Characterization of Oily and Non-Oily Natural Sediments in PalmOil Mill Effluent

Palm oil is one of the many vegetable oils widely consumed around the world. e production of palm oil requires voluminous amount of water with the concurrent generation of large amount of wastewater known as palm oil mill effluent (POME). POME is a mixture of water, oil, and natural sediments (solid particles and �bres).ere is a dearth of information on the physical properties of these POME sediments. is study intends to distinguish the physical properties of oily and non-oily POME sediments which include sediment size, particle size distribution (PSD), sediment shape, sediment surface morphology, and sediment density.ese characterizations are important for future researches because these properties have signi�cant effects on the settling process that occurs either under natural gravity or by coagulations. It was found that the oily and non-oily POME sediments have different sizes with nonspherical irregular shapes, and because of that, the aspect ratio (AR) and circularity shape factors were adopted to describe the shapes of these sediments. e results also indicate that the density of oily POME sediment decreases as the sediment size increases.


Introduction
Raw palm oil mill effluent (POME) is a thick, brownish, highly concentrated, and colloidal slurry with pH ranging from 4.0 to 4.5.It contains mainly water (95-96%), suspended solids (2-4%), and oil (0.6-0.7%) [1].Freshly discharged POME has temperature between 80 and 90 ∘ C. Palm oil production process does not utilize any chemical; hence, POME is considered as non-toxic wastewater.e chemical characterizations of POME that include biochemical oxygen demand (BOD), chemical oxygen demand (COD), total solids (TSS), oil and grease (O and G) and other chemical properties are shown in Table 1 [2].
Many studies are focused on raw POME physicochemical treatments like sedimentation by coagulation and �occulation [3], adsorption [4], electrocoagulation [5] and membrane �ltration [6], and they characterized the raw POME in their studies.But there is no information about the physical properties of the suspended solids (or can be called natural sediments) that exit in the raw POME and play an important role in the separation processes like sedimentation, �ltration and separation processes by the membrane.Allen (2003) mentioned that the particle settling behaviour is dependent on the particle density, particle morphology (shape, texture, etc.), size and particle size, distribution [7].
e most natural particles in industrial processes are nonspherical and irregular in shape which means it is impossible to accurately describe their sizes [8].e size determination of the natural particles is considered a difficult task, and to overcome this, various approaches are suggested to determine their average diameter such as sieving and microscope image analysis [9].Furthermore, there is a relationship between the average particle diameter and its shape.is can be done by multiplying the mean diameter by the shape factor to obtain the equivalent diameter [10,11].Bouwman et al. [12] de�ned the shape factor as a number which could characterize the particle's shape, and it is derived from a microscopic image of the particle.Many different shape factors are being used in previous studies to describe the particles, such as Corey shape factor (cfs) [13], aspect ratio (AR) [9], circularity [9], new projection shape factor, Stokes' shape factor, and new roughness factor [12].It was reported that the AR and circularity shape factors are suitable to describe the particle's shape in different aspects [14].Particle aspect ratio is expressed by the ratio of the maximum diameter to minimum diameter, and the �bre aspect ratio is expressed by the ratio of mean length to the mean diameter.On the other hand, the circularity shape factor is based on the projected area of the particle and the overall perimeter [12,15].Another important property is the particle density which plays a signi�cant role in the particle settling rate combined with the �uid density.Actually, the particle settling depends on the difference between particle and �uid densities.e volume determination of non-spherical irregular natural sediments can be done directly by either pycnometer or by a liquid or a gas sample displacement [16].
e objectives of this study were to investigate the main physical properties of oily and non-oily natural sediments in raw POME.ese properties include particle size, particle size distribution (PSD), particle density, and particle surface morphology.In addition to these properties, this study examines the suitability of aspect ratio (AR) and circularity shape factors to describe the POME particles and �bres.e output of this characterization is considered as a source material for future researches because these properties play an important role and more signi�cant impact on the separation processes like sedimentation, �occulation, �ltration and separation process by membrane.

Separation of Oily and Non-Oily Sediments from Raw POME.
Raw POME was allowed to naturally settle for 24 to 48 hours to obtain POME sludge.e upper clear layer "supernatant" was drawn out by a pump, and the bottom layer "POME sludge" was taken and dewatered using �bre materials with pore openings <20 m.e dewatered POME sludge was freeze-dried completely using a freeze dryer (Model LABCONCO, USA) to get oily POME sediments.
A portion of the oily POME sediments was treated with n-hexane (C 6 H 14 , min 99.0%,QReC) in a Soxhlet until all the oil has been removed to obtain non-oily POME sediments.

