Nutrient removal and microalgal biomass production from different anaerobic digestion effluents with Chlorella species.

Potential of microalgal cultivation as an alternative approach to the treatment of anaerobic digestion (AD) effluents was examined using two representative Chlorella species, Chlorella vulgaris (CV) and Chlorella protothecoides (CP). Both species effectively removed NH4+-N from the AD effluents from four digesters treating different wastes under different operating conditions. In all experimental cultures on the AD effluents, NH4+-N (initial concentration, 40 mg/L) was completely removed within 10 days without residual NO3--N or NO2--N in batch mode. Compared to CP, CV showed greater biomass and lipid yields (advantageous for biodiesel production), regardless of the media used. Prolonged nitrogen starvation significantly increased the lipid accumulation in all cultures on the AD effluents, and the effect was more pronounced in the CV than in the CP cultures. On the other hand, compared to CV, CP showed significantly faster settling (advantageous for biomass harvesting) in all media. Our results suggest that the Chlorella cultivation on AD effluents under non-sterile, mixed-culture conditions may provide a viable way to manage and valorize the problematic effluents. Diverse bacteria derived from the AD effluents co-existed and presumably interacted with the Chlorella species in the cultures.

Introduction

With growing concerns about energy crisis and global warming, the need for sustainable energy sources is steadily increasing. Converting waste n class="Chemical">organics inpan>to biogas (primarily pan> class="Chemical">methane and carbon dioxide) through anaerobic digestion (AD) has been gaining momentum in recent years more as a renewable energy technology than simply as a treatment option[1]. Although AD is widely used in practice, it still has some practical issues to be resolved for more robust applications. Foremost among them is the high ammonia content of the treated effluents from AD processes, which can cause serious eutrophication problems in water bodies if not properly treated before discharge. Conventional biological nutrient removal (BNR) processes, such as A2O process, have been widely used for nitrogen control in wastewater treatment for decades. Although it has been demonstrated as effective, conventional BNR has two major drawbacks that weaken the economic application of the process: the high consumption of oxygen and the need for an external carbon source. These limitations are of even greater concern when treating wastewater with high ammonia nitrogen concentration and low available organic content, such as AD effluents[2].

Several alternative BNR processes, for example, shortcut nitrificationpan>-denpan>itrificationpan>, simultanpan>eous nitrificationpan>-denpan>itrificationpan>, anpan>d anpan>ammox, have beenpan> developed anpan>d tested to provide a more pan> class="Chemical">feasible treatment option[3-5]. Recently, microalgae-based processes have attracted much attention as a viable alternative for BNR from wastewater. Microalgae can utilize nutrients from wastewater, while photosynthetically fixing carbon dioxide, a greenhouse gas, for growth[6]. This indicates the potential of microalgae-based treatment as an economically as well as environmentally appealing method for BNR, without the need for an external carbon source. Several studies have investigated the application of microalgae to the treatment of AD effluents, and various microalgal species have shown effective removal of nitrogen and phosphorus (>83%) from different AD effluents[7-10].

Microalgal biomass has been considered a potential n class="Chemical">feedstock for producinpan>g biodiesel, as micropan> class="Species">algae accumulate large amounts of assimilated carbon in the form of lipids in the cell. Microalgae have several advantages as a biodiesel feedstock over conventionally used terrestrial plants, including higher lipid content, faster growth, higher biomass yield, and little competition for arable land[11-13]. However, substantial amounts of water and nutrients (often added as chemical salts) are required for the cultivation of microalgae, raising concern over its economic feasibility[14]. Therefore, cultivating microalgae using AD effluents as a medium can provide a more sustainable option for producing energy feedstock, while simultaneously treating nutrient-rich wastewater. The benefits of microalgal cultivation on wastewater can be maximized by performing the cultivation under non-sterile, mixed-culture conditions. A point to consider in such mixed-culture processes is that the nutrient removal and microalgal growth properties will vary not only with the microalgal species (i.e., inoculum) and the characteristics of wastewater (i.e., substrate). Such effects can be more pronounced in cultures using effluents from complex mixed-culture systems such as AD processes. However, the literature addressing this complication is limited. Hence, the present study focuses on this gap and aims to compare different AD effluents for the cultivation of microalgae in terms of nutrient removal and biomass production.

In this study, two representative n class="Species">Chlorella species, pan> class="Species">Chlorella vulgaris and Chlorella protothecoides, were tested for their ability to remove nutrients (particularly ammonia nitrogen) from and grow on four AD effluents from different sources. Chlorella species have been reported to grow well with wastewater nutrients and produce high levels of lipids[6,15]. C. vulgaris and C. protothecoides both have promising potential in commercial applications for food, nutrition, and energy purposes, and their ability to grow both autotrophically and heterotrophically could contribute to further degradation of organic residues in AD effluents[16]. Both species have shown greater biomass production and higher lipid productivity or content in the presence of organic carbon sources than in autotrophic cultures[17,18], which indicates another potential benefit of using AD effluent as a culture medium. For a comprehensive understanding, besides the nutrient removal efficiency, microalgal growth and lipid accumulation were also monitored during cultivation. In addition, the differences in microbial community structure between the Chlorella cultures treating different AD effluents were comparatively analyzed by molecular fingerprinting. This is a unique study in that it involves a set of physicochemical and biochemical approaches that allow for a holistic view of microalgal cultivation, from growth monitoring to kinetic modeling, lipid production, and microbial community structure, for complex AD effluents of different characteristics.

