Food Processing WWTP Retrofit in Fresno, California

Scenario

Project Type: Industrial wastewater treatment retrofit for a tomato cannery. Location Context: Central Valley agro-industrial zone with high summer temperatures (avg. 32°C), seasonal operation (3-month harvest season), and stringent zero-discharge requirements due to groundwater protection ordinances. Constraints: Must reuse 95% of treated water for equipment washing; existing aeration tanks are undersized and corroded; CA State Water Board mandates ≥92% BOD removal and MLSS stability under shock loads (e.g., juice spill events).

Given Data

• Influent Flow Rate: 8,200 m³/d (peak harvest flow)
• Influent BOD Concentration: 1,420 mg/L (high-strength organic load from peel/skin waste)
• BOD Decay Rate Constant (k): 0.31 1/d (temperature-corrected upward for 30°C using θ = 1.047)
• Hydraulic Retention Time (HRT): 4.8 d (reduced vs. municipal standards due to high k and need for compact design)
• MLSS Concentration: 4,800 mg/L (elevated to handle shock loads and support nitrification)
• Yield Coefficient (Y): 0.65 g VSS/g BOD removed (higher due to readily degradable substrate)
• Endogenous Respiration Rate Constant (kd): 0.062 1/d (increased for elevated temperature)

Calculation

Reactor volume is determined by: $$ V = Q \times \theta_H $$

Substituting values: $$ V = 8{,}200\ \text{m}^3/\text{d} \times 4.8\ \text{d} = 39{,}360\ \text{m}^3 $$

Although high BOD loading could suggest larger volume, the elevated k and MLSS allow shorter HRT while maintaining >95% BOD removal (verified via solids retention time analysis: SRT ≈ 12.3 d, well above minimum required for nitrifier stability).

Result and Decision

Calculated reactor volume: 39,360 m³. Instead of constructing new basins, the engineering team retrofitted two existing 18,500 m³ concrete tanks with high-efficiency submerged aerators, real-time DO/MLSS probes, and a return activated sludge (RAS) booster pump. Total installed volume = 37,000 m³ — validated via computational fluid dynamics (CFD) to ensure effective mixing and avoid dead zones at peak flow.

Lesson

For high-strength industrial streams, reactor volume can be reduced safely through kinetic optimization (leveraging temperature-enhanced decay rates and elevated MLSS), but only when paired with robust instrumentation and adaptive control — the original design failed because it relied solely on volumetric rules without dynamic sensor feedback.