Particle Size and Particle Size Distribution (PSD).
In this study the sieve analysis was used to estimate the particle size and PSD within a given size range.Separate batches of oily and non-oily POME particles were sieved using a sieve shaker (Model Retsch AS 200, Germany) to determine the particle size and PSD.As much as 80 g of the sample was placed in a weighted sieve above a series of sieves arranged with descending aperture sizes.e sieves were arranged from the top to bottom with aperture openings as follows: 1400, 1000, 800, 500, 250, 125, 63, 45, 38, 20 m, and the �nal receiving pan at the bottom.e sieves were shaken horizontally for 20 min, followed by carefully removing each sieve for reweighing, and the weight of POME particles in each sieve was determined.e steps were repeated (2 to 5 times) until the weight of any sieve remained constant at ±5% of the previous weight of the sieve [17].e samples retained on each sieve were weighed again aer the sieving process �nished and were calculated by the following equation [18,19]: weight of retained particles on each sieve, g weight of total particles, g × 100%. ( To calculate the percentage of undersize particles, the calculation should be started with 100% of the sample and then subtract the percentage of cumulative oversize particles on each sieve as given in the following equation [19]: % cumulative undersize = %100 − % cumulative oversize. (2) 2.3.Shape Factor.e shape factors of oily as well as nonoily POME sediments were determined using a microscopebased on image analysis.e image analysis was done by a microscope (OLYMPUS SZX9 stereo, Japan), equipped with a video camera, to determine sediment diameter size greater than 800 m.For sediment size less than 800 m, the compound microscope (OLYMPUS BX41, Japan) was used to determine their sizes.e soware that combined with this analysis was called Analysis Image Processing "cell A. " For each size, a small amount of POME particles was spread on a clean glass slide.e agglomerated POME particles were dispersed with solvent; the oily POME particles was separated using drops of water, and the non-oily POME particles were dispersed with hexane.Five to seven individual POME particles were randomly chosen from an ensemble of particles of each sample to measure the following geometrical dimensions: minimum diameter ( min ), maximum diameter ( max ), particle projected area (), and overall perimeter of the particle () to determine two different shape factors: aspect ratio (AR) and circularity.AR is determined by using the following equation [9]: e circularity was measured by the following equation [9]:

Surface Morphology of POME Natural Sediments by Scanning Electron Microscopy (SEM)
. e surface morphology of both oily and non-oily POME sediments were examined by a scanning electron microscopy (SEM) (Model Leo Supra 50 VP Field Emission, Germany) combined with an Oxford INCA 400 energy dispersive X-ray (EDX or EDS).e SEM was operated at 20 kV, and the working distance was varied between 7 and 9 mm.e POME particles were mounted on a carbon tape attached to an aluminium stab and vacuumed for 5-10 min before being inserted into the SEM system.

Determination of POME Natural Sediments Densities.
POME natural sediments densities were determined from their mass and volume.e speci�c gravities of these sediments were calculated using the following formula [20]: where  1 is the empty weight of volumetric �ask (g),  2 is the weight of volumetric �ask �lled with POME sediments (g),  3 is the weight of volumetric �ask �lled with distilled water and POME sediments (g), and  4 is the weight of volumetric �ask �lled with distilled water (g).e speci�c gravity of POME sediments was measured under room temperature 27 ± 1 ∘ C by water displacement method.A 25 mL volumetric �ask was used in place of pycnometer to measure the volume of POME sediments.Boiled distilled water was used in this experiment to keep it free from dissolved gases which might hinder the settling of the POME particles in the �ask [21,22].Aer the determination of the speci�c gravity of POME particles, the density can be easily calculated by using the following formula: speci�c gravity sp.gr. = density of the sample g/cm 3  density of water g/cm 3  .
e water density depends on water temperature and atmospheric pressure.e water density at 28 ∘ C is 0.9955 g/ cm 3 .

