its reuse. Granulated activated carbon (GAC) is expedient to use at the final stages of water treatment. The advantage of GAC is a large adsorbing surface, allowing to use it for extraction of a wide range of organic substances from water. The disadvantage of GAC is the necessity of its regeneration and replacement. High efficiency of cyanotoxin removal from water is achieved by using electroactive polymers [22]. For example, iron (II, III) oxide nanoparticles in polypyrrole film effectively remove microcystis and cylindrospermopsin from water. The advantages of electro-active polymers in comparison with traditional sorbents include: higher sorption capacity (238-300 μg/mg), short contact time (8-15 minutes), increased number of use cycles. Reagent oxidation, ozonation and ultraviolet water treatment [5; 7; 23] may be accompanied by the formation of toxic by-products, such as trihalomethanes. Studies in the field of cyanotoxin oxidation allowed us to determine the efficiency of the method combining electrolysis and heterogeneous photocatalysis. Application of electrolysis method allows to reduce the concentration of microcystins by 49%, application of photocatalysis - by 41%, and their combined application - by 99% [24]. Despite the efficiency of ozonation application, its spreading is limited by the high cost of its realization. Reduction of ozonation costs is achieved by using ozone-on microbubbles generated by a low-temperature plasma reactor based on a dielectric barrier discharge with an integrated liquid oscillator [5; 25]. The effectiveness of using methods of eliminating odorants of different origin from water [6] is presented in Table 5. Table 5. Methods of removing odorants from water № Odorant Effective Not effective 1. Geosmin О3, UV /Н2О2, О3/ Н2О2, activated charcoal, biological method Cl2, ClO2, КМnО4, chloramines, aeration 2. 2-methylisoborneol О3, UV /Н2О2, О3/ Н2О2, activated charcoal, biological method Cl2, ClO2, КМnО4, chloramines, aeration 3. Dimethyl disulphide, dimethyl trisulphide Oxidation, activated carbon, biological method Chloramines 4. Chlorinated compounds Activated carbon Biological method 5. Hydrogen sulphide Aeration, oxidation - 6. Low molecular weight aromatic and aliphatic compounds Aeration, activated carbon Oxidation 7. Phenol, chlorophenols О3, ClO2, activated charcoal, biological method Cl2, chloramines, КМnО4, 8. Benthic cyanobacterial blooms Optimisation of water levels in reservoirs - The use of O3 in doses of 1-15 mgO3/dm3 or H2O2 with a concentration of 1-15 mg/dm3 leads to a reduction in the total content of odorants (geosmin and MIB) up to 50% and a reduction in odor intensity from 4 to 3 points. The combined application of O3 and H2O2 showed greater effectiveness [3]. In general, the use of oxidizing agents for water deodorization is less effective than the use of PACs. Final chlorammonization has no practical effect on the odor level of drinking water. Secondary chlorination of treated water without ammonization can lead to chlorine odor. Conclusion 1. Increased anthropogenic load, accompanied by the intake of nutrients from wastewater, and the Earth's climate change, which contributes to an increase in the content of cyanobacteria due to the expansion of their habitat, leads to an increase in the frequency of occurrence of odors in drinking water. 2. The most common cause of odors in drinking water is the development of odors in source water Aphanizomenon flos-aquae; Oscillatoria agardhii; Microcystis flos-aqua; Microcystis viridis. 3. To control the number of cyanobacteria in sources of drinking water supply it is necessary to carry out monitoring. The methods of remote sensing are included. 4. A combination of ozonation and sorption methods is appropriate for removal of most odorant species produced by cyanobacteria. 5. It is advisable to introduce powdered activated carbons at the beginning of the technological process of purification, granular activated carbons are usually used at the final stages.

About the authors

Elena V. Kalinina

Research Center of Ministry of Defence of the Russian Federation

Author for correspondence.
Email: Kalininaelena1@rambler.ru
ORCID iD: 0000-0001-6965-4895
SPIN-code: 9259-4503

Candidate of Technical Sciences, Аssociate Рrofessor, Department Environmental protection

29, Komsomolsky Ave., Perm, Perm Krai, 614990, Russian Federation

Larisa V. Rudakova

Research Center of Ministry of Defence of the Russian Federation

Email: larisa.rudakova.007@gmail.com
ORCID iD: 0000-0003-3292-8359
SPIN-code: 1705-6430

Doctor of Technical Sciences, Рrofessor, Head of Department Environmental protection

29, Komsomolsky Ave., Perm, Perm Krai, 614990, Russian Federation

References