By J. Michael Marlett
During the Obama Administration, new guidelines were promulgated for the removal of contaminants from FGD wastewater. The contaminants addressed were selenium, arsenic, mercury and nitrates. Now, with a new administration, these limits, how to achieve the limits and the time frame for implementation of processes to achieve these limits have been put into question and uncertainty reigns within the industry. Companies who had defined a course of action are now rethinking and delaying those decisions.
The ELG’s for FGD wastewater list two primary methods of treatment. One is Biological to remove Se, Hg, As and nitrates but does not address total dissolved solids or recover water for reuse within the plant. The second method is Zero Liquid Discharge. This method also addresses Se, As, Hg, and nitrates but also removes total dissolved solids and recovers water for reuse in the process.
The Biological Treatment process is perceived as the low-cost option to FGD treatment and the Zero Liquid Discharge process is perceived to be more expensive from a CAPEX and OPEX standpoint. While this is true, the difference is not as much as has been presented within the market by various engineering firms. In addition, there are options for achieving ZLD while reducing the costs associated with this alternative.
The new Effluent Limitation Guidelines gave a delayed implementation date for those plants implementing Zero Liquid Discharge. That date was 2021 while the implementation of the Biological Treatment option was to be 2018. Many companies have been able to get relief from that date due to the testing that is required to properly design and define the operating conditions and efficacy of the Biological process. Since the ZLD process relies on physical chemical treatment and evaporation, both which are well known and well proven, testing is not required for the ZLD process.
THE BIOLOGICAL PROCESS
The biological process consists of several steps. These steps consist of Feed Pretreatment, Biological Treatment and Post Filtration. These are the major steps and can vary both in methods and conditions depending on the supplier of the process.
The Pretreatment phase consists of equalization and metals removal. This is where hardness, arsenic and mercury are removed. It can consist of pH adjustment, clarification and ion exchange. The pretreatment also conditions the feed for the biological process by adjusting hardness and suspended solids. Then the wastewater is fed to the bioreactor. That can be fixed film, membrane or another type of biological process. It normally requires a carbon source as “food” for the biomass. This is typically methanol, acetic acid or some proprietary carbon source. After reaction, the effluent is filtered to remove the biomass which contains the residual metals and the elemental selenium.
THE ZERO LIQUID DISCHARGE PROCESS
There are several options available to achieve Zero Liquid Discharge. All of them require some type of thermal evaporation step. The list below is not all of the processes available but are ones that have been applied and are proven.
• Phys/Chem Treatment/RO/Crystallizer
• Phys/Chem Treatment/RO/FO/Crystallizer
• Spray Dryer Evaporator
• Phys/Chem Treatment/Evaporation/Solids Separation
• Phys/Chem Treatment/Evaporation/Solidification
This article will be limited to the last two ZLD thermal processes.
Due to the high hardness and abundance of suspended solids in the FGD wastewater, with the exception of the Spray Dryer Evaporator, a physical/chemical treatment is required for the further treatment of the FGD wastewater whether the downstream treatment process is membrane or thermal. This physical /chemical treatment can be as simple as clarification with a solids contact clarifier to as complex as 2 stage complete softening to remove the hardness and suspended solids or somewhere in between. Removal of suspended solids is required to prevent downstream pluggage of the ZLD process equipment. Clarification only would be required for evaporation with solidification. Partial softening would be required for Evaporation with Solids Removal. Full softening would be required for any process employing Reverse Osmosis as a treatment step but could also be employed with Evaporation with Solids Removal.
The thermal processes can be divided up into two categories, small flow and large flow. For small flows, ≤30 GPM, the thermal equipment would consist of a Forced Circulation Crystallizer followed by the solids separation unit, either a filter press or centrifuge. For larger flows >60 GPM, the thermal equipment would be a Brine Concentrator followed by a Forced Circulation Evaporator and solids separation unit or possibly a Spray Dryer if the flow was small enough.
FGD WASTEWATER PRODUCTION
The volume of FGD wastewater produced is a function of the size of the power plant, MW produced, coal burnt and scrubber design. FGD blowdown is controlled to maintain the chlorides of the wastewater below the maximum concentration allowed by the scrubber supplier. This is a function of the materials used in the scrubber. Decreased allowable chloride contents result in increased FGD blowdown volumes. The composition of the coal used in the production of power also has an impact on the
amount of FGD wastewater produced. High sulfur content and high chloride content will increase the amount of FGD wastewater. The high chlorides require more blowdown to maintain chloride levels in the scrubber and sulfur content will affect the amount of gypsum produced and the amount of FGD blowdown.
