Microalgae Co-Immobilized in Alginate Beads for Wastewater Treatment Notes

Removal of Endocrine Disrupting Compounds from Wastewater by Microalgae

Highlights

  • Co-immobilized microalgae's effect on EDC removal was evaluated.
  • Microalgae immobilization increased NH_4-N and TP removal.
  • EDC kinetic removal rates ranged from 0.013 to 2.131 d^{-1}.
  • Co-immobilizing microalgae increased some EDC removal.
  • NH_4-N and TP concentration decay correlated with EDC concentration decay.

Abstract

  • Microalgae systems effectively remove microcontaminants; immobilized microalgae's effectiveness on EDCs is assessed.
  • Free & immobilized microalgae mixed in 2.5 L reactors with treated wastewater to assess removal efficiency for 6 EDCs. Control reactors w/o microalgae were also used.
  • After 10 days:
    • Free microalgae reactors: 64% NH_4-N, 90% TP eliminated.
    • Immobilized microalgae reactors: 89% NH_4-N, 96% TP eliminated.
    • Control reactors: 40% NH_4-N, 70% TP eliminated.
  • Both free and immobilized microalgae reactors removed up to 80% of most EDCs within 10 days.
  • Free microalgae increased kinetic removal rate for Bisphenol A, 17-a-ethinylestradiol, and 4-octylphenol (25%, 159%, 41% respectively).
  • Immobilizing microalgae in alginate beads enhanced kinetic removal rate for Bisphenol AF, Bisphenol F, and 2,4-dichlorophenol.
  • Conclusion: Co-immobilized microalgae-based wastewater treatment boosts removal of nutrients & some EDCs from wastewater effluents.

1. Introduction

  • Endocrine Disrupting Compounds (EDCs) disrupt hormonal signaling systems; they include pesticides, flame retardants, pharmaceuticals, chemicals in personal care products & housewares.
  • Conventional wastewater treatment plants (WWTPs) don't treat EDCs, leading to their presence in natural water bodies, exerting ecotoxicological effects even at low concentrations.
  • 17a-ethinylestradiol (EE2) feminizes male fish (fathead minnow, Pimephales promelas) at 5-6 ng L^{-1}, nearly causing extinction.
  • Bisphenol A (BPA) induces feminization in Xenopus laevis tadpoles; 4-octylphenol (OP) has high estrogenic effects.
  • Importance of reducing EDCs discharge into aquatic environments.
  • Tertiary-treatment wastewater technologies exist for EDC removal, including advanced oxidation & constructed wetland systems, but microalgae technologies are less understood.
  • Microalgae-based wastewater treatment produces algal biomass for fertilizer, products (paraffin, olefin, glycerol, protein, anti-oxidants, pigment, plastic, biofuel), & high-quality effluent.
  • Few studies focus on EDC removal by microalgae, though microalgae-based systems can remove organic matter and nutrients.
  • Lab studies suggest microalgae treatment removes EDCs via evaporation, photodegradation, biodegradation, and/or microalgae uptake.
  • Co-immobilization (joint immobilization) of microalgae and bacteria enhances pollutant removal and biomass recovery.
  • Deterioration of alginate beads due to pH and water composition is a concern; alginate-degrading bacteria exist in wastewater.
  • Immobilizing microalgae in alginate beads boosts removal of heavy metals & organic pollutants (organotin, oil spill compounds).
  • One study found microalgae co-immobilization enhances nonylphenol (NP) removal briefly (12-24 h) but performs worse than free microalgae over longer periods (96-168 h).
  • Lack of knowledge regarding real wastewater and immobilized microalgae effects on other EDCs at real environmental concentrations (mg L^{-1}).
  • The study aims to quantify the effect of co-immobilization of microalgae in alginate beads on the removal efficiency of 6 EDCs at environmental concentration (10 mg L^{-1}) using secondary-treated wastewater.
  • This is the first study using microalgae technology to attenuate EDCs from secondary-treated wastewater effluents.

