O&M

ZLD Success Factors

The power industry as well as oil & gas, chemical, petrochemicals, mining and other industries generate large volumes of waste water that must be managed.

By Daniel Bjorklund

The power industry as well as oil & gas, chemical, petrochemicals, mining and other industries generate large volumes of waste water that must be managed. Commonly these wastewaters are discharged via a plant outfall to a surface water body, an evaporation pond, or in some cases deep well injected. However, there are growing environmental concerns regarding such discharge practices, which has resulted in the development of Zero Liquid Discharge (ZLD) processes.

ZLD can be defined broadly as a process for maximum recovery of water from a waste water source that would otherwise be discharged. This water is beneficially reused and the salts, and other solids contained in the waste water are produced and generally disposed in a landfill.

The drivers for ZLD include a growing concern by the public about the impact of such discharges on the environment, and in many areas of the world, water is a scarce resource. Such concern is resulting in increased regulation and limitation of waste water discharges. Even without regulatory push, many companies in various industries are mandating initiatives for reducing water discharge by recycle reuse, as well as ZLD, to reduce their environmental footprint and improve sustainability.

Zero liquid discharge can be achieved in various ways. There is no “one size fits all” solution, as the optimal system design is site specific. The waste water composition, various streams to be treated, site specific operating costs, foot print availability and other factors are determining factors for an optimal design. This article provides a brief primer on various typical ZLD configurations and focus on factors that are critical to the successful design and operation of a ZLD system.

The system objectives for a ZLD system are to eliminate a liquid waste water discharge, generate solids for landfill disposal or reuse, and to recycle a high-quality water that can be beneficially reused. The design objectives are to minimize the capital investment and system operating cost, while not significantly impacting the manpower required for operation. Further, the system must be designed with operational flexibility to meet the facility needs and be safe and reliable.

Waste Water Chemistry

Careful consideration of waste water chemistry is needed for the successful design and operation of a ZLD system. Sometimes prior experience with similar water chemistry is available to the ZLD designer. Where experience is lacking, proprietary water chemistry modeling software can be applied to understand the solubility limits of various species as the water is concentrated to a high TDS brine. Such software is also useful for estimating the chemical consumption of various chemicals that may be used in the ZLD process for conditioning and pH control. If water is available, bench top studies can also be useful to validate chemistry modeling; where water may not be available, synthetic analogues can sometimes be used. A sound water chemistry design basis is key to successful ZLD design.

In a ZLD system, the waste water being processed is concentrated to solubility limits of the dissolved salts. When the solubility limits are exceeded, salts crystallize and can then be harvested using an appropriate means. Brine chemistries in which monovalent cations such as sodium are balanced with sulfate and chloride, generally are limited to a maximum TDS of less than 30% and a chloride concentration (important factor in metallurgy selection) of less than 170,000 ppm.

Divalent cations such as calcium and magnesium are of primary concern for design of a ZLD system. High calcium and magnesium concentrations can lead to concentration of highly soluble species such as calcium chloride and magnesium chloride. High concentrations of these divalent cations can significantly contribute to the increase in boiling point elevation. As waste water brines concentrate the boiling temperature increases above that of pure water due to a physical property of the solution known as boiling point elevation (BPE). The design of an evaporator requires accurate knowledge of the boiling point elevation. Further, high concentrations of these divalent cations can result in high concentrations of chloride ions and lead to more costly metallurgy.

Calcium is generally sparingly soluble due to the presence of alkalinity and sulfate cations and must be properly considered to avoid scaling of a pre-concentrating membrane system, as well as brine concentration evaporators (Brine Concentrator). Membrane preconcentrators generally rely upon softening and antiscalants to control scaling. Brine concentrators are designed with seeded slurry scale control. By using seeded slurry scale control brine concentrators scaling is retarded by maintaining a proper concentration such that a high ratio of crystal surface area is maintained.

Generally forced circulation crystallizers receive the blowdown from upstream preconcentrating membrane systems or brine concentrators. Crystallizers are designed to manage precipitation of highly soluble species such as sodium chloride and sodium sulfate, as well as sparingly soluble salts such as calcium sulfate. High concentrations of sodium relative to divalent cations are beneficial in controlling the chloride concentration.

