Research Article |
Corresponding author: Veronika Yordanova ( veronika_emilova96@abv.bg ) Academic editor: Roumiana Metcheva
© 2022 Veronika Yordanova, Yovana Todorova, Mihaela Belouhova, Lyubomir Kenderov, Valentina Lyubomirova, Yana Topalova.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Yordanova V, Todorova Y, Belouhova M, Kenderov L, Lyubomirova V, Topalova Y (2022) Environmental impact assessment of discharge of treated wastewater effluent in Upper Iskar sub-catchment. In: Chankova S, Peneva V, Metcheva R, Beltcheva M, Vassilev K, Radeva G, Danova K (Eds) Current trends of ecology. BioRisk 17: 59-71. https://doi.org/10.3897/biorisk.17.77381
|
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.
Ecological status, Iskar River, microbial community, pollution, self-purification, treated water discharge, WWTP
The increasing release of different organic and inorganic pollutants, associated with rapid urbanization and industrialization, is one of the global environmental threats for the quality and ecological status of aquatic ecosystems. The prevention of pollution, and protection of the quality of natural and drinking water, is a primary concern for society in order to ensure healthy living conditions, as well as a high standard of public health (
In Bulgaria, the upper Iskar sub-catchment (Danube River Basin) is one of Bulgaria’s most important economic and socially significant water sources in Bulgaria because of its role in supplying the capital Sofia with drinking water and upholding the city’s urban water cycle. The biggest reservoir in Bulgaria (Iskar Reservoir) is situated in this sub-catchment and provides more than 70% of Sofia’s drinking water. According to official data, the waters of the Iskar River basin are in good/moderate ecological status to Sofia, after which the status is moderate except for the section after inflow of Vladayska River, where the status is categorized as bad (http://www.bd-dunav.org/uploads/content/files/upravlenie-na-vodite/ocenka-na-sustoianieto/povurhnostni-vodi/BDDR_analiz_SWB_2019-2020.pdf). The critical factors with potential high-risk levels for water quality in the upper Iskar aquatic ecosystems are the discharge from the WWTP in the town of Samokov, diffuse pollution from agriculture, and the penetration of untreated sewage from the small villages. Failing to make maximum use of the capacity of the treatment plants, exacerbated by an inflow of untreated water, as well as several diffuse sources, constitute some of the background explanations for pollution in the upper valley of Iskar (
This study aims to assess the environmental impact of treated effluent discharge from the WWTP in Samokov municipality in the upper part of Iskar River. We use a complex approach with a combination of indicators (chemical, microbiological, biological quality elements) to assess local changes in water quality, ecological status and microbial communities. The paper is structured as follows: (1) Firstly, we discuss the effect of WWTP discharge on physicochemical parameters and nutrient concentrations on the water of the disposal area; (2) An analysis of the concentrations of selected hazardous and specific pollutants (microelements) in water samples is also presented; (3) The abundance and activity of microbial communities are assessed in water and sediments by standard cultivation and fluorescence techniques; (4) Finally, an assessment of ecological status in river sector by quality element “macrozoobenthos” is conducted and then discussed.
The study area is located in the upper part of the Iskar River before the Iskar Dam in Northern Rila, Bulgaria. Iskar is the longest (wholly) Bulgarian river (368 km) with a river basin of 8 650 km2. The study river sector is 10 km in length, and 25–35 m in width; its depth ranges from 50 to 200 cm and the bottom substratum consists of pebbles, coarse and medium sands. The seasonal character of flow is determined with summer and winter low flow (1–3 m3/s), a little increase in flow during the autumn (6–10 m3/s) and very expressive spring high water level (15–25 m3/s). According to a map of the land use, agricultural land, pastures and forests dominate the area (
The sampling design included sites upstream and downstream of the WWTP. We carried out two sampling campaigns in November 2020 and March 2021 when the average water flow is 5–8 m3/s. Paired water and sediment samples were collected from four sampling sites (Fig.
Sampling site UI1 – Iskar above WWTP Samokov
Sampling site UI2 – Discharge of WWTP Samokov
Sampling site UI3 – Dragushinovo village, under WWTP Samokov
Sampling site UI4 – Iskar River, near the Villa zone Mechkata (site from monitoring system for surface water bodies in Bulgaria)
We analyzed the physicochemical parameters (temperature, oxygen concentration, conductivity and pH) of the water in situ immediately after sampling with a portable oxygen and pH meters. The hand net was used for collecting the macroinvertebrates – up to 10 sub-samples were collected on one site according to multihabitat approach (EN ISO 10870:2012, EN ISO 16150: 2012). The assessment of the ecological status was conducted by metrics “biotic index” in ranges for river types R4 (semi-mountain rivers in 12 Ecoregion “Pontic Province”). The sediment samples for chemical, microbiological and fluorescent analyses were collected by manual dragging. The all water and sediment samples were transferred in sterile containers at 4 °C storage and processed within 24 h.