2. Material and methods

2.1. Experimental design

  • The study assessed the effect of microalgae and co-immobilization with bacteria on EDC removal.
  • Set-ups included microalgae reactors, control reactors without microalgae, co-immobilized microalgae in alginate beads, and control alginate beads.
  • A total of 12 experimental units consisted of 3 replicates of each of the 4 types of reactors.
  • Reactors fed with secondary-treated wastewater from the Montcada i Reixac WWTP.
  • Wastewater composition: total suspended solids (TSS) 75.4 mg L^{-1}; chemical oxygen demand (COD) 49 mg L^{-1}; NH_4-N 36.3 mg L^{-1}; total phosphorous (TP) 0.46 mg L^{-1}; fecal coliforms, 4.5 log CFU/100 mL; conductivity of 1480 mS cm^{-1}.
  • Reactor systems consisted of 2.5 L pre-cleaned glass containers.
  • A standard solution containing the 10 EDCs in methanol solution was added to each reactor (final water volume of 2 L) to obtain a final concentration of 10 mg L^{-1} (200 mL of spiking solution at 100 mg L^{-1} for each compound in methanol).
  • Microalgae reactors were inoculated with a microalgae consortium from an experimental high-rate algal pond treating urban wastewater.
  • Main microalgae populations were Chlorella sp. and Nitzschia acicularis. The inoculum also contained bacteria, but the microalgae accounted for over 90% of the biomass.
  • The microalgae consortium was pre-acclimatized to the growth conditions (secondary-treated wastewater) for 20 days before the reactors were stocked with microalgae.
  • The free and co-immobilized microalgae were inoculated to a concentration of approximately 80 mg L^{-1} dry weight (dw) biomass per reactor (150 mL of pre-acclimatized microalgae of approximately 800 mg L^{-1} dw biomass).
  • The experiments were run simultaneously for 10 days.
  • Reactors were set up in a temperature-controlled growth room at 23 ± 5 °C and lit by fluorescent tubes at a photon flux density of 150 mmol m^{-2} s^{-1} in a 12 h light/12 h dark cycle (Philips Master TL-D, 36W/840).

2.2. Immobilization of microalgae in alginate beads

  • Microalgae were immobilized as described previously by de-Bashan and Bashan (2010).
  • 150 mL of pre-acclimatized microalgae was mixed with 2% sodium alginate and stirred for 60 min to obtain an alginate solution.
  • The mixture was then dropped into a 2% CaCl_2 solidification solution in secondary-treated wastewater using a peristaltic pump (Minipuls 2, Gilson, WI, U.S.A.) with a flow rate of 1.1 mL min^{-1} to produce uniform immobilized microalgal beads (3-5 mm in diameter each)
  • Approximately 4600 beads were automatically produced per reactor.
  • The beads were left for 1 h under a soft stirring for curing and then washed with secondary-treated wastewater.
  • Control beads were prepared in the same way as the microalgal beads, except using secondary-treated wastewater instead of microalgal cells.
  • Finally, 4600 microalgal or control beads were added to each reactor, which was then filled to 2 L with secondary-treated wastewater (approximately 2 beads mL^{-1}).

2.3. Sampling strategy

  • Aqueous samples of 125 mL were taken regularly during experiments at 0, 0.1, 1, 3, 6, and 10 days.
  • The incubation time was selected on the basis of the hydraulic retention time of microalgae wastewater treatment systems, normally set at 2-4 days.
  • No water loss was observed over the experimental period due to evaporation.
  • All water samples were collected in 250 mL amber glass bottles, which were stored at 4 °C until analysis; sample holding time was less than 12 h.
  • Water and alginate bead samples were taken during experimentation from reactors and were examined with an optical microscope (Motic BA-310, China) to evaluate the main populations.
  • Microalgae genus were identified from classical specific literature (John et al., 2011).

2.4. Chemicals and reagents

  • Gas chromatography (GC) grade (Suprasolv) hexane, methanol, acetonitrile, and ethyl acetate were obtained from Merck (Darmstadt, Germany).
  • Analytical-grade hydrogen chloride was obtained from Panreac (Barcelona, Spain).
  • Magnesium sulfate, primary secondary amine (PSA), C-18, N,O-Bis(trimethylsilyl) trifluoroacetamide (BSTFA), BPA, bisphenol AF (BPAF), bisphenol F (BPF), 2,4-dichlorophenol (2, 4-DCP), EE2, OP, triphenylamine (TPH), bisphenol A-d16, 3,4-dichlorophenol, and caffeine-13C3 were purchased from Sigma-Aldrich (Steinheim, Germany).
  • Strata-X polymeric SPE cartridges (200 mg) were purchased from Phenomenex (Torrance, CA, U.S.A.), and the 0.7 mm glass fiber filters (ø 47 mm) were obtained from Whatman (Maidstone, U.K.).

