About the oxygen influence on the efficiency of the wastewaters treatment from organic pollutions in bioreactors

Telyma S.V.

Institute of Hydromechanics of National Academy of Sciences of Ukraine

ECOLOGY (Environmental safety)

About the oxygen influence on the efficiency of the wastewaters treatment from organic pollutions in bioreactors

In biological wastewater treatment reactors in particular in aerotanks the removal of organic contaminants (OC) occurs in aerobic conditions that is when oxygen is consumed for biooxidation of OC and self-oxidation of cell material and also used in other processes that may occur at this time. Therefore the modeling and development of the aeration system consist in providing such an oxygen regime in the reactor in which the rate of the biological treatment process should not be limited by the amount of oxygen contained in the reactor. In such reactors the processes of dissolution and consumption of oxygen occur simultaneously and interconnected and under normal conditions OC removal occurs only at the expense of suspended biocenosis (active sludge).

In this paper the peculiarities of the modeling of the oxygen regime at the aerobic biological treatment of the wastewaters in aerotanks with suspended and fixed biocenoses are considered.

The aim of the work is to develop the mathematical model for evaluation and analyze of the oxygen regime in aerotanks-mixers and aerotank-displasers under given conditions on the base of corresponding equations of the material balance recorded relative to the concentration of oxygen C.

That to archive the goal the following prerequisites were take into account:

• the some limiting cases of aerotanks work in the system of biological wastewaters treatment were considered;


• at evaluation of the removal of the OC the linear kinetics of reactions of the first and the zero orders were used

The removal of organic contaminants by suspended and fixed biocenose in aerotanks  must  ensure  uninterrupted supply of oxygen for growth and livelihoods of the microorganisms and control of its consumption in the amount necessary  to  maintain the kinetics  of  reactions with a high rate of utilization of OC in the given conditions of the aerobic process. The proposed model is as for aerotank - mixer

and for the aerotank - displacer

In practical calculations  it is enough to consider equations (1) - (2) in stationary conditions and to evaluate their members taking into account the known diffusion criterion of  Peckle  according to [1].

In the above equations in the general case the value of the flow of oxygen entering into the biofilm through its surface is determined by the equation:

where  - the ratio of the surface area of the biofilm when in contact with air bubbles to the total surface area of the biofilm.

In other words the equation (3) takes into account the possible additional flow of oxygen into a biofilm both from the volume of liquid in the form of dissolved oxygen  and as a result of the so-called inter-surface transfer (IST) directly from the  bubbles adhered to the biofilm. In this case when implementing models it is necessary to put on  and take into account on only the flow of dissolved oxygen from the volume of liquid to the biofilm due to diffusion [1,2]:

Note that the determination of the flow of oxygen through the surface of the biofilm  is obtained as a result of solving the following equation which characterize the consumption of oxygen in the removal of OC by a fixed biocenose (biofilm):

So for practical calculations the solution of equation (5) is sufficient to solve for stationary conditions that occur fairly quickly that is when .

The solution of equation (5) is performed under the following boundary conditions, namely:  for , and  for z = δ which allows us to determine the change in the concentration of oxygen in the thickness of the biofilm and in subsequent calculations to  determine the concentration on the surface of the biofilm .

In the above equations and dependencies  ,С_δ , ,  - respectively the concentration of oxygen in the biofilm, on the surface of the biofilm, in the aerotank and in the wastewaters  at  the entrance to the aerotank;  - concentration of saturation (soluble) oxygen in the liquid; ,  - the rate of reactions of using oxygen in a biofilm and aerotanks  taking into account on the oxidation velocity of the isolated substances during the death of microorganisms; , ,  - respectively the aerotanks working volume, the volume of the liquid in the aerotank, the volume of the elements of the established loading with the fixed biocenose; ,  - respectively the total surface area of the biofilm in the aerotank (reactor) and  the surface area of the oxygen in the biofilm; , ,  - respectively the volumetric mass transfer coefficient, the coefficient between the surface transfer of oxygen in biofilm, mass transfer coefficient of oxygen in liquid film [1,3,4].

Thus the implementation of the proposed models allows with known geometric and other characteristics to evaluate the influence of oxygen regime on the processes of treatment in aerotanks in different conditions of their operation and to substantiate an economical and efficient technological supply system with oxygen taking into account on the features of extraction of OC in aerotanks with suspended and fixed biocenose. In this case it is possible to substantiate the criteria in which the biochemical oxidation process will be adequately supplied with oxygen that it will not limit the kinetics of biooxidation  both suspended and fixed biocenose.

References:

1.Olіynik O.J., Airapetian T.S. Modelling  of  the  wastewater treatment from organic contaminations in the bioreactors-aerotanks with suspended (free flowing) and fixed biocenosis// Reports  of  National Academy of  Sciences of  Ukraine. - 2015. - №5. P-56-61. (in Ukrainian).

1. Lee K.M., Stensel H.D. Aepation and substrate utilization in a sparged packed - biofilm reactor//JWPCF, vol. 58, №11. P-1065-1073.


2. Maslun G.S.Practical recommendations for calculation of the oxigene regime at the purification of waste waters on submerged filters.// Problemy vodopostachannya, vodovіdvedennya ta gіdravlіky. - 2013. - 21. P-123-142.(in Ukrainian).


3. B.N. Repin, V.I. Bazhenov. Modeling of the oxygen regime in aerotanks-displasers//Vodnye Resursy. - 1991.- №1. P-122-130(in Russian).