Emissions

Effluent Limitation Guidelines

On Nov. 3, 2015 the Environmental Protection Agency (EPA) released a new set of Effluent Limitation Guidelines (ELGs) the first update to the Clean Water Act rules in over 30 years as they relate to Steam Electric Power Plants.

What They Mean and Your Options

By Derek Laan and Rob Broglio

ZLD Pilot Plant for FGD wastewater at the Young Heung power plant in South Korea. Photo courtesy: Doosan Heavy Industries & Construction

On Nov. 3, 2015 the Environmental Protection Agency (EPA) released a new set of Effluent Limitation Guidelines (ELGs) the first update to the Clean Water Act rules in over 30 years as they relate to Steam Electric Power Plants. The new rules are mainly a reflection of changes to water use in the power industry. Air quality controls, mainly FGD systems have taken pollutants previously in flue gas and transferred them to plant wastewater streams.

These new guidelines were designed to address this concern but they also have many operators looking for what the best solution for their plant’s situation is as there are a wide variety of options for treatment. While there are a variety of wastewater streams addressed by the ELGs, FGD wastewater is one of the main points of concern for the guidelines. This article will focus on summarizing the ELGs as they relate to FGD wastewater treatment and the available treatment options.

The EPA began a study of existing water at Steam Electric Power Plants in 2009 and the results were published in 2013. The study looked at pollutants being produced by power plants, what wastewater treatment technologies were being used at power plants already and what future technologies were in development. Based on these results the EPA designated two levels of technology which must be used by all Steam Electric Power Plants between 2018 and 2023 depending on when a plants NDPES permit is renewed. The EPA has several levels of technology based guidelines and the levels selected for the ELGs are on the more stringent side. For existing plants the Best Available Technology Economically Achievable (BAT) must be implemented while for new sources, meaning new power plants or FGD systems, must implement Best Available Demonstrated Control Standard / New Source Performance Standards (NSPS). These technologies were considered to be both necessary to prevent pollutants from entering local bodies of water and economically feasible to be implemented by producers.

FGD wastewater is considered the primary concern of the ELGs. This can be seen in the fact that FGD wastewater has the most stringent treatment requirements and if any other streams in the plant are added to the FGD wastewater stream these combined streams must meet the guidelines for FGD wastewater. As a result many plants which previously mixed their low volume wastewater streams together are currently developing plans for separating them to minimize the amount of water that must be treated as FGD wastewater.

One major change is that surface impoundment or ponds has been severely restricted by the new CCR rule. This means that most plants will be closing their impoundment/ash ponds. As stated earlier the guidelines are technology based but effluent water also has limitations set by the EPA these can be seen in the Table A. The BAT for FGD wastewater has been established as Chemical + Physical treatment followed by Biological treatment. What that means is that all existing Steam Electric Power Plants need to have systems which meet or exceed these requirements set up by the time they renew their NDPES permit between 2018-2023. Chemical & Physical processes employed will likely be similar across most plants depending on their permit requirements and constituencies of the wastewater.

The Chemical + Physical treatment processes are used to removed TSS & many of the other metals which may be dissolved in the FGD wastewater with the exception of Selenium. The process will vary depending on the qualities of the FGD wastewater but adjustment in the amount of chemicals or retention times can be made to ensure the desired water quality. The first step is usually suspended solids removal. This involves adding a polymer to the water and produces gypsum which can be sold commercially. The solids produced are removed with a clarifier and the water moves on to the next process. The next step is metal removal which involves adding liquid chelate aluminum, hydrochloric acid & sodium hydroxide to remove mercury & arsenic. These metals react to the reagents added to the water, fall out of solution and are removed with another clarifier step. The final physical process used in most cases will be hardness removal which is necessary to lower the TDS (Calcium). The biological processes used to for removing selenium can’t operate in high TDS conditions. Hydrochloric acid, sodium hydroxide & sodium carbonate are added to the water followed by a polymer. These cause the dissolved calcium and magnesium to precipitate out and allow their removal with a final clarifier step. When the EPA was considering the ELG rules Chemical + Physical treatment processes only were considered but due to their inability to remove selenium and nitrogen as N from the water biological treatment was also included.

There are several available technologies for biological treatment which remove of selenium. They follow the same basic process by which dissolved Selenate (Se+6) and Selenite (Se+4) are reduced to elemental Selenium (Se) via the growth of micro-organisms. This process can only take place in environments where oxygen is absent. As the selenium is reduced to its elemental form it comes out of solution and is removed along with the biological sludge produced in a filtration step. However this process is not an easy one to maintain and the market has struggled to come up with solutions that consistently perform under the harsh conditions of FGD wastewater.

The temperature and conditions for growth of the micro-organisms fall into a specific range which should be maintained by having effective chemical & physical treatment processes preceding it. The main problem stems from high TDS levels impeding the micro-organisms ability to grow at an appropriate rate which can increase retention times or impede the biological process from happening at all. For this reason not all FGD wastewater streams will be appropriate for these processes and may need to be treated using evaporators or other disposal methods such as mixing with ash for disposal in a landfill. However several companies have come up with unique technologies to use biological treatment to precipitate selenium out of solution and then physically remove it from the wastewater.

