Creation of nature-like artificial ecosystems for wastewater treatment technology

. Biological activity of sludge systems at microscale level is extensively used for effective wastewater treatment. The creation of nature-like artificial ecosystems with a large species diversity is a key point for biocenosis adjusting ability in the municipal wastewater treatment structures. The aim of the study presented is to model at the laboratory scale and to study a stability of active sludge mixed biocenosis physiological state during its aging. The measurements performed using the flow model of aeration tank-mixer with built-in sump.


Introduction
Modern approaches to the development of wastewater treatment methods are based on ecological concepts of the species diversity of biocenoses.The science of ecology studies a hierarchical series of biological systems, including the interaction of biotic components with the surrounding physical environment (energy and matter).This concept generalizes systems of organisms, populations and ecosystems.Thus, an ecological approach to the study of biological systems should include the study of the relationship of an individual with the environment at the organismal or ontogenetic level (issues of autecology).Demecology and eideology study the roles of various environmental factors in the viability of populations and their adaptations to environmental changes in space and time at the population-species level.Synecology considers the interactions of all elements of the biocenosis at the biogeocenological level.On the other hand, biological wastewater treatment systems of municipal wastewater disposal are usually characterized by technological performance indicators of facilities.However, the stability of the biological system can be assessed using the main indicators of diversity and technological parameters at various levels of the biological organization of the biocenosis of activated sludge (Table 1).A special study requires a mixed biocenosis of activated sludge, which is a set of populations formed as a result of coevolution of metabolic transformations and functioning as an integral system in the space of treatment facilities allocated to it.The use of mixed biocenoses in single-stage wastewater treatment systems involves determining the optimal conditions for the interaction of various populations of microorganisms and reflecting these conditions through technological parameters.
The study of the ecological relationships of activated sludge microorganisms at each of the biological levels should serve one purpose -the development of technical methods and designs that allow to regulate the numerical ratios of populations and control biological wastewater treatment systems.
Ecological studies at the ontogenetic or organismal level are conducted mainly by biologists and include the following tasks: studying the influence of abiotic factors (temperature, active pH reaction, concentrations of substances limiting the development) in experimental and natural conditions, studying morphophysiological or phenotypic changes of an individual under the influence of biotic and abiotic factors.However, the consideration of individual individuals of microorganisms in the study of an integral system is not legitimate, since in relation to biological treatment facilities it is impossible to talk about fertility, mortality, age structure -characteristics that make sense only at the group level.
Consequently, it is bacterial populations that can be the object of theoretical and experimental studies using population dynamics methods.The study of the dynamics of microbial populations includes not only the establishment of patterns of growth, abundance, but also the mechanism of interaction.
When studying populations, specific and ecological density was used as the main characteristic, which is the population size per unit of habitable space (available area or volume that is actually occupied).The specific density was determined as the number of grown colonies when seeding a certain volume of wastewater of a given dilution on the medium, attributed to the concentration of sludge in the bio-oxidant at the same time.The change in population dynamics over a certain period of time is characterized by indicators of significance for each species.If we consider several populations of the same trophic level, then usually only a few of them can be distinguished as dominant, that is, having a significant number, most of them have lower values or generally belong to rare species.In other cases, dominant species may be absent, and all species are characterized by intermediate abundance.A more complete assessment of the biotic community is given by the species diversity of the cenosis, which consists of two components -species richness and alignment.To analyze species diversity, comparisons based on diversity indices are used, which are mathematical dependencies between the number of species and their significance.One of the main components of species diversity -the index of species richness is the ratio between the species of populations and the number of colonies in them.The Shannon index, which characterizes the system as a whole, provides a general characteristic of the probability of the contribution of each population species in the biotic community, and is applicable in cases where individual components of diversity are not interested [1].
Ideas and technologies for the use of biological treatment or post-treatment of wastewater are being developed by domestic and foreign scientists on the basis of various structural solutions of aerotanks, purification stages and the composition of groups of microorganisms [1][2][3][4].
Foreign researchers note the multipurpose use of microorganisms in biological wastewater treatment to reduce the amount of fat, oil and grease.With the help of a liquid culture of microorganisms distributed by injection and addition of oxygen to the biological treatment zone of the container tank, a high efficiency of oxygen saturation and mixing of wastewater is achieved 1.
A number of studies have been devoted to the design of devices for biological wastewater treatment with the creation of anaerobic conditions for the most efficient removal of organic matter at high load, while reducing the amount of air and significantly reducing the amount of sediment formed [5][6][7][8][9][10][11][12].Most of the works are devoted to the development of multi-stage wastewater treatment systems, where biological treatment is included in the composition [13][14][15][16][17][18][19][20].

