3.1. Functioning of Ravda WWTP in Winter
During the winter period, from October to April (inclusive), the wastewater treatment plant operates in winter mode. The reason for this is the small volume of wastewater due to the small number of mainly local inhabitants, numbering 28,968 [
34] (data for the municipality of Nessebar, part of which is the tourist resort Sunny Beach). The process is similar, but uses a different number of facilities. The remaining ones are shut down in stages according to the decreasing number of tourists and seasonal workers.
The incoming water passes through two coarse grates with openings of 27 mm. This is followed by three fine grids with 6 mm openings. The water also passes through an aerated sand filter, which removes any remaining coarse or fine sand particles, silt, and gravel. The finely suspended matter, mainly of organic type, is separated in the primary radial clarifier by gravity. The heavier particles settle to the bottom and the lighter ones to the surface.
The next stage of the water treatment passes through a dephosphatization basin, where the biosolids are removed by FeCl
3. The wastewater is then treated in a biobasin with a sequential anoxic/denitrification/zone and an aerobic nitrification zone. The technological scheme of water purification during the winter period is shown in
Figure 2, and only the biological removal of carbon, nitrogen, and phosphorus is shown in
Figure 4. It is necessary to emphasize that the removal of phosphorus is combined with FeCl
3 by coagulation, and with the carbon-containing and nitrogen-containing organics by successive heterotrophic denitrification and nitrification. The water passes through an anaerobic denitrification zone and then into an aerobic nitrification zone, after which it is stirred in the part of the biobasin without an oxygen supply.
The water then passes through a secondary settling basin and discharges through two pipes, 2.5 km long and 18 m deep, into the Black Sea. This is necessary because the sea area close to the wastewater outfall pipe is particularly sensitive and vulnerable [
35].
Regarding the sludge, it is stored in one methane tank during the winter after digestion for biogas production. Biogas with 60–68% methane content is produced in the anaerobic facilities/methane tanks. This energy source covers up to 30% of the energy needs of the entire facility of Ravda WWTP [
36].
The operation of the treatment plant in this mode during the inactive winter period for tourism ensures the appropriate and satisfactory treatment of wastewater. Below, we examine the purification process by presenting the actual technological indicators by year and dynamics.
In
Figure 5a,b, the COD and BOD
5 of the influent and of the effluent are shown. These indicators are compared to the standards for the discharge of treated water into the Black Sea.
The COD rates (a) in influent water during winter are between 105 mgO
2/L and 189 mgO
2/L. In effluent water, the rates drop until 15 to 23 mgO
2/L, which completely meets the legal rate of discharge in Bulgaria [
37]. The effectiveness of COD removal is high—between 80 and 90%. This indicates that the treatment plant is highly effective in reducing the organic pollution in the wastewater.
When it comes to the BOD5 (b), in influent water, it reaches 100 mgO2/L in winter, with the highest rates noted in 2019. But in effluent water, the BOD5 is lower than 10 mgO2/L, which means that the concentration of organic pollutants in the water is relatively low and meets the standards. It is an indication of good water quality that it is unlikely to cause significant harm to aquatic life and the environment when discharged into the Black Sea.
Figure 6a,b, respectively, show the removal of nitrogen and phosphorus in the course of water purification in the winter scheme of the treatment plant. The concentrations of nitrogen and phosphorus were tracked comparatively in the influent and effluent water. The figure also shows the dynamics of nitrogen and phosphorus removal effectiveness in the years 2018–2022.
