Biological Wastewater Treatment Reactors Research Articles

Research of the process of biological wastewater treatment under conditions of uneven load of the treatment system

The main purpose of the article is to develop a multifactorial model for rapid assessment of the efficiency of biological wastewater treatment reactors. A mathematical model of the process of biological wastewater treatment has been developed based on: changes in the concentration of organic contaminants in the bioreactor over time, taking into account the uneven flow of wastewater to the treatment plant, the process of substrate entering the bioreactor (different amounts may enter at different times). The software implementation of the proposed algorithm for solving the corresponding model problem in Python is carried out. The results of computer experiments on the study of the efficiency of wastewater treatment in biological treatment reactors for different operating conditions of facilities are presented. In particular, such processes were considered with taking into account the unevenness of the load, because the maximum cleaning loads are in the morning and in the evening. The task was solved to simulate a real situation and show how cleaning takes place at the maximum load at a certain time of the day. The results obtained will be useful for calculations in the design of biological treatment facilities or in the reconstruction of existing bioreactors for their prospective operation under new operating conditions.

Assessment of Cu(II) impact on aerobic sludge biomass and its post-exposure self-recovery potential

Heavy metals are released from various sources, including metal plating, potentially at a small-scale industrial level, and from automobile service and repair workshops, paint works, etc., spread across the city. Being part and parcel of the city, the wastewater from these metallic sources finds its way into the sewage system and can potentially disrupt biological sewage treatment plants. Therefore, conducting health and quality assessments of the sludge biomass becomes crucial to ensure the effective operation of these treatment plants and to maintain environmental and public health standards. In this study, impacts of Cu(II) (10, 20, 40, and 80 mg/L) exposure on sludge biomass have been investigated, and the potential of self-recovery of sludge biomass was explored when Cu(II) exposure is stopped. Reactors have been operated for 0–25 d with Cu(II) (termed as stressed phase) and then another 29 d without Cu(II) (termed as recovery phase) in the feed. During stressed phase operation, Cu(II) stressed reactors have shown 39–75 % chemical oxygen demand (COD) removal, compared to 91 % for control. Biomass activity and ECatalase activity have been inhibited under Cu(II) stress with changes in microbial community structure. During the recovery phase, Cu(II) stressed reactors have shown 41–82 % COD removal, whereas the control reactor has shown 91 % COD removal. A low sludge volume index (SVI) and high biomass interface settling velocity (ISV) with increased biomass activity have been obtained for biomass exposed to 10 and 20 mg/L of Cu(II). Lower Cu(II) doses have led to partial recovery of enzyme activity and microbial community structure, but higher doses have shown incomplete recovery. The study shows the impact caused by Cu(II) metal in a biological wastewater treatment reactor. Through comprehensive analysis of basic and advanced physico-chemical and biological parameters, the study recommends preliminary and confirmatory tests to know metallic ingress through wastewater influent in a treatment system. Advanced knowledge about metallic ingress will be useful for operators to take necessary steps for the sustainable operation of wastewater treatment systems.

Different Pathways of Microplastics Entering the Sludge Treatment System Distinctively Affect Anaerobic Sludge Fermentation Processes.

Microplastics in wastewater inevitably accumulate in waste activated sludge (WAS) via wastewater biological treatment, potentially affecting the subsequent sludge treatment unit. Nevertheless, all previous research studies focused on the impacts of the direct addition of one type of model microplastics on the sludge anaerobic treatment process. This approach actually cannot reflect the real situation where multiple different microplastics simultaneously get into the wastewater treatment unit prior to the sludge treatment unit. Herein, this work innovatively proposed a more realistic method to assess the real toxic influences of microplastics on anaerobic WAS fermentation for short-chain fatty acid (SCFA) production by initially adding four typical microplastics (i.e., polyethylene terephthalate, polystyrene, and polypropylene) to the biological wastewater treatment system. Results showed that four microplastics initially entering the biological wastewater treatment reactor had little influence on the subsequent anaerobic SCFA production since WAS solubilization increased but hydrolysis and acidification decreased. In contrast, when the four microplastics were directly dosed in a WAS anaerobic fermenter, although there was no effect on WAS solubilization, the bioprocess of hydrolysis-acidification was clearly suppressed, ultimately significantly (P = 1.86 × 10-7) inhibiting the maximal SCFA production from WAS by 21.5 ± 0.1% compared to the control without microplastic addition. The excessive oxidative stress and toxic leachates from these typical microplastics reduced the relative abundances of key anaerobes (e.g., Longilinea sp.) involved in the anaerobic fermentation. This work revealed that the different pathways of microplastics entering the sludge treatment system had different impacts on anaerobic sludge fermentation processes and selecting a more realistic and accurate approach was important to evaluate the true toxicity of microplastics on the sludge anaerobic treatment system.

