Water-Stressed Regions Provide Proving Grounds for Advanced ZLD Systems

Zero-liquid discharge (ZLD) water treatment is already required in some parts of the world, and the experience gained in those areas with new technologies may help power generators in other regions evaluate their options as regulations or resource issues make ZLD increasingly important.

In water-stressed regions outside the U.S., power producers and other industrial water users are incorporating higher levels of water reuse, some to the point of zero liquid discharge (ZLD), due to heightened regulatory pressures and for economic reasons. In China, new power plant and chemical plant project approvals require inclusion of ZLD water treatment technology as a result of directives in the Chinese government's 12th Five Year Plan. In India, based on the successful implementation of ZLD requirements on industrial plants in the southern state of Tamil Nadu, other regions of the country are introducing water reuse and ZLD requirements for power producers and refineries. In both countries, state-led technical agencies have taken on the role of guiding companies toward best available technologies to solve wastewater problems and to develop water reuse and ZLD capabilities.

The steps taken by China and India provide examples for U.S. industrial water users, power producers in particular, to evaluate the newest methods of high-recovery water treatment as they plan their paths to compliance with more stringent water regulations.

China's Focus on Industrial ZLD

As China builds out infrastructure to further exploit its massive coal reserves, it is incorporating technologies to curb the increase of toxic gas and particulate emissions via SO_x and NO_x abatement, and is mandating water reuse and ZLD processes for new and existing plants. New and existing coal-fired power plants, together with coal-to-chemicals refining facilities, are targets for these ZLD mandates, and in both sectors the rate of new plant construction is staggering. A World Resources Institute report in 2013 detailed China's plan to construct 363 new coal-fired power plants, adding more than 550 GW of capacity to the existing 758 GW of coal power. Additionally, the Financial Times in 2014 reported on 18 coal-to-chemical projects under construction in China, with a total of 54 in the pipeline for possible construction.

China's water treatment design institutes, which are consultants and technology decision-makers on the wastewater process technologies for these projects, have sought out best-in-class technologies for reducing the energy consumption and cost of wastewater pretreatment, brine concentration, and final crystallization of salts for disposal or resale. They have been early adopters of advanced technologies such as forward osmosis (FO) for high-recovery treatment of waste streams at lower capital cost and energy consumption than traditional, evaporative technologies. In fact, the treatment trains most commonly recommended by the design institutes for adoption in new ZLD projects have used either multi-effect evaporation for the brine concentration step or FO for the brine concentration step, as shown in Figure 1.

1. Common process flows for Chinese coal power and coal-to-chemical ZLD projects.Courtesy: Oasys Water

In coal power and coal-to-chemical plants in China the typical wastewater streams treated to ZLD are flue gas desulfurization (FGD) wet scrubber blowdown or coal syngas and other process wastewaters, which both contain high SO_4- levels in the presence of high hardness. These complex waters are also subject to changes in makeup on a regular basis, depending upon the coal composition and upstream process parameter changes. See the example raw water composition range in Table 1.

Table 1. Example raw water characteristics for a coal-to-chemical wastewater stream.Source: Oasys Water

Therefore, the ZLD process flows for these waters include conservative softening pretreatment designs and redundant brine concentration systems to protect against scaling in the high-recovery parts of the process as parameters change, and to provide a high level of turndown as either flow or total dissolved solids (TDS) levels shift over time.

Membrane-based brine concentrators can be designed as multiple trains, with each train having a turndown ratio of 30%, in order to dampen changes in wastewater composition and flow, to provide more stable water characteristics for downstream crystallizer systems. Evaporator brine concentrators can be designed to operate in a seeded slurry mode, in which calcium sulfate is able to circulate through the evaporator at higher concentrations than normally allowed, preferentially precipitating on seed crystals in the brine, reducing the potential for scaling on heat transfer surfaces.

Operating a system in this way requires a high level of operator attention and skill. In well-designed membrane or thermal systems, these waters can be concentrated from 35,000 mg/L of TDS to 220,000 mg/L, achieving greater than 80% recovery. Sulfate solubility is usually the limiting factor.

Boiler feedwater plant regeneration waste and cooling tower blowdown are two additional areas where high-recovery treatment technologies and ZLD are being applied in the Chinese power and chemical industries.

The feedwater treatment plant reject and regeneration streams are typically treated as a combined waste stream, with similar pretreatment, brine concentration, and crystallization steps as described above. Boiler feedwater plant waste streams are typically lower in TDS (10,000 to 20,000 mg/L) than coal power FGD blowdown-based waste streams, but they do contain a similar combination of sulfate, hardness, chloride, and sodium. In addition to hardness, iron, barium, and other metals must be accounted for in the pretreatment of these waters. With proper pretreatment, these streams can be concentrated to 220,000 to 250,000 mg/L of TDS and greater than 90% recovery prior to the crystallization step.

Cooling tower blowdown streams can be treated individually but have also been combined with other wastewater streams, especially in coal power or coal-to-chemical plants.

Overall, the size, water complexity, and number of projects in China present an exciting growth opportunity for advanced wastewater technologies and will serve as a valuable set of examples for companies in the U.S. and other parts of the world tackling similar wastewater challenges.

India Uses ZLD for Water Pollution Mitigation

India is second only to China in the number of new coal power plant projects anticipated, with nearly 520 GW of new capacity planned, according to Global Water Intelligence (GWI). But India's focus on industrial water treatment goes beyond the need to offset the growth in water use and wastewater discharge resulting from a coal power boom.

