The ACCEL Model for Accelerating the Detoxification Kinetics of Hydrocarbons Requiring Initial Monooxygenation Reactions

Abstract

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 DO_acc and the retention-time ratio, Θ_acc/Θ_total. Adding an accelerator tank dramatically increases DCP removal, and the best DCP removal occurs for 0.2 < DO_acc  < 0.5 mg/l and 0.08 < Θ_acc/Θ_total < 0.2. The rates of phenol and DCP utilization follow the multiplicative relationship with a maximum specific rate coefficient proportional to μ_acc. Finally, μ _acc increases rapidly for Θ_acc/Θ_total < 0.25, acetate removal in the accelerator fuels the high μ _acc, and the biomass’s NADH + H⁺ content increases very dramatically for DO_acc < 0.25 mg/l.