The determination of the organic loading in the water was performed by measuring the chemical oxygen demand (COD) by colorimetric dichromatic method. Nitrogen was measured as dissolved inorganic forms – ammonium and nitrate ions (colorimetric methods). The concentrations of phosphorus were defined as phosphates also by use of colorimetric method. Procedural details for measuring nutrients and organics were in line with standard methods recommended in EN ISO standards. The concentrations of selected microelements in the water samples were determined with inductively-coupled plasma mass spectrometry (ICP-MS, PerkinElmer SCIEX Elan DRC-e). The number of the decimal places is related to the precision of the measurements. The estimation of accuracy was conducted by the analysis of two water standard reference materials: SPS-SW2 (Reference Material for Measurement of Elements in Surface Waters, Spectrapure Standards, Norway) and NWTM-23.5 (Environmental matrix reference material, a trace element-fortified sample, Environment and Climate Change, Canada). The experimental results were in very good agreement with the certified values.
Data from chemical analyses of water and assessment of ecological status was compared with the requirements of Bulgarian legislation (Regulation No. H-4 of 14.09.2012 on characterization of surface water and Regulation No. 12 of 18.06.2002 on the quality of surface water intended for drinking and household purpose). Selecting microelements and ranking hazardous and specific pollutants was conducted on the basis of the EU-list of priority substances (Water Framework Directive 60/2000/EC (WFD) and Regulation on environmental quality standards for priority substances and some other pollutants, 2015) and the above-mentioned regulations.
The total microbial count and number of coliforms was determined by the use of standard count-plate technique on Nutrient agar and Lactose TTC Agar with Tergitol 7 (Merck Millipore). The sediment samples were preliminarily treated with ultrasonic disintegrator VCX 750, Sonics & Materials Inc. (3 times × 10 sec). The data for microbial counts were normalized and presented as ln CFU/mL or ln CFU/g dry weight.
For analysis of changes in microbial activity of sediment communities, we used a modification of CTC/DAPI staining method with fluorescence imaging. CTC (5-cyano-2,3-ditolyl tetrazolium chloride) enters in cells and is reduced to CTC-formazan (fluorescent red signal). The content of reduced compound depends on the electron transport activity (activity of the dehydrogenase enzymes in viable cells) and is considered as an indicator of their metabolic activity. DAPI (4′,6-diamidino-2-phenylindole) is fluorescent dye that specifically binds to nucleic acids but enters both in live and fixed cells. It is widely used for the enumeration of bacterial abundance. The combined staining with CTC and DAPI is applied to distinguish the active fraction in different microbial communities (
In the study area, the water temperature showed typical seasonal dynamics and varied from 4.2 °C at sampling site UI3 to 8.9 °C at sampling site UI2 in November (Table
Physico-chemical parameters of the waters in the upper valley of the river Iskar.
Sites | Temperature °C | Oxygen, mg/L | Conductivity µS/cm | pH | ||||
---|---|---|---|---|---|---|---|---|
Nov. | March | Nov. | March | Nov. | March | Nov. | March | |
UI1 | 4.7 | 5.0 | 12.14 | 7.71 | 114 | 295 | 8.21 | 7.63 |
UI2 | 8.9 | 8.8 | 9.39 | 6.04 | 291 | 341 | 8.00 | 8.80 |
UI3 | 4.2 | 5.3 | 12.80 | 8.25 | 90 | 209 | 8.19 | 7.68 |
UI4 | 4.9 | 6.2 | 12.33 | 8.73 | 177 | 422 | 7.75 | 7.73 |
In Fig.
Dynamics of ammonium ions, phospahates and nitrates in surface water of study area in upper Iskar River (A – November, B – March).
The values for chemical oxygen demand (COD) during the two samplings at three of the four sampling sites were below 10 mgO2/L. The highest concentration was measured in site UI2 in March – 11.98 mgO2/L.
The concentrations of the selected microelements in the water samples are presented in Table
Concentrations of selected microelements in the water of the study area (the values exceeding the maximum admissible concentrations are marked in gray).
Pb | Cd | Hg | Ni | Mn | Cr | As | Cu | Fe | Zn | ||
---|---|---|---|---|---|---|---|---|---|---|---|
µg/L | µg/L | µg/L | µg/L | µg/L | µg/L | µg/L | µg/L | mg/L | mg/L | ||
UI1 | Nov. | 0.150 | 0.031 | 0.12 | 1.04 | 2.20 | 1.830 | 1.15 | 21.60 | 0.120 | 0.062 |
UI1 | March | 0.113 | 0.028 | <LOD | 0.480 | 1.10 | 0.166 | 0.163 | 1.56 | 0.098 | 0.004 |
UI2 | Nov. | 0.032 | 0.073 | 0.13 | 0.90 | 0.18 | 0.820 | 0.85 | 44.00 | 0.051 | 0.045 |
UI2 | March | 0.011 | 0.352 | 0.27 | 2.470 | 0.43 | 0.526 | 0.408 | 1.48 | 0.006 | 0.019 |
UI3 | Nov. | 0.084 | 0.030 | 1.42 | 0.30 | 1.13 | 1.100 | 0.59 | 21.30 | 0.112 | 0.058 |
UI3 | March | 0.022 | 0.035 | <LOD | 0.548 | 0.37 | <LOD | 0.245 | 1.29 | 0.027 | 0.003 |
UI4 | Nov. | 0.008 | 0.071 | 0.24 | 0.46 | 0.90 | 1.039 | 0.50 | 24.20 | 0.094 | 0.076 |
UI4 | March | 0.167 | 0.072 | <LOD | 1.640 | 1.46 | 0.498 | 0.310 | 1.81 | 0.184 | 0.003 |
The data from determination of total microbial count and coliforms in water and sediments are presented in the figures below (Fig.
