3.1. Decrease in filters' discharge
Figure 2 shows the decrease in the discharge of each of the filters over the 1 h operation time. The reduction of the filter discharge depended on filter head loss and the changes in the inlet filter working pressure. The maximum discharge reduction at an equal working pressure was observed with the disc filter, which had the highest head loss (233 and 190 kPa) after 1 h of operation. Also, for each filter, the maximum discharge reduction was related to the lower working pressure (150 kPa) due to faster formation of filtration cake. The automatic screen filter worked differently from the other filters. The discharge through this filter was constant during operation time since filtration operation was not interrupted during backwashing. However, during backwashing, its discharge was reduced to the equivalent of backwash discharge. Capra and Scicolone (2007) also reported a greater reduction in the discharge of disc filters than screen filters when a municipal effluent was used.
3.2. Head loss of the filters according to the filtered volume
The initial head loss both disc and conventional screen filters at both working pressures was 40 kPa, while for the automatic screen filter was 5 kPa. The manufacturing companies have reported an initial head loss of 2.65, 3.24, and 1.47 kPa, respectively, with freshwater (Table 2). Thus, when trout fish farm effluent was used, the initial head loss was 15.1, 12.3, and 3.4 times, respectively, than that observed with freshwater. However, according to Bucks et al. (1979) since effluent TSS was below 50 mg l-1, it posed a minor physical risk of emitter clogging. Suspended solids in the effluent of this fish farm are mainly organic (Manbari et al., 2020), and their density is usually between 1.03 to 1.19 g cm-3 (Tchobanoglous and Schroeder 1985; Chen et al. 1993; Anon 1995; Patterson et al. 2003).
Figure 3 shows the filtered effluent volume versus the head loss in 1 h of filters’ operation. The performance of the automatic screen filter was the same at both working pressures. After reaching a head loss of 40 kPa (over the filter initial head loss) once 6 m3 were filtered, the automatic screen filter carried out a backwashing. The peak points on Figure 3 indicate the number of automatic backwashings in 1 h. In the other two filters, the evolution of head loss versus the filtered volume was almost linear. Adin and Alon (1986), when using freshwater with different suspended solid loads and constant discharge, concluded that with increasing the concentration of suspended solids, the slope of the head loss regarding filtered volume became steeper. The slopes of the regression equations (Fig. 3) show the amount of head loss per volume unit. For both filters, the filtered volume during 1 h at the working pressure of 300 kPa was about 12 to 15% higher than that observed at 150 kPa (Table 4). The reason was that, since most of the solids in trout fish farm effluent were organic (Manbari et al., 2020), they were deformed as pressure increases and can pass through the filter (Adin and Alon, 1986; Puig-Bargués et al., 2005). In the conventional screen filter, the increase in the filtered volume led to a 9% increase in head loss, which decreased up to 22% in the disc filter. This difference was due to the different performance of both filters and also to the nature of the organic matter in the effluent. In the disc filter, in addition to the surface of the discs, the suspended solids were also trapped inside the grooves of the discs, and an increase in working pressure caused the suspended solids to pass through the grooves and reduce head loss. In the screen filter, the suspended solids are trapped only on the screen surface.
The filtered volume per filtration cross-section of the disc filter was more than three times that of a conventional screen filter, which corresponds to their initial discharge rates (Table 4). At pressures of 300 kPa and 150 kPa, this higher filtered volume increased the head loss by 22% and 71%, respectively, compared to the conventional screen filter (Table 4). Figure 4 shows the average filtered volume per unit of filtration cross-section for a head loss of 10 kPa (V10). For both screen filters, V10 did not change with the increasing of working pressure, but in the disc filter it significantly increased (P<0.05) by almost 50% at 300 kPa inlet pressure. At both working pressures, V10 for the disc filter was significantly (P<0.05) larger than that for the other two filters. This increase in volume compared to the conventional screen filter at low working pressure (150 kPa) was about 80%, while at high working pressure (300 kPa) was 161%. For the automatic screen filter, these ratios varied from 18% to 75%. The value of V10 in the automatic screen filter was 27% larger than that for the conventional screen filter, being their difference significant (P< 0.05). When using municipal effluent at inlet pressures between 200 and 400 kPa, the disc filter worked similarly but the performance of the conventional screen filter has been reported to be either increasing or decreasing (Duran-Ros et al. 2014).
