Wastewater Treatment for Power Plants: Considering Zero Liquid Discharge
Water management is a high priority for power plant owners. There are many factors that can make it a particular concern, such as stricter environmental restrictions on wastewater discharge, regional water shortages, and the public perception of power plants. Implementing Zero Liquid Discharge (ZLD) can help solve these issues.The ZLD process eliminates discharge at the end of the wastewater treatment cycle and allows the processed water to be reclaimed and reused in a variety of advantageous ways.
While helpful in reusing water, the decision to implement ZLD is most likely driven by concern about difficult environmental discharge at the plant. It is often selected when water is full of difficult-to-dispose-of chemicals. It can also eliminate the need for discharge permits.
Zero liquid discharge is particularly relevant for the steam electric power industry, because coal-fired power plants have a large water demand and challenging water discharge. Large volumes of water are required for cooling towers, quenching bottom ash, and flue-gas scrubbing. Power plant wastewater can be high in heavy metals, due to the process of flue-gas desulfurization (FGD).
The power industry was recently confronted with the strictest guidelines for heavy metal discharge since 1982. The EPA’s current Effluent Guidelines were finalized in 2015, and they call for zero discharge of pollutants from the fly ash and bottom ash waste streams. They set the first federal limits on the levels of heavy metals in wastewater that can be discharged from power plants.
As power plants tried to meet more stringent federal and state air pollution requirements in the last few decades, FGD systems have been implemented to remove sulfur dioxide emissions. The wastewater from the FGD process contains high levels of heavy metals, such as arsenic, mercury, and selenium, as well as chloride, total dissolved solids (TSS), and nutrients – all harmful to the environment.
Settling ponds can remove suspended solids and metal particles from FGD wastewater. The ZLD process allows those metals to be concentrated in the salts and the wastewater to be distilled.
ZLD technology not only offers a solution to the environmental challenge of eliminating wastewater, but it also reduces the volume of water a power plant needs to draw from other sources – such as rivers, lakes, or aquifers – for continuing plant operations.
The strategy can help plants meet environmental stewardship goals and improve their perception in the community, relieving public concern about the plant’s impact on the affected water supply.
For example, a coal-fired power plant in West Central Florida modified its wastewater treatment technology in 2017 to add equipment that allows it to reuse its wastewater instead of discarding it, achieving zero liquid discharge and eliminating disposal into the local waterway.
Previously, the effluent would have been added to nearby Tampa Bay, where it would increase nitrogen levels that were potentially harmful to marine life. Instead, this plant collects the reclaimed water from Polk County and the cities of Lakeland and Mulberry to use for cooling water. It’s a win-win situation.
Globally, regulations for wastewater treatment are becoming more stringent, with the expectation that all regions will tighten governance in the years to come. As an example, to obtain approval for a new power plant in China, some companies are required to include ZLD water treatment technology.
How does it work?
A conventional ZLD system is a holistic approach to water and wastewater treatment. It generally includes an assessment of all sources of wastewater as a potential resource to the plant and values the water that can be reused at the end of the ZLD process. A ZLD approach often incorporates pretreatment with clarification, softening, filtration, reverse osmosis (RO), and associated thickening and dewatering.
Reverse osmosis removes wastewater dissolved solids by concentrating the majority of the stream through a series of membranes, creating a waste stream that is high in TDS but reduces the amount of overall wastewater requiring treatment. Using a reverse osmosis system helps reduce the size of the crystallizer. However, the RO concentrate normally requires pre-softening to avoid scaling in the membranes.
A two-step pretreatment process – cold lime softening and filtration – is advised. The lime softening reduces the high pH that comes from having elements in the water, such as calcium, magnesium, and silica, that may cause fouling or scaling of the reverse osmosis equipment.
- In the first step, a Solids CONTACT CLARIFIER™ can be used to remove the hardness from the water. The activity of the clarifier extends the life of downstream ultrafiltration membranes and reduces the amount of cleaning cycles. It combines mixing, internal solids recirculation, gentle flocculation, and gravity sedimentation in a single unit.
- Another pretreatment step is filtration. WesTech’s ultrafiltration membrane systems can be used to remove any remaining solid particles. The backwash water is recycled back to the clarifier.
Reverse osmosis is then used to remove the bulk of dissolved solids from the wastewater stream in the primary phases of concentration, if the initial salinity is not too high. WesTech has developed reverse osmosis systems over the last 10 years that have been used reliably by municipalities and industries. Reverse osmosis systems are designed for integration with other pretreatment process equipment.
An associated ZLD process step is sludge treatment. WesTech’s Solids CONTACT CLARIFIER can be used for this stage, where the underflow is pumped into a thickener, which increases the concentration of the sludge. While the overflow is sent back to the clarifier, the thickened underflow is sent to a filter press or pressure dewatering filter.
Evaporation plays a prominent role in ZLD because it eliminates the waste stream. This lowers disposal costs of the solidified waste. Though some ZLD systems continue to use thermal technology because it can handle a wider range of waste streams, today’s typical process engages membranes and traditional physical/chemical treatment options – pH adjustment, precipitation, clarification, and filtration – to remove trace metals and hardness.
A traditional ZLD system at a power generating station in Montana relied on WesTech equipment to reduce the concentrate and recover as much liquid as possible before the wastewater spray dryer that dewaters the last of the concentrated slurry.
Below is the process flow sheet for the Montana plant. (Click image to view PDF.)
In an effective ZLD system, all wastewater becomes a potential source of plant makeup water. Small flows from thickening and dewatering processes can add up to large net savings of water.
Membrane-based technologies reduce the cost and energy consumption of the wastewater treatment and make ZLD more feasible and sustainable for power plants. The brine crystallizers and evaporation ponds are not eliminated from ZLD processes with this technology, but RO reduces the brine feeding the crystallizers or evaporation ponds.
An Optimal Choice
The cost of ZLD technology can be partially offset by the sale of valuable byproducts, like calcium carbonate, critical metals and elements, that can result in a supplementary stream of income.
While there are many reasons to implement ZLD technology, the most significant is regulatory considerations for the discharge.
It’s true that ZLD is a highly technical process that requires more energy and capital investment than conventional wastewater treatment and disposal. However, concern about removing the FGD wastewater, looming fresh water shortages from accelerated growth of global water-intensive industries, cumbersome government discharge permits, drought, and climate change will prioritize the use of ZLD. In response to public opinion, decision makers will drive regulatory incentives that may make ZLD a fundamental choice for the future.