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1

Continuing long-term expansion of low-oxygen conditions in the Eastern Gulf of Finland

Stoicescu Stella-Theresa, Hoikkala Laura, Fleming Vivi, Lips Urmas

Oceanologia

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2024

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Vol. 66 (1)

139--152

EN

o develop an oxygen indicator for the eastern part of the Gulf of Finland (EGOF), a dataset covering 1900–2021 was compiled. The analysis revealed a long-term declining trend in dissolved oxygen concentrations in the EGOF deep layer of 0.022 mg L–1 a–1 and multi-decadal variations associated with the observed changes in hydrographic conditions. About 27% of the decline in oxygen concentrations for 1900–2021 and 40% for 1990–2021 can be explained by the decrease in solubility due to the temperature increase and changes in hydrographic conditions. The water volume and bottom area under low oxygen conditions in 2016–2021, characterized by dissolved oxygen concentrations <= 6 mg L–1, have increased, compared to the selected reference period with almost no human impact in the 1920s–1950s, from 9.8 km3 to 78.0 km3 (from 2.6% to 20.9% of the EGOF total volume) and from 1190 km2 to 4950 km2 (from 13.4% to 56.0% of the EGOF total area), respectively. The environmental status of the EGOF was assessed as not good based on the introduced oxygen indicator. We conclude that, in the long-term, low oxygen conditions have expanded mostly due to the excess load and accumulation of nutrients in the system and temperature-related changes in biogeochemical processes and fluxes. However, on a decadal scale, changes in hydrographic conditions, i.e. stratification and mixing, can significantly impact the sub-surface oxygen conditions in the EGOF and similar estuarine basins.

2

Stacking Artificial Intelligence Models for Predicting Water Quality Parameters in Rivers

Almadani Mohammad, Kheimi Marwan

Journal of Ecological Engineering

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2023

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Vol. 24, nr 2

152--164

EN

Scrutinizing the changes in the quality of river water is one of the main factors of monitoring the quality of natural flows, which plays a crucial role in the sustainable management of these ecosystems. The concentration of dissolved oxygen (DO) in river water is one of the most important indicators of quality management in such water bodies. From an environmental point of view, exceeding the permissible and natural decay capacity of pollutants in natural streams leads to a decrease in DO and, consequently, causes serious risks for the survival of aquatic life in related ecosystems. Hence, in the present study, 10 daily variables with the amount of dissolved oxygen on the same day were collected and evaluated from Allen County. Moreover, half of these variables were chosen as effective inputs to the model based on statistical analysis, so as to calculate the dissolved oxygen concentration parameter. Modeling with artificial intelligence approaches was implemented in the form of four individual methods: ANFIS-PSO, OS-ELM, Bagging-RF and Boosting CART, and two ensemble-stacking methods: SMA and Meta-learner MLP. The outcomes of estimating the DO with RMSE, MAE, GRI, r, and MBE criteria and marginal-scatter and subject profile diagrams were discussed. Moreover, the efficiency of the models in estimating the outlier of the observational data was scrutinized by subject profile diagram. Finally, it was found that the Meta-learner MLP model with RMSE of 0.965 mg/L had improvement in performance by 8.8%, 8.9%, 22.3%, 24.9% and 27.6%, respectively, compared to SMA, Boosting CART, Bagging-RF, ANFIS-PSO and OS-ELM methods. This remarkable improvement led to recommendations for using stacking techniques in water quality modeling and simulation.

3

Environmental Quality Management through Parshall Flume Aeration Efficiency Modelling

Hamed Mohamed Ahmed Reda

Journal of Ecological Engineering

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2023

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Vol. 24, nr 2

124--130

EN

The dissolved oxygen content in surface waters is one of the vital indicators for human water quality usage as well as the aquatic plant and animal environmental life sustainability. Parshall flumes are one of the important ejector devices that are successfully used for oxygen requirement satisfying in various irrigation, wastewater, and ecosystems. However, the present study aimed to manage and improve various waterworks aeration efficiency through integrated modeling of experimental and analytical analysis as well as their operation conditional parameters for the Parshall flumes configuration. On the basis of the experiment work data sets run results, the principal component regression (PCR), partial least squares (PLS), and ridge regression (RR) techniques are used to develop the required aeration efficiency prediction models for such aerators by interrelating the impact of Parshall flumes characteristics and configurations, as well as various water flow rates on aeration efficiency. The predictive models developed in the study were statistically compared to the experimental data. The comparison confirms a good reliability and high accuracy. Considering the proposed aeration models, the optimum design of the new Parshall flumes can be successfully facilitated.