Strict industrial water regulation already had been implemented to mitigate serious river pollution issues and to address water scarcity in many regions of the country. For the industrial sector, the Indian government has promoted ZLD and near-ZLD as wastewater mandates for the largest wastewater generators, which a 2013 International Water Management Institute report identified as the thermal power, steel, and petrochemical industries. There are additional water treatment, reuse, and ZLD mandates for manufacturing companies, from chemical and pharmaceuticals to food, beverage and textile processors, which discharge waste streams into river systems in northern and southern India.

What began in the arid and drought-prone southern Indian state of Tamil Nadu in the mid-2000s to address water stresses between agricultural and industrial interests has spread to other regions of the country and to larger industrial sectors. The streams requiring treatment have tended to be lower volumes, in the 10 to 30 gpm range, for hundreds of small manufacturers and across a wide range of industries.

The ZLD process flows have included reverse osmosis (RO) plus small brine concentrating evaporators, feeding either a last-stage crystallizer or discharging to a solar pond. Evaporator design and performance issues in the first generation of ZLD plants, developed in 2005-09, have opened the door for newer technologies such as high-recovery RO and FO systems feeding a new breed of thin-film, precipitating crystallizers. Example process flows are shown in Figure 2.

2. Typical process flows for Indian ZLD applications. Courtesy: Oasys Water

The trend for companies in the larger industrial wastewater markets such as petroleum, power, and steel is to move to higher levels of water recovery in their operations. Additional water recovery from RO reject streams and treatment to higher recovery for cooling tower blowdown are two examples. As a result of this trend toward higher levels of industrial water reuse, together with general industrial sector growth in India, GWI predicts a doubling of demand in India for industrial wastewater treatment spending, from $1 billion in 2013 to $2 billion in 2020. For the power industry in particular, growth will be higher-from approximately $150 million in 2013 to almost $500 million in 2020.

North America Wastewater Volume Reduction and Water Reuse

Until recent years, North American companies operating in water-intensive industries such as oil and gas extraction, thermal power production, and chemicals manufacturing entered into ZLD and high-recovery water treatment projects only when absolutely required. Mickley and Associates reports from 2003 and 2008, analyzing municipal RO reject ZLD projects in the U.S. Southwest, have provided baseline data on capital and energy costs for pretreatment, brine concentration, and crystallization processes comprising the traditional ZLD process flow. Operators and engineering firms have had good benchmarks for evaluating the costs of evaporative high-recovery and ZLD versus alternatives such as deep well injection of saline waste streams or construction of evaporation ponds, when basic treatment and discharge are not an option.

As newer technologies such as FO, other membrane-based technologies, or electrochemical technologies have proven themselves in other worldwide markets-and as water scarcity and water stewardship have become more important issues-the application space of industrial water recovery projects has grown. More plants in a wider range of industrial sectors are actively investigating advanced wastewater projects.

Industry needs such as minimization of discharge of saline effluents to evaporation ponds or treatment plants from cooling towers, process waste streams, or oil and gas produced waters, by recovering reusable water, are being considered in many more cases than just five years ago. Using FO, evaporation, or other processes to recover an additional 60% to 70% of freshwater from a saline cooling tower blowdown source becomes more viable as treatment costs diminish with new technologies.

Power and industrial users of recirculating cooling systems are increasingly exploring ways to recover and reuse cooling tower blowdown to reduce makeup water requirements and to reduce discharge volumes. Cooling water reuse is on the roadmaps of companies as diverse as the large petrochemicals producers, including Shell and BP, to large data center operators such as Google and Amazon. FO and other technologies that can economically process complex wastewater streams such as cooling tower blowdown and recover additional quantities of freshwater are in a position to provide significant value for new and existing industrial plants.

ZLD and high-recovery treatment processes in the U.S. power industry have been employed in recent projects to help streamline the path to project approval and to help more easily navigate the permitting process. Panda Power Funds, for example, recently built combined cycle plants in Temple and Sherman, Texas, and incorporated ZLD in the Sherman plant and high-recovery brine concentration in the Temple plant for cooling water blowdown and boiler feedwater plant waste streams. (See the September 2015 issue of POWER, where these plants are profiled as gas-fired Top Plant Award winners.) In both cases, the designs reduced the volume of intake water required to run the plants as well as the wastewater discharged. For one of its newly announced projects, in Leesburg, Va., Panda Power Funds will use Leesburg municipal wastewater as a portion of the intake and will use ZLD waste treatment to eliminate outflows, benefiting the Chesapeake Bay watershed in both cases.

The progress in high-recovery, saline wastewater treatment and ZLD being made in other parts of the globe, to improve the cost and reliability of treating complex industrial streams, provides new options for consideration by U.S. power and other industrial water treaters. As federal and local regulations tighten and as forward-thinking companies such as Panda Power Funds look to incorporate ZLD processes into new plant designs to meet the requirements of more stringent water regulations and tougher permitting processes, the technologies being adopted in other markets provide new options to be considered in order to optimize cost and performance. These innovative, high-recovery technologies can help to broaden the adoption of ZLD throughout the power industry and other water-intensive industries throughout the U.S. and the Americas.

- John Tracy (jtracy@oasyswater.com) is director of marketing for Oasys Water.

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