On-line fluorescence profile of aerobic sludge digestion

An online fluorometer designed for following intracellular NAD(P)H was used to monitor aerobic sludge digestion experiments. The fluorescence showed an initial rise to a high plateau, a sharp decline after staying at the plateau for 20-60 h, and a trailing very slow decrease. The characteristic fluorescence profile was shown to result mainly from the solids-associated fluorescence, after ruling out other factors such as pH, temperature, and supernatant fluorescence. The fluorescence profile was, however, not a mere result of the decreasing solids concentration. The varying sludge viability and population composition (e.g., the decay of heterotrophs and the increasing fraction of nitrifiers) played important roles. The fluorescence profile correlated well with the profile of the viable heterotrophic cell number concentration evaluated with TSB-agar plates. The initial increase of the number concentration was attributed to the growth of multiple small bacteria from the lysate of each large microorganism, which was demonstrated in the experiments with baker's yeast as the starting culture for digestion. The fluorescence profiles observed in the yeast experiments were similar to those in the sludge experiments. Responding to glucose additions and the switch from aerobic to anaerobic conditions, the yeast systems showed typical step increases of fluorescence as expected from the change of NAD(P)H level associated with heterotrophic metabolism. However, no such fluorescence responses were detectable in the sludge digestion systems. NAD(P)H were thus uncertain to be responsible for the online fluorescence observed. Nonetheless, the initial fluorescence plateau corresponded to the period of rapid digestion and, for the plant studied, the EPA regulation criteria of VSS reduction >38% and/or SOUR <1.5 mg of O(2) (g of TS)(-)(1) h(-)(1) were satisfied at the end of the plateau. The online fluorescence provides an effective means of monitoring the aerobic sludge digestion process.     

Modeling active mass in aerobic sludge digestion

Aerobic digestion of waste-activated sludge was carried out in lab-scale reactors for both batch and semicontinuous flow patterns. The reactors were monitored at three different temperatures: 10, 20, and 30 degrees C. During the course of digestion, significant solubilization of volatile suspended solids was observed, and its effect on the magnitude of kinetic coefficients was examined. Differences in metabolic activity and sludge stabilization were found between batch and semicontinuous flow patterns. An Arrhenius-type relationship was not found to apply to rate constants for the semicontinuous reactors.     

Microbial populations and sludge characteristics in thermophilic aerobic sewage sludge digestion

Summary Estimates of bacterial numbers from raw sewage sludge and sludge treated by thermophilic aerobic digestion were compared with simple indicators of sludge quality and concentrations of potential substrates. Significant differences were found between sludge types for all but one of the variables examined (frequency of dividing cells). During a stable period of digestor operation, the average number of viable obligate thermophiles present in digested sludge (1.63 × 10⁶ ml^–1) was approximately 10²-fold greater than in feed sludge (1.10 × 10⁴ ml^–1). Total numbers of bacteria were slightly greater in digested sludge (3.24 × 10¹⁰ ml^–1) than in feed sludge (2.39 × 10 ml^–10), as were viable counts of bacteria at incubation temperatures of 37°C and 55°C. Significant correlation was found between viable counts of bacteria at 37°C and 55°C for digested sludge, and 65°C and 55°C for feed sludge. The numbers of obligate thermophiles present and the total of bacteria present were related to the temperature and pH of the digested sludge and inversely related to the numbers ofEscherichia coli and coliforms present, which were not detected at temperatures greater than 50°C.     

Survival rates of parasite eggs in sludge during aerobic and anaerobic digestion.

