Control of Microthrix parvicella by aluminium salts addition.

Aluminium and iron chloride were added to a biological nutrient removal pilot plant (1,500 population equivalent) treating urban wastewater to investigate the control of Microthrix parvicella bulking and foaming by metallic salts. Monitoring plant performance over two 6-month periods showed a slight impact on the removal efficiencies. Addition of metallic salts (Me; aluminium or aluminium + iron) at a concentration of 41 mmol Me(kg MLSS·d) (MLSS: mixed liquor suspended solids) over 70 days allowed a stabilization of the diluted sludge volume index (DSVI), whereas higher dosages (94 mmol Me(kg MLSS·d) over 35 days or 137 mmol Me(kg MLSS·d) over 14 days induced a significant improvement of the settling conditions. Microscopic observations showed a compaction of biological aggregates with an embedding of filamentous bacteria into the flocs that is not specific to M. parvicella as bacteria from phylum Chloroflexi are embedded too. The quantitative polymerase chain reaction targeting M. parvicella further indicated a possible growth limitation in addition to the flocculation impact at the high dosages of metallic salts investigated. DSVI appeared to be correlated with the relative abundance of M. parvicella.

Critical review of activated sludge modeling: state of process knowledge, modeling concepts, and limitations.

This work critically reviews modeling concepts for standard activated sludge wastewater treatment processes (e.g., hydrolysis, growth and decay of organisms, etc.) for some of the most commonly used models. Based on a short overview on the theoretical biochemistry knowledge this review should help model users to better understand (i) the model concepts used; (ii) the differences between models, and (iii) the limits of the models. The seven analyzed models are: (1) ASM1; (2) ASM2d; (3) ASM3; (4) ASM3 + BioP; (5) ASM2d + TUD; (6) Barker & Dold model; and (7) UCTPHO+. Nine standard processes are distinguished and discussed in the present work: hydrolysis; fermentation; ordinary heterotrophic organisms (OHO) growth; autotrophic nitrifying organisms (ANO) growth; OHO & ANO decay; poly-hydroxyalkanoates (PHA) storage; polyphosphate (polyP) storage; phosphorus accumulating organisms PAO) growth; and PAO decay. For a structured comparison, a new schematic representation of these processes is proposed. Each process is represented as a reaction with consumed components on the left of the figure and produced components on the right. Standardized icons, based on shapes and color codes, enable the representation of the stoichiometric modeling concepts and kinetics. This representation allows highlighting the conceptual differences of the models, and the level of simplification between the concepts and the theoretical knowledge. The model selection depending on their theoretical limitations and the main research needs to increase the model quality are finally discussed.

Sulfur transformations during two-stage anaerobic digestion and intermediate thermal hydrolysis.

The formation of hydrogen sulfide (H2S) during anaerobic digestion (AD) imposes constraints on the valorisation of biogas. So far, inorganic sulfur compounds -mainly sulfate - have been considered as the main contributors to H2S formation, while the contribution of organic sulfur compounds is mostly neglected. This study investigates the fate of organic and inorganic sulfur compounds during two-stage anaerobic digestion with intermediate thermal hydrolysis for treatment of primary and secondary sludge in a WWTP treating domestic wastewater. The results of a seven-week monitoring campaign showed an overall decrease of organic sulfur compounds in both stages of anaerobic digestion. Further fractionation of organic sulfur revealed a high conversion of the particulate organic fraction during the first digestion stage and of the soluble organic fraction during the second digestion stage. The decrease of soluble organic sulfur during the second digestion stage was attributed to the solubilisation and hydrolysis of sulfur-containing organic compounds during thermal hydrolysis. In both digestion stages, more organic sulfur was taken up than particulate inorganic sulfur (metal sulfide) was produced, indicating the formation of other reduced sulfur forms (e.g. H2S). Further batch experiments confirmed the role of organic sulfur uptake in the formation of H2S during anaerobic digestion as sulfate reduction only partly explained the total sulfide formed (H2S in biogas and precipitated FeS). Overall, the conversion of organic sulfur was demonstrated to play a major role in H2S formation (and thus the biogas quality), especially in case of thermal hydrolysis pretreatment.