Particle Size and Particle Size Distribution (PSD) of POME Oily and Non-Oily Natural
Sediments.is study used sieve analysis to determine the particle size and PSD of oily and non-oily natural sediments of POME separately.is analysis can provide an average particle size with particle size distribution based on this average [17].e POME particles that passed through and were retained on each sieve gave an approximate particle size within the range of the aperture openings.It was found that the oily POME particles sizes ranged from 64 to 1000 m as shown in Table 2.No amount of particles greater than 1400 m or less than 45 m has been retained.Slightly more than 74% of the oily POME sediments are greater than 250 m, and almost 100% of the oily POME sediments are greater than 63 m.Hardly 2% of the sediments were larger than 800 m.e non-oily POME particles sizes ranged from 38 to 1000 m as demonstrated in Table 3.No amount of particles greater than 1400 m or less than 20 m has been retained (oversize particles).Five percent of these particles are larger than 800 m and almost 60% larger than 250 m.Only about 18% of the oily particles are larger than 500 m in comparison to almost 32% of the non-oily particles.
Light microscopy was used to determine both oily and non-oily POME particles that have size less than 38 m which could not be measured by sieving process.It was found that these small particles tend to agglomerate or adhere to larger particles and produce enlarged particles greater than their actual size.is could be due to the van der waals interactive forces, chemical reaction, mechanical bonding or electrostatic charging which agrees with the observations of Adi et al. [23].
e lognormal distribution plot of the cumulative undersize percent of oily and non-oily POME particles respectively, versus sediments sizes (or sieves openings sizes) demonstrates the broad distribution of particle sizes as shown in Figures 1 and 2.   e PSD graphs provide a wide range of statistical parameters that describe both types of the POME particle size distribution using only the percentiles taken from the cumulative frequency curves.McKenna et al. mentioned in their work that two important parameters are being used to describe the lognormal distribution, namely, geometric mean diameter (  ) and the geometric standard deviation (  ) [24].e geometric mean diameter (  ) is obtained from  16 and  84 .is procedure is considered standard and commonly drawn from PSD which is used even if the distribution deviates from lognormal [25].e geometric mean diameter (  ) is de�ned as follows [26,27]: To measure the degree of sorting, geometric standard deviation (  ) could be used and computed as follows [26,27]: where  84 , is the diameter for which 84% of the particles are �ner than  84 and  16 are the diameter for which 16% of the particles are �ner than  16 [26].Birta et al. [27] mentioned in his research that ( 8) is used to determine the geometric standard deviation for total reign (between  16 and  84 ), but there are other formulas is used to determine the geometric standard deviation such as Equation ( 9) is used to determine   of high region (between  84 and  50 ) and ( 10) is used to determine   of low region (between  16 and  50 ) [27].Beside these parameters, there are more statistical parameters like the median diameter ( 50 ) that represented the 50% of the distribution, mode particle size, mean particle size, and particle diameters such as  5 ,  10 ,  90 , and  95 at 5th, 10th, 90th, and 95th percentiles of cumulative mass distribution, respectively.e size characteristics represented by  16 ,  50 , and  84 from Figures 1 and 2  e sorting of the sediments around the average that is represented by (  ) shows that the oily POME sediments are moderately well sorted because they have values existing between 1.41 and 1.62.But the non-oily POME sediments have poorly sorted values because their (  ) value lies between 2.00 and 4.00 [28].

Scanning Electron Microscopy (SEM) Examination of Oily
and Non-Oily POME Sediments.SEM offers more advantages than the conventional light microscope such as greater �eld depth that allows more of the specimen to be focused at the same time, higher magni�cation so closely spaced specimens can be magni�ed, and clear images that provide shape and surface structure [29].
e details of the surface morphology with clear images of the oily and non-oily POME sediments can be provided by SEM and are seen in Figures 3 and 4. e SEM images in Figures 3(a) and 3(b) show no distinction between oily and non-oily POME sediments (particles) in their surface morphology.Both POME sediments have no symmetry and are irregular in form with unequal width and breadth, thus having a wide range of sizes with odd dimensions.e particles outline comprises varying twist and turns of edges known as angular.Some of these particles are �aky, and some are clumped together and appear like one mass having a branched crystalline shape [30].�nder greater magni�cation (1000x), it is evident that the oily POME particles have smoother surfaces as a result of complete coating with oily layer, and all the openings are �lled with oil and appear like a single mass seen in Figure 4(a).e smooth, yet uneven, surface exterior denotes that the particles are held together by the natural viscosity and thickness of the adhered oil.e vigour of this hold by the oil on the particles will affect the settling behaviour in the solution.
In comparison, the surface structure of the non-oily POME particles is more distinct, evidenced by its rugged features, rough edges, and numerous cavities as seen in Figure 4(b).Some particles have sharper edges while the majority possess more obtuse perimeter with rounder and thicker formations as opposed to �atter features belonging to other particles.Flatter POME particles appear �aky with grooves and others chunky whilst more tiny particles are loosely adhering to these surfaces.

