Urban wastewater treatment plants are continuously exposed to two types of shocks: organic shocks (from unauthorized industrial wastewater discharges) and hydraulic shocks (from rainfall, network leaks, or collection-line incidents). These shocks can destabilize the return activated sludge process.
Effects of hydraulic shock:
A sudden increase in inflow reduces hydraulic retention time, washes out biomass (increasing effluent MLSS/TSS), lowers dissolved oxygen, disrupts settling, and reduces nitrification efficiency. Moderate shocks (1.5–3x average flow) typically recover within 1–4 days, while severe shocks (over 4.5x) can affect the system for more than a week.
Effects of organic shock:
A sudden rise in BOD/COD increases the F/M ratio, boosts production of soluble microbial products (SMP), and can trigger filamentous bacterial growth and sludge bulking. In severe shocks (4x normal load), effluent COD can exceed 600 mg/L, with recovery taking up to 5 days.
Field experience at Qom Plants No. 2 and 3:
Both plants have inlet pH analyzers, and operators bypass inflow with abnormal pH, color, or odor. However, some industrial wastewaters with high COD — such as one case with a COD of roughly 928 mg/L and a strong petroleum odor — showed no clear change in pH or color and entered the system undetected, killing bacteria, causing foaming in the aeration basin, and washing floc out of the secondary clarifier weir. Microscopic observations also showed flagellates and amoebae dominating during organic shock events. A hydraulic shock event at Plant No. 2 similarly caused sludge to escape over the secondary clarifier weir.
Mitigation strategies:
For organic shocks, increasing primary and waste sludge disposal along with daily monitoring of DO, SV, MLSS, MLVSS, and F/M is recommended. For hydraulic shocks, preventing recurrence is the first priority, alongside the same continuous parameter monitoring.
Conclusion:
Hydraulic shocks mainly act through physical washout of biomass, while organic shocks degrade effluent quality through bacterial die-off and extracellular polymeric substance production. Combined shocks can cause prolonged system disruption, particularly in SBR or high-load systems.
Author: Fatemeh Pirouzfar — Process Unit, Wastewater Treatment Plants No. 2 and 3, Qom
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