Zero Liquid Discharge for Coal-Fired Power Plants

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When environmental regulations for coal-fired power plants change, effluent treatment methods currently being used may not be able to meet the new standards. Many power plant operators find that one of the new factors they must face is the EPA’s revised Effluent Limitations Guidelines (ELGs), issued in September 2015.

Recently at a mine-mouth coal-fired plant with a nominal capacity of 1,600 megawatts (MW), designers had done what they thought necessary to comply with regulatory expectations – they designed for zero discharge of process water. For blowdown water from the cooling tower, they made plans to discharge in a way that would meet National Pollutant Discharge Elimination System (NPDES) limits for the permitted outfall.

Process water is recycled internally for other plant processes, including the air quality control system (AQCS), which incorporates the flue gas desulfurization (FGD) system. However, as the wastewater continuously cycles through the two FGD absorber units, a buildup of chlorides and other constituents occurs. The level of total dissolved solids (TDS) in the purge water is controlled by blowdowns triggered by the TDS levels, and makeup water is then added to the system.

The FGD wastewater can contain TDS in excess of 31,000 parts per million (ppm), total hardness of 30,000 ppm, chlorides of 20,000 ppm, and total suspended solids (TSS) of 10,000 ppm. The blowdown wastewater is purged from the system whenever the chloride concentration in the water exceeds 20,000 ppm of chlorides.

The purged high chloride FGD wastewater is then disposed of by mixing it with fly ash and gypsum coal combustion residual (CCR) material in a pug mill. That mixture was originally disposed of at an off-site landfill, located about 20 miles away.

One seemingly small change caused this design to tip off-balance: the decision by the power plant to start its own landfill on site for disposal of the fly ash mixture. This action opened up the plant to the responsibility of managing the landfill leachate – water from precipitation flowing through the landfill, picking up contaminants along the way. The plant’s operators began pumping the leachate from the on-site landfill back to the plant’s recycle basin for re-use as make-up water. However, they found that internal recycling of wastewater is not sustainable at that location, as it results in the cycling up of chlorides and other factors that increase pipeline corrosion.

As a result, from 2014 to 2015, the chloride concentration in the recycle basin increased to three times the previous year’s concentration. The design for chlorides concentration for the recycle basin was set at less than 500 mg/L, but the data show levels approaching 3,000 mg/L within two years. These concentrations seemed likely to go on increasing unless measures were taken to manage the problem.

Problems such as these have been found at many coal-fired power plants. At the root of the problem is the fact that the water-management systems were set up to support the efficient combustion of coal to produce power. However, environmental regulations concerning water use and disposal have become more restrictive. This change has put an increasing operational focus on efficient water management to lessen the discharge of water from the plant and reduce materials of concern in that wastewater.

Thus environmental regulations, such as those intended to support zero liquid discharge (ZLD), now have an increased effect on operations, moving water management up on the priority lists of power plant operators.

One of the most common issues at many coal-fired power plants is the one seen in the story above – that, in many cases, the plant’s operators do not have a comprehensive plan for water use. They lack detailed, accurate data on which parts of the plant use water, how much those parts use, and what constituents the processes add to that water.

Many plants combine their wastewater inputs into a central flow and then treat the water that comes from that single pipe. In such cases, a more focused and cost-effective plan could be developed by segregating flows so that each stream receives only the level of treatment it needs. Segregation of wastewaters can generate substantial opportunities for recycling part of that water flow and limiting the most costly treatment and disposal methods to only the streams that need it.

For example, consider pump seal water. Many plants use clean water around the outside of the seals to reduce the possibility of the pumped fluid escaping. The pump seal water that drips out is gathered and then generally is just placed into the plant’s overall wastewater flow. Since this water is virtually clean, it would make more sense to capture this water separately so it can be treated at low cost, rather than being part of the larger, more complex wastewater flow.

Comprehensive analysis of the many water flows within a plant may be able to point to similar opportunities to segregate wastewater streams so that not all water needs to be treated with expensive methods. Specific data on water use at various points within the plant can help guide the choice of treatment approaches. At the plant described above, an astute review of the wastewater components saved the owner millions of dollars that would have been required for a new treatment plant. Instead, the plant managers were advised to use low-cost techniques for reducing the chlorides in their wastewater flow.

It is important to remember that as the plant’s operations change, the effects on the wastewater stream must be considered. The above-mentioned power plant was impacted by just such a change – the new landfill’s leachate forming a new source of chlorides to be managed.

Each coal-fired plant is different – the type of coal, equipment, and other factors such as local geology – so the following steps may be useful in finding an appropriate solution:

Analyze the current situation: One of the first steps for preparing for the new ELGs is to collect data on the flow and composition of wastewater streams and characterize typical wastewater flows.

Develop plans: Review various limiting strategies, such as reusing wastewater to reduce discharges, and then use mass balance and chemistry modeling tools to evaluate reuse, treatment, and discharge strategies to meet these new limits.

Choose management options: The choice for selecting the appropriate management tool depends on yet another wide range of factors that are better understood after carrying out the first two steps. The toolbox can include:

  • Discharge to a Publicly Owned Treatment Works (POTW)
  • Evaporation Ponds
  • Flue Gas Injection
  • Fixation
  • Deep Well Injection (depending on factors such as geology – experience has found that the tight rock formations of Pennsylvania, for example, are less useable for this purpose than the more appropriate geologic formations of other locations, such as Florida)
  • FGD wastewater treatment system (WWTS) Effluent Reuse/Recycle
  • Settling Ponds
  • Constructed Wetlands, Phytoremediation, and other Natural Based Systems
  • Vapor-Compression Evaporation
  • Physical/Chemical Treatment
  • Physical/Chemical with Added Biological Treatment
  • All the above can be components of a Zero Discharge approach

Other approaches utilities should consider include measures such as using existing evaporation (from cooling towers and FGD absorbers), using blowdown water for conditioning of fly ash, and other water reuse and conservation measures to reduce the amount of wastewater requiring treatment.
Working with a qualified professional with experience in each of these technologies can lead to wiser choices around which systems may be best, given the site-specific factors.

If you have any questions regarding your plant, please contact the post author, Ivan A. Cooper, P.E., BCEE, a principal based in CEC’s Charlotte, N.C., office, at icooper@cecinc.com; (704) 226-8074.