The effects of mesothermic anaerobic or aerobic sludge digestion on survival of eggs from the roundworms Ascaris suum, toxocara canis, Trichuris vulpis, and Trichuris suis and from the rat tapeworm Hymenolepis diminuta were studied. Destruction of eggs throughout a 15-day treatment period, as well as their viabilities after reisolation, was analyzed. The laboratory model digesters used in this study were maintained at a 15-day retention schedule, partially simulating a continuously operating system. Ascaris eggs were destroyed in the anaerobic (23%) or aerobic (38%) digesters, and 11% Trichuris eggs were destroyed in the aerobic digesters. Trichuris eggs in anaerobic digesters and Toxocara eggs in either anaerobic or aerobic digesters were not destroyed. Destruction of eggs in digesters was correlated with the state of the eggs before subjection to the treatment processes; i.e., some Ascaris and Trichuris eggs were already embryonated in host intestinal contents or feces and hence past their most resistant stage. The viabilities of Ascaris and Toxocara eggs that survived the digestion processes were greater in anaerobically treated than in aerobically treated material. Eggs from Hymenolepis were nonviable before use in the experiments. However, they were more effectively destroyed in aerobic digesters than in anaerobic digesters.     

Bacterial survival and association with sludge flocs during aerobic and anaerobic digestion of wastewater sludge under laboratory conditions.

The fate of indicator bacteria, a bacterial pathogen, and total aerobic bacteria during aerobic and anaerobic digestion of wastewater sludge under laboratory conditions was determined. Correlation coefficients were calculated between physical and chemical parameters (temperature, dissolved oxygen, pH, total solids, and volatile solids) and either the daily change in bacterial numbers or the percentage of bacteria in the supernatant. The major factor influencing survival of Salmonella typhimurium and indicator bacteria during aerobic digestion was the temperature of sludge digestion. At 28 degrees C with greater than 4 mg of dissolved oxygen per liter, the daily change in numbers of these bacteria was approximately -1.0 log10/ml. At 6 degrees C, the daily change was less than -0.3 log10/ml. Most of the bacteria were associated with the sludge flocs during aerobic digestion of sludge at 28 degrees C with greater than 2.4 mg of dissolved oxygen per liter. Lowering the temperature or the amount of dissolved oxygen decreased the fraction of bacteria associated with the flocs and increased the fraction found in the supernatant.     

Venturi aeration and thermophilic aerobic sewage sludge digestion in small-scale reactors

Summary The aeration performance of two venturi aeration reactors (operating volumes 381 and 251) was studied for an air-water system. It was found that the mass transfer coefficient (k _la) could be described in terms of the superficial gas velocity (V _s) alone by the simple expressionk _La=aV _infb ^supS with constantsa=0.313,b=0.579 for the 38-1 reactor anda=0.214,b=0.534 for the 25-1 reactor. A similar relationship was obeyed when the 38-1 reactor was aerated with a diffuser tile (a=17.0,b=1.52). A linear relationship betweenk _La and gas hold-up was observed for the 38-1 reactor with both venturi and diffuser aeration. The 25-1 reactor was used successfully for the thermophilic aerobic digestion of sewage sludge. A mean sludge temperature rise of 30°C was observed. Chemical oxygen demand, pH, and total solids content of the digested sludge differed significantly from the feed sludge and were similar to values obtained for full-scale thermophilic aerobic digestion. No significant differences between inorganic solids content, dissolved oxygen concentration, or redox potential were observed between feed and digested sludge.     

Transient response of aerobic and anoxic activated sludge activities to sudden substrate concentration changes

The state-of-the-art understanding of activated sludge processes as summarized in activated sludge models (ASMs) predicts an instantaneous increase in the biomass activity (which is measured, e.g., by the corresponding respiration rate OUR, NUR, etc.) under sudden substrate concentration changes. Experimental data (e.g., short-term batch respiration experiments under aerobic or anoxic conditions) collected for the calibration of the dynamic models (ASMs) often exhibit a transient phenomenon while attaining maximum activity, which cannot be explained by the current understanding of the activated sludge process. That transient phenomenon exhibits itself immediately upon addition of a substrate source to an endogenously respiring activated sludge sample and it usually takes a few minutes until the activated sludge reaches its maximum possible rate under given environmental conditions. This discrepancy between the state-of-the-art model and the experimental data is addressed in detail in this investigation. It is shown that the discrepancy is not caused by an error in the experimental set-up/data but it is rather due to model inadequacy. Among the hypotheses proposed, it appears that this transient response of the activated sludge most likely results from the sequence of intracellular reactions involved in substrate degradation by the activated sludge. Results from studies performed elsewhere with pure cultures (S. cerevisae and E. coli) support the hypothesis. The transient phenomenon can be described by a dynamic metabolic network model or by a simple first-order model, as adopted in this study. The transient phenomenon occurring in short-term batch respiration experiments is shown to interfere severely with parameter estimation if not modeled properly (2.8%, 11.5%, and 16.8% relative errors [average of three experiments] on Y(H), micro(maxH), and K(S), respectively). Proper modeling of this transient phenomenon whose time constant is on the order of minutes (1 to 3 min) is expected to contribute fundamentally to a better understanding and modeling of Orbal, carousel, and SBR-type treatment plants with fast-alternating process conditions, although such studies are beyond the scope of this report.     

