Common Secondary Treatment Problems and How to Fix Them

Recent Trends in Secondary Treatment

Operators of municipal and industrial wastewater plants are facing tighter effluent limits and higher volumes due to population growth and stricter environmental regulations. Many facilities are reporting recurring issues in their secondary treatment stage—mainly in activated sludge systems—that reduce efficiency and increase compliance risk. New monitoring tools and biological conditioners are emerging, but basic troubleshooting remains essential for stable operation.

Background: How Secondary Treatment Works

Secondary treatment uses microorganisms to break down organic pollutants after primary screening. Typical systems include activated sludge processes, trickling filters, and sequencing batch reactors. The biological community must be kept in a balanced state—adequate oxygen, proper nutrient ratios, consistent pH, and controlled solids retention time. When any of these variables drift, common problems arise.

Common User Concerns and Solutions

• Sludge bulking – Filamentous bacteria overtake floc-forming organisms. Fix: Adjust food-to-microorganism ratio, increase dissolved oxygen in aeration zones, or introduce selective wasting.
• Foam formation – Often caused by surfactants or slow-growing filaments (e.g., Nocardia). Fix: Reduce mean cell residence time, add defoamers, or install surface spray systems.
• Low effluent clarity (turbidity) – Pin floc or dispersed growth after upset. Fix: Check mixed liquor suspended solids (MLSS) levels, reduce hydraulic loading, or add coagulant.
• Nutrient deficiency – Insufficient nitrogen or phosphorus slows biological activity. Fix: Supplement with urea, ammonia, or phosphoric acid while maintaining a BOD:N:P ratio near 100:5:1.
• pH swings – Nitrification consumes alkalinity, leading to drops below 6.5. Fix: Add lime or sodium bicarbonate, and monitor alkalinity weekly.
• Oxygen transfer inefficiency – Clogged diffusers or high temperature reduce aeration. Fix: Clean diffuser membranes, consider fine-bubble retrofits, and upgrade blowers if needed.

Likely Impact on Operations

Unresolved secondary treatment problems can cause permit violations, higher energy usage, and increased sludge disposal costs. Frequent upsets also stress downstream units like disinfection and tertiary filtration, potentially leading to fines or mandatory plant upgrades. For facilities already near capacity, repeated noncompliance may trigger regulatory intervention or limit future discharge allocations.

What to Watch Next

• Real-time biological monitoring – Online respirometers and microscopy sensors allow earlier detection of filamentous shifts.
• Advanced process control – Automated wasting based on sludge settling characteristics can reduce human error.
• Combined membrane bioreactors – These replace clarifiers, but require careful membrane cleaning to avoid fouling.
• Bioaugmentation products – Commercial bacterial blends are being tested to quickly restore upset systems, though selection should be based on site-specific waste composition.