Screening the growth potential of the microalgae on

Flow cytometry, however, is compared to counting chambers significantly more expensive. Microplate based method can be used to observe microalgal growth rates in low density microplate cultures (van Wagenen et al., 2014), but is limited in too dense cultures. As for flow cytometry, it is not possible to distinguish microalgal species from each other.

This section reports unpublished methodological work for applications in the microalgal growth studies and details are found in Appendix 4.

Although the method has some limitations, the results suggest that flow cytometry, direct counting, and microplate-based method can all be used to monitor microalgal growth in wastewater accurately, and the fluorescence-based microplate method correlated well with the established techniques.

4.5.2 Screening the growth potential of the microalgae on

Table 4.1 Secondary effluent wastewater characteristics after UASB reactor and tight-micro filtration (T-MF) treatment that was used in screening tight-microalgal-based treatment in batch system for nutrient removal.

Parameters Unit

Effluent Quality DAF

Pre-treatment UASBR T-MF Physico-chemical Parameter:

sCOD mg∙l⁻¹ 721±128 240±79 272±86

tCOD mg∙l⁻¹ 917±179 504±141 301±120

TSS mg∙l⁻¹ 420±83 101±9 0.02 ±0.01

NH4+ mg∙l⁻¹ 57±8 55±8.6 55±9

NO3- mg∙l⁻¹ <0.5 <0.5 <0.5

PO43- mg∙l⁻¹ 24±6 24±4 24±4

TN mg∙l⁻¹ 59±7 57 ± 4 55±4

TP mg∙l⁻¹ 25±5 25±6 24±6

Alkalinity mg CaCO3∙l⁻¹ 478±69 684±128 691±45 Total VFA mg CH3COOH∙l⁻¹ 143±51 182±146 112±11 Microbiological Parameters:

Total Coliforms log10CFU∙(100 ml)⁻¹ 6.0±0.1 5.7±0.2 0.0 E. coli log10CFU∙(100 ml)⁻¹ 4.1±0.2 4.0±0.1 0.0 Enterococci log10CFU∙(100 ml)⁻¹ 1.3±0.3 1.0±0.2 0.0 Heterotrophs log10CFU∙(100 ml)⁻¹ 7.7±0.1 7.6±0.1 0.0

Filtered UASB effluent was used as media for growth studies on the pre-selected microalgal species. Calculated maximum specific growth rates based on data from identified logarithmic phases are shown in Table 4.2.

Results showed that secondary wastewater effluent enhanced growth for C. vulgaris, C. sorokiniana, T. obliquus, but not for H. pluvialis and M.

salina. Low salinity or possibly toxic substances present in the wastewater could be inhibitory for growth of M. salina in wastewater.

As mentioned in previous section, H. pluvialis never reached high cell density. To make this strain grow in wastewater, one should consider different growth media and other environmental conditions (Zhu et al., 2018).

Table 4.2 Established microalgal growth rates cultivated in pre-growth media and filtered UASB secondary effluent

Algal strains Growth rate (day^-1)

Pre-growth media¹ Wastewater²

C. vulgaris 0.77±0.12 1.23±0.33

C. sorokiniana 0.63±0.11 1.46±0.39

T. obliquus 0.62±0.32 1.26±0.48

H. pluvialis 0.37±0.20 0.02±0.001

M. salina 0.56±0.42 0.21±0.12

1Pre-growth media: MWC+Se media for C. vulgaris, C. sorokiniana, T.

obliquus, H. pluvialis; L1 media for M. salina

2Wastewater: UASB+membrane effluent

The results in Figure 4.17 imply that C. sorokiniana was the most efficient microalgal strain for removing nutrients in the tested wastewater with hydraulic retention time of 9 days. C. sorokiniana was able to remove up to 97±2% TP and 70±8% TN, while T. obliquus removed up to 83±4% TP and 49±5% TN. C. sorokiniana, C. vulgaris and T. obliquus significantly reduced ammonium (>70%). Similar results were obtained by (Wang et al., 2010), where Chlorella sp. removed up to 83% ammonium from municipal wastewater. Nitrate was produced by the culture (+26 - 43%; Figure 4.17), indicating limited ammonium oxidation. Furthermore, almost 100 % alkalinity reduction was observed by all strains in all batch tests, suggesting the nitrification process to be alkalinity limited and microalgae used in this experiment preferred ammonium as a nitrogen source. It is possible that some ammonium could have been removed by ammonia volatilization. The low alkalinity at the end of experiments implies that pH has decreased. C. sorokiniana removed more than 90% of phosphate and proved to be the most efficient phosphorous removal microalgae tested, followed by T. obliquus (83%).

