The River Dommel is a modest-sized and ecologically sensitive stream located in the southern region of the Netherlands, covering a catchment area of 153,000 hectares. Its primary water source is rainwater, supplemented by a minor contribution from groundwater, particularly notable in the upstream portion of the catchment. Nevertheless, rainwater remains the primary source feeding the river, with its influence most pronounced in the midstream and downstream portions of the Dommel. In addition to that, the River Dommel receives the Eindhoven Wastewater Treatment Plant’s effluent while flowing through the city of Eindhoven. Eindhoven WWTP is one of the largest wastewater treatment facilities in the Netherlands with a 750,000 Population Equivalent (p.e.) and its effluent contributes approximately 50% of the total flow in the River Dommel during dry summer periods (Weijers et al., 2012; Kamstra, 2018).
During dry summer seasons, the Rivel Dommel faces the problem of insufficient base flow and low water levels due to decreased precipitations and increased evaporation rates. Insufficient base flow and therefore low water levels causes water quality issues in the River Dommel. Moreover, inadequate water flow, compounded by warmer temperatures in the summer, exerts stress on aquatic ecosystems within the River Dommel.
The primary objective of this report is to present a comprehensive study aimed at identifying a technically and economically viable solution to feed the River Dommel base flow during dry weather flow (DWF) conditions. Possible solutions addressing these challenges are elaborated and discussed in this report. Consequently, the application of sewer mining technology is chosen to be further investigated. The idea of implementing the sewer mining technology in this study is based on the extraction of the wastewater from Riool-Zuid sewer system, followed by cleaning processes and feeding the River Dommel with reclaimed water. While there are several treatment methods that can be applied in sewer mining, a thorough review of the literature, as presented in the relevant sections of this report, has led to the conclusion that a combination of Forward Osmosis (FO) and Reverse Osmosis (RO) represents an effective and efficient approach to achieving the objectives of this study. However, it should be noted that, implementation of sewer mining technology directly impacts current sewer system conditions and potentially give rise to significant challenges such as corrosion, odour and health impacts. Therefore, the impact of the sewer mining concept on the Riool-Zuid sewer system is also investigated and discussed in this study, followed by proper risk-mitigation measures.
The analysis of sewer mining is conducted through Mega-WATS simulations, based on a number of separate scenarios composed of three different implementation locations and various extraction ratios. The pressure main plays a crucial role on the extent of sewer mining technology impact on the Riool-Zuid sewer system as it represents anaerobic conditions while the rest of the sewer system is composed of gravity pipes representing aerobic conditions. Therefore, de Meren and Aalst are chosen to reveal the impact of anaerobic conditions on the severity of water extraction from the sewer system as these locations are both located upstream of the pressure main. In addition to that, the juncture of Heeze, Sterksel and Leende domestic wastewater inflow is chosen as the third implementation location, which is located downstream of the pressure main.
The implementation of the sewer mining technology upstream of the pressure main resulted in high dissolved sulphide concentrations and therefore led to considerable corrosion and odour problems in Riool-Zuid. On the other hand, when it was implemented downstream of the pressure main, the corrosion and odour problems were observed to be minimal. However, it is worth emphasizing that the distance between the sewer mining technology and the River Dommel plays also a crucial role in selecting the implementation location since the primary objective of this study is to feed the base flow of the River Dommel in an efficient, reliable, and cost-effective manner.
De Meren represents the nearest location to the River Dommel. Moreover, the existing infrastructure at the de Meren control station, including an adequately sized building, presents a distinct advantage. This eliminates the need for additional construction costs, and concurrently minimizes the expenses associated with the installation of piping. Despite the fact that implementing the sewer mining technology in de Meren resulted in higher dissolved sulphide concentrations and more noticeable corrosion problems in Riool-Zuid, the required nitrate dosages for corrosion control were found to be less than those required for implementing the sewer mining technology in Aalst. Regarding the water quantities, sewer mining implementation in de Meren yielded flow rates in the River Dommel ranging from 0.5446 to 0.1004 m3/s slightly below the observed rates in Aalst, which ranged from 0.5454 to 0.1021 m3/s. Furthermore, in terms of cost analysis, the total treatment cost for the FO-RO system was determined to be approximately 1.67 €/m3.
In conclusion, this study reveals that FO-RO hybrid system can be applied as a sewer mining technology at the Meren control station to boost the base flow of the River Dommel during the summertime to minimize the water quality problems in River Dommel.