Biomedical subjects

M C M van Loosdrecht

Publications and source records attributed to M C M van Loosdrecht.

At least 19 recordsLinked to original sources

PubMed2026

A Mobile Glycosylation Locus Modulates Cell Wall Architecture in Lactobacillus crispatus.

Lactobacillus crispatus dominance in the vaginal microbiome is associated with beneficial health outcomes, yet strain-level variation and its implications remain poorly understood. Here, we resolve the genomic context of three glycosyltransferase gene fragments (GT1-3) previously linked with dysbiotic states. Long-read resequencing revealed that GT1-3 are part of a ~18.7 kb Wzx/Wzy-dependent cell wall polysaccharide (CWPS) locus, containing several IS256-family transposases. Serial propagation in vitro produced isolates with 4.1 kb excised via a composite transposon encompassing the GT3, UDP-galactopyranose mutase, flippase, and hypothetical protein, demonstrating structural plasticity. Transmission electron microscopy showed a ~20%-25% thinner peptidoglycan layer in the derived strains, while FT-IR and monosaccharide analysis indicated no gross changes. Molecular dynamics simulations suggest that GT3 contributes to the structural stability of the glycosyltransferase complex, without compromising catalytic function. Together, these findings establish the CWPS locus as a mobile, structurally plastic element that directly influences cell wall architecture in L. crispatus.

Lactobacillus crispatus↗

PubMed2003

Improved method for determination of ammonia and nitrite oxidation activities in mixed bacterial cultures.

A simple and reliable method to measure the activity of ammonia and nitrite oxidisers in mixed bacterial cultures was developed. The developed method differentiates between the ammonia and nitrite oxidisers by consecutive injection of NO2- and NH4+. The main advantage of this method is that it avoids the use of metabolic inhibitors for ammonia or nitrite oxidisers, as used by other methods. Moreover, it allows measuring of the short-term effect of an inhibitor on both the ammonia and nitrite oxidisers in one test under controlled environmental conditions (pH, temperature). The developed method was applied to determine the inhibitory effects of salt (NaCl up to 15 g Cl/l) on an enriched culture of nitrifying bacteria. The results of the method demonstrate its potential to accurately determine the individual activities of nitrite and ammonia oxidisers.

Ammonia↗

PubMed2003

Effect of dissolved oxygen concentration on sludge settleability.

This laboratory study presents a detailed evaluation of the effects of dissolved oxygen concentration and accumulation of storage polymers on sludge settleability in activated sludge systems with an aerobic selector. The oxygen and substrate availability regime were simulated in laboratory sequencing batch reactor systems. The experiments showed that low dissolved oxygen concentration (< or =1.1 mg O2 l(-1)) had a strong negative effect on sludge settleability, leading to the proliferation of filamentous bacteria (Thiothrix spp., Type 021N and Type 1851). This negative effect was stronger at high chemical oxygen demand loading rate. This indicates that a compartmentalised (plug flow) aerobic contact tank, designed at short hydraulic residence time to guarantee a strong substrate gradient, with low dissolved oxygen concentration, might be worse for sludge settleability than an "overdesigned" completely mixed contact tank. Contrary to the general hypothesis, the maximum specific acetate uptake rate, poly-beta-hydroxybutyrate production rate, and resistance to short starvation periods are similar in both poor- and well-settling sludge. The results of this study support our previous hypothesis on the importance of substrate gradients for the development of filamentous structures in biological flocs, from soluble organic substrate gradients to dissolved oxygen gradients in sludge flocs.

Acetates↗

PubMed2003

Degradation of polymers in a biofilm airlift suspension reactor.

The effect of degradation of polymeric substrates (starch and soy proteins mixture) on the structure of biofilms has been studied. The characteristics of the obtained biofilms were compared to those obtained on corresponding monomeric substrates (glucose and aspartic acid). Based on literature suggestions it was hypothesized that the polymeric substrates, which have a low diffusion rate in the biofilm matrix, would affect the biofilm structure if hydrolytic activity occurs in the biofilm. The obtained biofilm could be expected to present properties like low density and rough surface, facilitating transport and conversion of large polymeric molecules. From the present study it was concluded that the structure of the formed biofilms was influenced by the substrate degraded, however no unequivocal effect of degradation of a polymer on the biofilm structure could be observed. The hydrolytic activity with soy protein and starch as substrate was under stable conditions found to be mainly associated to the biofilm (more than 95% of the total activity). During unstable conditions or start-up significant hydrolytic activity occurred outside the biofilm.

Biodegradation, Environmental↗

PubMed2003

Study on the use of NADH fluorescence measurements for monitoring wastewater treatment systems.