When and why do gradients of the gas phase composition and pressure affect liquid-gas transfer?

Gas bubbles are introduced in water to absorb or strip volatile substances in a variety of unit operations, for example during (waste)water treatment. To calculate the transfer rate of substances between the liquid phase and the gas phase, different assumptions have been made in literature regarding the gas phase composition and hydraulic pressure, which both vary along the reactor height. In this study, analytical expressions were derived for the total (macroscopic) liquid-gas transfer rate, using either the complete gradients of the mole fraction and pressure (comprehensive approach) or a uniform value, for one or both of them. Simulations with the comprehensive model were performed to understand the effect of the type of volatile substance and of the reactor design and operating conditions on the total liquid-gas transfer rate. These effects were found to be highly interactive and often non-linear. Next, the simulation results of the comprehensive model were compared with those from models that assume either a uniform mole fraction or a uniform pressure in the complete reactor volume. This illustrated that for soluble substances, the mole fraction gradient strongly affects the total liquid-gas transfer rate, while the pressure gradient became only important under operating conditions that promote stripping (i.e., for a high concentration in the liquid phase and low concentration in the inlet gas). For very poorly soluble substances, the pressure became more important under conditions that promote absorption. These results on the importance of the mole fraction and pressure gradients remained equally valid when explicitly considering a typical variation of the volumetric overall transfer coefficient (KLa) along the reactor height. Finally, a simple and fast procedure was made available through a spreadsheet to select appropriate simplifying assumptions in reactor or plant-wide models. By applying the procedure to oxygen (O2), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and nitrogen gas (N2) in an aerobic biological wastewater treatment reactor, it was demonstrated that some common simplifications can lead to significant errors, for which corrections were proposed.

Key Microbes and Metabolic Potentials Contributing to Cyanide Biodegradation in Stirred-Tank Bioreactors Treating Gold Mining Effluent

ABSTRACT Microbial community and metabolic potential changes in a biological wastewater treatment reactor were investigated using phylogenetic and functional profile analysis from 16S rRNA-gene-based pyrosequencing. Stirred-tank bioreactors fed by cyanide-containing synthetic solutions were inoculated with a sewage sludge and the cyanide-degrading microbial consortium. A better cyanide degradation was observed in the reactors containing the phylum Proteobacteria, but lower efficiency was observed when Firmicutes and Bacteroidetes were dominant. This study shows the important role of phylum Proteobacteria for the cyanide-containing wastewater treatment in the gold processing plant and contributes the fundamentals about the microbial community exposed to the cyanide.

Comprehensive assessment of microbial aggregation characteristics of activated sludge bioreactors using fuzzy clustering analysis

Understanding microbial aggregation dynamics in response to the often violent environmental fluctuations is important for activated sludge wastewater biotreatment practice, yet remains poorly understood. We investigated microbial aggregation process of an activated sludge reactor in response to various operating conditions of resource limitations, disinfectant and pH stresses, and quantified aggregation characteristics by employing a fuzzy clustering analysis (FCA) method. The results revealed that the FCA provided a means for comprehensive assessment of microbial aggregation dynamics of the bioreactor relying solely on simple parameter estimation. Proper disinfectant stress (of NaClO 1.00% or 2.00%) is a promising strategy to improve the comprehensive performance of microbial aggregation and sludge settleability. Nitrogen– (of C/N ratio > 40) and dissolved oxygen–limitations (of DO < 0.2 mg/L) had medium influence on the comprehensive performance of the activated sludge system, while little impacts for acidic and alkaline conditions. These quantitative estimations offer insights into the underlying bio–physicochemical processes of an activated sludge bioreactor in response to practical fluctuations that is often beyond typical assessment practice. In addition, it may represent a step towards uncoupling the complex biophysical interactions that is essential for optimized designing and proper engineering practice of biological wastewater treatment reactors.

