# Removal of Volatile Solids from Greywater Using Sand Filters
## 4 Discussion  
The efficiency of VS removal was comparable to that obtained by [PERSON] et al. [17] for a down-flow sand filter--82%. The VS removal efficiency obtained in this study was in turn much higher than results obtained by other authors: [PERSON] et al. [44] for a study carried out using filter mesh of 0.130 mm diameter (28%), by [PERSON] [45] for sand filter (10%) and obtained by [PERSON] et al. [46] (15-20%--for TSS). Removal efficiency of organic compounds in this study (26.8%) was similar to that obtained by [PERSON] [45] (17%) and [PERSON] et al. [46] (20-25%), and also comparable to results obtained by [PERSON] et al. [44] in a study carried out using a filter mesh of 0.130 mm diameter (47%). COD removal efficiency was much lower than the removal efficiency obtained by other authors: [PERSON] et al. [18]--for conventional buried sand filters (92%), and [PERSON] et al. [17]--for a down-flow gravel filter (74%). Higher removal efficiency than this study was obtained by [PERSON] [47] for commercial sand used in the filtration of swimming pools--55%. In that research work, it was found that up-flow sand filters were more efficient in removing organic compounds (COD, BOD\({}_{5}\)) and total suspended solids (TSS). Mean inflow turbidity was 45.7 \(\pm\) 1.4 FAU and mean outflow turbidity was 14.0 \(\pm\) 0.23 FAU. A similar value of turbidity was detected by [PERSON] et al. [43] in greywater originating from lavatory sinks at a university campus (35.8 \(\pm\) 45.1 NTU). Turbidity removal efficiency (69.4% on average) was relatively high (compared to COD and VS removal). Figure 3: Result of analytical simulations for (a) change of specific deposition with depth, (b) change of concentration with depth. The mean EC in the inflowing greywater was 1025 \(\pm\) 5 and 1049 \(\pm\) 1.7 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 and 8.8 \(\pm\) 0.02 in the outflowing greywater. Both inflow and outflow values were higher than those commonly detected in greywater. The possible reason for this could be the homogenisation of the ingredients used to make the solution. A slight decrease in EC during filtration was observed. The dismantling of the filter column after 21 days (cumulative load of 3,279 cm) was comparable in regard to the cumulative load of 3,904 cm (COD--357.4 \(\pm\) 72.0, SS: 99.9 \(\pm\) 14.7) for dismantling of filters in the [PERSON] et al. [19] study. These authors demolished the columns with the aim of loss on ignition (LOI) determination after 60 days (the hydraulic load of 16.0 cm d\({}^{-1}\)). The vertical distribution of organic matter was typical for down-flow slow filters supplied with wastewater. VS content in the sand filter was comparable to values reported in the literature. Similar results were obtained by [PERSON] et al. [19]--the most notable difference in weight loss on ignition was observed in the top 0-2.0 cm depth (0.77%), much smaller at 5.0 cm--about 0.5% of weight loss on ignition; and between 15 and 50 cm of depth--about 0.3% of weight loss on ignition. It is worth noticing that the density of greywater could impact the amount of retained volatile solids (it also influences adsorption and mobility coefficients of constituents). However, under typical domestic (household) conditions, significant differences in density are not observed (temperature and concentrations of constituents vary to a small extent). The maximum specific deposit (8.29 \(\pm\) 0.66 mg cm\({}^{-3}\)) was achieved at 36,365 cm hydraulic load, what was much higher than that used by [PERSON] et al. [19] for eight months--a period during which clogging was not observed. At the very high hydraulic load used in this study, the maximum deposit of organic matter in sand was achieved at 36,365 cm, which corresponds to 238 days of operation.