Silica is present in varying concentrations in natural water sources. The solubility is very limited at near neutral pH; however, solubility is greatly enhanced if the pH is increased. If allowed to precipitate without control, silica can scale preconcentrating membrane systems and the heat transfer surface of evaporators. Such scales are difficult to remove by chemical cleaning and therefore need to be avoided and considered in the design of the system.

Ammonia when present will volatilize in an evaporator system and partition between the distillate and atmospheric vent. As the ammonia volatilizes the pH of the system may decrease and caustic may be needed to control the system pH. If ammonia is present additional controls on the vent may be required depending on the concentration of the vent to avoid a health hazard, an air permit violation or a nuisance odor.

ZLD System Design

Evaporation systems generally are more capital and operating cost intensive than membrane systems, with crystallizers the most costly. For that reason, and when possible, membrane systems can be utilized to reduce the capital and operating cost of the evaporation system.

Conventional membrane systems can concentrate up to 2 to 3 percent TDS, specially designed high recovery systems can concentrate to as high as 6 percent to 8 percent in some applications. Depending on the waste water composition, preconcentrating using a membrane system can dramatically reduce the sizing requirement of the backend evaporations system and thus the system capital and operating cost. As an example, if a waste water with a feed TDS concentration of 5000 is concentrated using a high recovery membrane system, the duty requirement of the evaporation system may be reduced by 90 to 95 percent. Note that to reach high recoveries in a waste water membrane system, appropriate pretreatment such as softening and pH adjustment is often required.

Vertical tube falling film brine concentrators are generally used to concentrate lower total dissolved solids (TDS) brine solutions up to 12 percent to as high as 25 percent total solids and are used to minimize the design capacity of a downstream forced circulation crystallizer. Brine concentrators are specifically designed to manage the scaling of sparingly soluble divalent salts such as calcium sulfate and calcium carbonate, as well as silica that is also commonly present. Forced circulation crystallizers are generally used to concentrate brine blowdown from upstream concentration equipment, although small waste water flows are sometimes treated directly with a forced circulation crystallizer. Such applications generally involve waste water flows less than 20 to 30 gpm. Crystallizers are designed to manage crystallization of all salts, sparingly soluble as well as highly soluble sodium salts such as sodium chloride and sodium sulfate, without excessive scaling and cleaning frequencies. This robustness comes at the expense of higher specific energy consumption and higher specific capital cost.

The solids generated by a forced circulation crystallizer are generally harvested and dewatered by either an indexing belt filter or by centrifuge. In such case the solids are collected and typically landfilled in a conventional landfill as long as the waste passes Toxicity Characteristic Leaching Procedure (TCLP) testing. However, in some applications involving, ZLD equipment the highly concentrated brine is discharged to an evaporation pond. Such a configuration reduces the footprint of the evaporation pond, and the labor and expense of operating the dewatering equipment.

ZLD Success Factors

Relevant Experience of the ZLD Supplier. ZLD systems must be custom designed based on the waste water chemistry and flow of waste water to be treated. ZLD system design is the intellectual property of the system supplier and is generally not available from text books, journals or Wikipedia. A successful implementation of a ZLD system requires that the supplier can demonstrate relevant successful experience. Just as important in the supply of the equipment is the support that the supplier provides after the system is started up. The ZLD supplier should have a strong organization to provide such support and be able to demonstrate the same.

Waste Water Chemistry Design Basis. There are no “one size fits all solutions”. It is critical to establish a waste water chemistry design basis that is representative of the average conditions, as well as the minimum and more importantly the maximum conditions. Care should be taken to not be overly heavy handed in applying margin to design chemistries as such practice may not achieve the desired result. It is better to estimate the expected chemistry and discuss implications of deviations with the ZLD system supplier.

Metallurgy. Metallurgy plays a significant role in the capital cost of a ZLD system. Alloys that provide corrosion resistance to the highly concentrated brines are required. There are options available that allow for cost optimization without sacrificing plant service life.

Conservative Design Margin. The ZLD system is the “end of the pipe” in most plants; anything that has washed into a waste sump concentrates in the ZLD system. Experience shows that actual waste water chemistry will deviate from the design chemistry.

Once properly designed operating issues can be handled by the plant operating team working with a ZLD system supplier that has demonstrated experience in operating similar facilities.

Author:

Daniel Bjorklund is vice president of Aquatech International, a global leader in water purification technology for industrial and infrastructure markets.

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