4

One-Dimensional Model Predictions of Carbonaceous Biological Oxygen Demand and Dissolved Oxygen for Hilla River Water Quality, Iraq

Al-Dalimy Shahad Z., Al-Zubaidi Hussein A. M.

Ecological Engineering & Environmental Technology

|

2023

|

Vol. 24, iss. 7

258--265

EN

The water quality of the Hilla River, located in Iraq, has been varying due to the presence of different illegal point and non-points sources along the river stream. This study highlighted the carbonaceous biological oxygen demand (CBOD) and dissolved oxygen (DO) levels for safe usage by applying the river and stream water quality model called QUẠL2K model, depending on the hydraulic and water quality data collected along 6.8 km passing through the main city (Hilla City) on October 2022 (low flow season) and January 2023 (high flow season). The modeling results showed that the simulated predictions are in good agreement with field data. The outputs revealed the two parameters (CBOD and DO) ranged between (1.425–3.075) mg/L and (9.5–10.65) mg/L, respectively, during low flow season and between (0.745–2) mg/L and (9.5–10.5) mg/L, respectively, during high flow season. The river CBOD levels along the river follow same pattern during the high and low flow seasons, but the DO levels behaved inversely. However, both parameters were within the acceptable limits. Thus, the river health can be considered good for basic human usage.

5

Observed variability in physical and biogeochemical parameters in the central Arabian Gulf

Rakib Fazle, Al-Ansari Ebrahim M. A. S., Husrevoglu Yusuf Sinan, Yigiterhan Oguz, Al-Maslamani Ibrahim, Aboobacker Valliyil Mohammed, Vethamony Ponnumony

Oceanologia

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2021

|

No. 63 (2)

227--237

EN

In situ measurements of physical and biogeochemical variables were conducted along a transect in the Exclusive Economic Zone (EEZ) of Qatar during late summer (September 2014) and winter (January 2015) to investigate their vertical, spatial and temporal variability. The study reveals that the water column is characterized by strong stratification during late summer in the deepest station, where the water depth is around 65 m and the surface to bottom temperature variation is around 9.1°C. The water column is vertically homogeneous during winter due to surface cooling and wind mixing. The surface to 23 m water column is characterized by ample dissolved oxygen (DO) during late summer and winter in the offshore regions, however, relatively low DO is found during late summer due to weak mixing and advection under weak winds and currents. Dissolved oxygen drops to hypoxic levels below the summer thermocline, and the winter high DO layer extends up to the bottom. Chlorophyll-a (Chl-a) is relatively high during late summer in the offshore region, while that in the nearshore regions is very low, which is linked to the anthropogenic stresses from the central east coast of Qatar. The results identified in this study fill an essential gap in the knowledge of regional primary production dynamics.

6

Features of the surface water oxygen regime in the Ukrainian Polesie Region

Aristarkhova Ella O., Fedoniuk Tetiana P., Romanchuk Ludmila D., Latushynskyi Sergii V., Kot Iryna V.

Journal of Water and Land Development

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2021

|

no. 49

104--110

EN

The research analyzed seasonal changes of the oxygen regime and related indicators on the example of water objects of the Ukrainian Polesie Region. The region shows different directions of economic use. Zebrafish (Danio rerio Hamilton–Buchanan) and the Prussian carp (Carassius auratus gibelio Bloch) were used as test objects to investigate survival responses. Dissolved oxygen (DO) concentration in water, pH values and temperatures were determined by standard methods. Based on research results, the main problems were determined pertaining to the oxygen regime of investigated waters, i.e. the increase in temperature and toxicity of the aquatic environment in the summer. A rather dangerous decrease in DO concentration, almost up to the levels of maximum allowable concentration (MAC) (4.10 mg∙dm–3 in group E1 and 6.07 mg∙dm–3 in group E2), was observed in August and it was typical for the reservoirs with a slow water movement. Flowing river waters (group E3) were eliminated due to their better aeration compared to other groups. The correlation analysis based on the presented data revealed a high and average degree of probable correlation between the DO concentration and water temperature, as well as an average degree of correlation with general toxicity determined on sensitive species of D. rerio, and in group E1 on the persistent species C. auratus gibelio as well. The interrelations and equations of the rectilinear regression can be used to predict the oxygen regime of the waters investigated and other surface waters having similar problems.

