This work evaluates the use of native microalgae consortia for a dual role: polishing treatment of municipal wastewater effluents and microalgae biomass feedstock potential for biodiesel or biofertilizer production. An initial screening was undertaken to test N and P removal from secondary effluents and biomass production by 12 consortia. A subsequent treatment was performed by selected consortia (01 and 12) under three operational conditions: stirring (S), S + 12 h of daily aeration (S + A) and S + A enriched with CO2 (S + AC). All treatments resulted in compliance with environmental regulations (e.g. Directive 91/271/EEC) and high removal efficiency of nutrients: 64–79% and 80–94% of total N and PO43−-P respectively. During the experiments it was shown that pH alkalinization due to microalgae growth benefits the chemical removal of ammonia and phosphorus. Moreover, advantages of pH increase could be accomplished by intermittent CO2 addition which in this research (treatment S + AC) promoted higher yield and lipid concentration. The resulting dry biomass analysis showed a low lipid content (0.5–4.3%) not ideal for biodiesel production. Moreover, the high rate of ash (29.3–53.0%) suggests that biomass could be readily recycled as a biofertilizer due to mineral supply and organic constituents formed by C, N and P (e.g. carbohydrate, protein, and lipids).

INTRODUCTION

The excessive discharge of nitrogen (N) and phosphorus (P) from wastewater to water bodies causes eutrophication, a world-scale environmental problem which reduces water quality and alters ecosystem structure and function (Correll 1998; Dodds et al. 2009). As a mitigation measure, nutrients should be removed from wastewater before they are discharged into the aquatic environment (Dodds et al. 2009; Martinez et al. 2000; Rasoul-Amini et al. 2014). Microalgae have been proven to be efficient in removing N and P (mainly NH4+, NO2, NO3 and PO43−), thus making them ideal candidates for nutrient recovery, providing a cost-effective treatment process compared to the chemical and physical processes, with the advantage of being environmentally friendly and sustainable as the process does not generate additional pollutants (Pittman et al. 2011). This important feature has led to investigations of the use of microalgae to treat municipal wastewater effluents, representing a major source of nutrient pollution (Wang et al. 2010; Pittman et al. 2011; Cai et al. 2013). As a result, not only P and N concentrations could be controlled, but also microalgae biomass with a high nutrient content and important secondary bioproducts such as fatty acids, pigments and anti-oxidants (Vigani et al. 2015) may be obtained using a virtually cost-free culture medium (i.e. treated water). Additionally, the complete biomass can be processed for applications such as human food, animal feed, fine chemicals, biofuels and biofertilizers (Barsanti & Gualtieri 2006; Pittman et al. 2011; Rashid et al. 2014; Garcia-Gonzalez & Sommerfeld 2016).

Prior to applying a technology based on dual application of microalgae for secondary effluent treatment and biomass production/valorization at industrial scale, experimental evidence is essential (1) to identify nutrient removal capacity of (native) microalgae species and their biomass composition and (2) to know about critical process parameters such as abiotic factors (e.g. light intensity, temperature, pH) and operational factors (e.g. stirring, aeration and CO2 enrichment) for optimal microalgae growth and nutrient removal (Heubeck et al. 2007; Park & Craggs 2010; Rashid et al. 2014; Rasoul-Amini et al. 2014). For these reasons, in the present study, a polishing of municipal wastewater effluent was tested using native microalgae for nutrient removal under three different conditions (stirring, aeration and CO2 enrichment), providing a qualitative physicochemical analysis (Kjeldahl nitrogen, phosphorus, lipids, ash (total mineral content) and organic matter) of the resulting microalgal biomass to identify preliminarily the potential in biodiesel or biofertilizer industries.

METHODS

Municipal wastewater treatment plant effluent sampling

Secondary effluents from a local municipal wastewater treatment plant (MWTP) located close to Pesqueria City, Nuevo Leon, Mexico (25°44′16.1″N, 100°04′09.3″W) were collected according to ISO 5667 (ISO 1991). In the present study, secondary effluent was directly used as the medium for microalgae growth propagation in the selected consortiums.