Ultrasonic pretreatment for thermophilic aerobic digestion in industrial waste activated sludge treatment

In order to enhance the degradation efficiency of waste activated sludge (WAS) by thermophilic aerobic digestion, an ultrasonic pretreatment was examined. It was observed that ultrasonic pretreatment increased the solubilization of organic matter in the WAS and that the solubilization ratio of the organics increased during the first 30 min but did not extensively increase thereafter. Therefore, a pretreatment time of 30 min was determined to be the economical pretreatment time from the experimental results. From the digestion experiments, which was conducted using the WAS collected from an oil refinery plant in Inchon, Korea, investigating the effects of an ultrasonic pretreatment on thermophilic aerobic digestion, it was confirmed that the proposed ultrasonic pretreatment was effective at enhancing the release of the cellular components in WAS and the degradation of released components in the thermophilic aerobic digestion.     

Mesophilic and thermophilic aerobic digestion of municipal sludge in an airlift U-shape bioreactor

Aerobic digestion of primary and secondary sludges was studied in airlift bioreactors at mesophilic and thermophilic temperatures. The experimental studies were conducted with a laboratory U-shape airlift reactor (operating volume 23 liters) and in a pilot U-shape airlift reactor of 1150 liters operating volume. In the laboratory reactor, with cold (6°C) and concentrated (3–4% solids) feed of primary and secondary municipal sludge, a 30% volatile suspended solids (VSS) reduction was achieved with a hydraulic retention time (HRT) of 2·5 days. A VSS loading rate of 8·2 kg VSS/m³/day was achieved. This loading is comparable to that obtained in a pure-oxygen sparged, mixed reactor.In the pilot-plant reactor at mesophilic temperature (31–33°C), a VSS loading rate of 7·9 kg VSS/m³/day and a VSS reduction of 40% were achieved with a HRT of 4 days.     

污泥厌氧消化产酸发酵过程中乙酸累积机制

[目的]研究污泥厌氧消化产挥发性脂肪酸(VFA)过程中的有机物碳流的转化机制,阐明乙酸累积机理。[方法]研究溴乙烷磺酸盐(BES)和氯仿(CHCl3)抑制模型下中间代谢产物和气体的累积,检测各产乙酸功能菌群数量,推断污泥产酸发酵过程中的有机物碳流方向和乙酸累积机理。[结果]BES模型乙酸浓度达27 mmol/L,fhs基因拷贝数比对照组高2-3倍,产氢产乙酸菌略有下降。CHCl3模型乙酸浓度达22 mmol/L,fhs基因拷贝数比BES组低一个数量级,产氢产乙酸菌下降明显。[结论]BES特异性较高,除产甲烷菌外对其他厌氧产酸细菌没有影响,乙酸浓度增加并且其主要来源于水解发酵产酸以及同型产乙酸过程。氯仿除抑制产甲烷菌外,对同型乙酸菌和产氢产乙酸菌也有强烈的抑制作用。     

Sequential anaerobic/aerobic digestion of waste activated sludge: analysis of the process performance and kinetic study

