Effect of Filtration Level and Pressure on Disc and Screen Filter Performance in Microirrigation Systems Using Effluents_2014_DURAN-ROS

Proceedings International Conference of Agricultural Engineering, Zurich, 06-10.07.2014 – www.eurageng.eu 1/7

Ref: C0406

Effect of filtration level and pressure on disc and screen fil-ter performance in micro irrigation systems using effluents

Miquel Duran-Ros, Jaume Puig-Bargués, Gerard Arbat, Francesc Ramírez de Cartagena, Department of Chemical and Agricultural Engineering and Technology, University of Girona, Campus Montilivi s/n, 17071 Girona, Catalonia, Spain

Javier Barragán, Department of Agricultural and Forestry Engineering, University of Lleida, Avda. Rovira Roure 177, 25198 Lleida, Catalonia, Spain

Abstract

Emitter clogging can be reduced with an appropriate design and operation of the filters, which are mandatory in any micro irrigation system. Filtration is especially important when biological effluents are used because of their high clogging risk. Micro-irrigation systems should be designed for supplying the pressure needed for an adequate operation under both normal filtration and backwashing filtration, which is used for cleaning the filtration media. The objective of this work was to assess the performance of screen and disc filters, which are widely used in micro-irrigation system, under different pressures and filtration levels when filtering a biological effluent. An experimental platform that was placed at the wastewater treatment plant of Celrà (Girona, Spain) was used in the experiments, which lasted more than 2200 h. A screen filter and a disc filtration system were studied at three working pressures: 200, 300 and 400 kPa. The filtration levels were 55 and 130 µm for the disc filter, and 50 and 120 µm for the screen one. Filtered flow rate and volume, inlet and outlet pressure, inlet pH, temperature, dissolved oxy-gen and turbidity and outlet dissolved oxygen and turbidity data were collected every minute with a supervisory control and data acquisition system. Pump pressure operation was con-trolled with a frequency controller drive which allowed changing operation regime without varying the backwashing conditions. Screen and discs filters with a filtration level of 50 and 55 µm filtered 97.7 and 96.7 %, re-spectively, less effluent volume to reach the same pressure loss increasing than the same filters with a filtration level of 120 and 130 µm. Screen filter had the same head loss than discs filter but only filtering the 24 and 33% of the volume filtered by the disc filter, at 120/130 and 50/55 µm, respectively. The best turbidity removals, which are related to a reduction of the physical clogging risk, were achieved in average for both filtration systems at 50/55 µm (10.9%) than in 120/130 µm (5.5%). Working pressure for both filters is suggested to be 400 kPa as it allowed reducing turbidity.

Keywords: filtration, clogging, drip irrigation, water pressure, wastewater

1. Introduction

Effluents are used in agriculture as a viable alternative to freshwater in areas where water is scarce or there is strong competition for its use. Although micro-irrigation is the most advan-tageous irrigation system for applying effluents, especially from public health and environ-mental points of view, the use of effluents can increase emitter clogging (Bucks, Nakayama,


Gilbert, 1979), which affects water distribution and, consequently, crop yields (Tajrishy, Hills, Tchobanoglous, 1994). As a result, filtration is an essential operation that can prevent emitters from becoming clogged (Oron, Shelef, Turzynski, 1979), although it does not avoid it completely (Tajrishy & Hills 1994). The three common filter types used in micro-irrigation systems are screen, disc and sand media filters. In screen filters, particles are trapped on the surface of a perforated cylinder. Disc filters have many grooved discs pressed together, and the particles are re-tained within the grooves of the discs. In media filters, solids are caught by the particles of gravel or sand. In screen and disc filters, the particles larger than the pores of the filtering media are retained on the surface of the media and accumulate layer by layer, forming a cake of increasing thickness that reduces the diameter of pores and allows smaller particles to be retained. In sand filters, the particles to be removed can be smaller than the filter pores since particle capture is controlled by both physical and chemical mechanisms (Adin & Alon, 1986). Screen and disc filters are simple, economical and easy to manage but sand filters are more complex and expensive, and they are only appropriate for farms with high technical and pro-fessional standards (Capra & Scicolone, 2007). As suspended particles are trapped by the filters, the filtration rate decreases because the filter becomes clogged and, then, it must be cleaned to recover operational conditions. Most filters are cleaned with automatic backwashing based on a fixed head loss across the filter and/or an operation time. Both options allow for easy system automation. Automatic back-washing of filters may require a minimum flushing pressure that pumping system must supply (Nakayama, Boman, Pitts, 2007). The filter performance in micro-irrigation systems using effluents has been studied by several authors (Adin & Elimelech, 1989; Ravina et al., 1997; Tajrishy & Hills, 1994; Puig-Bargués, Barragán, Ramírez de Cartagena, 2005; Capra & Scicolone, 2004, 2007; Duran-Ros, Puig-Bargués, Arbat, Barragán, Ramírez de Cartagena, 2009a; Ribeiro, Paterniani, Airoldi, Silvia, 2008). In these studies, inlet filter pressure was maintained between 250 and 400 kPa. How-ever, as energy saving in the pumping system is an important issue nowadays, it is preferred to work with the minimum inlet pressure at which filtration and backwashing are effective. The objective of this study was to assess the performance of two filtration systems (disc and screen) under different pressures and filtration levels when filtering a biological effluent.

