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Activated Sludge Design (Complete Mix Reactor)

Activated Sludge Design (Complete Mix Reactor). Activated Sludge Principles . Wastewater is aerated in a tank Bacteria are encouraged to grow by providing Oxygen Food (BOD) Nutrients Correct temperature Time As bacteria consume BOD, they grow and multiply

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Activated Sludge Design (Complete Mix Reactor)

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  1. Activated Sludge Design (Complete Mix Reactor)

  2. Activated Sludge Principles • Wastewater is aerated in a tank • Bacteria are encouraged to grow by providing • Oxygen • Food (BOD) • Nutrients • Correct temperature • Time • As bacteria consume BOD, they grow and multiply • Treated wastewater flows into secondary clarifier • Bacterial cells settle, removed from clarifier as sludge • Part of sludge is recycled back to activated sludge tank, to maintain bacteria population • Remainder of sludge is wasted

  3. Schematic of activated sludge unit

  4. Kinetics of Microbial Growth • Biochemical reaction • Biomass concentration. • The concentration of biomass, X (mg/L), increases as a function of time due to conversion of food to biomass: Where m is the specific growth rate constant (d-1). This represents the mass of cells produced/mass of cells per unit of time.

  5. Effect of substrate concentration on growth rate constant

  6. Monod Kinetics • Growth rate • Growth rate constant, m , is a function of the substrate concentration, S. • Two constants are used to describe the growth rate • mm (mg/L) is the maximum growth rate constant (the rate at which the susbtrate concentration is not limiting) • Ks is the half-saturation constant (mg/L) (i.e., concentration of S when m = mm/2

  7. Biomass production • Where kd represents the endogenous decay rate (d-1) (i.e., microorganism death rate). • Substituting the growth rate constant:

  8. Substrate utilization Where Y is the yield factor (mg of biomass produced/mg of food consumed) • Y range: • Aerobic: 0.4 - 0.8 mg/mg

  9. Food to microorganism ratio (F/M) • Represents the daily mass of food supplied to the microbial biomass, X, in the mixed liquor suspended solids, MLSS • Units are Kg BOD5/Kg MLSS/day

  10. Since the hydraulic retention time, q = V/Qo, then

  11. Typical range of F/M ratio in activated sludge units

  12. Design parameters for activated sludge processes

  13. Operational characteristics of activated sludge processes

  14. Activated Sludge Design Equations Mass balance of biomass production • Influent biomass + biomass production = effluent biomass + sludge wasted • Substitute biomass production equation • Assume that influent and effluent biomass concentrations are negligible and solve

  15. Mass balance of food substrate • Influent substrate + substrate consumed = effluent susbtrate + sludge wasted substrate • Substitute substrate removal equation • Assume that no biochemical action takes place in clarifier. Therefore the substrate concentration in the aeration basin is equal to the substrate concentrations in the effluent and the waste activated sludge. Solve:

  16. Overall equations • Combine the mass balance equations for food and biomass: • The cell residence time is: • and the hydraulic retention time is,q = V/Qo • Substitute and rearrange: • Compute the F/M ratio

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