An ecosystem analysis of the activated sludge microbial community

Abstract

This study was undertaken (i) to investigate the interactions of the activated sludge microbial community in a chemostat with the “environment”, such as the substrate composition and variations, (ii) to investigate how these interactions affect the quality of the treated effluent and (iii) to determine the limits or applicability conditions to the indicators and to the prediction potential of the treated effluent quality. This work presents (a) the experimental results obtained from a reactor fed municipal wastewater (Data Set2-DS2) concerning the reactor's operating conditions and the microbial community of the sludge (b) comparisons between DS2 and an older Data Set (DS1) obtained when the reactor was fed synthetic substrate, all other experimental conditions being identical, and (c) simulation results and sensitivity analyses of two model runs (R1 and R2, corresponding to DS1 and DS2). The first trophic level (P₁) of the DS2 microbial community consisted of bacteria, the second trophic level (P₂) of bacteria-eating protozoa, rotifers and nematodes and the third trophic level (P₃) of carnivorous protozoa and arthropods. Rotifers were an important constituent of the DS2 microbial community. The DS1 and DS1 communities differed in total size, trophic level sizes and species composition. Correlations between the major microbial groups of DS2 community and either loading rates or effluent quality attributes were generally low, but the correlation of bacteria with SVI and ammonia in the effluent was better. Also, the ratio of rotifers to protozoa in P₂ was correlated to BOD in the effluent. The results of this work indicate that predictions of the treated effluent quality based only on protozoa may not be safe. Sensitivity analysis of R2 run indicate that, when variation in Y and K_d biokinetic coefficients of the sludge are combined with fluctuations in composition and quality of municipal wastewater entering the reactor, then sufficient significant prediction of bacteria in the aeration tank is not possible. In order to avoid erroneous oversimplifications regarding phenomena taking place in the sludge and to understand “unexplained” process failures, more ecologically sound methods for studying wastewater treatment (WWT) processes are needed, since WWT are primarily ecosystems interacting with technological systems.

Keywords:

• Activated sludge
• microbial community
• protozoa
• rotifers
• indicators ecosystem
• Vensim

Notes

^a Means are based on eighteen samples;

^b grand mean is based on fifty four samples;

^c presence frequency corresponds to percent presence in the samples;

^d Phil: Philodina; Lec: Lecanes; Nem: Nematod; Arth: Arthopod.

^a Means are based on eighteen samples;

^b grand mean is based on fifty four samples;

^c Phil: Philodina; Lec: Lecanes; Nem: Nematod; Arth: Arthopod;

∗ Values less than 0.01.

^a See text for explanation.

^b Correlations are based on 54 values.

^a From refs.^{[ 17 , 18 ]}