# Removal of Volatile Solids from Greywater Using Sand Filters
## 3 Results  
### Volatile Solids, Turbidity and Organic Compounds' Removal Efficiency  
The removal efficiency of volatile solids (VS) quickly increased (on the third day of the experiment--equivalent to about 20 days at a hydraulic load equal to 16-17 cm d\({}^{-1}\)) to 62% and stabilized between 51% and 60%. From a practical point of view, it is significant that relatively high removal efficiency of VS was achieved in a short time; this information can be useful for mechanical greywater treatment or pre-treatment. VS removal efficiency is shown in Table 3.  
Mean inflow COD was 199.6 \(\pm\) 4.8 mg O\({}_{2}\) dm\({}^{-3}\) (n = 15). A similar value (145.8 \(\pm\) 79.1 mg dm\({}^{-3}\)) was reported by [PERSON] et al. [43] in greywater from laboratory sinks at a university campus.  
After 21 days, COD removal efficiency reached 27%, an increase from 14% recorded at the beginning of the experiment (first day). This can be accepted as relatively efficient, taking into account the fact that the mechanical treatment occurred mainly during the study.  
Mean inflow turbidity was 45.7 \(\pm\) 1.4 FAU (n = 9) and mean outflow turbidity was 14.0 \(\pm\) 0.23 FAU (n = 9). The turbidity removal efficiency (69.4% on average) was relatively high (compared to COD and TS removal).  
The mean value of EC in raw greywater was 1025 \(\pm\) 5 (n = 12) and after filtration, the value of EC increased to 1049 \(\pm\) 1.7 (n = 69) in the outflowing greywater. A slight increase in EC during filtration through the sand was observed.  
The mean pH in the inflowing greywater was 9.1 \(\pm\) 0.06 (n = 12) and in the outflowing greywater was 8.8 \(\pm\) 0.02 (n = 69).  
In the case of both EC and pH, the changes caused by filtration were very small (2% and 3%, respectively) and can be neglected due to their statistical insignificance.  
Organic matter deposit changed significantly with sand filter depth (Figure 2). As expected, the largest deposit was observed in the top layer of 4.0 cm depth: 3.2 mg of dry organic matter (d.o.m.) per cm\({}^{-3}\) on average (three columns: No. 2, No. 3 and No. 7) of sand after six days of the experiment. The average daily flows of these columns were 182.6 cm d\({}^{-1}\), 181.9 cm d\({}^{-1}\) and 188.8 cm d\({}^{-1}\), respectively, and the cumulative hydraulic loads of these filters were 1095.9 cm d\({}^{-1}\), 1091.7 cm d\({}^{-1}\) and 1084.9 cm, respectively. After 14 days of the experiment, the detected deposition was 4.2 mg of d.o.m. per cm\({}^{-3}\) on average (three columns: No. 1, No. 4 and No. 8) in the top layer of 5.0 cm depth of sand. These filters were loaded with greywater--144.4 cm d\({}^{-1}\), 151.0 cm d\({}^{-1}\) and 142.8 cm d\({}^{-1}\), respectively at cumulative hydraulic loads of 2021.1 cm, 2113.6 cm and 1999.6 cm, respectively. The last part of the filter columns was dismantled after 21 days of operation and 7.3 mg d.o.m. cm\({}^{-3}\) was detected on average (three columns: No. 5, No. 6 and No. 9) in the top layer of 7.5 cm depth of sand for the experiment running at hydraulic loads of 152.3 cm d\({}^{-1}\), 152.3 cm d\({}^{-1}\) and 163.5 cm d\({}^{-1}\), respectively.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Filter Operation Time (Day)** & **Mean Removal Efficiency (\%)** \\ \hline
6 & 60 \\
14 & 57 \\
21 & 51 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Volatile solids (VS) removal efficiency.  
Maximum specific deposit (8.29 \(\pm\) 0.66 kg m\({}^{-3}\)) was observed at the cumulative hydraulic load of 36,365 cm.