2.5. Analytical methodology

  • Conventional wastewater quality parameters, including ammonium nitrogen (NH_4eN), TP, and TSS, were determined in all the water samples.
  • The nutrients were measured with Hach Lange NH_4-N and TP cell tests (LCK 303, 304, and 349) on a spectrophotometer (Hach Lange DR 1900 Portable Spectrophotometer).
  • Measurements of water pH and dissolved oxygen (DO) were taken using Hach Lange sensors.
  • The microalgae growth rate was calculated by measuring the optical density at 680 nm using the Hach Lange spectrophotometer, as described by Wang et al. (2009).
  • The determination of EDCs in water samples was as follows:
    • All water samples were filtered and processed as previously reported (Matamoros et al., 2010).
    • A 100 mL sample was spiked with 100 ng of a surrogate standard (bisphenol A-d16, 3,4-dichlorophenol, and caffeine-13C3).
    • The spiked sample was percolated through a previously activated polymeric solid-phase extraction cartridge (200 mg Strata X).
    • Elution was performed with 10 mL of ethyl acetate.
    • The eluted extract was evaporated under a gentle nitrogen stream until ca. 100 mL remained, at which point 100 ng of TPH was added as an internal standard.
    • After that, the vial was reconstituted to 300 mL with ethyl acetate.
  • EDCs retained in the microalgae filters were extracted using the QuEChERS method.
    • Homogenized wet filters were fortified with 100 ng of the surrogate standard.
    • Then, 10 mL of acetonitrile was added to the mixture and extracted in an ultrasonic bath for 20 min twice.
    • The tube was centrifuged at 4000 rpm for 3 min, and 10 mL of the organic phase was transferred to a 15 mL clean-up tube containing 150 mg of PSA, 900 mg of magnesium sulfate, and 150 mg of C-18.
    • Finally, the tube was centrifuged at 4000 rpm for 3 min, and 6 mL of the organic phase was evaporated under a gentle nitrogen stream and processed as described above for water samples.
  • 50 ml of each of the both extracts was derivatized by adding 50 ml of BSTFA for 60 min at 70 °C.
  • Derivatized samples were injected into a TRACE GCeMS (Thermo-Finnigan, Dreieich, Germany) in the electron impact mode (70 eV ionization energy) fitted with a 30 m × 0.25 mm, 0.25 mm film thickness Zebron 35-HT Inferno capillary column coated with 35% diphenyl, 65% dimethyl polysiloxane from Phenomenex (CA, U.S.A.).
  • The recoveries for the water samples ranged from 80 to 107% and from 71 to 118% for the particulate phase (microalgae fraction).
  • The limit of quantification ranged from 0.02 to 0.48 mg L^{-1} for the water samples and from 0.1 to 1.9 mg g^{-1} for the filters.
  • Repeatability was lower than 15% for all of the cases (n = 3).

2.6. Data analysis

  • The experimental results were statistically evaluated using the SPSS v. 13.0 package (Chicago, IL, U.S.A.).
  • The Mann-Whitney U test was applied to compare differences between the experiments.
  • Linear regressions between the pairs of variables were performed using Pearson's correlation analysis.
  • The statistical significance was defined as p-value < 0.05.