2. Materials and methods

2.1. Experimental set-up

A filtration platform with two filtration systems was used to carry out the filtration experiments with the effluents at the wastewater treatment plant (WWTP) of Celrà (Girona, Catalonia, Spain), which treats urban and industrial wastewater using a sludge process. The first filtra-tion system consisted of one screen filter with a 50.8 mm diameter, 1,100 cm2 filtration sur-face while the second system was formed by two disc filters in parallel, both with 50.8 mm diameter and a filtration surface of 940 cm2. The screen and disc filters had an automatic backwashing system triggered by a pre-set pressure difference between each filtration sys-tem inlet and outlet pressure, which was measured by a pressure transmitter with flush dia-phragm. The flow at the filter outlets was measured by an electromagnetic flow metre. A tur-bine volume metre with impulse emitter was used to determine backwashing water volume. In order to obtain water quality data and their variation, pH, temperature, dissolved oxygen and turbidity sensors were installed. Hydraulic scheme of one of the units can be observed at Fig 1.


Figure 1: Hydraulic scheme of the mircroirrigation system and location of monitoring and control equipment These devices were connected to a supervisory control and data acquisition (SCADA) sys-tem (Duran-Ros, Puig-Bargués, Arbat, Barragán, Ramírez de Cartagena, 2008), which al-lowed the control of each filter performance and also to collect filter data performance con-tinuously. This required a computer and two programmable logic controllers (PLC).

2.2. Operational procedure

The filtration systems were studied at three working pressures: 300 kPa, 400 kPa (33% higher than 300 kPa) and 200 kPa (33% lower pressure). These pressures were enough for the irrigation of the experimental drip irrigation field. The filtration levels were 55 and 130 µm for the disc filter, and 50 and 120 µm for the screen one. Each filtration degree was studied using two screens or discs cartridges. Twelve experiments were carried out from the combination of the filtration level and working pressure. The test lasted 2264 h during the summer season. The different working pressures were reached changing the controls of the variable frequency pump drive. In order to obtain the minimum backwashing pressure, according to the manufacturers, the variable frequency pump drive also changed the controls when the backwashing took place. The tested pres-sures allowed irrigation to continue during backwashing, as it is desirable in micro-irrigation systems. During the experiment, each filtration system operated two 6-h periods per day up to the total amount of hours. After this time, the screen filter had filtered 6827 m3 and the disc filter 11070 m3. The reason why the total volumes were different is because with the screen filter and with the smallest grade (50 µm) the tests were not completed because the filter became clogged with a small effluent volume and it needed very frequent backwashings. Before the start of each experiment, all the screens and discs were cleaned with pressurised freshwater and submerged for 8 h in a solution of 5% NaClO. In both experiments the working time was rotated so that each filtration system operated at different times of the day. Thus, any hourly variation in effluent parameters should not affect always the same filtration unit. The average surface filtration velocity was between 0.017 and 0.028 m s-1 for the screen and disc filter (Table 1).


Table 1. Filtration velocities during the experiments with the filters

Filtration level (µm)

Inlet pressure

(kPa)

Filtration velocity (m s-1)

Disc Screen

120/130 200 0.020 ± 0.0005 0.017 ± 0.0007 300 0.026 ± 0.0009 0.022 ± 0.0009 400 0.028 ± 0.0017 0.023 ± 0.0010

50/55 200 0.021 ± 0.0013 0.012 ± 0.0060 300 0.026 ± 0.0009 0.020 ± 0.0004 400 0.024 ± 0.0015 0.020 ± 0.0005

Filters were cleaned automatically by backwashing when the head loss across the filter ex-ceeded 50 kPa, as suggested by Ravina et al. (1997), for more than 2 min. The results of effluent characterisation are shown in Table 2. Following the risk classification proposed by Bucks et al. (1979), the effluent had a low clogging risk for the turbidity (highly related with suspended solids) and a medium clogging risk for the pH.

Table 2: Mean and standard deviation of the effluent pH, temperature, turbidity and dissolved oxygen at filter inlet.

Inlet pressure

(kPa)

Filtration system

pH Temperature

(ºC) Turbidity

(FTU)

Dissolved Oxygen (mg l-1)

200 Screen 7.20 ± 0.04 28.00 ± 1.59 4.41 ± 1.28 3.53 ± 0.65

Disc 7.27 ± 0.11 27.61 ± 3.21 4.46 ± 1.49 3.54 ± 0.60

300 Screen 7.98 ± 0.20 27.25 ± 2.45 7.19 ± 2.90 4.63 ± 0.60

Disc 8.07 ± 0.17 26.35 ± 2.29 8.19 ± 5.21 4.15 ± 1.55

400 Screen 7.31 ± 0.39 19.17 ± 1.73± 6.10 ± 1.16 4.08 ± 0.96

Disc 7.19 ± 0.27 20.67 ± 2.90 7.92 ± 3.67 3.61 ± 1.31 Mean 7.49 ± 0.43 24.36 ± 4.45 6.40 ± 3.32 3.91 ± 1.10

