Forward osmosis in reverse osmosis concentrate management

University of Technology Sydney. Faculty of Engineering and Information Technology.The production of fresh water and the disposal of wastewater are the major challenges of the last few decades. Reverse osmosis (RO) membrane plants are used extensively for brackish water desalination and industrial water purification. These plants operate at about 75% product water recovery so that about 25% of feed water is wasted as concentrated brine. The large quantities of concentrated brine generated has a disposal problem especially when the plants are located inland. Because of high disposal costs there is need to re-use and conserve water. RO reject concentrate (brine) is being increasingly processed to recover additional potable water. In order to achieve higher recoveries, therefore, alternate processes are used. Out of them forward osmosis is attractive. FO water desalination technique uses the natural osmotic pressure of the draw solute to drive osmosis rather than hydraulic pressure. Fertilizer drawn forward osmosis (FDFO) has been applied as a low cost water desalination option for agriculture purposes. This technique is further investigated by applying pressure on feed solution to enhance water permeate flux, which is called pressure assisted fertilizer drawn forward osmosis (PAFDO). PAFDO can enhance final dilution of the fertiliser draw solution beyond osmotic equilibrium concentration. In simple terms, this technique can be considered combination of FO and low pressure reverse osmosis RO. Due to the low cost desalination potential, the FO and PAFO processes have gained attention of the research community. Reverse osmosis concentrate (ROC) produced in water reclamation and desalination plants can endanger the environment if it is not treated before discharge. Volume minimisation of ROC can help in its easy disposal. The study examined the use of forward osmosis (FO and PAFO) with and without granular activated carbon (GAC) fixed bed adsorption pretreatment for volume minimisation of ROC and removal of organic micropollutants. In this study FO and PAFO were assessed in treating reverse osmosis concentrate using a low concentration of (KCl) as fertiliser DS. A low concentration of KCl (0.25 M) was chosen as DS and it was diluted to 0.14 M KCl during the FO operation due to transport of water permeate flux from feed solution. This diluted KCl solution can be used for direct fertigation, as the past studies showed successful use of 10 Kg/m³ (≈0.13 M KCl) for fertigation. Forward osmosis (FO) and nanofiltration (NF) membranes were tested to treat the ROC for possible water reuse. Due to very small pore sizes of FO membranes were used in nano-filtration mode to treat ROC from water reclamation plant. Commonly used NF membrane was good option for removing for organic compounds including micropollutants from wastewater however, most of inorganic compounds passed through the NF membrane. Since the FO membranes have pore size less than most of NF membranes, they also removed inorganic ions present in ROC. In this way the resultant permeate flux was able to be recycled back to RO unit to increase overall efficiency of the plant. Fouling and scaling is an important and inevitable phenomenon in FO membranes as well. Lower membrane fouling and/scaling implies more product water, less cleaning and longer membrane life, thereby reducing operational and capital costs. It was observed that scaling and fouling were not fully reversed in FO/PAFO by physical cleaning. However, the physical cleaning followed by chemical cleaning could almost fully restore the activity of the membrane. In this study, the membrane bioreactor (MBR) and granulated activated carbon were used as pretreatment methods to curtail organic fouling of the membrane. Both these pretreatment processes were proved to be successful to reduce total organic carbon of ROC including a majority of micropollutants. Moreover, inorganic carbon of ROC was reduced by acid pretreatment. These pretreatment processes resulted in high permeate water flux and less membrane fouling