Oxygen uptake rates for determining microbial activity and application

The method presented involves using the oxygen uptake rates of activated sludges for determining the microbial activity and viability. The microbial activity correlates well with the substrate reduction first order rate constant. K₁, determined in batch operations, and with the specific substrate utilization rate, q, of steady state operations. The microbial viability determined based on respiratory activity would result in slightly higher values than other means such as plate counting, since some of the respiratory activity is contributed by nonviable yet active cells. The extent of respiratory activity attributed to the nonviable cells increases as the biological solids retention time increases (or net growth rate decreases). Since the loss of ability of multiplication is not necessarily associated with loss of biochemical activity, the viability determined by the respiratory activity would represent more realistically the activity of microbes and thus is suggested as the preferred technique.     

Microbial activity in aerobic sludge digestion

Laboratory experiments were performed to study variations in microbial activity during aerobic sludge digestion. The main objective was a critical evaluation of the most common measures of activity, these being: ATP, plate counts, dissolved oxygen uptake rate and activity of dehydrogenase enzymes. Their variation with digestion time was monitored for batch and semi-continuous flow regimes at three different operating temperatures: 10, 20 and 30°C. The relation between these measures and the mass content of the sludge was also investigated.It was found that ATP offers the best estimate of activity, while plate counts can result in erroneous and misleading interpretations of the metabolic state of the mixed liquor population. ATP was found to parallel the decrease in biodegradable suspended solids (BVSS) during batch digestion, when corrected for solids reductions from cell lysis. It was concluded that the decrease in ATP does reflect a decrease in active biomass from the endogenous metabolism of the bacterial population, and that the ATP content per active cell mass remains constant with detention time thus allowing the estimation of active fraction of the sludge.     

Evaluation of alternative sludge settleability indices

Proposed modifications of the standard sludge volume index (SVI) to eliminate its unpredictable variation with suspended solids (SS) concentration include stirring, conduct of the test at constant SS concentration, and dilution. We have tested these modifications by making correlations between the respective indices and the content of filamentous organisms in activated sludge expressed in terms of total extended filament length. The results indicate that diluted SVI is the best index of sludge settleability among those tested. Correlation of diluted SVI with specific functional relationships between settling velocity and SS concentration would enable the application of this parameter to prediction of thickening capacity in secondary clarifiers.     

Prediction of thickening capacity using diluted sludge volume index

Knowledge of the secondary clarifier's capacity to produce an adequately thickened underflow stream is essential for successful design and control of the activated sludge process. Accurate prediction of thickening capacity is possible using settling flux theory, but establishment of the required functional relationship between settling velocity and suspended solids concentration is a lengthy and tedious procedure. In this study, parameters characterizing such relationships are shown to be well correlated with an easily measured index of sludge settleability, the diluted sludge volume index (SVI^*). It is therefore possible to predict, using settling flux theory, limiting values of solids loading as a function of SVI^* and underflow velocity. This technique has numerous potential applications in activated sludge process control and design.     

Estimation of viable biomass in wastewater and activated sludge by determination of ATP, oxygen utilization rate and FDA hydrolysis

ATP content, oxygen utilization rate (OUR) and fluorescein diacetate (FDA) hydrolysis were tested for the ability to express the amount of viable biomass in wastewater and activated sludge.The relationship between biomass and these activity parameters was established in growth cultures made by inoculating a nutrient medium with either wastewater or activated sludge. Biomass was then determined directly by measurement of dry weight of growth culture (dw), and compared to data obtained by using the previously mentioned methods. In the exponential growth phase, ATP content showed the best correlation with biomass, while FDA hydrolysis in the sludge failed to show any such correlation. Conversion factors of 3 mg ATP/g dw, 300 mg O₂/h g dw and 0.4 A/h (mg dw/ml) for ATP, OUR and FDA methods, respectively, were calculated.When the methods were applied for in situ determinations in four different wastewater plants, it was found that ATP content and respiration rate estimated viable biomass to range from 81 to 293 mg dw/g SS for raw wastewater and from 67 to 187 mg dw/g SS for activated sludge with a rather weak correlation between ATP and respiration measurements. The FDA hydrolysis estimated viable biomass to be higher than suspended solids, for which reason this parameter could not be recommended for determination of viable biomass in wastewater and activated sludge.     

