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 SOxand NOx 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.

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 SO4- 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.

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.

Source: Power Mag

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• Zero Discharge
• Zero Liquid Discharge plants
• Zero Liquid Discharge