As electrodeionisation (EDI) technology becomes more widespread and accepted, new applications evolve. The application described here involves what is believed to be a new feedwater source for EDI. The application is a power plant water treatment system designed for zero liquid discharge (ZLD). The water treatment system includes an evaporator that processes cooling tower blowdown, mainly for wastewater concentration. High-quality condensate from the evaporator is further refined by EDI. This evaporator condensate presents new challenges in terms of feed composition and temperature range for the EDI process.
EDI replaces traditional on-site regenerated demineralisation with an ion exchange (IX) resin unit located after the evaporator. This represents a considerable improvement in operating and chemical cost and in the environmental liability associated with acid and caustic regeneration of the IX resin beds. The positive results of this new application hold promise for environmentally sound water treatment.
Guadalupe and ZLD
The Guadalupe power plant is located in Guadalupe County, Texas. It is fuelled by natural gas and is designed to deliver approximately 1000 MW of electric power.
Two identical power generation units are each composed of two combustion turbines, one steam turbine, and a cooling tower. The two cooling towers are supplied with softened raw water from two identical clarifiers. A common wastewater treatment and recovery system is integrated into the plant to supply high purity demineralised water for boiler feed, a small portion of which is used for cooling tower makeup. The power plant is a zero liquid discharge facility. ZLD has become a common approach to increasingly strict environmental wastewater discharge regulations. Evaporation followed by crystallisation reduces dissolved minerals to dry salts that can be easily disposed of in landfills.
Guadalupe water treatment system
The water treatment facilities consist of two distinct trains. The raw water supply system consists of two raw water clarifier type softeners. The wastewater treatment section consists of an evaporator, a crystalliser, and an EDI system followed by ion exchange polishing. The cooling towers receive makeup water from the softeners. As a result of the cooling process, the dissolved solids are concentrated before being discharged as blowdown. The cooling tower blow down (CTBD) is further concentrated by the evaporator and crystalliser. The recovered distillate from the evaporator/crystalliser is further processed in the EDI system to produce high purity water for the boilers. Excess distillate (condensate) becomes cooling tower makeup (CTMU).
Cooling tower operation sets key parameters such as flow rate and concentration that must be controlled by the water treatment systems to maintain zero liquid discharge status. In essence, the cooling tower evaporation rate determines the total volume of water that is supplied to the plant facility, and the chemistry of the raw water combined with the softening operation determines the volume of CTBD to be processed (by the evaporator). Other plant operations affect water treatment facilities but not to the extent that cooling tower operation does.
The raw water softener removes calcium from the CTMU in order to allow higher cycles of concentration to be achieved in the cooling water circuit. A small amount of magnesium and silica are also removed. The resulting CTBD is evaporated using a compressor driven seeded slurry evaporation process.
Evaporator operation
The treatment of cooling tower blowdown is accomplished in two evaporative steps. The first step evaporates the majority of the wastewater (greater than 97 per cent) to distillate in a falling film type evaporator. The distillate produced in the first evaporation stage is of high quality and is used as feed to the EDI system. Excess distillate is sent to the cooling towers as makeup. The evaporator utilises an energy efficient vapour compression cycle. The compressor energy source is electricity. The concentrated brine from the evaporator is evaporated to crystal cake in a calandria type crystalliser. Solids from this device are separated from the crystalliser brine slurry with a belt pressure filter for disposal off site as non-hazardous material. The crystallisation step is a slow evaporative process, necessary to initiate crystal growth. This process is heated with steam generated from a natural gas fired boiler system.
EDI operation
The EDI section of the water treatment system consists of a single skid containing controls and hydraulics. Connected to the skid are three EDI membrane stacks, each with a nominal product capacity of 50 gallons per minute. This 150 gpm unit is designed to operate continuously on demand. The EDI unit operation is initiated manually, but safety protocols are programmed to be automatic. Some critical feed specifications such as temperature and conductivity are monitored online to ensure that the EDI unit does not experience off-spec feed. High-purity EDI product is fed to a portable ion exchange resin bottle (regenerated off site), which acts as a final polishing step.
How EDI works
Electrodeionisation is a membrane-based separation process. An EDI system is composed of a stack of flow-directing spacers separated by anion and cation semi-permeable membranes. This stack of components is bounded on each end by electrodes, and the entire assembly is held together using tie rods, similar to a plate and frame structure.
