EVALUATION OF THE PERFORMANCE OF AEROTANKS DUE TO ADD-ON ATTACHED BIOCENOSIS

Іnstitute of Hydromechanics of the NAS of Ukraine, Zheliabov St., 8/4, Kyiv, Ukraine, 03057, tel. +38 (050) 559 72 89, +38 (044) 459 65 10, e-mail Oleynik2019Al@gmail.com, ORCID 0000-0002-9110-1709 Dep. «Water Supply, Sewerage and Purification of Waters», O. M. Beketov National University of Urban Economy in Kharkiv, Marshal Bazhanov St., 17, Kharkiv, Ukraine, 61002, tel. +38 (057) 707 33 40, +38 (057) 707 31 06, e-mail Tamara.Ayrapetyan@kname.edu.ua, ORCID 0000-0002-8834-5622 Іnstitute of Hydromechanics of the NAS of Ukraine, Zheliabov St., 8/4, Kyiv, Ukraine, 03057, tel. +38 (063) 917 87 84, e-mail kurganska@ukr.net, ORCID 0000-0002-7593-6458


Introduction
Among the most important environmental problems of the present day, the issue of protecting natural sources of water supply is of paramount importance. The discharging of raw and insufficiently treated wastewater in the surface water reservoirs constantly worsens their condition; these sources of pollution, as a rule, contain compounds of nitrogen and phosphorus. Therefore, deep purification of wastewater is one of the most important problems for today.
The accumulated experience and scientific research of many generations of specialists prove the priority of biological methods of purification. Today, the majority of wastewater, which is discharged in a centralized way, is cleared at aeration stations in aerotanks [1,2,4,5,12]. However, the existing conventional biological purification technology is characterized by inadequate efficiency (especially for the removal of phosphorus compounds) and is not always able to meet the set norms [3,7,14,16,19,23].
Therefore, in wastewater treatment plants, where biological treatment of effluent is carried out according to the traditional scheme, the important practical tasks are the issues of improvement of existing technologies of wastewater treatment on the basis of these structures, increase of their efficiency.
In general, the following solutions are used to improve the efficiency of aerotanks: the creation of sites with aerobic and anaerobic conditions of oxi- dation [2,13,21], the use of uneven dispersed effluent intake [2], the supply of technical oxygen [2,15], the addition of reagent preparations [2,15], etc. It is clear that the improvement of structures and technologies is delayed by economic factors. Also currently, the widely used effluent clarification method has become the process of simultaneous removal of organic pollutants (OP) and nitrogen by suspended and attached biocenosis [20,22]. By their activity immobilized biocenosis considerably exceed the oxidation capacity of suspended active sludge. In this case, the loading elements (nozzles, screens, etc.) can be located throughout the volume of the aerotanks or more densely and compactly only in its individual sections.
However, the possibility of evaluating the efficiency of aerotank due to add-on elements with attached biocenosis (biofilm) requires more complete and advanced mathematical models. Models that will take into account the processes of simultaneous oxidation of OP by suspended and attached biocenosis will allow to reasonably estimate the influence of various factors of purification in aerotanks. These processes are associated with the formation of a biofilm of different thickness and structure on the surface of the additional loading (screen) in the aerotank volume [13,21].
The parameters of wastewater treatment, determined by numerical methods, taking into account the influence of various purification factors will give an opportunity to obtain reliable results. Based on the results obtained, it is possible to evaluate the effectiveness of the aerotanks through the introduction of additional elements with the attached biocenosis (biofilm).

Purpose
The main purpose is to carry out calculations and obtain a comparative assessment of the efficiency of complete-mix aerotanks and continuousflow aeration tanks. Herewith wastewater is treated due to the additional arrangement in their volume of loading with the attached biocenosis (biomass) in the form of a biofilm with a high concentration of microorganisms.
To achieve this purpose, the following tasks need to be addressed: -Provide calculation dependencies and mathematical models that allow estimating the influence of various factors on the simultaneous removal of organic pollutants (OP) with suspended and attached biocenosis in aerotanks of various structures, namely in complete-mix aerotanks and continuous-flow aeration tanks; -On the basis of the proposed models and methods of calculation, to conduct an analysis and evaluation of the influence of the main factors, to identify the parameters of supply of the required amount of oxygen to the place of reaction and the disposal of OP; -To consider the specific examples for different process flow schemes of the location of the attached biomass reactor and to provide comparative results of studies of simultaneous OP purification; -To substantiate the increased efficiency of effluent clarification due to the arrangement of the attached biocenosis in the volume of the aerotanks.

