Zero liquid discharge

The use of RO to treat and recover reject from another RO system is becoming more common, particularly in ZLD applications. A secondary RO is used to treat the reject from the primary RO to reduce the thermal evaproative requirements of the ZLD system. 

The Nalco Water Handbook, Z ed., McGraw-Hill Book Company, New York, New York, 1988. 

Nalco Company, personal communication, October 17,2008. 

Guzman, Natalia, Morton Salt, personal communication, November 13,2008. 

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Figure 21 Refinery wastewater recycle/zero liquid discharge scheme. Pretreatment and reverse osmosis are used to recycle water, and brine concentrator and crystallizer are used to treat the rejects to achieve zero liquid discharge. (From Ref. 78.)...

Figure 15.1 Zero-liquid discharge (ZLD) system showing primary and brine recovery RO systems.

High efficiency reverse osmosis (HERO) is a patented process (US Patent 5925255, Debasish Mukhopadhyay, 1999) originally developed to treat high silica water for the microelectronics industry. Its use has expanded to power and zero liquid discharge applications (see Chapter 15.4.3). Features of the process include ... 

High efficiency RO is often used in zero liquid discharge (ZLD) applications. The HERO process is used prior to the thermal equipment to reduce the size and energy required by the thermal system. In many cases, the brine concentrator can be eliminated entirely, such that the concentrate from the HERO process feeds directly to the crystallizer. 

Membrane systems followed by an evaporator (for low-volume highly concentrated effluent) can enable industries to achieve zero liquid discharge goals. 

The Jarosite Atmospheric Leach process may be particularly suited to arid climates, where the only water source readily available is saline and it is possible to operate with zero liquid discharge. 

The design of drain points is brought to the extreme in the case of the so-called ZLD (zero liquid discharge) plants. As the name suggests, in such cases there is not any liquid purge permitted anymore. Hence, impurities can only be taken off either with the crystal mass in the adherent mother liquor after separation as dissolved impurity or as cocrystallized substances. Most of the time, the crystal mass recovered this way has to be wasted and the crystallization cannot be considered for separation of the compounds anymore (Figure 16.5). The plant is forced to cocrystallize - up to a lot of substances - at their invariant point in their relevant solubility systems. 

Figure 16.5 Drain point for impurities in a zero liquid discharge crystallization via residual mother liquor in the solid.

Aapo Saiisk (development of polygeneration and zero liquid discharge), CEO, Scarab Development AB, Stockholm... 

Deep well injection Evaporation pond Zero-liquid discharge... 

In arid regions, plants can be designed to create no effluent water so as to not affect the ecosystems. Such zero-liquid-discharge technologies are applied at some plants and require additional heat. The leftover fi-om the water treatment is a brine, which is concentrated to a solid waste salt by evaporation, is of lower quality, and must usually be land filled [32]. 

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