Article pubs.acs.org/est
Physical Disintegration of Toilet Papers in Wastewater Systems: Experimental Analysis and Mathematical Modeling Beytullah Eren† and Fatih Karadagli*,† †
Sakarya University, Faculty of Engineering, Department of Environmental Engineering, Esentepe, Sakarya, Turkey, 54187 S Supporting Information *
ABSTRACT: Physical disintegration of representative toilet papers was investigated in this study to assess their disintegration potential in sewer systems. Characterization of toilet papers from different parts of the world indicated two main categories as premium and average quality. Physical disintegration experiments were conducted with representative products from each category according to standard protocols with improvements. The experimental results were simulated by mathematical model to estimate best-fit values of disintegration rate coefficients and fractional distribution ratios. Our results from mathematical modeling and experimental work show that premium products release more amounts of small fibers and disintegrate more slowly than average ones. Comparison of the toilet papers with the tampon applicators studied previously indicates that premium quality toilet papers present significant potential to persist in sewer pipes. Comparison of turbulence level in our experimental setup with those of partial flow conditions in sewer pipes indicates that drains and small sewer pipes are critical sections where disintegration of toilet papers will be limited. For improvement, requirements for minimum pipe slopes may be increased to sustain transport and disintegration of flushable products in small pipes. In parallel, toilet papers can be improved to disintegrate rapidly in sewer systems, while they meet consumer expectations.
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INTRODUCTION Flushable consumer products (FCPs) such as toilet tissue, tampons, and wet wipes are commonly used around the world, and their consumption is increasing gradually. They are disposed of into wastewater collection systems, where they blend with other waste materials like human excreta; food waste, particularly, fat, oil, and grease (FOG); detergents; cleaning agents; pharmaceuticals; personal care products; and cosmetics.1−3 Because FCPs can absorb most of these waste materials, they play a central role in sewer processes, and subsequently, their transport and disintegration are affected adversely. In sewer networks, FCPs are assumed to move along with wastewater to treatment plants; however, their transport depends on factors such as pipe shape and slope, wastewater flow rate and velocity, frequency and amount of product discharge, and disintegration characteristics of products.4 Under unfavorable conditions, FCPsrelatively large solidshave the potential to accumulate and initiate pipe blockages particularly in small diameter pipes of building drains where low flow conditions are observed frequently.5,6 In main sewer pipes, accumulation of surface debris because of heavy rainfall and root penetration are among the main causes of blockages that lead to sanitary sewer overflows (SSOs).6 FCPs play a role in SSOs as large solids that move and disintegrate slowly in sewer pipes. Annual averages of 50 000 SSOs and 400 000 backups in basements occur as a result of pipe blockages in the United States of America (USA).6,7 Subsequently, SSOs lead to financial problems and significant public health concerns with © 2012 American Chemical Society
outbreaks of diseases such as chlorea, giardasis, cryptosporidiosis, and hepatitis.8 FCPs also are involved in chemical and microbial interactions in wastewater. For instance, absorption of hydrophobic substances like FOG by FCPs can lead to formation of FOG deposits that are critical causes of sewer blockages.3,9 FOG contains long-chain fatty acids that participate in complexation/ precipitation reactions with cationic species such as Ca2+ and Mg2+ that are commonly found in wastewater or that are released by microbial corrosion of concrete pipes.3,9,10 Depending on concentrations of reactants and pH, metal− fatty acid complexes form solid particles that initiate FOG deposits in sewer pipes.11,12 In this process, FCPs can accumulate fatty acids through absorption and can facilitate complexation/precipitation reactions with Ca2+ and Mg2+ in wastewater. Conversely, physical disintegration of FCPs produces small-size solids that can flow with wastewater. These solids can absorb long-chain fatty acids and can convey them to treatment plants and can minimize formation of FOG deposits. As indicated by this example, FCPs play key roles in sewer systems, and the mechanisms responsible for their transport and disintegration as well as chemical and microbial interactions must be addressed systematically. Elucidation of Received: Revised: Accepted: Published: 2870
October 13, 2011 January 30, 2012 January 31, 2012 January 31, 2012 dx.doi.org/10.1021/es203589v | Environ. Sci. Technol. 2012, 46, 2870−2876
