Kinetic evaluation of nitrification performance in an immobilized cell membrane bioreactor.

High rate membrane bioreactor (MBR) systems operated at extremely low sludge ages (superfast membrane bioreactors (SFMBRs)) are inefficient to achieve nitrogen removal, due to insufficient retention time for nitrifiers. Moreover, frequent chemical cleaning is required due to high biomass flux. This study aims to satisfy the nitrification in SFMBRs by using sponge as carriers, leading to the extension of the residence time of microorganisms. In order to test the limits of nitrification, bioreactor was run under 52, 5 and 2 days of carrier residence time (CRT), with a hydraulic retention time of 6 h. Different degrees of nitrification were obtained for different CRTs. Sponge immobilized SFMBR operation with short CRT resulted in partial nitrification indicating selective dominancy of ammonia oxidizers. At higher CRT, simultaneous nitrification-denitrification was achieved when accompanying with oxygen limitation. Process kinetics was determined through evaluation of the results by a modeling study. Nitrifier partition in the reactor was also identified by model calibration.

Nitrogen removal performance of intermittently aerated membrane bioreactor treating black water.

The study investigated the effect of intermittent aeration on the nitrogen removal performance of a membrane bioreactor (MBR) treating black water. A pilot-scale MBR with an effective volume of 630 L operating as a sequencing batch reactor (SBR) with intermittent aeration was used in the experiments. Substrate feeding was limited to the initial non-aerated phase. The MBR unit was sustained at a steady state at a sludge age of 60 d with a biomass concentration of around 10,000 mg/L for 3 months. The treated black water could be characterized with an average COD of 950 mg/L and total nitrogen of 172 mg/L, corresponding to a low COD/N ratio of 5.5. The selected MBR scheme was quite effective, reducing COD down to 26 mg/L, providing effective nitrification and yielding a total oxidized nitrogen concentration under 10 mg N/L. The nitrogen removal performance was substantially better than the level predicted by process stoichiometry, due to multiple anoxic configuration inducing additional nitrogen removal. Dissolved oxygen profiles associated with the cyclic operation of the system suggested that the incremental nitrogen removal could be attributed to simultaneous nitrification-denitrification, a commonly observed mechanism in MBR systems sustained at high biomass concentrations.

Impact of mixing on water quality in the Bosphorus - Implications on sustainable management of wastewater marine discharges.

The study established the water quality modelling of the Bosphorus system, based on hydrodynamic data as well as the results of the water quality survey carried out in the last five years. The model revealed significant decrease in the magnitude of pollutant loads in the upper layer at the exit into The Marmara Sea providing numerical proof that no pollutant transport would take place from sewage discharges to the upper layer. A similar modelling approach was implemented at the Bosphorus/Marmara interface, a significant hotspot as it included two major deep marine outfalls. The results asserted that the entire sewage flow would enter the lower flow in The Bosphorus through the interface without an appreciable mixing with the upper flow. This way, the study provided a significant scientific support for the sustainable management of marine discharges in this area, since they have no physical interference with The Marmara Sea.

Experimental evaluation and model assessment of coexistence of PAOs and GAOs.

The study evaluated the competition and co-existence of PAOs and GAOs in sequencing batch reactor (SBR) systems sustaining enhanced biological phosphorus removal (EBPR). The SBR operation used acetate as the sole external carbon source and covered a wide range of initial COD/P ratios between 6.5 to 25.9 g COD/g P. A mechanistic model, ASMGG, was adopted for this purpose, which basically incorporated model components and processes associated with GAO metabolism and glycogen metabolism of PAOs. Model calibration was successfully performed with the same set of stoichiometric and kinetic coefficients for all the acetate, phosphate, glycogen and PHA profiles obtained in different experiments. Interpretation of experimental results by means of model simulation indicated competition and co-existence of PAOs and GAOs within the EBPR process, numerically assessing the composition of the microbial community sustained and identifying the respective role and function of PAOs and GAOs on the fate of glycogen and PHA.

Pollution profile and biodegradation characteristics of fur-suede processing effluents.

