Biomass Retention Time Research Articles

Microalgae-bacteria consortia dynamics in a long term operated membrane-coupled high-rate algal pond (MHRAP)

Traditional activated sludge-based technologies have significant drawbacks, including high energy requirements and greenhouse gas emissions. Microalgae-based processes offer a promising, low-cost, and environmentally friendly alternative. However, the knowledge of treatment systems based on microalgae-bacteria consortia is limited, and even more so is their microbial composition and its relationship with operational parameters. Thus, this study explores the dynamics of microalgae-bacteria consortia in a long-term operated membrane-coupled high-rate algal pond (MHRAP) for wastewater treatment. For this, a pilot-scale MHRAP plant, located in a wastewater treatment plant in Valencia (Spain), was monitored under various hydraulic retention times (HRT) and wastewater influents: i) effluent from a primary settler and ii) effluent form pre-treatment. The biomass retention time was kept constant at 6 days. The composition of the bacterial community was studied through 16S rDNA sequencing, while 18S rDNA sequencing was used to study the microalgae. The results indicate that shorter HRTs significantly increased bacterial diversity, but not eukarya. Principal Co-ordinates Analysis (PCoA) revealed that the HRT and the incoming wastewater quality control the type of the bacterial populations. However, this effect was not observed in eukaryotic organisms. The dominant microalgae genera identified were Desmodesmus and Coelastrella, with Coelastrella becoming more prevalent at shorter HRTs. For bacteria, Verrumicrobiota dominated (18–56%) at high HRT while Proteobacteria was dominant (28–44%) at HRTs below 6 days. The changes observed in the bacterial composition, including the ammonia oxidizing bacteria (AOB) community (mainly Nitrosomonas), suggest that photo-inhibition could be taking place. The nitrite oxidizing bacteria (NOB) community was dominated by Nitrospira and Candidatus Nitrotoga. Operational parameters such as light intensity, pH, and nitrite concentration were found to significantly influence the microbial community structure. Higher light intensity and alkaline pH favored the growth of Desmodesmus, while Coelastrella thrived under lower HRTs. Bacterial diversity plays a crucial role in the treatment process, while microalgae primarily support aerobic bacterial processes by providing oxygen. These findings contribute to a deeper understanding of the complex biological processes in microalgae-bacteria consortia and offer insights into improving wastewater treatment technologies.

Urban wastewater treatment at ambient conditions using microalgae-bacteria consortia in a membrane high-rate algal pond (MHRAP): The effect of hydraulic retention time and influent strength

The effects of hydraulic retention time (HRT) and influent wastewater strength on microalgae-bacteria performance were studied in two sets of experiments in a 6-day biomass retention time membrane high-rate algal pond (MHRAP) pilot plant to assess its pollutants removal potential. In the first set of experiments, the MHRAP was continuously fed with effluent from primary settling and operated at 6-, 4- and 2-days HRT and in the second set, the MHRAP was operated at 4-, 2-, and 1-day HRT using pre-treatment effluent. Results showed a complex interaction between algae-bacteria consortium, ambient conditions and MHRAP operation. In set 1, the highest total nitrogen (T-N) and total phosphorus (T-P) removal efficiencies were reached with 6-day HRT (23.6±0.6 % and 32±4 %, respectively), partially due to free ammonia nitrogen stripping and phosphorus precipitation, because of abiotic factors, which reduces the activity of nitrite-oxidizing bacteria and led to partial-nitrification. On the other hand, the 4 and 2-day HRT T-N and T-P removal efficiencies were reduced (below 10 % and 5 %, respectively) because the abiotic factors limited the physicochemical processes, and less biomass was taken up. In set 2, a suitable equilibrium between the microalgae and bacterial communities at 2-days HRT achieved high T-N (91±5 %) and T-P (71±8 %) removal efficiencies. Mass balances indicated that the nitrogen was mostly removed by nitrification-denitrification (87 % of T-N) and T-P predominant removal mechanism was precipitation. The COD removal efficiencies were higher than 86 % in both sets, since the particulate COD and colloids were removed by ultrafiltration. The obtained results showed that MHRAP technology would allow reducing COD and nutrient loads in the effluent under certain operating conditions, demonstrating the potential for resource recovery and wastewater treatment of microalgae-bacteria consortia.

