# Preliminary Study on Greywater Treatment Using Nonwoven Textile Filters
## 4 Discussion
The relatively high efficiency of mechanical treatment of greywater on nonwoven textile filters was obtained by reducing the effective pore size of the filtration layer due to high biomass concentration and accumulation of suspended solids. Aerobic decomposition of dissolved organic compounds at relatively high efficiency was possible due to the sufficient concentration of dissolved oxygen greywater flowing in the immediate vicinity of the biomass fixed on the nonwoven fabric and due to the low flow rate with a high efficiency of oxygen diffusion from the atmospheric air. The reactors also played the role of settler; however, it is difficult to calculate the real retention time because the rate of mixing during dose application is unknown. On the other hand, the low hydraulic capacity of filters does not cause any local currents in the liquid phase of the reactor. During the studies, it was observed that filters were covered by a filter cake of low cohesion, susceptible to dropping away due to the vertically oriented filtration layer. Thanks to this, these filters are less susceptible to clogging (they can operate without rinsing for at least one to two years) and the possible rinsing can be carried out in a simple way--by washing the filter surface with a water jet. In order to maintain a high removal efficiency of organic compounds and nitrification, periodic removal of sludge from the bottom of the reactor is recommended. The basic pollution indicators removal efficiencies were satisfactory regarding simple construction and maintenance, low investment cost, and low operational costs (power and electric current consumption). No statistically significant difference between four-layer filter and eight-layer filter of set I was stated (Table 1). Similar results were obtained for filters treating STE [30]. In that studies the difference was observed only according to BOD\({}_{5}\) removal, but not according to TSS and COD\({}_{\text{Cr}}\) removal efficiency. The results of statistical analysis showed that the efficiency removal and outflow concentration of \(\text{COD}_{\text{Cr}}\), \(\text{P}_{\text{tot}}\), and \(\text{BOD}_{5}\) between filter 1 and filter 2 in set I and between filter 1 and filter 5 in set II did not constitute a statistically significant difference at the 95% confidence interval (Table 1). Moreover, the results of the statistical test of a different mean between filter 1 of set I and filter 1 of set II showed that only the concentration of \(\text{P}_{\text{tot}}\) in treated greywater showed a statistically significant difference (\(p=0.04\)). It was found that the filters worked, at least in the air-side layer, in aerobic conditions, which was indirectly confirmed by periodic removal of total and ammonium nitrogen. Obtaining aerobic conditions--even to a limited extent--enables the course of aerobic treatment processes and promotes a relatively high effectiveness of organic compound removal and also prevents the purified greywater from decomposing (rotting). It has been repeatedly confirmed in the research carried out that the nonwoven fabric and filter cake on the greywater side of filters, despite their considerable thickness, never showed any symptoms of anaerobic conditions--the observed color of the filter cake on the filter surface was always grey or brown-grey (not black), while the biomass smell was earthy (not decomposing). The aerobic conditions can be explained by the direction of filtration opposite to the direction of diffusion of atmospheric oxygen to greywater and living biomass, and allowing the relatively intensive contact of greywater with air at the air side, where the filtration layer is only moistened with greywater or covered with a thin layer of greywater flowing down the vertical filter surface. Similar results and observations were noted during studies on the filters treating STE. The availability of oxygen through the air-side surface of the filtration layer was favored by low outflow rates.