Monitoring the variations of the oxygen transfer rate in a full scale membrane bioreactor using daily mass balances.

Oxygen transfer in biological wastewater treatment processes with high sludge concentration, such as membrane bioreactor (MBR), is an important issue. The variation of alpha-factor versus mixed liquor suspended solids (MLSS) concentration was investigated in a full scale MBR plant under process conditions, using mass balances. Exhaustive data from the Supervisory Control And Data Acquisition (SCADA) and from additional online sensors (COD, DO, MLSS) were used to calculate the daily oxygen consumption (OC) using a non-steady state mass balance for COD and total N on a 24-h basis. To close the oxygen balance, OC has to match the total oxygen transfer rate (OTRtot) of the system, which is provided by fine bubble (FB) diffusers in the aeration tank and coarse bubbles (CB) in separate membrane tanks. First assessing OTR(CB) then closing the balance OC = OTRtot allowed to calculate OTR(FB) and to fit an exponential relationship between OTR(FB) and MLSS. A comparison of the alpha-factor obtained by this balance method and by direct measurements with the off-gas method on the same plant is presented and discussed.

Activated sludge modelling: development and potential use of a practical applications database.

This study aims at synthesizing experiences in the practical application of ASM type models. The information is made easily accessible to model users by creating a database of modelling projects. This database includes answers to a questionnaire that was sent out to model users in 2008 to provide inputs for a Scientific and Technical Report of the IWA Task Group on Good Modelling Practice - Guidelines for use of activated sludge models, and a literature review on published modelling projects. The database is analysed to determine which biokinetic model parameters are usually changed by modellers, in which ranges, and what values are typically used for seven selected activated sludge models. These results should help model users in the calibration step, by providing typical parameter values as a starting point and ranges as a guide. However, the proposed values should be used with great care since they are the result of averaging practical experience and not taking into account specific parameter correlations.

Plant-wide investigation of sulfur flows in a water resource recovery facility (WRRF).

Even though sulfur compounds and their transformations may strongly affect wastewater treatment processes, their importance in water resource recovery facilities (WRRF) operation remains quite unexplored, notably when it comes to full-scale and plant-wide characterization. This contribution presents a first-of-a-kind, plant-wide quantification of total sulfur mass flows for all water and sludge streams in a full-scale WRRF. Because of its important impact on (post-treatment) process operation, the gaseous emission of sulfur as hydrogen sulfide (H2S) was also included, thus enabling a comprehensive evaluation of sulfur flows. Data availability and quality were optimized by experimental design and data reconciliation, which were applied for the first time to total sulfur flows. Total sulfur flows were successfully balanced over individual process treatment units as well as the plant-wide system with only minor variation to their original values, confirming that total sulfur is a conservative quantity. The two-stage anaerobic digestion with intermediate thermal hydrolysis led to a decreased sulfur content of dewatered sludge (by 36%). Higher (gaseous) H2S emissions were observed in the second-stage digester (42% of total emission) than in the first one, suggesting an impact of thermal treatment on the production of H2S. While the majority of sulfur mass flow from the influent left the plant through the treated effluent (> 95%), the sulfur discharge through dewatered sludge and gaseous emissions are critical. The latter are indeed responsible for odour nuisance, lower biogas quality, SO2 emissions upon sludge combustion and corrosion effects.

Biodegradable organic matter in domestic wastewaters: comparison of selected fractionation techniques.

The first objective of this work was to evaluate the ability of the long-term BOD tests to provide the total biodegradable COD (BCOD) for domestic wastewaters. Results show that the method is repeatable (using two different volume samples, 97 and 164 mL) and that the fractions of BCOD determined were not statistically different from the one obtained by respirometry (at low S/X ratio). Respirometric tests were also repeatable. They were shown to be sensitive to the origin of the sludge. The results obtained are in the range of literature data. But they later indicated that long-term bioassays (>20 days) give higher biodegradable fractions than the other methods (BOD and respirometry). The second objective was to compare soluble fractions obtained with raw and pre-flocculated samples. Flocculated filtered COD is significantly lower than filtered COD, even if a pore size of 0.1 microm is used. The comparison to literature values shows that physicochemical RBCOD fractions are significantly higher than the ones obtained using bioassays. Comparison with the fractions used in calibrated models would help the choice of the more suitable method for modelling purposes.

