Phytodesalination of saline waters
Rationale for phytodesalination of saline water in a filtration and regeneration bioplato
Filtration and regeneration bioplato can be used for integrated treatment and associated membrane-free phyto-desalination of mineralized water. Demineralization of water is made by removing salts of higher aquatic plants in bioplato and circulation and regeneration water treatment using coagulants, enzymes, bioreagents and probiotics, suspension of natural minerals with electrochemical activation of reagents. Block of treatment and purification of circulating-regeneration water of bioplato should include flotation reactor- clarifier for flotation of suspension, the introduction of additional reagents and self-cleaning polystyrene foam filter with dosing of bioreagent into clarified water, which allows a stable growth of green biomass and intensification of salt excretion. In order to substantiate the possibility of simultaneous desalination of saline water with the help of higher aquatic plants, the phytotechnology of filter-regeneration hydroponic bioplato was used. It is analyzed that the placement of bioplato at the end of the technological scheme allows to provide deep purification of water from organic and inorganic impurities and restore its natural properties at the expense of contact with higher aquatic plants. It was also shown that the installation for treatment and purification of circulating-regeneration water by bio-plato should include a flotation reactor- clarifier for the introduction of additional reagents and a self-cleaning polystyrene foam filter with dosing of enzymes, bioreagents and probiotics into clarified water, which allows to provide a stable growth of green biomass and intensify the extraction of salts by higher aquatic plants.
Introduction. The problem of desalination of saline water is particularly acute in all countries of the world, especially in areas with limited freshwater supplies. At present, these are the countries of Central Asia, Middle East, densely populated countries of Indonesia, China, India, almost all countries in Africa and a number of countries in Europe, America and USA are suffering from shortage of fresh water for agriculture and water supply and need to address the shortage of clean water as soon as possible. According to
UNO, freshwater shortage in the world is increasing by 13-20 % annually and in 2050 more than five billion people will face water shortage [1]. Solving the problem of ensuring access to clean fresh water for the population of the world is the goal of all countries of the world without exception. An important way of solving this problem is to use saline water in drinking and technical water supply after it has been desalinated to the legally prescribed requirements, first of all to a salt concentration of no more than 1000 - 1500 mg/dm3.
The current state of saline water demineralisation. The main methods of desalination are reverse osmosis, ion exchange, evaporation (distillation), electro dialysis and their combinations. Currently, the most common desalination methods are reverse osmosis and ion exchange. Electro dialysis and distillation are less commonly used. In most cases ion exchange allows water to be desalted to almost complete removal of anions and cations. For this purpose, the water is sequentially passed through cation exchange and anion exchange filters and mixed filters loaded with a mixture of anion exchanger and cation exchanger. The number of filtration stages and the type of ion exchange material are determined by the depth of desalination, the qualitative and quantitative composition of the impurities, and the requirements for removal of pollutant ions. In most cases ion exchange is recommended for desalination of brackish water with initial concentration of 1500-2000 mg/dm3 [3], although some authors recommend more. The advantages of the ion exchange method include high desalination reliability. The disadvantages of this method are the large amount of reagents for periodic regeneration of ion-exchangers, which leads to discharge of spent regeneration solutions-eluates with salt content on average of 2.0-3.0 more by mass of salts contained in saline water [4]. A considerable problem is the processing of these solutions, which must not be discharged into water bodies.
In recent decades, the use of membrane processes such as reverse osmosis (hyperfiltration) has become widespread in desalination. These processes reduce the salt content of saline water by filtering under pressure through special membranes.
The salts are concentrated as concentrate or sent for further processing. The water pressure must be higher than the osmotic pressure of the salts in the water, which increases with increasing ion concentration. For example, if the osmotic pressure of drinking water reaches
0. 10 MPa, for seawater containing 35 g/dm3 salts the osmotic pressure is a significant value of 2,58 MPa, which requires the use of high pressure pumps. The desalinated water (permeate) yield reaches a maximum of 75 % [5]. Multistage filtration is used to increase the efficiency of desalination and reduce the volume of waste concentrates. At high salt concentrations it is economic to use two-step
using reverse osmosis or electrodialysis in the first stage and ion exchange in the second. This combined desalting scheme reduces the number of reagents and the concentration of salts to be removed.
Compared with ion exchange treatment and desalination, reverse osmosis has the following advantages: continuity of the process and smaller number of reagents for membrane regeneration. However, disadvantages of this technology, such as sensitivity of membranes to biological fouling, colloids, heavy metals and organic impurities, precipitation of insoluble salts on membrane surface, incrustation of membranes with hardness salts and higher energy costs should be noted. All this requires thorough pre-disinfection and purification of the water from sludge, heavy metals and organic impurities that can "poison" the membranes. In addition, special chemicals (scale inhibitors or anti-sealants) are added to the source water to prevent the deposition of insoluble compounds on the membrane surface.
