During the experimental process, various physico chemical parameters were measured at 11 hours HRT for all the concentrations of C. vulgaris (20%, 25%, 30%,35%, 40% and 45%). The average concentration of raw domestic wastewater collected from a nearby sewage pumping station for various parameters studied is tabulated in Table 1.
After the treatment of the raw domestic wastewater with microalgae, it was found that even though with low HRT of 11 hours in the present study, efficient removal was observed in all the parameters analysed at different microalgal concentration when compared with various literatures where the HRTs varied in days to weeks [21–23]. Chlorella vulgaris was used because of its characteristics features like (1) high growth rate, (2) fast nutrient removal rate; (3) strong adaptability to different types of wastewater and local climate; and (4) high biomass productivity [1].After the treatment of the raw domestic wastewater with microalgae, it was found that coupling WWT with algae can be a reasonable, cost-effective viable opportunity for water treatment, with an opportunity for clean water production in areas of water scarcity [24]. Removal in different parameters was observed in each concentration studies. However, maximum removal was found in the effluent of the 30% microalgae.
Nitrogen is transformed into N2 gas in conventional nitrogen removal methods, whereas in the algal treatment system, nitrogen compounds are taken up for their growth [25]. The uptake of nitrate is light energy-dependent, and also microalgae prefer to utilize already reduced nitrogen, such as ammonium in comparison to nitrate, which is less energy intensive conversion [26]. Ammonia was efficiently removed in the system as it is incorporated into protein via protein anabolism for microalgal growth [27, 28]. Variation in percentage removal efficiency for ammonia at different concentration considered in the study was 59.84 ± 24.29 for 20% concentration, 67.37 ± 22.44 for 25% concentration, 73.65 ± 24.79 for 30% concentration, 69.56 ± 27.92 for 35% concentration, 49.84 ± 19.50 for 40% concentration and 54.37 ± 20.08 for 45% concentration in case of non- filtered effluent respectively. However, when effluent was filtered with coarse filter to remove filamentous microalgae if any, percentage removal efficiency further increased to 65.63 ± 23.45 for 20% concentration, 71.13 ± 21.94 for 25% concentration, 77.66 ± 22.64 for 30% concentration, 75.27 ± 24.56 for 35% concentration,55.93 ± 18.42 for 40% concentration and 58.47 ± 19.09 for 45% concentration respectively shown in Fig. 2(a). Increase in nitrate concentration was observed during the study as ammonia was converted to nitrate in aerobic condition via nitrification [29]. Nitrate concentration raised to 3.47 mg/L, 3.82 mg/L, 5.87 mg/L, 5.82 mg/L, 3.85 mg/L and 3.82 mg/L, when wastewater treated with 20%, 25%, 30%, 35%, 40% and 45% microalgae respectively.
Microalgae have also shown the great potential to utilize phosphorus from wastewater, but mainly in the form of orthophosphate [30], and it is incorporated into organic compounds such as nucleic acids, phospholipids and proteins [31]. Variation in percentage removal efficiency for phosphate at different concentration considered in the study was 60.46 ± 18.62 for 20% concentration, 63.95 ± 16.17 for 25% concentration, 71.47 ± 17.92 for 30% concentration, 69.29 ± 18.22 for 35% concentration, 54.61 ± 13.16 for 40% concentration and 56.14 ± 13.22 for 45% concentration in case of non- filtered effluent. However, when effluent was filtered with a coarse filter to remove filamentous microalgae if any, percentage removal efficiency further increased to 66.27 ± 15.99 for 20% concentration, 69.94 ± 13.48 for 25% concentration, 74.56 ± 16.98 for 30% concentration, 71.33 ± 18.08 for 35% concentration, 57.84 ± 12.68 for 40% concentration and 60.05 ± 12.87 for 45% concentration. Removal efficiency phosphate at different microalgal concentrations is shown in Fig. 2(b).
COD removal is performed in the microalgae system in symbiotic relation with the heterotrophic bacteria [32], as the oxygen produced by algae as an end product is utilised by bacteria for their growth and survival [33]. Variation in removal efficiency for COD at different concentration considered in the study was 52.40 ± 17.32 for 20% concentration, 55.13 ± 18.88 for 25% concentration, 65.13 ± 20.21 for 30% concentration, 62.54 ± 22.15 for 35% concentration, 57.51 ± 14.94 for 40% concentration and 59.35 ± 13.77 for 45% concentration in case of non- filtered effluent. However, when effluent was filtered with a coarse filter to remove filamentous microalgae if present, percentage removal efficiency further increased to 59.91 ± 16.34 for 20% concentration, 63.59 ± 15.14 for 25% concentration, 69.90 ± 20.54 for 30% concentration, 66.01 ± 21.60 for 35% concentration, 62.01 ± 12.87 for 40% concentration and 64.53 ± 12.94 for 45% concentration as shown in Fig. 3.
