2.1. Material
Polyethersulfone (PES) polymer purchased from Solvay, Vadodara, India. Polyvinyl alcohol (PVA), polyethylene glycol (PEG) of different molecular weights, ethanol, methanol, hydrochloric acid (HCl), glutaraldehyde (GA) and dimethylformamide solvent (DMF), sodium hydroxide (NaOH), ethylene diamine tetraacetic acid (EDTA), sodium lauryl sulfate (SLS), and sodium metabisulphite (SMBS) procured from SD. Fine Chemicals Ltd., Mumbai, India. The non-woven polyester fabric purchased from Miki Tokushu Paper Mfg. Co. Ltd., Japan, to be used as the support for casting the HF-UF membranes. Hi-Media Lab. Pvt. Ltd supplied the nutrient broth and agar, Mumbai, India. Indigenous HF-UF membrane synthesis spirally wound with Permionics Pvt. Ltd., Vadodara, India. Glass wear such as a conical flask, beakers, Petri dishes, and measuring cylinders used to prepare the agar broth and analyze sample collected from Vasco scientific, Hyderabad, India. Equipment such as autoclave (Equitron Medica Instrument, Mumbai, India), laminar flow chamber (Lab Tech, Mumbai, India), weighing machine (Sartorius, Hyderabad, India), Refractive Index (AntonPaar, Type: Abbemat 200, Mumbai, India) and incubator supplied by Secunderabad India. The hardware items such as aerator, pressure gauge, valves, pump, T – joint, telfon, cotton, tubing, parafilm, strips, and the MBR system supplied by SVS water solution for experimental setup, Hyderabad, India. Deionized water for water bath prepared using the laboratory ultrapure cascaded RO system.
2.2. Methods
2.2.1. Synthesis of HF-UF 5 kDa membrane
HF-UF membrane synthesized using the immersion precipitation method by phase inversion technique. The blend polymer solution for casting prepared by adding the 2 wt% of PVA and 23 wt% of PES and 0.5 mL of GA to the 74.5 mL of DMF solvent (wt/vol as per polymer weight) under continues stirring for 12-18 h at 50 ˚C. The mixture was kept stagnant at room temperature (30 ±2 ˚C) to remove excess bubbles in the polymer solution. The bubble-free solution was poured on the polyester non-woven fabric support fixed on a glass plate using the doctor’s blade for the desired thickness and immediately immersed in the non-solvent bath (pure water) (30 ± 2 ˚C) to obtain HF-UF 5 kDa membrane.
2.2.2. Effect of molecular weight of PEG
PEG with molecular weights of 6,000 and 4,000 Da dissolved in deionized water to prepare 1 L aqueous solutions to assess molecular weight cut off (MWCO) and rejection of solvents through synthesizing HF-UF membrane. Rejection measurements were performed at a 3 bar pressure using the PEG solution as the basis for feed. The concentration of feed, permeate, and retentate solution determined via Refractive Index.
2.3. Sample collection
RGW used in the present work obtained from the Akshaya Patra Foundation, Hyderabad, India. Initially, the sample pretreatment with alcohol and HCl before subjected membrane filtration. The overall experimental manifold provided in (Fig. 1a).
2.3.1. Treatment of cold and hot RGW by spiral-wound HF-UF 5 kDa membrane
RGW, which was at room temperature and warm temperature (80 ° C), passed through the spiral wound UF membrane with an area of 1.2 m2 for 3 h and the feed, permeate, and retentate sample used for the physicochemical study.
2.3.2. Pretreatment of RGW with coagulants
Wastewater treatment processes involve a series of physical, chemical, and biological treatment techniques that further classified into the pretreatment, primary and secondary treatment. In the present study, the coagulation pretreatment for RGW domestic wastewater pretreated for soluble removal of organic matter. The coagulation step was carried out by alcohol and acids to stabilize the starchy colloidal content, eliminating moderate levels of TDS, COD, pH, conductivity, and turbidity. During the pretreatment step, the hot raw RGW was allowed to cool at room temperature, and 6.5 L feed was collected in a container and treated with a 1% HCl solution. The reaction mixture was stirred at 250 rpm for about 30 min and allowed 2 h to sedimentation, as shown in (Fig.1b). The same experiments repeated with methanol and ethanol as pretreatment coagulants. Among that methanol, coagulant shows higher removal efficiency in suspended from RGW.
2.3.3. Experimental setup for spiral wound UF membrane
After the preliminary pretreatment with the methanol, the supernatant liquid passed through the spiral wound HF-UF membrane module membrane area 1.2 m2 at 3 bar pressure using 300 gpd (gallons per day) pumps. The experiments performed continuously by measuring the permeate flux to time, whereas the concentrate was recycled back to the feed tank. The UF process flow diagram was provided in (Fig.1c) the pretreatment method and UF experimental system.