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mathematical models of sewer networks are improved to incorporate these processes and to predict the outcome of various scenarios successfully.16,17 Collectively, these studies addressed pollution effects and control strategies of suspended solids in sewer lines. A few research groups concentrated on large solids and studied their transport in sewer pipes. Large solids are defined as those with any dimension >8 mm,18 and previously, as >6 mm.19 They are described as those from garbage grinders in kitchens and from sanitary sources including tampons, toilet papers, sanitary towels, and feces.20 Littlewood and Butler5 studied their transport in small diameter pipes of 50−150 mm, like household connections. Their experimental results showed that they are conveyed with a flush of water to a certain distance in drains. The following flushes act like waves and move solids for short distances.4,5 Subsequently, solids move like a sliding dam which is a common transport mechanism of FCPs in small pipes under low flow conditions. A similar study by Walski et al.18 showed that movement of a large solid is affected by vertical dimension and specific gravity of that solid as well as by depth of water flow in the sewer pipe. On the basis of these parameters, they developed a method to estimate minimum flow velocities required for solid movement. Their experimental results indicated that velocities of 0.6 to 1 m/s are sufficient for most large solids to move in drains and sewer pipes. Karadagli et al.21 developed a theoretical approach for physical disintegration of FCPs. The physical breakup of a large solid depends on turbulence in water and solid characteristics such as mechanical strength, size, and density. Turbulence can be represented with Reynold’s number, while characteristics of a solid product are represented with a specific disintegration rate coefficient. They tested this approach experimentally with two different cardboard tampon applicators of the same manufacturer. Their experimental and modeling results showed that the approach was correct and can be used to capture disintegration patterns of FCPs. The physical disintegration theory is described mathematically by eq 1 as follows
these issues will provide long-term sustainability of wastewater collection and treatment systems. Among the flushable products, toilet papers are consumed more frequently and in more amounts than any other product; thus, they make up the largest proportion of FCPs in sewer networks. They are manufactured from cellulosic fibers with addition of various chemicals such as binders, fillers, adhesives, color pigments, and scents. They easily absorb constituents of wastewater and participate in sewer processes. However, not much is known about their fate in building drains and main sewer lines. Therefore, we studied their physical disintegration as the first step toward understanding their fate and impact in wastewater systems. We collected toilet papers from different parts of the world, and we identified their characteristic properties such as paper density, number of microscopic layers, paper thickness, and specific volume. On the basis of these parameters, we categorized the products as average and premium quality, and we selected our test products as those with critical and representative values from each category. The systematic characterization of toilet papers provided preliminary information about material composition and disintegration potential of each product, for example, dense product and slow disintegration. Thus, our first objective was to present that characterization of flushable products is useful and crucial to foresee how FCPs will break up in sewer lines. The second objective of this study was to identify disintegration rate coefficients for toilet papers. These kinetic coefficients are fundamental parameters of the disintegration process and are essential to elucidate the fate of FCPs in sewer systems. The third objective of this work was to estimate fractional distribution ratios among various size ranges. Physical disintegration produces small paper pieces in various shapes and sizes. Fractional distribution ratios specify how small pieces are allocated into proper size ranges. For instance, 40% of the main product is released as very small solids. The fourth objective of this study was to use our experimental and modeling results to assess disintegration potential of toilet papers in sewer pipes. We compared turbulence level of our experimental conditions with those of representative partial flow conditions in drain and sewer pipes. This comparison pointed out critical branches of sewer networks that should be improved for transport and disintegration of FCPs. In line with this assessment, the final objective of this study was to identify critical FCPs and to provide quantitative environmental data to toilet paper manufacturers for sustainable product design. By using our quantitative results, we compared toilet papers with each other and with tampon applicators that were studied previously. We identified the products with slow disintegration potential as they need improvement in terms of environmental concerns. Our interpretations of the results indicate a new direction to improve existing products and to develop a new generation of toilet papers that will meet consumer expectations and that will disintegrate rapidly in sewer networks.
