A flat-parallel type photobioreactor design for sewage water treatment

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National Aviation University

National Transport University

A FLAT-PARALLEL TYPE PHOTOBIOREACTOR DESIGN FOR SEWAGE WATER TREATMENT

Lesia Pavliukh, PhD, Associate Professor

Sergei Shamanskyi, Doctor of

Engineering. Associate Professor

Zaiats Olga, PhD, Associate Professor

Introduction

Organic and inorganic substances which were released into the environment as a result of domestic, agricultural and industrial water activities lead to organic and inorganic pollution. The normal primary and secondary treatment processes of these wastewaters have been introduced in a growing number of places, in order to eliminate the easily settled materials and to oxidize the organic material present in wastewater. The final result is a clear, apparently clean effluent which is discharged into natural water bodies. This secondary effluent is, however, loaded with inorganic nitrogen and phosphorus and causes eutrophication and more long-term problems because of refractory organics and heavy metals that are discharged. Microalgae culture offers an interesting step for wastewater treatments, because they provide a tertiary biotreatment coupled with the production of potentially valuable biomass, which can be used for several purposes. Microalgae cultures offer an elegant solution to tertiary and quandary treatments due to the ability of microalgae to use inorganic nitrogen and phosphorus for their growth. And also, for their capacity to remove heavy metals, as well as some toxic organic compounds, the refore, it does not lead to secondary pollution. Compared to higher plants, microalgae are simple in structure, being unicellular, filamentous or colonial, and energy is directed via photosynthesis into growth and reproduction; they do not need to establish and maintain complex tissues and organs [1]. More recently these photosynthetic microbes have also become the focus of considerable attention as a potential source of oils for biodiesel production [2-5].

In the current review we will highlight on the role of microalgae in the wastewater treatment processes.

Problem statement

photobioreactor wastewater treatment tank

Microalgae have been proposed as an option for wastewater treatment since the 1960s, but still, this technology has not been expanded to an industrial scale. The main reason for the small-scale use of microalgae in purification processes is their cultivation.

The purpose of this article is to develop a photobioreactor design for wastewater treatment in which the application of new elements and connections reduces material consumption for the manufacture of transparent flexible tanks, reducing labor costs for installation and dismantling of tanks, and preventing the mixing of immobilized algae and their removal from the working area of the photobioreactor.

Analysis of previous research

To design adequate photobioreactors, it is mandatory to understand the major phenomena limiting the performance of microalgae cells such as light availability, nutrients supply including CO2, environmental conditions including temperature and solar radiation, and mixing. To fulfill the requirements of microalgae cells, different technologies has been proposed such as raceway and thin-layer open reactors, in addition to tubular and flat-plate closed reactors. These technologies are still being upgraded and improved to maximize the biomass production capacity and to reduce the production cost. Additionally, the control and modeling of these reactors is a hot topic for the industrial development of microalgae-based processes [6].

The current market of microalgae biomass is estimated to be around 20,000 t per year; the price ranges between 30 and 300 € kg^-1. Mass cultures of microalgae have traditionally been cultivated in open ponds which are much cheaper and easier to operate than photobioreactors (PBRs). Several PBR designs have been proposed in recent years, to grow microalgae as a source of sustainable energy. For the cultivation of most species suitable for biodiesel production and for human consumption, the use of a closed system is advisable. Indeed, most of the species cultivated for oil production require strict control of the temperature between 20 and 30 °C, which is problematical to maintain in open ponds. PBRs are also recommended for the production of high- value compounds for which the strict control of culture variables is necessary in order to satisfy the good manufacturing practice (GMP) requirement for pharmaceutical products. However, it is worth pointing out that although a major advantage of closed PBRs is their ability to prevent contact of the microalgae culture with the atmosphere, the risk of pollution cannot be ruled out. Among the closed systems, tubular PBRs are the most common design developed at an industrial level [7]. The advantages and limitations of tubular PBRs have been discussed in several book chapters [8-10].Because of the high production cost usually reached with this culture system, the main R&D on PBR design is aimed at achieving high light conversion efficiency, i.e. at pushing productivity well beyond the one currently attained, which -- it seems -- is the main way to develop cost-effective tubular PBRs.

