The problem on site
Sewage treatment plants (STPs) and effluent treatment systems subject reinforced concrete to continuous chemical and biological exposure. On construction and maintenance projects across India, project engineers and civil contractors frequently observe that the inner surfaces of sewage retention pits, equalization tanks, and aeration basins begin to deteriorate far earlier than anticipated.
During plant inspections or planned shutdown routines, site teams often discover that the hard cement matrix has softened. When scraped with a hand tool, the concrete surface breaks away as a soft, paste-like sludge instead of remaining rigid. Over time, progressive pitting develops along the walls, particularly in the headspace zone above the normal water line. In advanced cases, the cement paste washes out completely, exposing coarse aggregate stones and leaving embedded reinforcing bars or metallic cast-ins prone to corrosion.
Allowing concrete to soften and pit inside an STP tank undermines the structural integrity of the facility. As the protective cover over the rebar diminishes, structural steel corrodes, which can lead to cracking, structural movement, groundwater contamination, or environmental compliance violations. Restoring these tanks requires more than a simple plaster wash; it demands a barrier system designed specifically to resist wastewater environments.
Why it happens
The softening and pitting of concrete inside sewage handling facilities is primarily driven by microbially induced chemical attack. Raw sewage contains organic matter and sulphur compounds. In stagnant, warm, or slow-moving wastewater, anaerobic bacteria reduce these sulphur compounds into dissolved hydrogen sulphide gas.
As the effluent moves or becomes agitated, hydrogen sulphide escapes the liquid phase and collects in the humid headspace of the enclosed tank. Here, moisture condenses onto the concrete roof and upper wall sections. Aerobic bacteria present on these damp surfaces metabolize the hydrogen sulphide gas and convert it directly into sulphuric acid.
Standard concrete is alkaline, drawing its strength from hydrated cement paste consisting largely of calcium hydroxide and calcium silicate hydrate. When biogenic sulphuric acid contacts the alkaline concrete surface, an aggressive chemical neutralization reaction takes place. The acid converts calcium hydroxide into calcium sulphate (gypsum). Further reactions with calcium aluminate hydrates produce ettringite.
These reaction products have no structural strength and exhibit significant volumetric expansion. The expansion disrupts the internal bonds of the paste, leading to loss of cohesive strength, crumbling, and deep pitting. Because the conversion is continuous as long as sewage flows through the system, unprotected concrete will continue to dissolve layer by layer.
How Space Arc Engineering approaches it
Treating sewage-damaged concrete requires technical assessment before applying any coating material. Space Arc Engineering operates as an authorised project distributor and solution provider, collaborating directly with structural consultants, maintenance contractors, and facility managers to address wastewater tank durability.
Rather than treating the issue as a simple product purchase, Space Arc Engineering helps site teams establish a clear, systematic remediation plan:
- Damage assessment: Our technical team evaluates the extent of chemical attack, identifying whether the concrete is sound or whether soft, carbonated, and acid-degraded layers need mechanical removal.
- Substrate preparation protocols: We advise on mechanical cleaning methods, such as abrasive blasting or high-pressure washing, to remove softened paste, expose solid concrete, and eliminate residual acidic compounds.
- Reconstruction planning: Where pitting has formed deep voids or exposed aggregate, we guide site engineers on repairing the profile using compatible repair mortars prior to coating.
- System selection: We supply heavy-duty protective systems manufactured to withstand industrial and domestic effluent, continuous immersion, and biogenic gas exposure.
- Application guidance: We provide detailed instructions on environmental parameters, surface dryness, recoat windows, and equipment handling to ensure proper adhesion on site.
The system we supply
To establish a durable barrier against wastewater gases and moisture, Space Arc Engineering supplies Fosroc Nitocote UP, manufactured by Fosroc. As an authorised distributor, we deliver this high-performance material directly to projects across India.
Fosroc Nitocote UP is a solvent-free, coal tar epoxy protective coating engineered for concrete and steel assets in water-immersed and effluent-exposed conditions. It provides crucial functional benefits for wastewater infrastructure:
- Made for effluent-exposed concrete: The dense coal tar epoxy matrix provides resistance against sewage, microbial acids, salts, and continuous immersion, preventing acidic gases from reaching the underlying cement.
- Protects both concrete and steel: STPs contain concrete walls alongside structural steel baffles, pipes, ladders, and brackets. Fosroc Nitocote UP bonds effectively to both properly prepared concrete and blasted steel surfaces, simplifying site inventory.
- Solvent-free, primerless application: Because it is formulated without volatile solvents, the coating reduces safety concerns associated with fumes in enclosed tank environments. It is designed for direct application to prepared concrete without requiring a separate primer, which reduces steps in the application schedule.
What to check before you order
Applying an STP tank protective coating requires strict attention to site conditions. Site engineers should verify the following points before placing an order and commencing work:
1. Substrate soundness
All deteriorated, acid-affected concrete must be mechanically removed down to sound, hard concrete. Applying a coating over soft or crumbly paste will result in delamination.
2. Surface dryness and moisture content
The concrete must meet the manufacturer-specified moisture thresholds. Trapped moisture inside the tank walls can cause blistering under high-build resin coatings.
3. Cleaning and decontamination
The surface must be free from grease, bio-films, residual sewage sludge, and loose dust. High-pressure washing followed by mechanical profiling ensures a clean anchor pattern.
4. Headspace ventilation
Even with solvent-free coatings, enclosed sewage tanks require proper mechanical ventilation during surface preparation, repair, coating application, and curing to maintain safe working conditions and remove humidity.
5. Surface area calculations
Measure the entire interior envelope, including sump pits, baffles, roof soffits, and pipe collars. Account for the concrete surface profile, as rough or repaired surfaces require more material than smooth screeds.
Talk to our technical team
Space Arc Engineering assists project owners, structural repair specialists, and civil contractors with technical guidance, product selection, and reliable supply for water and wastewater treatment facilities. Explore our solutions to learn more about how we help protect critical concrete infrastructure across India.
For technical inquiries, material schedules, or site evaluations, contact Space Arc Engineering:
Phone: +91 99991 55255 / +91 72900 89007
Website: www.space-arc.in
Address: 11/155, Sector 3, Rajendra Nagar, Sahibabad, Ghaziabad, Uttar Pradesh 201005, India