3.3. Operating time and filtered volume until backwashing
Figure 5 shows the operating time of the filters until they needed backwashing since filter head loss was 70 kPa (disc filter), 50 kPa (conventional screen filter) and 40 kPa (automatic screen filter) above the initial. The working time of both disc and conventional screen filters at a pressure of 300 kPa was longer than that for 150 kPa. This difference was about 5 min for the disc filter and less than 2 min for the conventional screen filter, which has also been reported in the use of treated municipal wastewater (Puig-Bargués et al., 2005). Kumar et al. (2017) also reported that an increase in the inlet working pressure from 250 to 400 kPa significantly diminished the number of backwashes required for disc and screen filters. The duration of the time a filter operates is dependent on the water quality and the type of filter, and when using low-quality municipal wastewater, the filters have a shorter operating time (El-Tantawy et al., 2009; Capra and Scicolone, 2004, 2005, 2007). The disc filter worked longer than the other two filters at both working pressures, and there were few differences between the operating time of the conventional and automatic screen filters. Capra and Scicolone (2001) reported that the operating time of the screen filter in the use of non-diluted municipal wastewater to reach the backwash was much shorter than that of the disc filter, and in the use of diluted wastewater, both the disc and screen filters had a similar operating time. Also, Puig-Bargués et al. (2005) argued that in the use of a more loaded effluent from a meat industry, the disc filter needed backwashing earlier than the screen filter. These performance differences may be due to the type of suspended solids in the effluent. In general, the operating time of these filters up to the backwash stage is very short and unacceptable in terms of operation.
Figure 6 indicates the filtered volume of effluent passing per filter cross-section (VB). In the disc filter, with higher working pressure, the volume increase was significant (P<0.01). In the screen filters, there was no significant difference in the volumes at either working pressure. VB for disc filter was in the range of 2.7 to 3.5 times as much as the automatic screen filter and 5.3 to 6.1 times as much as the conventional screen filter, being these differences significant (P<0.01). VB for the automatic screen filter was twice that for the conventional screen filter, but there were not significant (P>0.01) differences between both.
3.4. Mass retention in the filters (q)
Filtration efficiency regarding total suspended solids (Fig. 7) was -5% to 78%. Negative values, showing an increase in TSS at filter outlet, were observed for the automatic screen filter at 300 kPa during backwashing. The automatic screen filter did not interrupt the filtration operation during backwashing and, therefore, its efficiency dropped considerably. This efficiency reduction may be attributed to the mixing of the suspended solids released during backwashing with those of the filtered effluent, which has been reported by some researchers (Adin and Alon, 1986; Duran-Ros et al., 2008, 2009a, 2009b; Puig-Bargués et al. 2005). For the automatic screen filter, suspended solid concentrations in the backwash output at 300 kPa and 200 kPa were 28.2 mg l-1 and 34.6 mg l-1, respectively. A higher concentration of TSS on backwashing water at a working pressure of 200 kPa than at a pressure of 300 kPa implies a higher filtration efficiency at this working pressure.
Figure 8 shows that the mass retention of the filters tended to increase at lower working pressures. For the disc and automatic screen filters, this increase was only significant until backwashing was needed (P <0.01). Conversely, mass retention for conventional screen filter did not show any significant differences regarding pressure. Some researchers reported that using treated municipal wastewater, an increase in working pressure does not lead to any changes in the performance of disc and screen filters (Ravina et al. 1997; Duran-Ros et al. 2009a; Kumar et al. 2017) but Duran-Ros et al. (2014) found higher turbidity reduction in disc and screen filters at higher working pressures.
In conventional screen and disc filters, q increased over time at both working pressures due to the filtration cake. In disc filter, this increase remained significant (P<0.01) after one hour of operation (P <0.01) but was not significant under the other operating conditions (Fig. 8). Puig-Bargués et al. (2005) reported a similar observation for the screen filters in the use of treated municipal wastewater. The maximum value of q relates to the disc filter, which shows a significant difference (P<0.01) in performance compared to the conventional and automatic screen filters. The q value for both conventional and automatic screen filters was similar (Fig. 9). Some researchers have suggested the use of disc filter for low-quality effluents from wastewater treatment plants and screen filter for diluted effluents (Capra and Scicolone, 2004, 2005, 2007) but, conversely, others did not report noticeable differences of either filter when using urban effluents (Puig-Bargués et al. 2005). Using urban tertiary effluents, Duran-Ros et al. (2009b) found that the best water distribution uniformity was obtained by the emitters protected by a screen filter (83%), while disc filter achieved lower values (59%). Some researchers have also reported that screen and disc filters do not play a pivotal role in removing suspended solids from wastewater (Adin 1987; Adin and Elimelech 1989; Puig-Bargués et al., 2005; Taylor et al., 1995; Ravina et al. 1997; Ribeiro et al. 2008; Duran-Ros et al. 2009a, 2009b). The maximum q value at the working pressure of 150 kPa, and after one hour of operation was 32.7 g min-1 m-2 for disc filter, and the minimum q value reported for the automatic screen filter at the pressure of 300 kPa and during the backwash time was -1.4 g min-1 m-2. Overall, the q values for the disc, conventional, and automatic screen filters were 28.88, 9.11, and 7.72 g min-1 m-2, respectively. Therefore, less emitter clogging in the drip irrigation systems using fish trout farm effluent can be expected with disc filter.