Advanced Integrated ZLD Systems

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Courtesy of Aquatech International Corporation

The Hybrid ZLD approach pairs the best of membrane and thermal technology to make ZLD affordable, yet robust and reliable.

Over the past decade, the Hybrid ZLD systems i.e. integrated zero liquid discharge (ZLD) systems have emerged as die most effective ZLD approach for dealing with the most difficult to treat wastewaters in many industries including power, refining, and chemicals. The Hybrid ZLD approach pairs the best of membrane and thermal technology to make ZLD affordable, yet robust and reliable. The Hybrid ZLD process incorporates advanced membrane technology, HEROTM (High Efficiency Reverse Osmosis) based Reverse Osmosis system as a pre-concentration step followed by evaporation and crystallization technologies.

Hybrid ZLD
Wastewater streams are known to be rich in silica, high heavy metals, TOC, hydrocarbons, and sparingly soluble salts, etc. A conventional RO system is limited by its chemistry to provide high yields and produces large quantities of brine for either treatment or disposal.

The patented HERO Technology is used to overcome these chemistry related issues and generate high yields of product water, thereby reducing the quantity of die reject/brine stream to less than 5-10% of the total wastewater for die thermal based ZLD system diat follows. The high recovery is especially important for plant economics as the more water diat can be recovered, the lesser the evaporation requirement downstream. This leads to significant energy efficiency as compared to a more conventional system.

The evaporation technology provided is usually a Mechanical Vapor Compression (MVC) based Brine Concentrator followed by a Forced Circulation Crystallize^ where the concentration of the waste stream exceeds the solubility limits of die major dissolved salts, thereby producing salt slurry. Solids are removed from the slurry using a belt filter press. The mother liquor that is separated goes back to the crystalyzer while die salts separated arc suitably disposed.

The permeate from die advanced membrane based pre-concentration step can be used as cooling tower make-up or can be further polished and used as ultra high purity process makeup. The high purity distillate produced from the thermal pordon in this system is also acceptable for demineralizer makeup.

The feed to die thermal system consists of HERO reject. An advantage of this process is that it operates at relatively high recovery generating less reject compared to a conventional RO. This leads to the generation of a very small concentrate stream/feed for die thermal system or evaporation pond, therefore making it possible to achieve a true ZLD status.

Thermal Process
The plate and frame feed/distillate heal exchanger preheats the feed with outgoing hot distillate. The preheated feed is sprayed into the deaerator, which further heats the feed with low-pressure evaporator vent vapors and boiler steam.

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