Membrane performance
The water treatment installation was originally designed around “ultralow energy” seawater RO elements. However, after three years, these elements were replaced with Lewabrane® RO B400 HR elements since a performance projection (prepared with the LewaPlus® design software) showed that the required permeate quality could be achieved with standard test pressure BW elements operating at an even lower feed pressure.
After start-up, the predicted performance advantages of the Lewabrane® BWRO membranes were confirmed during routine daily operation. The RO feed pressure was throughout three years below 9.5bar (with the previous SWRO elements feed pressures were on average12 bar). The RO system salt rejection was approx. 98.6% (as compared to the 97.3% rejection achieved with the SWRO elements).
The new elements also provided an increased silica rejection (from 94% to 96.6%) resulting in an increase of the cycle time between regenerations for the mixed bed ion exchange system. After three years in operation (with monthly cleaning), the RO system rejection level is still in the order of 97.5%. Recently, the first stage RO elements were replaced with a Fouling Resistant Type (RO B400 FR) incorporating a 34mil feed spacer (recommended for wastewater application),which has resulted in a reduced cleaning frequency.
Textile Industry
The second industrial example is an innovative process in the textile industry. The dying process requires high concentrations of salt to fix the dyes onto the cellulose textile. At the end of the process, the wastewater stream consists of a high concentration of salts and organics. Discharging the untreated wastewater would not only be harmful for the environment, it may also impact existing drinking water sources. Since the textile industry is an important employer, the Indian government strongly supports projects with MLD processes.
The aim of this MLD process was to reuse the process water, and recovery of the salt to
• Save feed water costs
• Realize cost savings by recovery of the process salt
• Produce less waste
The key technologies to achieve a water quality, which could be reused in the process, were the application of a scavenger resin, a softening resin process, and a reverse osmosis process.
In this process, the wastewater is first treated with a biological treatment (tertiary treatment) to reduce the amounts of organics in the wastewater. This step is followed by quartz filtration. The process water is then passed through an ion exchange treatment stage consisting of a scavenger resin to remove the organic substances, followed by high TDS water softening. Since sodium sulfate is added during the process, calcium must be removed to avoid the precipitation of calcium sulfate.
After the ion exchange sequence, the water is passed through a reverse osmosis system to remove salts and other remaining organics. The RO system operates at a recovery rate of 80%. The RO brine reject is further treated by NF (nanofiltration) membranes to recover the sulfate, while the sodium chloride, which
passes through the NF membrane, is concentrated by a multiple effect evaporator. The sodium chloride salts are finally dried in a solar pan (as solid waste).
The concentrated sulfate from the NF process step is reused in the process.
The efficiency of the different treatment steps can be seen in the following table.
The MLD treatment process is extremely efficient, and this underlines the advantages of an ion exchange pretreatment for an RO process. The lifetime of the RO elements in this process is three to four years, with the added benefit of reduced COD content from the scavenger resin treatment step to reduce the potential of bio growth.
The above industrial examples show that the reduction of wastewater, even in challenging process industries, is possible using the right tools. Ion exchange and reverse osmosis is a smart and efficient combination to treat wastewater with a high organics load and high salinity.
Even with wastewater as feed solution a final water quality, that can be used for boiler feed water, can be achieved by the combination of an RO system followed by polishing ion exchange mix bed.
The use of scavenger resin as a pretreatment before RO should be considered, to remove organics from the wastewater that can pass through an ultrafiltration process and lead to potential biofouling in the RO process.
Finally, for MLD/ZLD processes, it must be noted that it is often challenging to achieve standards, but innovative combinations of existing technologies make it possible to reach the targeted goal.
KedarOke, Head–Marketing/Sales and Technical, Business Unit Liquid Purification Technologies, LANXESS India Private Limited
Dr. Jens Lipnizki, Head of Technical Marketing Membrane, Business Unit Liquid Purification Technologies, LANXESS Deutschland GmbH
