# A Modified Bio-Ecological Process for Rural Wastewater Treatment
## 3 Results and Discussion  
### Performance Evaluation of ABR  
Figure 2 shows HRT and COD removal relationship in the ABR. COD in the influent was between 180 and 330 mg/L\({}^{-1}\), and the variation of the COD concentration in the influent was impacted by the summer rain. In general, the COD removal efficiency increased with increasing HRT. The average COD removal efficiencies were 60%, 65%, 72%, and 74% for HRTs of 24 h, 48 h, 72 h, and 96 h, respectively. \begin{table}
\begin{tabular}{c c c} \hline \hline
**No.** & **Parameters** & **Test Methods** \\ \hline
**1** & COD & Closed reflux \\
**2** & TN & Potassium persulfate spectrophotometry \\
**3** & Ammonium & Salicylic acid spectrophotometry \\
**4** & TP & Molybdenum antimony anti points spectrophotometry \\
**5** & TSS & Gravimetric analysis \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Test methods used for analysis [26]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Phase** & **Time (Days)** & **Temperature (\({}^{\circ}\)C)** & **HRT (Hours)** & \
\begin{tabular}{c} **Volume Load** \\ **(kg COD/m\({}^{3}\)-day)** \\ \end{tabular} &
\begin{tabular}{c} **COD Removal** \\ **Rate \%** \\ \end{tabular} \\ \hline
**I** & 20 & 16–23 & 72 & 0.67–1.06 & 51 \\
**II** & 15 & 18–28 & 48 & 0.92–1.61 & 60 \\
**III** & 15 & 20–31 & 24 & 1.76–3.24 & 61 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Operational conditions during ABR start-up. HRT is one of the key factors affecting the operating results of the anaerobic reactor [32]. According to [33], higher HRT increases the wastewater contact time in the reactor and improves the removal efficiency of organic matter by promoting the growth of anaerobic microorganisms. However, longer HRT means a small flow rate and insufficiently stirred anaerobic sludge, resulting in an inability to maintain removal efficiency. Longer HRTs also require large areas and volumes of the reactor, which is not economically suitable. Selecting an accurate HRT for an anaerobic reactor is very important. The increase of the HRT from 24 h to 72 h increased the COD removal efficiency from 60% to 72%, however, when HRT increased from 72 h to 96 h no significant changes in the COD removal efficiency occurred. Excessively long HRTs reduce the up-flow velocity in each reactor compartment and the nutrition distribution for microbes; therefore, no significant changes occurred in the COD removal rate [34]. Extending HRT increased the reactor volume and construction costs [35]. An HRT of 72 h was optimal for the COD removal. Figure 3 shows the COD removal rate in different compartments of the ABR at different HRTs. The first compartment showed a higher removal efficiency compared to all of the other compartments, followed by the second, third, fourth, and fifth compartments. As the HRT increases, the removal rate also increases. This result was possibly caused by the elevated substrate concentration, which also increased substrate flux into the bioaggregates, thereby increasing microbial growth [36]. The removal rates in compartments I, II, III, IV, and V at HRTs of 24 h to 96 h were 28%-42%, 12%-15%, 8%-11%, 5%-7%, and 2%-4%, respectively. Compartments IV and V showed low removal rates because most of the COD had been removed in the first three compartments and anaerobic microbes have low nutrients to consume [35]. Figure 2: COD concentration and removal efficiency of ABR under different HRTs. Influent water; \(\blacktriangle\) Effluent water; \(\blacktriangledown\) COD removal efficiency. Organic loading rate (OLR) is a chief factor that indicates the amount of volatile solids to be fed into the reactor every day [37]. Figure 4 shows the relationship of OLR and COD removal efficiency. OLR fluctuates between 0.58 and 2.87 kg COD/m\({}^{3}\cdot\)day. At lower OLR, the removal efficiency was high. When the OLR was between 0.5 and 1.0 kg COD/m\({}^{3}\cdot\)day, the COD removal rate was above 70%. At the middle point between 1.5 and 2.0 kg COD/m\({}^{3}\cdot\)day, the removal efficiency dropped.