Sequential anaerobic-aerobic digestion was applied to waste activated sludge (WAS) of a full scale wastewater treatment plant. The study was performed with the objective of testing the sequential digestion process on WAS, which is characterized by worse digestibility in comparison with the mixed sludge. Process performance was evaluated in terms of biogas production, volatile solids (VS) and COD reduction, and patterns of biopolymers (proteins and polysaccharides) in the subsequent digestion stages. VS removal efficiency of 40%, in the anaerobic phase, and an additional removal of 26%, in the aerobic one, were observed. For total COD removal efficiencies of 35% and 25% for anaerobic and aerobic stage respectively, were obtained. Kinetics of VS degradation process was analyzed by assuming a first order equation with respect to VS concentration. Evaluated kinetic parameters were 0.44 ± 0.20 d(-1) and 0.25 ± 0.15 d(-1) for the anaerobic stage and aerobic stage, respectively. With regard to biopolymers, in the anaerobic phase the content of proteins and polysaccharides increased to 50% and 69%, respectively, whereas in the subsequent aerobic phase, a decrease of 71% for proteins and 67% for polysaccharides was observed. The average specific biogas production 0.74 m(3)/(kg VS destroyed), was in the range of values reported in the specialized literature for conventional anaerobic mesophilic WAS digestion.     

Nitrogen transformations during biological aerobic treatment of pig slurry: effect of intermittent aeration on nitrous oxide emissions

A laboratory scale aeration treatment system was built to study the fate of nitrogen during aeration of pig slurry. For each run evaluated, the nitrogen mass balance was determined including measurement of the nitrous oxide gas emissions. Intermittent aeration led to a nitrogen removal of about 53% of the total nitrogen content of the raw slurry. About 18% of the total nitrogen content of the raw slurry was emitted as N2O during aeration with an aerobic to anoxic ratio equal to 0.625. In contrast, the extension of the anoxic period (aerobic to anoxic ratio = 0.375) allowed complete denitrification and avoided N2O emissions.     

Effects of microwave and alkali induced pretreatment on sludge solubilization and subsequent aerobic digestion

Individual and combined effects of microwave (MW) and alkali pretreatments on sludge disintegration and subsequent aerobic digestion of waste activated sludge (WAS) were studied. Pretreatments with MW (600 W-85 °C-2 min), conventional heating (520 W-80 °C-12 min) and alkali (1.5 g NaOH/L - pH 12-30 min) achieved 8.5%, 7% and 18% COD solubilization, respectively. However, combined MW-alkali pretreatment synergistically enhanced sludge solubilization and achieved 46% COD solubilization, 20% greater than the additive value of MW alone and alkali alone (8.5 + 18% = 26.5%). Moreover, the results of the batch aerobic digestion study on MW-alkali pretreated sludge showed 93% and 63% reductions in SCOD and VSS concentrations, respectively, at 16 days of SRT. The VSS reduction was 20% higher than that of WAS without pretreatment.     

Detection of the microbial activity of aerobic heterotrophic, anoxic heterotrophic and aerobic autotrophic activated sludge organisms with an electrochemical sensor

The microbial activity of aerobic heterotrophic, anoxic heterotrophic and aerobic autotrophic microorganisms in biological wastewater treatment was determined by means of an electrochemical bioactivity sensor. The development of the sensor resulted in a system which can determine the microbial activities that are relevant for effective wastewater treatment. The signals of the sensor system are proportional to the substrate degradation and it can show inhibiting effects on the biomass. The most important advantages of the system are: it is independent of O₂ consumption, the three most important types of metabolic activities in wastewater technology can be measured with one sensor, furthermore the measurement is suitable for automation and it is on-line. The result is a potential for the optimization of processes based on microbial activity.     

