A pilot study on a step-feeding anoxic/oxic activated sludge system.

A four stage pilot plant of step-feed biological nutrient removal (BNR) was employed to investigate reactor performance and process stability. The results obtained showed that step-feed BNR is efficient and cost-effective for nitrogen and carbonaceous removal from municipal wastewater. The total average removal efficiencies of COD, NH3-N, TN and TP could reach as high as 89.5, 97.8, 73 and 75%, respectively, with 50% of return activated sludge (RAS), 9 h of hydraulic retention time (HRT) and 20 d of sludge retention time (SRT). Step-feed BNR is an alternative and effective technology of nutrient removal for municipal wastewater treatment.     

Performance analysis of anoxic selector in upgrading activated sludge process in tropical climate.

The paper describes and analyses the performance of anoxic selectors in upgrading activated sludge process in a municipal wastewater treatment plant under tropical climate, where poor sludge settleability is a problem due to elevated temperature. Site monitoring and laboratory experiment were conducted to study the denitrification, sludge settleability, kinetics, mass balance, pH and alkalinity variation in the system. The sludge settleability measured in Sludge Volume Index (SVI) was observed to improve with the increasing degree of denitrification in the anoxic selector. Under well-developed stable state, an average SVI value of 136 ml/g was achieved, which was significantly lower than the value of 250 ml/g before the application of anoxic selector. The specific reaction rates for denitrification and nitrification at 30 degrees C were 0.06 mg NO3-N/mg MLSS day and 0.08 mg NH4-N/mg MLSS day, respectively. The overall efficiencies of nitrification and denitrification were 86 percent and 55.4 percent, respectively, with an alkalinity recovery ratio of 15.4 percent. 32 percent of total COD removed was used up as electron donor in the denitrification process. However, due to absence of the internal Mixed Liquor Recirculation (MLR), a higher degree of denitrification occurred in the secondary sedimentation tank than in the anoxic zone. Issues for further studies are also discussed.     

Behaviour of activated sludge from a system with anoxic phosphate uptake

Activated sludge from a new activated sludge modification for biological phosphorus and nitrogen removal was studied. Population dynamics and the phenomenon of anoxic phosphate uptake with simultaneous denitrification were investigated.The ability of the process to remove nutrients and to suppress filamentous bulking was studied. The course of phosphate concentrations along the tested system showed an anoxic phosphate uptake with simultaneous denitrification. The mechanism of anoxic phosphate uptake was confirmed using kinetic batch tests. © 1998 IAWQ. Published by Elsevier Science Ltd     

Extracellular redox activity in activated sludge

The tetrazolium salts triphenyltetrazoltum chloride (TTC) and 2-(p-iodophenyl)-3-(p-nitrophenyl)-5phenylteirazolium chloride (INT) have been used extensively for the measurement of respiratory activity in natural waters, drinking water, sediments and activated sludge. Usually this is done spectrophotometrically after extracting the reduced formazan salt. Other tetrazolium salts like 5-cyano-2,3-di-4-tolyl-tetrazolium chloride (CTC) are reduced to formazan salts which are fluorescent. This property has been used to microscopically determine the proportion of metabolically active cells in a sample. The observation that CTC is also reduced extracellularly prompted an investigation of the role of this activity in activated sludge. In order to test the assumption that extracellular reduction is stimulated by recalcitrant molecules a bench-scale sequencing batch reactor (SBR) was fed with synthetic wastewater containing either lignin or amylose as well as hexanoic acid, glycerol and sucrose as organic constituents. About 4 times as much tetrazolium salt 3′-(I-|(phenylamino-)carbonyl]-3,4-tetrazolium)-bis (4-methoxy-6-nitro) benzene-sulfonic acid hydrate (XTP) was reduced by extracts containing extracellular polymeric substances (EPS) produced in the presence of lignin than by extracts derived from amylose-fed cells. These differences are statistically significant at P > 0.05. Extracellular redox activity m lignin-containing activated sludge accounted for approximately 2% of total redox activity. In addition to experiments involving reactors fed with different substrates, the extracellular redox activity was measured microscopically using a Zeiss confocal laser microscope. Planktonically grown cells of Escherichia coli or activated sludge flocs were treated with CTC and the nucleic acid stain PicoGreen which fluoresces in the red and green visible light regions, respectively. In Escherichia coli preparations, CTC-formazan crystals were found exclusively inside cells. By contrast, activated sludge flocs revealed crystals both inside and outside the cells. Formaldehyde-treated flocs did not show any reduction of CTC. The extracellular crystals accounted for up to 50% of the total CTC reduction. It follows that extracellular redox activity is associated with the production of EPS. Its role may be to help microbial systems to deal with organic materials which are difficult to utilize.     

