Methods using separation membranes to treat raw water in order to make it potable have been under development for several years. Feed water is injected under pressure into separation modules, generally constituted by bundles of tubular membranes (in particular hollow fibers) for nanofiltration, microfiltration, ultrafiltration, or reverse osmosis. The modules may operate in dead-end mode where all of the water to be treated passes through the membranes and is purified, or else they may operate in cross-flow mode where only a fraction of the water to be treated passes through the membranes, while the remainder that does not pass through the membranes is recycled (filter loop). In both cases, matter in suspension accumulates on the separation side of the membranes and may end up by clogging them. It is therefore essential to provide for the membranes to be periodically backwashed by injecting a backwash fluid in the direction opposite to the treatment direction to dislodge the matter deposited on the membranes and to wash it away. It is often advantageous to use previously treated water as the backwash fluid.
To improve backwashing, it is known that additives can be added to the backwashing fluid that serve to enhance detachment of the deposit while enabling it to retain a degree of cohesion (limited fragmentation) by performing various physico-chemical actions on the deposit that has accumulated on the membranes, which actions are not all fully understood at present. Chlorine type oxidizing agents are known for this purpose.
It is also found to be advantageous to add an additive to the feed water that is suitable for adsorbing matter in solution that would not otherwise be stopped by the membranes. A substance commonly used in the treatment of water is active carbon. When performing separation on membranes, active carbon is used in powder form, and in some cases it provides the advantage of increasing filtration flows appreciably (about 30% to 40%), perhaps because of its abrasive action on the deposit that forms on the membranes.
During backwashing, water containing chlorine and passing through the membranes in the reverse direction to the treatment direction washes away the mixture of active carbon and other matter deposited on the membranes, together with the active carbon in powder form in suspension in the water that is in contact with the deposited matter, and the water obtained during backwashing (backwash discharge) contains, inter alia, the separated solid matter, active carbon that has adsorbed various solutes, chlorine, and chlorinated derivatives formed by the action of the chlorine on the substances that come into contact therewith.
At present, since only small flow rates are treated in this way, the backwash discharge is dumped in the drainage network for rainwater or for sewage, or it is transported to a distant sewage works. Ecological risks and economic losses (backwashing often consumes 10% of the treated water) can thus result from using membranes to perform separation in installations of large production capacity.
It therefore appears necessary to treat the backwash discharge so as to recover as much reusable water as possible together with a residue having a high concentration of solid matter that can be disposed of ecologically and cheaply.
Conventional methods of treating sewage suffer from the drawbacks of being lengthy and complex because of the composition of the matter in suspension in the backwash discharge; and above all they go against the looked-for purpose of using separation membranes, namely treating water while avoiding the production of waste containing chemicals or their reaction products.