Comparison of Extraction Methods of Extracellular Polymeric Substances from Activated Sludge

Extracellular polymeric substances (EPSs) are biosynthetic polymers of microbial origin in the sludge activation process and crucially affect the properties of sludge in biological wastewater treatment reactors, such as the formation of sludge flocs, stabilization of sludge structure, and protection of microbes against noxious environmental conditions. However, the EPS extraction efficiency differs significantly according to the extraction method used. In this study, soluble EPSs and loosely bound EPSs can be extracted by centrifugation first and tightly bound EPSs in activated sludge require additional eight treatments for extraction, respectively. Three physical methods (centrifugation, sonication, and heating) and five chemical methods (cation exchange resin, NaOH, formaldehyde+NaOH, EDTA, and formaldehyde+EDTA) were tested, and the content and composition of TB-EPS were analyzed. Meanwhile, the functional groups and elements in TB-EPS were investigated. Results showed that the heating method did not introduce exogenous substances during the EPS extraction process and that the destruction of cells from this method was relatively slight. Heating was shown to be a gentle and efficient method in this study. The three-dimensional excitation and emission matrix (EEM) fluorescence spectroscopy showed that the cation exchange resin method had good extraction effect on humic-like and protein-like substances. As to fulvic-acid-like substances, NaOH was better than the other seven methods. Infrared spectroscopy showed that no notable difference appeared in the functional groups of the TB-EPS extracted by physical methods, whereas chemical methods induced big differences and showed particular bands that did not appear in the TB-EPS extracted by physical methods. Overall, the amounts of TB-EPS elements extracted using chemical methods were greater than those extracted using physical methods. In conclusion, a method must be selected and established for each case, taking into consideration the experimental purpose, and the most appropriate method should be chosen carefully.

Characteristics of ARG-carrying plasmidome in the cultivable microbial community from wastewater treatment system under high oxytetracycline concentration.

Studies on antibiotic production wastewater have shown that even a single antibiotic can select for multidrug resistant bacteria in aquatic environments. It is speculated that plasmids are an important mechanism of multidrug resistance (MDR) under high concentrations of antibiotics. Herein, two metagenomic libraries were constructed with plasmid DNA extracted from cultivable microbial communities in a biological wastewater treatment reactor supplemented with 0 (CONTROL) or 25mg/L of oxytetracycline (OTC-25). The OTC-25 plasmidome reads were assigned to 72 antibiotic resistance genes (ARGs) conferring resistance to 13 types of antibiotics. Dominant ARGs, encoding resistance to tetracycline, aminoglycoside, sulfonamide, and multidrug resistance genes, were enriched in the plasmidome under 25mg/L of oxytetracycline. Furthermore, 17 contiguous multiple-ARG carrying contigs (carrying ≥ 2 ARGs) were discovered in the OTC-25 plasmidome, whereas only nine were found in the CONTROL. Mapping of the OTC-25 plasmidome reads to completely sequenced plasmids revealed that the conjugative IncU resistance plasmid pFBAOT6 of Aeromonas caviae, carrying multidrug resistance transporter (pecM), tetracycline resistance genes (tetA, tetR), and transposase genes, might be a potential prevalent resistant plasmid in the OTC-25 plasmidome. Additionally, two novel resistant plasmids (containing contig C301682 carrying multidrug resistant operon mexCD-oprJ and contig C301632 carrying the tet36 and transposases genes) might also be potential prevalent resistant plasmids in the OTC-25 plasmidome. This study will be helpful to better understand the role of plasmids in the development of MDR in water environments under high antibiotic concentrations.

Fast anaerobic sludge granulation at elevated salinity

It is commonly accepted that high salt concentrations negatively affect microbial activity in biological wastewater treatment reactors such as upflow anaerobic sludge blanket (UASB) reactors. Microbial aggregation in such reactors is equally important. It is well documented that anaerobic granules, when exposed to high salinity become weak and disintegrate, causing wash-out, operational problems and decreasing process performance. In this research, the possibility of microbial granule formation from dispersed biomass was investigated at salinity levels of 5 and 20 g Na+/L. High removal efficiencies of soluble influent organics were achieved at both salinity levels and this was accompanied by fast and robust formation of microbial granules. The process was found to be stable for the entire operational period of 217 days. As far as we know this is the first time it has been demonstrated that stable granule formation is possible at a salinity level as high as 20 g Na+/L. Methanosaeta was identified as the dominant methanogen at both salinity levels. Streptococcus spp. and bacteria belonging to the family Lachnospiraceae were identified as the dominant microbial population at 5 and 20 and g Na+/L, respectively.

