Sample collection
The algal samples were collected from aquatic regions of various districts. The stock algal strains from the culture collection of the Department of Agricultural Microbiology, Tamil Nadu Agricultural University, Coimbatore were also used.
Isolation of microalgal cultures
The collected samples were serially diluted from 101 to 105 times and plated on sterile plates containing BG11 medium. The plates were incubated at an algal growth chamber supplying 16h: 8h of light: dark (1400 l), with a temperature of (28 ± 2ºC) for 7–10 days. Cold fluorescent lamps were used for illuminating the growth chamber. The colonies grown were picked and continuously sub-cultured in BG11 agar plate, supplemented with ampicillin (100 µg/ml) and kanamycin (100 µg/ml), cychloheximide (100 µg/ml) ensuring the absence of bacterial and fungal contamination respectively. The purified colonies were cultured in BG11 broth and used for further analysis.
Screening for electrogenic activity
Cyclic voltammetry (CV) method, under electrochemical work station (AMETEK, scientific instruments, USA) was performed using three-electrode system, namely glassy carbon electrode (3mm diameter) as a working electrode, a platinum wire and Ag/AgCl (3M KCl) as a counter and reference electrodes respectively. The electron transfer mechanism was analyzed using a working electrode covered with a concentrated microalgal coat. The coated electrode was shade dried and incubated at room temperature (28 ± 2ºC) for two days to induce the interaction between electrode and algae. CV traces were determined in the potential range of -0.8 to + 0.8 V at a scan rate of 10 mVs− 1. Deoxygenated, deionized water was used as an electrolyte to ensure the absence of oxidation reduction.
Authentication of electrogenic activity
Hindakia sp. was grown in BG 11broth and waste water separately along with stainless steel mesh (anode) and incubated in above-mentioned specifications of the light chamber. One end of the anode was let outside the rubber cork to measure the electron flow. They were connected to the cathode of the multimeter (MASTECH, MAS830L) and the circuit is closed by connecting its anode to a graphite plate (air cathode) as depicted in Fig. 1. The power density was recorded on alternate days up to the 25th day. The study level current and voltage were calculated using the formula I = V/R, where I is the current, V is the voltage measured and R is external resistance. The power density P was calculated from the measured voltage (P = V2/RA). The cathode area in our study used is 64 cm2 and the resistance provided is 500 Ω .
Estimation of Extracellular polymeric substances (EPS)
The algae at the log phase were harvested on the 20th day and the cells were heated at 45ºC for 20 min. It was then centrifuged at 8000 rpm for 20 min at room temperature. The supernatant was collected and added with three volumes of acetone and stored at − 20ºC for 24h. After 24h, it was spun down at 4 ºC for 20 min. Along the sides of the tube, the precipitated polysaccharides were dried to measure the EPS yield gravimetrically(15).
Microscopic analysis
The screened algal cultures were observed in a light microscope and their morphological characteristics were studied. Further, to analyze the live and dead cells, algal strains were stained with trypan blue(16).
Effluent collection
The raw industrial effluents from the processing plant were collected from Aavin Dairy Plant, Coimbatore District Co-operative Milk Producers Union Ltd., Pachapalayam, Perur, Coimbatore (10.9755° N, 76.9153° E).
Experimental setup
The screened algal strain was inoculated into wastewater at lab-scale reactor. The preliminary trials were carried out with lab scale raceway pond fabricated for the growth of microalgae of 190 L capacity made up of acrylic sheet. It is a transparent plastic material of 5mm thickness. The laboratory setup was fabricated with a length of 1.2m, width 0.41 m and height 0.3 m. The pond was assembled over a steel structure to the height of 0.62 m from the ground level. A paddle wheel of acrylic sheet and fixed to the axle. The axle is supported by two bearings in the structure. The gearbox is used for regulating the speed of an electric motor, which in turn maintains the speed of the paddle wheel. The operating speed is maintained in the range of 10 to 40 rpm throughout the study. The movement is transmitted from the motor to the axle by a drive chain. The operational view of constructed lab-scale raceway pond is given in Fig. 2.
The quantity of wastewater was maintained up to 20 cm height throughout the experiment. The experimental setup was maintained under sunlight (360 x 102 l). The trial was conducted for 15 days.
Effluent treatment with algae
The screened algal isolate was treated with wastewater. The raw wastewater was fed into raceway pond solely without dilution. To 100 l of wastewater, 2% of Hindakia cultures at the optical density of 0.5 at A680 nm were inoculated.
Measurements
The change in characteristics of the effluents namely pH, EC(17), Total dissolved solids (18), Total suspended solids(18), Dissolved oxygen(19) biological oxygen demand(18), chemical oxygen demand(18), total nitrogen(20), ammoniacal (NH4-N) and nitrate nitrogen (NO3-N) (18) total phosphorus total organic carbon(21) and chlorophyll(22) was assessed using standard protocols at a periodic interval of 3 days for 15 days.
Statistical analysis
The dataset generated from our study were subjected to one-way ANOVA, in SPSS (Version 20, IBM, N, USA). Statistically significant differences between algal treatments were analyzed using Tukey HSD. The graphs were plotted using Origin, 2018.