Iran's water and wastewater treatment plants — from municipal drinking-water plants to municipal and industrial wastewater treatment plants — rank among the most corrosive industrial environments for metal structures. The simultaneous combination of hydrogen sulfide gas (H2S) generated by the biological breakdown of wastewater, saturated humidity and constant vapor above the basins, disinfectants such as chlorine and ozone, and microbiologically induced corrosion (MIC) caused by acid-producing bacteria in the wastewater environment sharply shortens the service life of metal grating, handrails, and walkways over aeration, clarifier, and digester basins. The experience of treatment plant operators in Iran shows that galvanized steel structures in a wastewater environment typically suffer severe corrosion and require replacement within just a few years. Nabak Composite's FRP and GRP profiles, with full chemical resistance, electrical insulation, and light weight, offer a proven answer for every section of the plant — from the headworks and screening area to the sludge line, disinfection, and drinking-water treatment.

1Why the Treatment Plant Environment Is Harsh on Metal

2Six Key Engineering Properties for Treatment Plants

Selecting FRP for a water or wastewater treatment plant depends on six complementary properties. First, and the property most specific to this industry, is resistance to H2S gas and microbially induced corrosion (MIC). Hydrogen sulfide gas generated by the anaerobic breakdown of wastewater converts, in the presence of moisture, into biogenic sulfuric acid, which rapidly degrades galvanized steel and even concrete in channel headspaces and digesters; FRP vinyl ester resin resists this biogenic acid compound, and its surface provides no substrate for bacterial biofilm growth. Second, general chemical resistance to chlorine, ozone, and disinfection chemicals. Third, complete electrical insulation for safety in a damp, vapor-saturated environment. Fourth, a high strength-to-weight ratio for installation on existing concrete tanks and basins without adding structural load. Fifth, UV resistance for exposed structures in Iran's sunny regions. And sixth, long-term durability under the plant's continuous, round-the-clock operating cycle.

A Complete FRP Solution for Treatment Plants — Grating, Handrails, and Composite Structures

Resin selection (vinyl ester for direct exposure to raw wastewater and H2S gas, or isophthalic polyester for general walkway and handrail use in drinking-water units) is recommended by Nabak Composite's technical department based on the exact installation location.

3FRP Profile Applications in the Liquid Wastewater Treatment Train

The liquid wastewater treatment train, from the plant's inlet to the final effluent outlet, comprises several key units, each creating a specific application for FRP. Headworks & Screening, the first unit to receive raw wastewater and see the highest H2S gas concentration, requires grating and handrails that are both chemically resistant and slip-resistant. The aeration basin, with saturated humidity generated by air diffusion, is a site of extensive FRP walkway and bridge-crossing applications. Primary and secondary clarifiers, typically with circular structures and rotating bridges, require lightweight, corrosion-resistant grating. Transfer and overflow channels also use FRP covers to prevent H2S gas from escaping into the surrounding environment.

FRP Platform Walkway over Aeration and Clarifier Basins — Overview of the Liquid Treatment Train

4FRP Profile Applications in the Sludge and Digester Unit

The sludge and biosolids unit is the most corrosive section of a wastewater treatment plant. The anaerobic digester, where H2S gas concentrates in the tank headspace, requires a platform and handrails resistant to the highest concentrations of biogenic sulfuric acid. The dewatering unit — including belt presses, centrifuges, and filter presses — with its high humidity and corrosive polymer chemicals, has made FRP grating and handrails the standard choice. The sludge cake storage and handling platform also benefits from composite covers and grating due to its direct, continuous contact with acidic sludge.