Characterization, structure, and function of extracellular polymeric substances (EPS) of microbial biofilm in biological wastewater treatment systems: a review

A review concerning the definition, extraction, characterization, production, and functions of extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment reactors is given in this paper. EPS are a complex high-molecular-weight mixture of polymers excreted by microorganisms, produced from cell lysis and adsorbed organic matter from wastewater. The EPS fill and form the space between the biofilm constituents, e.g. prokaryotic and eukaryotic microorganisms which inhabit the aggregates. It seems that the EPS matrix may serve as a multipurpose functional element of microbial communities, including adhesion, structure, protection, recognition, and physiology. Due to the metabolic activity of the cells, gradients develop and create different habitats within small distances, allowing a wide variety of organisms to settle and grow in the aggregate. As many of these organisms produce their specific EPS, it is not surprising that an extremely wide variety of microbial aggregates result. All of them have in common their highly hydrated extracellular matrix which is the place the organisms shape their very own microhabitat in which they live. However, as EPS are very complex, the knowledge regarding EPS is far from complete and much work is still required to fully understand their precise roles in the biological treatment process.

Quorum quenching is responsible for the underestimated quorum sensing effects in biological wastewater treatment reactors

Quorum sensing (QS) and quorum quenching (QQ) are two antagonistic processes coexisting in various bacterial communities in bioreactors, e.g., activated sludge for biological wastewater treatment. Although QS signal molecules are detected in activated sludge reactors and known to affect sludge properties and reactor performance, there has been no direct evidence to prove the endogenous existence of QQ effects in activated sludge. In this study, for the first time, acyl homoserine lactones-degrading enzymatic activity, a typical QQ effect, was discovered in activated sludge and found to considerably affect the QS detection results. The coexistence of QS and QQ bacteria in activated sludge was further confirmed by bacterial screening and denaturing gradient gel electrophoresis analysis. The method developed in this study could also be used to evaluate QQ activities in bioreactors, and a possible way is provided to tune bioreactor performance through balancing the QS and QQ processes.

Finding the Optimal Aeration Profiles and DO Profiles in a Plug-flow Biological Wastewater Treatment Reactor with A Theoretical Approach

It is a common principle that aeration to a wastewater treatment reactor performing aerobic carbon and ammonium oxidation should be minimised while satisfactory carbon and ammonium removal is achieved. The implementation of this principle in a plug-flow like aerobic reactor has so far been in an ad-hoc manner in the absence of a theoretically optimal dissolved oxygen (DO) profile. In this study, the Pontryagin Maximum Principle is applied to derive the theoretically optimal DO profile along a plug-flow aerobic reactor. While it is difficult to implement this optimisation algorithm online in practice due to the requirement of detailed wastewater composition and model parameter information, the optimal aeration profiles revealed under various conditions provide guidance to the design and operation of aeration systems. The optimal KLa and DO profiles can be applied in benchmark model to not only achieve satisfactory effluent requirements but also aeration energy is saved.

Formation of soluble microbial products (SMP) by activated sludge at various salinities

Soluble microbial products (SMP) present a significant component of effluent organic matter from biological wastewater treatment reactors, and can affect the membrane fouling and formation of disinfection by-products. Thus, SMP have attracted increasing concerns in wastewater treatment and reclamation. In this work, the formation of SMP by activated sludge at various NaCl concentrations is investigated by using fluorescence excitation-emission matrix (EEM) spectroscopy with parallel factor analysis (PARAFAC) and fluorescence regional integration (FRI). The results show that a high level of salinity decreases substrate removal efficiency and leads to an accumulation of SMP, especially proteins. Three components of SMP, one protein-like and two humic-acid-like components, are identified by PARAFAC, which exhibit different trends with the variation of NaCl concentration. FRI analysis reveals that the majority of protein fluorescence is attributed to tryptophan and tryptophan-like proteins, rather than tyrosine and tyrosine-like proteins. With an increase in NaCl concentration, the normalized volume percentages of tyrosine and tryptophan region increase, while those of humic- and fulvic-acid-like region decrease significantly. This work demonstrates that salinity affects the formation of SMP, and that EEM with PARAFAC in combination with FRI analysis is a useful tool to get insight into the formation of SMP by activated sludge.