Bioaugmentation with Clostridium pasteurianum for high-yield continuous bio-hydrogen production in a dynamic membrane bioreactor

This study demonstrated the feasibility of achieving high-yield continuous bio-H2 production through bioaugmentation with Clostridium pasteurianum. C. pasteurianum was introduced during the initial granulation stage in a dynamic membrane bioreactor (DMBR) inoculated with heat-treated digester sludge. Following bioaugmentation, the average hydrogen yield (HY) reached 3.07 ± 0.13 mol H2/mol hexoseconsumed, with a hydraulic retention time (HRT) of 2 h. This high-yield and high-rate hydrogen production performance was achieved alongside a high concentration of suspended and granular biomass as 21.02 ± 1.01 g VSS/L. The DM played a crucial role in prolonging the solid retention time of the suspended biomass to 12.15 h. C. pasteurianum emerged as the predominant species, constituting 92.3 % of the total bacterial population. This dominance significantly contributed to achieving the high HY, reaching 77 % of the theoretical maximum HY (4 mol H2/mol hexoseconsumed) by shifting the metabolic pathway from non-H2 production pathways to hydrogen-producing acetate. However, during further continuous operation, the hydrogen production pathway was gradually shifted towards the butyrate pathway, resulting in a decreased HY of 2.26 mol H2/mol hexoseconsumed along with the decreased population of C. pasteurianum. Additional bioaugmentation of C. pasteurianum during the maturational granulation stage did not impact the metabolic pathways or microbiome significantly, as the added strain was supposed to be washed out. This study provides insights into achieving high-yield continuous bio-H2 production through bioaugmentation with C. pasteurianum during the initial granulation stage. Further studies are necessary to sustain C. pasteurianum-dominated high HY with the acetate pathway during prolonged operation.

Study of parameters for the design of a continuous flow laboratory biogas reactor

This study aimed to determine the parameters needed to design a continuos flow bioreactor. Developing studies that expand the quantity and efficiency of biogas currently produced on an industrial anaerobic wastewater treatment in a food processing plant in the Brazilian State of Goiás. The procedures developed in this work we used nine sets of bioreactors in batch and followed the German Standard VDI 4630. A special workbench was developed for that purpose, alternative procedures for determining the volume of the produced biogas were tested. The biomass studied in this work is the resulting sludge of the wastewater treatment from an industrial plant. The daily production is about nine tons, currently, which is totally discarded. Through this batch tests, it was possible to calculate the volume of biogas production and the ideal biomass retention time of the biomass. Obtained data to project a laboratory continuous flow bioreactor. With this data was made a projection of the biogas production potential by the use of this feedstock in an industrial bioreactor constructed for this purpose.

Influence of salinity on microalgae-bacteria symbiosis treating shrimp farming wastewater

Shrimp farming has strongly developed in recent years, and became an important economic sector that helps create jobs and increase incomes for Vietnamese. However, the aquatic environment has also been greatly affected by the development due to the amount of wastewater discharged from shrimp farms. Among biological processes used for treating shrimp farming wastewater, the application of microalgae-bacteria co-culture is considered high potential due to its treatment and energy saving. Consequently, a photobioreactor operated with microalgae-bacteria co-culture was employed to treat shrimp farming wastewater. The salinity of wastewater and the operating condition (ratio of biomass retention time and hydraulic retention time, BRT/HRT) are the major factors affecting pollutant removal. Thus, this study investigated the effects of salinities of 0.5–20 ppt and BRT/HRT ratios of 1.5–16 on the removal performance. The results indicated that the nutrient removal was reduced when PBR operated under salinity over than 10 ppt and BRT/HRT over 5.5. Particularly, the nitrogen and phosphorus removal rates were achieved 6.56 ± 1.33 gN m−3 d−1 and 1.49 ± 0.59 gP m−3 d−1, and the removal rates decreased by 2–4 times under a salinity >10 ppt and 2–6 times under a BRT/HRT ratio >5.5. Whereas, organic matter treatment seems not to be affected when the removal rate was maintained at 28–34 gCOD m−3 d−1 under various conditions.