This study investigated the effect of stream segregation on the biodegradation characteristics of wastewaters generated by fur-suede processing. It was conducted on a plant located in an organized industrial district in Turkey. A detailed in-plant analysis of the process profile and the resulting pollution profile in terms of significant parameters indicated the characteristics of a strong wastewater with a maximum total COD of 4285 mg L(-1), despite the excessive wastewater generation of 205 m3 (ton skin)(-1). Respirometric analysis by model calibration yielded slow biodegradation kinetics and showed that around 50% of the particulate organics were utilized at a rate similar to that of endogenous respiration. A similar analysis on the segregated wastewater streams suggested that biodegradation of the plant effluent is controlled largely by the initial washing/pickling operations. The effect of other effluent streams was not significant due to their relatively low contribution to the overall organic load. The respirometric tests showed that the biodegradation kinetics of the joint treatment plant influent of the district were substantially improved and exhibited typical levels reported for tannery wastewater, so that the inhibitory impact was suppressed to a great extent by dilution and mixing with effluents of the other plants. The chemical treatment step in the joint treatment plant removed the majority of the particulate organics so that 80% of the available COD was utilized in the oxygen uptake rate (OUR) test, a ratio quite compatible with the biodegradable COD fractions of tannery wastewater. Consequently, process kinetics and especially the hydrolysis rate appeared to be significantly improved.

Pollutant dynamics between The Black Sea and The Marmara Sea: Basis for wastewater management strategy.

The study evaluated pollutant dynamics between The Black Sea and The Marmara Sea using data collected during a marine survey of the region around The Bosphorus strait, in the last five years. A hydraulic model was utilized to define two-layered water exchange in The Bosphorus. Analysis of pollutant exchange indicated The Black Sea as major polluter for the marine environment in The Marmara Sea. Four wastewater outfalls are located along The Bosphorus; Mass balances between the two ends of The Bosphorus indicated losses of 44 t/d total N and 138 t/d COD in the lower layer before reaching The Black Sea. This was explained with a simultaneous nitrification-denitrification process sustained in the low oxygen or anoxic zones around the outfalls, implying that a sustainable wastewater strategy should preclude additional treatment for The Bosphorus discharges, since they do not have an appreciable impact on the water quality of the lower flow.

Effect of anoxic decay process on simultaneous nitrification denitrification in a membrane bioreactor operated without an anoxic tank.

This study was focused on evaluating the role and the effect of anoxic decay on the extent of simultaneous nitrification-denitrification (SNdN) process sustained in a single membrane bioreactor. The membrane bioreactor was fed with relatively strong domestic sewage and operated at steady state at a sludge age of 36 days at a corresponding suspended solids level maintained in the range of 17,500-21,000 mg/L. The SNdN could be sustained due to diffusion limitation of oxygen into the flocs. The evaluation identified an MLSS threshold level of around 17,000-18,000 mg/L below which nitrogen removal was essentially controlled by denitrification and above, the rate limiting mechanism shifted to nitrification maintaining total nitrogen removal efficiency of 85-95% for a typical domestic sewage. The contribution of anoxic decay process to the overall denitrification potential was evaluated as 60%, substantially higher than the remaining 40% associated with the anoxic growth during the SNdN process.

Optimization of alkaline hydrothermal pretreatment of biological sludge for enhanced methane generation under anaerobic conditions.

This paper investigated the effect of alkaline hydrothermal pretreatment (HTP) on the hydrolysis, biodegradation and methane generation potential of waste activated sludge (WAS). A multi-variable experimental approach was designed, where initial solids content (1-5%), reaction temperature (130-190 °C), reaction time (10-30 min.) and caustic concentration (0-0.2 mgNaOH/mgVS) were varied in different combinations to assess the impact of alkaline HTP. This process significantly enhanced the hydrolysis of organic compounds in sludge into soluble fractions, whereby increasing the chemical oxygen demand (COD) leakage up to 200-900% with the 17-99% solubility. It boosted volatile solids (VS) biodegradation up to 40%, which resulted in a parallel increase in methane generation from 216 mLCH4/gVS to as high a 456 mLCH4/gVS methane generation basically relied on the conversion of solubilized COD. Alkaline HTP process was optimized for the maximum methane production. Optimum conditions were obtained at 190 °C reaction temperature, 10 min. reaction time, 0.2 mgNaOH/mgVS and 5% dry matter content. Under these conditions, 453.8 mLCH4/gVS was predicted. Biochemical methane potential (BMP) value was determined as 464 mLCH4/gVS supporting predictive power of the BMP model. The biodegradability compared to the untreated raw WAS was enhanced 78.2%.

Respirometric evaluation of biodegradation characteristics of dairy wastewater for organic carbon removal.