Fluorescence measurement of intracellular nicotinamide adenine dinucleotide (NADH) provides information about the physiological response of microbes towards changing conditions in their environment and has been suggested to be useful for the control of wastewater treatment plants. In this study, the practical usefulness of such measurements was evaluated from batch experiments with a commercially available NADH sensor in a bench scale reactor. The sensor was linear in the NADH concentration, robust, almost maintenance free, and hardly sensitive to floc size distribution. Measured fluorescence intensity proved to depend strongly on the concentration of active heterotrophic biomass. The NADH level was supposed to be dependent on the ratio of electron donor/electron acceptor availability inside the cells; however, neither acetate nor ammonium addition was reflected by the measurement signal.A jump wise NADH signal change was observed at complete oxygen or nitrate depletion as also reflected by bends in the redox curve. In the near zero concentration ranges of oxygen and nitrate (0.1-0.5mg/l) the signal changes only slightly in the opposite direction to the redox trend.

Biomass↗

PubMed2003

Modification of Activated Sludge Model no. 3 considering direct growth on primary substrate.

This paper provides the structural framework for the proposed modified version of Activated Sludge Model No. 3 (ASM3), where direct heterotrophic growth on readily biodegradable substrate is included as a new process and provision is made so that growth on internal storage compounds is started sequentially, after the depletion of the external primary substrate pool. The results have provided strong indication that there was a need for considering direct growth on primary substrate as a significant biological mechanism.

Biodegradation, Environmental↗

PubMed2003

From waste treatment to integrated resource management.

Wastewater treatment was primarily implemented to enhance urban hygiene. Treatment methods were improved to ensure environmental protection by nutrient removal processes. In this way, energy is consumed and resources like potentially useful minerals and drinking water are disposed of. An integrated management of assets, including drinking water, surface water, energy and nutrients would be required to make wastewater management more sustainable. Exergy analysis provides a good method to quantify different resources, e.g. utilisable energy and nutrients. Dilution is never a solution for pollution. Waste streams should best be managed to prevent dilution of resources. Wastewater and sanitation are not intrinsically linked. Source separation technology seems to be the most promising concept to realise a major breakthrough in wastewater treatment. Research on unit processes, such as struvite recovery and treatment of ammonium rich streams, also shows promising results. In many cases, nutrient removal and recovery can be combined, with possibilities for a gradual change from one system to another.

Conservation of Energy Resources↗

PubMed2003

A proposed sustainable BNR plant with the emphasis on recovery of COD and phosphate.

Water problems have to be solved in an integrated way, and sustainability has become a major issue. For this reason, developing more sustainable wastewater treatment processes is needed. New discoveries and good understanding on microbial conversions of nitrogen and phosphorus make more sustainable processes possible. New options for decentralized sustainable sanitation are generally compared to conventional sewage systems, we think that for a proper comparison also innovative centralized treatment schemes should be evaluated. In this article, a more sustainable WWTP is proposed for municipal wastewater treatment, mainly based on the principles of denitrifying dephosphatation and anaerobic ammonium oxidation (ANAMMOX). The proposed system consists of a first stage of the A/B process in which maximal sludge production is achieved. In this way, COD is regained as sludge for methanation. The following BCFS and CANON processes can remove N and P with minimal or no COD need. As a potential fertiliser, struvite can easily be removed from the sludge water by adding magnesium compounds. A case study is done on the basis of the mass balance over the proposed plant. The effluent from the system has a good quality to be recycled. This could also make a contribution to meeting the world's water needs and lessening the impact on the world's water environment. Since all the separate units are already applied or tested on pilot-scale, no problems for technical implementation are foreseen.

Bacteria, Anaerobic↗

PubMed2002

Aerobic granulation in a sequencing batch airlift reactor.

Aerobic granular sludge was cultivated in an intensely mixed sequencing batch airlift reactor (SBAR). A COD loading of 2.5 kg Acetate-COD/(m3 d) was applied. Granules developed in the reactor within one week after inoculation with suspended activated sludge from a conventional wastewater treatment plant. Selection of the dense granules from the biomass mixture occurs because of the differences in settling velocities between granules (fast settling biomass), and filaments and flocs (slow settling biomass). At 'steady state' the granules had an average diameter of 2.5 mm, a biomass density of 60g VSS/I of granules, and a settling rate of > 10 m/h. The biomass consisted of both heterotrophic and nitrifying bacteria. The reactor was operated over a long period during which the granular sludge proved to remain stable. The performance of the intermittently fed SBAR was compared to that of the continuously fed biofilm airlift suspension reactor (BASR). The most importance difference was that the density of the granules in the SBAR was much higher than the density of the biofilms in the BASR. It is discussed that this could be due to the fact that the SBAR is intermittently fed, while the BASR is continuously fed.

Air Movements↗

PubMed2002

Modelling the start-up of a full-scale biological phosphorous and nitrogen removing WWTP.