Effect of low power ultrasound on aerobic/anoxic digestion of activated sludge

The effect of low power ultrasound on aerobic/anoxic digestion of activated sludge was investigated. First, dehydrogenate activity (DHA) and special oxygen uptake rate (SOUR) were detected to indicate the change of biological activity of sludge induced by ultrasound. Single-factor experiments showed that when the ultrasonic frequency was 28 kHz, the optimal ultrasonic density and exposure time were 0.15 W/mL and 10 min, respectively. Through orthogonal tests, the optimal ultrasonic parameters for aerobic/anoxic digestion were found to be a ultrasonic density of 0.15 W/mL, ultrasonic exposure time of 10 min and interval time of 12 h. The aerobic/anoxic digestion was improved by ultrasonic pretreatment, the VSS reduction at the optimal conditions was increased by 35.73%, compared with the control. It can be concluded that low power ultrasonic pretreatment is a valid method for improving aerobic/anoxic biodegradability of activated sludge.     

Microbial Fe(II)-oxidation by nitrate in activated sludge

The oxidation of Fe(II) to Fe(III) by addition of nitrate and nitrite to activated sludge was studied to determine whether the process was biological or chemical (chemodenitrification). It was shown that the process was mainly biological, although the microorganisms involved have not yet been described. Investigations in a full scale treatment plant suggested that the process most likely took place m the anoxic (denitrification) tank. Details of the kinetics and stoichiometry have not yet been determined, but the process may he of significance for keeping Fe(III) oxidized, which is important for P-removal and for floc structure. Furthermore, in some treatment plants, the oxidation may also be of significance for nitrate removal (denitrification).     

The effect of anoxic selectors on sludge bulking.

The effect of the substrate feeding pattern on sludge settleability in denitrifying systems was investigated. The feeding pattern was simulated in two sequencing batch reactors (SBR). SBR1 simulates a system with a limiting substrate (acetate) and a non-limiting electron acceptor (nitrate), while SBR2 simulates the opposite condition. Sludge volume index (SVI) was the main observed parameter. The stoichiometry, substrate uptake rate, storage capacity as well as sludge morphology and mixed culture population dynamics were also determined. Results show that independent of the feeding pattern, the settleability of the sludge remained good (SVI approximately 50 mL/g) and stable. Neither the concentration of the acetate nor of the nitrate offered any limitations on sludge settleability, assuring that a good bio-P activity is present in nitrate limiting systems. Filamentous bacteria belonging to the TM group were usually present, although in low numbers. Despite the diffused, irregularly shaped and open floc structure, good settleability was observed. The results obtained were in line with the hypothesis on diffusion based selection.     

Simultaneous biological removal of sulphide and nitrate by autotrophic denitrification in an activated sludge system.

The feasibility of an autotrophic denitrification process in an activated sludge reactor, using sulphide as the electron donor, was tested for simultaneous denitrification and sulphide removal. The reactor was operated at nitrate (N) to sulphide (S) ratios between 0.5 and 0.9 to evaluate their effect on the N-removal efficiency, the S-removal efficiency and the product formation during anoxic oxidation of sulphide. One hundred per cent removal of both nitrate and sulphide was achieved at a NLR of 7.96 mmol N-L(-1) x d(-1) (111.44 mg NO3- -N x L(-1) x d(-1)) and at a N/S ratio of 0.89 with complete oxidation of sulphide to sulphate. The oxygen level in the reactor (10%) was found to influence the N-removal efficiency by inhibiting the denitrification process. Moreover, chemical (or biological) oxidation of sulphide with oxygen occurred, resulting in a loss of the electron donor. FISH analysis was carried out to study the microbial population in the system.     

Measurement and modelling of ordinary heterotrophic organism active biomass concentrations in anoxic/aerobic activated sludge mixed liquor.

Ordinary heterotrophic organism (OHO) active biomass (ZBH) is a key parameter in models for activated sludge systems, which defines quantitatively the kinetic rates of relevant processes. However, ZBH has not been measured directly with consistent success: a simple respirometric batch test has provided varying correspondence between measured and theoretical concentrations. In this paper, the batch test is applied to mixed liquors drawn from well defined anoxic/aerobic parent systems at 10 and 20 d sludge ages, with consistent but poor correspondence between measured and theoretical values. In contrast, aerobic digestion batch tests on the same mixed liquors give good correspondences. It is concluded that the differences between theoretical and batch test measured values are due to the batch test method itself and its interpretation. It is found that the batch test conditions (particularly the substrate/ZBH ratio) influence the kinetic constants derived from the data, and hence the ZBH estimate. Two kinetic models with two competing OHO populations, a fast and a slow grower, are developed and applied to the batch tests and parent systems. The first model is based on kinetic selection only, while the second includes additional metabolic selection. Both models can account for the observations in the batch tests, but the second provides greater consistency between simulations of the parent systems and batch tests.     