R dis =
dM dC =V = kdis × Re × C × V dt dt
(1)
where Rdis = the rate of change in mass of the solid product because of physical disintegration (mg/s), M = mass of the product (mg), (dM/dt) = unit change in mass of the product (mg) per unit time (s), V = liquid volume containing the solid product (L), C = concentration of the solid product (mg/L), kdis = disintegration rate coefficient (1/s) that depends on the solid’s mechanical properties, and Re = Reynold’s number representing turbulence in water (dimensionless). When the liquid volume is constant over time, the total mass (M) of a solid in a system equals the product C × V, and therefore, the total mass in the system can be represented as mass concentration. In this work, we used the theoretical approach outlined by eq 1 to investigate physical disintegration of toilet papers. We characterized toilet papers and selected representative test products. We conducted disintegration experiments by adapting standard protocols to toilet papers with improvements. We developed a mathematical model to predict disintegration of toilet papers under various flow conditions. We simulated our experimental results with our model to estimate kinetic coefficients and fractional distribution ratios for
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BACKGROUND Although toilet papers have been disposed of into sewer systems for quite some time, limited information is available about their fate in sewer networks in the current literature. Most of the past studies on sewer solids focused on suspended solids that originate from sanitary sources and surface runoffs.13−15 They identified the mechanisms and conditions responsible for deposition, microbial degradation, erosion, and transport of suspended solids in sewers. Concomitantly, 2871
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Experimental Procedures. We characterized sample products on the basis of their sheet mass, paper density, number of microscopic layers, paper thickness, and sheet volume. Some of these properties are measured experimentally, while others are computed from the measurements. For instance, sheet mass is measured by weighing a sheet before and after drying at 104 °C for 24 h, and then, moisture content is estimated from weight differences. Paper density, mass per unit surface area, is estimated by dividing sheet mass with surface area. Sheet volume is reported as the product of surface area and paper thickness. Specific volume, paper volume per unit mass, is estimated as sheet volume divided by sheet mass. The number of microscopic layers in each sheet and their thicknesses are identified with the microscope Leica VMHT MOT (Leica Microsystems GmbH, Wetzlar, Germany). Details of microscopic paper thickness measurements, the results from physical characterization of toilet papers, and the systematic selection of our test products are presented in detail in the Supporting Information (SI) section of this manuscript. For the disintegration experiments, we used 1 sheet per 1 L water to represent per capita daily discharge rate of toilet papers and water. Daily consumption of water and toilet papers in water closets (WCs) was quantified by surveying number of households in the United Kingdom.23 The results showed that average of 11.7 sheets, ca. 12 sheets, and 30 L of water are discharged from toilets per capita per day, which corresponds to 0.4 sheets per litre of water. However, maximum daily discharges can reach up to 25−30 sheets along with 30 L of water to give a ratio of 1 sheet per 1 L of water as in our experiments. We conducted disintegration experiments according to standard protocols.22 We prepared identical mixtures of 1 sheet/L tap water in 2 L beakers that were fastened on a rotary shaker table with an orbital diameter of 25 mm (OS-20 shaker table, Biosan Research Technologies, Riga, Latvia). We rotated the mixtures at 200 rotations per minute (rpms) and collected samples at appropriate time intervals. For sampling, we removed a flask and passed all of its contents through a series of standard sieves. Standard protocols indicate that 8, 4, 2, and 1 mm sieves should be used for this step; however, separation of paper pieces was difficult when mesh sizes of sequential sieves were close to each other, for example, 1 and 2 mm. Therefore, our preliminary trials showed that standard sieves of 11.2, 8, 4.76, and 2 mm serve well for toilet papers. Accordingly, we improved the standard protocols with identification of proper sieves in this step. The standard protocols also indicated that sieves should be rinsed gently with a hand-held spray nozzle for solid removal. However, wet and delicate pieces of toilet papers broke up easily when sieves were rinsed with a spray nozzle. Thus, we placed each sieve in a traylike container that was partially filled with water. We swirled the sieve slowly in water and examined solids visually to ensure that they belonged to the corresponding size category. This visual examination was a critical step and an improvement of standard protocols to obtain reliable and accurate results. Then, we transferred the material on each sieve to the water body of traylike container. We filtered this mixture under vacuum with a filter paper with pore sizes of 1−2 μm to capture solids, and we determined the mass of solids from weight differences before and after drying at 104 °C. To use our experimental results to evaluate actual situations in sewer lines, we estimated Re for the experimental conditions and compared it with Re values for representative flow
the test products. We identified disintegration patterns of toilet papers and compared them with those of cardboard tampon applicators to identify critical FCPs with potential to persist in sewer lines. We compared turbulence level in our experimental setup to those in actual sewer pipes via our estimated Re values for each flow system.
