Keywords

1 Introduction

Sustainable development is questioned as 40% of water lack will be faced by 2030 in various regions globally. This setback may arise from uncontrollable water usage and release in nearby receiving bodies without adequate treatment [1]. The untreated/partially treated wastewater contains organic and inorganic matters, toxic to humans and the aquatic ecosystem.

In India, only 40% of the sewage generated is treated, resulting in 60% of untreated or partially treated sewage being released into the river. Nonetheless, the greater part of the current sewage treatment in agricultural-based nations faces troubles with nutrient expulsion at low costs [2]. Also, extensive methodology, carbon transmission, and additional sludge treatment generated during various treatments are severe setbacks of the conventional system [3].

Investigations have proposed microalgae as a biological treatment that proficiently eliminates high and fluctuating concentrations of organics and nutrients present in various wastewaters [4, 5]. In addition, nitrogen, phosphorus, and various trace metals such as Fe, Mn, and S present in municipal wastewater act as food to microalgae [4].

In the present study, C. vulgaris was utilized to treat domestic wastewater due to its attributes like (a) high development rate, (b) quick expulsion rate, and (c) highly riched biomass [5]. The disinfectant property of microalgae is also an added advantage to remove toxic microorganisms. The phycoremediation system has low capital and operating cost. These benefits make it more noticeably internationally. However, the literature also reveals that the significant expense related to the method is the harvesting of biomass [6].

After analyzing the gaps in the acceptability of the system, the present study was focused on working on a few challenges by investigating the viability of the phycoremediation system in removing nutrients along with other physicochemical parameters at lower HRT and in the open to sky conditions. In this study, the workability of a phycoremediation system was observed to treat raw domestic wastewater under light/dark conditions.

2 Methodology

C. vulgaris was used as a microalgal strain for the study, which was provided by an Ahmedabad-based consultancy. A batch study was performed with a total volume of 1800 mL, combining raw wastewater and microalgal concentrations (Fig. 1). In the study, six different concentrations of microalgae were studied, ranging between 20 and 45%. Each concentration has an increment in 5% microalgae than the previous. Six different microalgal concentrations studied were 20% (360 mL microalgae + 1440 mL raw wastewater), 25% (450 mL microalgae + 1350 mL raw wastewater), 30% (540 mL microalgae + 1260 mL raw wastewater), 35% (630 mL microalgae + 1170 mL raw wastewater), 40% (720 mL microalgae + 1080 mL raw wastewater), and 45% (810 mL microalgae + 990 mL raw wastewater), and their respective dry weight is illustrated in Table 1.

Fig. 1
figure 1

Experimental setup

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Table 1 Different microalgal concentrations and corresponding dry weight
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The study was conducted in an outdoor condition. The study was conducted in the month of June–July. Temperature variation during the study was between 21 and 24 °C. The number of trials conducted for each set of experiments (20–25%, 30–35%, and 40–45%) was 24, respectively. The retention time for the study was 11 h (aeration followed by settling). Physico-chemical analysis was done on the supernatant drawn after settling.

The sewage pumping station near the experimental site was the source of domestic wastewater for the study. Parameters such as BOD, COD, PO4-P, NO3-N, NH3-N, pH, EC, TS, TSS, and TDS were analyzed. Each sample was analyzed twice, and the average was considered for analysis to reduce the instrument and human errors in the study.

3 Result and Discussion

The study was conducted in three combinations (20% and 25%), (30% and 35%), and (40% and 45%). Variations in characteristics of domestic wastewater during the study are tabulated in Table 2.

Table 2 Variations in raw wastewater characteristics
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High COD (>92%), NH3-N (>85%), and phosphorus removal (30%) were obtained at 6 days and 10 days HRT [7]. The phycoremediation system effectively reduced the various parameters analyzed at low HRT during the present experimental study.

NH3-N concentration in the effluent lowered to 1.62 mg/L (84.58%), 1.00 mg/L (90.51%), 0.39 mg/L (96.60%), 0.43 mg/L (96.26%), 3.34 mg/L (77.11), and 2.71 mg/L (77.91%), when treated with 20%, 25%, 30%, 35%, 40%, and 45% microalgal concentration, respectively. Figure 2a depicts variation in the reduction of NH3-N at different concentrations. Microalgae burn-through NH3-N among all N structures accessible [8]; NH3-N consumed is fused into protein for microalgal development.

Fig. 2
figure 2

Reductions in nutrients at various microalgal concentrations

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Similarly, for PO4-P, promising reductions were observed in all concentrations. PO4-P concentration in the effluent lowered to 0.42 mg/L (80.00%), 0.34 mg/L (82.84%), 0.17 mg/L (91.73%), 0.25 mg/L (88.36%), 0.49 mg/L (73.59%), and 0.45 mg/L (75.35%), when treated with 20%, 25%, 30%, 35%, 40%, and 45% microalgal concentration, respectively. Figure 2b illustrates PO4-P reduction at different concentrations. Microalgae consumes phosphorus as orthophosphate [9].