The amount of nitrogen in influent water during winter varies between 9.4 mg/L and 24.3 mg/L. The highest rates were noted in 2019 and 2020. The reason is probably related to the beginning of the COVID-19 pandemic and the massive use of disinfectants, more frequent cleaning of rooms and common areas, and the greater amount of detergents used for washing and cleaning. It is important to note that the increased parameters can also be associated with a greater quantity of drugs discharged through the wastewater of all households. There are some assumptions that, in Nessebar, as early as the winter of 2019 COVID-19 was distributed because of intensive touristic exchange. This fact was not possible to prove in the early phase of COVID-19 distribution. There are no statistics to prove it. In 2021, there was a sharp decrease in the efficiency of nitrogen elimination in the course of purification, and it reached a low value of 40%. In all likelihood, this was due to the presence of inhibitors in the waters as a result of the measures to deal with the COVID-19 epidemic—namely, disinfectants, antibiotics, steroids, medicinal preparations, etc. These activated sludge toxic pollutants led to a decrease in nitrogen removal efficiency, mainly affecting denitrification and total nitrification. However, the rate of elimination was sufficient to maintain nitrogen within the catchment discharge standards.
In 2020 and 2021, a process of decreasing the efficiency of phosphorus elimination was also established. In 2020, it was 40%, and in 2021, it was 20%. This is most likely due to the more complex processes of phosphorus elimination, chemical and biochemical, on the one hand, but on the other hand, to the more difficult adaptation of polyphosphate-accumulating microorganisms in the activated sludge to the resistance and degradation of phosphorus-containing detergents and other xenobiotic substances used in the COVID-19 epidemic. Regardless, the concentration of phosphorus was within the range of discharge norms. In 2022, there was a trend of recovery of the efficiency of operation of the Ravda wastewater treatment plant in terms of the parameters of nitrogen and phosphorus elimination efficiency. Their concentrations were close to the discharge standard rates in the Black Sea.
3.2. Functioning of Ravda WWTP in Summer
One of the major challenges for the operation of treatment plants, which operate in a seasonal mode with contraction and multiple expansions of the volume of treated water and the pollutants within it, is the change in the mode of operation and the expansion and contraction of the station itself in terms of scale and types of facilities. An additional challenge is that, in the peak loading season, treatment must be the deepest, most controlled, and most sustainable because the discharge is adjacent to recreational areas. The load in these periods increases not only the quantity of pollutants in wastewater, but also the variety of pollutants. During this busy tourist season, there are numerous pollutants of a toxic nature in the wastewater, such as detergents, steroid hormones, food preservatives, antibiotics, and other biologically active substances and inhibitors which are actively used in tourist complexes.
In the complexes that we have examined in the period of 2018–2022, there was a significant fluctuation in overnight stays. This includes the active COVID-19 pandemic period of 2020–2021, which was accompanied by a strong reduction in tourist activities on the one hand, but with the imposition of specific measures to disinfect and prevent the spread of SARS-CoV-2. All of this had a significant impact on the structure of wastewater, and, as expected, on the treatment processes. These changes will be further discussed in this article.
Figure 7 shows the dynamics of the number of overnight stays in the period of 2018–2022.
In the period from November to April, there were no tourists registered in Sunny Beach according to the National Institute of Statistics. This is a resort working with extreme intensity from May to October.
According to the statistics, in 2018 and 2019, there were over 5 million overnight stays recorded for the season. In 2020, the number dropped drastically to 1,364,008 due to the COVID-19 pandemic. In the next season, in 2021, tourism began to recover again, with the number of overnights doubling. In 2022, it almost reached pre-pandemic levels, with 4,763,211 tourists.
In order to adapt to these fluctuations, the Ravda WWTP prepared a seasonal regime with the gradual inclusion of additional facilities prior to each season.
In April, a phased start-up of the remaining facilities began, with the SRB reactors starting operation in May (
Figure 2 and
Figure 8). The activated sludge from the pump station for returning sludge and recirculating active sludge was transferred to them.
For 3 consecutive days, samples were taken at the point of discharge to check for the presence of pathogenic microorganisms.
During the active tourist season, significantly more polluted water flows in. For purification, two more fine grates (five in total), and another sand trap (two in total) were added. The biological treatment also includes a second line of four SBR reactors where the water entered after the dephosphatization basin. Water from the biobasins and SBR reactors is collected in a total of three secondary settling tanks (two more than in the winter). In the summer, the water also goes through UV disinfection before being discharged into the Black Sea.