Integrated microalgae-bacteria modelling: application to an outdoor membrane photobioreactor (MPBR)

A mechanistic model describing the key interactions occurring in microalgae-bacteria consortia systems was developed and validated. The proposed model includes the most relevant features of microalgae, such as light dependence, endogenous respiration, growth, and nutrient consumption for different nutrient sources. The model is coupled to the plant-wide model BNRM2, including heterotrophic and nitrifying bacteria, and chemical precipitation processes, among others. A major novelty of the model is microalgae growth inhibition by nitrite. Validation was conducted using experimental data from a pilot-scale membrane photobioreactor (MPBR) fed with permeate from an anaerobic membrane bioreactor (AnMBR). Three experimental periods dealing with different interactions between nitrifying bacteria and microalgae were validated. The model was able to accurately represent the dynamics occurring in the MPBR, predicting the relative abundance of microalgae and bacteria over time. Specifically, >500 pairs of experimental and modeled data were evaluated, giving an average R2 coefficient of 0.9902. The validated model was also used to evaluate different offline control strategies for enhancing process performance. For example, partial-nitrification resulting in NO2-N accumulation (i.e., microalgae growth inhibition) could be avoided by increasing biomass retention time from 2.0 to 4.5 days. It has been also concluded that microalgae biomass growth rate could be also enhanced by punctually increasing the dilution rate, allowing to outcompete nitrifying bacteria.

Effect of biomass retention time on performance and fouling of a stirred membrane photobioreactor

Co-culture of microalgae-activated sludge has the potential to purify wastewater while reduce energy demand from aeration. In this work, a mechanically stirred membrane photobioreactor (stirred-MPBR) was used to evaluate the impact of the biomass retention time (BRT) on the treatment performance and membrane fouling. Results showed that stirred-MPBR was affected by BRT during treating domestic wastewater at a flux of 16.5 L m−2 h−1. The highest productivity was attained at BRT 7d (102 mg L−1 d−1), followed by BRT 10d (86 mg L−1 d−1), BRT 5d (85 mg L−1 d−1), and BRT 3d (83 mg L−1 d−1). Statistical analysis results showed that BRT 7d had a higher COD removal rate than BRT 10d, however, there is no difference in total nitrogen removal rate. The highest TP removal occurred when the biomass operated at BRT as short as 3d. Reduced BRTs caused a change in the microalgae-activated sludge biomass fraction that encouraged nitrification activity while simultaneously contributing to a higher fouling rate. The bound protein concentrations dropped from 31.35 mg L−1 (BRT 10d) to 10.67 mg L−1 (BRT 3d), while soluble polysaccharides increased from 0.99 to 1.82 mg L−1, respectively. The concentrations of extracellular polymeric substance fractions were significantly altered, which decreased the mean floc size and contributed to the escalating fouling propensity. At the optimum BRT of 7d, the stirred-MPBR showed sufficient access to light and nutrients exchange for mutualistic interactions between the microalgae and activated sludge.