Materials and Methods

Microalgal strains and pre-cultivation

n class="Species">Chlorella vulgaris KCTC AG10002 (pan> class="Species">CV) and Chlorella protothecoides UTEX 1806 (CP) were purchased from the Korean Collection for Type Cultures and the Culture Collection of Algae at the University of Texas at Austin, respectively. CV was pre-cultivated in a modified Bristol medium (BT) with NH4+-N instead of NO3−-N as the nitrogen source, whereas CP was cultured in a modified Proteose medium prepared by adding 1 g/L Proteose peptone to BT. BT contained (per liter distilled water (DW)) 157 mg NH4Cl, 25 mg CaCl2·2H2O, 75 mg MgSO4·7H2O, 75 mg K2HPO4, 175 mg KH2PO4, 25 mg NaCl, and 1 mL trace element solution. The trace element solution contained (per liter DW) 0.49 mg Co(NO3)2·6H2O, 8.82 mg ZnSO4·7H2O, 1.44 mg MnCl2·4H2O, 0.71 mg MoO3, 1.57 mg CuSO4·5H2O, 11.42 mg H3BO3, 50 mg EDTA, 31 mg KOH, 4.98 mg FeSO4·7H2O, and 0.001 mL H2SO4 (98%, v/v). The prepared media were sterilized by autoclaving at 121 °C for 15 min prior to inoculation. Pre-cultivation was performed in a 500-mL Erlenmeyer flask (working volume, 450 mL) at room temperature (around 20–25 °C) under white light-emitting diode (LED) illumination in a cycle of 16-h light (1000 lux) and 8-h darkness. Each flask was aerated by bottom bubbling with ambient air (170 mL/min) under vigorous magnetic stirring (200 rpm). Cultivation was continued until the NH4+-N in the culture was exhausted.

Microalgal cultivation tests with AD effluents

The AD effluents used in this study were collected from four difn class="Chemical">ferenpan>t lab-scale anpan>aerobic digesters treatinpan>g food waste at a high (HF; 5 g VS/L·d) anpan>d a low (LF; 1.5 g VS/L·d) organpan>ic loadinpan>g rates, Ulva (UL) anpan>d whey (WH), respectively. The operatinpan>g conpan>ditionpan>s of the four digesters are summarized inpan> Supplemenpan>tary Table 1. The effluenpan>t samples were takenpan> at a steady state anpan>d cenpan>trifuged inpan> a 50-mL conpan>ical tube at 3400 × g for 20 minpan>. The supernpan>atanpan>t was recovered anpan>d cenpan>trifuged againpan> inpan> the same manpan>ner. The finpan>al supernpan>atanpan>t was diluted to a pan> class="Chemical">NH4+-N concentration of 40 mg/L with DW and tested for cultivating the Chlorella strains in parallel with BT as the control medium for comparison. The NH4+-N concentration in the AD effluent media was set to be the same as the nitrogen concentration in BT, one of the most commonly used synthetic medium for the cultivation of Chlorella, to investigate the potential use of the AD effluents as a culture medium for microalgal cultivation in comparison with a conventionally used synthetic medium. The nutrient and organic concentrations in the prepared media and the pre-cultivated CV and CP inocula are presented in Supplementary Table 2.

The microalgal cultivation tests with the prepared media were performed under white LED illumination of 1000 lux with a 16-h light/8-h dark cycle in batch mode. Each cultivation trial was run in a 500-mL Erlenmeyer flask with a working volume of 400 mL. The pre-cultivated micron class="Species">algae were used as seed culture to inoculate the reactionpan> mixtures at a volume/volume ratio of 10%. The flasks were incubated under the same aerationpan> anpan>d mixing conpan>ditionpan>s as the pre-cultivationpan>. All cultivationpan> tests were performed in duplicate at room temperature without pH conpan>trol anpan>d sampled daily to monpan>itor the nutrienpan>t removal anpan>d pan> class="Disease">microalgal growth.

Physicochemical analyses

Chemical n class="Chemical">oxygen demand (COD) was colorimetrically analyzed using HS-COD-MR kit (HUMAS), and solids were measured according to the procedures in Standards Methods[19]. Anionpan>s and cationpan>s were measured using two ionpan> chromatographs (Dionpan>ex ICS-1100, Thermo Scienpan>tific) equipped with an Ionpan>Pac AS14 column and an Ionpan>Pac CS12A column, respectively. Samples for ionpan> analysis were prepared by filtrationpan> through a syringe filter with 0.22 μm pore size.

The n class="Disease">microalgal growth was monpan>itored by measurinpan>g the optical denpan>sity at a wavelenpan>gth of 680 nm (OD680) as well as the pan> class="Chemical">chlorophyll concentration during the batch cultivation. The total chlorophyll concentration (sum of chlorophyll a and b) was determined as previously described[20]. Briefly, 1 mL of microalgal culture was sampled and centrifuged at 12,000 × g for 10 min, and the supernatant was replaced by 1 mL of dimethyl sulfoxide to extract the pigments. The pigment-containing sample was then incubated at 65 °C for 60 min and centrifuged at 12,000 × g for 10 min. The concentrations of chlorophyll a and b were estimated from the absorbance at 649 nm (Abs649) and 665 nm (Abs665) of the resulting supernatant using the following equations.

