Galina Dimova

This study assesses the technical potential for wastewater heat recovery in Sofia and its contribution to domestic hot water supply and greenhouse gas reduction. Heat extraction locations were identified using temperature and flow data from the sewer network and the municipal wastewater treatment plant (WWTP) Kubratovo. Temperature thresholds were defined to ensure stable biological treatment ( ≥ 10°C influent) and environmental protection ( ≥ 5°C effluent). Four scenarios were analyzed, considering heat recovery at the WWTP inlet and outlet with ΔT = 2–4 K. The heat recovery potential is evaluated using 15-minute temperature and flow data by applying scenario-specific temperature thresholds, enabling a dynamic assessment that captures real variations in both flow and temperature and explicitly accounts for system shutdown periods. Heat recovery at the WWTP effluent provides higher and more stable yields than at the influent. The potential ranges from 238,536–380,314 MWh/year at the inlet and 264,828–529,705 MWh/year at the outlet. Under the most favorable scenario (effluent, ΔT = 4 K), the recovered heat can supply domestic hot water to over 76,000 households, reducing emissions by more than 200,000 t CO₂/year when replacing natural gas.

The present research focuses on the application of peanut and walnut shells, in their natural state, for the removal of methylene blue (MB) from water solutions in batch and dynamic (flow-through) conditions. Under batch conditions, at a 100 mg/L MB concentration of aqueous solution, the optimum dose of the studied biosorbents was determined to be 2.5 g/L, reaching about 95–97% efficiency of MB removal for both materials. Langmuir and Freundlich isotherms were used to model and evaluate the experimental data under different initial concentrations of MB (25 to 100 mg/L). The determined maximum adsorption capacities are 41.50 mg/g for walnut shells and 46.80 mg/g for peanut shells. Under flow-through conditions, breakthrough curves are determined for three sizes of fractions (<0.25 mm, 0.315–0.5 mm, and 0.8–1.6 mm). For peanut shells, the smaller the particle size, the higher the adsorption capacity and the column breakthrough time. For walnut shells, however, the particle size seems to have a more complex influence on the adsorption process parameters, and this phenomenon deserves future investigation. The adsorption capacity for one and the same fraction size of 0.315–0.5 mm and initial MB concentration of 50 mg/L is higher under dynamic flow conditions, i.e., 51 mg/g compared to 20 mg/g for walnut shells and 46 mg/g compared to 17.5 mg/g for peanut shells.

Rainwater harvesting attracts rising interest in solving the new challenges associated with climate change and socio-economic development. Rainwater harvesting is addressed in various policies, but standards related to the harvested rainwater quality (HRWQ) are currently set mostly for reuse for agricultural purposes. This paper discusses the necessity for the introduction of specific legislative requirements for the HRWQ tailored to the reuse purpose, which would reduce the health and environmental risks. Based on a literature review of research outcomes regarding HRWQ parameters and existing legislation, the paper discusses the complexity of the factors influencing HRWQ and provides some thoughts for possible actions forward that could be undertaken toward the development of specific legislation. The actions include the application of a risk-based approach, the development of a database and guidance with technological solutions.

The presented study aims at the development of an approach, which will enable selection of optimal measures for mitigation of water stress. The approach is based on two software pillars – Water Evaluation and Planning System (WEAP) and MatLab, which are applied in combination. The approach has three main steps: 1) modeling of the river basin with WEAP software 2) selecting mitigation measures and preparation of intervention curves and 3) running of an optimization using MatLab to select the mitigation measures. The set of the applied mitigation measures includes: reducing urban water demand and supply, reducing water demand for irrigation, and rehabilitation of the irrigation system. The result of the performed optimization with objectives set as minimal investment and water abstraction shows that in the Vit River basin the best combination of mitigation measures are a change in irrigation practice as well as reducing leakage in the irrigation distribution network and in the municipal distribution network. The optimization results show that 36% of the abstracted water could be saved if 2.1 million EUR were invested. The approach which was developed and applied in this work proved its suitability for facilitating decision making for water stress management at a river basin level. Doi: 10.28991/cej-2020-03091602 Full Text: PDF