3. Results and discussion

3.1. Reactor performance

  • Microscope observations showed that Chlorella sp. and Nitzschia acicularis remained predominant during the experiments in the microalgae reactors.
  • After 10 days of incubation, the biomass weight had nearly tripled in the reactors inoculated with microalgae, whereas no biomass development was observed in the reactors that had not been inoculated with microalgae.
  • The microalgae growth rate was 0.36 d^{-1}, which is within the range of values found in laboratory-scale experiments treating urban wastewater with microalgae (0.2-1.0 d^{-1}).
  • The control reactors with alginate beads showed low stability, as evidenced by the increase in TSS over the period This is in keeping with the observation made by Cruz et al. (2013), who demonstrated that natural bacteria affected the stability and strength of alginate beads in non-sterile wastewater.
  • In this study we have observed that the concurrence of bacteria and microalgae (Chlorella sp. and Nitzschia acicularis) can increase the stability of alginate beads, probably due to the microalgae interactions with the alginate chemical structure.
  • The pH increased from 8.0 ± 0.5 at the start of the experiment to 11 ± 0.3 at the end of the experiment for the free microalgae reactors. The control reactors had a pH of 8.5 ± 1.0 throughout the experiment.
  • The concentration of DO was higher than 5 mg L^{-1} for all reactors, including the control ones due to the enhanced air-water diffusion resulting from having stirred the reactor.
  • The removal of N-NH_4 after 10 days of incubation was significantly higher (p-value<0.05) in the presence of microalgae (64%, with a kinetic removal rate of 0.108 d^{-1}) than without (37%, with a kinetic removal rate of 0.049 d^{-1}).
  • Moreover, the co-immobilization of microalgae enhanced (p-value<0.05) the removal up to 90% (with a kinetic removal rate of 0.234 d^{-1}), whereas with the control alginate beads this enhancement was only 54% (with a kinetic removal rate of 0.073 d^{-1}).
  • Therefore, the presence of microalgae increased the removal rate by more than 100%, whereas the immobilization of microalgae increased it by more than 400%.
  • These results are in keeping with those reported by Samorì et al. (2013), who found similar N-NH_4 concentration decay in a batch culture system containing the microalgae Desmodesmus communis and fed with wastewater (>90% after 6 days of incubation).
  • Liu et al. (2012) reported that the immobilized cells had a higher ammonium removal rate (22% and 44%) than free living cells (14% and 39%) under autotrophic and heterotrophic conditions, respectively, after 5 days of incubation.
  • The main processes involved in this removal could be microalgae uptake, nitrification, and ammonia volatilization.
  • Whereas the first two processes are prevalent in almost all reactors, volatilization is expected to be only relevant in the case of free microalgae reactors due to the fact that NH_3 gas can easily be volatilized into the atmosphere, especially beyond pH 10 (dissociation constant pKa = 9.24)
  • TP concentration decay was higher in the presence of alginate beads than microalgae. The presence of microalgae increased removal efficiency from 72 to 89%.
  • The kinetic removal rate of TP was significantly higher (p-value<0.05) in microalgae reactors (0.220 d^{-1}) than control reactor (0.126 d^{-1}). Whereas the control alginate beads had a removal efficiency of 92% (with a kinetic removal rate of 0.229 d^{-1}), and the co-immobilized microalgae a removal efficiency of 97% after 10 days of incubation (with a kinetic removal rate of 0.306 d^{-1}).
  • The kinetic rate in the co-immobilized microalgae reactor was significantly higher than those observed in the other reactors evaluated (p-value <0.05).
  • These results are in keeping with previous studies that found that immobilized Scenedesmus sp. cells removed 94% of phosphate within 12 h of incubation, while free-living cells removed 30% of phosphate within 36 h of treatment (Fierro et al., 2008).
  • Similarly, Cruz et al. (2013) found that treatment reactors containing alginate beads were capable of removing between 85 and 95% of TP after 48 h of incubation, whereas control reactors in wastewater removed less than 15%.
  • The higher nutrient removal efficiencies with immobilized algae can be explained by both the increase in biodegradation processes due to algal-bacterial symbiosis and the ionic exchange between the nutrient ions and the immobilization matrix.
  • For example, it has been proved that organic matter removal in a microalgae based wastewater treatment systems is achieved by a mutualistic relationship between bacteria and algae.
  • The DO required for aerobic bacterial organic matter decomposition is provided through algal photosynthesis, whereas the carbon, nitrogen and phosphorus needed for algal growth are provided by bacterial decomposition of wastewater components (García et al., 2000).
  • Alginate beads, which are anionic in nature, are usually associated with the adsorption of cations (N-NH4), whereas the calcium ions of alginate beads are particularly efficient for the precipitation of phosphate ions (PO4^{-3}) from wastewaters (Eroglu et al., 2015).
  • In view of these results, immobilization of microalgae in alginate beads seems to be a good wastewater treatment solution for removing nutrients such as ammonium and phosphorous from WWTP effluents.