This study evaluates the biodegradation kinetics of an integrated dairy wastewater, with the main purpose of defining the experimental basis for modelling of the activated sludge process. Besides conventional characterization, the experiments involved detailed chemical oxygen demand (COD) fractionation and assessment of major kinetic and stoichiometric coefficients, by using respirometric methods. A multi-component model based on the endogenous decay concept was used for the kinetic interpretation. The results of conventional analyses and respirometric evaluations together with the assessment of residual components showed that the organic carbon content of the dairy wastewater was mostly soluble and biodegradable. The soluble, slowly biodegradable COD was the major COD fraction, representing around 50% of the total COD. Model calibration of the oxygen uptake rate profiles were consistent and revealed the existence of dual hydrolysis kinetics for soluble and particulate COD components. The hydrolysis rate associated with the main COD component--the soluble, slowly biodegradable COD fraction--was found to be 1.2 d(-1), which is quite low and underlines the role of this COD fraction as the rate-limiting factor for effluent quality. Simulation of process efficiency by the adopted model, calibrated with the experimentally determined parameters, indicated that effective control of the biodegradation of the soluble biodegradable COD components could be done by selection of appropriate values for the sludge age and hydraulic retention time. In this way, the total effluent soluble COD level could be lowered to 30-40 mg L(-1) range, in conformity with effluent limitations.

COD fractionation of tannery wastewaters--particle size distribution, biodegradability and modeling.

This study aims to establish the scientific link between particle size distribution (PSD) and biodegradability of different COD fractions of tannery wastewater, by means of sequential filtration/ultrafiltration, respirometric analysis and model evaluation. PSD profiles were determined in physical segregation experiments, using eight membrane discs, each with different pore sizes between 2 and 1600 nm. Biodegradability-related COD fractionation was determined at each size interval by model simulation and calibration of the corresponding oxygen uptake rate (OUR) profiles. Activated Sludge Model No. 3 (ASM3), modified for direct growth on hydrolysis products, was adopted for evaluation. PSD analyses defined a COD fingerprint with two significant portions at the two ends of size distribution, with 60% of the total COD at the particulate range, 25% at the soluble range and the remaining 15% well distributed among the colloidal range. Comparative evaluation of the sequence of OUR profiles yielded values of applicable model coefficients. It also enabled the assessment of size distribution for each major COD fraction, as an original tool for better interpretation of specific biodegradation characteristics of the selected tannery wastewater. Results also revealed a very slowly biodegradable/residual particulate COD component with a significant inhibitory effect. Model-based evaluation of the OUR profiles enabled quantifying the impact of inhibition in terms of changes in rate coefficients for growth, hydrolysis of soluble COD and endogenous decay.

How does shear affect aggregation in granular sludge sequencing batch reactors? Relations between shear, hydrophobicity, and extracellular polymeric substances.

The objective was to provide an answer to "how to grow/survive in aggregative physiology" through evaluating the relation between physical stress and observed biomass characteristics. For that, a lab-scale sequencing batch reactor was operated at an anaerobic-aerobic mode and under altered hydraulic selection pressures of settling time (10-1 min) and hydrodynamic shear rates due to mechanical mixing (15.5-12.0 cm/s) and/or aeration (1.76-0.24 cm/s). Main physical stress experienced by the biomass was mechanical mixing, which resulted in extreme shearing conditions at the first operational stage (days 1-86), during which first granules formed but settling properties deteriorated and biomass was almost totally washed out. After relaxing the overall shear stress at the second stage, biomass formation accelerated, settling properties enhanced and granulation proceeded (days 86-136), until disturbance of the process at the last month of operation (days 136-163). Aggregative physiology-related parameters, being cell surface hydrophobicity and extracellular polymeric substances (EPS), followed increasing trends parallel to the progress of granulation, and then decreased upon disturbance of the process. There was an increase in the EPS production also during the first stage under extreme shear, while a substantial amount of biomass was present in the system. A direct correlation was also found between %hydrophobicity and EPS-composition expressed as ExoPN/ExoPS.

Modelling of long-term simultaneous nitrification and denitrification (SNDN) performance of a pilot scale membrane bioreactor.