Propagation and selection of native microalgae consortia

Twelve microalgae samples (01–12) were collected from shorelines and surface water of different water bodies belonging to the San Juan hydrological sub-watershed on the northeastern Mexican State of Nuevo León (26°21′55″ N, 98°51′15″ W) during 2012–2013. Samples were collected manually according to ISO 5667 (ISO 1991) at a depth of 0–0.5 m and placed into 2 liter polypropylene vessels and maintained at 4 °C in darkness. Subsequently, the water samples containing microalgae communities were propagated in 500 mL beakers filled with nutrient solution media (Lopez-Chuken & Young 2010) for 14 d under continuous light (4,800 lux) using a jar tester at 100 rpm (Phipps & Bird PB 700).

After initial propagation, cell biomass was concentrated by centrifugation at 4,500 rpm for 5 min. Subsequently, inoculum cell was standardized by direct cell counting in a Neubauer chamber and dilution adjustment; the inocula resulted in a concentration of 1.06 × 103 cells/mL in 300 mL samples (3-fold replicates) of MWTP effluent, which were placed into 500 mL translucent plastic vessels with the lid adapted for gas exchange. Water volume loss by evaporation was compensated for on a daily basis during the 12 d trial (1 to 13 August 2013). The experimental area was located at 25°43′31.5″ N, 100°19′00.7″ W. Microalgae cultures were maintained under non-controlled environmental conditions (30.7 ± 7.4 °C). After 12 d selected microalgae consortia were chosen for the polishing treatment trial based on the following criteria: (1) removal of nutrients NO3-N and PO43−-P measured by Aquarium Pharmaceuticals, Inc. (API) assay kit, coupled to a spectrophotometer (Thermo Spectronic Genesys 20) and (2) microalgal biomass productivity directly measured as total suspended solids (TSS) dry weight (dw) (Whatman #40, 70 °C for 48 h).

MWTP effluent polishing experiment

Selected microalgae were propagated using a jar tester at 100 rpm (Phipps & Bird PB 700) in 1,000 mL beakers filled with 750 mL of non-modified MWTP effluent with continuous light at 4,800 lux. Every 8 d, 250 mL of the growth media was replaced with fresh MWTP effluent to keep cultures active and close to the exponential growth phase.

The experiment was conducted using six acrylic cylindrical photobioreactors (21 L, inner diameter 30 cm, height 30 cm) adapted for external aeration supply (either atmospheric air or atmospheric air enriched with CO2) and a mechanical stirring system (900 rpm) (Figure 1). Photobioreactors were filled with 18 L of MWTP effluent and inoculated with 41 g of fresh biomass (equivalent to 0.61 g dw). Experimental treatments were applied for 30 d as follows: (S) 24 h continuous stirring/day, (S + A) stirring and aeration 1% (v/v) per minute 12 h/day (during the sunlight 7:00 to 19:00 h) and (S + AC) stirring and aeration enriched with 0.06% of CO2 per minute. In order to observe physicochemical changes and the potential growth of non-inoculated wild microalgae, two controls of MWTP effluent (inoculum-free) under static conditions were included: one exposed to natural sunlight (SL) and the other covered with aluminum foil and non-exposed to sunlight (NSL), both using loose-cap 1.5 L PET bottles, each containing 1.2 L of MWTP effluent. Average temperature during the trial was 13.1 ± 4.9 °C. The experiment took place under natural conditions from 18 December 2013 to 17 January 2014 at 25°39′31.0″N, 100°11′17.3″W.
Figure 1

Schematic representation of the photobioreactor system.

Figure 1

Schematic representation of the photobioreactor system.

Physicochemical analysis of MWTP effluent and microalgae biomass

Physicochemical parameters monitored in the MWTP effluent during the experiments are shown in Table 1. At day 30, controls (SL and NSL) were analyzed for TSS (Whatman #40, 70 °C for 48 h). The nutrient removal was calculated by the following formula: ((C0 – C)/C0) × 100; where C0 is the initial concentration; C, concentration after treatment with microalgae. Total microalgae biomass was harvested by vacuum filtration and dried (60 °C, 48 h) using a convection stove (Quincy Lab. Inc. 40GL) to calculate biomass (dw). Dry microalgal biomass was analyzed using standard methods (AOAC 1997) to determine moisture, total Kjeldahl nitrogen, total phosphorus (TP), lipids, ash (total mineral content) and indirect determination of organic matter ((microalgal biomass dw 100%) – ash dw %).

