Industrial buyers often reuse a "water tank" drawing for sewage. Six months later the coating blisters, the mixer nozzle cracks, and the level transmitter reads foam as liquid. A wastewater tank is a process vessel with atmospheric pressure.
Medium parameters that decide steel
pH 2–4: acid equalization → glass-flake vinyl ester or 316L, not carbon steel + alkyd
pH 6–9: municipal effluent → epoxy coal tar or pure epoxy
H2S > 50 ppm: needs sulfide-resistant topcoat and vent-side carbon cartridge
Oil & grease high: add inclined baffle and removable skim manhole
TSS high: stilling tube for radar, flush connection at bottom
Lining systems compared
Epoxy coal tar: cheap, robust for raw water/sewage, dark, not food grade
Pure epoxy: cleaner, potable-adjacent, weaker in strong solvent
Epoxy phenolic: food/pharma and some chemical service, higher cost
Glass-flake vinyl ester: aggressive wastewater, pH 1–12, splash zone, FRP-compatible
304/316L stainless: when coating holiday risk is unacceptable and budget allows
Nozzle load discipline
A mixer, aerator or return-sludge pump imposes side load on the shell. ZN requires:
Mixer nozzle reinforcement plate ≥ shell thickness + 6 mm
Cantilever moment from mixer shaft entered into roof/shell FEA if > 1.5 kNm
Separate support bracket from tank structure, not relying on shell plate alone
Level instrumentation
Foam and surface turbulence fool radar. Solutions:
Stilling well / stilling tube
Guided-wave radar instead of open-air radar
Ultrasonic only for clean water, never for aerated sludge
FAQ snippet
"Do wastewater tanks need cathodic protection?" Buried sewage tanks: yes, bottom-side CP or dielectric liner. Aboveground tanks: coating is primary; CP only if soil contact or liner breach risk exists.
A well-specified sewage tank is not more expensive-it simply moves the cost from "emergency relining in year 2" to "correct lining in month 1."