The total n class="Chemical">lipid conpan>cenpan>trationpan> was measured by colorimetric pan> class="Chemical">sulfo-phospho-vanillin (SPV) method as previously described[21], and the lipid content of biomass (%, w/w) was calculated based on the concentration of volatile suspended solids. Fatty acid composition was determined by the direct fatty acid methyl ester (FAME) synthesis method. FAMEs were prepared from dried microalgal biomass by direct transesterification of fatty acids as previously described[22], and analyzed using a gas chromatograph (7820A, Agilent) coupled with a flame ionization detector and an Innowax column (Agilent). A standard FAME mix (C4–C24, Supelco) was used to identify and quantify FAMEs based on their retention times. All physicochemical analyses were carried out at least in duplicate.

Microalgal growth and nutrient consumption modeling

The observed chlorophyll accumulationpan> anpan>d pan> class="Chemical">NH4+-N removal profiles were modeled using the following modified Gompertz equations:where Ct is the total chlorophyll concentration (mg/L) at time t, CP is the maximum potential of chlorophyll production (mg/L), Pm is the maximum chlorophyll production rate (mg/L·d), N0 is the initial NH4+-N concentration (mg/L), Nt is the residual NH4+-N concentration at time t (mg/L), Rm is the maximum NH4+-N removal rate (mg/L·d), λ is the lag phase length (d), and t is the cultivation time (d).

Biomass harvesting efficiency

The harvesting efficiency of the cultivated biomass by gravity settling was determined as described previously by Darpito et al.[7]. An 8-mL volume was taken from each culture and vigorously shaken in an 11-mL glass vial (diameter, 1 cm; height, 14 cm). The optical density at a wavelength of 540 nm (OD540) of each culture sample was measured directly from the vial 2 cm above the bottom. The cultures were measured at 5, 15, 30 and 60 min for careful monitoring of settling profiles during the early period and at longer, irregular intervals thereafter along the profiles (3, 6, 10 and 14 h for the n class="Species">CV cultures, anpan>d 2, 4, 6, 7, 11, 25 h for the CP cultures). For each measured poinpan>t, the harvestinpan>g efficienpan>cy was calculated usinpan>g the followinpan>g equationpan>:where OD0 is the sample OD540 at time zero, anpan>d ODt is the sample OD540 at time t.

Prediction of fuel properties

The average degree of unsaturation (ADU) of microalgal oil was estimated based onpan> the pan> class="Chemical">fatty acid composition according to Hu et al.[23], using the following equation:where M is the number of carbon-carbon double bonds in each fatty acid constituent, and Yi is the mass fraction of each fatty acid constituent. The fuel properties of biodiesel transformed from the extracted microalgal oil were predicted as described previously, using the equations listed below[23].

Molecular fingerprinting

Total Dn class="Chemical">NA was extracted from the microalgal culture samples usinpan>g anpan> automated nucleic acid extractor (ExiProgenpan>, Bionpan>eer) as previously described[24]. Denpan>aturinpan>g gradienpan>t gel electrophoresis (DGGE) anpan>alysis was performed with the prepared Dpan> class="Chemical">NA samples to look at the microbial community structures. Bacterial 16S rRNA genes and eukaryotic 18S rRNA genes were amplified by polymerase chain reaction (PCR) using BAC338F/805 R and EUK1A/516 R primer sets, respectively, using a touch-down thermal protocol[24]. The resulting PCR amplicons were electrophoresed on 8% (v/v) polyacrylamide gels with 20–60% (bacteria) or 10–50% (eukaryotes) denaturant gradients for 16 h at 80 V in a D-code system (Bio-Rad, USA). The gels were stained with SYBR Safe dye (Molecular Probe) after electrophoresis and visualized under blue light transillumination. Bands of interest were retrieved from the gels for further sequencing analysis as previously described[24]. The obtained gene sequences were compared against the GenBank database using the BLAST program for phylogenetic affiliation. All nucleotide sequences reported in this study have been deposited in the GenBank database under accession numbers MH155232-MH155241.

Statistical analysis of DGGE fingerprints

The bacterial and eukaryotic DGGE fingerprints were each converted into a matrix based on the relative contribution of individual bands to the total band intensity in each lane, using TotalLab 1D image processing software (TotalLab). The constructed matrices were analyzed by cluster analysis using the unweighted pair group method with arithmetic means (UPGMA) to visualize the relatedness between the analyzed microbial community structures. Clustering calculations and dendrogram generation, with the n class="Disease">Bray-Curtis distance measure, were carried out using PAST 3.17[25].

Results and Discussion

Nutrient removal

n class="Chemical">NH4+-N was completely removed withinpan> 10 days of cultivationpan> inpan> all the cultures onpan> AD effluenpan>ts, while its residual conpan>cenpan>trationpan> remainpan>ed at high levels (>20 pan> class="Chemical">mg/L) in the cultures on BT, for both CV and CP (Fig. 1A,B). This can be related to the low N/P ratio of BT (<1) (see Supplementary Table 2). N/P ratio is an important factor that affects not only nutrient removal but also the extent of microalgal growth. The optimal N/P ratio may vary with substrate characteristics or microalgal strains, and it has been reported that Chlorella species grow well with efficient removal of nutrients at an N/P ratio between 5 and 15[26-28]. In this study, the N/P ratios of the AD effluents were in or near this range, which presumably contributed to the effective removal of NH4+-N.

Figure 1

Residual concentrations of NH4+-N and PO43−-P in Chlorella vulgaris (A and C) and Chlorella protothecoides (B and D) cultures. The curves are labeled with the corresponding culture media: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. Results are expressed as mean ± standard deviation (n = 2).