Table 1

Initial average values in MWTP effluent and frequency of parameters during the polishing treatment

Parameter Value (day 0) Frequency days Method 
pH 7.23 ± 0.01 2 to 30 AOAC (1997)  
EC 133.30 ± 0.20 2 to 30a 
TOC 18.28 ± 0.22 30 TOC analyzer (TOC-VCSH, Shimadzu, Japan) 
TN 17.33 ± 0.83 30 TN analyzer (Multi N/C 3000, Analytik Jena, Germany) 
Org-N 3.67 30 Indirect determination TN (NO2-N + NO3-N + NH4+
NO2-N 0.03 ± 0.00 2 to 30a Assay kit API, coupled to a Thermo Spectronic Genesys spectrophotometer 
NO3-N 13.40 ± 0.07 2 to 30a 
NH4+-N 0.23 ± 0.02 2 to 30a 
PO43−-P 2.29 ± 0.03 2 to 30a 
Parameter Value (day 0) Frequency days Method 
pH 7.23 ± 0.01 2 to 30 AOAC (1997)  
EC 133.30 ± 0.20 2 to 30a 
TOC 18.28 ± 0.22 30 TOC analyzer (TOC-VCSH, Shimadzu, Japan) 
TN 17.33 ± 0.83 30 TN analyzer (Multi N/C 3000, Analytik Jena, Germany) 
Org-N 3.67 30 Indirect determination TN (NO2-N + NO3-N + NH4+
NO2-N 0.03 ± 0.00 2 to 30a Assay kit API, coupled to a Thermo Spectronic Genesys spectrophotometer 
NO3-N 13.40 ± 0.07 2 to 30a 
NH4+-N 0.23 ± 0.02 2 to 30a 
PO43−-P 2.29 ± 0.03 2 to 30a 

EC: electrical conductivity; TOC: total organic carbon; TN: total nitrogen; Org-N: organic nitrogen.

Mean ± SE (standard error). All values are expressed in mg/L except pH (units) and EC (mS/m).

aAnalysis every 2 days. Note: pH, EC, TOC, TN and PO43−-P in controls SL and NSL were analyzed at 30 d. n = 3.

RESULTS AND DISCUSSION

Selection of native microalgae consortia cultivated in MWTP effluent

After 12 days of cultivation using MWTP effluents, all 12 native microalgae consortia showed removal rates ranging from 74.34 to 91.07% of NO3-N and 60.37 to 79.27% of PO43−-P, respectively. The removal capacity of N and P in this research was not the key criterion for the selection of the microalgae consortia; this was because microalgae are known to greatly consume other macronutrients such as S, Ca, Mg and K, as well as micronutrients, such as Mo, Fe, Ni, Cu, Zn, Co, B, Mn and Cl (Giorgos & Dimitris 2011), which would possibly contribute an extra polishing of MWTP effluents while a major generation of biomass is expected. Taking into consideration the second selection criteria (microalgal productivity), consortia 01 and 12 stood out particularly with 74.22 and 165.61 mg (dw)/L respectively, in contrast with the average of 20.7 ± 8.96 mg (dw)/L obtained from the rest of the consortia. Giving these results, native microalgae consortia 01 and 12 were selected for study of their polishing treatment.

Polishing treatment of MWTP effluent by native microalgae consortia

Figure 2(a) illustrates the pH kinetics in the MWTP effluent during the 30-d experiment. An observed initial stable period was followed by a sudden pH increase, which is known to occur concurrently with the start of microalgae photosynthetic activity and subsequent increased OH levels in water by consumption of HCO3, which provide the CO2 needed for the growth of microalgae, and NO3, which in assimilation produces equivalently OH, according to the following reactions (Martinez et al. 2000; Rashid et al. 2014):
  • Reaction 1:

  • Reaction 2: NO3 + 5.7 ( CO2) + 5.4 (H2O)→

    C5.7H9.8O2.3N + 8.25 (O2) + OH

Figure 2

Evolution of (a) pH and (b) EC during the tertiary treatment period by microalgae consortia 01 and 12. Treatments: 01S (— + —), 01S + A (- -○- -), 01S + AC (. . .. . .), 12S (— × —), 12S + A (- -♢- -), 12S + AC (. . .. . .). Error bars indicate SE, where n = 3. Absence of error bar indicates negligible SE.