Nutrient removal capability of the MBR process has attracted more attention than organics removal in the past few years. Apart from the conventional schemes for nitrogen removal in MBR process, simultaneous nitrification-denitrification (SNDN) requires the most attention for further research. In order to fully understand the fundemantals and mechanism of SNDN in MBRs, a pilot plant was set up. A mathematical model was adopted for investigation and calibration against the observed values. This paper reports a study focusing on evaluating major mechanisms that govern nitrogen removal from domestic wastewater in membrane bioreactors. Two items need to be emphasized in this evaluation: (i) an MBR is basically regarded as an activated sludge process-a suspended growth bioreactor with total biomass recycle and substantially higher biomass concentration; (ii) in this context an AS model, namely ASM1R modified for endogenous respiration, is used for dynamic modelling and calibration of experimental results. The impact of diffusion through biomass which obviously exerts a significant effect on system performance and denitrification is evaluated with success using the adopted model by means of switch functions that regulate nitrification-denitrification with respect to dissolved oxygen concentration in the bulk liquid.

High temperature pyrolysis of sewage sludge as a sustainable process for energy recovery.

This study explored the potential of high temperature pyrolysis for energy recovery from domestic sewage. It mainly defines optimum operating conditions to maximize syngas generation. A pyrolysis unit was operated in batch mode, at temperatures of 450, 600 and 850 °C, rotation speeds of 10, 40 and 60 Hz. The sludge had 6% moisture content; it contained 65% organic matter and involved a low calorific value of 13.535 kJ/kg dry matter. Pyrolysis at 850 °C and high rotation speed of 60 Hz yielded the highest conversion of sludge to syngas, with an average of 59% of the organic matter as syngas, 29% as tar and 12% as biochar. Pyrolysis enabled 74% of the energy recovery as syngas and tar. Continuous full-scale pyrolysis systems would further increase the syngas by recovering condensable gaseous products and/or recycling tar back into the pyrolysis unit. A unified approach for energy recovery management should equally consider what fraction of the energy contained in the wastewater was consumed and wasted before generating the sludge. Therefore, the adopted management scheme should also cover all design and operation parameters of the treatment plant, because this is how the energy is best conserved even before the sludge is generated.

Microbial endogenous response to acute inhibitory impact of antibiotics.

Enhanced endogenous respiration was observed as the significant/main response of the aerobic microbial culture under pulse exposure to antibiotics: sulfamethoxazole, tetracycline and erythromycin. Peptone mixture and acetate were selected as organic substrates to compare the effect of complex and simple substrates. Experiments were conducted with microbial cultures acclimated to different sludge ages of 10 and 2 days, to visualize the effect of culture history. Evaluation relied on modeling of oxygen uptake rate profiles, reflecting the effect of all biochemical reactions associated with substrate utilization. Model calibration exhibited significant increase in values of endogenous respiration rate coefficient with all antibiotic doses. Enhancement of endogenous respiration was different with antibiotic type and initial dose. Results showed that both peptone mixture and acetate cultures harbored resistance genes against the tested antibiotics, which suggests that biomass spends cellular maintenance energy for activating the required antibiotic resistance mechanisms to survive, supporting higher endogenous decay rates. ABBREVIATIONS: [Formula: see text]: maximum growth rate for XH (day-1); KS: half saturation constant for growth of XH (mg COD/L); bH: endogenous decay rate for XH (day-1); kh: maximum hydrolysis rate for SH1 (day-1); KX: hydrolysis half saturation constant for SH1(mg COD/L); khx: maximum hydrolysis rate for XS1 (day-1); KXX: hydrolysis half saturation constant for XS1 (mg COD/L); kSTO: maximum storage rate of PHA by XH (day-1); [Formula: see text]: maximum growth rate on PHA for XH (day-1); KSTO: half saturation constant for storage of PHA by XH (mg COD/L); XH1: initial active biomass (mg COD/L).

Critical appraisal of respirometric methods for metal inhibition on activated sludge.

This paper evaluates the merit of oxygen uptake rate measurements for the assessment of metal inhibition on activated sludge. For this purpose, experiments are conducted to calculate EC50 levels of nickel and hexavalent chromium using the ISO 8192 procedure, yielding results that are highly variable and difficult to correlate, depending on the type of substrate and the initial food to microorganism ratio. Similar experiments based on continuous respirometric measurements to give the entire oxygen uptake rate profile provide a much better insight on the impact of inhibition on different biochemical processes taking place in the reactor. The results indicate that percent reduction of the amount of dissolved oxygen utilized after an appropriate reaction time is a much better index for the assessment of the inhibitory effects.

Respirometric assessment of biodegradation characteristics of the scientific pitfalls of wastewaters.

