Effect of operation conditions on biological Fe oxidation with rotating biological contactors

Oxidation of ferrous iron (Fe²⁺) by acidophilic iron-oxidizing bacteria was conducted with a laboratory-scale rotating biological contactor (RBC) in order to investigate the effects of Fe²⁺ concentration, pH, temperature and rotation rate. Fe²⁺ oxidation rate on RBC was expressed as a first-order reaction with respect to Fe²⁺ concentration when it was below 150 mg 1⁻¹. Fe²⁺ oxidation rate was not affected by pH from 1.0 to 2.6 nor by temperatures from 10 to 40°C but was accelerated by increasing peripheral disk velocity from 4.7 to 28.2 m min⁻¹.     

Dynamics of nematodes in a high organic loading rotating biological contactors

Nematode diversity and dynamics of a full-scale rotating biological contactor plant (RBC) has been studied. Analysis of biofilm composition showed a well-established zoning of microfauna among the three RBC sections analysed. Nematodes appeared to be the dominant group within the larger microfauna populations with average abundances between 200 and 300ind/mg or 8000 and 17000ind/cm(2). The most abundant nematode species were Diplogasteritus nudicapitatus and Paroigolaimella coprophages and, to a lesser extent, Paroigolaimella bernensis and Steinernema intermedia. The relationship between nematodes and filamentous bacteria (specifically the genus Beggiatoa) was the most significant biotic relationship found, and to a lesser extent, nematodes with ciliates. The relationship between the abundance of nematode species and the physical-chemical variables suggests that nematodes may be good indicators of low pollutant load levels in the entry of the RBC system. Finally, the results indicate that nematodes may have a relevant role for a good biofilm development.     

Kinetic model of excess activated sludge thermohydrolysis

Thermal hydrolysis of excess activated sludge suspensions was carried at temperatures ranging from 423 K to 523 K and under pressure 0.2-4.0 MPa. Changes of total organic carbon (TOC) concentration in a solid and liquid phase were measured during these studies. At the temperature 423 K, after 2 h of the process, TOC concentration in the reaction mixture decreased by 15-18% of the initial value. At 473 K total organic carbon removal from activated sludge suspension increased to 30%.It was also found that the solubilisation of particulate organic matter strongly depended on the process temperature. At 423 K the transfer of TOC from solid particles into liquid phase after 1 h of the process reached 25% of the initial value, however, at the temperature of 523 K the conversion degree of ‘solid’ TOC attained 50% just after 15 min of the process.In the article a lumped kinetic model of the process of activated sludge thermohydrolysis has been proposed. It was assumed that during heating of the activated sludge suspension to a temperature in the range of 423-523 K two parallel reactions occurred. One, connected with thermal destruction of activated sludge particles, caused solubilisation of organic carbon and an increase of dissolved organic carbon concentration in the liquid phase (hydrolysate). The parallel reaction led to a new kind of unsolvable solid phase, which was further decomposed into gaseous products (CO₂). The collected experimental data were used to identify unknown parameters of the model, i.e. activation energies and pre-exponential factors of elementary reactions. The mathematical model of activated sludge thermohydrolysis appropriately describes the kinetics of reactions occurring in the studied system.     

Effect of HRT,SRT and temperature on the performance of activated sludge reactors treating bleached kraft mill effluent

Laboratory scale research on the effects of hydraulic retention time (HRT), solids residence time (SRT), high operating temperatures and temperature shocks on activated sludge (AS) treatment of kraft pulping effluent was performed using two 51 continuously fed bioreactors. Baseline performance of the reactors was established at 35°C by operating the reactors at steady state (HRT 10–12 h; SRT 12–15 d) for a period of two months. During this period percent removal of BOD, COD, and toxicity averaged 87.9 ± 4.3, 32.4 ± 9.0, 97.7 ± 0.4, respectively. Reactor MLVSS was 1675 ± 191 mg/l, effluent VSS was 45.5 ± 11.2 mg/l and specific oxygen uptake rate was 16.5 ± 3.3 mg O₂/g MLVSS·h. Varying HRT between 12 and 4 h and SRT between 5 and 15 d indicated that HRT had more of an effect on treatment performance than SRT. Longer HRTs led to improved BOD, COD, toxicity and AOX removal, while longer SRTs were not shown to significantly affect performance. Shorter HRTs and longer SRTs led to significant increases in specific oxygen uptake rates (SOURs). For reactors operated at temperatures between 41 and 50°C, removal of BOD and acute toxicity was comparable to that observed at mesophilic temperatures. COD removal was improved over that observed at mesophilic temperatures, possibly as a result of improved dissolution of organic compounds at the higher temperatures. The effect of temperature shocks (decreases of 7°, 16.5°, 32° and 40.5°C) on reactor performance was proportional to the size of the disturbance. Reactor performance returned to pre-shocking levels within 12–24 h for the two smaller temperature shocks. Approximately 72 h was needed for the system to recover from the two larger temperature shocks (32° and 40.5°C).     

