The Vacom One-step ZLD system operates as a concentrator and/or a crystallizer depending on the specific heat exchanger and blower designs.

Heavy industrial processes can generate oily wastewaters containing a variety of other toxic contaminants such as heavy metals, surfactants and ammines. The system can concentrate the oily contaminants into a high BTU waste fuel recovering clean water.

Because the Vacom One-step ZLD can be used in a variety of heavy industrial processes utilizing wastewaters of varying chemical consistencies and levels of corrosiveness, the materials with which to design and manufacture each system must be chosen carefully.

Material selection is critical. The system has operated under chloride concentrations exceeding 300,000 ppm while at pH levels ranging from 1 to 14. Additionally, the system can be used to concentrate wastewater without any pretreatment. This means there are often high levels of abrasive suspended solids. The system handles corrosion, abrasion and pitting by ensuring that the right metals have been selected for the application.

Molybdenum

Molybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting corrosion especially by chlorides and sulphur chemicals. When added to low alloy steels, molybdenum improves high temperature strengths and hardness. When added to chromium steels it greatly diminishes the tendency of steels to decay in service or in heat treatment.

Chromium

Chromium is added to the steel to increase resistance to oxidation. This resistance increases as more chromium is added. 'Stainless Steel' has a marked degree of general corrosion resistance when compared with steels with a lower percentage of chromium. When added to low alloy steels, chromium can increase the response to heat treatment, thus improving hardenability and strength.

Nickel

Nickel is added in large amounts, over about 8%, to high chromium stainless steel to form the most important class of corrosion and heat resistant steels. These are the austenitic stainless steels where the tendency of nickel to form austenite is responsible for toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low temperatures when added in smaller amounts to alloy steels.

The circulation loop continually circulates the preheated wastewater from a holding vessel through a 'primary' heat exchanger and back into the vessel. By circulating at extremely high rates in a pressurized system, the heat exchange efficiency is at the maximum possible and is within the turbulent and submerged boiling flow regimes. The benefits of this design include maximum heat exchange efficiency.

The high circulation rate creates shear and velocity forces in the heat exchanger that effectively minimizes solids fouling and keeps the system self-cleaning. Operating the circulation loop at pressures that keep the heated wastewater from boiling in the heat exchanger, the release of clean steam in the vessel is controlled by letting down the pressure. This creates a zone where the heated wastewater flashes into steam in the vessel only, which eliminates scaling in the heat exchanger which is common in most boilers.

The recovery loop works in combination with the second, by evacuating the steam from the vessel with a steam pump that blows the steam across the clean side of the primary heat exchanger used in the circulation loop. There the steam condenses into clean water. When the steam condenses, it gives up the energy it took to convert the preheated wastewater into steam. The latent heat of vaporization is completely recovered. After the water condenses, it is still very hot, but it cools as it exits the system across the preheat exchanger described in the feed loop.

The steam make-up loop introduces steam from an outside source into a separate section within the primary heat exchanger to heat the system to initial operating temperatures and to make-up any minor heat losses from the difference between the feed temperature and the distillate exit temperature.

Xylem’s Vacom One-step ZLD can be operated both as a concentrator and as a crystallizer in the same system.