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.