Methodology
Mathematical models were constructed and methods for calculating the parameters of purification [6,7,11,17,18] were developed for the estimation and analysis of the simultaneous removal of organic pollutants (OP) with suspended and attached biocenosis in aerotanks of various structures. In this case, for various hydrodynamic schemes of aerotanks, namely complete-mix aerotanks and continuous-flow aeration tanks. The specific examples showed and substantiated a significant increase in the efficiency of purification due to the add-on attached biocenosis.
First, let us consider the most advisable from the practical point of view technological scheme of a complete-mix aerotank, which consists of two parts (reactors 1 and 2) [7]. In reactor 1, the OP is removed only due to the suspended biocenosis (active sludge), that is, it works as an ordinary complete-mix aerotank. In the second part (reactor 2), the OP removal occurs mainly by the attached biocenosis, which is formed at the add-on load, with the possible consideration, if necessary, of the action of the active sludge in the reactor 2. It is believed that the biochemical oxidation process is adequately provided with oxygen, that is, the flow of oxygen will not limit the kinetics of oxidation of both the suspended and attached biocenosis. Let us consider also the most widespread case in which the removal of OP by an active sludge occurs by a zero-order reaction, and in a biofilmby the first-order reaction. Then, according to [7], to determine the concentration of OP at the output from the 2nd reactor (of the aerotank) La2, the following general dependence is proposed For the 1st reactor, the volume of the liquid is assumed to be equal to the reactor volume 1 The specific loading area with the biofilm in the reactor 2 is determined by the formula under the assumption that the reaction rate in the biofilm is taken of the first order, are given in the papers [7,9]. Here . The results presented in Fig. 1 show that post-treatment by the attached biomass can significantly improve the quality of wastewater treatment. However, as shown by Fig. 2, with the increase of the parameter γ, which depends on the flow N and mainly on the specific loading area n F  , that is, on the load element density, this improvement is significantly slowed down. Therefore, in this case, as in other process flow schemes of aerotanks with attached biocenosis, an important issue is the choice of the most appropriate loading area  F based on the additional analysis.
As can be seen from the data in Fig. 1 and Fig. 3 the active sludge in the second reactor with the same values of the parameter τ does not significantly affect the overall clarification effect. However, clarification in the free volume of the 2nd reactor allows for some increase in the value of the The paper [10 ( Fig. 2, 3)] presents the comparative results of the studies of simultaneous OP clarification in complete-mix aerotank, in which the elements of additional loading are evenly arranged. When constructing these graphs, the dependence (5) is used, in which the parameters τ and γ are taken in accordance with the adopted process flow scheme of the complete-mix aerotank, namely: Fδl is the total area of loading elements in the complete-mix aerotank with length l and volume Wal. According to [10 ( Fig. 2, 3)], only at significant values of the parameter τ and minor values of γ one should also take into account the removal of OP due to the active sludge.
The obtained dependencies and performed calculations allow substantiating the increase in efficiency of the aerotank using the arrangement of loading elements with the attached biocenosis in the second half of its volume.
The works [7,10] present theoretical studies and methods for calculating the parameters of biological wastewater treatment from organic pollutants (OP) in continuous-flow aeration tanks with simulatneous purification of suspended and attached biocenosis. To evaluate the efficiency of such aerotanks, let us consider the scheme of their work with the suspended and attached biocenosis when the loading elements (nozzles, screens, etc.) are evenly spaced along the entire length l [10].
As a result of the theoretical studies carried out under the condition that the removal of the OP by the suspended and attached biocenosis occurs by the first-order reaction, the following dependence is obtained to determine the change in the concentration of La along the aerotank X where   , , is the loading surface area (biofilm) per unit length of aerotank l (m), is the biofilm parameter determined according to the recommendations given in the papers [7,9]. At the exit of the aerotank we have In order to carry out further analysis and obtain comparative estimates of the influence of various factors on the clarification processes in this case, we represent the dependence (10) in the form  The results of the calculations presented in [8 ( Fig. 3)] and in Fig. 4 show that the additional introduction into the aerotank of loading with the attached biocenosis (biofilm) allows to significantly improve the work of the continuous-flow aeration tank, especially with BL < 1.
The work [13] presents an experimental evaluation of the efficiency (EF) of the complete-mix aerotank due to add-on plastic screens, the results of which at 110  a Q ml/min,  Table 1.
The Table 1 shows a significant increase in EF, a decrease in La when using 6 screens and a gradual increase in efficiency with increasing number of screens. In this case, the total increase in efficiency is 25%. As noted earlier in Fig. 2, 3 and in [10 ( Fig. 2)], the intensity of the increase in efficiency is stabilized upon achievement of the minimum concentration of pollutants at the output La .

Findings
The comparative calculations on the presented dependencies make it possible to estimate the intensity of increasing the clarification efficiency in aerotanks due to additional elements with attached biocenosis. The constructed models take into account the special features of the intake of organic pollutants and oxygen in a biofilm, its parameters, as well as the velocity of kinetic reactions with the simultaneous removal of organic contamination by suspended and attached biocenosis.
Implementation of the calculation methods for the proposed dependencies allows more fully and reasonably to consider the important processes of wastewater treatment, which occur with the additional involvement of the attached biocenosis. These processes significantly affect both the disposal of OP and the efficiency of aerotanks.

Originality and practical value
As part of the study described in this article, it is proposed to increase the efficiency of the aerotanks by allotting its part for additional loading with attached biocenosis. In this part there is simultaneous oxidation of organic pollutants with active sludge and biofilm. The mathematical model describing this process is presented. There are carried out calculations, performed comparative evaluation of the unit efficiency, depending on the degree of influence of the additional loading. Such an evaluation allows us to justify the rational parameters of the biological treatment unit of improved design. This means that the obtained scientific result in the form of the results of calculations based on the mathematical model of biological oxidation of organic pollutants by suspended and attached biocenosis, including an assessment of the influence of these components on the purification efficiency, is interesting from a theoretical point of view. From the practical point of view, the conducted researches allow to justify the use of additional loading with attached microorganisms in order to improve the quality of removal of organic pollutants in aerotanks.

Conclusions
The results obtained in the article give grounds for asserting the possibility of introducing into the actual technological process of wastewater treatment the use of attached biocenosis in aerotanks, which will allow getting effluent at the output from the facilities with a concentration of organic substances within the permissible values.