Systematic study of the effect of operating variables on reactor performance and microbial diversity in laboratory-scale activated sludge reactors

Biological treatment processes are “complex systems” where many different kinds of microbes grow and interact in a dynamic manner. Understanding the relationship between microbial diversity and bioreactor performance could facilitate the optimisation of bioreactor design and enable the solution of bioreactor-related problems. However, systematic studies of the effects of operating variables on microbial diversity and reactor performance are rare. In this study, we determined the effects of different operating conditions and system configurations on the performance of laboratory-scale activated sludge reactors and microbial diversity, based on experiments designed using the factorial design approach. We found that the overall system performance and the diversity of the microbial communities in the reactors were affected by changes in the operating parameters. However, the relationship between diversity and performance was sometimes counterintuitive, as increases in system performance were not always associated with increased community diversity. Reactor configuration and addition of soil had the biggest effects on reactor performance, while the effects of organic loading rates and feed composition were less marked. Of all these parameters, reactor configuration was the only one that had a consistent effect on reactor community diversity.     

The use of dimensionless groups in the design of activated sludge reactors

Dimensional analysis techniques were applied to the differential equations for a well mixed activated sludge reactor to obtain generalized information on the effect of process variables on conversion of substrate. The Monod model was used to represent the reaction kinetics since it also contains first and second order kinetics as special cases. The performance of a steady state reactor could be described in terms of five dimensionless groups involving the important combinations of process variables. A simple criterion was developed to predict washout conditions for reactors without recycle of biological solids. Under certain operating conditions, activated sludge reactors were shown to possess an inherent degree of self control to compensate for changes in feed concentration.     

Effect of settling on performance of the upflow anaerobic sludge bed reactors

The effects of settler volume on the start-up and steady-state performance of 41. laboratory upflow sludge bed reactors treating bean blanching waste of 10,000 mg COD l⁻¹ were determined. The rate of start-up, as well as the maximum loading rate, increased with increased settler volume and performance. A loading rate of 30 kg COD m⁻³ day⁻¹ (based on reactor volume alone) and a COD removal of 95% was obtained with a 21. settling flask and a 4 to 1 recirculation rate. Without a settler, the maximum loading rate was 10 kg COD m⁻³ day⁻¹. The sludge was flocculent rather than granular. Sludge profiles and characteristics in the reactors and settlers were determined.     

Cultivation of anaerobic granular sludge in UASB reactors with aerobic activated sludge as seed

Methanogenic bacteria of 10⁸ g SS⁻¹ in the activated sludges from an aeration tank treating sewage and from a secondary sedimentation tank of an activated sludge plant treating textile dyeing wastewater were enumerated by the Most Probable Number (MPN) technique. By using the two activated sludges as the seed material, anaerobic granular sludges were obtained at 35°C in two lab-UASB reactors having volumes of 29 and 481, and treating a glucose molasses solution of 1000–3500 mg COD 1⁻¹ and citrate wastewater of 20,000–36,000 mg COD 1⁻¹ respectively. The characteristics of granulation using the activated sludge as the seed were similar to those using digested sewage sludge as the seed. It is shown that activated sludge is readily available seed material for an anaerobic reactor. The growth of methanogenic bacteria in the activated sludge can be attributed to the existence of some anaerobic nuclei in the activated sludge flocs. The factors for the cultivation of granular sludge by using the activated sludge are also discussed.     

