# Effect of pH Change on the Microalgae-Based Biogas Upgrading Process
## 1 Introduction  
Today, producing biogas from different wastes is a highly recommended strategy contributing to the production of non-conventional renewable energy, so that organic matter contained in effluents and wastes can be used as substrates in anaerobic reactors. In this sense, produced biogas can be injected into the natural gas grid, used as vehicular fuel, or burned to obtain thermal/electrical energy. Depending on final use, purification of biogas can be required, so a biogas upgrading system must be coupled to a biogas reactor to assuring the high methane content and removal of carbon dioxide (CO\({}_{2}\)), hydrogen sulfide (H\({}_{2}\)S), and other gases (nitrogen (N\({}_{2}\)), ammonium (NH\({}_{3}\)), siloxane, etc.) [1]. A biological technology for upgrading biogas is microalgae culture, where due to photosynthetic activity, these microorganisms can uptake CO\({}_{2}\) from biogas as a carbonsource. A special configuration for biogas upgrade systems through microalgae culture is the two-stage system, where an absorption unit is coupled with microalgae culture. In this configuration, the contact between the biogas and microalgae takes place in the absorption unit. Thus, the operation of an open photobioreactor connected to a counterflow bubble column (absorption unit) for carbon dioxide (CO\({}_{2}\)) absorption represents a feasible alternative for biogas upgrade. The result of this two-stage system is an upgraded biogas with low CO\({}_{2}\) and oxygen (O\({}_{2}\)) levels. Figure 1 shows a schematic representation of such a process. To date, there are no operating full-scale installations for algal biogas upgrading, and researchers have focused their work on enhancing the performance of algal biogas upgrading systems in indoor/outdoor conditions at lab/pilot scale, modifying the operational parameters of the absorption unit (L/G) ratio [2], decreasing the O\({}_{2}\) concentration in biogas treated through the nitrification bacteria process in an algal photobioreactor [3], the addition of a trickling filter for bacterial oxygen removal [4], biogas diffusers [5], simultaneous CO\({}_{2}\) and H\({}_{2}\)S removal [6], biogas supply regime [7], etc. All this research indicates efforts carried out to achieve a successful implementation of biogas upgrading through microalgae at full scale. However, although it has been widely reported that the utilization of a counterflow column improves the quality of upgraded biogas, the microalgal culture is exposed to important pH changes when circulating through the column [8]. Such pH changes are higher when reducing the flow, and pH variations are likely to produce a metabolic change in the microalgae culture. In this sense, the pH is an important parameter in the operation of a photosynthetic biogas upgrading system because the pH influences the inorganic carbon equilibrium [9] and the microalgae activity [10]. When CO\({}_{2}\) is dissolved in the aqueous phase, its inorganic carbon species depends on the pH [11; 12; 13]. The inorganic carbon dissociation causes the release of H\({}^{+}\), and as a result, pH decreases. The pH reduction affects the microalgae activity because most microalgae culture grows at a pH range of 7-9, with an optimal pH between 8.2 and 8.7 [14]. Most of the carbon is in bicarbonate form in a pH range of 7-9. Although CO\({}_{2}\) is the substrate of the Rubisco enzyme, microalgae cells can use bicarbonate as a carbon source [15]. Bicarbonate can be transformed into dissolved CO\({}_{2}\) by the enzyme carbonic anhydrase (CA) [16; 17; 18]. Although CO\({}_{2}\) dissolution causes a pH decrease, the activity of CA causes pH increases outside the cell due to the transport of hydroxide ions outside the cell in association with the capture of H\({}^{+}\) ions for the interior of the thylakoid membranes [19]. In this sense, research for biogas upgrading through microalgae culture had shown a pH drop in the absorption unit, where the pH has decreased up to 2 points as a result of CO\({}_{2}\) dissolution in the liquid phase [3; 20].