Low-cost optical sensor to automatically monitor and control biomass concentration in microalgal cultivation

Low productivity of microalgal cultures leads to a high cost of the fuel feedstock. Turbidostat operation, which automatically monitors and controls biomass density, is a mean to manage biomass density and internal light intensity, so that biomass productivity can be maximized. Available versions of turbidostat control are expensive and not amenable to large-scale operation. We designed a system that costs less than $250 and that can be used for any type of microbiological system. It includes an in-line, infrared turbidity sensor connected to an Arduino ATmega microcontroller and auxiliary power replays. The target biomass density is adjustable, and key operating data – such as time stamps, pump status, and set and measured values of biomass density – are available in real time and logged continuously. The sensor's output was linear for OD₇₃₀ from 0.5 to 4.5, which brackets the realistic ranges for microalgae culturing. We tested the turbidostat with step-down and step-up experiments with Synechocystis cultures. The turbidostat maintained stable biomass concentrations for all steps. The results of the turbidostat experiments demonstrated how turbidity control leads to systematic management of average internal light intensity, specific growth rate, and biomass production rate. This open-design, low-cost system should promote higher productivity and help make microalgae biomass an affordable fuel feedstock.