Shapes of Oily and
Non-Oily POME Sediments.e determination of oily and non-oily POME sediments shapes was done by image analysis (IA) or optical microscope based on two-dimensional images combined with special soware called analysis Image Processing "cell A" used to measure the speci�c dimensions adopted to determine different shape factors.Although this approach is time consuming, it is considered the most efficient method that provides many details on the size and surface morphology of these sediments.Geometric determination was difficult especially for the �ne oily and non-oily POME particles (<20 m) since they are bound together by particulate interactive forces in an unmethodical assemblage, forming larger particles [23].
As mentioned before, the POME particles have nonspherical and prevalently irregular shape, and rough in formation.e wide-ranging variation of the uneven forms is inherent and inadvertently formed as a result of the way the fruits were processed in the factories.e POME sediments are fundamentally residuals from the solid components of the fresh fruits and escaped parts of fresh bunches, in the form of mesocarp �bres and other �bres that passed through the processing during the extraction of palm oil.
In both POME types oily and non-oily, there are different shapes and lengths of �bres.ese �bres sometimes appeared very clearly as standalone without being attached to any particle, and sometimes they were covered with particles making the �bres almost invisible.e presence of varying lengths and thicknesses of the �bre strands is visible among the oily POME particles of different sizes ( O ≤ 250 m,  O ≤ 500 m, and  O ≤ 800 m) shown in Figures 5(c), 5(d), and 5(e).ese �bres covered with oily particles can be noticed partially or totally but at oily POME particle diameter,  O > 1000 m, �bres could not be observed because probably they were covered totally by other particles.
e larger strands have approximate diameter of 185 m and the smaller ones are about 63 m in diameter, with lengths ranging from 700 m to 7900 m.No �bre strand was found in samples sieved through 63 m, 125 m, and 1000 m.e absence of �bre strands in samples sieved through smaller pores might be due to the inability of the strands to make it through the holes.Most of the strands were found in samples sieved through 500 m, which potentially is the reason for the obvious absence of any �bre strands in samples sieved through 1000 m openings.Figures 6(a) to 6(i) show the range of non-oily POME particles with diameter,  NO < 38 m to ≤1000 m.e particles are nonuniform in shape, width, and thickness.�n the oily POME samples, �bres strands were sporadically observed among the other POME particles.On the contrary, the presences of �bres are not easy to detect among the non-oily POME particles.in wiry �bre-like strands can be observed in Figures 6(g) to 6(i).
POME �bres have different shapes such as simple �bre, brunched �bre, and �ake �bre.e �bres present in natural sediments of POME are partially embedded in the particles or stand alone.e different shapes of oily POME �bres at different sizes covered with POME particles are shown in Figure 7.A closer look at the �bre as demonstrated in Figure 7 shows different shapes, and length of the oily POME �bres with smaller POME particles adhered to their length.e non-oily POME �bres at different sizes and shapes are shown in Figure 8. is �gure shows an e�ually different shapes and lengths of �bre strands found among the non-oily POME samples.

Shape Factors of Oily and Non-Oily POME Sediments.
To describe the degree of divergence of non-spherical irregular POME sediments from the spherical shape, the aspect ratio (AR) and circularity shape factors were used [31].
Many studies used the average diameter, aspect ratio and circularity in their studies to characterize the particles [12,14,32].Zhengmin et al. [14] used AR and circularity to compare between three brown corundum powders and found that these shape factors were suitable to describe the particle shapes in different aspects.Other studies mentioned that AR can be used when the particles have different shapes (such as �bre-like or �aky) and can provide a concept about the granules being spherical or cubical [12].Other works used AR to determine the shape factor of minerals that have �ak and �bre structure [33].
e values of AR and circularity for oily and non-oily POME particles at different range of particle sizes are shown in Table 5.From Table 5, the AR values decreased when POME particle size increased, while the circularity values increased when POME particle sizes increased.is could  indicate that the POME particles become closer to spherical shapes more than the elongate one when their sizes increase [32].It is observed that the circularity of the oily POME particles was higher than the circularity of non-oily POME particle of the same size.A possible explanation for some of these results might be that the oily POME particles were covered with an oily layer making the shape of these particles smoother and more spherical than non-oily POME particles and as shown in SEM images in Figure 4(a).is �nding is in agreement with Almeida-Prieto et al. (2007) who studied the relationships between 16 pellets of different shapes having diverse properties.eir study showed that AR increases with increasing sample irregularity [9].On the contrary, Abouel-Kasem (2011) studied the effect of particle size on slurry erosion and observed that both the aspect ratio and circularity values increase with the diameter of the particles [32].
As mentioned earlier, the POME natural sediments are a mixture of particles and �bres; to determine the �bre�s shape, AR was used.Table 6 presents the AR values of non-oily POME �bres.
It can be concluded from this table that the �bres that have needle-shaped or rod-shaped structure have large aspect ratios compared with other �bres that have �aky shapes.
e non-oily POME �bres have a wide range of dimensions depending on the particles elongation.In some cases although the �bre had a long diameter, it was still retained within particles having small size ranges because it had a short diameter within this particle size range; that is, the nonoily POME �bre longest length was around 4680 m but the shortest length was 100 m as shown in Table 6, so it could be retained in a particle size range of 63-125 m.e AR for oily POME �bres could not be determined since it was very di�cult to obtain a single �bre from oily POME samples as the particles were attached to the �bre and covering it as shown in Figure 7.