3. Results and discussion

Screen and discs filters with a filtration level of 50 and 55 µm filtered 96.8 and 97.7%, re-spectively, significantly (P<0.05) less effluent volume to reach the same pressure loss than the same filters with a filtration level of 120 and 130 µm (Fig. 3). Screen filter had the same head loss than disc filter but only filtering 24 and 33% of the volume filtered by the disc filter, at 120/130 and 50/55 µm, respectively. This difference was not observed by Duran-Ros et al. (2009a) when, working with 120/130 µm disc and screen filters and with similar water quality and filtration velocity, found that screen filter needed 17 and 8% more volume than disc filters to achieve the backwashing head loss level. The volume filtered for increasing 10 kPa the head loss was the less at 200 and 400 kPa for disc and screen filter, respectively.


Figure 2. Volume of effluent needed to increase 10 kPa the filter head loss. The 120/130 µm filtration level is shown at the a) figure whereas 50/55 µm filtration is shown at figure b). Although turbidity and dissolved oxygen were measured at filter inlet and outlet, we will focus only on turbidity removal since this is a parameter related to physical clogging. Due to their characteristics, screen and disc filters cannot reduce the biological clogging risk. For this reason, values of dissolved oxygen did not supply valuable information. The highest turbidity reduction at 120/130 µm filtration level was obtained for 400 kPa with disc and screen filters, i.e. 12.2 and 13.2%, respectively (Fig. 4). With 50/55 µm filtration lev-el and for the disc filter the best turbidity reduction (23.8%) was also achieved at 400 kPa. However, at this filtration level, the best turbidity removal for screen filter was found at 200 kPa (13.5%) not so far from that achieved at 400 kPa (12.9%). On the other hand, turbidity slightly increased at filter outlet with 300 kPa for the filtration grade of 120/130 µm. Duran-Ros et al. (2009a), working with a similar effluent, also found that with 120/130 µm disc and screen filters, higher turbidity reductions were achieved at higher working pressures. Moreo-ver, turbidity removals for screen and disc filters observed in the present experiment were higher than previous observed values (3.86 and 7.14% for 130 µm disc and 120 µm screen filters, respectively) (Duran-Ros et al., 2009a). However, the high variability of the results in the present and previous works should be highlighted. It should be pointed out that turbidity removals of screen and disc filters were lower than those usually achieved by sand filters, above 60% (Elbana et al., 2012). The lower turbidity removal found with screen and disc fil-ters is due that their filtration mechanism does not allow retaining as much particles as sand filter does. Only the filtration degree and working pressure had significant effects (P<0.05) on the turbidi-ty reduction, but neither filter type nor interactions between effects were reported to be signif-icant. Both filtration systems showed significant (P<0.05) turbidity reductions, in average, when 50/55 µm were used instead of 120/130 µm (10.9 and 5.5 %, respectively). These re-sults could be anticipated as the lowest the filtration degree the lowest the particles retained and, thus, outlet turbidity could be reduced greater. However, the use of these lower filtration degrees with the effluents used in this experiment has been not practical due to the frequent


backwashing needed. On the other hand, higher pressure allowed significantly higher turbidi-ty reductions: 1.78, 9.13 and 17.88% at 200, 300 and 400 kPa, respectively. A possible ex-planation could be that less size particles could be easily trapped on filtration cake at higher pressures due to higher flow velocities within the filter. However, further research is needed to deeply analyse particle dynamics in screen and disc filters.

Figure 3. Turbidity reductions obtained with disc and screen filtration at 50/55 µm and 120/130 µm filtration levels combined with 200, 300 and 400 kPa of working pressure.

4. Conclusions

Filtration behaviour could be described as the capacity of the removal of the clogging parti-cles (measured as turbidity) and the volume of water filtered to increase the head loss. In an experiment with screen and disc filters working with a reclaimed effluent, increasing the working pressure from 300 to 400 kPa allowed higher reductions on turbidity. With this pres-sure increasing, the 130 µm disc filter was able to filter more volume to obtain the same head loss. Filtration levels of 50/55 µm filtered 97 % less volume than 120/130 µm disc and screen filter. On the other hand, turbidity reduction was 5.5 % with 120/130 µm and 10.9 % with 50/55 µm. So that, in order to obtain better turbidity removals 55/50 µm filtration levels and 400 kPa of pressure have to be applied. However, as working at 55/50 µm yields shorter filtration cycles, would be more practical to work at 120/130 µm filtration levels at 400 kPa for both screen and disc filters

5. Acknowledgements

The authors would like to express their gratitude to the Spanish Ministry of Education and Science for their financial support for this experiment through grant CGL2012-31180. The authors would also like to thank Regaber and the Municipality of Celrà for their help in carry-ing out this experiment.

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Effect of Filtration Level and Pressure on Disc and Screen Filter Performance in Microirrigation Systems Using Effluents_2014_DURAN-ROS