Performance of rapid sand ﬁ lter – single media to remove microplastics

Microplastics (MPs) have been detected in drinking water and raw water sources. Therefore, it is important to know the performance of drinking water treatment process. The rapid sand ﬁ lter (RSF) is one of the water treatments that can be an alternative treatment in removing MPs after several con ﬁ guration processes (pre-sedimentation, coagulation- ﬂ occulation, and sedimentation). This study aims to determine the effectiveness of RSF to remove MPs. The arti ﬁ cial samples were made from plastics bags and tyre ﬂ akes, with sizes from 10 μ m to more than 500 μ m. Bentonite was added to represent turbidity in the water. The average removal ef ﬁ ciency of plastics ﬂ akes before entering the ﬁ lter was 50.48% (using bentonite) and 47.78% (without bentonite). Overall, the removal ef ﬁ ciency for the tyre ﬂ akes was 90.72% (using bentonite) and 93.03% (without bentonite). The ﬁ ltration used in this study was varied between 4 and 10 m/h. Removal ef ﬁ ciency using RSF for plastic ﬂ akes on which the Effective Size (ES) ﬁ lter media 0.39 mm was 97.7% and on which ES 0.68 mm was 94.3%. Meanwhile, the removal ef ﬁ ciency of the tyre ﬂ akes for ES 0.39 mm were 90.6% and ES 0.68 mm was 85.2%. However, in this study, RSF mostly removed MPs particles greater than 200 μ m in size.


GRAPHICAL ABSTRACT INTRODUCTION
At present, the occurrence of plastic waste is increasing in the aquatic environment, including oceans and water bodies. Plastic waste includes not only larger plastic debris, but also smaller plastic particles commonly referred to as microplastics (Eerkes-Medrano et al. ). Microplastics (MPs) are plastics less than 5 mm in size, which can be formed from industrial production or fragmented from larger plastics (Crawford & Quinn ). As RSF is used intensively in Indonesia, this paper will investigate the performance of RSF in DWTP.
Prior to rapid sand filtration, the samples are treated in the operation unit and processes such as pre-sedimentation, coagulation and flocculation, and sedimentation.
Therefore, this study also conducted a preliminary study to show the removal of MPs in processes prior to RSF. The filter media used is silica sand because silica sand is easy to get and has a smaller porosity (Droste ). This study aims to analyse the effectiveness and mechanism of the RSF process in MPs removal and analyse the effect of research variables on the performance of filter media. The variables in this study were MPs type and size, filter media size, filtration time and filtration rate.

Filter media and rapid sand filter reactor
The filter media used in this study was silica sand. Silica sand was chosen because it is common in the water supply and it is inexpensive. This study used an RSF as most of Indonesian DWTPs apply RSFs. Initially, the silica sand was screened to size 20-40 mesh and 40-70 mesh to get two variations of effective size (ES). Then, before the media was used, sieve analysis was carried out to determine the ES and uniformity coefficient (UC) of silica sand. Then, the result of sieve analysis was depicted in a distribution accumulation curve to find the ES and UC values. In addition, the values of porosity and density were also determined. Porosity and density of filter media can be calculated with the equation below: Density (ρ) ¼ filter media mass filter media volume The reactor used was cylindrical, made of acrylic, 10 cm in diameter and 100 cm in height. The reactor was designed with continuous flow (downflow) using a submersible pump. The flow rate was managed by a flow meter. The filtration process scheme for the reactor is depicted in Figure 1.

Microplastics artificial sample
The microplastics samples used in this study were artificially made from plastic shopping bags and motorcycle tyres.

Microplastics identification
The sample was collected using a glass bottle with a volume of 500 mL. Then the sample was filtered with a Whatman GF/C paper filter using a vacuum filter. The filter paper with MPs above was transferred into a petri dish and dried in an oven at 105 C for approximately 30 minutes to remove the moisture content on the filter paper. and also their form, such as flake, fragment or fibre.

Pre-treatment
Pre-treatment aims to simulate a series of water treatment units before water enters through the filtration unit. Pretreatment includes pre-sedimentation, coagulation-flocculation, and sedimentation. In this study, the samples were given two treatments, namely the addition of bentonite and without the addition of bentonite. In the pre-sedimentation stage, the sample was settled for two hours, then the settling MPs were counted.
The first stage in this preliminary study was pre-sedimentation. Microplastics removal at the pre-sedimentation stage was similar to the provision of particulates in the processing of raw water and wastewater utilizing gravity. Discrete particle deposition occurs in the pre sedimentation unit (Metcalf and Eddy ; Kawamura ). Deposition of particles is affected by particle size, particle shape (flat, round or irregular), density, liquid specific gravity, liquid viscosity, particle concentration in the liquid, particle properties in the suspension and temperature. The size and shape of the par- The coagulation, flocculation and sedimentation processes were conducted in standard jar test apparatus. In coagulation, 1% alum (30 ppm) was added and the sample was rapidly mixed (120 rpm) then the mixed speed was reduced to slow mixing (40 rpm). pH was monitored to remain within the optimum range for coagulation (5.5-7.0).
The sample was then allowed to stand for two hours at the sedimentation stage. Also, the settling MPs in this stage were counted to calculate the difference between initial MPs and the final stage of pre-treatment as MPs removal efficiency.

