The ACCEL model for accelerating the detoxification kinetics of hydrocarbons requiring initial monooxygenation reactions

Elizabeth P. Dahlen, Bruce Rittmann

Research output: Contribution to journalArticlepeer-review


The two-tank accelerator/aerator modification of activated sludge significantly increases the biodegradation of hydrocarbons requiring initial monooxygenation reactions, such as phenol and 2,4-dichlorophenol (DCP). The small accelerator tank has a controlled low dissolved oxygen (DO) concentration that can enrich the biomass in NADH + H+. It also has a very high specific growth rate (μ acc) that up-regulates the biomass's content of the monooxygenase enzyme. Here, we develop and test the ACCEL model, which quantifies all key phenomena taking place when the accelerator/aerator system is used to enhance biodegradation of hydrocarbons requiring initial monooxygenations. Monooxygenation kinetics follow a multiplicative relationship in which the organic substrates (phenol or DCP) and DO have separate Monod terms, while the biomass's content of NADH + H+ has a first-order term. The monooxygenase enzyme has different affinities (K values) for phenol and DCP. The biomass's NADH + H+ content is based on a proportioning of NAD(H) according to the relative rates of NADH + H+ sources and sinks. Biomass synthesis occurs simultaneously through utilization of acetate, phenol, and DCP, but each has its own true yield. The ACCEL model accurately simulates all trends for one-tank and two-tank experiments in which acetate, phenol, and DCP are biodegraded together. In particular, DCP removal is affected most by DOacc and the retention-time ratio, Θacc/ Θtotal. Adding an accelerator tank dramatically increases DCP removal, and the best DCP removal occurs for 0.2 < DOacc< 0.5 mg/l and 0.08 < Θacctotal < 0.2. The rates of phenol and DCP utilization follow the multiplicative relationship with a maximum specific rate coefficient proportional to μacc. Finally, μ acc increases rapidly for Θacc/ Θtotal < 0.25, acetate removal in the accelerator fuels the high μ acc, and the biomass's NADH + H+ content increases very dramatically for DOacc < 0.25 mg/l.

Original languageEnglish (US)
Pages (from-to)237-250
Number of pages14
Issue number3
StatePublished - Jun 2006


  • Activated sludge
  • Dichlorophenol
  • Monooxygenation
  • Nicotinamide adenine dinucleotide
  • Phenolics
  • Specific growth rate

ASJC Scopus subject areas

  • Environmental Engineering
  • Microbiology
  • Bioengineering
  • Environmental Chemistry
  • Pollution


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