Biodegradation kinetics of stored wastewater substrates by a mixed microbial culture

The anaerobic accumulation of several organic pollutants from industrial wastewaters, as storage substrates, and their subsequent aerobic biodegradation using a wastewater treatment mixed microbial culture for biological nutrient removal has been studied. The amount and the kinetics of substrate accumulation in the anaerobic stage depended on the characteristics of the wastewater fed to the anaerobic stage. Depending on the substrate used, levels of between 27 and 86% of storage polymers were accumulated with respect to the level obtained on feeding with acetate. The biodegradation kinetics were studied by modelling respirometry results. During the aerobic stage, oxygen-consumption data obtained in the respirometric tests were fitted to a model using a non-linear fitting estimation method. The simulation data obtained correlated well with the experimental oxygen-consumption data. The estimated kinetic parameters obtained indicate that each storage polymer was degraded at a different rate. However, the values obtained for the storage polymer half-saturation coefficient, K_S: 16 mg COD l⁻¹, and for the coefficient for endogenous respiration, b: 0.008 h⁻¹, were similar in all the experiments. The results indicate that each substrate produces the synthesis of a specific storage polymer that is degraded at a different rate.     

Spatial and temporal changes in the microbial community in an anaerobic swine waste treatment lagoon

Microorganisms are central to both the beneficial (organic degradation, nutrient removal, biogas production) and detrimental (odor production, pathogen contamination) effects of swine waste storage systems. In this study, both quantitative (real-time polymerase chain reaction) and qualitative (denaturing gradient gel electrophoresis, cloning, sequence analysis) molecular analyses were used to track spatial and temporal changes in the microbial community of swine slurry from a 0.4 ha anaerobic lagoon. The lagoon, located in a region of western Kentucky which has a humid, subtropical environment, was sampled on a monthly basis (n = 10) over a period of one year at four different depths (top, 51 cm from the top, 152 cm from the top, and bottom >198 cm). The concentration and diversity of Bacteroides sp. was seasonal (up to 90% decrease between March and June). Hespellia sp. and other clostridial species, on the other hand, were endemic in the slurry (concentrations up to 1.0 × 10⁷ cells mL^?1 slurry) regardless of time of the year or lagoon depth. Results suggest that there were seasonal effects on the microbial community in the swine lagoon, while the effect of depth was not as pronounced. Seasonal changes in the microbial community in stored wastes may be (directly or indirectly) correlated with changes in malodor emissions from lagoons.     

Identification of microorganisms in the granules generated during methane fermentation of the syrup wastewater produced while canning fruit

The wastewater produced in the process of canning fruit contains a syrup that consists mainly of sucrose. This syrup wastewater was treated by methane fermentation in an upflow anaerobic sludge blanket reactor. The organic loading rate of syrup wastewater was increased gradually as fermentation progressed. The higher the organic loading rate, the more methane gas evolved until the organic loading rate reached 30.3 kg COD m^?3 d^?1, at which point methane generation abruptly diminished because the loading rate was too high to stably operate the reactor. The changes in the microbial community, that of both bacteria and archaea in the granules, were analyzed simultaneously using PCR-DGGE during the fermentation process. Methanosaeta spp., which are methanogenic archaea that produce extracellular polymers indispensable for the formation of granules, were dominant when the methane gas vigorously evolved, and the iron-reducing bacterium belonging to genus Geobacter, which outcompetes methanogens, grew proportionally with the deterioration of methane fermentation.     

Microbial processes and bacterial populations associated to anaerobic treatment of sulfate-rich wastewater

A pilot-scale (1.2 m³) anaerobic sequencing batch biofilm reactor (ASBBR) containing mineral coal for biomass attachment was fed with sulfate-rich wastewater at increasing sulfate concentrations. Ethanol was used as the main organic source. Tested COD/sulfate ratios were of 1.8 and 1.5 for sulfate loading rates of 0.65–1.90 kgSO₄²⁻/cycle (48 h-cycle) or of 1.0 in the trial with 3.0 gSO₄²⁻ l⁻¹. Sulfate removal efficiencies observed in all trials were as high as 99%. Molecular inventories indicated a shift on the microbial composition and a decrease on species diversity with the increase of sulfate concentration. Beta-proteobacteria species affiliated with Aminomonas spp. and Thermanaerovibrio spp. predominated at 1.0 gSO₄²⁻ l⁻¹. At higher sulfate concentrations the predominant bacterial group was Delta-proteobacteria mainly Desulfovibrio spp. and Desulfomicrobium spp. at 2.0 gSO₄²⁻ l⁻¹, whereas Desulfurella spp. and Coprothermobacter spp. predominated at 3.0 gSO₄²⁻ l⁻¹. These organisms have been commonly associated with sulfate reduction producing acetate, sulfide and sulfur. Methanogenic archaea (Methanosaeta spp.) was found at 1.0 and 2.0 gSO₄²⁻ l⁻¹. Additionally, a simplified mathematical model was used to infer on metabolic pathways of the biomass involved in sulfate reduction.     

