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Journal of soil science and plant nutrition

versión On-line ISSN 0718-9516

J. Soil Sci. Plant Nutr. vol.16 no.1 Temuco mar. 2016  Epub 18-Ene-2016 

Recycling pulp mill sludge to volcanic soil: a column leaching study


F. Gallardo1,3*, G. Briceño1,3, M.J. Flores3, M.C. Diez2,3

1Chemical Science and Natural Resources Department, La Frontera University, Avenida Francisco Salazar 01145, Temuco, Chile.

*Corresponding author:

2Chemical Engineering Department, La Frontera University, Avenida Francisco Salazar 01145, Temuco, Chile.

3Scientific and Technological Bioresource Nucleus, La Frontera University, Avenida Francisco Salazar 01145, Temuco, Chile.


A leaching experiment was conducted to determine the influence of four successive applications of pulp mill sludge on some parameters of an Andisol. Undisturbed soil columns were treated with pulp mill sludge (0 to 30 Mg ha-1) every three months during one year with a hydraulic regime of 1200 mm per year. Leachates of each treatment were analyzed periodically and, at the end of the period, columns were fractionated at three depths (0-20, 20-40 and 40-60 cm) and soil chemical parameters were analyzed. Sludge added successively to Andisol increased the nitrate concentration in leachates. After the fourth application of 30 Mg ha-1 sludge, nitrate leaching was about 1.7 times higher than in the control column, however, this level is still lower than the limit under Chilean regulations. Low ammonium concentrations were observed in leachates as a result of nitrification processes and retention by clay and organic matter in the Andisol. In fractionated soil columns, pH, organic matter, total nitrogen and phosphorus increased as the sludge rate increased,with the highest values found in the upper part of the columns. The concentration of nitrate and ammonium in soil columns followed an inverse pattern: while ammonium concentration was the highest at 0 to 20 cm, nitrate concentration was the lowest in that fraction; sludge addition caused a linear increase in ammonium and nitrate content at the three depths analyzed.

Keywords: Andisol, leaching, pulp mill sludge, repeated application

1. Introduction

The pulp and paper industry has become one of the most important industries in the world. However, this productive activity is directly associated with environmental problem related with wastewater generation, solid wastes including sludge generating from wastewater treatment plants and air emissions (Hossain and Ismail, 2015). Nowadays, application of industrial residues to soil is increasingly being investigated as an alternative option to chemical fertilizers and as a soil amendment. It may also reduce treatment and disposal costs, enhance soil fertility, improve growth conditions for crops and increase crop yields (Yuan et al., 2011; Gallardo et al., 2012; Masud et al., 2014). Chile is the second largest producer of cellulose in Latin America, with current production of 4.7 million tons per year. The biological wastewater treatment plants of the pulp and paper industry produce large quantities of sludge (0.2-1.2 kg dry matter (DM)/kg biological oxygen demand (BOD) removed), that require disposal. At present,the conventional uses of sludge include industrial use, landfill, combustion and composting for application to farmland (Sánchez et al., 2004). This sludge generally contains organic substances, such as cellulose, lignin and microorganisms; and inorganic substances, such as nitrogen (N), phosphorus (P), potassium (K), silt, clay, calcium carbonates and metals. Pulp mill sludge improves the physical, chemical and biological properties of soils; its use increases soil pH and organic matter (OM) content, improves soil structure, and consequently increases soil productivity (Aravena et al., 2007; Nunes et al., 2008; Gallardo et al., 2010). In addition, it facilitates nutrient transport, increases water-holding capacity and serves as a partial replacement for the most expensive chemical fertilizers. Sludge also may improve the cation exchange capacity and microbial activity of soils (Nunes et al., 2008; Gallardo et al., 2012).

Several researchers have demonstrated the beneficial effect of pulp mill sludge as a soil amendment. However, little information has been published on the application of pulp mill sludge to volcanic soil, and how soil and groundwater could be affected by repeated applications. The feasibility of applying sewage sludge to soils increases with both the amount of sludge applied and the number of re-applications (Egiarte et al., 2005). According to Egiarte et al. (2005), the ability of soil to assimilate N, the cumulative effect of nutrients and contaminants on the soil, the changes in the bioavailability of pollutants with time and the history of previous sludge applications must all be taken into account to ensure that repeated sludge applications are properly managed. On the other hand, Tamir et al. (2013) demonstrated that the addition of organic residues to enhance soil fertility results in N mineralization and transformation, which may have important effects on chemical processes in the amended soils.