In the sludge treatment route, the difference is that three digesters are in operation (two more than in the winter).
In
Figure 9a,b, the COD and BOD
5 of the influent and the effluent water in the summer regime of functional WWTP Ravda are shown. These indicators are compared to the standards for the discharge of treated water into the Black Sea.
Data analysis by season (summer/winter) clearly showed the difference in influent and effluent water quality, affected by mass tourism.
The calculated average increase in water pollution during the summer season due to the development of mass tourism, measured using the aforementioned indicators, was as follows. On average, COD increased by about 3.2 times, BOD5 by about 3.1 times, total nitrogen by about 3 times, and total phosphorus by about 4 times. This shows that both the expansion of the treatment plant and the inclusion of specialized equipment, such as sequencing batch reactors with intensive, controlled denitrification/nitrification phases, are appropriate and effective. The improvement in summer operation is also indicated by the fact that the pollutant elimination efficiencies measured by the indicators mentioned above (COD reduction, BOD5, nitrogen, and phosphorus elimination) remained high, at 90–95%, for all years studied. A decrease in nitrogen elimination efficiency was recorded in 2020, and a slight decrease in the elimination of degradable carbon-containing organic matter (measured as COD and BOD5) was found in 2021 and 2022. As these decreases were within statistical fluctuations, we can consider them to be insignificant. More significant, however, is the decrease in phosphorus elimination efficiency, a trend that we also recorded for the winter operating regime of the facility. The concentrations of the two groups of substances were directly proportional to the number of tourists registered during the seasons. This may be associated with the use of cosmetics, laundry and cleaning products, and medicines. Although unproven, many managers have shared that restaurants dispose of some food waste down the drain to save on disposal fees. This also has a serious impact on phosphorus and nitrogen levels in wastewater.
- -
Permissible level of COD in treated wastewater: 125 mgO2/L;
- -
Permissible level of BOD5 in treated wastewater: 25 mgO2/L;
- -
Permissible level of nitrogen in treated water: 10 mg/L;
- -
Permissible level of phosphorous in treated water: 1 mg/L.
This further confirms that phosphate elimination should be subjected to tighter control in the future, and biological phosphorus elimination should be enhanced by activating the polyphosphate-accumulating microbial complex in the activated sludge. It should not rely solely on chemical coagulation with ferric trichloride. The functional analysis of the treatment during the summer tourist season notwithstanding, it is necessary to firmly underline two risk-eliminating facts for the marine ecosystem: 1: The legal norms for discharges into the marine ecosystem were not violated, although they were extremely close to the maximum permission level in 2021 (for the nitrates) and 2022 (for the phosphates), shown on
Figure 10. 2: Deep-sea discharges into water layers with intense anaerobic biodegradation and intense mass exchange ensure that ecosystem health and the cleanliness of the recreational area are maintained.
During the period under review, 2018–2022, there were two risk situations and problems in wastewater treatment processes that we would like to discuss in more detail.
During the examined annual time interval of the operation of Ravda WWTP in two different modes with two different scales and mechanistic technologies, we discovered these critical issues: 1: The dynamic epidemic situation that was limiting tourism and imposing travel restrictions in the period of 2020–2021 underlines that this situation had already started in 2019, but it was difficult to readjust the tourism industry, and most of the bookings were not canceled. The real decrease in touristic activities occurred in 2020–2021.
2: During the years of epidemic measures, actions needed to be carried out related to the use of more water for hygienic needs and the requirement for more disinfectants, medicines, and other pharmaceutical preparations such as synthetic vitamins, steroid hormones, more plastics for disposable packaging, more protective agents with low biodegradability, etc. The structure of wastewater underwent changes, which were expected: 1: A higher volume of wastewater was noted. 2: More low biodegradable and non-biodegradable pollutants were found in the wastewater, some of them with pronounced xenobiotic character, i.e., reduction in the BOD5/COD ratio, and the inhibition of certain sensitive processes of the complex biological treatment process was expected. 3: Deterioration of the treated effluent water quality was expected.