Dynamics of the microfauna community in a full-scale municipal wastewater treatment plant experiencing sludge bulking

We investigated the dynamics of the microfauna community in activated sludge, with special reference to sludge bulking, in two parallel municipal wastewater treatment systems in Beijing, China over a period of 14 months. Annual cyclic changes in microfauna community structures occurred in both systems. RELATE analysis based on Spearman's Rank correlation indicated that microfauna community structures were highly correlated with the sludge volume index (SVI) (p < 0.001), which indicates sludge settleability. Nutrient conditions of raw sewage (p < 0.01) and hydraulic retention time (HRT) (p < 0.05) were also related to microfauna community structures. Abundances of the species Epistylis plicatilis and Vorticella striata increased significantly with an increase in SVI (p < 0.001) and decrease in water temperature (p < 0.001), suggesting that sludge bulking may have created favorable conditions for the two species, even under unfavorable temperature conditions. Sludge de-flocculation primarily due to the excessive growth of Microthrix parvicella-like filaments could be an important driving force for the microfauna community changes. The release of flocculated non-filamentous bacteria may represent a suitable food source for these species. The two species may be considered as potential bioindicators for sludge bulking.     

Distribution and survival of faecal indicator bacteria in the sediments of the Bay of Vidy,Lake Geneva,Switzerland

The purpose of this study was to determine the concentrations and the horizontal distribution of faecal indicator bacteria (FIB) including Escherichia coli and Enterococcus sp. in the bottom sediments of the Bay of Vidy, City of Lausanne, Switzerland. A vertical distribution of FIB in sediments near the municipal wastewater treatment plant (WWTP) outlet was evaluated and their persistence in those sediments was monitored for a period of 90 days. High FIB levels were measured in the sediments sampled near the WWTP outlet pipe and the mouth of the Chamberonne River, at concentrations ranging between 10⁵ and 10⁷ CFU 100 g^?1. FIB levels at 10 cm depth in the sediments near the WWTP outlet pipe ranged between 10⁴ and 10⁵ CFU 100 g^?1, and were still detected in the top 6 cm after 90 days. Results of this study indicate that freshwater sediments of the Bay of Vidy constitute a reservoir of faecal indicator bacteria, which can persist in certain areas of the bay. Possible resuspension of FIB and pathogens may affect water quality and may increase health risks to sensitive populations during recreational activities. FIB survival in sediments for long periods is of considerable significance for the understanding of microbial pollution in water and for the management of risk at specific recreational coastal sites.     

Combined industrial and domestic wastewater treatment by periodic allocating water hybrid hydrolysis acidification reactor followed by SBR

A pilot-scale hybrid hydrolysis acidification reactor (HHAR) with periodic water allocation mode operation followed by sequencing batch reactor (SBR) in anoxic and aerobic metabolic function was evaluated for the treatment of low-biodegradable combined industrial and domestic wastewater. The HHAR combines the advantages of both the UASB reactor and AF, omitting the three-phase separator. Furthermore, it has lower average up-flow velocity (0.38–0.92 m/h) and higher periodic up-flow velocity (6 m/h), which made the reactor keep higher MLSS concentration (more than 10,000 mg/L) and sludge-bed is in periodic “expansion-sedimentation-expansion” state. When HRT less than 10 h, the B/C variation was positive and reached the maximum value of 0.07 at 8 h. SBR with a total cycle period of 4.5 h was applied as the post-treatment process to remove residual COD, NH₃-N and TN. At steady stage, the pilot-scale SBR effluent COD, NH₃-N and TN concentration was 65, 0.75 and 17.71 mg/L, corresponding in this case to full-scale SBR plant effluent was 93, 16.4 and 34 mg/L. Comparison results indicated that the application of HHAR–SBR system to treat combined industrial and domestic wastewater can improve effluent quality significantly.     