This paper provides an overview of common problems encountered when using oxygen uptake rate (OUR) measurements for the assessment of wastewater characteristics and process kinetics. Emphasis is placed upon pitfalls that would lead to significant errors. It covers model dependency of the OUR measurements and the need to select appropriate models; interpretation of OUR perturbations as a way to identify new model components and processes; the need for simultaneous observation of relevant model components and multicomponent modelling for appropriate evaluation of OUR measurements; parameter identifiability problems and the effect of active biomass concentration and the endogenous decay rate on model simulation and calibration. Relevant experimental OUR data from previous studies are presented to illustrate and underline common scientific pitfalls.

Effect of biochemical storage on the denitrification potential of acetate in sequencing batch reactors.

This study evaluates the effect biochemical storage on the denitrification potential (N(DP)) of acetate. The fate of bacterial storage is evaluated in a sequencing batch reactor system operated in a sequence of anoxic/aerobic phases, fed with acetate as a pulse and continuously under anoxic conditions. N(DP) is defined based on system stoichiometry both for direct growth and storage on acetate. Experimental results do not support conceptual calculations based on system stoichiometry, yielding a higher denitrification potential, N(DP), for continuous feeding than the N(DP) obtained with pulse feeding, due to partial utilisation of the stored PHB within the anoxic phase. The nitrate, acetate and poly-beta-hydroxybutyrate (PHB) profiles obtained in the experimental studies were used in model calibrations for two different feeding patterns. Results of model simulations confirm the experimental results and evaluate the effects imposed on the denitrification potential by sludge age and the anoxic volume ratio.

Effect of chemical and biological treatment on COD fingerprints of textile wastewaters.

Particle size distribution (PSD) via sequential filtration/ultrafiltration was used as the tool for COD fractionation and colour profiling of textile wastewaters before and after treatment. Profiles prior to treatment suggested PSD-based COD fingerprints characteristic for the influents. Treatment efficiencies were determined via comparing the profiles of the effluents from chemical- and biological-treatment to those of the corresponding influents. COD fingerprints of the wastewaters from the textile plants, applying different treatment alternatives, were different especially at the upper size range; yet profiles after treatment were similar, with the soluble fraction (< 2 nm) being almost the only apparent one. Half of the overall COD-removal via chemical treatment was at the particulate- and upper colloidal-ranges, revealing that this alternative was effective at higher ranges, but not at the soluble fraction. In contrast, biological treatment was effective at both ends of size distribution, with total removal at the particulate range and 50% elimination at the soluble portion. Overall colour content and PSD-based colour profiles of the influents were also different. Chemical treatment was successful in removing colour originating from the entire colloidal range, but was not efficient at the soluble fraction. Conversely, colour removal efficiency of biological treatment was moderate throughout the entire size spectrum.

Respirometric assessment of biodegradation for acrylic fibre-based carpet finishing wastewaters.

The paper evaluates biodegradation characteristics of wastewaters generated from acrylic fibre-based carpet processing and manufacturing. It involves detailed characterisation, respirometric modelling and kinetic description of dyeing and softening wastewater streams and the composite effluent. The wastewaters exhibit different COD content and fractionation. The resulting composite effluent has a total COD of 775 mgL(-1), predominantly soluble and with a biodegradable fraction of 86%. In respirometric studies, the OUR profiles can only be calibrated with a dual hydrolysis model with rates significantly slower compared to domestic sewage and other textile plant effluents. Kinetic information derived from the experiments is applied for the conceptual evaluation of the treatability of the composite wastewater using two different continuous-flow activated sludge configurations.

Effect of photochemical pre-treatment on COD fractionation of a non-ionic textile surfactant.

The work presented in this paper is focused on the effect of photochemical (H2O2/UV-C) pretreatment on COD fractionation and degradation kinetics of a non-ionic textile surfactant. In the first part of the study, the COD of non-ionic surfactant was adjusted to 1000 mg/L in order to simulate real effluent originating from the textile preparation stage featuring desizing, scouring, washing and rinsing operations. The surfactant was subjected to H2O2/UV-C pretreatment for up to 120 min at a dose of 30 mM (980 mg/L) H2O2. The biodegradability studies for untreated and photochemically treated samples were evaluated on the basis of modeling of oxygen uptake rate (OUR) profiles. Modelling of OUR profiles conducted for untreated sample showed that single complex substrate was subjected to enzymatic breakdown and disintegrated into one readily and two types of slowly biodegradable substrates. After modelling the biodegradation of photochemically pretreated sample, the readily biodegradable COD fraction was reduced, on the other hand, more slowly biodegradable organics were generated. A higher disintegration rate was obtained for chemically pretreated samples. However, other kinetic constants of growth and hydrolysis processes were not affected considerably.