2.4. Preparation of culture and nutrient agar
Microbial consortia developed to prepare a culture media using a nutrient agar and broth. The agar medium made by dissolving 28.0 g of agar in 1L distilled water and sterilized using autoclaved at a pressure of 15 lbs for 45 min by maintaining the temperature 121°C for sterilization. The agar medium was allowed to cool for 1 h at room temperature and poured into a petri dish under laminar airflow until it solidifies. The medium was subsequently streaked on the petri dish and kept in an incubator at 37 0C for 24 h
2.4.1. Preparation of nutrient broth
13.0 g of nutrient broth was uniformly mixed with 1L of distilled water and subjected to the autoclave for sterilization at 121°C using 15 lbs pressure for 30 min. The grown culture (50 mL) from nutrient agar was added to the nutrient broth and kept in the incubator for 24 hat 37 0C. The culture was added to the feed tank and allowed to stay overnight for further microbial growth, which aid bacteria to adapt with the RGW environment.
2.5. Experimental setup and procedure for pretreatment coagulation and integrated AMBR
In this experimental study, after the preliminary pretreatment of the RGW step with the chemicals (HCl, ethanol, and methanol), the supernatant was passed through the side-stream AMBR to remove suspended solids, turbidity, and color. Fig. 1d represents the process flow diagram AMBR, where the feed tank capacity of 2.5 L connected to the spiral UF membrane module with membrane area 1.2 m2 connected to 300 gpd pressure pumps. The module arranged in a cross-flow manner, and the permeate collected into the permeate tank, the retentate was fed back to the feed tank with the pressure gauge and control were fixed at the retentate line.
Initially, a mixed culture aerobic medium added to the pretreated RGW wastewater. A micro-bubble diffuser assembled in the feed tank to provide the oxygen and control the aerobic conditions for the growth of biomass. The feed fed to the membrane by maintaining the feed pressure of 3 bar throughout the experiments. Initially, feed, permeate, and retentate samples collected for the analysis. The operations were continued for 18 days in aerobic conditions to remove the TDS, color, conductivity, and COD.
2.6. Membrane characterization
Indigenous membranes were characterized by fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) analysis, and scanning electron microscope (SEM). For SEM studies, the membrane was cut into the liquid nitrogen and subjected to the SEM (JEOL JSM 5410, Japan) with a model number for 20 and 200 µm.
The X-ray pattern of the membrane was determined by XRD D 5000 (NJ, USA) to understand the crystalline nature of the polymer membrane. The X-ray diffract gram was obtained by the Bragg’s equation to determine 2 θ valves of the polymer.
FTIR spectroscopy carried out using a Shimadzu, Japan instrument for analyzing the formation of new functional groups and intermolecular interactions after membrane formation.
2.7. Membrane fouling
The membrane fouling caused by suspended particles, microbes, inorganic and organic components present in the feed that can accumulate, salts, and organic compounds present in the feed water collected on the surface of the membrane and pores. Therefore, the membrane fouling was reduced by removing the module from the system and cleans at regular time intervalsa fter every batch experiment using chemical washing followed by water washing at low pressure. The chemical washing conducted using1% citric acid, 1% NaOH+ 0.5% EDTA + 0.1% SLS for 30 min followed by water wash (30 min) after acid and alkaline wash. After chemical cleaning, the scalant have been completely removed from the surface of the membrane [13, 14]. After chemical cleaning, the membranes stored in SMBS (0.5% w/v) aqueous solution to avoid further biological fouling and extend the life span of the membrane.
2.8. Analytical methods
Raw industrial wastewater, pretreated supernatant (after coagulation), permeate samples were analyzed to pH, color, conductivity (mS cm-1), turbidity (FAU), TDS (mg L-1) according to the standard procedure for wastewater analysis. The sample pH determined through a digital pH-meter (model DPH-504) at room temperature. Color (Co-Pt), turbidity (FAU) analysis performed at (DR 800, HACH), TDS determined using TDS meter with model HM TDS 0-999, Hyderabad, India, and conductivity measured using model DCM900 conductivity meter obtained from Global Electronics, Hyderabad, India.
2.9. Mathematical tools
2.9.1. Permeate flux
During the separation process, the permeate volume determined by considering the active membrane area and time, as shown in Eq. (1).
See equation 1 in the supplementary files section.
Where J is the permeate flux (L m-2 h-1), V is the collected volume of permeate (L) in time T (h), and A is the membrane area (m2).
2.9.2. Porosity
We calculated the overall porosity (Ɛ) of the membrane by gravimetric method, as defined in the following equation Eq. (2):
See equation 2 in the supplementary files section.
Where m1, m2 is the weight of the wet and dry membrane respectively, whereas s1 is the surface area, δ is the cross-section thickness, and ρ is the density of the demineralized water.
2.9.3. Rejection efficiency
Rejection is another factor in which the membrane separation performance was evaluated by considering the turbidity, COD, and PEG rejection in the permeate by Eq. (3).
See equation 3 in the supplementary files section.
The percentage rejection denotes as R, Cp, and Cf are the solute concentration in permeate and feed (mg L-1).
2.9.4. COD
The quantity of the pollutes present in the permeate was determined after wastewater treatment is known as COD. The presence of higher organic contaminates presents in the water the higher value of COD. Hence, the COD can calculate from Eq. (4).
See equation 4 in the supplementary files section.
B, S is the volume consumed with blank and sample preparation 8000 being the equivalent weight of oxygen per L; N is the normality of standardized ferrous ammonium sulfate solution; D.F and V specified as dilution factor and sample volume (mL).