3.2. EDC response curves

  • The studied EDCs can be classified into three groups based on the effect of the free and co-immobilized microalgae on EDC removal over the different incubation times assessed
    • higher removal in free microalgae reactors than in the control reactors (BPA, EE2, and 4-OP) and higher removal
    • in the reactors containing alginate beads than in free microalgae reactors (BPAF, BPF, and 2,4-DCP).
3.2.1. Effect of microalgae
  • Microalgae reactors in the presence of microalgae were compared to control reactors without microalgae to determine the relevance of microalgae to EDC removal from secondary-treated wastewater effluents.
  • Only 3 out of the 6 studied EDCs were affected by the presence of microalgae (BPA, EE2, and 4-OP).
  • Although BPA concentration was lower at Day 6 in the microalgae reactors (23% removal) than in the control reactors (9% removal) (p-value<0.05), after 10 days of incubation the concentration was similar in both reactors.
  • In keeping with higher BPA removal efficiency observed by Gattullo et al. (2012) in the presence of microalgae (35-48%) than in control reactors without microalgae (<20%) after 2 days of incubation.
  • Different microalgae species were shown to have different BPA removal (from 20 to 80% after 9 incubation days) (Nakajima et al., 2007).
  • EE2 and OP concentrations were lower in the microalgae reactors than in the control reactors after 3 days of incubation (p-value
  • Similarly, Hom-Diaz et al. (2015) found a greater decline in EE2 concentration in the presence of microalgae, 30% after 1 day of incubation, and no removal under sterile control conditions.
  • The same authors observed that the addition of centrifuged liquid (centrate) from sludge anaerobic digestion to the grown medium increased the attenuation of EE2 from 60 to 95% after 7 days of incubation in the presence of Selenastrum capricornutum.
  • In keeping with the high removal efficiency observed in our study for EE2 (97% after 10 days of incubation) due to the presence of microalgae or the high EE2 biotransformation (60% after 3 incubation days with a biomass concentration of 600 mg L^{-1} and EE2 concentration of 200 mg L^{-1}) (Della Greca et al., 2008).
  • Therefore, the results suggest that microalgae may increase EDC removal efficiency by either releasing exudates, which aid the biodegradation processes carried out by bacteria, or through microalgae uptake.
  • Nevertheless, further studies are necessary to provide evidences of the presence of exudates and their effect on the removal of EDCs.
  • The increase in the indirect photodegradation rate for EDCs promoted by the presence of microalgae exudates cannot be disregarded, as it has already been demonstrated for EE2 (Ge et al., 2009).
  • The concentration of these EDCs in the microalgae was very low at the end of the experiment (<5%), except for OP, for which it was around 20%.
  • In keeping with previous studies that found that the accumulation of BPA in Chlorella sp. was lower than 1% after 10 days of incubation (Ji et al., 2014).
  • Similarly low retention rates for BPA and OP (from negligible to 4%) were found for aerated reactors with microalgae after 90 h of incubation due to air-stripping losses (Abargues et al., 2013).
  • Nevertheless, we can conclude that EDCs were not volatilized in our studies due to their low Henry's law constant values and the fact that reactors were not aerated by air-stripping.
  • The OP concentration retained in the biomass declined over time. The increase in pH from 10 (Day 6) to 11 (Day 10) in the free microalgae reactors may lead to the ionization of the OP (pKa = 10.4) retained in the biomass and, consequently, to its release into the water phase, where it can ultimately be removed by biodegradation or photodegradation.
3.2.2. Effect of microalgae co-immobilized in alginate beads
  • The effect of co-immobilization was assessed by comparing the reactors containing alginate beads and free microalgae reactors.
  • The co-immobilization of microalgae enhanced the removal efficiency of BPAF, 2,4-DCP, and BPF.
  • The BPAF concentration in the free microalgae reactors declined until Day 3, but after 6 days of incubation, it returned to the initial level; in contrast, BPAF attenuation was greater than 60% after 6 days of incubation in the reactors containing algal beads.
  • This shift in the concentration decline for the free microalgae reactors can be explained by the increase in the pH of these reactors to 11, whereas the pH remained between 8 and 9 throughout the experimental incubation period in the reactors containing alginate beads.
  • Given that BPAF is a hydrophobic compound (log Kow>4) with a pKa of 9.2, the shift in the pH from 8.8 to 11 between Day 3 and Day 6 may produce the ionization of the BPAF and, consequently, its release from the biomass into the water phase. In keeping with the decrease in the BPAF retained in the microalgae (from 16% at Day 3 to 3% at Day 6).
  • Conversely, microalgae immobilized in alginate beads did not show this behavior. The high effectiveness of alginate beads for removing BPAF (80% at Day 10) is in keeping with the high adsorption onto the alginate matrix and immobilized algal cells found for NP, a highly hydrophobic compound (log Kow = 4.48) (from 64% at 12 h to 23% at 168 h of the spiked NP) (Gao et al., 2011).
  • Therefore, the removal of BPAF in alginate beads may be due to sorption processes onto the alginate matrix and algal cells.
  • 2,4-DCP was removed at similar rates in the control and free microalgae reactors (32 and 33%), whereas the alginate bead reactors increased the removal efficiency of this compound after 10 days of incubation (67% for the control bead reactors vs. 76% for the alginate bead reactors).
  • Difficult to know whether this compound was removed by biodegradation or sorption onto the alginate beads.
  • BPF removal was more efficient after 10 days of incubation in the reactors containing alginate beads and the control reactors than in the free microalgae reactors (69 vs 87%).
  • This may suggest a light-blocking effect of free microalgae and, consequently, that photodegradation plays a relevant role in the removal of these compounds. Nevertheless, alginate control reactors showed lower removal (50-59%), probably due low alginate stability, as evidenced by the increase in TSS over time in such reactors.
  • That, in turn, suggests that immobilizing microalgae in alginate beads may increase EDC removal efficiency through sorption onto the alginate beads or by enhancing the biodegradation and photodegradation processes.

