In recent years it has regularly surfaced in our media: water is becoming a scarce commodity. There are various sustainable alternatives to save on expensive drinking water. The projects De Nieuwe Dokken in Ghent and Hof Van Saeys in Dendermonde are textbook examples of what is possible in this area.
Rainwater recovery is not new. Water falling on roofs and paved surfaces is collected in a pit and used for flushing toilets and possibly washing machines. Yet wastewater streams from buildings can also be given new life. Water runoff from showers, sinks and baths - gray water - normally disappears into the public sewage network. Fecal water from toilets - black water - goes to a septic tank, whose overflow also leads to the sewer system.
However, it is perfectly possible to recover grey water and upgrade it for reuse in the same building. This requires a separate wastewater line to avoid mixing with greasy water from sinks and dishwashers. The gray water flows through that drain line to a holding tank. There it is cleaned in two stages: first by coarse filtration, and then by membrane filtration where the water is pumped through several membranes. The cleaned water is then stored in a second buffer, from where it is pumped to be used for toilet flushing or any outdoor faucets.
The inflow of gray water to the wastewater buffer is fairly continuous according to a particular profile specific to the type of building. If the tank is too small, part of the dirty water would flow into the sewage system. To avoid this, the buffer must be sufficiently large. The buffer of cleaned water should also be large enough. If the level of that supply is too low, it is replenished with water from the rainwater buffer.
Heat can also be recovered from collected gray water. This, in turn, is injected into the collective heat grid to which homes and other spaces are connected.
Reclaimed gray water as process water
When gray water is treated, it can also be used as process water. In project The New Docks, this is done by making it available to Christeyns, the nearby producer of hygiene products.
Residential units, stores, offices, can be equipped with vacuum toilets. These require only 1.3 liters of water per flush, 4 to 5 times less than conventional toilets. This has the advantage that the residual fractions that end up in the fecal buffer are much less diluted. Thus, they can be easily fermented into biogas that can serve as fuel for a biogas boiler. This releases its heat to a heat grid. The residual product of the fermentation is struvite that can be used as fertilizer in green areas or vegetable gardens.
So gray and black water do not have to go to waste at all. Grey water can be perfectly cleaned and reused as rinse, wash and process water. To recover the heat from it, however, a collective heat supply is needed on the site or in the neighborhood, just as for heat recovery from black water. From the latter we also extract fertilizer that can be reused in the food cycle.
In De Nieuwe Dokken and Hof Van Saeys, we combine these possibilities, and get the most out of the (wastewater) can. In this way, we provide enormous savings in the use of city water, which is an asset in the current problem of fresh water scarcity.
Want to know more about smart and sustainable reuse of wastewater? Contact our expert Rubert Putman at ruben.putman@ingenium.be.