Residual concentrations of n class="Chemical">NH4+-N anpan>d PO43−-P inpan> pan> class="Species">Chlorella vulgaris (A and C) and Chlorella protothecoides (B and D) cultures. The curves are labeled with the corresponding culture media: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. Results are expressed as mean ± standard deviation (n = 2).

For all the AD effluents tested, CP achieved the complete removal of n class="Chemical">NH4+-N inpan> less time thanpan> pan> class="Species">CV, whereas NH4+-N in BT was consumed faster by CV than by CP. Correspondingly, the maximum NH4+-N removal rate (Rm) was significantly higher (10–22%) in the CP cultures than in the CV cultures for all AD effluents (Table 1). Interestingly, for both Chlorella species, the culture on UL showed the highest Rm (8.5 mg/L·d for CV and 10.4 mg/L·d for CP), followed by those on WH, LF, and HF. On the other hand, the maximum specific NH4+-N removal rate (qm), calculated by dividing Rm by the initial chlorophyll concentration, was higher in the CV cultures than in the CP cultures. For instance, when cultivated on UL, Rm was 8.5 mg/L·d for CV and 10.4 mg/L·d for CP, whereas qm was 9.0 d−1 for CV and 6.3 d−1 for CP. These results indicate that, although chlorophyll is a rough measure of microalgal biomass, CV might have performed superior to CP in terms of the specific activity (per cell or unit biomass) to remove NH4+-N from the AD effluents. The observed qm values (4.1–11.5 d−1 for the CV cultures and 4.7–7.2 d−1 for the CP cultures) are significantly higher than previously reported values for Chlorella species cultivated on synthetic or real wastewaters (≤3.8 d−1, calculated based on chlorophyll a only)[29,30]. For both species, the lag phase was significantly longer when cultivated on BT (≥3.1 days) than on the AD effluents (≤1.7 days) (Table 1). CP showed shorter lag periods (<1 day) than CV did for all AD effluents, which could be due to the higher initial microalgal concentration in the CP cultures, as mentioned above. It is notable that CP achieved only marginal NH4+-N removal, with a much longer lag time compared with CV, when cultivated on BT. This appears to be attributable to the absence of organic carbon sources in BT (see Supplementary Table 2). CP is metabolically versatile, can grow both autotrophically and heterotrophically, and is reported to grow significantly better under heterotrophic conditions[31]. This explains why CP showed much more effective and faster NH4+-N removal in the AD effluents than in BT. Another possibility is that CP may be more sensitive than CV to a low N/P ratio of the substrate.

Table 1

Kinetic parameters of nitrogen removal and chlorophyll production estimated using modified Gompertz equations in the Chlorella cultures.

Parameter	Unit	CV					CP
		BT	HF	LF	UL	WH	BT	HF	LF	UL	WH
Maximum nitrogen removal rate (Rm)	mg/L·d	3.1	6.3	6.9	8.5	8.4	nda	7.5	8.4	10.4	9.2
Lag phase (λ)	d	3.1	1.7	1.6	1.2	1.6	nd	0.6	0.6	0.9	0.6
Maximum specific nitrogen removal rate (qm)	d−1	4.1	5.0	6.9	9.0	11.5	nd	4.7	5.9	6.3	7.2
Chlorophyll production potential (CP)	mg/L	23.9	25.5	25.9	13.8	17.0	5.4	12.0	16.8	13.3	14.3
Maximum chlorophyll production rate (Pm)	mg/L·d	2.0	2.9	3.1	2.6	3.5	1.1	2.0	2.2	1.6	2.2
Lag phase (λ)	d	1.0	1.2	1.1	1.5	1.8	—b	—	—	—	—

Note: Chlorella vulgaris (CV) and Chlorella protothecoides (CP) were cultivated on five media: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively.

aNot determined due to insignificant nitrogen removal.

bNot observed.

Kinetic parameters of nitrogen removal anpan>d pan> class="Chemical">chlorophyll production estimated using modified Gompertz equations in the Chlorella cultures.

Note: pan> class="Species">Chlorella vulgaris (CV) and Chlorella protothecoides (CP) were cultivated on five media: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively.

an class="Chemical">Not determined due to insignpan>ificanpan>t pan> class="Chemical">nitrogen removal.

In all n class="Species">Chlorella cultures onpan> the AD effluenpan>ts, PO43−-P was removed alonpan>gside pan> class="Chemical">NH4+-N during the cultivation (Fig. 1C,D). PO43−-P removal was also much more rapid and significant in the cultures with AD effluents, and there was almost no removal of PO43−-P in the cultures on BT, for both CV and CP. The PO43−-P removal efficiency was comparable or higher in CP (48–72%) than in CV (24–69%). The nutrient removal profiles showed that both CV and CP were effective in the microalgal treatment of residual nutrients, particularly NH4+-N, in the AD effluents.

Microalgal growth

Both n class="Chemical">NO3− anpan>d pan> class="Chemical">NO2− concentrations remained negligible (<1 mg/L) during the cultivation under aerobic conditions (data not shown), suggesting that the removed NH4+-N was mostly assimilated into biomass, given the aerobic culture conditions. Figure 2 shows the temporal changes in total chlorophyll concentration and OD680, which are widely used to estimate microalgal abundance, for the microalgal cultures. Although both the chlorophyll and OD680 levels increased with the consumption of NH4+-N with time, the two indicators showed significantly different temporal profiles, particularly during the later period of cultivation. This appears to be because OD680 cannot differentiate between microalgal cells and other microorganisms or even non-biological suspended particles, potentially leading to confounding results. Such errors can be more pronounced in non-sterile, mixed-culture systems, which are prone to bacterial contamination. Therefore, microalgal growth was monitored using the concentration change in chlorophyll to avoid such errors in this study, although OD680 may better represent the total biomass.