In addition to nutrient removal, phycoremediation is quite effective in organics removal. COD removal reached to 79.17% (48 mg/L), 84.72% (35.2 mg/L), 85.13% (40.00%), 84.27% (46.55 mg/L), 73.89% (61.20 mg/L), and 72.92% (56.20 mg/L) after treatment with different microalgal concentrations, respectively. Figure 3a illustrates COD reductions during the study. COD reduction in the presence of microalgae results in cooperative connection with the heterotrophic microscopic organisms [10] as microbes use the oxygen delivered by algae as a finished result for their development and endurance. In any case, COD evacuation was marginally lesser than the nutrients, as the development of nitrates interferes with COD reduction [11]. COD and TKN removal were about 86% and 97% at an HRT of 2 days when the microalgal-bacterial consortium was used to treat municipal pre-settled wastewater [12].

Fig. 3
figure 3

Reductions in organics at the different microalgal concentration

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Phycoremediation technique effectively reduced PO4-P, NH3-N, and COD concentration to 97.89%, 98.81%, and 88.24%, respectively, from the primary-treated effluent's of an urban STP at the identical HRT, practiced for secondary treatment [9]. A lab-scale mode in SBR mode (HRT = 48 h.) comprising Chlorella vulgaris and activated sludge used to treat synthetic wastewater led to 88% and 15% removal of COD and TN, respectively [13]. Chlorella vulgaris reduced PO4-P, NH3-N, and COD concentration to 98.32, 97.26, and 84.71%, respectively, without any external aeration from the primary-treated municipal wastewater [11].

Likewise, BOD removal reached to 85.20% (12.96 mg/L), 89.14% (9.50 mg/L), 89.56% (11.61 mg/L), 88.83% (12.56 mg/L), 82.01% (20.30 mg/L), and 82.19% (20.50 mg/L) after treatment with different microalgal concentrations, respectively. Figure 3b illustrates reductions in BOD at different concentrations. However, the reduction in BOD was much higher than in the conventional system because of the ample amount of DO present, enhancing the degradation of organic matter.

Among solids, TSS had the greater reduction, i.e., up to 39.28% (895 mg/L), 34.70% (890 mg/L), 30.87% (912 mg/L), 33.67% (970 mg/L), 26.94% (925 mg/L), and 25.35% (940 mg/L) after treatment with 20% to 45% microalgal concentration, respectively. The formation of algal bacterial flocs resulted in a reduction in solids. TDS and EC had a similar reduction. C. Vulgaris used for domestic wastewater treatment at 3 days HRT led to COD, and nutrient and TS removal during the study was around 85% [14].

The study revealed that challenges of the conventional system could be resolved if the phycoremediation technique is incorporated into the traditional system. As the nutrients and organics concentration in the effluent was far below the desirable limit, the study also revealed that the increment in microalgal concentration led to increased removal efficiency of physicochemical parameters to a certain extent. Excess microalgae can harm the treatment process. Light availability and temperature varying between 20 and 30 °C are the primary conducive environmental conditions required for the system.

NH3-N had the highest removal among all the parameters because N comprises 10% algal biomass and phosphorus only 1%. DO reached 7 mg/L in the system comprising 20%, 25%, and 30% microalgal concentration. Whereas for the systems with 35% and 40% microalgal concentration, DO reached 7.6 mg/L and 7.8 mg/L in the case of 45% microalgal concentration. The increase in DO was due to the external aeration and microalgal presence.

Similarly, a pH increase was found in the effluents of all the systems. The pH increase in the effluent was contributed because of the photosynthetic effect, which was to a certain extent counterbalanced by nitrification [15]. However, increased pH contributed to nutrient removal as DO increment enhanced the organic removal. Effluent pH in all the concentrations studied was higher than 8.

The study revealed that combining microalgae in conventional treatment can be eco-friendly and helpful in minimizing the electricity cost (as microalgae can lead to aeration) and resolving other setbacks of the conventional system. In addition to this, the study also addressed that the phycoremediation technique can effectively work at lower HRTs.

4 Conclusion

The study also revealed that the increase in microalgal concentration does not lead to higher removal of pollutants. Among various microalgal concentrations analyzed, 30% microalgal concentration had the highest reductions in both nutrients and organics. Removal efficiency reached >90% for nutrients and >85% for organic matter when 30% concentration was used to treat raw municipal wastewater. Thus, 30% microalgal concentration was observed as the optimum concentration for the study. Reductions in the algal system were contributed by high pH and DO concentration in the effluent. Thus, the algal system reduces the eutrophication in the receiving bodies and can enhance the self-cleansing capacity of the water body. An appropriate treatment unit includes limited human and financial resources, reliable treatment, system economic feasibility, less energy demand, and social acceptance. Therefore, research combining microalgae with conventional wastewater treatment can be the best-suited treatment option. In addition to this, this system also eliminates the need for tertiary treatment and can answer various challenges of the present treatment systems practiced in developing countries.