Effects of integrated fixed film activated sludge media on activated sludge settling in biological nutrient removal systems

Integrated fixed film activated sludge (IFAS) is an increasingly popular modification of conventional activated sludge, consisting of the addition of solid media to bioreactors to create hybrid attached/suspended growth systems. While the benefits of this technology for improvement of nitrification and other functions are well-demonstrated, little is known about its effects on biomass settleability. These effects were evaluated in parallel, independent wastewater treatment trains, with and without IFAS media, both at the pilot (at two solids residence times) and full scales. While all samples demonstrated good settleability, the Control (non-IFAS) systems consistently demonstrated small but significant (p < 0.05) improvements in settleability relative to the IFAS trains. Differences in biomass densities were identified as likely contributing factors, with Control suspended phase density > IFAS suspended phase density > IFAS attached phase (biofilm) density. Polyphosphate content (as non-soluble phosphorus) was well-correlated with density. This suggested that the attached phases had relatively low densities because of their lack of anaerobic/aerobic cycling and consequent low content of polyphosphate-accumulating organisms, and that differences in enhanced biological phosphorus removal performance between the IFAS and non-IFAS systems were likely related to the observed differences in density and settleability for the suspended phases. Decreases in solids retention times from 8 to 4 days resulted in improved settleability and increased density in all suspended phases, which was related to increased phosphorus content in the biomass, while no significant changes in density and phosphorus content were observed in attached phases.     

Response of biological reactors to the addition of powdered activated carbon

A biological reactor operating on a synthetic feed was subjected to impulse and step disturbances of glucose and phenol. Powdered activated carbon was applied either as an impulse or as a step change to control reactor conditions. With glucose as the substrate, the addition of powdered carbon reduced the steady state TOC concentrations significantly but only slightly moderated the concentration transients after an upset. For impulse inputs of phenol, the simultaneous addition of carbon greatly reduced the magnitude of the transient changes in concentration. With step inputs of phenol, carbon addition rapidly lowered the phenol concentration in the reactor and permitted continuous operation with inlet phenol concentrations above 1000 mg 1⁻¹. Thus, powdered carbon can provide an effective control method for maintaining effluent quality in the presence of toxic upsets.     

Thickening of waste activated sludge by biological flotation

Waste activated sludge was thickened by biological flotation without polymer flocculant dosage. The BIOFLOT® process utilizes the denitrifying ability of activated sludge bacteria. Gaseous products of anaerobic nitrate reduction cause spontaneous flotation of the sludge suspended solids. Laboratory tests confirmed the dependence of sludge thickening efficiency on available nitrate concentration, flotation time and temperature. Full-scale experiments were performed in a fully automatized unit for discontinuous sludge thickening from wastewater treatment plants with a capacity of up to 5000 I.E. Waste activated sludge from wastewater treatment plants at Pisek. Milevsko and Björnlunda was thickened from 6.2, 10.7 and 3.5 g/l MLSS to 59.4, 59.7 and 66.7 g/t MLSS, respectively. Concentrations of COD, ammonium and phosphate ions were decreased in sludge water. The average nitrate consumption for bioflotation was 21.2 mg NO₁⁻ per 1 g of MLSS of activated sludge. Flotation time ranged from 4 to 48 h.     

Effects of free cyanide on microbial communities and biological carbon and nitrogen removal performance in the industrial activated sludge process

The changes in process performance and microbial communities under free cyanide (CN⁻) were investigated in a lab-scale activated sludge process treating industrial wastewater. The performance of phenol degradation did not appear to be adversely affected by increases in CN⁻ concentrations. In contrast, CN⁻ was found to have an inhibitory effect on SCN⁻ biodegradation, resulting in the increase of TOC and COD concentrations. Nitratation also appeared to be inhibited at CN⁻ concentrations in excess of 1.0 mg/L, confirming that nitrite-oxidizing bacteria (NOB) is more sensitive to the CN⁻ toxicity than ammonia oxidizing bacteria (AOB). After CN⁻ loads were stopped, SCN⁻ removal, denitrification, and nitrification inhibited by CN⁻ were recovered to performance efficiency of more than 98%. The AOB and NOB communities in the aerobic reactor were analyzed by terminal restriction fragment length (T-RFLP) and quantitative real-time PCR (qPCR). Nitrosomonas europaea lineage was the predominant AOB at all samples during the operation, but an obvious change was observed in the diversity of AOB at the shock loading of 30 and 50 mg/L CN⁻, resulting in Nitrosospira sp. becoming dominant. We also observed coexisting Nitrospira and Nitrobacter genera for NOB. The increase of CN⁻ loading seemed to change the balance between Nitrospira and Nitrobacter, resulting in the high dominance of Nitrobacter over Nitrospira. Meanwhile, through using the qPCR, it was observed that the nitrite-reducing functional genes (i.e., nirS) were dominant in the activated sludge of the anoxic reactor, regardless of CN⁻ loads.     

