Modeling hydraulic characteristics and nitrogen removal in a fixed-bed reactor for septic tank effluent treatment

Tian C. Zhang

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


This paper presents information on modeling of (1) hydraulic characteristics, (2) the nitrification, and (3) denitrification processes in the down-flow sulfur/limestone autotrophic denitrification (SLAD) fixed-bed biofilm reactors that were used for septic tank effluent treatment. To simulate the lateral field, the reactors were filled, from bottom to top, with 2 inches of soil, different heights of sulfur/limestone (S/L), different heights of sand, 6 inches of gravel, and 4 inches of topsoil. Due to the 2-inch layer of soil (loamy clay) at the bottom of the reactors, the flow in the SLAD reactors followed a pattern representative of a plug flow reactor (PFR) followed by a continuously stirred tank reactor (CSTR) if the column was fed continuously for greater than 10 h. However, when the column was fed intermittently (e.g., six times a day, each time with a 0.5-1-h feeding duration), the flow was found to follow a PFR pattern. The nitrification process in the gravel/sand layer was found to follow first-order kinetics based on concentration changes in bulk solution with an average rate constant of 0.19 (1/h). For the S/L layer, the reaction rate order in bulk solution appeared to depend on how the nitrate-N concentration, C, compares to a critical nitrate-N concentration, CCritical at the biofilm-liquid interface. If the C > CCritical a zero-order reaction in bulk solution with a rate constant k0 of 7.25 mg/L-h was good for modeling denitrification; otherwise, a half-order reaction in bulk solution with a rate constant k1/2 of 1.14 mg1/2/L1/2-h should be used. The intrinsic zero-order reaction rate constant, k0B within the biofilm for SLAD was found to be 8.58 mg-N/L-min. The results of this study were obtained from the specific reactor configurations and experimental conditions. However, the parameters and empirical models may be useful for future studies and process design related to SLAD processes.

Original languageEnglish (US)
Pages (from-to)347-360
Number of pages14
JournalEnvironmental Engineering Science
Issue number4
StatePublished - 2003


  • Biofilm
  • Denitrification
  • Modeling
  • Nitrification
  • Nitrogen
  • Septic tank effluent
  • Sulfur/limestone

ASJC Scopus subject areas

  • Environmental Chemistry
  • Waste Management and Disposal
  • Pollution


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