International Journal of Environmental Science and Technology
Recovery and water quality of plants were changed in cor- relation with stages and as well scale formation tendency (Bonné et al. 2000; Hoek et al. 2000). The effectivity of acid treatment in this work was benchmarked using Langelier Saturation Index (LSI) value which is widely used in an industrial setting for examining scale formation mostly cal- cium carbonate.
forward osmosis (Al Hawli et al. 2019), photocatalysis (Tran et al. 2020), nanofiltration (Shahriari and Hosseini 2020), capacitive deionization (Qin et al. 2019), and reverse osmo- sis. In reverse osmosis (RO) plant, water passes through a semi-permeable membrane from a higher concentration of salts to lower the concentration by applying force through the use of a high-pressure pump (Qasim et al. 2019). Product water is collected in the middle stream, and rejected water which has high salt concentration is collected in another stream. RO consumes relatively low energy as compared to other approaches (Park et al. 2020). Scaling and fouling in RO depend on the feed quality of water (Čuda et al. 2006). Collection of deposition of particles on the surface of membranes is called membrane fouling. Fouling causes a decline in flux flow of RO and results in membrane damage requiring replacement due to permanent adsorption over the membrane surface. Significant types of fouling are organic, inorganic, and biological. There are different techniques which are used to control fouling on the membrane surface, including pretreatment, water softening, coagulation, and flushing. Usage of soft water for the RO plant enhances plant recovery and resolves the problem of scaling and fouling. To control fouling, residual chlorine removal and disinfec- tion are frequently used. Oxygen removal from feed water can also reduce fouling in RO. The proper dosage rate of antiscalant and biocides help to control fouling and scaling. Till now, the mechanism of scaling and fouling has not been correlated with operating conditions like temperature, flow velocity, pH, and total dissolved solids (TDS) (Al-Ahmad and Aleem 1993). Precipitation of hard minerals such as deposition of CaSO 4 , CaCO 3 , and silica on the surface of membranes is known as scale formation. Several types of membranes have been developed to slow down the scaling such as spiral wound membranes offer slower scale formation than cross flow membranes (Lee and Lee 2005). In another development, Pramanik et al. (Pra- manik et al. 2017) used polyaspartic acid and its deriva- tive as an anti-scaling agent in lab-scale RO followed by examination of the used membrane using a scanning electron microscope (SEM) and x-ray diffraction XRD for determin- ing the type of fouling. Tong et al. (2020) studied the charac- teristic of fouling (e.g., biofouling, inorganic, organic) in a two-stage industrial RO system for reclamation of wastewa- ter. Yin et al. (2019) focused on the silica scaling and studied its relationship with membrane surface chemistry to enhance surface flux in a RO plant. In this paper, the membrane distillation process was examined, and fouling techniques were used for the distilla- tion process in a real industrial setting. Fouling mechanism was discussed by the usage of brackish water. Moreover, a three-stage filtration RO plant was used in which the trend of fouling and scaling was different from two-stage or single- stage RO plants operated at the same feed water quality.
Material and methods
Experiments were carried out on a three-stage RO plant (Fig. 1) installed at water treatment plant Cogen-2 in paper and Board mills (PBM) Limited, Pakistan. Water testing was performed in a PBM water lab, Pakistan. The RO plant which was chosen for experimentation has a capacity of 20 tonnes/h product flow and 6.6 tonnes/h flow discharge in the reject water stream. This RO plant has raw water, high-pressure pump, and pre-filtration including multimedia filter and cartridge filter of 5 and 1 microns, respectively. Feed, product, and reject flow meter was installed to measure the flow rate. Scaling and fouling have been controlled by reducing feed velocity. Another treatment suggested was the usage of caustic regime instead of lime for brackish water. The concentration of feed water was changed by changing the softening process. In this way, permeate flow remained constant over a long period (Amiri and Samiei 2007). Sulfuric acid (98%) and antiscalants were added at 0.28 and 3 ppm, respectively, to the RO feed to moderate the Langelier Saturation Index (LSI) value in the concentrate stream to < 2, and the plant was safely operated up to recov- ery of 85% without any BaSO 4 scale formation. Daily con- sumption of sulfuric acid and antiscalant was 10 and 2 kg per day. Sulfuric acid is preferred over HCl due to environmen- tal impact and less cost consumption. An organophosphate biopolymer antiscalant, HDC-ASI-ECO1, which is a biode- gradable antiscalant, was provided by Hatenboer-water Rot- terdam, Netherlands. Pressure gauges were installed on all stages, as shown in Fig. 1. LSI factor was used to calculate the tendency of scaling. This equilibrium model is derived from the theoretical concept of saturation, and it provides an indicator of the degree of saturation of water in CaCO 3 . The LSI was calculated using the equation provided by Antony et al. (2011). The product was collected in the middle stream and stored in a 400-tonnes capacity RO tank, while reject water was collected in the concentrated stream. Membranes were installed in the RO plant with the specifications provided in Table 1. Each stage had an indi- vidual sample point, as shown in Fig. 1. Water samples were collected from the feed of first, second, third stage, and total reject of RO. The temperature was calculated with a simple thermometer while for pH and TDS testing,
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