Effects of Aeration Volume on Velocity Distribution of Fluid in Multi-Flora Biological Wastewater Treatment Reactor

The flow characteristics and effects of aeration volume on velocity distribution of the fluid in primary reaction zone of multi-flora biological wastewater treatment reactor were studied with clean water. Experimental results demonstrated that the fluid in primary reaction zone formed non-symmetrical cycle flow in the vertical plane with lower flow velocity in the central area and higher flow velocity in the peripheral area under the lifting action of the bubbles from perforated aerated pipe. As the aeration volume varied from1.0 m3/h to 2.2 m3/h, the flow velocity of central points had lower increased rate contrary to the peripheral points with higher increased rate in primary reaction area. Thus, the decreasing gradient of flow velocity of the primary reaction zone from the outer race to the center could be changed by controlling the aeration volume. For the design and operation of multi-flora biological wastewater treatment reactor, the results had important guiding significances that were listed out.

Bacterial Community Structure in Geographically Distributed Biological Wastewater Treatment Reactors

Current knowledge of the microbial communities within biological wastewater treatment reactors is incomplete due to limitations of traditional culture-based techniques and despite the emergence of recently applied molecular techniques. Here we demonstrate the application of high-density microarrays targeting universal 16S rRNA genes to evaluate microbial community composition in five biological wastewater treatment reactors in China and the United States. Results suggest a surprisingly consistent composition of microbial community structure among all five reactors. All investigated communities contained a core of bacterial phyla (53-82% of 2119 taxa identified) with almost identical compositions (as determined by colinearity analysis). These core species were distributed widely in terms of abundance but their proportions were virtually the same in all samples. Proteobacteria was the largest phylum and Firmicutes, Actinobacteria, Bacteroidetes were the subdominant phyla. The diversity among the samples can be attributed solely to a group of operational taxonomic units (OTUs) that were detected only in specific samples. Typically, these organisms ranked somewhat lower in terms of abundance but a few were present is much higher proportions.

Determination of proteins and carbohydrates in the effluents from wastewater treatment bioreactors using resonance light-scattering method

A simple and sensitive method was developed for the determination of low-concentration proteins and carbohydrates in the effluents from biological wastewater treatment reactors using resonance light-scattering (RLS) technique. Two ionic dyes, Congo red and Neutral red were, respectively used as an RLS probes for the determination of proteins and carbohydrates. This method is based on the interactions between biomacromolecules and dyes, which cause a substantial increase in the resonance scattering signal of dyes in the wavelength range of 200–650 nm. The characteristics of RLS spectra of the macromolecule–dye complexes, influencing factors, and optimum analytical conditions for the measurement were explored. The method was satisfactorily applied to the measurement of proteins and carbohydrates in the effluents from 10 aerobic or anaerobic bioreactors, and a high sensitivity were achieved.

Operating strategies for variable‐flow sequencing batch reactors

AbstractSequencing batch reactors (SBRs) are variable‐volume, non‐steady‐state, suspended‐growth biological wastewater treatment reactors. The treatment process is characterised by a repeated treatment cycle consisting of a series of sequential process phases: fill, react, settle, decant and idle. The design and operation of an SBR must take into account (1) the biological process requirement for treating influent wastewater and (2) the hydraulic requirement to enable throughput of the water through the reactor without compromising on the quality of biological treatment. During routine operation, the priority between the process and hydraulic consideration can change depending on the influent flow rate and its rate of change. The importance of the interaction between these considerations will vary depending on the fill strategy and the cycle time control strategy. Where flow‐proportional cycle times are utilised to optimise the treatment process, the operating strategy must be capable of accurately adjusting the intercycle phase times to prevent loss of biological treatment or volumetric capacity. This paper considers various operating strategies and describes the specific strategy used at the SBR at Avonmouth wastewater treatment works.

