Application of forward osmosis for the reduction of pre-treatment sludge volume in desalination: modelling and experiments.
Creators: name: Liyanaarachchi, Susanthi
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400409
subject: Separation technologies
classificationCode: 400410
subject: Wastewater treatment processes
classificationCode: 401101
subject: Air pollution modelling and control

description: Forward osmosis (FO) is a novel water treatment process that potentially can be used as an alternative technology for both sludge and brine treatment due to its low energy requirement. In the FO process, a solution of high salt concentration (known as draw solution) is utilized to generate an osmotic pressure gradient across a semipermeable membrane to extract freshwater from a solution with lower salt concentration (known as feed solution). The FO process requires low energy to operate as it capitalizes on the phenomenon of natural osmosis. FO has been given significant attention over the past few years due its superior characteristics. However, this technology is still in the developmental stages. A few studies have been carried out using FO for the treatment of industrial wastewater, landfill leachate and food industry effluents. However, to date, there has been no research on sludge treatment and brine management using the FO process, other than this research, which could be another promising application of FO. Therefore, in this study FO was proposed as an additional process to the seawater reverse osmosis (RO) process to dilute the brine before it is discharged back to the ocean and to reduce the volume of pre-treatment sludge before mechanical dewatering. Diluting of brine have number of advantages depending on the industrial requirements such as (1) it can increase the brine diffusion rate as the concentration is low (2) it can keep the same diffusion rate however adverse effect to flora and fauna near the diffusers are low as the salt concentration is low (3) it can be sent back to the RO desalting process to increase the overall water recovery as the diluted brine is already pre-treated. Most of the current seawater desalination plants have two-stage reverse osmosis (RO) processes. <br><br>Therefore, the proposed FO systems utilize 1st stage RO concentrate (brine) as the draw solution (since osmotic pressure of brine is higher due to higher salinity) and pre-treatment sludge as feed solution (lower salt concentration). After passing through the FO system, as a consequence of water permeation from feed to draw solution, the pre-treatment sludge volume becomes lower and the brine gets diluted. Diluted brine can either be sent back to the 1st pass RO process to increase the overall water recovery or blended with the 2nd pass RO brine before being discharged to the ocean. By doing the latter, the diffusion rate of the brine within the water body can be increased. In this study, laboratory experiments to assess the viability of applying the FO process for an RO desalination system at different sludge conditions (pH, temperature) were conducted. Further, biofilm growth on the membrane surface up to 8 weeks of continuous filtration was analysed. In addition, mass balance calculations were used to predict the reduced sludge volume and power requirement arising from large scale (340 ML/day intake) and small scale (15 ML/day intake) hybrid FO/RO desalination plants. The electrical conductivity (EC) of the brine and seawater EC were 73.0 mS/cm and 44.5 mS/cm, respectively. As EC is directly proportional to osmotic pressure, there was a sufficient osmotic pressure difference between the draw and feed solutions to have adequate water flux through the FO system. However, the total organic carbon (TOC) of the brine and sludge were 3.10 mg/L and 8.92 mg/L, respectively. Therefore, there is a potential for biofilm growth on the membrane surface. When the pH of feed solution was increased from 6 to 8, there was a marginal change in water flux. Therefore, the as is pH (normally pH 8) of feed solution is recommended for the hybrid system. When the temperature increased from 20 to 40 oC, the average water flux slightly increased (5.6 to 6.0 LMH). However, considering the economic benefits, it is recommended to operate at room temperature. The water flux of continuous filtration experiments declined with time due to fouling as well as dilution of the draw solution.<br><br> However, flux increased when the draw and feed solutions were replaced with the fresh solutions. This increased flux was lower than the initial flux of the previous batch and was due to fouling on the membrane. After one week of filtration, the flux declined further due to the thickened fouling layer deposited on the membrane. The layer may have contained microorganisms in addition to salt deposits as both draw and feed solutions contained salt ions. However, scanning electron microscopy (SEM) spectrum showed salt deposits on the membrane surface after 5 week of continuous filtration without cleaning in between. This fouling can easily be overcome by providing regular flushes at high cross flow velocities as deposited layers are thin and loose and therefore readily removed. After 8 weeks of continuous filtration large salt deposits were observed. Further, after 8 weeks there was no water permeation through membrane. After analysing SEM EDX images and spectra, weekly membrane cleaning is recommended to avoid biofouling and inorganic fouling. After analysing water flux values and the fouling behaviour during FO filtration, mathematical modelling was carried out for the proposed RO/FO systems. Since, daily pre-treatment sludge generation varies (both in volume and solids content) with the desalination plant size, calculations were made for two plant sizes: large-scale plants (LSP, 340 ML / day intake) and small-scale plants (SSP, 15 ML / day intake). When the membrane area is 100 m2 (minimum area considered in this study) it can reduce sludge volume up to 7% in a SSP, however this depends on the water flux through FO. When the membrane area increases, sludge volume reduction increases in both large and small scale plants. The sludge solids content can be increased from 3 to 10% TS with a small power requirement (17.3 kW h /day). Interestingly, when the membrane area of a LSP is increased to 900 m2, the sludge volume is reduced by 50%. This yields a sludge stream having a final solids content of 7.6%. <br><br>Proposed system requires lower OPEX and CAPEX compared to existing system; however, they are marginal. With all the results obtained through laboratory scale experiments and mathematical modelling, it is evident that the proposed hybrid system is a promising technology to reduce the volume of pre-treatment sludge and increase the overall water recovery of RO process.