3.3. Kinetics rates

  • The kinetic behavior of the selected EDCs was estimated from the concentration data over time.
  • The decay rates of the EDCs in the different reactor configurations were fitted to a pseudo-first-order removal kinetics model with Pearson correlation coefficients higher than 0.60 (p-value < 0.05)
  • Pseudo-first-order removal rates were compound-dependent and ranged from 0.013 to 2.131 d^{-1}, with half-lives between 0.3 and 54 days.
  • The presence of free microalgae increased the kinetic removal rates by as much as 25% for BPA, 159% for EE2, and 41% for OP, in comparison with the control reactors. Furthermore, the BPAF, BPF, and 2,4-DCP removal rates were significantly higher in immobilized microalgae reactors than in free microalgae reactors (620, 55, and 329%, respectively).

3.4. Correlation analysis

  • A correlation analysis was performed between the EDC concentration decay rates and conventional water quality parameters (NH_4$N, TP, TSS, and pH).
  • The Pearson correlation test revealed that NH_4$N, TP, and TSS were useful for assessing the effectiveness of microalgae for removing EDCs.
  • NH4-N showed better results in both free and immobilized microalgae reactors (the concentration decay rates of 7 of the 10 EDCs (70%) correlated positively with the concentration decay rate of NH4-N), whereas it was lower for the other quality parameters (50 and 70% for TP, and 60 and 80% for TSS, in free and immobilized microalgae reactors, respectively).
  • In this sense, NH4-N concentration decay was useful for assessing the concentration decay of the EDCs, except for those EDCs whose concentration did not decrease over time (BPAF in the free microalgae reactors).

3.5. Comparison with other available tertiary wastewater treatment solutions

  • The removal efficiency rates for the studied EDCs after 10 days of incubation can be classified in three groups in accordance with their removal efficiency rates in the reactors containing alginate beads:
    • high removal (>95%: EE2 and 4-OP)
    • moderate-to-high removal (76-85%: BPAF, BPF, and 2,4-DCP)
    • low removal (46%: BPA).
  • In comparison with other tertiary wastewater treatment technologies, microalgae co-immobilized in alginate beads have been shown to be at least, if not more, effective for the removal of nutrients and EDCs as polishing ponds or constructed wetlands.
  • Nevertheless, they still perform worse than advanced tertiary treatment systems such as membrane or oxidation technologies. Even so, the low energy requirements and the possibility of using the generated biomass make the microalgae technology highly attractive.

4. Conclusions

  • The results of this study show that the presence of microalgae and their co-immobilization in alginate beads enhanced the removal of both nutrients and most of the EDCs studied.
  • Key conclusions:
    • After 10 days of incubation, 64 and 89% of the NH4eN and 90 and 96% of total phosphorous (TP) had been eliminated in the free microalgae and immobilized microalgae reactors, respectively, while the control reactors eliminated only 40% and 70% of the NH4eN and TP, respectively.
    • Hydrophobic compounds (OP and BPAF) were retained (>10%) in the microalgae, but the increase in the pH (pH > pKa) led to the ionization of these compounds and, consequently, to their release into the water phase.
    • The kinetic removal rates of EDCs ranged from 0.01 to 0.34 d^{-1}, with half-lives between 2 and 54 days.
    • The presence of free microalgae increased the kinetic removal rates by as much as 25% for BPA, 159% for EE2, and 41% for OP, in comparison with the control reactors. In contrast, the kinetic removal rates for BPAF, BPF, and 2,4-DCP were significantly higher in the immobilized microalgae reactors than in the free microalgae
    • NH_4-N and TP concentration decay in microalgae reactors correlated with the concentration decay of most of the EDCs studied.
  • Therefore, the use of microalgae technology is postulated as a suitable alternative for EDC removal from treated wastewater that can be enhanced by immobilizing the microalgae in alginate beads. Nevertheless, further research is needed to address technical issues, such as the hybridization of different polymers to create a stronger, more efficient matrix for algal cells.