7

Assessment of Lower Zab river water quality using both Canadian Water Quality Index Method and NSF Water Quality Index Method

Ahmed Sarah M., Taha Omer M.E., Najemalden Mohamed A., Ahmed Rehab T., Abedulwahab Ali A.

Przegląd Naukowy Inżynieria i Kształtowanie Środowiska

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2020

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Vol. 29, No. 2

155--171

EN

Rivers are considered the most important sources of surface water on Earth. They are play a significant role in all human activities and the quality of river water is needed. Therefore, the importance of the water quality index is arising through providing data base about quality of the water source, and explain the change in the water quality over a period of time continually. This study involved determination of physicochemical and biological parameters of Lower Zab river in Kirkuk city at two different points. The objectives of the study are to assess the present water quality, through analysis of some selected water quality parameters like pH, TDS, BOD, dissolved oxygen, turbidity, EC, alkalinity, and salinity etc. and to compare the results with the Canadian Council of Ministers of the Environment and National Sanitation Foundation Water Quality Indices. Raw water samples were collected from the Lower Zab river twice a month by one sample every 15 days from each station. The water quality data include 16 different parameters. Tests were carried out following the American Public Health Association standard methods. The results show that all parameters values were within the standards of drinking water proposed by the CCME standards and Iraqi standards or the World Health Organization standards for drinking purpose, except turbidity, DO, nitrate, calcium, which were mostly higher than the standards and sometimes BOD and potassium. The results of WQI showed that the water quality at LZ3 station is lower than LZ2 station due to the polluting activity of the Lower Zab river. Furthermore, for the years 2014, 2015, and 2016, the water quality was degraded due to the ISIS war. Also, it was noted in the 2013 year that the water quality degraded more in fall and winter seasons due to that the earth has exposed to the long-dried season and then suddenly exposed to a high rainfall season which in turns leads to increase some parameters very high (i.e. turbidity). Finally, the Lower Zab river water cannot be use for drinking directly. However, a pretreatment is needed before the drinking use.

8

Argo floats in the southern Baltic Sea

Walczowski Waldemar, Merchel Małgorzata, Rak Daniel, Wieczorek Piotr, Goszczko Ilona

Oceanologia

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2020

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No. 62 (4PA)

478--488

EN

This work aims to familiarize the reader with issues related to modern oceanographic measurement techniques performed by Argo autonomous profiling floats. The opportunity for this is the three years of innovative activity on the part of Argo-Poland in the Baltic Sea. Based on the experience and results acquired by the Institute of Oceanology of the Polish Academy of Sciences (IO PAN), we can say that a revolution in the Baltic Sea monitoring is underway. During three years of activity, the floats launched by IO PAN provided more than 1600 CTD profiles, including 600 O2 profiles. Together with synoptic data from ships, data from moorings and surface buoys, the Argo float measurements are an important part of the southern Baltic monitoring system. Two Argo floats launched by IO PAN collected enough data to determine the dynamics of the oxygen content in various layers, the extent of hypoxic and anoxic zones, and to detect small baroclinic inflows to the Gotland and Gdańsk Deeps.

9

Microstructure and Corrosion Behaviour of Co-Mo/TiO2 Nano-Composite Coatings Formed on Disk and Wire Electrodes by Electrodeposition

Krawiec H., Vignal V., Latkiewicz M.

Archives of Metallurgy and Materials

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2019

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Vol. 64, iss. 1

23--28

EN

The influence of the electrode geometry on the microstructure and corrosion behaviour of Co-Mo nano-crystalline coatings elaborated by electrodeposition is studied. The corrosion behaviour was determined in the Ringer’s solution at 25°C. Electrodeposition mechanisms are also discussed as a function of the electrode geometry. The electrode geometry was found to affect the growth rate and, under certain conditions, the microstructure (existence of channels and pores). It does not have influence on the corrosion behaviour.

10

Influence of Dissolved Oxygen on the Corrosion Behaviour in Ringer's Solution and Artificial Saliva of Co-Mo Nano-Crystalline Coatings Electrodeposited on Co and 316L Substrates

Vignal V., Krawiec H., Latkiewicz M.