Influence of nitrogen species and biomass retention time on nutrient removal and biomass productivity in a microalgae-based bioreactor

This study examined the effects of nitrogen species (NH 4 + N/NO 3 − N ratio) and biomass retention time (BRT) on nutrient uptake and biomass productivity in Chlorella sp . In batch photobioreactors (PBR), microalgae were cultivated with varied nitrogen species ratios (100% NH 4 + N, 50% NH 4 + N:50% NO 3 − N, and 100% NO 3 − N). It was observed that a medium containing 100% NO 3 − N boosted algae growth, with a maximum biomass concentration of 3188 mg/L. However, the lowest nutrient removal rates were obtained under the 100% NO 3 − N condition, according to the reverse logistics model. In contrast, the highest removal rates occurred for substrate containing 100% of NH 4 + N species based on first-order decay models, although yield biomass was lower (2940 mg/L). The presence of the ammonia nitrogen resulted in the flocculation of the microalgae into large flocs, suggesting that ammonia nitrogen was rapidly consumed to produce flocculation-related metabolic products rather than for biomass production. Using the 100% NH 4 + N medium, different BRT values (7, 5, 3, 2 days) were examined under continuous operation using a membrane photobioreactor (MPBR). At 3-day BRT, the maximum biomass productivity, nitrogen, and phosphorous removal rates were 214 ± 4, 63.1 ± 4.1, and 2.1 ± 0.6 mg/L d, respectively. The biomass yield increased as the BRT decreased, leading to an increase in pollutant removal rates. Finally, this study provides some essential information for improving the operating conditions of membrane photobioreactor system under different biomass retention times and various nitrogen sources in the feed. • The highest biomass productivity was achieved in the 100% NO 3 − N medium. • The highest nutrient removal occurred in the 100% NH 4 + N medium. • The highest biomass productivity and nutrient removal was observed at 3-day BRT. • Lower fouling rates were achieved at higher BRTs.

Simulation Experiment of Environmental Impact of Deep-Sea Mining: Response of Phytoplankton Community to Polymetallic Nodules and Sediment Enrichment in Surface Water.

In this paper, simulation experiments were conducted to study the response of phytoplankton biomass and community composition to the influence of polymetallic nodules and sediment at four stations in the western Pacific in 2021. Chlorophyll a, pico-phytoplankton cell abundance, and metal concentration were measured before and after 24 h of deck incubation. The results show that there were three different patterns of response, namely, restrained, stimulated, and unaffected patterns. The restrained pattern appeared in the filtered treatments at station Incub.01, and the stimulated pattern appeared in the unfiltered treatments at station Incub.02. The response of the phytoplankton was not detectable at stations Incub.03 and 04. Regardless, positive and negative responses were found in the dominant pico-phytoplankton group—Prochlorococcus—and with slight variation in Synechococcus. The concentration of manganese varied among the treatments compared to that of iron and other metals. The factors affecting the growth of the phytoplankton in this study were metal concentrations and turbidity. The phytoplankton biomass baseline may also have played an important role: the lower the biomass, the higher the growth rate. This study proved that deep-sea polymetallic nodule mining will have a specific impact on surface phytoplankton biomass, but turbidity and particle retention time could be important factors in mitigating the extent of the impact.

A Review on Development of Aerobic Granular Sludge Membrane Bioreactor for the Industrial Effluent Treatment

The technology of aerobic granular sludge and Membrane Bioreactor (MBR) were combined to develop an Aerobic Granular Sludge Membrane Bioreactor (AGMBR). Biomass content is high, higher the retention time of biomass, and the capacity of phosphorus and nitrogen removal has been shown by aerobic granular sludge; now a day it is a promising option for wastewater treatment. Reclaiming wastewater one of the favourite choices is Membrane technology in removing organic and inorganic impurities they have recognised as moderately operative with biological entities removal from wastewater. Consequently, the wastewater treatment membrane bioreactor and aerobic granular sludge technology are dynamic and new trending cost-efficient technologies. This article aims to review all the potential applications and principles in AGMBR technology.

A novel bioscrubber for the treatment of high loads of ammonia from polluted gas.

