Biotechnical approach for a continuous simultaneous increase of indoor and outdoor air quality

Phytofiltration is the most sustainable way to achieve a better quality of inlet air in buildings in a polluted environment. But they don’t take into account the biorhythms of plants and pollute the inlet air with CO2 during breathing only time. We collected and analysed data about the biorhythms of plants. As a result, a new bi-directional phytofilter was offered for cleaning and oxygenation of the inlet ventilation air, and also to protect the environment by cleaning the exhaust air from different pollutants. The device has spaces with shifted illumination rhythms and a valve system. A controller directs the inlet air to the space(s), where plants release CO2. The outlet air runs through other spaces. Literature data show that in the less favourable case, the CO2 and oxygen emissions are balanced per day without overall CO2 gain to the environment. When plants are growing, they sequestrate CO2 to catch greenhouse gas emissions. Either natural light, artificial light, or a combination of the two can be used. While the second option simply demands one plant metabolism type, the first option needs a combination of CAM metabolism and other plants


Introduction
The fundamental issue with contemporary dense construction and high levels of traffic is their detrimental impact on the environment, which lowers the quality of the outdoor air.Outdoor air in industrial areas or near congested roadways is too filthy to be used for ventilation.The air is passed to an air handling unit, which can remove dust, and change temperature and humidity, but can't effectively eliminate the gas pollutants.Thus, dirty inlet air pollutes the indoor one.The last one is additionally polluted by indoor sources such as furniture, water supply, equipment, finishing materials, etc.And finally, the more polluted indoor air is released into the environment through exhaust ventilation and makes the environment dirtier.To solve the problem, we can use a biotechnological approach -green structures that connect living plants and building structures [1][2][3][4][5][6][7].The most perspective solution for inlet/internal ait treatment is fytofilters with living plants.Modern phytofiltration systems in rooms or ventilation systems run independently on the biorhythms of plants.At the time when plants don '

The current state of phytoremediation of indoor air 2.1. Mechanisms of removal of air pollutants by plants
Indoor air pollution can be remedied using methods such as chemical cleaning, assimilation by ventilation, insulation, and plant decontamination (phytoremediation).
Plants are autotrophic organisms that perform intensive gas exchange to carry out cellular vital activities, thanks to which air pollutants can be absorbed or accumulated inside [8][9][10].The term "Phytoremediation" means bioremediation of polluted air, soil and water with the help of plants [11].
Plants reduce the mobility, toxicity, and volume of pollutants through a variety of mechanisms, such as accumulation, volatilization, and degradation.
These biological processes ultimately require light energy (solar, high-efficient LED phytolamps), and therefore phytoremediation is a cheaper method than engineering ones.Phytoremediation is considered an alternative or additional air purification technology [11].
More than 35 years ago, the US National Aeronautics and Space Administration (NASA) faced health problems related to poor indoor air in fully enclosed systems in outer space.More than 300 volatile organic compounds (VOCs) were found in the environment of the spacecraft, which were cause the deterioration of the astronauts' health.To solve this problem, NASA researched the use of plants to remove pollutants and maintain a safe breathing environment.The first studies on the purifying properties of plants were conducted within the framework of NASA's Clean Air project in cooperation with the Association of Landscape Contractors of America (ALCA) [12].
The results of these studies have shown that some common indoor plants in combination with activated carbon as a substrate contribute to the natural purification of the air from several of toxic substances for humans, such as benzene, formaldehyde and trichloroethylene, helping to neutralize the sick building syndrome.The first list of air-purifying plants was formed by NASA and published in 1989 [12][13][14], which was aimed at research on air purification in the closed conditions of space stations.The plants included in the first list, in addition to the absorption of carbon dioxide and the release of oxygen, also eliminated a significant amount of benzene, formaldehyde and trichloroethylene.