In most cases when large volumes of seawater are desalinated in significant quantities, concentrated salt solutions are returned to the environment together with the antisolvents and flushing chemicals, which is extremely harmful to the environment. Coastal ichthyofauna and coral reefs are particularly affected by the discharge of concentrates after hyperfiltration into seawater. In particular, the US Department of Environmental Protection found that a huge number of seawater desalination plants caused more than 3.4 billion harm to fish and other marine life and caused more than $212.5 million in economic losses to the country's fishing industry over a one-year period. Desalination plants can also destroy around 90% of plankton in a short period of time [6].
The current trend is towards natural methods of water treatment and conditioning using renewable natural resources, in particular by means of higher aquatic plants (HAPs). Among natural water treatment systems, bioengineered bioplato type facilities [7; 8], which are used to treat domestic, industrial wastewater, natural water in water bodies and polluted surface run off, are becoming widespread. The essence of the operation of most of the bio-plant is that the phytoremediation of water in them is carried out by filtration of water through the root system of the PDA, due to photosynthesis in plants with the provision of their absorption, cumulative, oxidative and the ability to synthesize oxygen during biodegradation of carbon dioxide. Closed hydroponic type bioplantos (ZBGT) [9, 10], in which the root system of higher aquatic plants is fixed in a porous (gravel) filtering load and is constantly washed by water moving vertically from top to bottom or from bottom to top, are quite common.
In addition to efficient removal of suspended solids, organic impurities, biogenic nitrogen and phosphorus compounds, and soluble salts, higher aquatic plants also remove from water. Thus, when using water hyacinth (Eichornia crassipes) along with wastewater purification from organic impurities in biofilter (bioreactor) removal of chlorides up to 32%, sulphates up to 43% was observed [11]. Cane at a yield of 44 t/ha of dry matter can accumulate up to 419 kg/ha of potassium, 408 kg/ha of chloride, 450 kg/ha of sodium [12]. In bioengineered constructions like Constructed Wetlands using higher aquatic plants, sulphate purification efficiency reached 25-30% and sodium ions purification efficiency 10-15% [13].
At the same time in the bioplato there is a gradual collimation of pore space of filter bed, inter-root space of NAP and drainage by biofilm and mineralized sludge. In addition, the roots of higher aquatic plants and algae are constantly dying off, which further clogs the backfill and drainage. These processes reduce the supply of oxygen and nutrients to the ATS root system, which disrupts photosynthesis, transpiration and phytoremediation of water.
As these biofloors do not provide sludge removal, sludge starts to accumulate and compact in the filter bed and in the inter-root space. Anaerobic biological processes begin to take place, resulting in reduced efficiency of mineral salt extraction, sorption and detoxification of organic impurities.
Suffusion processes begin to occur, peptization of multicomponent colloidal impurities occurs and as a consequence, secondary contamination of treated water is observed, higher aquatic plants die, efficiency and productivity of bioplato facilities is reduced.
As experimental data show, sulphate recovery is high up to 144 hours of operation of bioplato facilities and is 0.404-0.837 mg/h, then the intensity of absorption decreases to 0.121-0.046 mg/h. [11]. The same is observed for chloride removal. Periodic shutdown of the facilities for repair and recovery works related to washing and regeneration of filter bed and drainage required to restore operation of the bioplato, creates stressful conditions for HAP and negatively affects the subsequent phytoremediation processes.
These disadvantages of bioplato are not present when using the filter-regeneration hydroponic
type of bioplato (FRHTB) [14], which ensures constant flushing of filter bed, HAP root system and drainage. Flushing and regeneration of the bioplato is carried out by means of a hydro-automated drainage of special medium drainage of contaminated circulation-washing water from the filter bed of the bioplato and its subsequent cleaning on a self-cleaning Styrofoam filter.
1 - bioplat body, 2 - upper drainage of water supply to the bioplat, 3 - top layer of filter bed, 4 - bottom layer of filter bed, 5 - middle drainage of circulation and regeneration water collection and discharge, 6 - bottom drainage of collection and discharge of treated water, 7 - circulation and regeneration water pump, 8 - collector of initial mineralized water supply for treatment, 9 - flotation clarification reactor, 10 - self-cleaning foam filter, 11 - installation of hydro automatic filter washing, 12 - higher water plants, 13 - reagent facilities, 14 - sediment drainage, 15 - reservoir for accumulation of filter wash water.
