Abstract
The biodegradation of chemicals in sewage treatment plants is a key issue of environmental risk assessment. To predict the residual concentration the rate of the biodegradation process has to be estimated. This rate is the result of microbial adaptation of the micro-flora in the system. Therefore the adaptability of the
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treatment system to chemicals is to be described.
A principal condition for adaptation is that a specific flora is able to grow at a growth rate higher than the average growth rate of the viable biomass in the system. Analysis of viable biomass fractions in activated sludge under various process conditions showed that these systems tend to a steady state with an average growth rate of about 0.3 per day. Therefore adaptability is limited to those substances that allow a growth rate higher than 0.3/d.
Maximum growth rates on single substances have been measured in various test systems for several hundreds of chemicals. Growth is observed at a maximal rate between 1 and 10/day. Thus, adaptability of the treatment system is possible for most substances and the average growth rate in the process is far below their maximum. This implies that the reaction rate will approximate first order.
A first modification of the classical equation of Monod is developed to give a first order rate constant for biodegradation as a function of adaptation of the system to the load of the substance. For this equation it is assumed that each chemical is degraded by a specific biomass fraction that is proportional to the load of the system with that chemical. This equation also includes an extrapolation factor that enables to assess the adaptability on the basis of the maximum growth rate on the substance.
If adaptation occurs, the rate constant will develop until the low residual concentration becomes rate limiting. This rate limitation is usually quantifies on the basis of the half-saturation value in the Monod equation. In this thesis it is shown that in addition there is a limitation due to mass transfer. Therefore, a second modification of the equation is developed that includes a rate constant for mass transfer.
Adaptability is also described in a probabilistic way. The specific fraction before adaptation is determining the time needed to reach a steady state and also the chance that biodegradation may be observed in a standard test. This thesis describes various simple procedures to estimate this fractions using current standard methods. The high variability observed for these fractions (up to 8 orders of magnitude) supports the proportionality concept and explains the observed high variability between laboratories and between test methods.
A new set of criteria is proposed for assessment and classification of biodegradability of chemicals. This set includes a lower limit for the maximum growth rate and a lower limit for the specific fraction before adaptation in addition to the present criterion for complete mineralisation. With this set of criteria, the current classes of readily degradable, inherently degradable and persistent can be maintained, but in a definition with a sound scientific basis.
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