Figure 2

Evolution of (a) pH and (b) EC during the tertiary treatment period by microalgae consortia 01 and 12. Treatments: 01S (— + —), 01S + A (- -○- -), 01S + AC (. . .. . .), 12S (— × —), 12S + A (- -♢- -), 12S + AC (. . .. . .). Error bars indicate SE, where n = 3. Absence of error bar indicates negligible SE.

At the end of the trial pH values of 10.9 to 11.3 in treatments S and S + A, respectively, were obtained. This average pH increase (≈ 54%) is in accordance with early studies using a high rate algae pond (HRAP), where pH values above 11.0 caused by depletion of CO2 and HCO3 are commonly reported (Heubeck et al. 2007; Park & Craggs 2010). Conversely, our study indicated that treatment S + AC resulted in pH values around 8.5 (Figure 2(a)) which was possibly induced by a buffering effect in the CO2/HCO3 ratio (Martinez et al. 2000; Giorgos & Dimitris 2011; Craggs et al. 2013).

It is important to underline that nutrient (N and P) assimilation by microalgae is also regulated by external factors such as pH, which in our study reached values above 9.3 during the trial (treatments S and S + A), affecting concentration and ratio of NH4+/NH3 in solution. For instance, pH values over 9.3 have been shown to have NH3 predominance effects over NH4+, thus facilitating NH3 removal by pH increase whereas NH4+ is unavailable for microalgae assimilation (Giorgos & Dimitris 2011; Craggs et al. 2013); Likewise, pH has a strong effect on P bioavailability, causing precipitation of P with cations (e.g. Ca3(PO4)2) at pH values of 9.0 and 11.0 (Heubeck et al. 2007; Giorgos & Dimitris 2011; Cai et al. 2013). It is noteworthy that the ammonia volatilization and phosphate precipitation may be reduced by CO2 addition and buffering effect on pH; however, this reduction in the treatment can be offset by the increased algal production and associated nutrient assimilation into this biomass (Park et al. 2011). Nonetheless, in this study the pH in treatment S + AC was not completely regulated due to the intermittent addition of CO2, indicating a potential advantage by allowing increased pH in the treatment and providing C necessary for the development of microalgae. Furthermore, the key role of alkaline pH in Escherichia coli removal was confirmed by Heubeck et al. (2007), who observed significantly higher E. coli removals at pH 9.5 (≈100%) than at pH 8 (≈50%) in an HRAP treating domestic wastewater, which is a significant result giving the potential benefit of providing a chlorine-free water disinfection method for treated MWTP effluents prior to discharging into natural water reservoirs.

Figure 2(b) shows a general decrease of electrical conductivity (EC) during the trials. The observed EC decline due to microalgae activity, as pointed out by Oswald (1988), could represent a parameter highly correlated with increased microalgal biomass production, since the consumption of basic elements for microalgal growth, i.e. N, P and S in ion forms (e.g. NO3, PO43− and SO42−), and ionic components such as Na+, K+, Ca2+, Mg2+ and Fe2+/Fe3+ and trace elements (Cai et al. 2013) contribute to the overall EC of the solution.