Archives of Metallurgy and Materials

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2019

|

Vol. 64, iss. 1

39--43

EN

The influence on the corrosion behaviour of Co-Mo nano-crystalline coatings of dissolved oxygen is studied in the Ringer’s solution and artificial saliva at 25°C. This was done by means of potentiodynamic tests and surface observations. It was shown that dissolved oxygen has no influence on passivity, oxidation of the coating and selective dissolution of cobalt. By contrast, dissolved oxygen affects corrosion. General corrosion was observed in the Ringer’s solution whereas pitting corrosion was found in artificial saliva.

11

Wielopoziomowy system sterowania stężeniem tlenu i wyznaczania trajektorii zadanej stężenia tlenu w biologicznej oczyszczalni ścieków

Piotrowski R.

Pomiary Automatyka Robotyka

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2018

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R. 22, nr 4

19--23

PL

Napowietrzanie ścieków jest jednym z najważniejszych i najdroższych procesów w oczyszczalni ścieków. Stężenie tlenu jest natomiast głównym parametrem wpływającym na efektywność zachodzenia procesów biologicznych w oczyszczalni ścieków. W artykule przedstawiono wielopoziomowy nieliniowy system sterowania stężeniem tlenu i wyznaczania trajektorii zadanej tej wielkości. W testach symulacyjnych pokazano wyniki sterowania dla oczyszczalni ścieków typu SBR.

EN

Aeration of wastewater is one of the most important and expensive process in a wastewater treatment plant. Dissolved oxygen concentration is the main parameter influencing the effectiveness of biological processes in wastewater treatment plant. In the paper, a multilevel nonlinear control system for dissolved oxygen control and determining the set point trajectory of dissolved oxygen was designed. The control results for the SBR wastewater treatment plant were presented in simulation tests.

12

Influence of Sedimentary Fe and Mn on the Oxygenation of Overlying Waters in Dam Reservoirs

Koszelnik P., Bartoszek L.

Journal of Ecological Engineering

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2018

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Vol. 19, nr 5

180--185

EN

This paper reports the work to determine the impact that concentrations of manganese (Mn) and iron (Fe) in the bottom sediments of shallow dam reservoirs exert upon the dissolved oxygen concentrations of overlying waters. Specifically, the work was conducted in the period of 2013–2014 in six shallow artificial reservoirs located in SE Poland. The waters of all the reservoirs studied are relatively well oxygenated, though both supersaturation and anoxic conditions are observed seasonally across the 50–150% range. The reported reactions of bottom sediments were between pH 7.7 and 8.2, while the mean concentrations of Mn and Fe were found to vary widely from site to site (Mn from 0.068 to 1.48 g/kg d.w. and Fe from 2.48 to 24.0 g/kg d.w.). It was not possible to demonstrate any direct relationship between sediment Fe and Mn concentrations, the Mn/Fe ratio or pH on the one hand and the oxygenation of reservoir waters on the other. However, multiple regression analysis did allow for the identification of a significant influence of both Mn content and sediment pH on the oxygen concentration in reservoir waters. In simple terms, both an increase in Mn and a decrease in pH can be said to result in more fully aerobic conditions in waters.

13

Optymalizacja procesów biologicznych w oczyszczalni ścieków typu SBR

Piotrowski R.

Gaz, Woda i Technika Sanitarna

|

2018

|

Nr 9

343--347

PL

Biologiczna oczyszczalnia ścieków jest skomplikowanym systemem sterowania. Jednym z istotnych i kosztownych procesów tam zachodzących jest napowietrzanie ścieków. Prawidłowy jego przebieg ma decydujący wpływ na stopień oczyszczenia ścieków i koszty operacyjne pracy oczyszczalni. W związku z tym prowadzone są prace badawcze nad optymalizacją procesów biologicznych, pozwalającą redukować koszty, np. zużycie energii. W artykule opracowano model decyzyjny, a następnie rozwiązano zadanie optymalizacji - wyznaczenia optymalnej wartości zadanej stężenia tlenu. W badaniach symulacyjnych przedstawiono wyniki optymalizacji dla oczyszczalni ścieków typu SBR.

EN

A biological wastewater treatment plant is a complex control system. One of the important and costly processes is the aeration of wastewater. The correct of the process has a decisive influence on the degree of sewage treatment and the operating costs of the treatment plant. Therefore, research works are being carried out on optimisation of biological processes, allowing to reduce costs, e.g. energy consumption. In the paper, a decision model was developed and then the task of optimization - determining the optimal set point of dissolved oxygen was solved. In the simulation studies the optimization results for wastewater treatment plant type SBR were presented.