In water samples from the studied part of the river, we enumerated stable values for total culturable microflora – in the range of 103÷104 CFU/mL. The coliforms were constantly presented in water samples with abundance of 10÷102 CFU/mL. According to site location, the numbers of two indicator groups showed a slight increase in the sampling site of WWTP discharge during the each season studied. In the sediment samples, the total microflora was more abundant and variable. The numbers fluctuated between 105÷107 CFU/g. In the sediments of discharge area of WWTP, the increase in numbers of two indicator groups was significant, especially for coliforms (3 000÷11 000 CFU/g).
The mean fluorescence intensity and percent live cells were calculated from the total area and the area of fluorescent objects on images from CTC/DAPI analysis (Table
Sites | mean fluorescence intensity, CTC | percent live cells, CTC/DAPI | ||
---|---|---|---|---|
November | March | November | March | |
UI1 | 80.25 | 99.67 | 0.40 | 0.83 |
UI2 | 79.67 | 96.67 | 3.07 | 2.81 |
UI3 | 182.33 | 130.00 | 2.20 | 0.21 |
The ecological status of the studied river sector was assessed as “good” according to WFD and Regulation N-4/2012 using macroinvertebrate bioindicators. The metrics Biotic index had a score of 4 in both sites for macrozoobenthic analyses (UI1, UI3). Despite similar ecological status, in sampling site UI1 species tolerant to deterioration in environmental conditions were found – Hydropsyche sp. (Trichoptera), Chironomidae (Diptera), Erpobdella octoculata/monostriata (Hirudinea). After discharge of WWTP Samokov, in sampling site UI3, the total taxa number was higher and Baetis sp. (Ephemeroptera) was also found.
Many previous studies have discussed the response of aquatic ecosystems to WWTP effluents by assessing various indicators, but in most cases, the analyses were performed on specific communities or groups of indicators (
The data about nitrates, ammonium ions and phosphates also are of interest when we refer them to the measured low organic content in the waters of the studied area. The discharge of the WWTP does not lead to additional organic loading and in the aqueous phase there is a imbalance in the ratio C:N:P, which further complicates the utilisation of nitrogen and phosphorus by heterotrophs. However, if we look at the river ecosystem in its heterogeneity, namely the sediment component, the high abundance of microorganisms in the sediment microbiome is impressive. The sediments are stable, active habitat where the predominant part of the transformation processes is probably quickly realized. The more diverse redox and oxygen regimes, the different ecological niches, and the longer retention of organic matter suggest that the sediments are the habitat where the full variety of self-purification processes unfolds. This is confirmed by the high metabolic activity and the share of live cells in sediments – these indicators have the higher values in the discharge area of WWTP and at the sampling site located downstream. The registered high fluorescence intensity at the site under the treatment plant shows that the activity of sediment microbiome remains high downstream, despite the fact that the number and share of live cells decreased. The assessment of the ecological status confirms the role of the sediments for the retention of the organics and the fast realization of the self-purification processes after the discharge. At the same time, the fact that the sediment habitat also serves as a potential refuge for opportunistic and pathogenic microorganisms must be taken into account. The high abundance of coliforms shows the potential role of sediments as a “natural depot” and bring to the fore the recommendation for including the sediment component into the system of water quality monitoring in the upper subcatchment of Iskar River.
Compared to our earlier studies in this part of river subcatchment (
Along with the positive aspects of the increased number of WWTPs worldwide, the associated environmental risks of their operation must be taken into account, especially in terms of the functioning of urban water cycles. The importance of the studied sub-catchment of Upper Iskar for the quality of drinking water of Sofia enforces the extension of an existing monitoring program with a more detailed assessment of the environmental impact of wastewater discharge.
This work was financially supported by Project BG05M2OP001-1.002-0019 ‘Clean Technologies for Sustainable Environment – Waters, Waste, Energy for a Circular Economy’, financed by Operational Program ‘Science and education for smart growth’, co-financed by the European Union through the European structural and investment funds and by Project 80-10-84/25.03.2021 ‘Functional profile of sediment microbiome in discharge areas of Wastewater Treatment Plants in Iskar catchment’, Scientific Research Fund of Sofia University.