Measuring the activities of higher organisms in activated sludge by means of mechanical shearing pretreatment and oxygen uptake rate

A pretreatment method was developed to assess the activities of higher organisms. The method is based on mechanical shearing to damage the large cells of the protozoan and metazoan community in activated sludge. The procedure was confirmed through experimentation to be effective in determining the activities of higher organisms by comparing oxygen uptake rates (OURs) before and after the higher organisms were eradicated. Shearing led to disintegration of flocs, which could be effectively reconstituted by centrifugation. The reconstitution of the sludge flocs was essential since otherwise the activity of the floc mass would be too high due to lack of diffusion limitation. Mechanical shearing had no influence on the morphology, quantity and specific activity of yeasts, and it was inferred that bacteria smaller than yeasts in size would also not be influenced by the applied shearing procedure. Moreover, the effect of filamentous organisms on the measured activities of higher organisms was experimentally demonstrated and analyzed, and determined to be so weak that it could be ignored. Based on these tests, five typical activated sludge processes were selected to measure the contribution of higher organisms to the original OUR. The measured activities of higher organisms ranged from 9.4 to 25.0% of the original OURs.     

Model system of bulking and flocculation in mixed culture of Sphaerotilus sp. and Pseudomonas sp. for dissolved oxygen deficiency and high loading

A mixed continuous culture system was made up as a model for bulking and flocculation phenomena of the activated sludge to study the effect of dissolved oxygen (DO) deficiency and the effect of high organic loading. The system consisted of a floc forming bacterium and a filamentous bacterium which were isolated from the activated sludge and were identified as Pseudomonas sp. and Sphaerotilus sp., respectively. Sphaerotilus sp. had potential to cause a filamentous bulking phenomenon on the activated sludge. It was observed that the filamentous microorganism showed three kinds of growth form, filamentous form, pellet form and dispersed form, and that the floc former showed two kinds of growth form, good floc form and dispersed form. In the model system, these changes of growth form of two microorganisms, which could be thought as the cause of settling characteristics changes, depended on the DO level and the dilution rate (as a substitution for organic loading). The DO level also influence the aggregative ability of each microorganism and the maximum oxygen uptake rate, Q_O2max, of filamentous microorganism. The proportions of both microorganisms in model system were inverted reversibly by the DO level or the dilution rate changes.     

The activities of some fermentation enzymes in activated sludge and their relationship to enhanced phosphorus removal

A method to determine lactate, malate, β-HO-butyrate dehydrogenase and phosphotransacetylase activities in activated sludge was developed. The method was used to monitor two of the enzymes, which showed correlation with phosphorus removal in two activated sludge plants. The phosphorus removal capacity of the two plants showed different sensitivities to changes in the activities of β-HO-butyrate dehydrogenase and phosphotransacetylase. The reasons for this are discussed.     

Effect of water composition, distance and season on the adenosine triphosphate concentration in unchlorinated drinking water in the Netherlands

The objective of our study was to determine whether water composition, distance to the treatment plant and season significantly affect the adenosine triphosphate (ATP) concentration in distributed drinking water, in order to resolve the suitability of ATP as an indicator parameter for microbial regrowth. Results demonstrated that the ATP concentration in distributed water averaged between 0.8 and 12.1 ng ATP L⁻¹ in the Netherlands. Treatment plants with elevated biofilm formation rates in treated water, showed significantly higher ATP concentrations in distributed drinking water and ATP content was significantly higher in the summer/autumn compared to the winter period at these plants. Furthermore, transport of drinking water in a large-sized distribution system resulted in significantly lower ATP concentrations in water from the distal than the proximal part of the distribution system. Finally, modifications in the treatment significantly affected ATP concentrations in the distributed drinking water. Overall, the results from our study demonstrate that ATP is a suitable indicator parameter to easily, rapidly and quantitatively determine the total microbial activity in distributed drinking water.