Based on the factors above, expected FGD blowdown, when limiting chloride content of FGD wastewater to 10,000 PPM, for various coals and plant capacities would be as shown below:
Sub-bituminous coal, aka PRB coal, and bituminous coal makes up 95% of the coal burnt in power plants as reported in 2016 by the US Energy Information Administration.
COST COMPARISON OF BIOLOGICAL WWT TO ZERO LIQUID DISCHARGE
Studies by both CH2M (now Jacobs) and Burns & McDonnell resulted reports comparing the cost of a ZLD FGD Wastewater Treatment plant to a Biological FGD
Wastewater plant. The results of their study is shown below.
Data from “Review of Available Technologies of Removal of Selenium from Water” CH2MHILL for North American Metal Council, June 2010
Industry presentations by the Engineering Firm Burns & McDonnell have postulated that the cost of Biological Treatment plants and ZLD plants are closely competitive up to 200 GPM where the above graph from CH2M indicates the installed cost of ZLD to be three times that of Biological treatment.
The equipment costs and estimated installation costs of the ZLD system generated by Aquatech paint a much more positive depiction of the relative installed costs of ZLD compared to Biological treatment.
The graph below has the estimated costs of a ZLD thermal system imposed on the graph above. It indicates the estimates by Aquatech of the cost of thermal equipment only and the installed cost of the thermal equipment. While much lower than the CH2M estimate, it still represents a 40-percent premium compared to Biological treatment.
This can be expected for several reasons. These mostly result in a difference in the materials of construction of the various pieces of equipment, details of the equipment construction, the types of equipment provided and the level of redundancy expected.
Biological Treatment systems are specified as 1×100% where ZLD systems are typically specified as 2×60% or 3×50% capacity systems.
Historical data from actual installations of ZLD plants for treating FGD wastewater have shown the shape of the curves for both the installed costs and the equipment only costs are correct and properly represent the costs that we could expect at the time of manufacture.
The costs represented by the CH2M curves are overly inflated when compared to our costing structure and estimates. The Aquatech costs include the full pretreatment and full ZLD. These costs can be significantly impacted if full ZLD is achieved not by using a Forced Circulation Crystallizer and solids removal device but by taking the blowdown from the Brine Concentrator and using it to wet the ash prior to disposal in an approved landfill. Pretreatment would be simplified to a Solids Contact Clarifier and the Forced Circulation Crystallizer and Solids Separation Equipment would be eliminated at the expense of a pug mill and mixing equipment.
OPERATING COST OF ZLD SYSTEMS
There is another component of the cost of a ZLD system and that is the operational cost of the equipment. The values in the tables below are for a plant that is generating 750 MW, is burning bituminous coal and generating 200 GPM of FGD wastewater. They demonstrate how savings can be achieved with the choice of one ZLD method over another.
With full ZLD where a solid is generated for disposal in a landfill, the costs of disposal would be $32.82/1000 gallons.
This is a significant cost for treatment but there are methods that result in significant savings to both operating costs and capital costs. This would be going to evaporation to reduce the volume of the wastewater to manageable levels and then either using it for dust suppression or mixing it with fly ash and disposing of it in an approved landfill. Tests in our laboratory have indicated that a fly ash mixture of no less than 20% FGD wastewater to fly ash mixture would still pass the EPA paint filter test. This process option would eliminate steam and the softening chemicals reducing the OPEX to $4.37/1000 gallons from $32.82/1000 gallons.
THE CASE FOR ZLD
The studies by CH2M (Jacobs) indicated that the capital cost for ZLD when compared to Biological Treatment is extraordinarily high. Burns & McDonnell agrees that ZLD is higher but can be competitive at low flows (<200 GPM). Costing studies by Aquatech supports the Burns & McDonnell position that ZLD plants can be cost competitive in these ranges.
The operating costs of ZLD plants can be excessively high depending on the chemistry of the feed, flow and method of operation. If the plants are run at full ZLD generating a solid for disposal, the cost of operation can be as high as $32.82/1000 gallons. However, operating the plants in a mode where the discharge is reduced to match the available fly ash for mixing and disposal, a treatment facility designed to take 200 GPM feed and reduce the volume, operation savings of $2.8 million can be achieved. In addition, the capital cost can be reduced by 40-50 percent, making it very competitive with Biological treatment.
The ZLD option also has positive attributes that cannot be met with Biological Treatment.
Zero Liquid Discharge is a viable option for treatment of FGD wastewater both from an OPEX and CAPEX standpoint. While ZLD can be more costly than Biological Treatment, process arrangements are available to make it more cost effective. It is also the only process that provides water for reuse in the plant and guarantees compliance with any liquid discharge regulation that may be passed in the future since it has no liquid discharge that can be regulated.