In this paper we simulated the start-up of a full-scale biological phosphorus and nitrogen removing (BPNR) wastewater treatment plant (wwtp). For the simulation we used a metabolic phosphorus model integrated in ASM2d, referred to as the Technical University Delft Phosphorus (TUDP) model. In a previous study it was shown that under steady state conditions the model is determined by stoichiometry rather than kinetics. To evaluate the kinetics of the metabolic biological phosphorus model, we recorded and simulated the start-up of a full-scale upgraded BPNR process. The initial state of the start-up was the simulated steady state of the process prior to the start-up. We could evaluate the SRT during the start-up on the basis of the accumulation of total-phosphorus in the sludge. During the start-up, the process changed from partly nitrifying to fully nitrifying, denitrifying and phosphate removing. Growth of PAO only showed sensitive for the glycogen formation rate kGLY. Disregarding a 20% in- or decrease of the kinetic rates of biological phosphorus removal (BPR), all start-up simulations converged to comparable steady states. This underlines that BPR is determined by stoichiometry rather than kinetics. Previous simulation studies with the TUDP-model showed that glycogen accumulated to unrealistic high concentrations with an increasing sludge retention time (SRT). We modelled the glycogen kinetics with a maximum glycogen fraction, which proved an effective and straightforward method to avoid unrealistic glycogen accumulation. In the steady state simulations, the glycogen concentration was determined by the value of the maximum glycogen fraction (fGLY(max)), whereas the overall sludge composition showed insensitive towards this parameter. Temperature appeared highly sensitive and therefore should not be neglected when modelling BPR.

Bioreactors↗

PubMed2002

Mathematical modelling of biofilm structures.

The morphology of biofilms received much attention in the last years. Several concepts to explain the development of biofilm structures have been proposed. We believe that biofilm structure formation depends on physical as well as general and specific biological factors. The physical factors (e.g. governing substrate transport) as well as general biological factors such as growth yield and substrate conversion rates are the basic factors governing structure formation. Specific strain dependent factors will modify these, giving a further variation between different biofilm systems. Biofilm formation seems to be primarily dependent on the interaction between mass transport and conversion processes. When a biofilm is strongly diffusion limited it will tend to become a heterogeneous and porous structure. When the conversion is the rate-limiting step, the biofilm will tend to become homogenous and compact. On top of these two processes, detachment processes play a significant role. In systems with a high detachment (or shear) force, detachment will be in the form of erosion, giving smoother biofilms. Systems with a low detachment force tend to give a more porous biofilm and detachment occurs mainly by sloughing. Biofilm structure results from the interplay between these interactions (mass transfer, conversion rates, detachment forces) making it difficult to study systems taking only one of these factors into account.

Bacteria↗

PubMed2002

Modelling of activated sludge processes with structured biomass.

Bacterial communities when subjected to feast-famine conditions as occurring in many wastewater treatment systems store substrates as reserve polymers. Including storage polymers in a description of microbial growth processes makes important the choice of kinetic relations. Presently there is no sound description for the diversion of substrate towards biomass growth or substrate storage. Based on observations with pure cultures and mixed cultures growing under dynamic conditions a model is proposed to describe such behaviour. This description is based on the observation that bacteria in order to grow fast have to induce a high level of RNA and proteins in order to allow fast growth. We assume that this protein synthesising system is only induced in the presence of external substrates. Based on this assumption a model structure is proposed and evaluated. It seems that this model can predict the turnover of PHA in the cells correctly, and describes well the overall behaviour mixed culture SBR systems. However especially the growth rate in the famine phase seems to be overestimated. The model defined here is a contribution to a further development of mechanistically based models for activated sludge processes.

Biomass↗

PubMed2002

Error diagnostics and data reconciliation for activated sludge modelling using mass balances.

Research on activated sludge models is mainly directed towards the reliability and estimation of model coefficients. Model calculations however, rely heavily on accurate determination of operational conditions. Accurate measurement of operational conditions and mass flows is difficult, caused by large (full-scale) process flows and the absence of reliable measurements. Therefore operational data should be verified on (gross) errors before being implemented in model studies. Calibrating a model on erroneous mass flows leads to laborious calibration procedures and moreover, unjustified adaptation of the model (kinetic and stoichiometric) parameters. Gross error detection is possible when there are more measurements than strictly required to solve a system of linear conservation relations (mass balances). A simple method for error detection is evaluating the mass balance residuals. For over determined systems data accuracy can be improved using balancing methods (i.e. minimising balance residuals). This is referred to as data reconciliation. A reconciled data set contains fewer errors and is exactly in line with the mass balances of the system. In this paper we describe a method for gross error detection and data reconciliation. It is shown how data reconciliation improves the accuracy of the data set and how the use of a balanced data set simplifies the model calibration procedure. This is demonstrated on the basis of a modelling study of a full-scale WWTP.

