Variations of growth yield coefficient with substrate concentration of a mixed microbial population was studied. Substrates used for growth were 2,4-dichlorophenoxyacetic acid (2,4-D) and para-chloro-ortho-cresol (PCOC). The synthetic medium used was designed so that substrate was the limiting-nutrient. The microbial culture was obtained from an activated sludge system treating effluent containing 2,4-D and PCOC. This was acclimated to the particular substrate metabolised in the investigations. Growth was conducted in batch and chemostat configurations.
Experimental data obtained indicated variations in growth yield did occur and were dependent on substrate concentration. Growth yield and specific growth rate biokinetics were directly calculated from the data obtained. Analysis of specific growth rate help in understanding culture-substrate systems. Specific growth rate increased to a maximum then decreased with increasing substrate concentration in batch configuration. Decrease in growth rate began above 500 mg/I for 2,4-D and above 60 mg/I for PCOC. This is an indication of substrate inhibition. The design of the chemostat maintained a constant specific growth rate.
Growth yield decreased with increasing substrate concentration with growth on either 2,4-D and PCOC in batch and chemostat configurations. A review of the literature indicated maintenance coefficient is a key parameter in explaining variations in observed growth yield. Data analysis for determination of the biokinetic constants of maintenance coefficient, me, and half-saturation constant, K , was performed. Analysis techniques for these constants are traditionally derived from Monod kinetics. Monod kinetics adequately explains growth on innocuous substrates. However analysis of specific growth rate had indicated the substrates used were inhibitory. Determination of maintenance coefficient and half-saturation constant by Monod derived techniques was unsatisfactory.
The effect of maintenance coefficient on growth yield was considered. The literature indicated maintenance coefficient is constant for growth on innocuous substrates. The substrates used in the investigations have phenolic structures. Phenolic compounds are recognised to be destructive to cell membranes. It was proposed that maintenance coefficient increased with increasing inhibitory substrate concentration as a result of increasing cell damage. An explanation for the decreasing growth yield with substrate concentration is adequately given by considering the
variable maintenance coefficient. Substrate used for maintenance is substrate that is not available for growth.
The relationship between growth yield and substrate concentration is essentially linear. Linear regression of yield on substrate was performed for growth on 2,4-D and PCOC in batch configuration and PCOC in chemostat configuration. Fitting of the linear functional form was considered appropriate. Analysis of the linear models for the three biosystems indicated yield variations with substrate concentration are significant. The models for the three biosystems have been given:
For growth on 2,4-D in batch configuration
Y(s) = 0.334 - (2.8xl04 )s
For growth on PCOC in batch configuration
Y(s) = 1.03 - (5.6xl0-3)s
For growth on PCOC in chemostat configuration
Y(s) = 0.799 - (6.0xl0.3)s
Growth yield models may be incorporated into an overall growth model when similar biosystem configuration and substrates are studied. It is considered that this will give a growth model with greater accuracy in design and operation of biological treatment plants.