## 4 Discussion  
### Changes in Physicochemical Profile under Different Aeration Regimes  
The liquid fractions of animal manure contain high concentrations of nitrogen, phosphorus, organic compounds and micronutrients, such as copper and zinc [10; 27]. The high concentrations of the two later derive from feed additives also containing iron, manganese, molybdenum and selenium [27]. The chemical and biological processes occurring within the wetland (triggered or hindered by biotic and abiotic factors) influence the concentration and/or the state of those compounds. In this study, attention was given to the effect of artificial aeration adapted to IBC systems to enhance the efficiency of COD removal from the effluent of activated sludge systems treating the liquid fraction of piggery manure. During the presented experiment, great fluctuations in the physicochemical variables were observed. For instance, the decrease of pH values could have been influenced by the presence of organic matter, nitrification, precipitation and buffering reactions or the loss of carbon dioxide. One reason of the decrease of pH can be explained considering aerobic respiration and the corresponding CO\({}_{2}\) production. Nitrification is an important factor determining the pH drop. It results in the formation of carbonic acid, potentially occurring in the treatment systems A, B and C which lead to a decrease in pH. The optimal pH range for a nitrification process to occur in suspended-growth systems, such as aerated CWs or lagoons, is between 7.2-9.0 [28]. In non-wetland or conventional treatment systems, optimum nitrification has been achieved even at a pH of 6.6. By comparing the pH values with NO\({}_{3}\)-N concentrations after 2 weeks of the experiment (sixth sampling instance), it was seen that some of the lowest NO\({}_{3}\)-N values were determined when pH was approximately 6.6. The low alkalinity levels in the wastewater to resist changes in pH and neutralise acids generated by the active biomass could have also been the reason of the pH drop in treatments A, B and C. This parameter was not measured during the experiment, though it is noteworthy to consider in an optimised system for further research. In the presence of oxygen, ions are oxidized and form precipitates, which, in consequence, decrease pH. For example, the increased concentrations of iron (Fe\({}^{3+}\)) dissolved in the slurry reduce the pH if Fe\({}^{3+}\) precipitates as iron phosphate (FePO\({}_{4}\)(s)) or ferric hydroxide (Fe(OH)\({}_{3}\)) [29]. Other possible occurring precipitates are aluminium phosphate, theoretically precipitating at pH of 6.3, or iron phosphate that ideally precipitates at a pH of 5.3 [30]. Furthermore, the precipitation processes could have been facilitated by the presence of the substrate used (Argex(r)). Reported studies by [PERSON] et al. [7; 10] and [PERSON] and [PERSON] [31] concluded that high levels of phosphorous removal (up to 98%) were achieved at pilot scale CWs filled with Argex(r). The results also suggest that prolonged aeration periods provided to IBC A, B and C inhibited the denitrification process. Therefore, the NO\({}_{3}\)- concentrations were significantly reduced only in the IBC without aeration. Another reason that could have inhibited the denitrification process is the potential lack of readily degradable organic material serving as electron donors to promote the reduction of NO\({}_{3}\)- to molecular nitrogen (N\({}_{2}\)) [4]. Some of the main microbial communities related to nitrogen removal are ammonia-oxidizing, denitifying, and anammox microbial communities. Their presence, growth and reaction processes are affected by organic matter, redox condition, temperature, pH, the presence of plants and media characteristics. Most denitifying bacteria require an anaerobic environment and readily available organic compounds for energy and as a carbon source [32]. The fractions of organic nitrogen may eventually become unavailable for microbial processes if incorporated or absorbed in the substrate.