Removal of inorganic nitrogen species NO3-N, NO2-N, and NH4+-N during the 30-d trial are shown in Figure 3(a)3(c). In aquatic environments, the most oxidized, thermodynamically stable and predominant form is NO3 (Barsanti & Gualtieri 2006), being in accordance with the present study, where 98.1% of total nitrogen (TN) in the secondary effluent was present in inorganic form with high removal rates: 92.5 to 98.5% and 77.0 to 97.8% by microalgae consortia 01 and 12, respectively (Figure 3(a)). In contrast, lower TN removal from water was observed: values of 52.6, 64.2 and 52.8% were found in treatments 01S, 01S + A and 01S + AC, respectively. Similarly, TN was removed by more than 50% from the MWTP effluent (>8.7 mg/L TN), representing 64.4, 67.5 and 75.5% by treatments 12S, 12S + A and 12S + AC, respectively. The different removal rates between inorganic N (NO3) and TN is explained by the relatively constant Org-N averages during the treatments, 39.5 ± 6.0 and 16.2 ± 5.4% using consortia 01 and 12, respectively.
Figure 3

Evolution of (a) NO3-N, (b) NH4+-N, (c) NO2-N and (d) PO43−-P during the tertiary treatment period by microalgae consortia 01 and 12. Treatments: 01S (— + —), 01S + A (- -○- -), 01S + AC (. . .. . .), 12S (— × —), 12S + A (- -♢- -), 12S + AC (. . .. . .). Error bars indicate SE, where n = 3. Absence of error bar indicates negligible SE.

Figure 3

Evolution of (a) NO3-N, (b) NH4+-N, (c) NO2-N and (d) PO43−-P during the tertiary treatment period by microalgae consortia 01 and 12. Treatments: 01S (— + —), 01S + A (- -○- -), 01S + AC (. . .. . .), 12S (— × —), 12S + A (- -♢- -), 12S + AC (. . .. . .). Error bars indicate SE, where n = 3. Absence of error bar indicates negligible SE.

Nitrogen, as NO2 and NH4+ concentrations, during the trial is shown in Figure 3(b) and 3(c). NO2 and NH4+ concentration increased as NO3 decreased, an effect previously reported by Wang et al. (2010) growing Chlorella sp. in secondary MWTP effluents. This was explained by N absorption by photosynthetic organisms, involving a two-step reduction for production of NH4+ at chloroplast level, thus producing NO2 during the process of NO3 reduction to NH4+, in which a fraction of the produced NO2 could be reintroduced to the media (Burhenne & Tischner 2000). Moreover, despite the sum of NO2-N and NH4+-N representing a small fraction of inorganic N in MWTP effluent, its monitoring could represent a potential indirect way to estimate the microalgae activity during the treatment. For instance, in the present study, it was shown that NH4+-N removal occurred first using consortia 01 (days 10–16) (Figure 3(b)) probably due to being the preferred nitrogen source (prior to NO3-N) (Rashid et al. 2014). However, it is important to mention that the NH4+-N could also be modified by biological activity and external conditions (e.g. high pH and temperature) (Giorgos & Dimitris 2011; Cai et al. 2013) and, as a result, cannot be distinguished directly from the removal of inorganic N by assimilation. Nevertheless, the behavior of NO2-N could be discriminated reliably from the biological activity of microalgae and the different external conditions since nitrite is strongly dependent on NO3-N uptake, which is converted into nitrite intracellularly by nitrate reductase (Barsanti & Gualtieri 2006). According to this approach the microalgae consortia in this trial showed different removal rates by the treatments with the following order: S + AC > S + A > S (Figure 3(c)).

During the experiment, P was efficiently removed (Figure 3(d)). In general, P is regarded as the limiting nutrient factor for microalgae growth (Cai et al. 2013). However, it has been also reported that 15 μg/L of PO43−-P is sufficient to promote and saturate algal growth in most aquatic systems (Correll 1998). It has to be remarked that the polishing treatment with microalgae used in the present study resulted in P and N levels far lower than strict environmental regulations such as the Directive 91/271/EEC that established a limit of 1 mg/L TP and 10 mg/L TN for effluent discharge into areas sensitive to eutrophication (European Commission 1991). In this study all treatments resulted in levels of P and N in compliance with Directive 91/271/EEC. The results of this study indicated that treatments using consortia 01 showed similar removal rates of PO43−-P (90.0% for 01S + A and 01S + AC, and 93.5% for 01S). Moreover, treatments 12S and 12S + A removed 94.3 and 89.5% PO43−-P in MWTP effluent, respectively. In contrast, treatment 12S + AC exhibited less removal (79.9% PO43−-P), due to a slight increase in P by the end of the experiment (Figure 3(d)). This effect could be explained theoretically by cell decay by-products generated during the cell decline phase (Barsanti & Gualtieri 2006), as well as nitrate depletion, which is reported to produce polysaccharides in the medium (Thepenier et al. 1985), giving the possibility of contributing to the huge rise of total organic carbon (TOC) obtained (76.7 mg/L representing an increase of 419.15% as compared to the initial concentration). Moreover, the TOC for the rest of the treatments decreased slightly during the trial (15.3 to 25.6%).