This allows for a self-replicating mode of operation of the bioretention phytoconstructions without creating stressful conditions for the growth of HAP on the bioretention bioretention. Due to the developed technology and design of FRHTB, the operation mode of bio plateau allows, regardless of the concentration of contamination in the source water, the cyclicity of its supply for treatment, the presence of service personnel and in any climatic conditions to achieve a higher quality and stability of water treatment using higher aquatic plants. Therefore, the use of FRHTB for the complex removal of impurities and salts from saline water can be promising in the case of reagent-free and membrane-free desalination.
The aim of this work is to analyse the water treatment on a bioplato and justify the possibility of using a filter-regenerative hydroponic bioplato (FRHTB) for simultaneous desalination of saline water using higher aquatic plants (HAPs).
Research results. In order to substantiate the possibility of concomitant desalination of saline water using higher aquatic plants was used phytotechnology filtration-regenerative bioplato hydroponic type, the principal scheme of which is shown in the figure.
According to the technological scheme of filtration-regeneration hydroponic bioplato (FRBGT) phyto desalination and water purification is carried out in several stages Si The main degree of desalination and water purification occurs in the bioplato itself (1) due to the use of photosynthesis processes in higher aquatic plants (HAP) with absorption of salts and biogenic compounds from water and their accumulation in the biomass of higher aquatic plants. Thus, according to research by V. Kravetz [15] it was found that on existing phytotreatment systems of water using higher aquatic plants, removal of sulphates and chlorides on bioplato filters is 58-35 % and 67-49 %, respectively, depending on the structure of bioplato, time of year and species of higher aquatic plants. The total extraction of dissolved salts from the mineralised water can be on average 40-55 % of the total salt content of the initial brackish water with salt concentration 2500-3500 mg/dm3.
Additional desalination of saline (brackish) water using the FRHTB technology is carried out by water purification and simultaneous extraction of salts from circulating and regeneration water of bioplato using reagents and probiotics in a flotation reactor- clarifier (9) and then in a self-cleaning foam filter, which are combined into a single water-treatment complex [16]. Coagulants, metal hydroxides, filtering materials and suspensions of the natural minerals clinoptilolite, diatomaceous earth, bast, tuff, bentonite, peat [17] and their combinations may be used for treatment and desalination of circulation and regeneration water.
When using aluminium hydroxochloride coagulants with sodium aluminate for treatment of mineralised water
with sulphate concentration of 500-700 mg/dm3 the purification degree from sulphates reaches 83-88 % [18]. Magnetite, iron and aluminium hydroxides obtained by electrolysis using metal anodes, known as electrocoagulation process, have high sorption properties in relation to dissolved salts. Experimental data obtained by the authors show that during electrocoagulation the degree of extraction of chlorides from saline brackish water reaches 13-15 % and that of sulphates - 20-31 % and more due to high sorption capacity of metal hydroxides at the moment of their formation after ionization of metal anodes by effect of electric current or internal electrolysis of metal chips [19; 20]. Probiotics [21] and activated natural suspensions based on zeolite (clinoptilolite) and other natural minerals or their complex mixtures [22] are dosed into the source water to intensify photosynthesis processes in HAP with the provision of a constant growth of green biomass in the bioplato.
Activation of natural zeolite suspension can be carried out by effective micro-organism-enzymes, probiotics and catholyte produced in the cathodic zone of the membrane cell [19; 20], or by complex activation [23]. Suspension activation ensures more intensive accumulation of biogenic compounds of nitrogen and phosphorus by zeolite and other natural filtration materials and suspensions and stimulates intensive growth of the root system of BPS, which promotes photosynthesis and salt immobilization by higher aquatic plants.
Technological scheme of treatment and simultaneous desalination of mineralized water in the WWTP works as follows. Mineralized water through a collector (8) is fed into the bioplato (1) and through the upper drainage (2) is evenly distributed in the upper layer of filtering backfill (3), where the most active zone of root system of higher aquatic plants (12) is located.
Due to constant contact of higher aquatic plants with water, there is active mass exchange between water and root system of WAPs, photosynthesis in bioplata complex biochemical processes of transformation of organic and mineral impurities present in water, absorption of dissolved salts by plant biomass. Then water is filtered from top to bottom through the filter bed layers (3, 4), evenly collected through the filtration zone by the bottom drain (6) and discharged to its destination.
In the process of water movement in the layers of filter bed and root zone of bioplato plants there is a constant accumulation of activated sludge film, microscopic algae, suspended mineral and organic impurities, which leads to clogging of bed and drainage layers. By increasing the hydraulic resistance of the filtering layers, suffosion processes begin to occur, which leads to a decrease in the quality of the treated water.