Inactivation of Salmonella during anaerobic digestion of sewage sludge

The inactivation of Salmonella duesseldorf in sewage sludge during anaerobic digestion was investigated at 35 and 48°C with mean retention periods of between 10 and 20 days. Digesters were fed daily with raw sludge containing added Salm. duesseldorf after removal of digested sludge. During steady operation, the levels of Salm. duesseldorf in the digested and the feed sludge were determined and their specific rates of decay were estimated. The latter were: (i) greater at 48°C than at 35°C for the same retention time; (ii) similar for retention periods greater than 15 d, but lower for 10 d; (iii) greater when the level of salmonellas in the feed was lower. Gas production, a measure of steady state, was gradually lost when the mean retention period was reduced to 6.7 d. In experiments in which a single dose of Salm. duesseldorf was added to digesting sludge, the inactivation appeared to follow first-order kinetics at 35°C and the decimal decay rate, 1.6/d, was similar to that in the daily feeding experiments (1.4/d) with larger and similar inocula of Salm. duesseldorf. At 48°C, however, the rate of inactivation declined with decreasing time from inoculation suggesting that the culture contained cells differing in thermal resistance. The degrees and rates of inactivation of salmonellas in those experiments were greater than in full-scale digesters, because the latter seldom operated under conditions ideal for inactivation or because indigenous salmonellas are more resistant.     

Evolution of bioaggregate strength during aerobic granular sludge formation

This work investigated the modification of aggregate properties during the formation of granular sludge in a sequencing batch airlift reactor (SBAR). The cohesion of biological aggregates was quantified by subjecting sludge samples to two different controlled shear stresses in a stirred reactor. For reference sludge (without granules), flocs broke and reformed easily, indicating that floc size was controlled by the turbulence micro-scale (Kolmogorov scale, here from 17 μm to 62 μm). In contrast, granules showed high strength which enabled them to resist turbulence and their size was no longer imposed by the Kolmogorov micro-scale. Different steps were observed during the granulation process: a first increase of aggregate cohesion associated with a decrease in sludge volume index (SVI), a growth of aggregates with detachment of fragile particles from the surface and, finally, an increase in the sizes of small and large granules to reach a pseudo-stable size distribution. Results suggest that small particles could have formed the seeds for new granules, as they were maintained in the bioreactor. Here, granular sludge was formed in an SBAR with a conventional settling time (30 min), i.e. without particle washout, and with a low superficial air velocity (SAV = 0.6 cm s⁻¹): it is thus demonstrated that high SAV and low settling time are not necessary to produce granules, but probably only accelerate the accumulation of granules. It is shown that the increase of cohesion is the initial phenomenon explaining the granule formation concomitantly with bacterial aggregates densification. It seems important, in the future, to investigate the reasons for this cohesion increase, which is possibly explained either by bacterial bounding interactions or the excretion of extracellular polymeric substances (EPS).     

Activated sludge transformation in anoxic condition

Summary Substantial increase of methanogens and obligate hydrogen producing acetogens is occurs when activated sludges are allowed to remain under anoxic conditions at either 25°C and 35°C, without oxygen free atmosphere or any external addition of reductants. Acetate addition to activated sludge stimulates anoxic growth of all anaerobes except for butyrate users. During this period, the sludge volume index decrease.     

Enrichment of denitrifying glycogen-accumulating organisms in anaerobic/anoxic activated sludge system

Denitrifying glycogen-accumulating organisms (DGAO) were successfully enriched in a lab-scale sequencing batch reactor (SBR) running with anaerobic/anoxic cycles and acetate feeding during the anaerobic period. Acetate was completely taken up anaerobically, which was accompanied by the consumption of glycogen and the production of poly-beta-hydroxy-alkanoates (PHA). In the subsequent anoxic stage, nitrate or nitrite was utilized as electron acceptor for the oxidation of PHA, resulting in glycogen replenishment and cell growth. The above phenotype showed by the enrichment culture demonstrates the existence of DGAO. Further, it was found that the anaerobic behavior of DGAO could be predicted well by the anaerobic GAO model of Filipe et al. (2001) and Zeng et al. (2002a). The final product of denitrification during anoxic stage was mainly nitrous oxide (N(2)O) rather than N(2). The data strongly suggests that N(2)O production may be caused by the inhibition of nitrous oxide reductase by an elevated level of nitrite accumulated during denitrification. The existence of these organisms is a concern in biological nutrient removal systems that typically have an anaerobic/anoxic/aerobic reactor sequence since they are potential competitors to the polyphosphate-accumulating organisms.