C. vulgaris and M. salina removed 63% and 50% respectively in wastewater. Wang et al. (2010) reported 90 % phosphorous removal of by Chlorella sp. in municipal wastewater. The N/P ratio for microalgal-based wastewater

Figure 4.17 Microalgal nutrient, COD and alkalinity removal from secondary wastewater effluent in the batch system after reaching stationary phase. Error bars show standard deviations.

treatment has been proposed to be in the range of 6.8 to 10.0 (Wang et al., 2010). The measured N/P ratio in the UASB secondary wastewater effluent were slightly lower from this proposed optimal condition by 4.3, however, growth does not seem to be severely limited by phosphorous or nitrogen indicating another growth limiting factor to be decisive. Low alkalinity and limited nitrification suggest that to be CO2.

TSS production of microalgal strains cultivated in wastewater after reaching the stationary phase represented dry microalgal biomass production. The microalgal production yield result is presented in Table 4.3. and show C. sorokiniana to have a relatively high yield compared to C. vulgaris, M. salina, and T. obliquus. The amount of TSS was measured before and after microalgal growth tests, and may be used as an indication of total microalgal biomass production (Ramaraj et al., 2015). Comparing these values with results gained in other studies indicates a high and effective biomass production (biomass per liquid volume). Ramaraj et al. (2015) reported values ranging from 0.07 gTSS∙l

-1 to 0.26 gTSS∙l^-1 for microalgae consortia cultivated in a natural water

-60 -40 -20 0 20 40 60 80 100

CODs TN NH₄-N NO₃-N TP PO₄-P Alkalinity VFA

Removal efficiency (%)

C. vulgaris C. sorokiniana T. obliquus M. salina

Table 4.3 TSS production and yield of microalgal strains cultivated in wastewater after reaching the stationary phase (±standard deviation).

C. vulgaris C. sorokiniana T. obliquus M. salina Unit 0.75±0.23 1.05±0.34 1.35±0.18 0.60±0.12 gTSS∙l^-1 0.03±0.01 0.04±0.01 0.02±0.005 0.02±0.01 gTNremoved∙gSS^-1 0.02±0.003 0.02±0.004 0.01±0.005 0.01±0.006 gTPremoved∙gSS^-1

media. Biomass productivity of microalgae cultivated in effluent from a submerged membrane anaerobic bioreactor by Ruiz-Martinez et al.

(2012) resulted in a maximum biomass level of 0.6 gTSS∙l^-1.

Microalgae can remove organic carbon through mixotrophic or heterotrophic metabolism (Cai et al., 2013). As presented in Figure 4.17, C. vulgaris demonstrated the highest dissolved COD removal by 45±2%, followed by C. sorokiniana and T. obliquus, with removal efficiencies of 44±3% and 40±4%, respectively, likely the result of co-cultured with heterotrophs which would growth symbiotically with the microalgae.

Moreover, organic and nutrient removal by C. vulgaris and C.

sorokiniana indicated they are mixotrophic culture as it has been known that acetate has been used as the main carbon source in some industrial mixotrophic cultivations of Chlorella (Richmond, 2003). CO2 can be the limiting nutrient in microalgal cultivation when using atmospheric CO2

as an inorganic carbon source. As shown in Figure 4.17, all microalgal species removed all the alkalinity in the wastewater, implying nitrification. Therefore, addition of an external CO2 or co-culturing with ordinary heterotrophic bacteria could enhance microalgal growth and nutrient removal. For example, biogas produced in the UASB system could be used as CO2 source with a proper gas collecting system.

However, in this presented study, there was no biogas handling, and CO2

was likely stripped by the membrane system.

4.5.3 Nutrient-limited kinetic growth analysis of Chlorella