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MATERIALS AND METHODS Mathematical Model. Equation 1 is the basis of our mathematical model. We extended this equation to account for all solid sizes produced during disintegration. We established solid size categories of (>11) mm, (8−11) mm, (5−8) mm, (2−5) mm, and (11)mm] = −k1Re[(> 11)mm] dt
(2)
d[(8−11)mm] = (f1 )k1Re[(> 11)mm] dt − k2Re[(8−11)mm]
(3)
d[(5−8)mm] = (f2 )k1Re[( > 11)mm] dt + (f5 )k2Re[(8−11)mm] − k3Re[(5−8)mm]
(4)
d[(2−5)mm] = (f3 )k1Re[( > 11)mm] dt + (f6 )k2Re[(8−11)mm] + (f8 )k3Re[(5−8)mm] − k4Re[(2−5)mm]
(5)
d[( 11)mm] dt + (f7 )k2Re[(8−11)mm] + (f9 )k3Re[(5−8)mm] + k4Re[(2−5)mm]
(6)
where the concentration of solid mass in the specified size range is represented in [...] in units of (mg/L). The disintegration rate coefficient, kdis of eq 1, is shown here with ki in units of (1/h) for the specified size range as 1 for (>11) mm, 2 for (8−11) mm, 3 for (5−8) mm, and 4 for (2−5) mm. The distribution ratios, f j, of intermediate size solids are indicated with subscripts as shown in eqs 2 − 6. For example, eq 4 shows that disintegration of the (>11) and (8−11) mm size solids produces (5−8) mm solids at ratios of f 2k1Re[(>11)mm] and f5k2Re[(8−11)mm]. The rate at which the (5−8) mm solids disintegrate is represented with k3Re[(5−8)mm] in the same equation. We estimated values for all (kiRe) and f j parameters by best-fitting our experimental results from batch system with the model predictions. 2872
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conditions in sewer pipes. To estimate Re for the experimental system, rotational water motion is simulated by using computational fluid dynamics (CFD) software, FLOW-3D version 7.2 (Flow Science Inc., Santa Fe, New Mexico). The general outline of rotational flow simulations and details of Re computations for sewer flows are provided in the SI. FLOW-3D simulations will be published as a separate manuscript shortly after this work.
disintegration potential, and their exact disintegration patterns can be identified with experimental results as presented below. Table 2 shows our experimental results and mass balance closures for disintegration of TP-1. Its initial mass, 440 mg, is Table 2. Experimental Results and Mass Balance Closures for TP-1
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RESULTS AND DISCUSSION We categorized toilet papers systematically as premium and average quality on the basis of their properties presented in Tables S1 and S2 of the SI. Characteristic properties of average quality products ranged as sheet mass of 350−500 mg per sheet, paper density as area density of 33−45 g/m2, two superimposed microscopic layers, sheet thickness of 75−175 μm, and sheet volume of 800−2100 mm3. Alternatively, the same values for the premium products were sheet mass of 550− 720 mg per sheet, paper density of 50−57 g/m2, three microscopic layers, sheet thickness of 165−250 μm, and sheet volume of 2000−3000 mm3. As the next step, we selected representative products from each category for experimental testing. Table 1 presents
product name characteristic parameter
TP-1
TP-2
465 440 5.4 120 × 100 12 000 39 2 85−90 175 2100 4.8
720 680 5.5 125 × 100 12 500 57 3 65−70 200 2500 3.7
(>11) mm (mg)
(8−11) mm (mg)
(5−8) mm (mg)
(2−5) mm (mg)
(11) mm size solids, we first fit (k1Re) for this curve because disintegration of the main product does not depend on smaller sizes. Then, we fit k2Re, k3Re, and k4Re for (8−11) mm, (5−8) mm, and (2−5) mm solids, respectively. This initial adjustment of the kiRe values matched the width of a model curve to that of experimental data. Then, we matched peak heights by adjusting the f j values. This systematic approach allowed us to find good fits for all of the size categories. Figure 1a shows the results for the main product, (>11) mm, and the next two size categories, (8−11) and (5−8) mm. The main product disintegrates by 50%, 90%, and 100% in 3, 12, and 24 h, respectively. The (8−11) mm solids are formed subsequently with a peak value of 115 mg/L at 8 h, when 85% of the main product disintegrated. Similarly, (5−8) mm solids are produced next with a peak concentration of 100 mg/L around 12 h. This concentration is maintained until 30 h because of continuous disintegration of (>11) and (8−11) mm size solids. Figure 1b shows the results for (2−5) mm and (11) mm (mg)
(8−11) mm (mg)
(5−8) mm (mg)
(2−5) mm (mg)
(11) mm, and the next intermediate size solids, (8−11) mm, disintegrated completely. Figure 2 presents our experimental and modeling results for TP-2. Figure 2a shows that TP-2 disintegrates completely in 48 h, whereas TP-1 disintegrated in 24 h (Figure 1a). The (8−11) mm solids that are formed with a peak value of 120 mg/L at 14 h disintegrate by 96 h. The (5−8) mm solids follow a similar pattern as their peak concentration, 130 mg/L at 48 h, decreases slightly to 110 mg/L at 96 h. Figure 2b shows that the (2−5) mm solids are produced gradually with a maximum concentration of 150 mg/L. In comparison, (
