Lipids (characterized as oils, greases, fats and long-chain fatty acids) are important organic components of wastewater. Their amount, for example, in municipal wastewater is approximately 30-40% of the total chemical oxygen demand. The concern over the behavior of lipids in biological treatment systems has led to many studies, which have evaluated their removal, but still the exact behavior of lipids in these processes is not well understood. In this review, we discuss the current knowledge of how lipids/fatty acids affect both aerobic and anaerobic processes and specific methods that have been used in an attempt to enhance their removal from wastewater. Overall, the literature shows that lipids/fatty acids are readily removed by biological treatment methods, inhibitory to microbial growth as well as the cause of foaming, growth of filamentous bacteria and floc flotation.
Soluble microbial products (SMP) are organics produced by microorganisms as they degrade substrates. The available literature does not reveal how SMP affect and regulate microbial activities. In this study, we monitored variations in pH, dissolved oxygen concentration, soluble biological and chemical oxygen demands (sBOD5 and sCOD) as a measure of microbial activity in synthetic wastewater. Aerobic degradation tests were carried out under the following conditions: aeration, 1,500 cm3 /min; initial sBOD5, 515+/-5 mg/l; initial sCOD, 859+/-6 mg/l; initial biomass concentration (defined as mixed liquor suspended solids), 1,200+/-25 mg/l; sludge retention time, 24 h; and temperature, 20+/-1 degrees C. The study involved non-acclimated biomass (R0 flora), biomass developed in the presence of SMP (R1 flora), and biomass developed in reduced level of SMP (R2 flora). We also determined which of these flora produced more refractory SMP. The results showed that R2 flora utilized the synthetic feed more quickly, and produced less refractory organic matter than R0 and R1 flora. The production of more refractory organics by R0 and R1 flora shows that not all the biomass was active. R1 flora degraded the substrates irregularly, suggesting that some microbes were dependent on the metabolic products of those that could utilize the feed components. These results show that production of SMP also depends on the prior substrates and on the ability of the flora to respond to changes in substrate composition.
We carried out two experiments to study the influence of soluble microbial products (SMP) on biomass concentration [defined as mixed liquor suspended solids (MLSS)] and removal of soluble biological and chemical oxygen demands (sBOD5 and sCOD): (1) SMP were allowed to accumulate, and (2) SMP content was artificially reduced by washing the biomass. The daily initial sCOD in both experiments was kept constant at 859+/-6 mg/l for 16 days. In experiment 1, the highest sCOD removal (80%) occurred during the first day. Thereafter, it decreased successively to 40% [sludge retention time (SRT), 12 days], after which it increased steadily to 50+/-4%. Variations in residual sCOD were accompanied by variations in sBOD5, showing that the biodegradability of the accumulated SMP components was changing. MLSS fluctuated within the range 1,200+/-25-1,993+/-58 mg/l. We attributed the irregular accumulation of the biomass to variations in the biodegradability of SMP components. The initial sBOD5/MLSS ratio varied according to variations in initial sBOD5 and MLSS, whereas the residual ratio was constant at 0.025+/-0.008. This indicated a direct relationship between the concentrations of biomass and SMP produced. In experiment 2, MLSS increased from 1,200+/-25 to a constant value (2,810+/-16 mg/l; SRT, 12 days). After this time, no decrease or increase in MLSS was observed. Correspondingly, sCOD and sBOD5 removal increased from 80-97 to 84-99%. A stable microbial community that could consume organic matter efficiently was developed under these conditions.
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