Introduction
The scarcity of potable water has emerged as a global challenge, with around 25% of the world population lacking access to safe drinking water. Industrialization and urbanization have further aggravated water pollution, necessitating clean water for both domestic and industrial purposes. Consequently, water-related mortality has surged, emphasizing the urgency of innovative water purification technologies.
Reverse osmosis (RO) is a pressure-driven membrane process that effectively removes dissolved impurities. However, its performance is limited by membrane fouling, scaling, and deterioration. Ultrafiltration (UF), with pore sizes ranging from 0.01–0.1 µm, offers a promising alternative for pretreatment due to its high efficiency in removing macromolecules, microbes, and suspended particles, with minimal chemical usage. Integrating UF with RO provides a dual-barrier system that enhances water quality while optimizing operational parameters like pressure, energy, and membrane life.
Materials and Methods
Sample Collection
Water samples were systematically collected from various residential and industrial zones across Chennai, Tamil Nadu, to ensure a wide range of water quality for evaluation. The samples were obtained in pre-cleaned, sterilized polyethylene bottles and transported to the laboratory under refrigerated conditions to prevent any physical, chemical, or biological changes during transit. Each sample was categorized based on its Total Dissolved Solids (TDS) concentration into three classes: low (Sample 1), moderate (Sample 2), and high salinity (Sample 3).
Sample Preservation and Storage
The samples intended for chemical analysis were preserved using standard methods as prescribed by APHA (American Public Health Association). Acidification with nitric acid to pH < 2 was carried out for metal analysis, and samples were stored at 4°C for microbial and COD analyses.
Physicochemical Analysis
Comprehensive physicochemical characterization of the collected samples was performed using standard analytical protocols outlined by the Bureau of Indian Standards (BIS) and the American Public Health Association (APHA).
1. Total Suspended Solids (TSS):
Determined gravimetrically using the following procedure:
- A known volume (25 mL) of sample was filtered through pre-weighed Whatman filter paper placed in a porcelain crucible.
- The filter with retained solids was dried in an oven at 103°C for 24 hours.
- After cooling in a desiccator, the crucible was weighed again to determine the net increase due to suspended solids.
- TSS (mg/L) = (W2 – W1) × 1000 / Volume of Sample (mL)
2. Total Dissolved Solids (TDS):
- 50 mL of the sample was taken in a clean evaporating dish and heated to dryness at 180°C until a constant weight was obtained.
- The residue weight was used to calculate TDS:
TDS (mg/L) = (Final weight – Initial weight) × 1000 / Sample volume
3. pH Measurement:
- The pH was measured using a calibrated digital pH meter (Eutech Instruments, Model pH700) using standard buffer solutions of pH 4, 7, and 9.
- The probe was rinsed with deionized water before and after every measurement to ensure accuracy.
4. Conductivity and Salinity:
- Electrical conductivity was measured with a conductivity meter (Elico CM-180).
- Salinity was calculated based on the conductivity reading using the device’s calibration for NaCl equivalents.
5. Turbidity:
- Turbidity was measured using a calibrated nephelometric turbidity unit (NTU) meter.
Chemical Analysis
Quantitative analysis of key ions and elements in the water samples was conducted using the following protocols:
- Total Hardness and Calcium Hardness (as CaCO₃): Determined by EDTA titrimetric method using Eriochrome Black T as indicator.
- Chlorides (Cl⁻): Measured by Argentometric titration using potassium chromate as indicator and silver nitrate as titrant.
- Sulfates (SO₄²⁻): Determined using turbidimetric method employing a spectrophotometer at 420 nm.
- Total Iron (Fe): Quantified using atomic absorption spectrophotometry (AAS).
- Calcium and Magnesium: Determined separately via complexometric titration.
- Manganese and Zinc: Measured using AAS for trace-level accuracy.
- Chemical Oxygen Demand (COD): Analyzed by open reflux method using potassium dichromate and ferroin indicator.
Treatment System Configuration and Operating Parameters
A pilot-scale integrated water purification unit was set up, combining UF, RO, and UV modules in a sequential flow system. The configuration included:
- Sediment Filtration Stage – Removal of particulate matter using a 5-micron pre-filter.
- Ultrafiltration Module – A flat-sheet PVDF (polyvinylidene fluoride) membrane with pore size of 0.1 µm, operating under crossflow mode at 1.5–2 bar pressure. The module had an effective surface area of 6 m².
- Reverse Osmosis Module – Thin-film composite polyamide RO membrane with a pore size of 0.0001 µm operating at 30–40 psi.
- UV Disinfection Chamber – Equipped with an 11-watt mercury vapor lamp emitting UV-C rays (254 nm wavelength) for microbial disinfection.
Process Flow and Monitoring
- The feedwater was sequentially passed through the filtration units and collected post-treatment in a storage tank.
- Key process variables including flow rate, pressure, and flux were monitored continuously using pressure gauges, rotameters, and digital flow meters.
- Permeate and retentate were collected at regular intervals to measure recovery rate and membrane performance.
Performance Evaluation Metrics
- Permeate Volume (mL): Measured using a graduated cylinder at 1-minute intervals.