Behavior of methanogens during start-up of farm-scale anaerobic digester treating swine wastewater

The aim of this study was to monitor the changes in methanogenic community structures in an anaerobic digester (250 m³ working volume) during start-up including prolonged starvation periods. Redundancy analysis was performed to investigate the correlations between environmental variables and microbial community structures. The anaerobic digester was operated for 591 days at alternating operating temperatures. In initial start-up period at stage I (35 °C), growth of various species of mesophilic aceticlastic methanogens (AMs) and hydrogenotrophic methanogens (HMs) was observed. Methanobacteriales species survived better than other methanogens under long-term starvation conditions. In stage II (50 °C), HMs became dominant over AMs as the operating temperature changed from mesophilic to thermophilic due to increase of ammonia inhibition. In stage III (35 °C), only the Methanomicrobiales population significantly increased during 50 days of HRT while Methanobacteriales dominated over 15 days of HRT. The influent pH negatively correlated with all methanogenic populations especially in stage II.     

Research on soybean protein wastewater treatment by the integrated two-phase anaerobic reactor

The start-up tests of treating soybean protein wastewater by the integrated two-phase anaerobic reactor were studied. The results showed that the soybean protein wastewater could be successfully processed around 30 days when running under the situation of dosing seed sludge with the influent of approximately 2000 mg/L and an HRT of 40 h. When the start-up was finished, the removal rate of COD by the reactor was about 80%. In the zone I, biogas mainly revealed carbon dioxide (CO₂) and hydrogen (H₂). Methane was the main component in the zone 2 which ranged from 53% to 59% with an average of 55%. The methane content in biogas increased from the zone I to II. It indicated that the methane-producing capacity of the anaerobic sludge increased. It was found that the uniquely designed two-phase integrated anaerobic reactor played a key role in treating soybean protein wastewater. The acidogenic fermentation bacteria dominated in the zone I, while methanogen became dominant in the zone II. It realized the relatively effective separation of hydrolysis acidification and methanogenesis process in the reactor, which was benefit to promote a more reasonable space distribution of the microbial communities in the reactor. There were some differences between the activities of the sludge in the two reaction zones of the integrated two-phase anaerobic reactor. The activity of protease was higher in the reaction zone I. And the coenzyme F₄₂₀ in the reaction zone II was twice than that in the reaction zone I, which indicated that the activity of the methanogens was stronger in the reaction zone II.     

Decolorization of Reactive Red 195 by a mixed culture in an alternating anaerobic–aerobic Sequencing Batch Reactor

The mono-azo dye Reactive Red 195 (RR 195) is a widely used color compound in the textile industry. As many other colors, it is persistent and difficult to be removed from water with conventional processes. The present study investigates biological decolorization of RR 195 under alternate anaerobic–aerobic conditions in a laboratory scale Sequencing Batch Reactor (SBR) containing a mixed culture and fed with a biodegradable carbon source. Different values of the Sludge Retention Time (SRT), Hydraulic Retention Time (HRT), influent color and organic carbon loadings were adopted during the experimental activity and their effects on color and Chemical Oxygen Demand (COD) removal efficiencies and process kinetics determined. The optimal operating conditions were found to be: 800 mg l⁻¹ influent COD, 50 d SRT and a 24 h-cycle. Under these conditions, the maximum color efficiency of 97% was achieved for a 40 mg l⁻¹ RR 195 in the feed. Some inhibition was present at influent color loadings above 40 mg l⁻¹, which was confirmed by the application of the Haldane model.     

Biodegradation of 2,4,6-trinitrophenol (picric acid) in a biological aerated filter (BAF)

This study demonstrated the microbial purification of a model wastewater containing 2,4,6-trinitrophenol (TNP), which was carried out in a continuously working biological aerated filter (BAF). The main emphasis was on the operating performance of the reactor as a function of the pollution load. TNP was degraded at a maximum volumetric removal rate of 2.53 g TNP/L d, with low residual COD and TNP concentration. Overloading of TNP inhibited the nitrite-oxidizing activity, resulting in poor TNP degradation performance in the BAF system. The inhibition depended on some factors, such as influent concentrations and flow rates of the influent. It is assumed that nitrite-oxidizing occurred spontaneously during TNP degradation in the BAF system, could have significant influence on TNP wastewater treatment. One year after the reactor start-up, the dominance of Rhodococcus, which was initially inoculated in the reactor, was confirmed by analysis of 16S rDNA sequence of the PCR products separated by DGGE.     