Of course, these processes were not unidirectional or unambiguous. Firstly, the number of overnight stays was decreasing; second, the amount of pollutants and the structure of pollution was increasing; and third, the volume of water used per overnight stay was increasing. We cannot ignore the numerous environmental and economic considerations that have necessitated the modernization of hotel complexes, leading to water savings, the introduction of circular water reuse solutions, and the replacement of chemical tourism aids with their biological alternatives. Taking this into account, we made the following analysis. It has reached this stage based on the available data, and we do not claim it to be complete. Biological analyses and evidence will be added to this analysis in our future publications.
Figure 11 shows the proportion and dynamics of overnight stays and the quantity of water used.
During the summer season, tourists number more than 5 million. A considerable drop was noted in 2020 because of COVID-19, but starting from 2021, the number has been slowly increasing.
The water quantity (m3) used for drinking, pools, SPA centers, and irrigation also decreased during the period of 2020–2021, but then increased again in 2022. However, the amount of water was relatively constant in 2019, 2021, and 2022, and significantly lower compared to 2018. This is because of the introduction of sustainable practices in hotels that limit its consumption. Sink faucets are being replaced with aerated faucets, and sensor faucets are being installed in common areas. Showers in common areas are also being replaced with push-button showers. The introduction of sustainable water-saving practices, albeit in phases, is having a major impact. We, therefore, will use the year 2019 as the control year in subsequent studies and comparative analyses.
It is interesting to note as well that two large hotel resorts began operating in 2018 (Aqua Paradise Resort) and 2019 (Wave Resort). The first has the capacity to accommodate 1500 guests, and the second 1100 guests. They are both equipped with sustainable solutions that minimize their carbon footprint, such as [
38]: (1) bathrooms with low-flow showers, rainforest showers (equipped with a flow rate limit of 9.5 L/min), and additional flow-restricting laminar technology and an eco-saving hand shower equipped with a flow restriction of 8 L/min. Sink taps have aerators included, which limits the water usage to 5 L/min. (2) Gardens are only watered during nighttime with a dropping and sprinkler system to avoid vaporization. (3) The staff are actively training to notice and prevent leakages in the room toilets, and they expect to be informed of such occasions by guests. (4) The toilets are equipped with two-stage buttons (6 L/3 L) to control the amount of water used. (5) The urinals in the public restrooms are equipped with motion detector sensors, which avoids unnecessary water consumption. (6) Towels and linens are changed on an eco-principle, which means they are not changed daily, but upon guests’ request.
An innovative solution for Sunny Beach is utilized by Aqua Paradise Resort, which is equipped with a rainwater collector with a capacity of 300 m3 to irrigate gardens. Wave Resort, located in Aheloy, close to WWTP Ravda, uses another great resource that helps them to save even more water. Sunny Beach and the area are marshy areas with a significant amount of underground water. They install boreholes to extract this resource, using it for irrigation and maintenance of common areas. Unlike the rainwater tank, which covers less than 1% of the hotel’s needs, this solution satisfies them completely. This is why the amount of potable water used has not risen in 2019 despite the launch of the facility.
This is proof of the importance of taking measures and introducing innovative solutions in all hotels in the resort area.
Another important fact is that hotels in Sunny Beach began replacing plastic packs, cups, and straws with reusable ones in 2019. But this was implemented in only a few hotels as a test. In 2020, the initiative was paused because of safety measures due to COVID-19. The replacement of plastic restarted again in 2021, step by step, in numerous hotels. More resorts applied the measure in 2022. As a consequence, the use of water increased because of the need to wash reusable recipients. The higher consumption of water in the period of 2020–2022 can also be linked to COVID-19 practices, including more frequent cleaning of rooms, pools, and common areas.
In
Figure 12, the overnights in August are compared with the water quantity in August. The number of local citizens is not included as it is constant during the whole year.
During the winter period, only 30,000 people live in Sunny Beach [
29]. By comparison, there are an average of 222,000 people per day in the summer, which includes seasonal workers. This large difference necessitates the adaptation of the Ravda WWTP using different numbers and types of equipment.