At the end of the trial (day 30), the NSL control had no presence of TSS; in contrast, the SL control resulted in 160 ± 10 mg (dw)/L of TSS. The observed increase in TSS was due to the growth of non-inoculated microalgae already present in the MWTP effluent. The latter reaffirms the potential of MWTP effluent as a culture medium for microalgae and source of nutrient pollution in water bodies (Wang et al. 2010; Pittman et al. 2011; Rasoul-Amini et al. 2014). Moreover, a remarkable side-effect of microalgae growth is shown in control SL, exhibiting a higher pH level (10.82 ± 0.01), and decrease in EC (18.4%), NT (40.5%), PO43−-P (66.81%) and TOC (19.7%), whereas control NSL (microalgae-free) resulted in a lower pH increase (8.39 ± 0.01), without changes in EC, a minor decrease of NT (10.8%), an increase of PO43−-P (14.41%) and higher decrease of TOC (43.4%). In addition the differences obtained in NSL control for PO43−-P and TOC are probably a result of oxidation and further dissolution of residual organic matter in the MWTP effluent (Correll 1998).

Microalgal biomass productivity and physicochemical characterization

Table 2 shows the microalgae biomass recovery after the polishing treatment. Results suggest that the intermittent enrichment with 0.06% v/v CO2 (i.e. S + AC) compared to continuous supply of 1% v/v CO2 recommended by Taher et al. (2015) caused a positive effect on biomass and lipid production in microalgae, showing agreement with Jiang et al. (2011). This effect is possibly due to CO2 enrichment that produced more bioavailable inorganic carbon in the MWTP effluent for the synthesis of organic molecules (i.e. carbohydrate, protein, and lipids) through the chemical reduction process of photosynthesis (Rashid et al. 2014). Likewise, the use of emissions from an industrial process unit (e.g. from fuel-fired power plants), as source of CO2, and municipal wastewater reuse as culture medium for the microalgae growth, offers a high potential for reduction of CO2 and costs for microalgae biomass production (Jiang et al. 2011; Pittman et al. 2011; Cai et al. 2013; Taher et al. 2015).

Table 2

Microalgal biomass harvest and composition after the treatment of MWTP effluent

Treatment Biomass harvest (g)* Composition of biomass expressed in dw (%)
 
Total Kjeldahl nitrogen TP* Lipids Ash Organic matter 
01S 6.63 1.00 ± 0.01a 0.57 1.15 ± 0.42b 44.13 ± 0.47c 55.87 
01S + A 7.69 0.78 ± 0.00c 0.34 0.78 ± 0.03b 39.97 ± 0.21d 60.03 
01S + AC 12.34 0.48 ± 0.01e 0.35 3.40 ± 0.61ª 29.26 ± 0.30e 70.74 
12S 6.04 0.98 ± 0.00a 0.65 0.93 ± 0.26b 48.38 ± 0.01b 51.62 
12S + A 6.10 0.83 ± 0.00b 0.51 0.45 ± 0.19b 53.04 ± 1.11a 46.96 
12S + AC 8.40 0.58 ± 0.00d 0.54 4.33 ± 0.26ª 42.97 ± 0.15c 57.03 
Treatment Biomass harvest (g)* Composition of biomass expressed in dw (%)
 
