# Demonstration of Wastewater Recycling in a Slaughterhouse
## 3 Results and Discussion
### Reverse Osmosis  
After the biological process has treated the SWW to irrigation-water quality, the RO system should treat the water to process-water quality. The transmembrane flux is a key performance indicator for RO systems. Figure 12 shows the flux and the permeability of the RO during the monitoring period. The flow decreases sharply in the first 20 days and drops to 5 L-h-1.m-2. The transmembrane flux could be restored by cleaning with 1% NaOH solution and subsequent acid cleaning with citric acid solution at pH 3, but the flux drops sharply again. Chemical cleaning was carried out regularly to restore the flux. From day 50, the flux appears to be more stable. This is because, from that point onward, the MBR was optimized, resulting in improved effluent quality. However, the flux continues to decrease over time and can only be regenerated by acid cleaning. This leads to the conclusion that at the beginning, biological fouling strongly reduced the flux, which could be improved by a better MBR performance. At the end of the monitoring period, mineral scaling seems to predominate. As this can be negated by acid rinsing, it is reasonable to assume that these are acid-soluble compounds. During the monitoring period, the chemicals required to clean the RO system were 6 kg of citric acid, 2 kg of NaOH and around 600 L of washing water per month. In the work by [PERSON] et al. [8], a flux of 30 L-h-1.m-2 was achieved at a TMP of 20 bar. In a comparable study by [PERSON] et al. [40], a flux of 20 L-h-1.m-2 was achieved at a TMP of 20 bar. Compared to this work, a lower flux was achieved here. On the one hand, this can be attributed to the maximum TMP of 13 bar, which was due to the system, on the other hand, it can be assumed that scaling effects took place on the membrane surface [8; 40]. Figure 12: Flux and permeability of the reverse osmosis membrane during the testing period with “cleaning-in-place” (CIP) chemical cleanings with citric acid and NaOH recorded. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Parameter** & **Unit** & **Feed** & **Outflow** & **Removal** & **Theoretical Outflow** & **Theoretical Removal** & **Irrigation Limit** \\  & & & & [7; 17; 37] & [7; 17; 37] & [29; 39] \\ \hline COD & mg-L\({}^{-1}\) & 1000 & 80 & 92\% & 25 & 95\% & 90 \\ TOC & mg-L\({}^{-1}\) & 280 & 5 & 98\% & - & - & - \\ TN & mg-L\({}^{-1}\) & 65 & 10 & 80\% & 8 & 85\% & - \\ TP & mg-L\({}^{-1}\) & 15 & 5 & 65\% & 4 & 70\% & - \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Performance of the membrane bioreactor and comparison with the Tunisian limits for agricultural irrigation. In order to take a closer look at the possible mineral scaling, the saturation index of potential minerals was calculated using the PHREEQC software on the basis of the measured ion composition in the concentrate. Table 6 lists the saturation index of the minerals likely to be present in the solution. The saturation index indicates the degree of saturation of a mineral in water, with positive values suggesting supersaturation (and potential for precipitation) and negative values indicating undersaturation, where dissolution is more likely. The saturation index is only positive for hydroxyapatite, but the saturation index for calines, aragonites, and gypsum is already higher than \(-1\). All of these minerals are calcium composites, which are typical for the region. It is therefore reasonable to assume that the reduction in flux is due to mineral precipitation on the membrane surface. The mineral species also explain why the flux could be restored by acidic cleaning. For long-term operation of the system, the use of anti-scaling agents or prior removal of calcium should be considered. Table 7 shows the performance of the RO system at the end of the monitoring period and compares it with the limit values for drinking water. Most parameters can be reduced by more than 90%, except COD and TN, where a reduction of more than 80% was achieved.