In Chile, there are approximately 3.1 million ha of soil derived from volcanic ash (Andisol and Ultisol). Their main characteristics are acid pH, high extractable aluminum (Al) and high P fixation, which increases when the pH decreases (Mora et al., 2005), affecting soil productivity. In acidic soils moreover, soluble Fe, Al, and Mn are usually present in higher concentrations, and P reacts with these ions to produce insoluble compounds unavailable to plants (Gallardo et al., 1999). Moreover P availability decreases due to P fixation by hydrous oxides of Fe and Al and also by adsorption. The use of pulp mill sludge may improve the content of available P (Gallardo et al., 2012), by increasing soil pH and thus reducing Al saturation. According to Masud et al. (2014), Al toxicity and infertility are the main factors limiting plant growth and development in acidic soils.

Pulp mill sludge has a high N content that may be transformed into plant-available forms, either as ammonium (NH4+-N) or nitrate (NO3--N) (Ribeiro et al., 2010). In general, NH4+-N is tightly bound by the negatively charged surfaces of clay complexes and OM, and thus immobilized. By contrast, NO3-- N with a negative charge can readily be lost from negatively charged soils, resulting in environmental contamination (Gilmour et al., 2003; Ribeiro et al., 2010). Large amounts of N applied in paper mill sludge could potentially cause NO3--N leaching (Feldkinchner et al., 2003). Therefore, in order to protect soil and humans from pollution, the recommended sludge dose should set so as to avoid excessive leaching of NO3-- N into groundwater and excessive loading of heavy metals and organic pollutants in soils (Egiarte et al., 2005). The environmental risks of pulp mill sludge may be estimated by monitoring leaching of certain elements that can contaminate groundwater. The aim of this work was therefore to evaluate the nutrient supply to soil, potential for NO3-- N and NH4+-N leaching, and P availability with successive applications of pulp mill sludge in increasing doses to an Andisol.

2. Materials and Methods

2.1. Soil and sludge

The sludge used in this assay was obtained from a bleached Kraft pulp mill wastewater treatment plant (aerated pound) and, was collected from a landfill one year after disposal. Under this condition, sludge becomes stable naturally and complies with the requirements established in Chilean Standard NCh 2880 (INN, 2004). The soil used for this study was an Andisol derived from volcanic ash, belonging to the Freire Series, located in Southern Chile (38°42,S, 73°35,W) at Maquehue Experimental Station of Universidad de la Frontera. The soil sample for the characterization was taken from 020 cm depth, air dried at room temperature, sieved through a 2 mm mesh and stored in plastic bags under refrigeration (4 °C).

2.2. Soil column assay

The leaching assay was carried out using undisturbed soil. PVC cylinders (diameter 14 cm, height 60 cm) were buried in the soil, using a hydraulic device which pushed the cylinder down by small increments to minimize disturbance to the soil structure inside. Once the cylinders reached the desired depth (60 cm), they were lifted out of the collection site and taken to the laboratory without disturbing the soil. A plastic disc was used to close the lower end of each cylinder and an L-shaped tube (1/4 inch diameter) was placed in the center of the plastic disc to receive the leachates. The columns were installed in the laboratory under controlled temperature conditions (20 ± 2°C). The sludge was applied to the upper part of the columns, in the range 0 to 30 Mg ha-1 (DW sludge equivalent), and gently mixed with the first 5 cm of soil. Re-applications were made every three months until one year of treatment. There were three replicates per treatment and columns without sludge addition were used as controls. Distilled water was added daily to each column, simulating a rainfall regime of 1200 mm per year. Leachates were collected every week and stored under frozen conditions (12 subsamples per application). After the last subsample was collected, the frozen subsamples were defrosted and homogenized for the analysis of N (in the form of NO3--N and NH4+-N), pH and P. At the end of the experiment, the soil was extracted from the cylinders and was fractionated in depths from 0-20, 20-40 and 40-60 cm. These samples were air dried and sieved through a 2 mm sieve and analyzed to determine pH, OM, available P, total N, NO3--N and NH4+-N.

