# A Modified Bio-Ecological Process for Rural Wastewater Treatment
## 3 Results and Discussion
### Overall Reactor Performance  
The complete A2O-wetland unit consisted of an anaerobic baffled reactor, anoxic tank, oxic unit, and constructed wetland as a combination of biological and ecological processes, was operated as a consecutive flow model. The HRTs in the ABR, anoxic tank, and oxic unit were 3 days, 5 h, and 4.4 h, respectively. The reflux ratio in the anoxic tank was 100%, and the hydraulic loading of the wetland was 0.2-0.4 m\({}^{3}\)/day. Water spinach was selected for the constructed wetland due to its easy survival and well developed root system. Figure 6 shows the concentration and removal efficiencies of COD, TN, NH\({}_{4}\)-N, and TP in a bio-ecological system. The combined overall removal efficiency of the A2O-wetland system during the monitored period was 91% for COD, 78% for TN, 85% for NH\({}_{4}\)-N, and 92% for TP. The mean, minimum, and maximum for the concentration and removal efficiencies of COD, ammonia, TN, and TP are shown in Table 4. The influent COD concentration was between 185.3 and 321.2 mg/L\({}^{-1}\) and after treatment, the effluent COD concentration was 10.4-28.7 mg/L\({}^{-1}\). The removal efficiency for COD reached up to 90%. TN concentrations in the influent and effluent were 22.4-42.1 mg/L\({}^{-1}\) and 3.6-11.5 mg/L\({}^{-1}\), respectively, and the total nitrogen removal efficiency was 75%-85%. Whereas, the average effluent concentration of NH\({}_{4}\)-N and TP were 2.35 \(\pm\) 0.99 and 0.36 \(\pm\) 0.10 mg/L\({}^{-1}\) with removal efficiency of 85% and 92%, respectively. Figure 6: Removal efficiency of COD, TN, NH\({}_{4}\)\({}^{+}\)-N, and TP in A2O–wetland system. Influent water; \(\blacktriangle\): Effluent water; \(\blacktriangle\): COD removal efficiency. Figure 7 illustrates the removal efficiencies of the bio-ecological system for tested parameters in different stages of the A2O-wetland system. The most effective COD removal took place in the ABR under an HRT of 3 h, and it was supposed that the chief quantity of the organic matter was removed by heterotrophic bacteria [42]. The average TN removal efficiency in the anoxic tank was 40.1%, and the other major part was constructed wetland for TN removal. Nitrogen removal can occur through nitrification, denitrification, and volatilization, and nitrogen can also be taken up by plants and become part of plant biomass [43]. Nitrogen uptake by plants is an important function of CW (constructed wetland) [44]. TP removal efficiency was between 87% and 95%. The main part of TP was removed by the CW. The processes that can remove nitrogen in constructed wetlands are nitrification, denitrification, nitrogen fixation, NH\({}_{3}\) volatilization, plant and microbial uptake, nitrate reduction to ammonium (nitrate-ammonification), mineralization (ammoniumication), anaerobic ammonia oxidation (ANAMMOX), sorption, fragmentation, desorption, burial, and leaching, whereas phosphorus removal processes in constructed wetland are adsorption, desorption, dissolution, precipitation, plant and microbial uptake, leaching, fragmentation, sedimentation, and mineralization [45; 46]. The bio-ecological reactor was proposed for rural domestic wastewater treatment. In comparison with other decentralized or on-site technologies, such as intermittently aerated MBBR (TN = 47%-55%; COD = 42%-67%) [47], filter packed with recycled glass (TN = 10%-26%; COD = 72%-83%) [48]; step feed biofilm process (TN = 25%-75%; COD = 80%) [49]; hybrid bioreactor (TN = 59.1%; COD = 93%) [50]; reed bed (TN = 33%-66%) [51]; and integrated A/O biofilm reactor (TN = 66%-84%; COD = 62%-85%) [52], the organic pollutant removal efficiency of A2O-CW was higher