Toxicity evaluation of parboiled rice effluent using sperm quality of zebrafish as bioindicator

This study aimed to assess the acute toxicity of raw and treated wastewater generated by the rice parboiling industry using zebrafish (Danio rerio) sperm quality as a bioindicator. Toxicity bioassays were conducted comparing physicochemical parameters of sperm quality for zebrafish at sublethal conditions (n = 150 fish, 50 per treatment). Acute toxicity was detected in all sperm quality parameters assessed for both raw and treated wastewater, when contrasted to the control (p < 0.05). For zebrafish exposed to raw effluent, negative correlations with parameters of sperm quality were observed for the concentration of iron, phosphorus and total suspended solids (p < 0.05). Salinity, the biochemical oxygen demand and the concentration of total suspended solids were negatively correlated with parameters of sperm quality for zebrafish exposed to treated effluent (p < 0.05). In comparison with the levels observed for the raw effluent, most physicochemical parameters of the treated effluent were reduced to levels within the limits required by the environmental legislation. Despite the physical and chemical parameters measured in the treated wastewater meeting environmental legislation thresholds, acute toxicity persisted. These results show that the sperm quality can be used as a bioindicator for wastewater toxicity and release of wastewater to surface water could affect the fertility of fishes.     

Application of anaerobic–aerobic sequential treatment system to real textile wastewater for color and COD removal

A sequential anaerobic packed column reactor and an activated sludge unit was operated continuously for treatment of a textile industry wastewater, in Izmir, Turkey. Metal sponges were used as support material in anaerobic unit and pre-activated textile dyestuff biodegrading PDW facultative anaerobic bacterial culture was immobilized on the support particles. Effects of hydraulic retention times in anaerobic unit (θ_H anaerobic = 12–72 h) and initial COD concentration (COD₀ = 3000 ± 200 mg/L and 800 ± 100 mg/L) at θ_H anaerobic = 24 h on color and COD removal performance of the system were investigated. The results indicated that over 85% decolorization and about 90% COD removal efficiency can be obtained up to θ_H anaerobic = 48 h but higher retention times causes decreasing in decolorization efficiency. Operating the system with real wastewater without adding any nutrients at θ_H anaerobic = 24 h resulted in over 60% improvement in color removal in studied wastewater compared to existing treatment plant.     

Impacts of monosodium glutamate industrial wastewater on plant growth and soil characteristics

Research into utilization of monosodium glutamate industrial wastewater (MSGW) as a plant nutrient source was undertaken. The physico-chemical and microbiological characteristics of MSGW were analyzed in detail. Effect of MSGW on early growth of Chinese cabbage (Brassica rapa L. cv. Pekinensis) and maize (Zea mays L. cv. Bright Jean) was tested by the seed germination bioassay. Subsequently, in a greenhouse pot experiment using the same plant species, effects of MSGW application rates on the plant biomass yield, nitrogen content and soil properties were analyzed. The MSGW was characterized by high levels of N (56.7 g l^?1), organic C (344.6 g l^?1), total solids (600 g l^?1) and other minerals. At MSGW concentrations below 1%, germination indices for both the plant species were significantly (p < 0.01) higher than the control. Further, the greenhouse study results indicated significant increase in the plant biomass yield at MSGW application rates of 5000 and 7500 l ha^?1. As the MSGW dose increased, the biomass yield decreased, decreasing the N-use efficiency. Maize showed significantly higher wastewater N-use efficiency compared to the Chinese cabbage. Although the total culturable bacterial and fungal counts in the raw MSGW were low, addition of MSGW to the soil increased the soil microbial activities and soil respiration. Soil organic C was also increased by the addition of MSGW, due to the presence of significant amounts of organic C in the wastewater. This preliminary study demonstrates that by proper management of the pH and optimization of application rate, MSGW can be utilized as a nutrient source for plant growth. Further long-term field studies to evaluate the environmental impact of MSGW usage in agriculture are being designed to reduce the environmental risks associated with the reuse of this underutilized wastewater in the agriculture.     