Total Kjeldahl nitrogen TP* Lipids Ash Organic matter 
01S 6.63 1.00 ± 0.01a 0.57 1.15 ± 0.42b 44.13 ± 0.47c 55.87 
01S + A 7.69 0.78 ± 0.00c 0.34 0.78 ± 0.03b 39.97 ± 0.21d 60.03 
01S + AC 12.34 0.48 ± 0.01e 0.35 3.40 ± 0.61ª 29.26 ± 0.30e 70.74 
12S 6.04 0.98 ± 0.00a 0.65 0.93 ± 0.26b 48.38 ± 0.01b 51.62 
12S + A 6.10 0.83 ± 0.00b 0.51 0.45 ± 0.19b 53.04 ± 1.11a 46.96 
12S + AC 8.40 0.58 ± 0.00d 0.54 4.33 ± 0.26ª 42.97 ± 0.15c 57.03 

*n = 1; Average (n = 2) ± SE. One-way ANOVA. Different letters indicate significant difference (Tukey, α = 0.05).

A low concentration of lipids was obtained in all treatments (<5%) (Table 2), implying that the microalgae biomass generated from MWTP effluents used as culture media may not be suitable for biodiesel production. In addition, it has been reported that biodiesel production from microalgae, to be cost-effective, needs microalgae rapid growth rates and high biomass lipid content (>70%) (Rashid et al. 2014), in addition to the design of subsystems that include lipid extraction and its conversion into biodiesel (Cai et al. 2013; Craggs et al. 2013). Alternatively, the ability of microalgae to efficiently remove inorganic metal species from aqueous solution, through incorporating them as organic material into the cell, should be considered (Barsanti & Gualtieri 2006; Park et al. 2011). In the present study, the results show high inorganic metal recoveries (measured as ash content) ranging from 29.26 to 53.04% dw (Table 2) and the probable assimilation of metal ions such as Na+, K+, Ca2+, Mg2+, Zn2+, Al3+ and Fe2+/Fe3+, as described previously by Wang et al. (2010), who observed decreases of these metal ions using a culture of Chlorella sp. in MWTP effluent. On the other hand, the composition of the resulting microalgal biomass (Table 2) suggests that biomass could be used as a biofertilizer due to its high mineral content (micronutrients) and macronutrients such as C, N and P incorporated in organic forms (i.e. carbohydrate, proteins, and lipids). However, it has to be considered that microalgae also contain plant growth regulators (auxins, gibberellins, and cytokinins) which could enhance the growth of agricultural crops (Barsanti & Gualtieri 2006; Garcia-Gonzalez & Sommerfeld 2016). Likewise, from the sustainability perspective, unlike chemical fertilizers, microalgae biofertilizers are important resources still not widely used, especially using MWTP effluents. This process could certainly be cost-effective with little contribution to environmental pollution, and could provide a beneficial stimulatory effect on plant growth (Pittman et al. 2011; Garcia-Gonzalez & Sommerfeld 2016). Another advantage of using microalgae as biofertilizers is the possibility of saving energy through avoiding the drying of the biomass, through the direct use of the living culture or wet microalgae extracts.

CONCLUSIONS

The polishing treatment of MWTP effluent using native microalgae consortia 01 and 12 under different operating conditions (S, S + A, S + AC) produced results in compliance with environmental regulations (e.g. Directive 91/271/EEC) and high removal rates of TN and PO43−-P: 64–79% and 80–94% respectively. In general, secondary effluents in the presence of microalgae exhibited pH alkalinization and a decrease in the EC, and nutrients (N and P). This last condition could represent a potential use for removal of ammonia by stripping and of phosphorus via chemical precipitation bound to the intermittent CO2 addition, which in this study did not limit the increases of pH, in addition to generating more microalgae biomass with higher yield and lipid concentration. For all treatments, dry biomass showed low lipid content (0.5–4.3%) that hampers the sustainable use as a feedstock for biodiesel production. On the other hand, the high-rate recovery of ash (29.3–53.0%) suggests that biomass could be readily recycled as a biofertilizer due to its mineral supply and organic compounds of C, N and P (e.g. carbohydrate, protein, and lipids).

ACKNOWLEDGEMENTS

The authors wish to acknowledge the support obtained from the Mexican National Council for Science and Technology (CONACYT), Facultad de Ciencias Quimicas of Universidad Autónoma de Nuevo León and Departamento de Ciencias Básicas of Universidad Autónoma Metropolitana-Azcapotzalco.

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