Materials and techniques
In laboratory conditions, an attempt was made to form a stable physiological state of the mixed biocenosis of activated sludge by changing its age.The research was carried out on a flow model of an aerotank mixer with a built-in sump.The waste water was fed evenly throughout the day to the lower part of the aeration tank, with a volume of 8 liters, where it was almost instantly mixed with the sludge mixture and saturated with oxygen in the air.The concentration of dissolved oxygen was maintained at the level of 5.2-6.0 mg0/l.
In the center of the upper part of the aeration tank there was a built-in sump in the form of a tray with a slot in the lower part, through which the deposited sludge was lowered into the working area under the influence of gravity, and the clarified liquid was poured into a semicircular tray in the center of the sump and discharged into the container.The established age of sludge was maintained by removing the calculated amount of sludge mixture during the day according to the methodology of the VODGEO Research Institute.The VODGEO/SamSTU methodology, the authors of which are Doctor of Technical Sciences V.N.Shvetsov, Candidate of Technical Sciences K.M. Morozova and Doctor of Technical Sciences S.V. Stepanov.Calculations of aerotanks are based on formulas of enzymatic kinetics.The technological calculation of biological treatment facilities is based on the concept of describing the processes of organic matter oxidation, biological phosphorus removal, nitri-denitrification in anaerobic, anoxic and aerobic conditions using the equations of enzymatic kinetics and the equations of material balance, providing a given degree of wastewater treatment for all dictating components.The duration of aeration was 12 hours.
In each studied mode, the duration of adaptation was the value of two or three times the established age of silt.In a stable mode, the installation worked for at least one week before taking control samples.As a substrate, an imitation of wastewater from a sugar factory with dilution with household wastewater was used.This composition refers to biologically difficult-to-oxidize mixtures.The main initial characteristics: COD =1250 mg/l; mg/l; nitrates and nitrites were absent.
The following modes were set: • with unfixed selection of the sludge mixture while maintaining a dose of sludge in the aeration tank of about 2 g/l in dry matter; • with a fixed selection of sludge mixture and a fixed age of sludge T = 7 days; • with a fixed selection of the sludge mixture and a fixed age of sludge T = 20 days.

Results
Microbiological studies of nitrogen-transforming biocenosis were carried out in a model aerotank-nitrifier, taking into account the age of activated sludge according to the specific density of populations at various regulated (7 and 20 days) and unspecified ages of activated sludge, the results are presented in Table 2.The designation system is the same.For the experimental data obtained, a statistical analysis was carried out using the Micro Cal Origin ver software package.2.8 MicroCAL Origin 7.5 Pro Microcal Origin v4.10.Confidence estimates of the specific density of populations with unknown measurement accuracy were verified using the Student's t-test.With the number of series of experiments equal to three and a confidence probability of 0.95, the critical value of the Student's t-test is 2.353.
The percentage distribution of the specific density of populations (1 -ammonifiers, 2nitrifiers, 3 -denitrifiers, 4 -nitrogen fixators) in logarithmic values at the ages of silt T = 7 and T = 20 days.on the same substrate is shown in Fig. 1.At the age of silt T = 7 days, the dominant position is occupied by ammonifying groups of microorganisms, exceeding the corresponding group in the compared variant by 18%.The specific density of populations of nitrifiers with low growth rates is significantly lower.The percentage of denitrifiers characterizing heterotrophic biomass of a wide spectrum has practically not changed.
Based on the results of microbiological analysis, it was established that the specific density of all groups of populations of nitrogen-transforming microorganisms at an unspecified age of sludge significantly differs from each other in each series of observations, whereas at an established age, there is a discrepancy of data by no more than 5-7%.
The results of the aeration tank are presented in Table 3. Analysis of the work of the laboratory aeration tank showed that with an increase in the age of activated sludge in the system, deeper purification is achieved, both for organic substances and nitrogen compounds (purification efficiency increased by 7 and 9%, respectively).When the operating mode was changed, the oxidizing capacity of organic matter facilities increased by 10%.With an increase in the dose of sludge from 1.8 to 2.3g/l, the sedimentation properties of activated sludge improved and the sludge index decreased from 110 to 89 cm3/g.In this mode, there is a decrease in the specific rate of oxidation and the load on sludge for organic substances with unchanged quantitative and qualitative indicators of the source wastewater.Such changes can be explained by an increase in the dose of sludge and a simultaneous decrease in the proportion of rapidly growing highly active species of microorganisms in the total biomass with a slight increase in ash content.At the same time, almost complete nitrification is observed.
The ratio of OVM to OM in nitrogen, which is an indicator of the effectiveness of denitrification, increases by 4.4 times.Thus, the aerotank-nitrifier fully corresponds to its purpose with the formed mixed biocenosis.To reduce the nitrogen content in oxidized forms, it is necessary to create optimal conditions for denitrification.
The analysis of the species diversity of the studied biocenosis confirms the above conclusions (Table 4).

Discussion
With an increase in the age of sludge in the system, the density of populations of microorganisms that can be attributed to slow-growing species significantly increased, and the index of significance of the contribution of nitrifiers to the nitrogen transformation process exceeded this indicator for ammonifiers by 47 times.The alignment index for all types of populations has also increased.The significance of the contribution of ammonifiers has been reduced.With an increase in the age of sludge from 7 to 20 days, the total index of biocenosis diversity increased by 1.8 times.Thus, an increase in the residence time of a mixed, including nitrogen-transforming biocenosis in a bioreactor, corresponding to an increase in its total diversity, reveals a tendency to decrease negative types of interactions, due to positive ones that increase the stability of the ecosystem as a whole.Consequently, the high species diversity of the mixed biocenosis of activated sludge indicates an increase in the stability of the biological wastewater treatment system from complex organic substances and compounds.

Conclusion
In the study presented it is shown that the stability of artificially created ecosystems is based on a high species diversity that is similar to natural ones.The total diversity index increase is of particular importance for biocenosis adjustment in a municipal wastewater treatment structure.In a stable operating mode of biooxidant the system is characterized by wide range of species and its individual importance equality.When the system is unstable the periodic changes occurs, the competitive species dominance is more obvious and the biological processes inhibition exists.Thus, taking into account the population stability theory, it is assumed that the species diversity of biocenosis is a functional characteristic that affects biological wastewater treatment stability.The effect requires further experimental studying at various biological levels and chould be used in practical tasks solution.

Table 1 .
Biological, ecological and technological characteristics of the diversity of sludge systems by levels of biological organization

Table 2 .
Results of studies of the specific density of populations with a change in the age of silt

Table 3 .
Performance indicators of the aeration tank when the age of silt changes

Table 4 .
Assessment of the species diversity of the biocenosis at different ages of silt