14

Estymacja respiracji wykorzystująca pomiar stężenia tlenu w SBR

Piotrowski R.

Gaz, Woda i Technika Sanitarna

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2018

|

Nr 4

138--144

PL

Biologiczna oczyszczalnia ścieków jest złożonym nieliniowym systemem przemysłowym. Jednym z istotnych i kosztownych procesów tam zachodzących jest napowietrzanie ścieków. Z procesem tym związana jest respiracja, czyli szybkość zużywania tlenu przez bakterie oczyszczające ścieki. Jest to jeden z najważniejszych parametrów, decydujący o efektywności oczyszczania ścieków. Niestety koszt zakupu urządzeń do pomiaru respiracji - respirometrów jest bardzo wysoki i w oczyszczalniach ścieków nie są one instalowane. W artykule dokonano estymacji respiracji, w oparciu o pomiar stężenia tlenu w biologicznej oczyszczalni typu wsadowego. W tym celu wykorzystano rozszerzony filtr Kalmana. W badaniach symulacyjnych przedstawiono wyniki estymacji respiracji dla biologicznej oczyszczalni ścieków typu SBR.

EN

Biological wastewater treatment plant is a complex, nonlinear, industrial system. One of the significant and costly process taking place there is aeration of wastewater. This process involves the respiration - the rate of oxygen consumption by the bacteria. It is one of the most important parameter deciding on the efficiency of wastewater treatment. Unfortunately, the cost of buying respiratory equipment - respirometers is very high and in wastewater treatment plants they are not installed. The paper presents estimation of respiration based on the measurement of dissolved oxygen. For this purpose, the extended Kalman filter is used. Simulation results for the biological wastewater treatment plant type SBR are presented.

15

Treatment processes based on the molecular weight distribution of textile dyeing wastewater

Yaman F. B., Çakmakci M., Karadag D., Özkaya B., Bali V., Dora B.

Environment Protection Engineering

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2018

|

Vol. 44, nr 2

147--158

EN

Rinsing wastewater from dyeing and bleaching processes in a cotton dyeing facility has been separately characterized to evaluate the suitable treatment processes and reuse options. Alternative treatment processes were proposed based on molecular weight distribution (MWD), ultraviolet absorbance of 254 nm (UV254) and specific ultraviolet absorbance (SUVA). Rinsing wastewater samples were sequentially filtrated to determine the MWD of 5-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), dissolved organic carbon (DOC), total Keldahl nitrogen (TKN) and ammonium (NH4+). Bleaching rinsing wastewater had higher organic and nitrogen contents than dyeing rinsing wastewater and concentrations of pollutants decreased after each membrane filtration step. During the sequential filtration, BOD5/COD ratio in bleaching rinsing wastewater slightly decreased while it significantly increased in dyeing rinsing wastewater. SUVA values indicated that organic matters in the rinsing wastewaters have hydrophilic characteristics in all conditions. The evaluation of all experimental data indicates that combination of anaerobic treatment and NF membrane filtration could provide high quality water for reuse within the facility and discharge into receiving environments.

16

Key factors contributing to simultaneous nitrification-denitrification in a biological aerated filter system using oyster shell medium

Hwang C.-C., Weng C.-H.

Environment Protection Engineering

|

2017

|

Vol. 43, nr 1

75--86

EN

Factors contributing to nitrogen removal in a biological aerated filter (BAF) using oyster shell medium have been investigated. The system was operated in parallel with a bio-ball filter. Both filters were fed with a synthetic domestic wastewater containing approximately 25 mg N/dm3 of total nitrogen (TN). The COD of wastewater was 200 mg O2/dm3. The sizes and dissolved oxygen (DO) of the voids within both filters were measured. Results indicated that the oyster shell system performed better with a nitrogen removal of 64.3%. The two systems exhibited a similar COD removal efficiency of approximated 80%. The oyster shell filter showed higher degree of variability in both sizes and DO levels of its void spaces. The condition provided a favorable environment for nitrogen removal through simultaneous nitrification and denitrification (SND). The release of carbonates from oyster shells were minimal, as judged from mass balance analysis of the system using calcium. It is concluded that the function of a SND reactor can be enhanced by using non-uniform filter media such as oyster shells. On the other hand, alkalinity is not a major concern when treating wastewaters with moderate ammonia concentration, such as that of domestic wastewaters.