Densities of Oily and
Non-Oily POME Sediments.e densities of oily and non-oily POME sediments are shown in Table 7.
It is obvious that the density of oily POME sediments decreases as the sediment size increases.is is due to the oil content percentage which increases with the POME sediments sizes as shown in Figure 9.
Oil density is 0.89 g/cm 3 at 27 ± 1 ∘ C (room temperature), which is less than that of the POME supernatant density of 0.998 g/cm 3 at the same temperature.e relation between the oily POME particle size, and its density can be explained as follows: if we assume that the oily POME particles have ideal spherical shapes, then their surface areas will be augmented with increasing POME particle size and consequently the attached oil will be mounting too.But indeed the POME particles have non-spherical irregular shape as mentioned   before which means that the POME particle surface area was bigger than the spherical particle.e increasing surface areas led to an increase in the amount of attached oil inside the pores and on the surface of the POME particles.is explains the reason behind the decrease in oily POME particle densities with the increase in oily POME particle size.On the other hand, the density of non-oily POME particles is increased with increasing the particles' size because there are no oil droplets to the POME particles.

Conclusion
e raw POME is a mixture of water, oil, and sediments (solid particles and �bres).e oil covered some of these particles and �bres and conse�uently affected their characteristics such as particle size distribution, particle shape, surface morphology, and their densities.e particle size of oily POME ranged between 63 m and 1000 m and the nonoily POME particles between 38 m and 1000 m.e size characteristics ( 16 ,  50 , and  84 ) and the statistical parameters ( g ,   ) are different between oily and non-oily POME particles.e POME particles have nonspherical irregular shape because of that aspect ratio (AR) and circularity as shape factors have been used to describe the degree of divergence of these natural sediments from spherical shape.e AR was found to be more relevant to POME �bres, and circularity shape factor was found to be more suitable for POME particles.e �bres found among these particles have different sizes and shapes such as simple �bre, branched �bre, and �a�y �bre.For oily POME sediments, the �bres are seen among particles sized 250 m ≤  O ≤ 1000 m.However, these �bres can be obscured when completely covered by the particles.Non-oily POME �bres can be observed among particle sized between 500 m and 1000 m.e oily POME particles have lower density than that of non-oily POME because of the adhered and embedded oil and grease within the particles.e oily POME sediment density was found to decrease with the increasing of size because of the increasing surface area of these particles and the increasing amount of attached oil.Conversely, the density of non-oily POME particles increased with the increasing of particle size.

F 2 :
Lognormal distribution plot of cumulative undersize percentage of non-oily POME particles.

F 5 :
(a)  O > 63 m (b)  O > 125 m (c)  O > 250 m (d)  O > 500 m (e)  O > 800 m (f)  O > 1000 m Presence of �bres among the oily POME particles observed form sieving at different opening size.T 5: Shape factors for oily and non-oily POME sediments.
T 1: Chemical properties of palm oil mill effluent (POME).
*Unit for all parameters is mg/L except pH.
combined with the statistical parameters   and   are summarized in Table4.It can be seen that the size of oily POME sediments at  5 ,  10 and  16 is bigger than that the non-oily POME sediments T 4: e size characteristic and statistical parameters extracted from PSD graphs of oily and non-oily POME sediments.
at the same particle size.iscouldbe due to the agglomeration of �ne oily sediments that produce �ocs of large sizes.On the other hand, the geometric median diameter ( 50 ) of oily POME sediments (355 m) is very much similar to the  50 value of non-oily POME sediments (325 m), and it ( 50 ) can be used to determine the settling velocity of the POME natural sediments in POME suspension.eaverageparticle size represented by the geometric mean diameter (  ) can be observed from Table4; the   for oily POME particles (322.5 m) is larger than   of non-oily POME particles (279.285m).