Filtration tests
The main research was conducted using a reactor as shown in Figure 1. Water from the sample tank was pumped to the storage tank using a submersible pump and was filled into the reactor through the filter media to the outlet. The initial height of the water surface above the filter media was 5 cm.
The sample flowed for 10 hours for each batch with four variations of loading rate (4; 6; 8; and 10 m/h consecutively). One litre of inlet and outlet samples was collected at the specified filtration times, which were at 0.5; 1; 5 and 10 hours. The experiment was duplicated for each variation loading rate.
The objective was to find the average MPs removal efficiency after several reactors operating times, as well as the size distribution and the number of MPs that affect the performance of the filter.

RESULTS AND DISCUSSIONS
Filter media analysis  () explain the arrangement between filter media will cause strain when the ratio of particle diameter to filter media diameter is greater than 0.15. The effective size of the smallest media specified in an RSF is usually around 0.5 mm, where at that size the filter media is unable to hold particles smaller than 30-80 μm. can be seen in Figure 2.

Pre-treatment
The pre-treatment process consists of pre-sedimentation, coagulation, flocculation and sedimentation. The pre-treatment  flakes have a greater specific gravity so more tyre debris settles in the pre-sedimentation process, up to 89%.

Coagulation and flocculation
Alum is used as a coagulant in this study. Alum is easily soluble in water and easily forms Al 3þ and sulfate (SO 4 À ) ions.
Al 3þ ions in water are hydrolyzed to Al(OH) 3 in colloidal form. According to Manurung (), the mechanism of coagulation is categorized into chemical and physical processes. A chemical process states that colloids obtain an electric charge on their surface by chemical ionized group and coagulation occurs because of the chemical interaction between colloidal particles and coagulants.
The charge of colloidal particles that cause turbidity in water is the same, therefore if the ionic strength in water is low, the colloids will remain stable. In theory, coagulation occurs through reduction of force.

Sedimentation
The supernatant of the coagulation-flocculation process was filtered to identify the number of MPs that were settled and which were still floating in the sample. The settling process was carried out for two hours. The sedimentation process for plastics flake removal with additional bentonite was 29.03%, whereas removal without bentonite was 29.01%.

Observations on MPs removal and size distribution and
MPs number in the preliminary study can be seen in  However, the trend shows that the removal percentage tends to decrease with increasing filtration rate.
Similar to the plastic flake removal, the percentage of MPs removal from tyre flakes also fluctuated ( Figure 6).
The percentage of removal ranged from 77.8 to 95.5%.
The highest removal occurs at a filtration rate of 6 m/h after reactor operation for 1 hour, which is 95.5%, while the lowest removal occurs at a filtration rate of 8 m/h after operation of the reactor for 5 hours, which is equal to 77.8%. By looking solely at the trend, the removal of tyre flakes decreases with increasing filtration rate.
The percentage of MPs removal of plastic flakes (96.4%-99.2%) is higher than that of tyre flakes (77.8%-95.5%). The MPs particles of plastics flakes were bigger than that of tyre flakes so it is more likely that plastic flakes removal is higher.
Research on the relationship between filtration rate and the efficiency of MPs removal has not been found, so the    there was no effect of filtration rate on MPs removal.      The process of mechanical straining that occurs in MPs removal using RSF is influenced by the size of the ES, porosity and MPs size. This is basically because the process of mechanical straining occurs by utilizing the pore size of the filter media so that MPs of a larger size will be strained. Crittenden et al. () explained that if particles have a size greater than the size of the voids in the filter, the particles will be removed through straining, but if the particle size is smaller, then the particles will be set aside when contacted and attached to the filter media due to Van der Waals forces. The diameter of the opening between the filter media pores can be determined mathematically in reference to Huisman and Wood () in Figure 9.
It is known that e is the diameter of the opening in the filter media, which can be used as the basis for the size of particles that can be retained or which can still pass through the pores of the filter media, while d is the diameter of the filter media grain. The value of e can be determined by looking for a comparison of the valued with 6.46. Crittenden et al. () also explained that for round filter media, a tightly closed arrangement would cause strain when the ratio of particle diameter to grain diameter was greater than 0.15, meaning that smaller particles would pass through the filter media.
However, this does not apply to all conditions. So, based on this explanation, the MPs size that can pass through the filter media can be determined. The calculation results can be seen in Table 4.
Based on the results of calculations in

CONCLUSIONS
Microplastics removal in conventional water treatment processes consisting of sedimentation, coagulation and flocculation, sedimentation and followed by filtration, were  considered capable of removing certain size of MPs. Pretreated water followed by the rapid silica sand filter process with ES 0.39 mm and 0.68 mm can remove 85% to 97% MPs that are mostly greater than 200 μm in size.