The second and third lists were formed later by Wolverton in the book [15] and article [16].These works provide information about plants that remove more specific chemicals from the air.NASA researchers suggest that effective air purification can be achieved with at least one plant per 100 square feet (approximately 9.29 square meters) of a home or office space.At the initial stage of research, only plants grown hydroponically (that is, without soil) were considered, in later studies it was shown that microorganisms in a soil mixture in a pot can remove benzene from the air.Certain types of plants also contribute to the removal of benzene.Given the surface area required, using leaves alone was impractical, so the NASA team investigated the possibility of toxin removal by microorganisms in the rhizosphere.In addition, research conducted at NASA's National Space Technology Laboratories has shown that phytoremediation is effective in treating residential wastewater and in removing compounds containing radioactive components from both soil and wastewater [16,17].
In 1985, fundamental research that revealed the ability of indoor plants and their root microorganisms to clean indoor air from chemical pollutants received further development.However, the removal time of pollutants can be long [18].
There are different forms of plant pollution removal.One of them is absorption of pollutants by roots from soil and water.Root uptake is related to contaminant concentration and properties, plant species/composition, time of exposure, and other system variables.When organic pollutants (such as trichloroethylene) are present in the shoots, they can be transported to the roots by the phloem.The transformation or degradation of the pollutant by plant tissues is another important issue in phytoremediation [19] gaseous pollutants and particles such as dust and bioaerosols are adsorbed on the leaf surface, gaseous pollutants are absorbed by stomata and accumulate in various internal structures, photosynthesis can remove CO 2 and produce O 2 .Leaf transpiration and evaporation from the environment for rooting (soil, precipitation, sludge, sewage) [20] increase the moisture content.

Removal of volatile organic compounds (VOCs) from indoor air by plants
Volatile organic compounds (VOCs)are an important class of indoor air pollutants because they cause a wide range of harmful effects on human health, even at low concentrations.Despite this, currently available techniques for removing VOCs from indoor air are often ineffective, especially when it comes to gaseous compounds [21].
The idea of the removal of volatile organic compounds from indoor air by plants was also presented by Wolverton et al [12] in 1989, as discussed above.These studies confirmed that potted plants can remove significant or large amounts of gaseous VOCs in sealed chambers, reducing VOCs from 10 % to 90 % in 24 h [22].
Wolverton et al studied 12 plants for removing VOCs and demonstrated the possibility of improving indoor air quality by removing traces of organic pollutants from the air in energyefficient buildings.They showed that contact of the root area with air has a higher efficiency in removing organic pollutants [12,23].
The work [24] describes the following possible ways of removal of volatile organic compounds by plants: • removal of the above-ground zone of plants; • removal of VOCs by microorganisms in the soil; • removal through the root system; • removal by the plant environment (substrate).
Highlighting the method of VOC removal by microorganisms in the soil is worthwhile, which is a subspecies of phytoremediation and phytodegradation, which describes the breakdown of complex organic pollutants into simple compounds or the metabolism of pollutants in the phyllosphere and rhizosphere [11].
Transformation in the rhizosphere, also called rhizodegradation (or considered phytostimulation), is carried out by soil organisms such as bacteria, fungi or enzymes released from plants or microorganisms.Pollutants are degraded in the rhizosphere as well as inside plants by specific plant enzymes such as nitroreductases, dehalogenases, and laccases [25].Compounds secreted from plants, such as sugars, amino acids or enzymes, can stimulate the growth of bacteria in the soil and vice versa -stimulate microbial and fungal degradation by releasing enzymes into the rhizosphere.That is why rhizodegradation is also called phytoremediation or bioremediation with the help of plants [11].
During photosynthesis, plants absorb VOCs through stomata and cuticle wax, subsequently converting them into amino acids through the Calvin cycle [26].In the intercellular spaces of plants, absorbed pollutants are stored or react with the inner surface of leaves and the water film, then decompose or are released to the outside [26].
Other common gaseous air pollutants such as SO 2 , CO 2 , N O x and O 3 also accumulate in plant cells and tissues, mainly through stomata, wax and cuticle.Photosynthesis is the basic plant mechanism in which plants absorb CO 2 and convert it into oxygen.Stomata located on the epidermis of leaves and stems of plants are the primary zone where the process of gas exchange takes place [27].
