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The sustainable management of water resources is crucial for maintaining high-quality tourism, as it ensures the availability and reuse of water through effective wastewater treatment processes. This requires the implementation of technologies and circular measures for managing water resources. In this context, the present study focuses on the Ravda Wastewater Treatment Plant (WWTP), which treats the wastewater of the largest coastal resort in Bulgaria, Sunny Beach. Data from seven consecutive years (2018–2024), including the years of COVID-19 measures, have been analyzed. Particular attention has been paid to analysis during the last two years of this study. For the period 2018–2022, hydrochemical parameters (total nitrogen and total phosphorus, volumetric load of activated sludge/volumetric organic load) and technological parameters (treatment efficiency, COD/BOD5) were examined. In 2023 and 2024, additional parameters such as the SVI, comparative microscopic analysis of activated sludge, dynamics and quantities of aerobic and anaerobic heterotrophic bacteria and denitrifying bacteria, the abundance of genera Pseudomonas and Acinetobacter, and the elemental composition of the water at the inlet and outlet of the treatment plant, were studied. Samples were taken from five critical control points in the course of the treatment process during the off-season, in April and November, when the plant operates with a reduced number of facilities. The aim of this study is to determine the efficiency of water treatment during the off-season and the possibility of its enlargement and improvement to meet the needs during the high season. Overall, the results of the comprehensive analyses show that the biotechnological system has significant biodegradation potential but requires improvement due to large fluctuations in the number of residents. The socio-economic and environmental situation in the area is extremely dynamic, necessitating the introduction of innovative wastewater treatment measures to balance the negative impact on the environment and ensure the sustainability of high-quality tourism. [ABSTRACT FROM AUTHOR] more...

Mass tourism puts enormous pressure on wastewater treatment plants due to its expansive growth during the summer months. To adapt to the fluctuations, the Ravda wastewater treatment plant (WWTP) uses innovative methods and technologies, allowing for "shrinking" and "expanding" of the facilities according to the season. This has been built in stages over the years, with two separate biological treatment lines adapting to different numbers of tourists and to the quantity of influent wastewater. The aim of this study is to make a comparative assessment of the work of WWTP Ravda in the summer and winter seasons and its effectiveness, as well as to compare them. In addition, it examines the years of the COVID-19 pandemic, when a much higher consumption of water per person was noted. Data were analyzed for the period of 2018–2022 inclusive, comparing influent and effluent BOD5 and COD in the summer and winter. Nitrogen and phosphorus removal efficiencies were also tracked. The study shows that municipal wastewater treatment is effective, but much higher values, close to the maximum permissible discharge values, are observed during the tourist season. With the continued growth of the tourism sector, the Ravda wastewater treatment plant would not be able to cope with the discharge standards set by the Ministry of Environment and Water, so measures need to be taken promptly. [ABSTRACT FROM AUTHOR] more...

One of the most widespread and risky pollutants in the environment is phenol. It is a by-product of many industrial, agricultural, and other anthropogenic activities. Microbial-assisted transformation, known as bioremediation, is an effective and cheap method for treating groundwater, soil, and sediments contaminated with phenol and its derivates. This study aims to assess the effect of the addition of a selected, pre-adapted bacterial strain Pseudomonas aureofaciens AP-9 on key kinetic, microbiological, and enzymological parameters of simulated bioremediation processes for the removal of phenol (250 mg/kg). The early effect of adding this microbial biodegradant in contaminated sediments is insignificant. The effect of added bacteria is manifested at the 48th hour by a restructuring of the microbial sediment communities and an increase in the number of cultivated microorganisms. This preparation of the sediment communities for a prolonged detoxification process is also confirmed by the repeated induction and very high increase in the activity of the enzymes directly involved in the cleavage of the benzene ring. The effectiveness of phenol removal at the 48th hour is increased by 15%, too. Considering this stimulation of a sustainable long-term bioremediation process, we can conclude that microbiological pre-adapted inoculants are an important mechanism for the management of bioremediation detoxification processes and can increase their effectiveness. [ABSTRACT FROM AUTHOR] more...