A systematic approach for model verification: application on seven published activated sludge models.

The quality of simulation results can be significantly affected by errors in the published model (typing, inconsistencies, gaps or conceptual errors) and/or in the underlying numerical model description. Seven of the most commonly used activated sludge models have been investigated to point out the typing errors, inconsistencies and gaps in the model publications: ASM1; ASM2d; ASM3; ASM3 + Bio-P; ASM2d + TUD; New General; UCTPHO+. A systematic approach to verify models by tracking typing errors and inconsistencies in model development and software implementation is proposed. Then, stoichiometry and kinetic rate expressions are checked for each model and the errors found are reported in detail. An attached spreadsheet (see http://www.iwaponline.com/wst/06104/0898.pdf) provides corrected matrices with the calculations of all stoichiometric coefficients for the discussed biokinetic models and gives an example of proper continuity checks.

Activated sludge modelling in practice: an international survey.

The Good Modelling Practice Task Group (GMP-TG) of the International Water Association (IWA) is developing guidelines for the use of Activated Sludge Models (ASM). As part of this work the group created and sent out a questionnaire to current and potential activated sludge model users in 2007. The objectives of the questionnaire were (i) to better define the profile of ASM users, (ii) to identify the tools and procedures that are actually used and (iii) to highlight the main limitations while building and using ASM-type models. Ninety-six answers were received from all over the world, from several types of organisation. The results were analysed to identify the modellers' perceptions of models depending on their profile. The results also highlighted the main topics of interest for improving modelling procedures which are standardisation of the available modelling guidelines and better experience and knowledge transfer.

Wastewater treatment modelling: dealing with uncertainties.

This paper serves as a problem statement of the issues surrounding uncertainty in wastewater treatment modelling. The paper proposes a structure for identifying the sources of uncertainty introduced during each step of an engineering project concerned with model-based design or optimisation of a wastewater treatment system. It briefly references the methods currently used to evaluate prediction accuracy and uncertainty and discusses the relevance of uncertainty evaluations in model applications. The paper aims to raise awareness and initiate a comprehensive discussion among professionals on model prediction accuracy and uncertainty issues. It also aims to identify future research needs. Ultimately the goal of such a discussion would be to generate transparent and objective methods of explicitly evaluating the reliability of model results, before they are implemented in an engineering decision-making context.

Prediction of alpha factor values for fine pore aeration systems.

The objective of this work was to analyse the impact of different geometric and operating parameters on the alpha factor value for fine bubble aeration systems equipped with EPDM membrane diffusers. Measurements have been performed on nitrifying plants operating under extended aeration and treating mainly domestic wastewater. Measurements performed on 14 nitrifying plants showed that, for domestic wastewater treatment under very low F/M ratios, the alpha factor is comprised between 0.44 and 0.98. A new composite variable (the Equivalent Contact Time, ECT) has been defined and makes it possible for a given aeration tank, knowing the MCRT, the clean water oxygen transfer coefficient and the supplied air flow rate, to predict the alpha factor value. ECT combines the effect on mass transfer of all generally accepted factors affecting oxygen transfer performances (air flow rate, diffuser submergence, horizontal flow).

Predicting oxygen transfer of fine bubble diffused aeration systems--model issued from dimensional analysis.