Potential of activated sludge ozonation.

The disposal of sewage sludge and the agricultural use of stabilised sludge are decreasing due to more stringent regulations in Europe. An increasing fraction of sewage sludge must therefore be dewatered, dried, incinerated and the ashes disposed of in landfills. These processes are cost-intensive and also lead to the loss of the valuable phosphate resources incorporated in the sludge ash. The implementation of processes that could reduce excess sludge production and recycle phosphate is therefore recommended. Partial ozonation of the return sludge of an activated sludge system significantly reduces excess sludge production, improves the settling properties of the sludge and reduces bulking and scumming. The solubilised COD will also improve denitrification if the treated sludge is recycled to the anoxic zone. However, ozonation partly kills nitrifiers and could therefore lead to a decrease of the effective solid retention time of the nitrifier, thus reducing the safety of the nitrification. This paper discusses the effect of ozonation on sludge reduction, the operating stability of nitrification, the improvement of denitrification and also presents an energy and cost evaluation.     

Potential of activated sludge disintegration.

The disposal of sewage sludge and the agricultural use of stabilised sludge are decreasing due to more stringent regulations in Europe. An increasing fraction of sewage sludge must therefore be dewatered, dried, incinerated and the ashes disposed of in landfills. These processes are cost-intensive and also lead to the loss of valuable phosphate resources incorporated in the sludge ash. The implementation of processes that could reduce excess sludge production and recycle phosphate is therefore recommended. Disintegration of biological sludge by mechanical, thermal and physical methods could significantly reduce excess sludge production, improve the settling properties of the sludge and reduce bulking and scumming. The solubilised COD could also improve denitrification if the treated sludge is recycled to the anoxic zone. However, disintegration partly inhibits and kills nitrifiers and could therefore shorten their effective solid retention time, thus reducing the safety of the nitrification. This paper discusses the potential of disintegration on sludge reduction, the operating stability of nitrification, the improvement of denitrification and also presents an energy and cost evaluation.     

Protein extraction from activated sludge.

Two methods for the separation of protein originating from activated sludge were compared. In one method, the total protein was isolated out of the activated sludge (crude extract). These samples included all dissolved proteins originating from the bacterial cells and biofilm made up of extracellular polymeric substances (EPS). Every time polyacrylamide gel electrophoresis (PAGE) was done, the protein bands from samples of crude extract were covered by polymeric substances including carbohydrates, uronic acids or humic compounds. Using the immunoblot technique it was possible to demonstrate the presence of the heat shock protein HSP70 in crude extracts of activated sludge. The comparison of protein fingerprints required that clear and distinct bands appear on the PAGE analysis. To this end, a procedure to separates bacterial cells from the EPS was developed. Bacterial cells were separated by incubation with EDTA and subsequent filtration. The isolated cells were directly incubated in a sample buffer.     

基于LabVIEW的推流式活性污泥法动态仿真研究

介绍了利用LabVIEW6.1及其数学子模板中一阶微分方程组的龙格库塔算法函数,根据活性污泥法1号数学模型一阶微分方程组的特征,开发出适用于推流式活性污泥法仿真系统的方法。应用该仿真系统模拟了重庆某污水处理厂的实际运行记录,结果表明仿真效果良好。该方法降低了软件的编程难度。     

Co-conditioning and dewatering of alum sludge and waste activated sludge.

Co-conditioning and dewatering behaviors of alum sludge and waste activated sludge were investigated. Two different sludges were mixed at various ratios (2:1; 1:1; 1:2; 1:4) for study. Capillary suction time (CST) and specific resistance to filtration (SRF) were utilized to assess sludge dewaterability. Relatively speaking, waste activated sludge, though of higher solid content, was more difficult to be dewatered than alum sludge. It was found that sludge dewaterability and settlability became better with increasing fraction of alum sludge in the mixed sludge. Dosage required of the cationic polyelectrolyte (KP-201C) for dewatering was reduced as well. It is proposed that alum sludge acts as skeleton builder in the mixed sludge, and renders the mixed sludge more incompressible which is beneficial for sludge dewatering. Implications of the results of the study to the sludge management plan for Taipei City that generates both alum sludge and waste activated sludge at significant amount are also discussed. The current sludge treatment and disposal plan in Metropolitan Taipei could be made more cost-effective.     

