## 4 Discussion
### Moving Towards Sustainable Concepts  
The efficient decrease of the COD concentrations through the aerated wetlands could facilitate meeting the COD discharge limit of 125 mgO\({}_{2}\)/L imposed to CWs treating animal manure. [PERSON] et al. [39] assessing the sustainability of CWs for wastewater treatment, concluded that several tests have resulted in advanced designs, such as hybrid and/or enhanced CWs. In addition, optimal operation settings regarding the hydraulic loading rate, residence time, plant and substrate selection, number of treatment compartments, etc., have been delineated and reviewed. All of them resulted in a great increase of removal efficiencies and the sustainable application of CWs. Nonetheless, they highlighted that among the remaining gaps to meet fully optimized treatment techniques, aerated CWs should be considered for further exploration and development. In addition, more than a post-treatment of the liquid fraction of animal manure, aerated CWs can also be optimal for the treatment of high-strength digestate, and other types of agro-industrial wastewater. In addition, the experience and literature, which focused mostly on the treatment of urban wastewater, have shown that aerated wetlands are cheaper than non-aerated wetlands. In most practical cases, relatively low aeration periods and hydraulic retention time (HRT) of two days are general treatment considerations applied for the purification urban wastewater through CWs. Consequently, higher efficiencies and lower sludge production can be reached as compared to non-aerated systems. However, the amount of easily biodegradable organic matter in urban wastewater is higher than in the liquid fraction of pig manure. Thus, to degrade persistent organic matter (recalcitrant COD). longer retention times are required in aerated wetlands treating the liquid fraction of piggery manure. Considering the overall manure treatment process, first, the manure is separated in solid and liquid fractions. This latter is then treated in an activated sludge system which is mostly an intensive system. Here, high aeration rates and short retention time are needed to reduce a great amount of easily biodegradable COD. In the polishing step, pig manure treatment wetlands become particularly extensive systems. The effluent of the activated sludge system containing high concentrations of non-easily biodegradable COD needs to reach concentrations below 125 mgO\({}_{2}\)/L. To meet this criteria, longer HRT and larger treatment areas are needed in such systems. Nonetheless, to tackle land availability requirements and constraints to meet discharge limits, as discussed by [PERSON] et al. [6], aerated wetlands have shown to be an effective solution. Almost half of land required could treat the same amount of wastewater at higher removal efficiencies. Wastewater contains many contaminants that are in fact mixtures. Generally, water quality parameters are measured by procedures that combine individual chemical compounds into an overall or total concentration for that class of materials. Some examples of these parameters are COD, BOD or TSS. When water passes through the wetland, its composition can change as the different fractions of the mixtures can reduce at different rates. As described by [PERSON] and [PERSON] [30], the mixture then becomes weathered. Therefore, the lower persistent COD concentrations that are aimed to be reached, the more difficult it gets to decrease their levels. However, if meeting the discharge limits is the key objective of the treatment, then one should consider that the additional costs of blowers and aeration piping would not exceed the costs of a large treatment area requirement. The use of aerated wetlands results in the reduction of the treatment area which implies that less liner, less substrate and less excavation works are needed. As a result, the overall price of CWs' installation decreases. Compared to vertical flow wetlands, aerated wetlands are 30-50% cheaper for the same capacity.