FRP Walkway Alongside Sludge and Treated Water Pipelines — Biosolids Unit

Specific Applications in the Sludge and Digester Unit

5FRP Profile Applications in Disinfection and Chemical Dosing

The chlorination, ozonation, and chemical dosing rooms differ fundamentally from other sections of the plant in terms of corrosivity: here, the primary hazard is not H2S gas but direct vapor and leakage of chlorine, sodium hypochlorite, ozone, and coagulant polymers — substances that corrode galvanized steel and even ordinary stainless steel within a short time. FRP grating, handrails, and chemical tank support structures, built with a chlorine-resistant resin selection, offer a durable answer for this acidic, oxidizing environment. The coagulant and polyelectrolyte dosing pump room and the acid and base storage tank for pH adjustment are other applications in this unit.

6FRP Profile Applications in Drinking-Water Treatment Plants

Unlike wastewater treatment, where the main challenge is biological and H2S-driven corrosion, drinking-water treatment plants face a different requirement: safety of direct contact with potable water. Grating over sand and activated-carbon filters, handrails around clearwells and treated-water storage tanks, and access platforms for the final chlorination unit must comply with safety standards for materials in contact with drinking water (such as NSF/ANSI 61) — a requirement that FRP profiles, with the appropriate resin, satisfy. Open storage tank cover structures are also an expanding application, preventing secondary contamination of treated water.

Key Difference in Technical Approach: Wastewater Treatment vs. Drinking-Water Treatment

In wastewater treatment, resin selection is prioritized based on resistance to H2S and biogenic acid; in drinking-water treatment, the priority is focused on the safety and inertness of the material in contact with potable water. Nabak Composite's technical department applies these two approaches separately for each project to ensure both technical performance and health requirements are met.

7In-Depth Technical Analysis: The Mechanism of H2S and Microbial Corrosion (MIC)

From a treatment plant corrosion engineering perspective, structural degradation occurs in three steps. First, sulfate-reducing bacteria (SRB) in the anaerobic wastewater or sludge environment reduce available sulfate and produce hydrogen sulfide gas (H2S). Second, this gas, in the headspace of a channel, digester, or basin, in the presence of moisture and sulfur-oxidizing bacteria on the metal or concrete surface, converts into biogenic sulfuric acid with a pH sometimes below 1; an acid that can completely destroy galvanized steel within months to a few years and even cause severe corrosion (crown corrosion) of reinforced concrete in the headspace of sewer channels. Third, direct microbially induced corrosion (MIC) occurs on surfaces submerged in wastewater or sludge, where bacterial biofilm attaches directly to the metal surface and drives localized electrochemical reactions.

FRP profiles resist all three mechanisms: vinyl ester resin remains stable against low-pH biogenic sulfuric acid, and the composite's smooth, non-porous surface — unlike rough concrete and steel — provides no suitable substrate for bacterial biofilm attachment and growth; as a result, both chemical corrosion and microbial corrosion are controlled simultaneously.

8Focus on Iranian Treatment Plants: Applications and Retrofit Logistics

Iran's major treatment plants each have their own climatic conditions and pollutant loads that must be considered when selecting and installing FRP profiles. Wastewater treatment plants in metropolises such as Tehran and Mashhad, with high urban pollutant loads and some aging lines, prioritize rapid retrofitting of sludge and digester lines without long operational shutdowns. Isfahan and Shiraz, with intense sun exposure through most of the year, require high UV resistance for exposed structures. Industrial wastewater treatment plants in Ahvaz and the oil regions of Khuzestan, due to proximity to industrial and petrochemical effluent, typically experience higher concentrations of corrosive chemical compounds alongside H2S and require more careful resin selection.

The Logistical Advantage of Retrofitting Existing Treatment Plants with FRP

Many of Iran's treatment plants were built with galvanized steel structures decades ago and are now at various stages of corrosion. Fully replacing these structures with a complete shutdown of the treatment line carries high cost and operational risk. The light weight of FRP profiles enables phased, localized replacement — unit by unit or basin by basin — without shutting down the entire plant; installation with hand tools and no need for heavy cranes also resolves access constraints at treatment plants located in dense urban areas.

9Reference Standards for the Water & Wastewater Industry

10Cross-Disciplinary Engineering Acceptance of This Solution