Figure 2

Biomass growth measured as chlorophyll concentration and optical density in Chlorella vulgaris (A and C) and Chlorella protothecoides (B and D) cultures. The curves are labeled with the corresponding culture media: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. Results are expressed as mean ± standard deviation (n = 2).

Biomass growth measured as n class="Chemical">chlorophyll conpan>cenpan>trationpan> anpan>d optical denpan>sity inpan> pan> class="Species">Chlorella vulgaris (A and C) and Chlorella protothecoides (B and D) cultures. The curves are labeled with the corresponding culture media: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. Results are expressed as mean ± standard deviation (n = 2).

Both n class="Species">CV anpan>d CP grew well without a substanpan>tial lag onpan> the AD effluenpan>ts (Fig. 2A,B). This inpan>dicates that the micropan> class="Species">algae readily adapted to the AD effluents and utilized the nutrients, resulting in the short lag periods in NH4+-N removal (Table 1). Proper dilution of AD effluents is an important factor to consider when using them for microalgal cultivation. If the medium is too dense, the light penetration is limited, whereas if it is too dilute, the nutrient levels are insufficient[32]. In this study, the effluents were diluted to a NH4+-N concentration of 40 mg/L, which appears to have provided favorable conditions for the growth of both Chlorella species.

Interestingly, for both n class="Species">CV anpan>d CP, the pan> class="Chemical">chlorophyll accumulation patterns did not correlate well with the NH4+-N removal patterns. CV showed markedly greater cumulative production of chlorophyll in the cultures on LF and HF, but the NH4+-N consumption was the slowest than in other cultures (Figs 1A and 2A). The amount of chlorophyll produced in the CV cultures on UL and WH was even lower compared with the culture on BT, which showed incomplete removal of NH4+-N in the medium. These results suggest that CV grew with different yields (per NH4+-N removed) in the cultures on different media. This is supported by the modeling results, where different CV cultures showed comparable maximum chlorophyll production rates (Pm), but significantly different values for maximum potential of chlorophyll production (CP) (Table 1). Such differences in apparent yield might be associated in part with the potential of NH4+-N consumption by bacteria co-existing with the microalgal species in the experimental cultures. Although less pronounced, similar trends were observed in the CP cultures. The culture on LF showed the highest production of chlorophyll, followed by those on WH, UL, and HF, which is very different from the order of their NH4+-N removal rates (UL > WH > LF > HF). Unlike in the CV cultures, the chlorophyll production in the CP culture on BT was much lower than those in the cultures on the AD effluents, correlating well with the NH4+-N removal results (Figs 1B and 2B). This can be related to the characteristics of CP, which, as mentioned above, grows better in heterotrophic or mixotrophic cultures[31].

The increase in n class="Chemical">chlorophyll conpan>cenpan>trationpan> stopped uponpan> the exhaustionpan> of pan> class="Chemical">NH4+-N, and no apparent growth was observed afterwards, in most of the cultures on the AD effluents, except the CP cultures on UL and WH. The growth of CP after NH4+-N exhaustion can be explained by the ability of microalgae to uncouple nutrient uptake from growth with internal nutrient storage[33,34]. On the other hand, OD680 continued to increase for a while after NH4+-N exhaustion in all cultures on the AD effluents (not seen clearly in the CP culture on HF within the cultivation period). This contrast can be attributed to the growth of bacteria, particularly of heterotrophic bacteria (using cell debris and extracellular secretion as carbon and nitrogen sources) or nitrogen-fixing bacteria, which can cause significant errors in the measurement of microalgal biomass based on optical density.

The culture experiments demonstrated that both n class="Species">CV anpan>d CP grew well onpan> all the AD effluenpan>ts tested anpan>d, inpan> most cases, showed signpan>ificanpan>tly higher rates of pan> class="Disease">microalgal growth and NH4+-N removal. This could be partially attributed to the presence of residual organic compounds in AD effluents, which could promote the growth of microalgae[35,36]. Our results suggest a great potential of microalgal cultivation on AD effluents, not only for the treatment of AD effluents but also for the production of microalgal biomass.

Biomass settling

Harvesting cultivated biomass accounts for at least 20–30% of the total costs in producing microalgal biomass, and therefore, the biomass settleability is an important factor to consider, particularly in large-scale cultivation[7,37]. n class="Species">Chlorella species reportedly have good auto-settlinpan>g properties, which, although not completely clarified, seems to be inpan>fluenpan>ced by ecophysiological factors such as pH, ionpan>s, anpan>d co-existinpan>g bacterial species[38,39]. The microalgal culture samples for auto-settlinpan>g tests were takenpan> onpan> days 11 anpan>d 9 for the pan> class="Species">CV and CP cultures, respectively. CV and CP showed completely different settling behaviors (see Supplementary Fig. 1). The CV cultures on the AD effluents had lag periods of several hours and significantly slower settling rates than those on BT. However, all CV cultures reached comparable high harvesting efficiencies (83.5–93.1%) in 24 h. On the other hand, the CP cultures settled quickly without lag, thus showing significantly greater initial settling rates than those of the CV cultures. The CP cultures showed high harvesting efficiencies of 76.6–89.6% in 25 h, with more than 85% of the maximum harvesting efficiency determined at 25 h being achieved within 6 h in all CP cultures. These results suggest that CP can have advantage over CV when the time for harvesting (or volume of settling basin) is limited, although all cultures on the AD effluents eventually achieved fairly comparable harvesting efficiencies. Interestingly, for both CV and CP, the highest harvesting efficiency among the cultures on the AD effluents was observed when cultivated on UL. Ulva biomass has a high content of metals such as Ca, Fe, and Mg, whose multivalent cations can help to neutralize the negative surface charge of microalgae and promote the aggregation of microalgal cells and other particles[40,41].