This work presents a novel bioscrubber configuration for the treatment of high ammonia loads at short contact times. The biological reactor was designed to work as a moving-bed biofilm rector (MBBR) increasing biomass retention time. This configuration is still unexplored for the treatment of waste gases. Long-term operation of a lab-scale bioscrubber under different inlet concentrations of ammonia (60-570 ppmv) and a gas contact time of 4s was performed to study the system operational limits during 250days. The effect of the dissolved oxygen concentration on the nitrification rate was also evaluated. Under these conditions, a critical elimination capacity (EC) of 250 NH3·m-3·h-1 and a maximum EC of 300g NH3·m-3·h-1 were obtained. The maximum nitrification rate obtained was 0.5kgN·m-3·day-1. However, this nitrification rate only was possible to be achieved under partial nitrification. For complete nitrification, the critical nitrification rate was 0.3kgN·m-3·day-1. These results confirm that bioscrubber coupled to a MBBR is a good alternative to treat high ammonia loads with remarkable advantages, such as the retention of properly biomass concentration without auxiliary equipment.

Unaerated feeding alters the fate of dissolved methane during aerobic wastewater treatment

In municipal wastewater treatment plants, some dissolved methane can enter the aerobic bioreactors. This greenhouse gas originates from sewers and return flows from anaerobic sludge treatment. In well-mixed conventional activated sludge reactors, methane emissions are largely avoided because methane oxidizing bacteria consume a large fraction, even without optimizing for this purpose. In this work, the fate of dissolved methane is studied in aerobic granular sludge reactors, as they become increasingly popular. The influence of the characteristic design and operating conditions of these reactors are studied with a mathematical model with apparent conversion kinetics and stripping: the separation of feeding and aeration in time, a higher substrate transport resistance, a high retention time of granular biomass and a taller water column. Even for a best-case scenario combining an unrealistically low intragranule substrate transport resistance, a high retention time, a tall reactor, an extremely high influent methane concentration and no oxygen limitation, the methane conversion efficiency was only 12% when feeding and aeration were separated in time, which is lower than for continuous activated sludge reactors under typical conditions. A more rigorous model was used to confirm the limited conversion, considering the multi-species and multi-substrate biofilm kinetics, anoxic methane consumers and the high substrate concentration at the bottom during upward plug flow feeding. The observed limited methane conversion is mainly due to the high concentration that accumulates during unaerated feeding phases, which favours stripping more than conversion in the subsequent aeration phase. Based on these findings, strategies were proposed to mitigate methane emissions from wastewater treatment plants with sequentially operated reactors.

Kinetic modeling of autotrophic microalgae mainline processes for sewage treatment in phosphorus-replete and -deplete culture conditions

A kinetic model of autotrophic microalgal growth in sewage was developed to determine the biokinetic processes involved, including carbon-, nitrogen- and phosphorus-limited microalgal growth, dependence on light intensity, temperature and pH, light attenuation and gas exchange to the atmosphere. A new feature was the differentiation between two metabolic pathways of phosphorus consumption according to the availability of extracellular phosphorus. Two scenarios were differentiated: phosphorus-replete and -deplete culture conditions. In the former, the microalgae absorbed phosphorus to grow and store polyphosphate. In the latter the microalgae used the stored polyphosphate as a phosphorus source for growth.Calibration and validation were performed with experimental data from a pilot-scale membrane photobioreactor (MPBR) fed with the permeate obtained from an anaerobic membrane bioreactor (AnMBR) pilot plant fed with real urban wastewater. 12 of the model parameters were calibrated. Despite the dynamics involved in the operating and environmental conditions, the model was able to reproduce the overall process performance with a single set of model parameters values. Four periods of different environmental and operational conditions were accurately simulated. Regarding the former, light and temperature ranged 10–406 μmol·m−2·s−1 and 19.7–32.1 °C, respectively. Concerning the later, the photobioreactors widths were 0.25 and 0.10 m, and the biomass and hydraulic retention times ranged 3–4.5 and 1.5–2.5 days, respectively. The validation of the model resulted in an overall correlation coefficient (R2) of 0.9954. The simulation results showed the potential of the model to predict the dynamics of the different components: the relative proportions of microalgae, nitrogen and phosphorus removal, polyphosphate storage and consumption, and soluble organic matter concentration, as well as the influence of environmental parameters on the microalgae's biokinetic processes. The proposed model could provide an effective tool for the industry to predict microalgae production and comply with the discharge limits in areas declared sensitive to eutrophication.

