A Low-Cost Two-Stage UV + Activated-Charcoal Filter for E. coli Removal in Household Drinking Water

1.1 Project Abstract

Roughly 200 million people in India rely on drinking water from sources that intermittently exceed the WHO threshold for E. colicontamination. Commercial UV purifiers cost ₹8,000–₹15,000, placing them out of reach for most rural households. This project proposes, prototypes, and evaluates a two-stage filter — an activated-charcoal pre-filter followed by a low-power UV-C exposure chamber — built entirely from off-the-shelf plumbing fittings, a 6 W UV-C lamp, and a food-grade plastic housing. Target unit cost is under ₹1,200. Over a planned 30-day trial, the two-stage design will be compared against a UV-only control and a charcoal-only control, using membrane-filter culture on m-ColiBlue24 media to count colony-forming units (CFU/100 mL) before and after treatment.

1.2 Introduction & Objective

UV-C irradiation inactivates E. colireliably at doses above ~30 mJ/cm², but turbidity and dissolved organic matter shield bacteria from the light, degrading real-world performance in untreated village water. Activated charcoal is widely used to absorb organics and improve clarity but does not by itself deactivate pathogens. Prior work has explored each stage independently; few published studies quantify the marginal benefit of combining a cheap charcoal pre-filter with a low-wattage UV lamp under realistic Indian source-water conditions.

Primary objective: quantify the reduction in E. coliCFU/100 mL achieved by a two-stage charcoal + UV-C filter compared to UV-only and charcoal-only controls, when tested on artificially contaminated water at three turbidity levels (5, 20, and 50 NTU).

Secondary objectives: (i) measure lamp-hour energy cost per litre treated, (ii) estimate the per-unit build cost at household scale, and (iii) identify the failure mode most likely to cause a health-relevant miss (false negative) in field use.

1.3 Innovation

The novelty in this project is not the UV lamp or the charcoal cartridge — both are well-known — but the combination and the constraint envelope: total bill of materials under ₹1,200, no moving parts, no line power required (runs on a 12 V battery + small solar panel), and a flow rate that matches real household draw (~2 L/min). Existing published designs meet one or two of these constraints but not all four simultaneously. The project also introduces a simple visual clogging indicator— a transparent viewing port after the charcoal stage — so users can see when the pre-filter needs replacement without lab testing.

1.4 Methodology

Build

1. Assemble the charcoal stage: a 50 mm PVC housing packed with 200 g of coconut-shell activated carbon between two nylon mesh disks (25 µm).
2. Assemble the UV stage: a 300 mm stainless steel chamber with a 6 W UV-C lamp (254 nm) mounted axially in a quartz sleeve.
3. Connect stages in series with food-grade tubing; add a peristaltic pump to maintain a controlled 2 L/min flow.

Test protocol

• Prepare 30 L batches of dechlorinated tap water spiked with laboratory E. coliK-12 to a target of 10⁶ CFU/100 mL. Adjust turbidity with sterile kaolin to 5, 20, and 50 NTU.
• For each turbidity level, run 30 daily trials across three configurations: (a) two-stage, (b) UV-only, (c) charcoal-only.
• Sample 100 mL pre- and post-treatment. Filter through a 0.45 µm membrane and culture on m-ColiBlue24 at 35 °C for 24 h. Count blue colonies as E. coli.

Analysis

Compute log₁₀ reduction values (LRV) per trial. Compare configurations at each turbidity using a two-way ANOVA (factors: configuration, turbidity) with a significance threshold of α = 0.05. Report 95% confidence intervals for LRV.

1.5 Expected Results, Numerical Validation & Conclusions

Based on published dose–response data for UV-C and preliminary bench tests, the following results are expected:

If observed LRV values fall within these ranges, the two-stage design will meet the WHO “highly protective” threshold (≥4 log reduction) up to 20 NTU — the vast majority of real-world household source-water conditions.

1.6 Scope, Falsifiability & Limitations

In scope: laboratory testing against a single organism (E. coliK-12) under controlled turbidity, at a fixed 2 L/min flow rate, over 30 days.

Out of scope: virus and protozoan inactivation (which UV-C also addresses but is not tested here); real-world field deployment; long-term lamp aging beyond 30 days; regulatory certification.

Falsification criteria:the central hypothesis (“the two-stage design outperforms UV-only at ≥20 NTU by at least 1 log”) is falsified if the mean LRV difference is <1.0 or if the ANOVA interaction term is not statistically significant at α = 0.05.

Known limitations: (i) lab strains behave better than environmental strains; (ii) kaolin is a simplified turbidity proxy and does not fully represent dissolved organic carbon; (iii) the peristaltic pump is more expensive than a hand pump and will be replaced in the paper phase.

1.7 Acknowledgements & References

Thanks to the biology lab at my school for access to the autoclave and incubator, and to Dr. R. Iyer at the local municipal water testing lab for guidance on membrane-filter technique.

1. WHO (2017). Guidelines for Drinking-water Quality, 4th ed.
2. Hijnen, W. A. M., Beerendonk, E. F., & Medema, G. J. (2006). Inactivation credit of UV radiation for viruses, bacteria and protozoan (oo)cysts in water: A review. Water Research, 40(1), 3–22.
3. Bureau of Indian Standards. IS 10500:2012 — Drinking Water Specification.
4. UNICEF/WHO Joint Monitoring Programme (2023). Progress on household drinking water, sanitation and hygiene 2000–2022.