Lipid production and composition

To evaluate the potential for n class="Chemical">oil recovery, each culture was examinpan>ed for the pan> class="Chemical">lipid content and composition of cultivated biomass. The productivity and properties of lipids vary greatly with microalgal species and culture conditions[42]. Several studies have recently reported that lipid productivity increases under nitrogen starvation[43-45]. Therefore, in this study, the lipids were analyzed for each culture before (i.e., after the complete NH4+-N removal) and after a nitrogen starvation period of 7 days (i.e., incubation with no additional medium or nutrients): on days 11 and 18 for the CV cultures and on days 9 and 16 for the CP cultures. All cultures on the AD effluents produced greater amounts of lipids compared to those on BT (1.7–2.3-fold difference for CV and 2.0–2.8-fold difference for CP) (Fig. 3A). This may be due to the effect of residual organic carbon in the AD effluents[46]. Both CV and CP showed the largest lipid production on UL before nitrogen starvation (137.8 mg/L for CV and 86.1 mg/L for CP). Interestingly, the lipid concentration increased significantly after the starvation in all cultures. The increase was more pronounced in the CV cultures (1.8–5.0-fold) than in the CP cultures (1.7–2.8-fold), and the largest lipid concentration was observed in the CV culture on LF after nitrogen starvation. These results correspond to previous observations, and a recent study even reported an increase by up to 20-fold in lipid accumulation for Chlorella sorokiniana under nitrogen-depleted conditions[43].

Figure 3

Total lipid concentration (per unit volume of culture; (A) and content (per unit mass of biomass; (B) in Chlorella vulgaris (CV) and Chlorella protothecoides (CP) cultures before (-bs) and after (-as) prolonged cultivation under nitrogen starvation. The x-axis represents five different media used for microalgal cultivation: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. Results are expressed as mean ± standard deviation (n = 2).

Total n class="Chemical">lipid conpan>cenpan>trationpan> (per unit volume of culture; (A) anpan>d conpan>tenpan>t (per unit mass of biomass; (B) inpan> pan> class="Species">Chlorella vulgaris (CV) and Chlorella protothecoides (CP) cultures before (-bs) and after (-as) prolonged cultivation under nitrogen starvation. The x-axis represents five different media used for microalgal cultivation: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. Results are expressed as mean ± standard deviation (n = 2).

With the increase in n class="Chemical">lipid accumulationpan>, the pan> class="Chemical">lipid content increased in all cultures, except the CP culture on BT after starvation (Fig. 3B). The lipid content ranged from 11% to 15%, which is comparable to those previously reported for Chlorella[47], before starvation in the experimental cultures. After the starvation, it increased by up to 2-fold, with the highest lipid content of 54% observed in the CV culture on LF. This is a significantly greater value than the normal range for autotrophic Chlorella cultures (14–30%)[47]. A previous study, although using a synthetic medium, reported a remarkably high lipid content of 89% (of dry weight) for a CV strain under nitrogen-depleted conditions[44]. These suggest that nitrogen starvation can provide a simple way to improve the lipid yield, and thus, the economic feasibility of microalgal biodiesel, particularly when wastewater is used as a culture medium. However, there are several reports of limitation of microalgal growth under nitrogen-depleted condition, which can eventually reduce the total productivity of microalgal lipids[48]. Therefore, careful attention is required to avoid such a negative effect while applying the nitrogen starvation strategy, for example, by employing a two-stage process with separate microalgal growth and lipid production stages[39].

n class="Species">CV anpan>d CP showed signpan>ificanpan>tly difpan> class="Chemical">ferent lipid composition, largely due to the difference in the composition of C18 fatty acids (Fig. 4). C16, C18:2, and C18:3n3 were the major fatty acids commonly found in the CV cultures. On the other hand, in the CP cultures, C18:1 formed one of the major C18 fatty acids, in addition to C18:2 and C18:3n3, with C16 being the dominant fatty acid in all cultures. It is notable that the proportion of C16 decreased with an increase in the proportion of C18 fatty acids after nitrogen starvation in the CV cultures, whereas starvation led to a marked increase in the proportion of C16, with a decrease in the proportion of C18 fatty acids in the CP cultures. These results mean that CV and CP had different lipid production characteristics and responded differently to nitrogen depletion stress in the experimental cultures, supporting the fact that the properties of microalgal lipids vary among different strains[42]. Both CV and CP yielded microalgal lipids suitable for biodiesel production, in terms of fatty acid composition, from the AD effluents. Taking into account the content of monounsaturated fatty acids, particularly oleic acid (C18:1), and the ratio of saturated to unsaturated fatty acids (see Supplementary Table 3), lipids from the CP cultures have superior properties for the oxidative stability and low-temperature fluidity of biodiesel[49,50]. Several fuel properties of biodiesel, related to combustion quality and stability, engine performance, low-temperature properties, and heating value, were calculated based on the fatty acid composition (see Supplementary Table 4). All lipid samples from the experimental cultures met the European (EN14214) and American (ASTM D6751) biodiesel standards, further confirming that the CV and CP cultures on the AD effluents produced microalgal lipids suitable for biodiesel production.