EDI effects a separation of dissolved ionisable components of the feed water by means of a DC electrical field across the stack of components. Ion exchange resin in the diluting compartment assists in scavenging the dissolved salts from the continuous-flow feed stream. Under the influence of the applied DC field, these ionic salts then leave the ion exchange resin and pass through the membranes segregated by attraction to their respective oppositely charged electrodes. Once the strongly charged dissolved ions have been removed from the feed stream, only relatively pure water remains in the diluting compartment process stream, along with weakly-charged and non-ionised species.
Some of this water is then continuously dissociated by the DC electrical field into hydrogen (H+) and hydroxyl (OH-) ions. These positively or negatively charged species migrate to the respective oppositely charged cation or anion ion exchange resin beads. These sites then take part in ionising reactions of neutral or weakly charged ions, such as silica, ammonia, and carbon dioxide. For instance, when a neutral silica molecule contacts the high-pH micro-region of an anion ion exchange bead, it becomes a negatively charged silicate molecule. Since it has acquired a charge, it then falls under the influence of the DC field and is transferred through the anion membrane.
The EDI process removes and ionises dissolved minerals and gases on a continuous basis with continuous in-place ion exchange regeneration. Feed requirements for the EDI process include limits on divalent ions such as calcium and magnesium that can lead to scaling on the membranes. Other feed limits include total organic carbon (TOC), which can foul IX resin, and iron, which will irreversibly bond to IX membranes and resin. Temperature and pH also have prescribed ranges. See Table 1 for a partial list of general EDI feed requirements.
EDI performance
The Guadalupe EDI system, operated intermittently, is on-line for approximately 8-12 hours per day as determined by power plant and evaporator requirements. To date it has operated for close to 1000 hours.
The main concerns with operating an EDI unit using evaporator condensate as feed are:
• Temperature excursions (potential damage to stack components)
• Slugs of high conductivity from the evaporator (resin and membrane fouling, depolarising of ion exchange resin)
• High organic levels (potential fouling of EDI resin).
EDI product quality has consistently averaged 15-16 M?.cm, with a required specification of 10 M?.cm. Figure 4 shows (figures not available at present) EDI product resistivity plotted against time. Conductivity removal has also remained at 95 per cent or higher. Figure 5 shows (not available) EDI conductivity removal plotted against time. Feed conductivity to the EDI unit varied from 0.6 to 14 µS/cm. This variability in feed conductivity was a function of changes in evaporator operating conditions.
Evaporators can be subject to periodic events called “foam-overs” in which a slug of high-conductivity material mixes with the normal product stream. When such an event occurs, a programmed safety protocol (high conductivity alarm) shunts the feed to off-spec and the EDI unit shuts down until the feed quality is back within feed requirements.
Temperature is another factor challenging EDI operating limits at the Guadalupe site. The upper limit for feed temperature for EDI stacks is 40°C, so the design accommodates temperatures up to 45°C. The feed to this EDI unit typically runs in the 28-34°C range. As the Texas summer approaches, this ambient temperature will no doubt increase. If the EDI feed temperature exceeds the limit programmed into the plant logic, the EDI feed is diverted and the EDI unit is shut down until the feed temperature range is within specifications.
Table 2 shows per cent removal levels of different feed components by the EDI process. At this site high levels of ammonia (483 ppb) and TOC (1070 ppb) are present in the feed. (EDI has been used previously in power plant applications for high levels of ammonia removal from process water.) The average per cent removal values in Table 2 show key performance values of the EDI unit. Average conductivity to the EDI was just below 3 µS/cm. Since feed conductivity was fairly low, conductivity removal was approximately 98 per cent. The ammonia came from the original feed source. The 99.7 per cent ammonia removal represents the highest removal rate of any of the feed constituents. Sodium and chloride are normally removed by the EDI process in the 99 per cent or higher range, but the low removal levels observed are a function of the relatively low levels in the feed. The TOC removal of 56 per cent is typical of EDI TOC removal rates at other plants. The TOC compounds present in this evaporator condensate are typically small since higher molecular weight compounds would be left behind in the evaporator concentrate. This EDI unit operates at 580 V DC at 2-4 A, depending on the brine stream conductivity. EDI product flow is set at 160 gpm. In keeping with the zero liquid discharge goal of the power plant facility, EDI brine blowdown and electrode steam waste are recycled back to the softening clarifiers. This increases the EDI per cent recovery from the typical 95 per cent up to 100 per cent.
To date, the EDI system has demonstrated excellent performance on evaporator feed when faced with the challenges of high temperature and high TOC.
TablesTable 1. Recommended EDI feed requirements at 95 per cent recovery Table 2. EDI feed and product composition
