Material
Polyethersulfone (PES) polymer was 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) were purchased from S.D. Fine Chemicals Ltd., Mumbai, India. The non-woven polyester fabric was procured from Miki Tokushu Paper Mfg. Co. Ltd., Japan to be used as the support for casting the hydrophilized (HF) – UF (ultrafiltration) membranes. The nutrient broth and agar were supplied by Hi-Media Lab. Pvt. Ltd. Mumbai, India. The indigenous synthesized HF – UF membrane was spirally wound with assistances of Permionics Pvt. Ltd., Vadodara, India. Glass wear such as a conical flask, beakers, Petri dishes, measuring cylinders, were used to prepare the agar broth and analysis of the collected samples obtained from vasco scientific from Hyderabad, India. Equipment’s 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 were supplied by Secunderabad India. The hardware items such as aerator, pressure gauge, valves, pump, T – joint, telfon, cotton, tubing, parafilm, strips, for installation of MBR system which was supplied by SVS water solution for experimental setup, Hyderabad, India. Deionized water for water bath was prepared using the laboratory ultrapure cascaded reverse osmosis (RO) system.
Methods
Synthesis of hydrophilized (HF) – ultrafiltration (UF) 5 kDa membrane
HF – UF membrane was synthesized using immersion precipitation method by phase inversion technique. The blend polymer solution for casting was 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 the presence of 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) at room temperature (30 ± 2 ˚C) to obtain HF – UF membrane.
Effect of molecular weight of Polyethylene glycol (PEG)
PEG with molecular weights of 6,000 and 4,000 Da were dissolved in deionized water to prepare 1 L aqueous solutions to assess MWCO and rejection of solvents through synthesize HF – UF membrane. Rejection measurements were performed at a pressure of 3 bar using the PEG solution as the basis for feed. The concentration of feed, permeate and retentate solution was then determined via Refractive Index.
Sample collection
Rice gruel wastewater (RGW) used in the present work was obtained from the Akshaya Patra Foundation, Hyderabad, India. Initially, the sample was pretreatment with alcohol and HCl before subjected membrane filtration. The overall experimental manifold was provided in Fig. 1a.
Treatment of cold and hot RGW by spiral wound HF – UF 5 kDa membrane
The RGW which was at room temperature and hot (80 ° C) were passed though the spiral wound UF module with membrane area 1.2 m2 for 3 h and the feed permeate and retentate samples were used for physicochemical analysis, respectively.
Pretreatment of RGW with coagulants
Wastewater treatment (WWT) processes involves a series of physical, chemical and biological treatment techniques were further classified into pretreatment, primary and secondary treatment [23]. In the present study, the coagulation pretreatment for RGW domestic wastewater was pretreated for soluble removal of organic matter. The coagulation step was carried out by alcohols and acids to stabilize the starchy colloidal content, which eliminates moderate levels of total dissolved solids (TDS), chemical oxygen demand (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 1% HCl solution. The reaction mixture was stirred at 250 rpm for about 30 min and allowed for 2 h to sedimentation as shown in Fig.1b. The same experiments were repeated with methanol and ethanol as pre-treatment coagulants. Among that methanol, coagulant shows higher removal efficiency in suspended from RGW.
Experimental setup for spiral wound UF membrane
After the preliminary pretreatment with the methanol the supernatant liquid was passed through the spiral wound HF – UF membrane module membrane area 1.2 m2 at 3 bar pressure using 300 gpd (gallon per day) pump. The experiments were performed in a continuous mode, by measuring the permeate flux with respect 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.
Preparation of culture and nutrient agar
For the development of microbial consortia, it is necessary to prepare a culture media using a nutrient agar and broth. The agar medium was prepared by dissolving of 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 370C for one day.
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 one day at 370C. The culture was added to the feed tank and stayed overnight for further microbial growth followed by adaptation within the RGW environment.
Experimental setup and procedure for pretreatment coagulation and integrated aerobic membrane bioreactor (AMBR)
In this experimental study, after the preliminary pretreatment of RGW step with the chemicals (HCl, ethanol, and methanol) the supernatant was passed through the side-stream AMBR for to remove suspended solids, turbidity, color. Fig. 1d represents the process flow diagram AMBR where the feed tank capacity of 2.5 L was connected to the spiral UF membrane module with membrane area 1.2 m2 connected to 300 gpd pressure pumps. The module was arranged in a cross-flow manner and the permeate was collected into 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 was added to the pre-treated RGW wastewater present in the feed tank (2L). A micro-bubble diffuser was assembled in the feed tank for providing oxygen and controlling the aerobic conditions for the growth of biomass. The feed was fed to the membrane by maintaining the feed pressure of 3 bar throughout the experiments. The initially feed, permeate and retentate samples were collected for the analysis. The experiments were continued for 18 days in aerobic condition to remove the TDS, colour, conductivity and COD.
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 was carried out using a Shimadzu, Japan instrument for analysing the formation of new functional groups and intermolecular interactions after membrane formation.
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 that accumulate on surface of membrane and pores. Therefore, the membrane fouling was reduced by removing module from the system and clean at regular time intervals, after every batch experiment using chemical washing followed by water washing at low pressure. The chemical washing was conducted using 1 % citric acid, 1% sodium hydroxide (NaOH) + 0.5 % ethylene diamine tetraacetic acid (EDTA) + 0.1% of sodium lauryl sulfate (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 [24,25,26,27]. After chemical cleaning the membranes was stored in sodium metabisulphite (SMBS) (0.5 % w/v) aqueous solution to avoid further biological fouling and extend the life span of the membrane.
Analytical methods
Raw industrial wastewater, pretreated supernatant (after coagulation), permeate samples were analysed in relation to pH, colour, Conductivity (mS/cm), turbidity (FAU), TDS (mg/L) according to the standard procedure for wastewater analysis [28]. Sample pH was determined through a digital pH – meter (model DPH-504), at room temperature. Colour (Co- Pt), Turbidity (FAU) analysis was performed at (DR 800, HACH), TDS was determined using TDS meter with model HM TDS0-999, Hyderabad, India and Conductivity was measured using model DCM900 conductivity meter obtained from Global Electronics, Hyderabad, India.
Mathematical tools
Permeate flux
During the separation process, the permeate volume was determined by considering the effective membrane area and time, as shown in Eq. (1).
Where J is the permeate flux (L/m2.h), V is the collected volume of permeate (L) in time T (h) and A is the membrane area (m2).
Porosity
We calculated the overall porosity (Ɛ) of the membrane by gravimetric method, as defined in the following equation:
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.
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).
Where % rejection denotes as R, Cp and Cf are the concentration of the solute in permeate and feed (mg/L).
Chemical oxygen demand (COD)
The quantity of the pollutes present in the permeate was determined after wastewater treatment is known as COD. The presence of higher organic pollutes present in the water the higher value of COD. Hence, the COD can be calculated from Eq. (4).
Where B is the blank volume consumed in titration, S is the volume consumed in titration with sample preparation 8000 is the equivalent weight of oxygen per L, N is the Normality of standardized ferrous ammonium sulfate solution, D.F and V is known as dilution factor and the volume of the sample (ml).