Considering the potential of plants to remove VOCs, there is some research on benzene removal.In [28], it was shown that the main agents for removing benzene, as one of the model VOCs, are microorganisms of the rhizosphere of potting mix.These studies were the first demonstration of microbial degradation of soil VOCs from the gaseous phase.The results demonstrate the ability of the microcosm created in the pots to contribute to indoor air purification and lay the foundation for the development of the plant and substrate system as an additional biofiltration system.
Sriprapat and Thiravetyan [29] provided experimental data for eight types of plants, including Sansevieria trifasciata, Euphorbia milii, Epipremnum aureum, Syngonium podophyllum, Hedera helix, Chlorophytum comosum, Dracaena sanderiana and Clitoria ternatea used to remove benzene from air and water.These houseplants are well known for their high tolerance to toxic pollutants.Within 96 hours, C. comosum was found to have the greatest potential among other plants to remove benzene from indoor air.The addition of bacteria showed a lower rate of benzene removal than in the absence of bacteria.Also, the efficiency of benzene removal by Chlorophytum comosum under conditions of light and darkness was considered in the work [23].
Some studies revealed that bacteria, which are concentrated on the surfaces of plant leaves, contribute to the biodegradation of VOCs [30].Wetzel and Doucette [31] state that wax coating of cuticles on leaves can provide a simple, cost-effective way to sample indoor air for VOCs and help improve indoor air quality.
Correct selection of plant species plays an important role in removing VOCs from indoor air.Thus, in work [32], about 120 species of indoor plants were analysed by various researchers for the degree of VOC removal, which made it possible to draw the following conclusions: • tropical indoor plants such as Dracaena Janet Craig and Spathiphyllum have low VOC removal activity [33]; • the best plants with high VOC removal: Hemigraphis Alternata, Hosta Purpleheart, Hedera helix, Asparagus fern, Senecio Macroglossus 'Variegatus', and Crassula portulacea.
There is an interesting study that evaluated the potential of a Spathiphyllum wallisii to remove cigarette-derived VOCs and all particle size fractions [34], achieving a single-pass removal efficiency of 43.26 % for the total VOCs and 34.37 % for the total number of suspended particles.A botanical biofilter with a selected type of vegetation allowed the reduction of the concentration of many of harmful chemicals in tobacco smoke, including nicotine, limonene and toluene.
Usually, among volatile organic compounds, the main research of the scientific community is focused on formaldehyde.And phytoremediation is still the simplest and most environmentally safe technology for removing formaldehyde from indoor air [35].Being toxic to humans and other living organisms due to, for example, DNA damage, formaldehyde is also an indispensable substance in plant metabolism [36].According to the mass balance of water-air plant systems, the main mechanism of formaldehyde loss is the decomposition of formaldehyde in plant tissue caused by enzyme and redox reactions [37].The redox mechanism shows that the rate of formaldehyde removal can be accelerated by increasing reactive oxygen species caused by ambient pressure [38].
Historically, the above-mentioned NASA studies on the purification of plants from formaldehyde in a closed environment used methods of formaldehyde absorption by plants in hanging pots, which also contributed to the study of formaldehyde absorption by leaf stomata [39] These methods have evolved into a modern automatic system for monitoring formaldehyde uptake by plants, which shows how fumigation increases hydroponic plants that are the most efficient for formaldehyde uptake [40].Other factors affecting formaldehyde removal are the type of plants and the environment for their cultivation [28].
In [41], a number of experiments are given that demonstrate the absorption of formaldehyde by 376 different types of plants.Only five species (∼1.3%) absorb more than 10 mg/m 2 of formaldehyde, while 90% of plants absorb less than 5 mg/m 2 of formaldehyde, with the majority of plant species belonging to the genus Tillandsia having a level of 97%, so can be recommended for indoor use.
The work [42] showed that Epipremnum aureum is one of the most effective plant species for formaldehyde absorption due to its high stomata and leaf area.In addition, in [43], an experiment was conducted on the removal of formaldehyde by such plant species as Epipremnum aureum and Rohdea japonica in a hermetic chamber.The results showed that the stems can also effectively remove formaldehyde.The results also showed that when plant leaves were exposed to formaldehyde, CO 2 concentration increased with decreasing formaldehyde concentration, and the change in CO 2 concentration could be used as an indicator of the degree of formaldehyde decontamination by plants.