Calibration↗

PubMed2002

Model-based evaluation of a new upgrading concept for N-removal.

Mathematical modelling is considered a time and cost-saving tool for evaluation of new wastewater treatment concepts. Modelling can help to bridge the gap between lab and full-scale application. Bio-augmentation can be used to obtain nitrification in activated sludge systems with a limited aerobic sludge retention time. In the present study the potential for augmenting the endogenous nitrifying population is evaluated. Implementing a nitrification reactor in the sludge return line fed with sludge liquor with a high ammonia concentration leads to augmentation of the native nitrifying population. Since the behaviour of nitrifiers is relatively well known, a choice was made to evaluate this new concept mainly based on mathematical modelling. As an example an existing treatment plant (WWTP Walcheren, The Netherlands) that needed to be upgraded was used. A mathematical model, based on the TUDP model and implemented in AQUASIM was developed and used to evaluate the potential of this bioaugmentation in the return sludge line. A comparison was made between bio-augmentation and extending the existing aeration basins and anoxic tanks. The results of both modified systems were compared to give a quantitative basis for evaluation of benefits gained from such a system. If the plant is upgraded by conventional extension it needs an increase in volume of about 225%; using a bioaugmentation in the return sludge line the total volume of the tanks needs to be expanded by only 75% (including the side stream tanks). Based on the modelling results a decision was made to implement the bioaugmentation concept at full scale without further pilot scale testing, thereby strongly decreasing the scale-up period for this process.

Ammonia↗

PubMed2002

Experience with guidelines for wastewater characterisation in The Netherlands.

In this paper experiences and results are presented with guidelines for wastewater characterisation that are standardised in The Netherlands for modelling purposes with ASM. The wastewater characterisation is based on a physical-chemical method to characterise the soluble and particulate fractions, combined with a BOD-analysis for characterising the biodegradable fraction of the influent COD. By following the guidelines, a sufficiently detailed and practical characterisation is obtained, and the results can be used for simulation studies on treatment plants for process optimisation, trouble-shooting and design assistance. At present, five years of experience is gained with the guidelines and they were used for the simulation of circa 100 treatment plants. The guidelines are evaluated as simple and easy to implement in routine analysis programs.

Biodegradation, Environmental↗

PubMed2002

Experimental assessment and modelling of nitrate utilisation for primary sludge.

Electron acceptor utilisation potential of filtered primary sludge under anoxic conditions was experimentally investigated. Major kinetic and stoichiometric parameters were assessed by means of model evaluation of nitrate profile obtained in batch reactors. ASM1, modified for endogenous decay, and ASM3 were used for model simulation. Both models provided consistent interpretation of experimental data. ASM1 yielded mu(H) and Y(HD) values of 6.1 d(-1) and 0.64 g cell COD(g COD)(-1) respectively for heterotrophic anoxic growth. The corresponding storage mechanism associated with ASM3 could be characterised by a k(STO) of 13 g COD (g COD d)(-1) and a Y(STO) of 0.78 g COD(g COD)(-1). The high k(STO) value suggests re-evaluation of the concept of readily biodegradable substrate as defined in ASM3 and tested in the study.

Biodegradation, Environmental↗

PubMed2002

Respirometric assessment of storage yield for different substrates.

A new procedure has been defined for the respirometric assessment of bacterial storage yield as defined in the Activated Sludge Model No.3. The procedure was used to determine the storage yield, Y(STO), associated with acetate, glucose and domestic sewage, together with mixtures of acetate/glucose and acetate/domestic sewage at different initial F/M ratios. Y(STO) was calculated as 0.78 gCOD(gCOD)(-1) for acetate, 0.87 gCOD(gCOD)(-1) for glucose and 0.96 gCOD(gCOD)(-1) for domestic sewage. The Y(STO) of substrate mixtures was found to reflect the characteristics of the dominant fraction in the mixture.

Acetates↗

PubMed2002

Gas chromatographic analysis of polyhydroxybutyrate in activated sludge: a round-robin test.

Polyhydroxyalkanoates (PHA) and poly-beta-hydroxybutyrate (PHB) in particular have become compounds which is routinely investigated in wastewater research. The PHB analysis method has only recently been applied to activated sludge samples where PHA contents might be relatively low. This urges the need to investigate the reproducibility of the gas chromatographic method for PHB analysis. This was evaluated in a round-robin test in 5 European laboratories with samples from lab-scale and full-scale enhanced biological phosphorus removal systems. It was shown that the standard deviation of measurements in each lab and the reproducibility between the labs was very good. Experimental results obtained by different laboratories using this analysis method can be compared. Sludge samples with PHB contents varying between 0.3 and 22.5 mg PHB/mg sludge were analysed. The gas chromatographic method allows for PHV, PH2MB and PH2MV analysis as well.

Bioreactors↗