- Permeate Flux (L/m²·h): Calculated using membrane area and time using the formula:
J = V / (A × t)
where J = flux, V = permeate volume, A = membrane area, t = time. - Removal Efficiency (%): Calculated for each parameter using:
% Removal = [(C_in – C_out) / C_in] × 100
Results
Table 1: Physical Analysis of Raw Water Samples
| Parameter | Sample 1 | Sample 2 | Sample 3 |
|---|---|---|---|
| TDS (ppm) | 500 | 1000 | 1500 |
| TSS | 3 | 4 | 8 |
| pH | 6.81 | 6.71 | 7.23 |
| Conductivity (µS/cm) | 1250 | 1460 | 1700 |
| Salinity (ppm) | 450 | 730 | 810 |
| Turbidity (NTU) | 1 | 1 | 1 |
| Odour | NIL | NIL | NIL |
Table 2: Chemical Analysis of Raw Water Samples
| Parameter | Sample 1 | Sample 2 | Sample 3 |
|---|---|---|---|
| Total Hardness (CaCO₃) | 320 | 421 | 541 |
| Calcium Hardness | 251 | 327 | 421 |
| Chlorides (Cl⁻) | 250 | 451 | 1100 |
| Sulfates (SO₄²⁻) | 280 | 320 | 540 |
| Total Iron (Fe) | 0.31 | 0.42 | 0.48 |
| Calcium (Ca) | 104 | 202 | 284 |
| Magnesium (Mg) | 21 | 45 | 49 |
| Manganese | 0.1 | 0.12 | 0.15 |
| Zinc | 1.0 | 1.0 | 1.0 |
| COD | 3 | 2.4 | 2.1 |
Table 3: Integrated UF and RO System Specifications
| Specification | UF | RO |
|---|---|---|
| Material | Hollow Fiber | PTFE |
| Flow Rate | 500 L/h | 500 L/h |
| Automation | Automatic | Automatic |
| Pore Size | 0.1 µm | 0.0001 µm |
Table 4: Permeate Volume & Flux Over Time
| Time (min) | Volume (mL) | Flux (×10⁻³ L/m²h) |
|---|---|---|
| 1 | 65–82 | 0.8–0.82 |
| 3 | 200–215 | 0.74–0.75 |
| 6 | 425–490 | 0.72 |
| 9 | 624–710 | 0.71 |
Table 5: Post-Treatment Results (UF and UF+RO+UV)
| Parameter | After UF Only | After UF+RO+UV |
|---|---|---|
| TDS (ppm) | 400–1260 | 82–92 |
| Total Hardness | 250–320 | 122–136 |
| Sulfates (SO₄²⁻) | 180–240 | 10–13 |
| Iron (Fe) | 0.10–0.13 | 0.1 |
| Manganese | 0.09–0.15 | 0.01–0.03 |
| COD | 2.1–2.8 | 1.0 |
Discussion
UF membranes effectively remove particulates and microorganisms but are limited in removing ionic contaminants. RO compensates for this by rejecting dissolved salts and heavy metals. The combination improves purification performance, reduces chemical use, and extends membrane lifespan. Trends indicate that increasing TDS correlates with higher conductivity and salinity, and pretreatment via UF significantly enhances RO performance.
Conclusion
The integrated UF-RO system demonstrated superior performance in removing suspended solids, dissolved ions, and microorganisms, thereby producing high-quality potable water. UF serves as an effective pretreatment step, reducing membrane fouling and operational costs of RO. This combined process is energy-efficient, sustainable, and suitable for direct drinking water applications.
Acknowledgement
The authors acknowledge the Faculty of Chemistry Department for the valuable support and guidance.
References
- Ardita Berisha, Leonard Krasniqi, Nora Islami, & Flamur Zeka (2016). A comprehensive study published in the Oriental Journal of Chemistry, 32, 2391–2400.
- Priya Menon & Rajeev Sharma (2015). Research findings reported in Current Science, 109(7), 1247–1254.
- Priya Menon & Rajeev Sharma (2015). Analysis and engineering insights featured in the International Journal of Engineering Science and Technology, 7(8), 267–278.
- Priya Menon & Rajeev Sharma (2014). Chemical technology exploration presented in the International Journal of ChemTech Research, 6(5), 2628–2638.
- Priya Menon & Rajeev Sharma (2015). Further developments in chemical research published in the International Journal of ChemTech Research, 8(11), 211–220.
- Sandeep Patel & Rajeev Sharma (2019). Proceedings presented at an IEEE conference, IEEE Publications, 1, 275–281.
- Sandeep Patel & Rajeev Sharma (2020). A scholarly article featured in Studia Rosenthaliana: Journal for the Study of Research, 12(2), 66–71.
- Sandeep Patel & Rajeev Sharma (2020). Architectural and technological research published in the Journal of Xi’an University of Architecture & Technology, 12(3), 5411–5417.
- Sandeep Patel & Rajeev Sharma (2020). Technical advancements detailed in the Journal of Xidian University, 14(4), 696–701.
- Sandeep Patel & Rajeev Sharma (2020). Innovations in science and technology explored in the International Journal of Advanced Science and Technology, 29, 4450–4454.