Landfill leachate is generated from waste degradation in landfill sites, and its treatment includes biological or combined treatment with physico-chemical methods. A frequently applied technology in the biological stage of landfill leachate treatment plants is based on denitrification and nitrification. Nevertheless, with the availability of a huge number of scientific reports, the management of denitrification and nitrification (D/N) processes in the frame of real wastewater treatment plants (WWTPs) is rather difficult and always remains a critical technological problem. This study aims to perform a comparative assessment between denitrification and nitrification processes during landfill leachate treatment in three different situations in the WWTP of the Municipal Enterprise for Waste Treatment in Sofia City, Bulgaria. The comparative assessment is based on the biotransformation effectiveness of ammonium ions in the course of nitrification and of nitrates in the course of denitrification. Mixed samples (wastewater and activated sludge) were taken from the sequencing batch reactors operating in the mode of denitrification and nitrification. All physico-chemical (pH, dissolved oxygen, organics concentration, BOD5:COD, nitrogen and phosphorous) and biological (sludge volume index, sludge biotic index, quantity of aerobic heterotrophs, denitrifiers and nitrifiers, total dehydrogenase activity) indicators were investigated in the samples from the biobasins in the mode of denitrification and nitrification. The conditions for the implementation of the two processes were compared, along with the state of the activated sludge in the different reactors. The obtained results showed that denitrification was the critical process in the technology on the base of D/N. A major factor that managed the denitrification was the lower concentration of biodegradable organic matter which led to a deformation of the activated sludge structure, a decrease in the count of heterotrophic microorganisms, a decrease in the total activity of the activated sludge and an inhibition of the nitrate reductase activity. The nitrification processes were accomplished with high intensity. The results confirmed that D/N could be optimized and controlled using specific wastewater treatment technology parameters and purposely applied indicators. [ABSTRACT FROM AUTHOR] more...

Detoxication technologies require the combination of theoretical and practical knowledge of xenobiotic biodegradation, wastewater treatment technologies, and management rules. The purpose of this complicated combination is to propose specialized strategies for detoxication, based on lab- and pilot-scale modeling. These strategies include preliminary created algorithms for preventing the risk of water pollution and sediments. The technologies and algorithms are essentially important outcome, applied in the textile, pharmaceutical, cosmetic, woodtreating, and oiltreating industries. In this paper four rehabilitation technologies for pretreatment of water contaminated by pentachlorophenol (PCP) have been developed in the frame of the European and Bulgarian National projects. Emphasize is put on the biological systems and their potential of detoxication management. The light and transmission electron microscopy of the reconstructed activated sludges the microbial, kinetic and enzymological indicators are presented and approved as critical points in the biocontrol. more...

The upper Iskar sub-catchment is one of Bulgaria's most important economic and socially significant water sources because of its role in supplying Sofia with drinking water. Among the critical factors that carry potential high-risk levels for water quality in this hydrosystem are the discharge from the Samokov Wastewater Treatment Plant (WWTP), diffuse pollution from agriculture, and the percolation of untreated sewage from the small villages. In this study, we assessed the effect of treated wastewater effluent on water quality, and on the ecological state and microbial communities in the river sector of Samokov's WWTP discharge area. The assessment was based on the complex use of chemical and microbiological indicators and biological quality elements. The concentrations of organics, nutrients and microcomponents were determined with results confirming the expected increase for parameters associated with the discharge of urban wastewater. The ecological state, according to macrozoobenthos indicators, was "good" throughout the river sector but local deterioration was registered in a proximal location downstream of the WWTP outfall. The analysis of stream water and bed sediment microbial communities by a fluorescent technique showed the high metabolic activity and intensive transformation processes in addition to high abundance registered with standard cultivation methods. The importance of the studied sub-catchment for the functioning of the urban water cycle, and for the quality of Sofia's drinking water, underlines the need to extend an existing monitoring program with a more detailed assessment of the environmental impact of wastewater discharge. [ABSTRACT FROM AUTHOR] more...

Plasma-based technologies take an increasing place in the new conceptions of wastewater management as a promising tool for the treatment of persistent organic pollutants with low biodegradability. Plasma major advantage is the synergy of diverse active components with high oxidative action and additional benefits as disinfection of treated water. But the bactericidal effect of plasma can influence the treatment effectiveness when this technology is used in combination with biological methods for the removal of pollutants. The aim of this paper is to study the effect of non-thermal atmospheric plasma torch on key enzymes from phenol biodegradation pathways in Pseudomonas aureofaciens (chlororaphis) AP-9. The strain was isolated from contaminated soils and had a high potential for biodegradation of aromatic compounds. The used plasma source is surface-wave-sustained discharge operating at 2.45 GHz in argon produced by an electromagnetic wave launcher surfatron type. The enzyme activities of phenol 2-monooxygenase (P2MO), catechol 1,2-dioxygenase (C12DO), catechol 2,3-dioxygenase (C23DO), protocatechuate 3,4-dioxygenase (P34DO) and succinate dehydrogenase (SDH) were measured in control and after plasma treatment of 10, 30 and 60 s. At short-time treatment, the activities of intradiol dioxygenases increased with 26% and 59% for C12DO and P34DO, respectively. Other oxygenases and SDH were inhibited with 35% even at 10 s treatment. Longer treatment times had a clear negative effect but SDH kept the higher activity at 60 s treatment compared to the oxygenases. Our data suggest that plasma-based technologies are a useful approach for post-treatment of aryl-containing wastewater in order to increase the effectiveness of biological removal. [ABSTRACT FROM AUTHOR] more...