It is known photobioreactor design for wastewater treatment [11], containing: flowing rectangular open-topped container, inside which are transparent tubes, with are interconnected and fixed in a rectangular container by the knees in such a way as to form a continuous zigzag coil into which a mixture of wastewater with microalgae prepared in a mixer and carbon dioxide is fed, the transparent tubes being fixed at an angle to horizon, and placed in a row on one side of the coil elbows are located above the elbows on its opposite side and they are valves for the release of accumulated gases there, while at the outlet of the pipeline is a separator and a guide tray for supplying separated from microalgae wastewater into body capacity.

The disadvantage of this design is that the design occupies a large area due to the horizontal location of the coil tubes, the need to use a long coil due to the impossibility of immobilization of microalgae in the working area, the need to disassemble the coil with significant labor costs and stopping the photobioreactor for a long period to clean the transparent tubes covered with sediment inside.

Also it is known photobioreactor design [12] which is made in the form of a transparent flowing rectangular open-topped tank, inside of which are vertically attached to the bottom of the tank by quick-release fasteners transparent flowing flexible hoses, to which at the bottom by means of nonreturn valves are connected pipelines for wastewater and microalgae supply and tubes for carbon dioxide supply and are connected by means of shut-off valves pipelines for drainage of a mixture of microalgae with residual wastewater, and in the upper hermetic part, where there are valves for drainage of accumulated gases, while the pipeline for the purified wastewater discharge is connected to a guide tray purified wastewater supply inside of a flowing rectangular open-topped tank, and at the outlet of the pipeline for drainage of a mixture of microalgae with residual wastewater is a microalgae separator to separate return and excess biomass.

The disadvantage of this design is that the design has a cylindrical arrangement, namely transparent flexible hoses are made in the form of a large number of cylinders of small diameter, which leads to overconsumption of material, as well as significant labor costs during installation and dismantling of hoses for cleaning or replacement. gas occurs directly in the working area of the hoses, which promotes mixing and removal of immobilized microalgae from the working area.

It is known airlift and bubble-column bioreactors are simple devices that have been used in bioprocessing, wastewater treatment, and the chemical process industry. These vertical column photobioreactors are compact, low-cost, and easy to operate axenically [13; 14]. Pneumatically agitated bubble columns and airlift devices attain the requisite mass transfer coefficient of 0.006 * s^-1 and liquid circulation velocity at a relatively low power input for practicable culture of microalgae [14; 15]. Camacho et al. reported that the maximum biomass volumetric productivity of Phaeodactylum tricornutum obtained in vertically oriented concentric-tube airlift photobioreactors was about half of that obtained by a horizontal-loop tubular photobioreactor [16]. A similar result was obtained in a bubble column, where the volumetric biomass productivity was about 60 % of that in horizontal-loop tubular photobioreactor for Phaeodactylum tricornutum grown under identical conditions [17].

Compared to bubble column photobioreactors, air-lift photobioreactors showed superior growth of microalgae. At a low aeration rate of 1 - L min^-1, cultures of Undaria pinnatifida and Porphyridium Sp. grown in an airlift reactor attained 33-50 % higher growth rates than when grown in a bubble column [18; 19]. Diatom yields were also about double in an airlift device than in a bubble column [20]. It appears that, in contrast to a bubble column where cell flow patterns are more random, the airlift system produces a more homogeneous flow pattern that moves cells from dark (riser) to light (downcomer) zones [21]. Thus, cells in a bubble column may reside in high or in low light intensities for a long time without circulation. Furthermore, cell sedimentation occurred in the bubble column while cells remained more uniformly suspended in the medium of the airlift photobioreactor [18; 21]. When large-scale diatom cultivation was compared in both bubble column and airlift photobioreactors, there was no significant different in the specific growth rate, which was 2.46M0^-2 h^-1 and 2.58M0^-2 h^-1 for bubble column and airlift reactor, respectively [20]. This may be the result of the non-ideal flow pattern in the airlift system and the internal circulation within the riser itself.

Different photobioreactor designs for microalgae cultivation are presented in the Table.

Table

Different Photobioreactor Designs for Microalgae Cultivation [22]

The photobioreactor-wastewater treatment plant of flat-parallel layout works as follows.

