# Demonstration of Wastewater Recycling in a Slaughterhouse
## 3 Results and Discussion
### Membrane bioreactor  
A MBR with a cross-flow membrane was used as the second cleaning stage. The observation period begins with the installation of the new blower and the changeover to 45 min of aeration followed by 15 min of no aeration. Figure 8 shows the oxygen concentration in the aeration tank after the accumulation of sufficient active biomass with the aforementioned aeration cycle. Particularly noticeable is the maximum oxygen concentration at the end of the aeration cycle of 2.5 mg\(\cdot\)L\({}^{-1}\) and the rapid oxygen degradation in the non-aerated phase. Based on the non-aerated phases, the oxygen consumption in the tank \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Unit** & **Feed** & **Outflow** & **Removal** & **Theoretical Removal [17,31]** \\ \hline COD & mg\(\cdot\)L\({}^{-1}\) & 1800 & 1000 & 45\% & 70\% \\ TSS & mg\(\cdot\)L\({}^{-1}\) & 600 & 120 & 75\% & 80\% \\ TN & mg\(\cdot\)L\({}^{-1}\) & 130 & 65 & 50\% & 55\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Performance of the physical flotation unit. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Unit** & **SWW Analyzed** & **BREF [18]** & **[PERSON] et al. [17]** \\ \hline TOC & mg\(\cdot\)L\({}^{-1}\) & 500–900 & & \\ TN & mg\(\cdot\)L\({}^{-1}\) & 50–150 & 200–475 & 150–350 \\ COD & mg\(\cdot\)L\({}^{-1}\) & 1500–3000 & 2800–4200 & 2000–4000 \\ BOD\({}_{\text{5}}\) & mg\(\cdot\)L\({}^{-1}\) & & 500–2500 & 1000–2500 \\ TP & mg\(\cdot\)L\({}^{-1}\) & 5–15 & & 5–30 \\ pH & & 6.7–7.2 & 7.0–8.0 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Slaughterhouse wastewater characteristics. can be estimated to be approximately 8 mgO\({}_{2}\cdot\)L\({}^{-1}\)h\({}^{-1}\), which is in the normal to high range for activated sludge tanks [32]. Figure 9 shows the progression of SV and TSS over the observed period. During intermittent aeration, neither the SV nor the TSS could be sufficiently increased. For this reason, the aeration tank was aerated continuously from day 12 to day 27. The oxygen concentration was kept above 2 mg\(\cdot\)L\({}^{-1}\) throughout the entire period, including at night and at weekends. After an increase in TSS was achieved, intermittent aeration was carried out again from day 27 to enable biological removal of nitrogen. From day 30 onwards, the SV increases  
Figure 8: Oxygen concentration in the aeration tank with 45 min aeration followed by 15 min of no aeration. Figure 9: TSS and the sludge volume over the testing period in the aeration tank. until it reaches a plateau at 250 mL\(\cdot\)L\({}^{-1}\). Compared to other aeration tanks, the SV is still relatively low [32]. The TSS remains below 100 mg\(\cdot\)L\({}^{-1}\) until day 10, then rises to 600 mg\(\cdot\)L\({}^{-1}\) with the switch to continuous aeration. In the second phase of intermittent aeration, it continues to increase until reaching a plateau at 1000 mg\(\cdot\)L\({}^{-1}\). This may be attributed to a temporary overflow of the aeration tank during periods of low transmembrane flux of the UF. However, since the COD load was already sufficiently reduced at these biomass concentrations and the majority of the ammonium could be oxidized, the sludge concentration did not increase further. Figure 10 shows the course of the ammonium and nitrate concentrations over the same period. At the beginning, most of the nitrogen is present in the form of ammonium, which indicates that there are not enough nitriifying bacteria in the sludge to oxidize the nitrogen. This changes with continued aeration. The ammonium concentration decreases and the nitrate concentration increases. Most of the nitrogen in the tank at the end of this period is in the form of nitrate. This suggests that the nitriying bacteria could be enriched in the tank. Therefore, intermittent aeration was subsequently carried out to achieve biological nitrogen removal. At the end of the monitoring period, the total nitrogen content in the tank was greatly reduced, which can be attributed to successful biological nitrogen removal.