Process Water Reclaim
For high-volume users of process water in particular
Numerous industries generate steam at high temperature and pressure, recycling the condensate after the steam has done its work. As a precaution against corrosion and scaling it is essential that only water of very high purity is used in for boiler feed and reclaiming the condensate offsets the cost of producing extremely purified water.
A leading example is the electricity industry, which traditionally cools the exhaust from steam turbines, condensing it and returning the water for purification. This process links back to the boiler feed system, reducing the requirement for boiler make-up from an external water supply. Petroleum and gas extraction sites, refineries and paper mills also frequently employ condensate recovery systems.
Recycling waste water
Reclaimed process waste water may be reused in a variety of applications, such as boiler feed and cooling tower make-up water, or washing and rinsing operations. Depending on the contaminants introduced during the initial process, the waste water normally requires appropriate pre-treatment. This is certainly the case if it is to be recycled back to the process water purification sequence.
The waste water may undergo oil separation, metal precipitation, flocculation, sludge removal, ozonation, microfiltration – to name just a few potential treatments. However, with the right treatment regime it is possible to eliminate excessive waste water disposal, even achieving Zero Liquid Discharge (ZLD) levels.
One of the advantages of the ZLD approach is that an industrial operation can operate within very stringent effluent discharge limits. Simultaneously, the treatment system can often recover metals and chemicals for reuse in the production process. Normally, such a rigorous recovery process has two stages: Pre-concentration and evaporation.
Electro Dialysis and Electro Dialysis Reversal (ED/EDR) can be an effective in the pre-concentration stage, removing a high percentage of water from the waste stream.
Blowdown recovery with EDI
Ionpure hot water sanitizable (HWS) IElectro Deionization (EDI) modules have been successfully used to recover water from steam generator blowdown, as a green alternative to conventional ion exchange resin bed demineralizers.
The blowdown flow from power industry steam generators has a high temperature and pressure (potentially as high as 260°C and 65 Bar). It is always depressurized before further processing or disposal, and its temperature reduced, often by cooling tower condensate via a heat exchanger. Using condensate as coolant only reduces the blowdown flow temperature to around 55°C. However, as Ionpure HWS modules are sanitizable to temperatures above 85°C, they can be effectively applied to blowdown deionization at a temperature of 60°C continuous.
Recovery from RO purification
Desalination and purification of water for industrial process applications frequently employ a reverse osmosis stage. It is possible to reduce the overall waste water stream by recovering water from the RO reject.
The percentage of permeate separated from the feed water (RO recovery rate) normally ranges from 50-75%, so the remaining 25-50% of the RO feed water is discharged as wastewater.
So how is the overall recovery rate improved?
One option is to reuse the water rejected by a first stage RO system and introduce it as feed to a second RO stage. However the high salinity and Total Dissolved Solids (TDS) of the first stage RO reject / RO concentrate means that a further treatment is required to prevent excessive scaling in the secondary RO membrane.
With the right membrane filtration (MF) system between the RO-RO sequence it is possible to extend the recovery rate beyond 95%, even though the TDS concentration in the initial RO reject may be in excess of 45,000 mgl-1. Rather than moving to an entirely new desalination system, it can therefore be advantageous to simply add a second RO stage, with pre-filtration, as a retrofit.
The high TDS RO reject typically contains very high levels of dissolved ions, silica, organic compounds and hardness-causing calcium and magnesium salts. It is important to take the bulk of these out of solution before the reject stream is membrane filtered.
In a 2-stage RO system the membrane filtration step normally adds softener and flocculants, inducing solids to settle out of solution in a concentration vessel. In the case of a Zero Liquid Discharge (ZLD) design, the excess solids are sent to a filter press for de-watering and disposal. Further solids removed by the membrane filters are also returned to the concentration tank, while the MF filtrate goes to the second stage RO.
Drawbacks – Recovering water from RO reject / RO concentrate using this approach is very feasible but it has a high maintenance component. The system is continuously combating silt fouling, biofouling and scaling of membrane surfaces and necessitates adding chemicals as part of the process. A far simpler alternative involves Electro Dialysis:
Pure Water Group’s Electro Dialysis (ED) and Electro Dialysis Reversal (EDR) modules can produce a high recovery rate of reverse osmosis (RO) reject water, without membrane fouling and scaling problems. It was once cost-prohibitive to use Electro Dialysis in this role but the new technology follows a decade of research and development that has removed the problem. Today’s new generation Electro Dialysis modules have a cross-flow fluid path, high performance membranes and lower cost control technology. Modulating the applied DC power also allows tuning of the process to select a desired quantity of salt removal.
The advantage of EDR over ED is that a periodic reversal of electrode polarity produces the equivalent of backwashing of the Electro Dialysis membrane, which also reduces anti-scaling pre-treatment requirements. Our EDR modules have already been used to increase the overall RO recovery to 96%.
How Does Electro Dialysis Work?
Today’s advanced ED (and EDR) units are often described as ‘tunable’. In other words, the electrical energy applied can be changed, according to the amount of salt that needs to be driven out of the feed water. When the feed quality improves, the energy requirement reduces and that energy is saved. Consequently, the process is self-adjusting to fluctuations in the RO reject quality.
Experience, expertise and attention
We believe there are only a few companies in the world to rival what we do.
Thanks to our extensive experience in water treatment solutions for a wide range of applications, we developed essential insights into the EDI, MD and EDR technologies and how they meet specific requirements ‘in the field’. This is reflected in the quality of technical/commercial support, training, service and documentation we provide.
So why not contact us? Our advice is free!