Figure 4

Fatty acid composition of total lipids in Chlorella vulgaris (A) and Chlorella protothecoides (B) cultures before (-bs) and after (-as) prolonged cultivation under nitrogen starvation. The x-axis represents five different media used for microalgal cultivation: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. Results are expressed as mean ± standard deviation (n = 2).

Fatty acid compositionpan> of total pan> class="Chemical">lipids in Chlorella vulgaris (A) and Chlorella protothecoides (B) cultures before (-bs) and after (-as) prolonged cultivation under nitrogen starvation. The x-axis represents five different media used for microalgal cultivation: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. Results are expressed as mean ± standard deviation (n = 2).

Microbial community structure

The eukaryotic and bacterial communities in the microalgal cultures were characterized by DGGE of total Dn class="Chemical">NA samples takenpan> from each culture before the period of prolonpan>ged pan> class="Chemical">nitrogen starvation: on day 11 for the CV cultures and on day 9 for the CP cultures (Fig. 1). It is worth noting that several eukaryotic bands other than those in the seed cultures were detected in the cultures on the AD effluents (Fig. 5A). These bands probably represent the eukaryotes derived from the AD effluents, given the non-sterile, mixed-culture conditions of cultivation. A unique eukaryotic band (E1) detected with high intensity in the CV culture on LF was closely related (≥97% sequence similarity) to several Colpodea ciliates (Table 2). This suggests that the culture contained high levels of bacteria to support the growth of the bacterivorous protozoan associated with E1[51]. The Chlorella-related bands derived from the seed cultures (E2 and E3) appeared as a major band in their corresponding cultures. Both bands showed high similarities (≥90%) to several Chlorella species including CV and CP. Although not robustly quantitative, the DGGE fingerprints further support the fact that both the Chlorella species adapted to and grew well in the AD effluents.

Figure 5

Denaturing gradient gel electrophoresis profiles (A) and cluster dendrogram (B) of the eukaryotic communities in Chlorella vulgaris (CV) and Chlorella protothecoides (CP) cultures. The lanes and nodes are labeled with the corresponding microalgal species and culture media: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. The Chlorella seed cultures are labeled as Seed. The full-length gel for (A) is a presented in Supplementary Fig. 2.

Table 2

Phylogenetic affiliation of eukaryotic 18S and bacterial 16S rRNA gene sequences retrieved from denaturing gradient gel electrophoresis bands.

Domain	Band	Closest relative	Accession No.	Similarity (%)	Query coverage (%)
Eukaryotes	E1	Colpoda aspera	KF111344	100	96
		Kreyellidae sp. MD-2012	JQ723971	100	96
		Kalometopia duplicata	KJ873050	98	96
	E2	Chlorella sp. WT1	KX109776	90	100
		Chlorella vulgaris	KJ561358	90	100
		Chlorella saccharophila	X63505	90	100
	E3	Chlorella emersonii	KF673368	99	97
		Chlorella zofingiensis	KM985403	97	100
		Chlorella protothecoides var. acidicola	AJ439399	95	100
Bacteria	B1	Sphingopyxis macrogoltabida	NR_113720	99	100
		Sphingopyxis soli	NR_116739	99	100
	B2	Nubsella zeaxanthinifaciens	NR_114146	99	100
		Pedobacter nanyangensis	NR_137392	99	100
	B3	Blastomonas natatoria	NR_040824	100	100
		Sphingomonas ursincola	NR_040825	100	100
	B4	Sphingomonas kyeonggiensis	NR_134182	100	100
		Sphingomonas leidyi	NR_025324	100	100
	B5	Tabrizicola aquatica	NR_117979	99	100
		Gemmobacter lanyuensIS	NR_109450	99	100
	B6	Carbophilus carboxidus	NR_104931	99	100
		Aminobacter aminovorans	NR_025301	99	100
		Mesorhizobium cantuariense	NR_137373	99	100
	B7	Corynebacterium vitaeruminis	NR_121721	99	100
		Corynebacterium ulcerans	NR_117211	99	100

Denaturing gradient gel electrophoresis profiles (A) and cluster dendrogram (B) of the eukaryotic communities in n class="Species">Chlorella vulgaris (pan> class="Species">CV) and Chlorella protothecoides (CP) cultures. The lanes and nodes are labeled with the corresponding microalgal species and culture media: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. The Chlorella seed cultures are labeled as Seed. The full-length gel for (A) is a presented in Supplementary Fig. 2.

Phylogenetic affiliation of eukaryotic 18S and bacterial 16S rRn class="Chemical">NA genpan>e sequenpan>ces retrieved from denpan>aturing gradienpan>t gel electrophoresis banpan>ds.

The eukaryotic cluster dendrogram shows a clear separation of the n class="Species">CV anpan>d CP cultures onpan> the AD effluenpan>ts, implyinpan>g that seed biomass had more decisive efpan> class="Chemical">fect than the characteristics of the effluents on the evolution of eukaryotic community during the cultivation (Fig. 5B). On the other hand, the cultures on BT were most closely related to their seed cultures and formed a distinct cluster. This suggests that the type of media (i.e., synthetic or effluent) also had a critical influence on the eukaryotic community structure. A possible reason for such difference is the unintended introduction of eukaryotic microorganisms from the AD effluents.

