What are the Concepts behind Closed-Loop Wastewater Recycling?

Typically a recycling process that uses deionization (DI) or another mineral reduction process is less expensive to operate because the contaminated water returning to the mineral reduction process has far less dissolved minerals than using tap water as the source water.

The initial water make-up to the AquaCycler (recycler) is typically treated depending upon the water purity requirements of the process. After the water recycling system is started, the contaminants from PCB assembly cleaning (flux), or precision cleaning (oils) or from either process (particulates) must be removed continuously or periodically. If this is not done, the washed parts become contaminated again and will accumulate in the piping system. Also, as the water evaporates, the dissolved minerals eventually deposit scale on all of the surfaces that the water contacts.

For PCB assembly cleaning with deionization, the water purity from an AquaCycler starts at 15-17 megohm-cm (18.3 megohm-cm is absolute pure water). The water purity decreases as the contaminants accumulate on the ion-exchange resin. For most applications, users continue to run the system until the purity is from 10,000 ohm-cm to 1 megohm-cm.

Important facts:

• Deionized (DI) or reverse osmosis (RO membrane) water has a low ion content that does not cause scale but does provide continuous removal of the ingressing contaminants
• High mineral content tap water to an RO system must use DI to achieve water purity in excess of 100,000 ohm-cm water purity
• Membrane vs. deionization
  • all membrane processes have a continuous reject stream (concentrated waste) that is from 5 to 50% of the water going to the membrane goes down the drain, therefore, aqueous processes cannot be closed-loop, but semi-aqueous processes (PCB assembly) can, because the solvent phase can continuously remove the contaminants
  • deionization (ion-exchange resins) does not have a continuous reject stream and can be closed-loop (resins can be discarded or regenerated)
• Total cost of water from a utility (tap water cost plus sewer discharge costs) varies from 1 to 5 cents/gal. When this water is deionized (1 megohm-cm plus), it costs the user an additional 1 to 12 cents/gal
• Typical separation technologies used:
  • mechanical filtration (particles)
  • activated carbon (organic molecules)
  • deionization (ions)
  • membrane
• Reverse osmosis (particles + organic + ions)
• Ultrafiltration (particles + organic molecules)
• Microfiltration (particles + large organic molecules)
• Distillation (particles + organic + ions)