# Preliminary Study on Greywater Treatment Using Nonwoven Textile Filters
## 4 Discussion  
The value obtained for set I start-up period was comparable to results of investigations carried out on filters treating STE (3-4 months for the removal of organic compounds) [36] while the start-up period for set II was much longer. The significantly longer start-up period of set II (comparing to start-up period of set I) was probably related to lower biodegradability of greywater (COD\({}_{\text{Cr}}\)/BOD\({}_{5}\) equal to 3.2) comparing to septic tank effluent biodegradability (COD\({}_{\text{Cr}}\)/BOD\({}_{5}\) equal to 2.8) and limited concentration of nutrients in the greywater. The hydraulic capacity did not change significantly during the period over the last several weeks. The results related to this technology, mentioned in the previous papers [41] suggested the long-term hydraulic capacity stabilization at a value of about 1-2 cm d\({}^{-1}\). This phenomenon can be explained by the excess biomass gravitational removal from the vertical situated filtering textile layer. Figure 12: Total suspended solids (TSS) inflow and outflow concentrations (**a**) and removal efficiencies (**b**) for set I.  
Figure 13: Total suspended solids (TSS) inflow and outflow concentrations (**a**) and removal efficiencies (**b**) for set II. The greywater average retention times in both reactors were comparable. The current retention times were changeable for both sets of filters (reactors) due to the changeable greywater table level in the reactors, especially in the case of set II reactor (11-23 cm greywater table level). It is worth noting that these filters, investigated previously (for septic tank effluent treatment), after a much shorter time of operation were equally effective according to the flow rate. This shows that for a long-term operation the flow rate does not decrease and is about 0.9-1.0 cm d\({}^{-1}\). This is caused by the gravitational removal of excess sludge (biomass) from the vertically situated filtering textile layer. The efficiencies of COD\({}_{\text{Cr}}\) (59-72%) and BOD\({}_{5}\) (57-80%) removal from greywater by nonwoven textile filters were comparable to the efficiency of nonwoven textile filters treating septic tank effluent [30; 34]. Such values can be justified as satisfactory according to the lower bio-degradability of greywater compared to septic tank effluent. Until now there have been no limiting values in Poland for greywater treatment as a separate type of wastewater, therefore (until its establishment) the values required for domestic wastewater (mixed black and greywater) have to be taken into consideration. Most COD\({}_{\text{Cr}}\), BOD\({}_{5}\) outflowing (treated) greywater concentrations meet the Regulation of the Minister of Environment of 18 November 2014. The presented technology and construction would be a useful means of limiting pollutant discharge into the environment or before the reuse of greywater. The relatively wide range of COD values in treated greywater was related to the very wide range of COD in inflowing greywater (e.g., 98-449 mg O\({}_{2}\) dm\({}^{-3}\) in case of set I). Maximum permissible values of pollution indicators for domestic wastewater or municipal water being introduced into waters or into the soil according to this regulation are: 40 mg O\({}_{2}\) dm\({}^{-3}\) for BOD\({}_{5}\), 150 mg O\({}_{2}\) dm\({}^{-3}\) for COD, and 50 mg dm\({}^{-3}\) for TSS. Due to the changeability of treated greywater values (COD\({}_{\text{Cr}}\), BOD\({}_{5}\)) it is difficult to draw the conclusions related to filters and sets types (filter layer thickness). The effectiveness of the removal of total phosphorus on non-woven filters (53-73%) was much lower than the efficiency of the removal of organic compounds. The removal of phosphorus compounds, mainly in the form of phosphates, proceeded with very variable efficiency.