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Primary clarifier

Primary clarifier

Introduction and Model

The primary clarifier separates wastewater into two streams - primary effluent (overflow) and primary sludge (underflow) - through a settling process.

The implementation is based on the Otterpohl model (1995), which represents the primary clarifier as a single homogeneous tank that accounts for hydraulic retention time and concentration smoothing within the clarifier. This tank is separated into the overflow and underflow stream through a mass balance and an empirical separation approach, that produces reasonable concentrations of particulate material in both streams. The empirical separation equation, dependent on the hydraulic retention time, is used to calculate the state variable concentrations in the effluent stream. The state variable concentrations in the sludge stream are determined by using a mass balance for each component. The flow rates for over- and underflow are calculated through a mass balance, where the sludge flow rate is assumed to be proportional to the influent sludge flow rate.

Primary clarifier flowchart
Flowchart of the Otterpohl model1
(pu: primary underflow; po: primary overflow; pc: primary clarifier)

Equations

COD removal efficiency

\[ \eta_{COD_p}(\mathrm{t}) = f_{corr}(2.88 f_x - 0.118) - (1.45 + 6.15 ln( t_h(\mathrm{t}) \cdot 24 \cdot 60)) \]
Symbol Description Unit
\(\eta_{COD_p}\) COD (chemical oxygen demand) removal efficiency in percent -
\(f_{corr}\) Dimensionless correction factor for the COD removal efficiency -
\(f_x\) Mean fraction of particulate COD -
\(t_h(\mathrm{t})\) Hydraulic retention time d

Primary effluent concentration of each state variable

\[ f_i = 1 - \frac{\eta_{COD_p}}{100} f_{SX,i} \]
Symbol Description Unit
\(f_i\) Primary effluent concentration of each state variable in percent -
\(f_{SX,i}\) Structural factor defining the fraction of particulate matter of each -

  • \(f_{SX,i} = 0\) for all soluble components

  • \(0 < f_{SX,i} \ll 1\) for all particulate fractions

  • In the benchmark case, all the particulate state factors are set to 1.

Source code documentation

mod primclar_bsm2

mod primclarinit_bsm2

  1. Benchmarking of Control Strategies for Wastewater Treatment Plants, chap. 4.2.4.1 Primary clarifier ↩

  2. Benchmark Simulation Model no. 2 (BSM2), chap. 4. Modeling of the primary clarifier ↩