During the study, removal in TS and TSS was also observed to a certain extent due to the formation of algal bacterial biomass that settles down and leaves a clear supernatant. Variation in percentage removal efficiency for TS at different microalgal concentration considered in the study was 11.74 ± 7.2 for 20% concentration, 12.36 ± 6.6 for 25% concentration, 13.72 ± 4.13 for 30% concentration, 11.14 ± 4.85 for 35% concentration, 11.23 ± 4.77 for 40% concentration and 11.53 ± 5.00 for 45% concentration in case of non- filtered effluent. However, when effluent was filtered with a coarse filter to remove filamentous microalgae, removal efficiency further increased to 18.17 ± 6.71 for 20% concentration, 17.64 ± 6.71 for 25% concentration, 22.27 ± 5.68 for 30% concentration, 19.98 ± 6.62 for 35% concentration, 18.44 ± 5.52 for 40% concentration and 18.69 ± 5.51 for 45% concentration as shown in Fig. 4(a). Variation in removal efficiency for TSS at different concentration considered in the study was 15.28 ± 11.65 for 20% concentration, 16.64 ± 10.18 for 25% concentration, 19.49 ± 6.85 for 30% concentration, 15.97 ± 7.83 for 35% concentration, 16.13 ± 7.74 for 40% concentration and 15.49 ± 8.28 for 45% concentration in case of non- filtered effluent. However, when effluent was filtered with a coarse filter to remove filamentous microalgae, removal efficiency further increased to 25.69 ± 10.95 for 20% concentration, 24.60 ± 11.20 for 25% concentration, 32.73 ± 10.19 for 30% concentration, 29.75 ± 10.98 for 35% concentration, 26.47 ± 8.00 for 40% concentration and 25.51 ± 7.89 for 45% concentration as shown in Fig. 4(b). The problem of separating algae from water may be solved by using attached algae because these attached algae are often observed in the form of algal biofilm in sufficient sunlight [34].
When compared to TS and TSS removal in TDS was low. Variation in removal efficiency for TDS at different concentration during the study was 7.29 ± 3.99 for 20% concentration, 7.11 ± 4.53 for 25% concentration, 5.83 ± 2.64 for 30% concentration,4.48 ± 2.11 for 35% concentration, 3.62 ± 1.59 for 40% concentration and 5.45 ± 2.39 for 45% concentration in case of non- filtered effluent. However, when effluent was filtered with a coarse filter to remove filamentous microalgae increase in removal efficiency was observed up to 8.68 ± 4.08 for 20% concentration, 8.98 ± 4.16 for 25% concentration, 8.05 ± 2.56 for 30% concentration, 6.59 ± 2.96 for 35% concentration, 6.13 ± 2.14 for 40% concentration and 8.65 ± 2.62 for 45% concentration as shown in Fig. 5(a). Variation in removal efficiency for EC at different microalgal concentration was 6.95 ± 4.27 for 20% concentration, 6.4 ± 3.69 for 25% concentration, 7.27 ± 6.97 for 30% concentration, 5.68 ± 6.28 for 35% concentration, 3.88 ± 2.51 for 40% concentration and 5.49 ± 3.35 for 45% concentration in case of non- filtered effluent. However, when effluent was filtered with a coarse filter to remove filamentous microalgae, removal efficiency further increased 8.84 ± 4.05 for 20% concentration, 8.79 ± 3.51 for 25% concentration, 9.27 ± 6.92 for 30% concentration, 6.97 ± 6.41 for 35% concentration, 6.11 ± 2.53 for 40% concentration and 7.08 ± 3.09 for 45% concentration as shown in Fig. 5(b).
Microalgal system lead to increase in DO concentration, which raised up to 6.6 mg/L, 6.8 mg/L, 7.2 mg/L, 7.6 mg/L, 7.6 mg/L and 7.8 mg/L respectively when influent was treated with microalgal concentration of 20%, 25%, 30%, 35%, 40% and 45% respectively. The increase in DO was mainly because of the photosynthetic effect of algae in addition to external aeration provided for the mixing of algae in the influent. Algal growth increases alkalinity to a remarkable level regardless of the initial pH [35]. The rise of pH by photosynthesis was impeded due to the production of H + ions by nitrification and by the use of ammonium as a nitrogen source for the photosynthesis process itself [36]. The elevated pH also enhances Ammonical-N removal. pH increased to 8.40 when treated with 20% microalgal concentration and 8.56 when treated with both 25% and 30% microalgae, whereas pH reached to 8.81, 8.53 and 8.57 when treated with 35%, 40% and 45% microalgal concentration respectively.