Nutrient recovery and microalgae biomass production from urine by membrane photobioreactor at low biomass retention times

Urine has been considered as an ideal nutrient source for microalgae cultivation thanks to its composition containing the high concentrations of nitrogen and phosphorus. Herein, the microalgae growth in urine was evaluated in a lab-scale membrane photobioreactor (MPBR) system. This work aimed to validate the influence of low biomass retention times (BRT) (10, 7, 5, 3, 2 d) on nutrient remediation and biomass productivity. It revealed that BRT of 7 d resulted in synergistically high biomass production (biomass productivity of 313 mg/L.d) and removal rates (TN of 90.5 mg/L.d and TP of 4.7 mg/L.d). Notably, the short BRT of 2–5 d was not sufficient to trigger actively growing microalgae and thus reduced biomass production rate. In addition, as operated at a low flux of 2 L/m2.h, MPBR system required no physical cleaning for 100 days of operation. The BRT-dependent biomass concentration played a pivotal role in changing the fouling rate of MPBR; however, the fouling is reversible in the MPBR system under the low flux condition.

Membrane aerated biofilm reactors for mainstream partial nitritation/anammox: Experiences using real municipal wastewater

This study investigated the potential of Membrane-Aerated Biofilm Reactors (MABRs) for mainstream nitrogen removal via partial nitration/anaerobic ammonium oxidation (anammox). Four laboratory-scale MABRs were operated with real municipal wastewater characterized by low concentrations of nitrogen (varying between 31 and 120 mg–NH4–N L−1) and the presence of biodegradable organic carbon (soluble COD (sCOD) between 7 and 230 mg-O2 L−1). Two reactors were operated with different aeration strategies (intermittent vs. continuous), the other two with differences in biomass retention (recirculation or removal of detached biomass). Keeping a constant HRT caused instabilities due to difficulties with setting the optimal oxygen flux for the respective surface loadings (1.6–6 g–NH4–N m−2 d−1). Operating the MABRs with a constant surface loading (2 g–NH4–N m−2 d−1) resulted in higher and more stable total nitrogen (TN) removal independent of the aeration strategy. The intermittently aerated MABR improved from an average TN removal of 23%–69%, the continuously aerated MABR from 20% to 50% TN removal. Independent of the feeding strategy, the continuously aerated reactor removed slightly more ammonium (80–95%) compared to the intermittently aerated reactor (74–93%).Limiting the oxygen supply by intermittent aeration proofed successful to favor partial nitritation and anammox. Continuous aeration did not achieve stable suppression of nitrite oxidizing bacteria (NOB). Of the removed ammonium, approx. 26% were left in the effluent as nitrate (only 10% with intermittent aeration).Recirculation of the detached biomass resulted in reattachment onto the biofilm or membrane surface. This recirculation led to significantly higher biomass retention times and thus to better performance. Removing detached biofilm from the reactor caused a slightly lower TN removal of 33% compared to 45% with reattachment, while average ammonium removal was 58% compared to 63%, respectively. Scouring events had a significant impact on the overall operation, resulting in short term losses of TN removal capacities of 50–100%.The microbial community composition was different depending on the aeration strategy and biomass retention. The continuously aerated reactor contained significantly more AOB than the intermittently aerated MABR. The reactor with biomass retention contained less ammonium oxidizing bacteria (AOB), compared to the reactor with low biomass retention. In all MABRs, anammox bacteria established in the biofilm after an initial drop in abundance.

Anaerobic phenol biodegradation: kinetic study and microbial community shifts under high-concentration dynamic loading.