Water Treatment 1
Process flow of a basic Phys-Chem + Biological Treatment System which meets the BAT for existing sources of FGD wastewater.

One of these technologies is a fixed bed bioreactor. In this process the microorganisms grow on a fixed media and the wastewater flows with gravity over the media which serve as the surface area for the microorganisms to grow. Once the fixed media is completely covered with a thick layer of biofilm a backwash cycle is performed which removes this biofilm (sludge) and the elemental selenium along with it. These reactors have the advantage of being simple to operate and having the biological & filtration processes in the same step.

Other manufacturers attempt this process in a 2-step process. The first is again a bioreactor of which there are 2 main types: fluidized & moving bed. A fluidized bed bioreactor (FBR) works by having the media surface (typically sand) on which the microorganisms grow pressurized causing it to move around the bioreactor like a fluid. The biofilm sluffs off of the sand as it grows. A Moving Bed Bioreactor or MBBR works by having plastic media pieces to increase the total available surface area for the microorganisms’ growth. These media flow around the bioreactor and the biofilm sluffs off once it has become too thick. Once the selenium has all been reduced to its elemental form it must be physically separated from the water without going back into solution.

There are several different methods of filtration available to use after the biological step. A rapid clarifier is one approach that has been used. Normal clarifiers take a long time period but rapid clarifiers use higher amounts of polymers to cause the sludge to separate out more quickly. UF membranes have also been used by several companies. However UF membranes can become clogged preventing clean water from entering the membrane. This is fouling is avoided by bubbling air thru the membranes. However if oxygen is introduced to the water the selenium can reionize and come out of solution. Dissolved selenate and selenite are not blocked by UF sized membranes meaning selenium’s reionization must be avoided. However Doosan Heavy Industries & Construction has developed a unique MBR system which does not use air to scour the membranes. Doosan’s Low Energy No Aeration (LENA MBR) works by creating the scouring force of air bubbles with mechanical reciprocation. The membranes are mounted on a moving bed that reciprocates slowly back and forth, shaking foulants from the membrane. Replacing air scouring with mechanical movement ensures an oxygen free environment preventing the selenium from coming out of solution. Doosan’s LENA MBR has already been tested for selenium removal applications at mining sites and the company is working on testing it for FGD wastewater applications.

While Physical-Chemical + Biological Treatment is the BAT for existing sources of FGD wastewater new sources are required to use Zero Liquid Discharge (ZLD) systems. In addition to new sources the ELGs are offering the option of delaying their implementation until 2023 for plants who agree to install ZLD systems by that time. What ZLD means is that no water leaves the boundaries of the plant. In practicality this typically involves using brine concentrators and crystallizers to create a solid sludge for disposal at a landfill. There have been installations in power plants for this application since the mid 1970s. While ZLD is necessary for all new sources of FGD wastewater it may also be the best option for treating especially concentrated streams of wastewater for which other treatment options might not be feasible.

There are 3 main kinds of evaporators used in ZLD systems; Vertical Type Falling Film (VTFF), Forced Circulation (FC), & Spray Dry Evaporators (SDEs). VTFF’s work by having the FGD wastewater pass in a thin film over tubes which are heated by waste steam from the power plant. This creates a thickened brine stream and vapor which is condensed into a pure distillate for reuse in the plant. Typically after a VTFF process the thickened sludge is passed thru an FC evaporator which uses high pressure and steam to crystalize the stream followed by a centrifuge or other dewatering process to remove the final moisture. The thickened sludge from the VTFF can also be mixed with fly ash to create a thick cement like past for disposal at a landfill. Spray Dry Evaporators are used on their own and work by utilizing hot flue gas. FGD wastewater is sprayed over the flue gas which causes the water to flash evaporator and the solids to drop out. The vapor is then collected for reuse and the flue gas goes to the FGD system.

ZLD systems are the most stringent form of wastewater treatment but they are expensive both to purchase and can be difficult to operate. Scaling is the main operating problem in most evaporators due to the high salt concentrations in the FGD wastewater. This problem can be avoided by using seeded slurry techniques or thru softening of FGD wastewater prior to entering the evaporator. Cost is another important factor and in general a full ZLD system will be at least 1.5 times more expensive than the BAT option. One option to reduce the cost of these systems is to use an RO process before sending the water to the VTFF evaporator. This can significantly reduce the amount of water that needs to be processed by the evaporator reducing capital costs. Doosan Heavy Industries & Construction is currently constructing such a system at the Yong Dong Power Plant in South Korea and has been successfully piloting a similar system at another facility (see picture). While ZLD may be more expensive it may be the only option available to meet the requirements and most greenfield coal plants being constructed around the world are installing ZLD systems.

With the wide variety of options available for meeting the ELGs it’s important for each plant to understand what their needs are. Considerations like whether the plant might be shut down shortly after the final implementation of the rule in 2023 or other local discharge requirements will have mean similar plants might have different plans for meeting ELGs. The high cost of ZLD systems and the complicated nature of operating both biological selenium removal treatment and evaporators means it’s important for plant operators to begin evaluating their options early to ensure they are not caught off guard when they need to renew their NPDES permit.

Authors

Derek Laan is business development manager for the Water Business Development Team at Doosan Heavy Industries & Construction Co. Rob Broglio is senior sales manager at Doosan Power Services Americas.

 

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