Steady-state vs upset conditions in wastewater systems

When a biological wastewater system is running a near full efficiency, it is said to be in steady-state. While this is actually this is a zone between stationary and decline phase growth depending upon the system, the goal is to remain in a set point with respect to biological reproduction, cell lysis, and biopolymer production that maintains desired effluent quality.

Where is this point on the biological growth curve?

Note that most conventional activated sludge systems as seen in municipal wastewater treatment are designed to run in the stationary growth phase. If solids are not wasted which increases MLVSS and lowers F/M ratios, the aeration system and secondary clarifiers can become over loaded with associated problems. The wasted solids from the secondary clarifier still have substantial insoluble organics entrapped in the floc biofilm. The insoluble organics are then degraded in the digester – either anaerobic or aerobic system which by rule operates in decline phase

Most industrial systems are designed to operate at the end of stationary to full decline phase growth. This section has the lowest F/M ratios and in theory most of the insoluble/recalcitrant organics are degraded by the microbes into their component soluble fractions.

What can cause a shift from target steady-state?

• Rapid increase (or decrease) in influent soluble organic concentrations (BOD5)
• Hydraulic washout or secondary clarifier problems reduce aeration tank MLVSS
• Shock loadings - a spill of an insouble organic (ex. grease, oils)
• Toxic or quasi-toxic influent (ex. cyanide, phenol, solvents) ~ usually seen in industrial wastewater
  
• Environmental changes such as pH, temperature, dissolved oxygen, alkalinity especially if change is rapid