The anaerobic biodegradation of phenol has been realised in a sequencing batch reactor (SBR) under anaerobic conditions with phenol as sole carbon and energy source and with glucose as co-substrate. A step-change increase of phenol loading (from 100 up to 2000mg/L of phenol concentration in the feed solution) has been applied during the acclimation phase in order to progressively induce the development of a specialised microbial consortium. This approach, combined with the dynamic sequence of operations characterising SBRs and with the high biomass retention time, led to satisfactory phenol and COD removal efficiencies with values > 70% for the highest phenol input (2000mg/L) fed as the single carbon and energy source. Analysis of removal efficiencies and biodegradation rates suggested that the use of glucose as co-substrate did not induce a significant improvement in process performance. Kinetic tests have been performed at different initial phenol (400-1000mg/L) and glucose (1880-0mg/L) concentrations to kinetically characterise the developed biomass: estimated kinetic constants are suitable for application and no inhibitory effect due to high concentrations of phenol has been observed in all investigated conditions. The microbial community has been characterised at different operating conditions through molecular tools: results confirm the successful adaptation-operation approach of the microbial consortium showing a gradual increase in richness and diversity and the occurrence and selection of a high proportion of phenol-degrading genera at the end of the experimentation. Key Points • Anaerobic phenol removal in the range of 70-99% in a sequencing batch reactor. • Negligible effect of co-substrate onremoval efficiencies and biodegradation rates. • No biomass inhibition due to phenol concentration in the range of 400-1000mg/L. • Increasing phenol loads promoted the culture enrichment of phenol-degrading genera.

Nitrite inhibition of microalgae induced by the competition between microalgae and nitrifying bacteria

Outdoor microalgae cultivation systems treating anaerobic membrane bioreactor (AnMBR) effluents usually present ammonium oxidising bacteria (AOB) competition with microalgae for ammonium uptake, which can cause nitrite accumulation. In literature, nitrite effects over microalgae have shown controversial results. The present study evaluates the nitrite inhibition role in a microalgae-nitrifying bacteria culture.For this purpose, pilot- and lab-scale assays were carried out. During the continuous outdoor operation of the membrane photobioreactor (MPBR) plant, biomass retention time (BRT) of 2 d favoured AOB activity, which caused nitrite accumulation. This nitrite was confirmed to inhibit microalgae performance. Specifically, continuous 5-d lab-scale assays showed a reduction in the nitrogen recovery efficiency by 32, 42 and 80% when nitrite concentration in the culture accounted for 5, 10 and 20 mg N·L−1, respectively. On the contrary, short 30-min exposure to nitrite showed no significant differences in the photosynthetic activity of microalgae under nitrite concentrations of 0, 5, 10 and 20 mg N·L−1.On the other hand, when the MPBR plant was operated at 2.5-d BRT, the nitrite concentration was reduced to negligible values due to increasing activity of microalgae and nitrite oxidising bacteria (NOB). This allowed obtaining maximum MPBR performance; i.e. nitrogen recovery rate (NRR) and biomass productivity of 19.7 ± 3.3 mg N·L−1·d−1 and 139 ± 35 mg VSS·L−1·d−1, respectively; while nitrification rate (NOxR) reached the lowest value (13.5 ± 3.4 mg N·L−1·d−1).Long BRT of 4.5 d favoured NOB growth, avoiding nitrite inhibition. However, it implied a decrease in microalgae growth and the accumulation of nitrate in the MPBR effluent. Hence, it seems that optimum BRT has to be within the range 2–4.5 d in order to favour microalgae growth with respect to AOB and NOB.

