Recovery of proteins from piggery wastewater using Scenedesmus spp. photobioreactors

Abstract

Algal-bacteria process is a low-cost and environmentally friendly alternative process for wastewater treatment. Currently, Algae biomass as biofuels and high value-added compounds have attention to be produced with wastewater treatments. The macromolecular composition of this biomass grown in wastewater treatment photobioreactors is a key information in order to select alternatives of valorization. The presence of bacteria and the wastewater components such as high concentration of carbon, nitrogen and heavy metals or the environmental conditions influence remarkably the algal biomass compositions. In this study, we set up a photobioreactors system and studied the evolution of algae-biomass, and the efficiency of wastewater treatment. We aimed to clarify the fluctuations of biomass based on green algae Scenedesmus spp. grown in non-sterilized piggery wastewater (PWW) and in synthetic water. Results from small 3L laboratory scale indoor photobioreactors were compared with 360L outdoor pilot plant thin layer (TL) photobioreactors and closed tubular (TB) reactor located in Almeria using also Scenedesmus species. Four laboratory experiments, 3L small open photobioreactors (15.8 cm depth, 15.5 cm internal diameter) were designed and performed with high repeatability. They were placed inside a water bath under artificial lights, 1053±32 μmol m-2 sec-1, 12h light: 12h dark. Scenedesmus sp. was used as inoculum and two reactors were fed with Modified Bristol medium (MBM) and other two with PWW. Feeding rate was set 500 mL d-1 (Hydraulic retention time; HRT 6 day), and pH 8 was maintained in the photobioreactors by supplying CO2. The removal rate of COD and TN were 83.6 % and 74.5%, respectively, in reactors treating PWW. The dissolved oxygen (DO) concentration in PWW resulted 0 mg L-1 while DO 14.8-17.1 mg L-1 were determined in MBM condition. This process did not require oxygen supply to remove pollutants by bacteria. It is thought that low-cost operation was possible due to the presence of bacteria for algae based wastewater treatment. The results showed high protein productivity on algae-bacteria biomass grown in PWW, related with high growth rate of microalgae. TSS during steady state achieved values of around 0.5 L-1 with synthetic medium and 1.8 g L-1 with PWW, respectively. Biomass compositions analysis showed similar values between MBM and PWW in laboratory reactors. In PWW feed photobioreactors, biomass with 54.6 % of proteins and 14.7 % of lipids was obtained. In MBM feed photobioreactors, biomass with 51.6 % of proteins and 14.2 % of lipids was obtained. Combining biomass growth and composition, protein productivities of 0.97 g L-1 were obtained from non-sterilized PWW and 0.27 g L-1 from MBM. However, biomass from TL and BL photobioreactor achieved 4 high carbohydrate content 43.5%, 43.0% of proteins contents, 9.6% of lipids contents, probably because the different light precedence and intensity. The pigments contents in PWW were lower than medium substrate (MBM, BM) in both of reactors. Chl-a, Chl-b, Carotenoids were 9.4%, 3.2%, 2.6% in MBM and 1.5%, 0.8%, 0.3% in PWW in laboratory reactors. The presence of heavy metals in PWW probably worked as the oxidative stress. Oxidative stress can degrade protein and lipids such as pigments. But these results were required further research. The extracted protein contents by enzymatic hydrolysis were maximum 12% with Protamax, 90 min of retention time in PWW biomass from TL photobioreactor. Lipids contents showed similar values before the extraction. Therefore, the all of extraction methods were not enough for the algae samples in PWW.