Investigation on the formation and kinetics of glucose-fed aerobic granular sludge

Aerobic granular sludge was cultivated in a glass sequencing batch reactor (SBR) with glucose synthetic wastewater. The spherical shaped granules were observed on 4th day with the mean diameter of 0.1 mm. With the increase of chemical oxygen demand (COD) concentration of the influent, aerobic granules grew matured, the size of which ranged from 1.2 to 1.9 mm. The aerobic granular sludge could sustain high organic loading rate (about 4.0 g COD L⁻¹ d⁻¹), with good settling ability (settling velocity 36 m/h) and high biomass concentration (MLSS 6.7 ±0.2 g/L). Experimental data indicated that the substrate utilization and biomass growth kinetics followed Monod's kinetics model approximately. The corresponding kinetic coefficients of maximum specific substrate utilization rate (k), half velocity coefficient (K_s), growth yield coefficient (Y) and decay coefficient (K_d) were 13.2 d⁻¹, 275.8 mg/L, 0.183–0.250 mg MLSS/mg COD and 0.023–0.075 d⁻¹, respectively, which made aerobic granules have short setup period, high rate of substrate utilization and little surplus sludge.     

Distribution of anammox bacteria in domestic WWTPs and their enrichments evaluated by real-time quantitative PCR

Enrichment of anaerobic ammonium oxidation (anammox) bacteria using five activated sludges in three domestic wastewater treatment plants (WWTPs) were processed in a short term of 70 days and evaluated by real-time quantitative PCR (RTQ-PCR). Before the enrichment, building phylogenetic trees of Planctomycetes phylum in four reactors of sequencing batch reactor (SBR), anoxic and oxic reactors of anaerobic–anoxic–oxic (A2O) process, and rotating biological contactor (RBC) revealed six groups of distantly relative genera of Planctomyces, Pirellula, Gemmata, Isophaera, Candidatus and putative anammox bacteria. All clones of Candidatus sp. were affiliated with anammox bacteria and the majority of anammox clones were related to Planctomycete KSU-1 (AB057453). The discovery of anammox bacteria in raw activated sludges provided a partial rationale for the utilization of activated sludge as a seeding source of the anammox process. To verify the activity of anammox bacteria in the activated sludges, enrichment cultivations were conducted using SBRs. The enrichment of anammox bacteria resulted in the significant anammox activity of three samples. Quantification of 16S rRNA gene of anammox bacteria using RTQ-PCR showed the highest concentration of anammox bacteria of 2.48 ± 0.22 × 10⁹ copies of 16S rRNA gene/mg-volatile suspended solids (VSS), which was the same order of magnitude as that of the referential granular anammox sludge, 6.23 ± 0.59 × 10⁹ copies of 16S rRNA gene/mg-VSS, taken from an anammox upflow anaerobic sludge blanket (UASB) reactor. The doubling time of anammox bacteria enriched in this study was 1.18 days. The growth yield of anammox bacteria enriched in this study was 4.75 ± 0.57 × 10⁶ copies of 16S rRNA gene/mg of ammonium- and nitrite-nitrogen, which was similar to 4.50 ± 0.61 × 10⁶ copies of 16S rRNA gene/mg of ammonium- and nitrite-nitrogen for the referential anammox sludge. Substrate uptake rates of three successful enrichments at the end of the enrichment were comparable to those of granular and suspended anammox sludges. Rapid enrichment of anammox bacteria using activated sludge could offer an alternative method for obtaining a large volume of seeding anammox sludge.     

Electrically conductive magnetite particles enhance the kinetics and steer the composition of anaerobic TCE-dechlorinating cultures

The interaction of anaerobic dechlorinating cultures with soil and aquifer geochemical components is largely unknown, although this has potentially a major impact on the bioremediation of chlorinated solvent-contaminated sites. In this study, we found that addition of magnetite (Fe₃O₄) – the end-product of Fe(III)-reduction by dissimilatory iron reducing bacteria – to anaerobic dechlorinating cultures enhances the kinetics of trichloroethene dechlorination up to 1.5-times, compared to unamended controls. Specifically, a low concentration (approx. 10 mg/L as total Fe) of small size particles (200 nm-filtered) resulted in a greater stimulatory effect compared to the addition of a higher concentration (approx. 300 mg/L as total Fe) of unfiltered particles. Notably, Desulforomonas spp. were substantially enriched in microcosms supplemented with magnetite, whereas Dehalococcoides mccartyi spp. was found to be markedly inhibited or outcompeted. Multiple lines of evidence, including the direct visualization of microbial cells and magnetite particles via Confocal Laser Scanning Microscopy (CLSM), suggest that electrically conductive particles promoted the establishment of a cooperative metabolism, based on direct interspecies electron transfer, between dechlorinating and non-dechlorinating microorganisms.     

