Simultaneous microalga biomass production and wastewater treatment in various pond geometries

Open raceway ponds are most common commercial microalgae cultivation systems currently used for production of algal biomass. The produced algal biomass is used as feedstock for production of human nutritional, animal feed and biofuel energy among various potential applications such as wastewater treatment and CO2 mitigation options. The treated wastewater can then be used for irrigation particularly in arid areas such as in the Middle Eastern countries. New algae technologies are considered an integrated approach for CO2 mitigation, wastewater treatment and biomass production. However, production of biomass by microalgal cultivation system requires improvement in overall productivity and also to be cost-competitive with petroleum fuel-derived energies. Various groups of variables such as pond geometry, fluid hydrodynamics, environmental and geographical conditions, microalga specifications and influent wastewater quality affect the efficiency of the cultivation system. In the present study, the geometry of a high rate algal pond (HRAP) system was considered for optimization as an effective variable, both in biomass productivity and wastewater treatment efficiency. The microalga Chlorella volgaris was used in cultivations. The biomass productivity of 12 different pond configurations in 3 sets of L/W = 1, 2, 3 and 4 with 1, 3 and 5 channels were compared. The cultures were run in a batch mode with raw wastewater to study the effects of shapes and dimensions with constant area, water depth, and mixing systems. The lighting, pH and temperature were monitored but not controlled during experiments. The average liquid velocities in the ponds were 0.2 m s-1. Results showed that the length-to-width ratio of 4:1 and 5 channels is the most effective measure to improve overall biomass productivity and biological oxygen demand (BOD) removal. In addition, it showed that productivity increased with increasing the number of channels in the ponds. The highest BOD removal of 95.6% and cell mass of 1.0 g L-1 were obtained in the pond with an L/W ratio of 4 and 5 channels. Results also show that higher L/W ratio is more efficient to improve cell productivity and effluent water quality. The current results obtained can be used for calibration of numerical models for large scale open raceway pond designs and optimization.