Figure 12 shows the dynamics of COD and BOD
5 in 2018–2022 per cubic meter of wastewater per night. It can be clearly seen that the values of this indicator were lower in 2018. In 2019, they increased due to increasing measures against COVID-19, but also due to the large amount of overnight stays, since they had not yet been canceled. It is noteworthy that, in 2019, non-degradable pollutants increased (based on the comparison of COD and BOD
5) and remained at these higher values in 2021 and 2022. This can be seen more clearly below in
Figure 13, where the BOD
5/COD ratio during the winter and summer operating periods of the Ravda WWTP is examined.
In 2020, this ratio (economic biodegradation factor K) decreased during both the winter and summer operations. The decrease was more significant in the summer during the tourist season, when the volume of xenobiotic pollutants was larger and more diverse. Despite the ratio increasing slightly in 2021 and 2022, K continued to be lower than 0.5. This indicates an increased presence of difficult-to-degrade pollutants in the wastewater. It is only in 2022, under both the summer and winter operating regimes, that the K was restored to the levels registered in 2018 (0.5 in effluent water). This reduction in the complex biodegradable pollutant due to COVID-19’s tourism impact is shown and clearly illustrated by the data in
Figure 14.
Figure 14 shows that, in 2020, the COD of water used per overnight decreased by 65% compared to the 2019 pre-pandemic control year. The BOD
5 in the same year decreased by 70%. In 2021, tourism began to recover. The number of tourists increased, which is reflected in the graph. In 2021, the decrease compared to 2019 was 27%. Even better, in 2022, the drop in COD/H
2O/overnight was only 10% compared to 2019. The data are similar for BOD
5. This trend also shows the growing and rapidly developing tourism in Sunny Beach and the need to make forecasts for its development. This must necessarily be taken into account when building and implementing new technologies to cope with this expansion.
Figure 14 shows that the reductions in complex pollutants and complex biodegradable pollutants deepened in 2020, 2021, and 2022, accounting and adjusting for water volume and number of nights. All of the above confirms that the tourism system and wastewater treatment technology are recovering from the cautionary crisis, albeit slowly, and that the various modern resource, energy, water efficiency and circular solutions taken in modern tourist large resorts are showing their positive impacts in economic and environmental terms.
To apply more detailed evidence COD, BOD
5, pollutant elimination efficiency, and compliance with discharge standards for the marine ecosystem, data according to month and year are shown in
Figure 15 below.
The data represent the period 2018–2022 and show the COD reduction efficiency in the influent water. Wastewater data during the critical summer treatment regime are shown in
Table 2. This shows that the treatment efficiency of COD reduction was sufficiently high in the months of active tourism during the COVID-19 epidemic, and subsequent recovery of water treatment technologies and processes was observed.
The plasticity of these treatment technologies and processes was sufficiently large to provide between 85.98% efficiency for July 2019 and about 95% for July in the remaining years of 2020–2022. This plasticity of technologies, eco-sustainable tourism measures, and professionalism of the treatment facility managers ensures compliance with discharge standards and maintenance of the ecological health of the marine ecosystem over the 5-year study period, including the period of tourism restriction and loading of low-biodegradable pollutants in the water, measures which were used to address the critical COVID situation.
The data on the summer and winter season, presented above in these graphs, are separated by month, where the changing conditions over the two periods are clearly visible. The COD and BOD5 data changed in direct proportion to the number of tourists. The efficiency was about 90% for all months of the year.
The ratio between the chemical oxygen demand (COD) and the biological oxygen demand over 5 days (BOD
5) indicates the level of biodegradability of a sample. The ratio of COD/BOD
5 was less than 2.0 or 2.5, which indicates a high biodegradability. This means that it can be treated easily using a biological treatment technique [
39].
According to the results shown in
Figure 14, the BOD
5/COD ratio was low, which means that the wastewater was easily biodegradable by microorganisms.