Diuron biodegradation in activated sludge batch reactors under aerobic and anoxic conditions

Diuron biodegradation was studied in activated sludge reactors and the impacts of aerobic and anoxic conditions, presence of supplemental substrate and biomass acclimatization on its removal were investigated. Diuron and three known metabolites, namely DCPMU (1-(3,4-dichlorophenyl)-3-methylurea), DCPU (1-3,4-dichlorophenylurea) and DCA (3,4-dichloroaniline), were extracted by solid-phase extraction (dissolved phase) or sonication (particulate phase) and determined using High Performance Liquid Chromatography-Diode Array Detector (HPLC-DAD). During the experiments only a minor part of these compounds was associated with the suspended solids. Under aerobic conditions, almost 60% of Diuron was biodegraded, while its major metabolite was DCA. The existence of anoxic conditions increased Diuron biodegradation to more than 95%, while the major metabolite was DCPU. Mass balance calculation showed that a significant fraction of Diuron is mineralized or biotransformed to other unknown metabolites. The presence of low concentrations of supplemental substrate did not affect Diuron biodegradation, whereas the acclimatization of biomass slightly accelerated its elimination under anoxic conditions. Calculation of half-lives showed that under aerobic conditions DCPMU, DCPU and DCA are biodegraded much faster than the parent compound. In the future, the sequential use of anoxic and aerobic conditions could provide sufficient removal of Diuron and its metabolites from runoff waters.     

The effect of inorganic salts on the activated sludge process performance

The effect of inorganic salts such as sodium chloride and sodium sulfate on the performance of the activated sludge process was examined. When proper acclimation procedures were followed, the adverse effects of salts on the process were minimized. One of the parameters monitored, effluent suspended solids, had very low values (less than 10 mg l⁻¹) up to an inflow sodium chloride concentration of about less than 35 gl⁻¹. The chemical oxygen demand of the effluent increased steadily with increasing sodium chloride concentrations, but biochemical oxygen demand values remained very low (less than 5 mg l⁻¹) which indicated that the increase in chemical oxygen demand was due to the portion that cannot be degrated biologically. The effect of sodium sulfate on the system was even less profound. In addition to the effluent being very clear and low in suspended solids, the chemical oxygen demand removal efficiency remained high.     

Optimization of activated sludge reactor configuration: : kinetic considerations

To evaluate and design staged activated sludge systems it is necessary to determine the biomass requirement for a given configuration. This depends on both kinetics and treatment requirements. We present a procedure to determine the optimum reactor configuration for a range of influent and effluent substrate concentrations, half saturation coefficients, and number of tanks in series for both inhibitory and non-inhibitory substrates. Dimensionless plots of the results show the minimum biomass requirement of the series relative to that for a single CSTR and the optimal relative sizes of the tanks. The plots may be used directly for staged system design and lead to the following conclusions: three tanks in series is generally best, high influent substrate concentrations and stringent discharge requirements increase the benefit of staging, and optimal tank sizing is significantly better than using equal sized tanks.     

