###### Abstract
Since both nitrifiers and microalgae are CO\({}_{2}\) dependent, the CO\({}_{2}\) mitigation ability of the photo-BNR process is potentially higher than that of other BNR processes. An anoxic dark period is added after the light period, to allow denitrification to occur. During the dark anoxic period, when no external carbon source is added, denitfifying PAOs are expected to perform denitrification by using the anaerobically stored polyhydroxyalkanoates (PHAs). The main mechanisms of nutrient removal observed in the photo-BNR were ammonia assimilation by the microbial biomass, phosphorus accumulation as poly-P by PAOs, and nitrate removal by denitrification. Due to the interaction between microalgae and bacteria, cells can aggregate as flocs more easily and settle very fast, resulting in a solids-free effluent and thus, solving one of the main problems of using microalgae for wastewater treatment. The goal of the photo-BNR process is to remove BNR aeration requirements and mitigate CO\({}_{2}\) without the need for costly external COD dosing, thus reducing the operational costs and ecological footprint of the WWTP [14]. Real-time bioprocess monitoring is of crucial importance for efficient operation and effective bioprocess control [15]. In contrast with offline, retrospective and time-consuming reference analytical methods, which do not provide a real-time knowledge of process performance, the use of fast, non-destructive, robust and sensitive online spectroscopy probes, in combination with chemometrics, have great potential for the real-time monitoring of key bioprocess parameters, significantly reducing the time required for bioprocess control and optimization [16]. Raman spectroscopy can provide a wide range of information, from molecular structure to chemical environment, being among the most interesting spectroscopic-based techniques reported for the online monitoring of microbiological processes [16]. In fact, in addition to representing a rapid, eco-friendly and economic alternative to reference analytical methods (e.g., chromatography), Raman spectroscopy is particularly suitable for the in situ quantitative monitoring of multiple component bioprocesses, owing to the incorporation of fiber optic-based probes, as well as due to its insensitivity to water [17]. Nevertheless, applications of online bioprocess monitoring and control using Raman spectroscopy coupled with chemometrics are still scarce. Examples in clude nitrate and nitrite monitoring in a wastewater treatment bioreactor [18], the real-time prediction of glucose concentration during microalgae cultivation in a photo- bioreactor [19] and during mammalian cell cultivations [20], as well as the monitoring substrates and products during bacterial and yeast fermentation processes [15; 16], especially for pharmaceutical industrial application [21; 22]. In this context, the application of Raman spectroscopy for the online monitoring of a photo-BNR is of upmost interest, facilitating optimization, fault detection and decision-making during the wastewater treatment process. Specifically, real-time knowledge on key-parameters such as NH\({}_{3}\), nitrate (NO\({}_{3}\)), phosphate (PO\({}_{4}\)) and total organic content (TOC), VFA and CO\({}_{2}\) can be crucial for optimizing nutrient and carbon removal, namely by controlling the CO\({}_{2}\) dosing and the time length of the anaerobic (dark), aerobic (light) and anoxic periods. In addition to PO\({}_{4}\), polyphosphate (poly-P), or total P (which allow a more direct monitoring and control of the P removal performance), PHAs and carbohydrates are key functionally relevant intracellular polymers also involved in the EBPR process, as their real-time quantification significantly contributes to understanding the dynamics and optimizing the nutrient removal process.