In general, the ability of succulent plants and plants with lanceolate, hard, leathery leaves to absorb formaldehyde is relatively low [44].
Similarly, 50 terrestrial and 15 hydroponic plant species exhibiting high formaldehyde uptake capacity were also found to be related to light intensity, photosynthesis, and biomass [45].Substrates for potted plants with high porosity, such as diatomite, peat, bark, sawdust, perlite, and bacterial residues, have a strong capacity to absorb formaldehyde [46].In addition, it has been confirmed [47] that leaf age is very important for formaldehyde absorption efficiency, since young leaves have low stomatal conductance, while mature leaves have higher stomatal conductance for formaldehyde absorption.
Formaldehyde gas accumulates in the leaves of plants and transported to the roots from the rhizosphere zone.This transport mainly depends on the absorption time and formaldehyde content in the leaves.Microorganisms on the surface of the leaves also absorb formaldehyde, further enhancing this process.
Indoors with continuous release of formaldehyde, early-stage uptake is mainly by roots and soil, while plant leaves play a smaller role in the early stage and a greater role in the later stages [48].
Plants are known to be associated with symbiotic microbes such as fungi and bacteria that mitigate abiotic and biotic stresses in them and enhance their growth.Plant-microbe interactions also play an important role during phytoremediation by degrading, detoxifying, or sequestering pollutants [49].
Therefore, the composition of soil rhizosphere microorganisms also plays an important role in the absorption and metabolism of gaseous formaldehyde by plants [50].In systems without rhizosphere microorganisms, formaldehyde is not removed, while in systems with a rhizosphere microbiome (10-45 mg/dm 3 ), all formaldehyde is removed [51].
Understanding the influence of rhizosphere microflora on formaldehyde metabolism is important to increase the efficiency of uptake and removal by houseplants [52].
The use of microorganisms can improve the remediation capacity of potted plants.The addition of cultivated microorganisms to the rhizosphere of such plant species as Aloe vera and Tradescantia zebrine improved the efficiency of formaldehyde removal [53].In addition, a hydroponic system was developed that consisted of plants of the species Ophiopogon japonicus, phenol-degrading bacteria such as Staphylococcus epidermis and Pseudomonas spp.and air compressor.This system showed a high capacity for phenol decomposition of about 1000 g/dm 3 daily [53].
The authors [54] showed that the rate of formaldehyde removal is not only related to the size of plants in pots but also closely related to air dynamics.They used the parameter of equivalent clean air supply rate, which is commonly used to quantify air cleaning capacity, and showed that the investigated parameter is only 5.1 m 3 /h per m 2 room for static air, and 233 m 3 /h per m 2 room when the air was transported through potted plants.
On the other hand, work [55] shows that, according to the ASHRAE standard 62.1-2016, the minimum ventilation rate in the working zones of office premises is 0.3 dm 3 /s for every m 2 of space (1.08 m 3 /h for every square meter of space) [56]; similarly to the NEN-EN 15251-2007 standard, the minimum ventilation rate for new or reconstructed buildings is 0.35 dm 3 /s for each m 2 of the room area (1.26 m 3 /h for each square meter of area) [57].
In the standard [56], it was established that the rate of supply of clean air for formaldehyde removal by potted plants is 0.03 m 3 /h, therefore, to meet the standards without an additional ventilation system, it is necessary to have 42 plants for every square meter of the room area.
In addition, the authors established [55] that the rate of supply of clean air for CO 2 depletion by plants in pots is 0.01 m 3 /h (Reace lily) and 0.02 m 3 /h (Nephrolepis exaltata).Therefore, it is necessary to have >100 plants for every square meter of room area to meet the standards without an additional ventilation system.
Despite the large number of works devoted to the study of the removal of VOCs from indoor air by plants [58,59], there is still a need for additional research on the modification of plant physiology by, for example, rhizosphere modification to increase phytoremediation of indoor gaseous formaldehyde.