17

Comparison of Two Nonlinear Predictive Control Algorithms for Dissolved Oxygen Tracking Problem at WWTP

Piotrowski R.

Journal of Automation Mobile Robotics and Intelligent Systems

|

2016

|

Vol. 10, No. 1

8--16

EN

The wastewater treatment plant is classified as a complex system due to its nonlinear dynamics, large uncertainty in the disturbance inputs, multiple time scales in the internal process dynamics and multivariable structure. Aeration is an important and expensive activity that is carried out during wastewater treatment plant operation. A precise aeration control in biological processes for all the operating conditions is necessary in order to guarantee adequate biological conditions for microorganisms. Therefore, the most important is to operate the biological processes so that a minimal energy was consumed and minimal DO concentration demand was applied. The paper proposes a two advanced control systems to track the dissolved oxygen reference trajectory. A decentralized and multivariable nonlinear predictive control algorithms are designed and compared. Simulation tests for the case study wastewater treatment plant are presented.

18

Adaptacyjny system sterowania stężeniem tlenu i instalacją napowietrzającą w oczyszczalni ścieków typu SBR

Piotrowski R.

Gaz, Woda i Technika Sanitarna

|

2016

|

Nr 5

176--183

PL

Biologiczna oczyszczalnia ścieków jest złożonym nieliniowym systemem sterowania. Jednym z istotnych i kosztownych procesów tam zachodzących jest napowietrzanie ścieków. Prawidłowy jego przebieg ma decydujący wpływ na stopień oczyszczenia ścieków i koszty operacyjne pracy oczyszczalni. W związku z tym prowadzone są prace badawcze nad nowymi strukturami i algorytmami sterowania. Powinny się one odznaczać wysoką efektywnością i niewielkim zapotrzebowaniem na energię elektryczną. W artykule zaprojektowano nowy adaptacyjny układ sterowania stężeniem tlenu uwzględniający instalację napowietrzającą jako system dostarczający powietrze do oczyszczalni ścieków. W badaniach symulacyjnych przedstawiono wyniki sterowania dla biologicznej oczyszczalni ścieków typu SBR.

EN

Biological wastewater treatment plant is a complex, nonlinear control system. One of the most important and expensive processes conducted is aeration. It’s valid flow has a deciding impact on the level of wastewater purification. Therefore a number of research are carried out to develop new control structures and algorithms. They should be characterized by high efficiency and low energy consumption. In the paper a novel adaptive control strategy of dissolved oxygen is employed. It considers aeration system as a source of the oxygen to the plant. Simulation tests presents control results for the biological wastewater treatment plant type SBR.

19

Removal of nitrates from wastewater using pond bottom soil

Erbanová E., Palarčík J., Slezák M., Mikulášek P.

Environment Protection Engineering

|

2016

|

Vol. 42, nr 2

145—154

EN

The possibility of the final treatment of mine water from chemical leaching has been investigated. Despite water purification, high nitrate levels remain in these waters and must be removed. The main requirements are the lowest economic, operating and staffing levels. These requirements are best achieved by the removal of nitrates using biological methods, in our case using the pond bottom soil. Experiments were carried out in a batch mode. The effect of the environment on the denitrification process, the influence of the type and amount of the organic substrate and basic parameters of the denitrification such as redox potential, pH and dissolved oxygen were studied.

20

Wavelet Characteristics of Hydrological and Dissolved Oxygen Time Series in a Lowland River

Rajwa-Kuligiewicz A., Bialik R. J., Rowiński P. M.

Acta Geophysica

|

2016

|

Vol. 64, no. 3

649--669

EN

In this study, we investigated the temporal variability of dissolved oxygen and water temperature in conjunction with water level fluctuations and river discharge in the Narew lowland river reach. For this purpose, high resolution hydrologic and water quality time series have been used. Spectral analyses of time series using continuous wavelet transform scheme have been applied in order to identify characteristic scales, its duration, and localisation in time. The results of wavelet analysis have shown a great number of periodicities in time series at the inter-annual time scale when compared to the classical Fourier analysis. Additionally, wavelet coherence revealed the complex nature of the relationship between dissolved oxygen and hydrological variables dependent on the scale and localisation in time. Hence, the results presented in this paper may provide an alternative representation to a frequency analysis of time series.