The bacterial DGGE fingerprints showed much more complicated and dynamic structures than the eukaryotic DGGE results (Fig. 6A). The retrieved band sequences (B1 to B7) were all closely related to known bacterial species (Table 2). As in the eukaryotic cluster analysis, the n class="Species">CV anpan>d CP cultures onpan> the AD effluenpan>ts were clearly separated from each other inpan> the bacterial cluster denpan>drogram (Fig. 6B). This inpan>dicates that seed micropan> class="Species">algae had the dominant influence on the differentiation of the bacterial community structure. It has already been reported that microalgae interact with many other organisms in their growing environment and affect the diversity of bacterial community[52]. It should be noted that the CP seed culture was contaminated with bacteria, particularly with the bacterium corresponding to B3 (Fig. 6A). B3 was closely related to metabolically versatile Sphingomonadaceae species[53], including Blastomonas natatoria, which is a photosynthetic bacterium capable of using peptone as a carbon source[54]. This suggests the possibility that this bacterial species was enriched during the pre-cultivation of CP in a medium containing proteose peptone (refer to Subsection 2.1). This is supported by the fact that B3 appeared as a major band with strong intensity in all CP cultures on the AD effluents, but as a faint band in the culture on BT without organic carbon. This may explain why the CP seed culture was clustered together with the CP cultures on the AD effluents in the cluster analysis (Fig. 6B).

Figure 6

Denaturing gradient gel electrophoresis profiles (A) and cluster dendrogram (B) of the bacterial communities in Chlorella vulgaris (CV) and Chlorella protothecoides (CP) cultures. The lanes and nodes are labeled with the corresponding microalgal species and culture media: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. The Chlorella seed cultures are labeled as Seed. The full-length gel for (A) is a presented in Supplementary Fig. 3.

Denaturing gradient gel electrophoresis profiles (A) and cluster dendrogram (B) of the bacterial communities in n class="Species">Chlorella vulgaris (pan> class="Species">CV) and Chlorella protothecoides (CP) cultures. The lanes and nodes are labeled with the corresponding microalgal species and culture media: one synthetic (Bristol medium, BT) and four prepared using effluents from different anaerobic digesters treating food waste at a high (HF; 5 g VS/L·d) and a low (LF; 1.5 g VS/L·d) organic loading rates, Ulva (UL), and whey (WH), respectively. The Chlorella seed cultures are labeled as Seed. The full-length gel for (A) is a presented in Supplementary Fig. 3.

The remaining sequences were assigned to bacterial groups commonly found in various natural and engineered systems: Sphingomonadaceae (B1 and B4), Sphingobacteriaceae (B2), Rhodobacteraceae (B5), Rhizobiales (B6), and Corynebacteriaceae (B7). B2, detected as the dominant band in the culture on WH (effluent from an anaerobic digester treating n class="Chemical">lactose-rich whey), was closely related to pan> class="Species">Nubsella zeaxanthinifaciens, which is capable of utilizing lactose[55]. B5 was closely related to the photosynthetic Rhodobacteraceae species. Some members of the family are able to accumulate fatty acids[56], and the bacterium related to B5 may have, in part, contributed to the production of lipids. B6 was closely related to several nitrogen-fixing Rhizobiales species, which may have provided nitrogen for other microbes to use in the cultures, particularly under nitrogen starvation conditions[57,58]. The molecular analysis results confirm that microalgae and bacteria co-existed and interacted in the experimental culture. de-Bashan et al.[59] have suggested that the co-immobilized microalgal and bacteria can be beneficial for the growth of microalgae, and other studies also have reported that co-existing bacteria play an important role in nutrient removal in microalgae-based wastewater treatment processes[60,61]. Supply of phytohormones, exchange of carbon and nutrients, and elimination of pathogenic microorganisms have been suggested as possible interactions between microalgae and bacteria that are beneficial for the growth of microalgae[58].

Conclusions

n class="Species">CV anpan>d CP were tested for their potenpan>tial to grow onpan> anpan>d treat difpan> class="Chemical">ferent AD effluents, i.e., HF, LF, UL, and WH, in comparison to a synthetic medium BT, in batch mode. Both CV and CP cultures showed significantly higher nutrient removal efficiencies in the AD effluents than in BT, with the substrate NH4+-N being completely removed within 10 days of cultivation on all the AD effluents. Correspondingly, both microalgal species generally had better growth on the AD effluents than on BT. Cultivation under nitrogen starvation significantly increased the lipid accumulation in all cultures, regardless of the media used. The increase in lipid content was even more pronounced in the CV than in the CP cultures. The CV cultures demonstrated higher microalgal growth and lipid production, whereas the CP cultures showed better auto-settling properties for biomass harvesting. For all cultures, the fuel properties estimated based on the fatty acid composition proved that the produced lipids were suitable as a feedstock for biodiesel production. The microbial community analysis results showed that both CV and CP adapted and grew well on the AD effluents under non-sterile, mixed-culture conditions in the presence of diverse bacteria. The overall results suggest a high potential for microalgae-based treatment and valorization of AD effluents. A point to consider, particularly in large-scale applications, is the great need for water to dilute AD effluents, which limits the economic feasibility of the microalgal cultivation process. Therefore, further studies on the microalgal adaptation to higher nitrogen loading (or less dilution) conditions and the minimization of water use by harvest water recycling are needed.

supplementary information