There is an inaccuracy related to volatile organic compounds.There is a subclass of it called volatile phytoorganic compounds, which contain healthy substances -phytoncides, odorants etc.But gas analysers often display total volatile organic compounds (TVOC) without division.When the authors perform experiments with plants, D91 gas analyser alerts the deadly content of TVOC, but the experimenters feel very good and stay healthy.Thus, we should be very careful interpreting the measuring results in greened rooms.When measuring the absorption of total volatile organic compounds by plants, the result will be always strictly underestimated or inverted.Thus, we can perform experiments only on individual compounds and only after testing the measuring sensors for the influence of volatile phytoorganic compounds.For example, we can bring the sensors to plants in a clean room and check the insignificance of rising the readings.
Phytoremediation was evaluated in [64] as a solution for trimethylamine (TMA) removal.Trimethylamine (TMA, N(CH3)3) is a volatile tertiary amine, which is a decomposition product of nitrogen-containing organic substances of animal and plant origin.It is a colourless gas that has a fishy odour at low concentrations and an ammonia-like odour at higher concentrations.Eight species of pot plants were used in the research: Opuntia, Dracaena sanderiana, Dieffenbachia camilla, Tradescantia spathacea, Peperomia magnoliifolia, Chlorophytum comosum, Cereus hexagonus (L.) Mill., and Scindapsus aureus, which were selected as candidates for TMA removal under light and dark conditions.
According to the research results, the authors divided the selected plants into two groups: plants with high TMA removal efficiency under light conditions and plants with high removal efficiency under light and dark conditions.The results showed that S. aureus had the highest TMA removal efficiency under light conditions after 72 hours (> 95 %).However, it had very low efficiency under dark conditions, suggesting that S. aureus should be housed in areas with 24-hour light sources.On the other hand, cactus species (C.hexagonus (L.) Mill.and Opuntia) remove TMA highly efficiently in both light and dark conditions after 72 hours (> 90 %) and therefore may be more suitable for real-world use.In addition, the ANOVA (analysis of variance) results confirm with 95 % confidence that plant type (chosen from 8 types) and lighting conditions (e.g., LED lighting, fluorescent lighting, or dark conditions) have a significant effect on TMA removal after 24 h.

Reducing the level of CO 2 in a room
Regarding the reduction of carbon dioxide (CO 2 ) levels and the expected level of air quality, the results showed a positive effect of indoor green spaces in reducing CO 2 levels [60].In addition, the authors also found that the concentration of CO 2 changes depending on human activity inside the living space.
Thus, work [61] shows that in non-industrial premises, such as offices, schools and houses, the main source of CO 2 is human metabolism.In addition, it is justified that CO 2 is no longer considered a pollutant, but an indicator of the presence of pollutants associated with the presence of people indoors.
It has been proven [62] that indoor plants improve the indoor atmosphere, relieve anxiety and reduce CO 2 concentration.In addition, there are a number of studies that confirm the removal of other types of indoor air pollutants by potted plants.For example, in [63] the mechanisms of nitrogen oxide removal by dry deposition by potted plants were considered.The principle of absorption of pollutants by leaves and the factors affecting the removal of nitrogen oxides are substantiated, providing a theoretical basis for the selection of urban green vegetation.

Microorganisms
The work [1] shows the results of research based on the microbiota factor.The impact of plants on air quality was determined by monitoring the microbiological state of the air in several office premises in order to select promising plant species for creating health-improving interiors.Observational data demonstrate the bactericidal effect of the investigated phytoncide plants.It has been experimentally established that the microbial count in rooms with plants is significantly lower than in a reference room without plants.In the room with plants, the number of microorganisms was 1.5 ... 5 times less, depending on the period of observation.In addition, work [1] offers an assortment of plants, namely: Ficus benjamina, Ficus benjamina Wiandi, Zamioculcas Zamiifolia, Dracaena marginata, Dracaena fragrans, Yucca elephantipes, Schefflera digitata, Aspidistra elatior, Crassula ovata, Spathiphyllum wallisii.

Technology of active "green" structures for indoor air purification
Active "green" structures are one of the newest varieties of phytoremediation systems, which include ventilation, heating and cooling of the house using evaporative cooling -by the same principle as the cooling effect of outdoor green structures [3].It operates on full recirculation, taking and returning the air to the room (figure 1).The system of "green" walls, which cleans the air inside, also acts as a temperature and humidity control.
