Modelling and pH Control in Raceway and Thin-Layer Photobioreactors for Wastewater Treatment

María José Rodríguez-Torres,José Luis Guzmán,Francisco Gabriel Acién,Ainoa Morillas-España

doi:10.3390/en14041099

Abstract

One of the most critical variables in microalgae-related processes is the pH; it directly determines the overall performance of the production system especially when coupling with wastewater treatment. In microalgae-related wastewater treatment processes, the adequacy of pH has a large impact on the microalgae/bacteria consortium already developing on these systems. For cost-saving reasons, the pH is usually controlled by classical On/Off control algorithms during the daytime period, typically with the dynamics of the system and disturbances not being considered in the design of the control system. This paper presents the modelling and pH control in open photobioreactors, both raceway and thin-layer, using advanced controllers. In both types of photobioreactors, a classic control was implemented and compared with a Proportional–Integral (PI) control, also the operation during only the daylight period and complete daily time was evaluated. Thus, three major variables already studied include (i) the type of reactors (thin-layers and raceways), (ii) the type of control algorithm (On/Off and PI), and (iii) the control period (during the daytime and throughout the daytime and nighttime). Results show that the pH was adequately controlled in both photobioreactors, although each type requires different control algorithms, the pH control being largely improved when using PI controllers, with the controllers allowing us to reduce the total costs of the process with the reduction of CO2 injections. Moreover, the control during the complete daily cycle (including night) not only not increases the amount of CO2 to be injected, otherwise reducing it, but also improves the overall performance of the production process. Optimal pH control systems here developed are highly useful to develop robust large-scale microalgae-related wastewater treatment processes.

Highlights

Nowadays, three of the great problems that concern society focus on the search for new systems of production based on renewable energies, the reduction of greenhouse gases, and the availability of clean water
The different tests present results for the On/Off controller and PI controller, both during the daytime period and all day, for comparison purposes. Both thin-layer and raceways are operated in one reactor with the classic On/Off control and in another one with the PI control, operated only during the daytime period
The modelling and control of pH by CO2 injections in thin-layer and raceway photobioreactors utilized for producing microalgae using wastewater as a culture medium are analyzed

Summary

Introduction

Three of the great problems that concern society focus on the search for new systems of production based on renewable energies, the reduction of greenhouse gases, and the availability of clean water. All of these aspects are directly related to existing pollution worldwide. Conventional purification systems have an economic cost and an environmental cost mainly due to the pollution derived from the form of generation of the energy required in this activity. In the last two decades, low-cost and environmentally friendly alternatives to convection-activated sludge processes have emerged [1,2,3,4]

Methods

Results

Conclusion