## 1 Introduction  
In Northwest Europe (NWE), several projects regarding waste management and bio-based resource recovery processes have been interlinked and developed based on circular economy principles, in opposition to the traditional linear economy. The aims have been to minimise natural resource needs, the emissions of waste, energy leakage, and promote their practice elsewhere. Flanders in Belgium, being part of the NWE area, is classified as a nitrate vulnerable zone (NVZ), where nutrients resulting from the excess of animal manure should be reduced. The surplus of farm wastes has largely affected the soil, ground and surface water of coastlines and watercourses [1]. Nitrate vulnerable zones are restricted on the use of fertilisers and are obliged to reduce nitrate concentrations in water below the 50 mgNO\({}_{3}\)-/L. Since the 2000s, manure management activities involve the capture, storage, spread on the field, treatment, and its re-use as a source (e.g., as biomass for energy production, growth medium for algal cultivation). Land spreading can be a viable option where there are large areas of available farmland. Alternatively, pig slurry should be treated either for further use (e.g., fertiliser) or proper disposal. One of the available manure management practices consists of a three-stage process. In the primary treatment, pig slurry is separated into a thin/liquid fraction and a thick/solid fraction [2; 3]. In secondary treatment, the solid fraction is converted into an exportable product, such as soil enhancers and fertilisers [3]. The liquid fraction can be treated by biological reactors where nitrogen and phosphorus concentrations are depleted, hence, the separation of pig manure into liquid and solid fractions as the primary treatment. The activated sludge treatment and anaerobic digestion (AD) as the secondary treatment are considered the most common and among the best options available amongst manure management techniques [2; 4]. Given the high concentrations of nutrients in soil and strict implemented regulations, additional treatments are needed for both fractions of manure [5; 6]. One of them is the use of constructed wetlands (CWs) as the tertiary treatment of the liquid fraction of piggery manure. The liquid fraction coming from activated sludge reactors and the liquid digestate from the AD process, can be treated by CWs to reduce the concentrations of remaining nutrients and other pollutants [4; 7]. In Flanders, CWs treating wastewater of animal manure can discharge effluents with a maximum of 15 mg/L of total nitrogen (TN), 125 mgO\({}_{2}\)/L of chemical oxygen demand (COD), and pH between 6.5 to 8.5 units [8]. There are no specified values regarding the microbial parameters. However, to meet the COD discharge limit, CWs sometimes must be over-dimensioned depending on the mass loadings. Consequently, in regions with limited land availability (e.g., Flanders) the implementation of CWs becomes restricted. As described by [PERSON] et al. [6], regardless of the size a CW, its discharge must meet the standards limits set by the EU Water Framework Directive. These limits were defined on basis of CWs larger than 50 ha and large waterbodies, but not to actual conditions. Therefore, there is a need to investigate the effect of emerging technologies that are adaptable to the treatment process and allow higher COD removal efficiencies to avoid the over dimensioning of CWs, and help meet the standards. In this regard, this study evaluates the effect of implementing artificial aeration in CWs to reduce COD concentrations. It is hypothesised that aerated systems favour the depletion of slowly biodegradable or non-biodegradable organic compounds and facilitate meeting discharge limits. However, considering that constant or intermittent aeration could also favour ammonium (NH4\({}^{+}\)) volatilisation and would therefore hinder the denitrification process, the evolution of total nitrogen (TN) and nitrate (NO3\({}^{-}\)) concentrations are also studied.