Performance of a membrane-coupled high-rate algal pond for urban wastewater treatment at demonstration scale

The objective of this study was to evaluate the performance of an outdoor membrane-coupled high-rate algal pond equipped with industrial-scale membranes for treating urban wastewater. Decoupling biomass retention time (BRT) and hydraulic retention time (HRT) by membrane filtration resulted in improved process efficiencies, with higher biomass productivities and nutrient removal rates when operating at low HRTs. At 6 days of BRT, biomass productivity increased from 30 to 66 and to 95 g·m−3·d−1 when operating at HRTs of 6, 4 and 2.5 days, respectively. The corresponding nitrogen removal rates were 4, 8 and 11 g N·m−3·d−1 and the phosphorous removal rates were 0.5, 1.3 and 1.6 g P·m−3·d−1. The system was operated keeping moderate specific air demands (0.25 m3·m−2·h−1), resulting in reasonable operating and maintenance costs (€0.04 per m3) and energy requirements (0.29 kWh per m3). The produced water was free of pathogens and could be directly used for reusing purposes.

Degradation and mineralization of 2-chlorophenol in a single-stage anaerobic fixed-bed bioreactor

Chlorophenols (CPs) have drawn great attention due to their high toxicity and ubiquitous presence in the environment. However, the practical application of anaerobic biodegradation to remove CPs is limited by low degradation rate and incomplete mineralization. This work aims to apply a single-stage anaerobic fixed-bed bioreactor (AnFBR) for complete anaerobic dechlorination and mineralization of CPs. Results showed that 2-CP removal efficiency of 99.4%, chemical oxygen demand (COD) removal efficiency of 93.0%, and methane yield of 0.22 L-CH4/g-COD could be obtained for a wide range of 2-CP loading rates (3.6–18.2 mmol L–1 d–1). Nearly complete anaerobic mineralization of 2-CP was achieved even in the absence of co-substrates, thereby greatly reducing the operation cost. This may be partly related to the attached-growth microorganisms in AnFBR, allowing a higher biomass concentration and longer biomass retention time for enhanced 2-CP removal. Moreover, 16S rRNA gene sequence analysis suggests that the AnFBR harbored the potential dechlorinators (e.g., Anaeromyxobacter ), phenol-degrading microbes (e.g., Comamonas and Syntrophobacter ), and methanogens (e.g., Methanobacterium and Methanosaeta ) after acclimation, which could cooperate effectively for 2-CP dechlorination and mineralization. Based on the identified intermediates, the possible mineralization pathway of 2-CP was proposed. These findings should be valuable to facilitate the engineering applications of AnFBRs for removing CPs from wastewater.

Continuous 3-year outdoor operation of a flat-panel membrane photobioreactor to treat effluent from an anaerobic membrane bioreactor

A membrane photobioreactor (MPBR) plant was operated continuously for 3 years to evaluate the separate effects of different factors, including: biomass and hydraulic retention times (BRT, HRT), light path (Lp), nitrification rate (NOxR), nutrient loading rates (NLR, PLR) and others. The overall effect of all these parameters which influence MPBR performance had not previously been assessed. The multivariate projection approach chosen for this study provided a good description of the collected data and facilitated their visualisation and interpretation.Forty variables used to control and assess MPBR performance were evaluated during three years of continuous outdoor operation by means of principal component analysis (PCA) and partial least squares (PLS) analysis. The PCA identified the photobioreactor (PBR) light path as the factor with the largest influence on data variability. Other important factors were: nitrogen and phosphorus recovery rates (NRR, PRR), biomass productivity (BP), optical density of 680 nm (OD680), ammonium and phosphorus effluent concentration (NH4, P), HRT, BRT, air flow rate (Fair) and nitrogen and phosphorus loading rates (NLR and PLR). The MPBR performance could be adequately estimated by a PLS model based on all the recorded variables, but this estimation worsened appreciably when only the controlled variables (Lp, Fair, HRT and BRT) were used as predictors, which underlines the importance of the non-controlled variables on MPBR performance. The microalgae cultivation process could thus only be partially controlled by the design and operating variables.A high nitrification rate was found to be inadvisable, since it showed an inverse correlation with NRR. In this respect, temperature and microalgae biomass concentration appeared to be the main factors to mitigate nitrifying bacteria activity.