Housing 1 is located in an open area with natural light. The work cycle is divided into three stages: the start operation; wastewater treatment cycle; cycle completion.

At the start stage, the housing 1 is filled with clear water so that the flat transparent containers 2are attached to the bottom of the housing with the floats 3 attached to their upper parts are in a vertical position. The pre-clarified wastewater to be treated is saturated with carbon dioxide from the tank 9 by means of a carbon dioxide injection compressor 10. The carbon dioxide-saturated wastewater is mixed with microalgae from the tank 16 pumped by the microalgae injection pump 17.

Then the mixture by means of wastewater injection pump 7 is fed to the offshoot of the wastewater supply pipeline4, by meansof which through nonreturn valves 6 is distributed over the volume of flat transparent containers 2 until they are filled. After that, the supply of microalgae is stopped.

In the purification step, the carbon dioxide- saturated wastewater is continuously supplied to the offshoot of the wastewater supply pipeline 4 by means of a wastewater injection pump 7 and enters the lower parts of flat transparent tanks 2 through non-return valves 6.

An excess of carbon dioxide that can enter the wastewater during saturation, are discharged into the environment by means of nipples 8 mounted on offshoots 4.

Inside the flat transparent tanks 2 wastewater moves from the bottom up and in their upper parts through the valves 19 is discharged by means of the treated wastewater pipeline 20. The vertical velocity of wastewater is selected so that the microalgae inside the tanks are in a suspended state and are not carried away with water.

Part of the carbon dioxide that can be released from the saturated wastewater inside the flat transparent tanks 2 is discharged into the environment through the nipple 18.

The treated wastewater is collected by pipe 20 and through the treated wastewater return pipe 21 and the guide tray 22 is fed into the distribution tray 23 of the housing 1.

The distribution tray 23 evenly distributes the flow of treated wastewater over the internal volume of the housing. At the opposite end of the housing 1, the wastewater is collected by the collecting tray 24 and removed from the photobioreactor using the drain tray 25. At the stage of cycle completion, the wastewater supply by the pump 7 is stopped.

The mixture of microalgae with wastewater through the elements of the check valve 11 is discharged from flat transparent containers 2 and through the microalgae drainage pipe12 is fed into the microalgae separator 13.

The recirculated biomass is piped to the microalgae tank 16, and the excess biomass is discharged through the excess biomass pipe 14.

After the loss of transparency of the walls of the flat transparent tanks 2 at the end of the cycle, the inner tank of the housing 1 is emptied, attached to the bottom of the housing with quick-release fasteners the flat transparent containers 2 are disconnected and replaced with new or previously cleaned.

Conclusions

During the past decades, great progress has been made in bioengineering and biotechnology for efficient microalgae mass production. Thus, several types of closed culture systems, such as tubular or flat plate photobioreactors show promise for application in large scale microalgae culture for bioenergy, aqua- and agricultural uses.

The proposed construction of photobioreactor can have good perspectives to be use in communal services for sewage water purification from biogenic elements.

Improved photobioreactor design also can be used in sewage systems of enterprises of different branches of industry, when it is necessary to purify sewage water.

The proposed flat-parallel type photobioreactor design for sewage water treatment differs by changing the geometric shape of the tanks, which serves a mixture of water and microalgae, resulting in a reduction in material costs for the manufacture of tanks and labor costs for their maintenance.

The future of microalgae biotechnology appears promising, and innovative processes and products are expected to emerge over the next few years.

References

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Анотація

Павлюх Л., Шаманський С., Заяц О.

ФОТОБІОРЕАКТОР ПЛОСКО-ПАРАЛЕЛЬНОГО КОМПОНУВАННЯ ДЛЯ ОЧИЩЕННЯ СТІЧНИХ ВОД

Вступ

Органічні та неорганічні речовини, які потрапили в навколишнє середовище в результаті побутової, сільськогосподарської та промислової діяльності з води, призводять до органічного та неорганічного забруднення. Стоки завантажуються неорганічним азотом і фосфором і викликають евтрофікацію. Культура мікроводоростей пропонує цікавий крок для очищення стічних вод, оскільки вони забезпечують третинне біоoчищення у поєднанні з виробництвом потенційно цінної біомаси, яку можна використовувати для кількох цілей.

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