A model of the redox measurement in aerated activated sludge.

A number of investigations have shown that pH, oxygen concentration, and the organic load affect the measured redox potential in aerated activated sludge. In this investigation, a model of the redox measurement is proposed. It is a kinetic model based on the necessary balance between the number of electrons accepted by and donated to a metal electrode in the sludge. The potential changes in order to maintain this balance. The model variables are pH, oxygen concentration, temperature and the activity of reduced compounds. Data from a wastewater treatment plant is analyzed using this model and it is shown that the model can be fitted to results from other published investigations. However we cannot as yet demonstrate a correlation between the activity of reduced compounds and the organic load on the sludge. The ultimate objective of this work is to extract more information from the redox measurement. This is a practical interest because the redox measurement is inexpensive and requires little maintenance.     

A comparison of conventional activated sludge and low sludge production strategies for advanced treatment of kraft pulp mill effluent.

Parallel laboratory investigations were conducted to examine aspects of two distinct but related bioprocess strategies for low sludge production in the treatment of the same TCF kraft pulp mill effluent. The purpose of this article has been to compare the performance results from these two bench-scale trials with respect to nutrient demands, nutrient discharge, COD removal, and waste sludge characteristics. The LSP (Low Sludge Production) process can be used to significantly reduce sludge yield with excellent sludge characteristics. These sludge characteristics seemed to be related to elevated protozoan grazing pressures. The BAS (Biofilm-Activated Sludge) process achieves similar reduced sludge yields and sludge characteristics while at the same time significantly reducing the nutrient demands and discharge levels. For both LSP and BAS process optimization, the selector nutrient loading is critical to the overall process performance. Selector nutrient requirements are distinct from the overall process nutrient requirements.     

Bacterial composition of activated sludge--importance for floc and sludge properties.

Activated sludge flocs consist of numerous constituents which, together with other factors, are responsible for floc structure and floc properties. These properties largely determine the sludge properties such as flocculation, settling and dewaterability. In this paper we briefly review the present knowledge about the role of bacteria in relation to floc and sludge properties, and we present a new approach to investigate the identity and function of the bacteria in the activated sludge flocs. The approach includes identification of the important bacteria and a characterization of their physiological and functional properties. It is carried out by use of culture-independent molecular biological methods linked with other methods to study the physiology and function, maintaining a single cell resolution. Using this approach it was found that floc-forming properties differed among the various bacterial groups, e.g. that different microcolony-forming bacteria had very different sensitivities to shear and that some of them deflocculated under anaerobic conditions. In our opinion, the approach to combine identity with functional analysis of the dominant bacteria in activated sludge by in situ methods is a very promising way to investigate correlations between presence of specific bacteria, and floc and sludge properties that are of interest.     

A comparison between model and rule based control of a periodic activated sludge process

Two strategies for control of nitrogen removal in an alternating activated sludge plant are compared. One is based on simple model predictions determining the cycle length at the beginning of each cycle. The other is based on simple rules relating present ammonia and nitrate concentrations. Both strategies are close in efficiency measured as effluent total (inorganic) nitrogen and both perform better than using fixed phase lengths for a test scenario describing a typical dry weather diurnal variation. After modifying the criterion functions of the rule based strategy the two strategies are found to produce equivalent controls, which means that the optimal criteria of the model based strategy can be fulfilled without the need for model predictions.     

Hydrogen sulphide removal by activated sludge diffusion.

Odours from wastewater treatment plants comprise a mixture of various gases, of which hydrogen sulphide (H2S) is the main constituent. Microorganisms commonly found in wastewater can degrade sulphurous compounds. Therefore, the use of activated sludge (AS) for odour control offers an alternative to traditional waste gas treatment processes, such as biofilters, bioscrubbers and biotrickling filters, both in practical terms (use of existing facilities) and economically (minimal capital cost). The performance of AS diffusion as a bioscrubber for removing H2S at concentrations at 25, 75 and 150 ppmv was evaluated. Pilot-scale trials were undertaken using parallel 60-L aeration tanks and 20-L clarifier reactors at the Bedford Sewage Treatment Works, Carington, UK. Olfactometry measurements were also carried out to determine whether there was any increase in odour concentration owing to H2S diffusion. Hydrogen sulphide removal rates of 100% were obtained, with no noticeable increase in odour concentration throughout the trials as measured by olfactometry. Odour concentration was highest at the beginning of the trials and lowest during the high H2S dosing period, with similar values being obtained for test and control. It was concluded that AS diffusion is an effective bioscrubber for the removal of H2S odour.