The standard oxygenation performances of fine bubble diffused aeration systems in clean water, measured in 12 cylindrical tanks (water depth from 2.4 to 6.1m), were analysed using dimensional analysis. A relationship was established to estimate the scale-up factor for oxygen transfer, the transfer number (N(T)) The transfer number, which is written as a function of the oxygen transfer coefficient (k(L)a(20)), the gas superficial velocity (U(G)), the kinematic viscosity of water (nu) and the acceleration due to gravity (g), has the same physical meaning as the specific oxygen transfer efficiency. N(T) only depends on the geometry of the tank/aeration system [the total surface of the perforated membrane (S(p)), the surface of the tank (S) or its diameter (D), the total surface of the zones covered by the diffusers ("aerated area", S(a)) and the submergence of the diffusers (h)]. This analysis allowed to better describe the mass transfer in cylindrical tanks. Within the range of the parameters considered, the oxygen transfer coefficient (k(L)a(20)) is an increasing linear function of the air flow rate. For a given air flow rate and a given tank surface area, k(L)a(20) decreases with the water depth (submergence of the diffusers). For a given water depth, k(L)a(20) increases with the number of diffusers, and, for an equal number of diffusers, with the total area of the zones covered by the diffusers. The latter result evidences the superiority of the total floor coverage over an arrangement whereby the diffusers are placed on separate grids. The specific standard oxygen transfer efficiency is independent of the air flow rate and the water depth, the drop in the k(L)a(20) being offset by the increase of the saturation concentration. For a given tank area, the impact of the total surface of the perforated membrane (S(p)) and of the aerated area (S(a)) is the same as on the oxygen transfer coefficient.

DNA synthesis and endomitoses in the giant nuclei of the silkgland of Bombyx mori.

At the first symptoms of organisation of the silkgland in the embryo, mitoses stop and nuclei start to grow. Through autoradiographic studies, performed after sequenced labeling with [3H] and [14C] thymidine, the durations of the different phases of DNA synthesis cycles (T = 42 to 48 h, S = 22 to 25 h, G = 20 to 23 h) are established. These durations are in fact identical during the second and the third instar, and the same, whatever region is concerned : posterior, middle or anterior parts. A model has been established to account for the distribution of the S phases during the second and third instars. The DNA synthesis in all nuclei of a given region has been followed during the first four instars by labelling with [3H]thymidine. The activity goes through maxima and minima, depending on the percent of nuclei synthesizing DNA and their synchronism, both being characteristic of each region; long resting periods are observed during the molting stages of the first three instars in the middle and the anterior parts. The coincidence is obvious between the maxima and minima and respectively the S and G phases of the model. DNA assays agree with the distribution of cycles established by autoradiography if one admits that each cycle does lead to a doubling of the amount of DNA. The overall DNA synthesis from the diploid value is estimated to correspond to 18-19 endomitoic cycles in the nucleic of the posterior part, 19-20 cycles of those of the middle part and 13 in those of the anterior part. The analysis of chromosome structures, by squashing the nuclear content, shows that existence of a complete endomitotic cycle: the doubling of chromosomes is associated with condensed structures, alternating with a decondensed state of chromatin, responsible for the DNA synthesis. The female heterochromatin undergoes a restricted morphological cycle delayed with respect to bulk chromatin. It is characterized by a late DNA duplication and by non dispersed daughter chromosomes. Some of these aspects are, to a lesser extent, reproduced in groups of autosomic chromosomes.

Robustness and economic measures as control benchmark performance criteria.

The objective of this paper was to investigate the usefulness of new multi-criteria measures to evaluate a control strategy through the Benchmark protocol. Using a case study in which respirometry-based control strategies are evaluated, the proposed measures were calculated. An economic index including weighted investment and operating costs (termed Total Cost Index--TCI) appears more powerful than a grey-scale presentation approach. Using the latter approach, it is hard to reflect the relative importance of the criteria investigated, which makes practical decisions rather difficult. In addition, a Robustness Index (RI) is proposed that allows us to evaluate the transferability of control strategies to situations different from the ones defined in the benchmark protocol. Finally, the case study shows that it may be advisable to replace the currently used open loop benchmark reference by a plant in which dissolved oxygen is controlled in all aerated reactors. This quite simple strategy also turned out to be the best one among all evaluated strategies.