To prevent this process, part of the contaminated water is removed from the upper filter layer by means of the
middle drain (5), which ensures constant regeneration of the upper layers of the filter bed and the BWP root system. Initially contaminated circulation and regeneration water is sent for further treatment to the flotation clarification reactor (9), where reagents for coagulation and sorption of suspended solids and extraction of dissolved salts as well as compressed air for flotation of suspended solids and water oxygenation are also supplied. The resulting sludge and suspended solids are periodically removed from the clarifier (9). The clarified water is removed from the sludge at the polystyrene filter (10). The filter bed is flushed periodically, as it becomes clogged, in hydro-automatic mode using a special siphon device (11). The rinsed water is collected in a tank (15) and together with the circulation and regeneration water is sent to the flotation clarifier (10) for treatment. If necessary, solutions of probiotics, enzymes and suspensions of effective microorganisms can be dosed into the treated recirculation and regeneration water after the Styrofoam filter. After treatment in the bioflotation clarifier-reactor (9) and in the self-cleaning polystyrene filter (10) the circulating regeneration water is mixed with the brackish water stream and fed to the bioplato head (1).
Thus, circulation-regeneration water during a day, circulating through the upper layer of filtering backfill (3) of bioplato, repeatedly washes the root system of BWW, which provides their constant flushing and allows to stabilize the process of brackish water treatment and demineralization in FRHTB technology.
Depending on the degree of contamination and salinity of the water, the FRHTB complex unit can operate in several combined versions, according to the preset operating modes of the treatment and phytodesodorization system. At relatively high concentrations of pollutants and mineral salts, the entire water flow can be fed directly to the flotation clarifier reactor (9) and the self-cleaning Styrofoam filter (10) for pre-treatment and desalination before the bioremediation water together with the circulating regeneration water. For relatively medium contaminant concentrations, the water can be divided into two streams, one of which flows directly to the bio-plateau and the other is mixed with recycled water and treated in the clarifier-reactor and filter. The flow ratio is determined by parameters such as pollutant concentrations, water supply mode for treatment, quality requirements for treated and desalted water as well as the design of the Bio Plateau.
The operating modes of FRHTB bioplato plants also determine the types of reagents, activated natural slurry, effective enzymes and probiotics to be dosed or synthesised by the reagent farm (13). This allows the properties of the water to be regulated or
It makes it possible to modify them in the direction required by the user of the treated water and to purify water under any climatic conditions. In particular, reagents can be used to remove particularly toxic impurities (heavy metals, complex organic impurities of an industrial nature) and to condition treated water.
FRHTB bioplato can be a part of the overall complex scheme of physical-chemical technological scheme of water treatment. In this case the Bioplato can be placed at the beginning or at the end of the overall contaminated water treatment scheme. If the water contains high concentrations of readily oxidisable organic and mineral impurities and requires a deep reduction in salinity, the Bioplato can be placed at the beginning of the process flow diagram. This allows to remove the bulk of organic and especially toxic mineral impurities and reduce the concentration of salts, and at the next stage to achieve a deep purification of clarified water. Placing a bioplato at the end of the technological scheme allows to ensure deep purification of water from organic and inorganic impurities and to restore its natural properties due to contact with higher water plants.
Conclusions.
The analysis of a possibility of mineralized water phytodeselting in filtration-regenerative bioplato of FRHTB type shows:
1. FRHTB-based bioplato can be used for hydro-automated treatment and simultaneous membrane-free phyto-desodorization of mineralized water. Phytodesalination of mineralized water in FRHTB is carried out by extraction of salts by higher aquatic plants in bio-plateau and treatment of recycled reclaimed water with coagulants, magnetite, metal hydroxides, suspensions of natural minerals with additional electrochemical activation of reagents.
2. Installation for treatment and purification of circulating regeneration water of bioplato should include flotation reactor- clarifier for additional reagents and self-cleaning polystyrene foam filter with dosing into clarified water of enzymes, bioreagents and probiotics, which allows providing stable increase of green biomass and intensify salt extraction by higher aquatic plants.
3. Calculations show that under individual use of FWGTB the total demineralization of brackish water with initial salt concentration of 2500-3500 mg/dm3 can reach up to 40-60% depending on FWGTB design while ensuring the required degree of water recirculation-reclamation.
4. The next stage of the work is to simulate the process of integrated treatment and accompanying phytosaline desalination of brackish water by FRHTB technology and determine the main parameters of functioning of the technological scheme objects depending on the degree of water recirculation and concentrations of polluting components.