微生物活性助剤を添加した生物処理反応槽に出現する微小動物相の特徴

Acomparative study of construction and function of microfauna (protozoa and metazoa) communities in biological domestic wastewater treatment reactors with and without a microorganism activator (Saponin) was carried out. It was showed that the addition of Saponin did not change the construction of microfauna communities, but promoted the growth of microfauna, especially attached rotifer that was famous as useful metazoa in biological treatments. The variation of species of metazoa was smaller than that of protozoa. When water temperatures dropped in the range of 10-13°C, the growth of protozoa highly decreased, but the growth of metazoa increased, showed that metazoa was highly tolerant to low temperatures than protozoa. The biomass of a special group of microfauna with the habits of attached living and SS feeding in the reactor with Saponin was the highest in microfauna communities and remarkably higher than that in the reactor without Saponin. Moreover, rotifer Philodina sp. was the most prosperous in all species of that special group. Statistical analysis showed the close connection between biomass of attached microfauna with the habit of suspended particle feeding and effluent SS concentrations, indicating these microfauna played an important and useful role in the removal of suspended solids.

Hydrodynamics of Biological Aggregates of Different Sludge Ages: An Insight into the Mass Transport Mechanisms of Bioaggregates

Internal permeation and its role in mass transport to biological aggregates are investigated through detailed settling experiments. Three groups of bacterial aggregates (0.8-3.2 mm), which were different in fractal dimension and porosity, were generated in batch activated sludge reactors with biomass residence times (BRTs) of 5, 10, and 20 days. An apparatus of vertically connected double settling columns, which were filled respectively with water and an EDTA solution of a higher density, was used to characterize the settling and permeability features of individual aggregates. The settling velocities observed in water were in good agreement with those predicted by Stokes' law for porous but impermeable aggregates. The BRT10 and BRT20 aggregates, with porosities that were generally less than 0.92, had fluid collection efficiencies ranging from 0 to 0.1, while the BRT5 aggregates with porosities that were generally higher than 0.97 were almost completely impermeable. It is suggested that bioaggregates with either a tightly or a loosely packed structure cannot be as highly permeable as characterized for nonbiological fractal aggregates. While falling from water into the denser EDTA liquid, many BRT10 and BRT20 aggregates stopped and stayed below the interface of the two liquids for a period that was more than an order of magnitude shorter than the prediction based on the assumption of molecular diffusion as the predominant mass transport mechanism to the aggregates. The results indicate that limited intra-aggregate convection, which may not be important to the hydrodynamic properties of bioaggregates, could significantly enhance mass transport to suspended aggregates in biological wastewater treatment reactors.

Spatial distribution of total, ammonia-oxidizing, and denitrifying bacteria in biological wastewater treatment reactors for bioregenerative life support.

Bioregenerative life support systems may be necessary for long-term space missions due to the high cost of lifting supplies and equipment into orbit. In this study, we investigated two biological wastewater treatment reactors designed to recover potable water for a spacefaring crew being tested at Johnson Space Center. The experiment (Lunar-Mars Life Support Test Project-Phase III) consisted of four crew members confined in a test chamber for 91 days. In order to recycle all water during the experiment, an immobilized cell bioreactor (ICB) was employed for organic carbon removal and a trickling filter bioreactor (TFB) was utilized for ammonia removal, followed by physical-chemical treatment. In this study, the spatial distribution of various microorganisms within each bioreactor was analyzed by using biofilm samples taken from four locations in the ICB and three locations in the TFB. Three target genes were used for characterization of bacteria: the 16S rRNA gene for the total bacterial community, the ammonia monooxygenase (amoA) gene for ammonia-oxidizing bacteria, and the nitrous oxide reductase (nosZ) gene for denitrifying bacteria. A combination of terminal restriction fragment length polymorphism (T-RFLP), sequence, and phylogenetic analyses indicated that the microbial community composition in the ICB and the TFB consisted mainly of Proteobacteria, low-G+C gram-positive bacteria, and a Cytophaga-Flexibacter-Bacteroides group. Fifty-seven novel 16S rRNA genes, 8 novel amoA genes, and 12 new nosZ genes were identified in this study. Temporal shifts in the species composition of total bacteria in both the ICB and the TFB and ammonia-oxidizing and denitrifying bacteria in the TFB were also detected when the biofilms were compared with the inocula after 91 days. This result suggests that specific microbial populations were either brought in by the crew or enriched in the reactors during the course of operation.