## 2 Materials and Methods
### Relevant Factors for Chemical Oxygen Demand (COD) Removal Effect
#### 3.3.2 Correlation between Chemical Oxygen Demand (COD) Removal Efficiency and Flow Rate  
It has been found that too low a flow rate would reduce the mixture degree between the sludge and wastewater, but too high a flow might render the bacteria unable to degrade the substrates effectively [15]. In this study, the variation of COD removal efficiency and the influent flow rate with time is shown in Figure 6a. It can be seen that the COD removal efficiency increased with the increase of flow rate in most of the days. However, it is worthy to note that the COD removal rate was very low at day 31 and day 68, probably caused by the sudden rise of flow rate and toxicity of the suspended solids in the influent. The value of flow rate could directly influence the anaerobic treatment effect on dyeing wastewater. The smaller the flow rate was, the longer the HRT was, which would ensure that HABR had sufficient time to effectively degrade the pollutants in the wastewater [13]. However, in real project, the flow rate should be controlled as large as possible, which might reduce the land area of the reactor so as to diminish the construction and operation investment. Figure 5: Numbers of the organic compound type in the hybrid anaerobic baffled reactor (HABR) influent and effluent. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Influencing Factor** & **Temperature** & **Flow Rate** & \begin{tabular}{c} **Organic Loading** \\ **Rate (OLR)** \\ \end{tabular} & \
\begin{tabular}{c} **Influent Chemical** \\ **Oxygen Demand (COD)** \\ \end{tabular} \\ \hline
\begin{tabular}{c} Generalized grey \\ relational grade \\ \end{tabular} & 0.58 & 0.94 & 0.87 & 0.02 \\ \hline
**Influencing factor** & **Influent pH** & **Influent SS** & **Influent VFA** & **Influent alkalinity** \\ \hline
\begin{tabular}{c} Generalized grey \\ relational grade \\ \end{tabular} & 0.14 & 0.63 & 0.14 & 0.13 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Generalized Grey Relational Grade of Respective Factors on Chemical Oxygen Demand (COD) Removal Effectiveness. During the operational period of 73-107 days, activated microorganisms performed evident functions of hydrolyzing and acidifying dyeing wastewater, when the flow rate was 1.5 m\({}^{3}\)/h, the corresponding up-flow rate was 1.7 m/h, and the HRT was 12 h. Compared with the optimal parameters obtained by other studies employing anaerobic technologies for dyeing wastewater treatment (Table 1), this HABR realized a relatively low HRT and thus a high up-flow velocity, mainly due to the effect of the cross-flow packing on stabilizing the microbes and enhancing the contact between the sludge and wastewater [16]. 3.3 Correlation between Chemical Oxygen Demand (COD) Removal Efficiency and Organic Loading Rate (OLR)  
OLR performed strong correlation with the COD removal effect, shown in Figure 6b. In general, too low OLR is not beneficial for the proliferation of various microbial communities in the inoculated sludge; because of the stronger adaptability and fecundity of acid-producing bacteria, too high or too fluctuant OLR might lead to systemic acidification and excessive proliferation of acid-producing bacteria, which would further result in the maladjusted proportion of bacterial communities and ruinate stabilization of granule sludge [37]. During the start-up period, the COD removal efficiency increased with the increase of OLR, which indicated that the OLR was controlled in a proper range so that the activity of anaerobic microbes was continuously promoted to degrade organic matter. During 73-107 days, the variation range of OLR was 1.5-2.0 kg COD/(m\({}^{3}\)-d). Compared with other studies which achieved a similar COD removal efficiency (Table 1), the OLR range in this 18 m\({}^{3}\) reactor was  
Figure 6: Time series of chemical oxygen demand (COD) removal efficiency with (**a**) flow rate; (**b**) organic loading rate (OLR); (**c**) influent suspended solid (SS); (**d**) water temperature.