Laboratory Evaluation of Biofilm and Mineral Deposition on Concrete Surfaces

Concrete storage tanks are marketed as virtually maintenance-free. A 30-day controlled study says otherwise. Download the free white paper to see what raw concrete surfaces actually do in potable water service, and what protective coatings change.

Why Read This: Uncoated Concrete Grew Six Times More Bacteria Than Coated Concrete in 30 Days

Concrete is everywhere in potable water infrastructure: clearwells, reservoirs, ground storage tanks, pipe. And the overwhelming majority of it is placed in service uncoated.

The assumption behind that practice is that concrete needs no protective treatment for potable water contact. But biological fouling in storage creates real compliance exposure: loss of disinfectant residual, increased disinfection by-product formation, elevated nitrification risk, and coliform occurrence as biofilms mature.

This study set out to test the assumption directly. Researchers at Water Systems Engineering, Inc. hypothesized that the porosity and unevenness of raw concrete would favor bacterial attachment and biofilm growth, then built a laboratory system to measure it.

The result is data you can take into a capital planning meeting: side-by-side biological activity and mineral deposition numbers for coated and uncoated concrete, on the four surface finishes most commonly specified in this industry.

What’s Inside

1. A regulatory and standards primer. How ANSI/NSF Standard 61 and the NSF Concrete Site Mix Design Evaluation Program apply to concrete in potable water contact, plus where AWWA D110, D115, and ACI 301/350 fit.
2. Full test methodology. A 200-gallon municipal-water test system with drain-and-fill cycling to mimic operational tank turnover; 4″ concrete coupons in cast, wooden float, light broom, and heavy broom finishes; a sacrificial carbon steel coupon as a controlled iron source; and Pseudomonas aeruginosa introduced as the challenge organism.
3. ATP as the measurement tool. Why culture-independent ATP quantification was used instead of heterotrophic plate counts, and how surface swab results were converted to cell population estimates.
4. 30 days of comparative data. 78 water and swab samples, charted across six sampling intervals, comparing raw concrete against two NSF/ANSI 61 certified epoxy coatings.
5. Iron deposition results. Mineral accumulation on each surface type, benchmarked against iron concentrations in the bulk tank water.

Key Findings

• Attachment starts immediately. Within 24 hours, populations on uncoated concrete rose to roughly 300,000 cells/mL while coated surfaces held at or below 30,000. That is a tenfold difference in the first sampling interval.
• The gap held for 30 days. At Day 29, uncoated coupons averaged 11.1 million cells/mL. Coated coupons averaged 2.1 million and 1.4 million, depending on the coating.
• Roughly six times more bacteria overall on raw concrete than on coated concrete across the study period.
• Roughly five times more iron deposition on uncoated coupons. Iron concentrations on those surfaces essentially matched the concentration in the tank water, indicating minerals deposit readily out of solution when nothing interrupts attachment.
• Coatings did not prevent biofilm entirely. Bacteria did initiate biofilm on coated coupons in later intervals. The finding is inhibition and delay, not elimination.

Who This is For

• Utility operations and water quality managers investigating residual loss, DBP formation, or nitrification in storage
• Engineers specifying new concrete storage or rehabilitating existing structures
• Asset managers weighing coating capital cost against recurring cleaning, disinfection, and offline time
• Anyone who has been told a concrete tank is maintenance-free

How to Use These Findings

1. Download the white paper and review the methodology section before citing the numbers. The study used bacterial and iron loading above what finished water typically carries, deliberately, to compress a fouling timeline into 30 days.
2. Compare against your own storage data. If you are tracking residual decay or DBP formation at a concrete facility, the mechanisms described here are a reasonable place to start looking.
3. Consider ATP monitoring. Biofilm quantification is not a regulatory requirement, which is precisely why it rarely appears in routine monitoring, and why fouling tends to be discovered late.
4. Factor coating into lifecycle cost, not just construction cost. Physical removal and chemical cleaning are usually deployed after fouling is already a problem, when they are most expensive and require taking the structure out of service.