Evaluation of empirical formulae for estimation of the longitudinal dispersion in activated sludge reactors

Tracer studies are widely applied to characterize the hydraulic properties of reactors. In the case of activated sludge reactors, however, tracer test results are difficult to interpret due to internal and returned activated sludge recirculation. Empirical formulae can be considered as an alternative method of estimating the hydraulic conditions within the activated sludge reactor. The aim of this study is to evaluate accuracy of four empirical formulae for the full-scale conditions based on the results of tracer studies performed at the Rock Creek Wastewater Treatment Plant (WWTP) in Hillsboro, OR (USA). Values of the dispersion coefficient, E(L), were first estimated using a 1-D advection-dispersion equation and setting a sum of squares of differences between the observed and calculated tracer concentrations to a minimum. The estimated values of E(L) coefficient remained within the range of 1043-1580 m2/h. The best approximation of dispersion was obtained from the formula of Fujie et al. (1983, J. Ferment. Technol. 63(3), 295). Also the formula of Murphy and Boyko (1970, J. San. Eng. ASCE 96(2), 211) generated E(L) values of the same order as the optimum E(L). The accuracy of these formulae was further confirmed based on the results of studies reported in the literature.     

Flocculation of activated sludge flocs by stimulation of the aerobic biological activity

Activated sludge flocs are known to deflocculate under short-term anaerobic conditions, but little is known about possible reflocculation under subsequent aerobic conditions. When activated sludge flocs from two wastewater treatment plants deflocculated under anaerobic conditions with well-defined shear conditions, they could be almost, but not completely, reflocculated by aeration for 1-2 h under the same shear conditions. If the biological activity was reduced by adding azide, chloramphenicol or by decreasing the temperature, no or only very little reflocculation took place. This indicated that the reflocculation was under direct or indirect microbial control. Only a small part of the reflocculation was due to improved flocculation properties obtained by oxidation of Fe(II) to Fe(III), which is a better flocculant. Fe(II) was produced under the anaerobic conditions by microbial iron reduction, and it was oxidized to Fe(III) within less than one hour after the aeration was started. However, by comparing two different sludges with different capabilities for iron reduction, iron oxidation and responses to substrate addition, it was found that the aerobic biological activity most likely was of greatest significance for the observed reflocculation and floc formation under aerobic conditions. This was further supported by adding organic substrates (glucose or ethanol) during the aerobic reflocculation phase, which promoted reflocculation. However, some substrates had the opposite effect (acetate and lactate), where a deterioration of the reflocculation was observed, probably due to different responses from different groups of microorganisms in the sludges.     

Observations on the attachment of Thiothrix to biological surfaces in activated sludge

The attachment of Thiothrix to stalks of the ciliated protozoan Opercularia has been observed and photographed in oxygen activated sludge. Correlations between the level of dissolved hydrogen sulfide in wastewater and the proliferation of Thiothrix are made.     

Control of phenol in biological reactors by addition of powdered activated carbon

A biological reactor containing a pure culture of E. coli was subjected to a variety of feed upsets involving phenol with powdered activated carbon applied as a control mechanism. The amount and rate of addition of the carbon was varied to evaluate the effectiveness of this strategy for maintaining the effluent quality. It was found that the carbon addition greatly reduced the magnitude of concentration transients and permitted operation with an input phenol concentration above 1000 mg 1⁻¹.     

Modelling biological nutrient removal activated sludge systems - a review

The external nitrification (EN) biological nutrient removal (BNR) activated sludge (ENBNRAS) system shows considerable promise for full-scale implementation. As an aid for this implementation, a mathematical simulation model would be an invaluable tool. To develop such a model, a study was conducted to select the most suitable simulation model to serve as a starting point for further development. For this, the existing available simulation models for BNRAS systems are compared with one another and evaluated against experimental observations in the literature and on ENBNRAS systems. One process immediately apparent to be crucially important is the anoxic growth of phosphorus accumulating organisms (PAOs), with associated PAO denitrification and anoxic P uptake for polyP formation. These linked processes are lacking in the earlier kinetic simulation models for BNRAS systems, which were based on aerobic PAO growth and P uptake only, but have been incorporated into the more recent kinetic models. This provides a substantive body of information on modelling this aspect. Other processes of significance identified to require consideration are anaerobic slowly biodegradable COD (SBCOD) hydrolysis to readily biodegradable COD (RBCOD), and COD loss. Both processes have significant impact on the predicted BEPR performance. Due to the uncertainties associated with the mechanisms and quantification of these two processes, it is concluded that the most extensively validated kinetic simulation model should be selected for development, and that the omissions in this model should be addressed progressively, using the relevant information drawn from the existing models, the literature and observations on ENBNRAS systems.