Deeper Blue
Circular water use: from wastewater to drinking water in an underground seasonal battery

How can we make water consumption in Flanders more sustainable? That’s the question Waterunie, in collaboration with Aquafin, set out to answer in the ‘DEEPER Blue’ project. The solution? Further purifying effluent—treated wastewater—from the Aalst wastewater treatment plant (WWTP) into drinking water, then injecting it underground during periods of water surplus as a reserve for later recovery and use during water scarcity. This approach helps reduce pressure on groundwater and surface water resources.
Climate-resilient seasonal battery for drinking water
A new water reuse facility is currently being built alongside the Aalst WWTP. Using membrane technology, it will purify a portion of the WWTP effluent to serve as a new source for drinking water production. Once operational, the treatment plant will produce around one million litres of drinking water per day.
During wet seasons such as autumn and winter, surplus water will be locally injected into the Sokkel, a deep underground aquifer. This process, known as Aquifer Storage & Recovery (ASR), builds a strategic reserve of injected water (Aquifer Storage), which can then be recovered and supplied to the drinking water network during dry seasons like spring and summer (Aquifer Recovery).
A key advantage of this approach is the short distance between the project site and the existing drinking water supply network. This means the water can be distributed not just to Aalst but also across a wider area in Flanders. The ASR system effectively acts as a seasonal battery for drinking water, ensuring water availability during shortages. In doing so, DEEPER Blue contributes to Flanders’ circular water management goals and the Strategic Water Supply Plan (SPW).
Safety & ecological sensitivity
Safety is a top priority for this project. The reuse facility is designed according to the multi-barrier principle, with reverse osmosis (RO) serving as the final safeguard against microcontaminants, bacteria, and viruses.
The project also considers the ecological sensitivity of nearby water bodies. By only using and injecting a portion of the WWTP effluent during wet periods, sufficient effluent remains available for other uses, such as agriculture and ecological needs, even during dry periods.


Project partners
DEEPER Blue is an initiative of Waterunie, a subsidiary of water companies Farys and De Watergroep. The project is being developed in collaboration with Aquafin, with support from the research platform Capture. Engineering consultancy Witteveen+Bos is assisting the project partners with environmental permitting, water management assessments, regulatory frameworks, and discharge evaluations.
The project forms part of a programme by the Flanders Environment Agency (Vlaamse Milieumaatschappij) within the framework of the “Blue Deal”, the Flemish Government’s plan to tackle water scarcity and drought.
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