The authors [65] studied the system over 300 days.Satisfactory efficiency was found to remove formaldehyde and toluene (90 % and more than 33 %, respectively, during the first four days).In addition, this ACBB system successfully reduced the indoor air temperature by 0.5 °C in the real environment, while the temperature reduction was 1 °C in the laboratory.The increase in relative humidity for realistic and laboratory conditions was 17.7 and 9-13 %, respectively.In addition, a 20 % reduction in outside air supply was achieved with the ACBB system, saving the energy required by the building, given that the toluene and formaldehyde concentrations determined the standard ventilation rate for this case.Also, the analysis of the studied data confirmed the reduction of particulate matter (PM) concentrations to a level that the current indoor HVAC system could not achieve under normal conditions.
The same authors [65] also conducted many studies in laboratory conditions.The efficiency of the system in removing PM was evaluated exclusively in a static chamber, where a general observation recorded that the efficiency of PM removal increased with an increase in the airflow rate.Experiments were conducted with five different air flow rates to evaluate the efficiency of the ACBB system for filtering total suspended particles, PM 2.5 and PM 10.Based on the collected data and further analysis, this study estimates that for four workers in an office space, 1 m 2 of the system can sufficiently provide the necessary ventilation.
Therefore, since indoor air quality in buildings contributes to human health problems on a large scale, implementing sustainable solutions to maintain indoor air quality has become a particular challenge for the scientific community.Active botanical biofiltration systems are effective in removing pollutants along with the potential to increase humidity and cool the air.It is also well-studied that these systems do not promote the spread of fungi if they are maintained in a well-controlled state [66].Moreover, the aesthetic value of the systems is an additional advantage for positive mental effects.
Since there is currently no software or computational methods capable of recognizing plants as indoor "air cleaners", the impact of phytoremediation on the operation of HVAC systems and the energy sector is still poorly researched.
In the work [67], Azolla filiculoides was integrated into building walls for bioremediation of indoor air and a new protocol for assessing the effectiveness of a "green" wall using the "Indoor Air Quality" and "Ventilation Rate" techniques was proposed, which are recognized as practical engineering methods for determining the efficiency of air exchange using mechanical equipment.Azolla filiculoides has been observed to help absorb indoor CO 2 , while under the Indoor Air Quality methodology, this results in a reduction in the building's ventilation rate and, as a result, a reduction in the building's overall energy consumption.In addition, the cooling effect of a green wall with Azolla filiculoides as a natural shading component was evaluated by applying its latent heat capacity to the thermal convection/conductivity of the facade, compared to aluminium louvres of the same size and spacing to computationally assess the thermal benefits of plants for improvement of cavity/inner space overheating.
So, based on the work [55], it is possible to define advantages regarding the effectiveness of the use of vegetation in rooms, which are based on scientific research over the past 30 years, of which the main ones are: • Biophilic design and vegetation have a positive effect on people who spend a long time indoors, which is manifested in an increase in the general psychological satisfaction and mood of people.• Evaporation of plants has been found to help lower ambient temperature, so this property of plants can be used to cool air and control humidity.• It has been shown that green plants can be used to reduce the sound level -as a passive acoustic insulation system.• The effect of vegetation on improving indoor air quality was analysed.However, there is still a lack of reliable and relevant data to understand the true pollutant removal mechanisms and factors in these systems (plant species, microorganism species, gas composition, light source, number of plants).• It has been shown that in an active vegetation system ("green" systems in combination with mechanical fans) the rate of air purification can be significantly higher than in a passive vegetation system (potted plants).
Nevertheless, no attention is paid to the time, when plants don't release oxygen.The daily biorythms of plants weren't analysed, which can principally change the conclusions.Thus, we need to focus on this challenge to avoid troubles in the corresponding daytime.

Materials and methods
The literature review shows the most naturally promising approach for air quality controlplants [1,2,11,68,69].They continuously humidify the air, remove volatile chemicals, dust, and other impurities, and release valuable volatile phytoorganic substances such as odorants and phytoncides.The last ones are the immunity substances that kill viruses and bacteria.