Kinetic modelling and performance prediction of a hybrid anaerobic baffled reactor treating synthetic wastewater at mesophilic temperature

A modelling study on the anaerobic digestion process of a synthetic medium-strength wastewater containing molasses as a carbon source was carried out at different influent conditions. The digestion was conducted in a laboratory-scale hybrid anaerobic baffled reactor with three compartments and a working volume of 54 L, which operated at mesophilic temperature (35 °C). Two different kinetic models (one model was based on completely stirred tank reactors (CSTR) in series and the other an axial diffusion or dispersion model typical of deviations of plug-flow reactors), were assessed and compared to simulate the organic matter removal or fractional conversion. The kinetic constant (k) obtained by using the CSTR in series model was 0.60 ± 0.07 h⁻¹, while the kinetic parameter achieved with the dispersion model was 0.67 ± 0.06 h⁻¹, the dispersion coefficient (D) being 46. The flow pattern observed in the reactor studied was intermediate between plug-flow and CSTR in series systems, although the plug-flow system was somewhat predominant. The dispersion model allowed for a better fit of the experimental results of fractional conversions with deviations lower than 8% between the experimental and theoretical values. By contrast, the CSTR in series model predicted the behaviour of the reactor somewhat less accurately showing deviations lower than 10% between the experimental and theoretical values of the fractional conversion.     

Relationship between microbial activity and microbial community structure in six full-scale anaerobic digesters

High activity levels and balanced anaerobic microbial communities are necessary to attain proper anaerobic digestion performance. Therefore, this work was focused on the kinetic performance and the microbial community structure of six full-scale anaerobic digesters and one lab-scale co-digester. Hydrolytic (0.6–3.5 g COD g^?1 VSS d^?1) and methanogenic (0.01–0.84 g COD g^?1 VSS d^?1) activities depended on the type of biomass, whereas no significant differences were observed among the acidogenic activities (1.5–2.2 g COD g^?1 VSS d^?1). In most cases, the higher the hydrolytic and the methanogenic activity, the higher the Bacteroidetes and Archaea percentages, respectively, in the biomasses. Hydrogenotrophic methanogenic activity was always higher than acetoclastic methanogenic activity, and the highest values were achieved in those biomasses with lower percentages of Methanosaeta. In sum, the combination of molecular tools with activity tests seems to be essential for a better characterization of anaerobic biomasses.     

Two ways to achieve an anammox influent from real reject water treatment at lab-scale: Partial SBR nitrification and SHARON process

A comparative study to produce the correct influent for Anammox process from anaerobic sludge reject water (700–800 mg NH₄⁺-N L⁻¹) was considered here. The influent for the Anammox process must be composed of NH₄⁺-N and NO₂⁻-N in a ratio 1:1 and therefore only a partial nitrification of ammonium to nitrite is required. The modifications of parameters (temperature, ammonium concentration, pH and solid retention time) allows to achieve this partial nitrification with a final effluent only composed by NH₄⁺-N and NO₂⁻-N at the right stoichiometric ratio. The equal ratio of HCO₃⁻/NH₄⁺ in reject water results in a natural pH decrease when approximately 50% of NH₄⁺ is oxidised. A Sequencing batch reactor (SBR) and a chemostat type of reactor (single-reactor high activity ammonia removal over nitrite (SHARON) process) were studied to obtain the required Anammox influent. At steady state conditions, both systems had a specific conversion rate around 40 mg NH₄⁺-N g⁻¹ volatile suspended solids (VSS) h⁻¹, but in terms of absolute nitrogen removal the SBR conversion was 1.1 kg N day⁻¹ m⁻³, whereas in the SHARON chemostat was 0.35 kg N day⁻¹ m⁻³ due to the different hydraulic retention time (HRT) used. Both systems are compared from operational (including starvation experiments) and kinetic point of view and their advantages/disadvantages are discussed.