During active oxygen release time, plants emit oxygen while absorbing CO 2 [70,71].At other times, plants breathe only and pollute the surrounding air by CO 2 .Except for [72] Crassulacean acid metabolism (CAM), which releases oxygen at night, most plants produce oxygen during the day.
To check the overall effect, we can analyse the outcomes of the Aglaonema roebelinii testing conducted by Intell House [73].To control illumination, the researchers use two packets -lighttight and transparent.The measurements were performed using NetAtmo Weather Station.In the first packet, the CO 2 content increases from 796 to 2311 ppm within 40 minutes.The concentration in the transparent packet under the sun decreases from 1060 to 315 ppm.The concentration starts to rise at 16:06, reaches 1572 ppm at 6:54 and decreases to 263 ppm in the morning.The researchers concluded of no overall oxygenation during the day.The experimental method is very simple and didn't take into account many factors such as the dependency of photosynthesis on CO 2 content.But we can conclude that in the less favourable case, the CO 2 and oxygen emissions are balanced per day without overall CO 2 gain to the environment.It's good for us because phytofiltration will never cause greenhouse gas emissions.The second important conclusion is that we should develop a new phytofilter that operates accordingly to the plant's biorhythms to avoid temporary pollution of the inlet/indoor air.To continuously reduce indoor and outdoor air pollution and oxygenate the inlet/indoor air, active oxygen release should be carried out in the inlet air and breathing only in the exhaust.

Results and discussion
We suggest a design for an inlet-exhaust ventilation phytofilter module (figure 2).The phytofilter contains a valve system 4, which controls the airflow through spaces with plants 3 and the corresponding illumination 5. Ducts 1 should be connected to inlet and outlet ventilation ducts or an air handling unit.Controller 6 switches illumination 5 by rotation to keep artificial biorhythms of the plants inside.Some spaces 3 can be equipped with glazing to utilize natural light.The valves are opened-closed together with the illumination to pass inlet air through space(s) 3 with oxygen release and outlet one -through other spaces 3.For 24/7 operation with most plants, the phytofilter should contain at least three cameras.But for working time up to 16 h/day two cameras will be enough.
The main problem is the energy consumption of phytolamps.It's impossible to split oxyden and carbon from CO 2 without energy consumption because in the opposite case, we will obtain perpetum mobile by burning the carbon.Thus we should find opportunities to use free accessible energy for illumination or utilise the heat from the lamp.There are some ways: • utilizing natural illumination only through transparent walls demands the combination of CAM and other plants, but this raises biomass requirements due to CAM's reduced efficiency as a result of its overnight energy discharge and daily energy charge, so the exergy destruction will occur twice; • the use of natural illumination for spaces with daily oxygen release; • the phytofilter should be installed after an air-handling unit with the heater, cooler, and re-heater (figure 2), thus the heat from illumination will reduce the requirement for energy We require tiny plants with highly efficient photosynthesis and cleaning for a compact design.
If some of the suggested plants grow larger than necessary, trimming can readily fix their crown size.It's possible to use the secondary materials for substratum, for example, broken bricks or wood after shelling from the russian federation.The main condition is to avoid secondary pollution.For example, particle boards are not suitable.

Conclusions
Phytoremediation is a perspective technology for increasing outdoor and indoor air quality.It allows for decreasing VOCs and microbes.But known indoor phytoremediation systems don't take into account plant biorhythms, which causes secondary CO 2 air pollution.We propose a new ventilation phytofilter, which can organise airflows to achieve two goals simultaneously: cleaning and oxygenating inlet air 24/7 and cleaning exhaust air to make a building more environment-friendly, which is very important for densely-built districts.
The continuation of the research is creating and testing experimental models of the phytofilter.Now we are researching gas exchange in plants using a gas-exchange camera in the Laboratory of Heat-Mass Exchange in Green Structures.The results will allow calculation the required biomass.The most throttling factor is the war of the russian federation against Ukraine, which significantly reduce sponsorship possibilities.