2.3. Analyses of soil, sludge and leachates

Soil samples and sludge were analyzed according to Sadzawka et al. (2004). Organic carbon (OC) content was determined by the dichromate oxidation method and colorimetric determination of the reduced chromate (Cr+3); OM content was obtained by multiplying the OC content by the factor 1.724; pH was measured in 1:2.5 (w/v) soil/water mixture; total N content by the Kjeldahl method; mineral N (NO3--N and NH4+-N) by extraction with KCl 2 M and titration with HCl and specific electrode respectively; and available P by extraction with sodium bicarbonate (0.5 M, pH 8.5) and determined colorimetrically with the molybdate-ascorbic acid method. Available macro and microelements were determined by atomic absorption spectrophotometry (Shimadzu GBC SensAA). Ca, Mg, K, and Na were quantified after extraction with ammonium acetate 1 M, pH 7.0; Fe, Mn, Cu and Zn were quantified after extraction with a solution composed of DTPA (diethylenetriaminepentacetic acid), calcium chloride and TEA (triethanolamine) buffered at 7.3 pH; and Al was quantified after extraction with KCl 1M (1:10 soil/solution ratio) by shaking for 24 h. All the analyses were done in triplicate. In the column leachates, pH was measured by potentiometry; NO3--N and NH4+-N by specific electrode after Kjeldahl distillation, and P was determined by the colorimetric method.

2.4. Statistical analysis

All the experiments were carried out in triplicate. The data were statistically analyzed by one-way analysis of variance (ANOVA), where statistical differences were observed, and means were separated using Tukey,s minimum significant difference test (p< 0.05).

3. Results and Discussion

3.1. Chemical characteristics of soil and sludge

The principal chemical characteristics of the soil and sludge used in this study are shown in Table 1. This soil presented typical characteristics of Andisols, with moderate acidity (pH 5.5) resulting in low Al saturation probably due to changes in the equilibrium of neutral species of Al (Al(OH)3) as pH increased. The soil presents basal levels of N, P and K for crop production, therefore, no supplements of these elements were considered in this study. The sludge presented a higher content of OM (68%), macronutrients (N, P, K, Ca and Mg) and micronutrients (Mn, Cu and Zn) than the soil. Conversely, the contents of Fe and Al were lower than in the soil. Considering the properties of the sludge, it was expected that its addition would help to improve soil characteristics and hence fertility. The application of recycled paper mill sludge with similar values of pH and OM that the sludge used in this study has been associated with substantial benefit such as increases the OM and other essential nutrients, and soils pH neutralization specially in acid soils (Abdullah et al., 2015). Previous studies done in our research group have shown positive effects on the physical, chemical and biological properties of several Andisols amended with pulp mill sludge (Aravena et al., 2007; Gallardo et al., 2007 and Gallardo et al., 2012). However, it is important to establish the best sludge-soil dosage in order to minimize the inhibitory effect on mobility of some elements through the soil due to successive applications.

Table 1. Chemical characteristics of soil (Freire Series) and pulp mill sludge.

CEC= Cation exchange capacity (Σ Ca, Mg, K, Na)
Aluminum saturation (%)= [Al/(Σ Ca, Mg, K, Na and Al) * 100]
* Available elements
Values for sludge and soil are in base to dry weight
Values are the average of three replicates (n=3)

3.2. Analysis of pH in leachates

The pH values obtained from leachates after four successive sludge applications (0 to 30 Mg ha-1) during a year were no different from those of leachates obtained from the control treatment without sludge (Figure 1); no significant differences (p<0.05) between treatments were obtained after the applications. In general, pH values increased with both the dose and the number of applications, due to the contribution of Ca, Na, Mg and K present in high concentrations in the sludge used in this study (Table 1). The main increase in pH was observed after the fourth application at a rate of 30 Mg ha-1 sludge with a pH value close to 6.2. Nevertheless, these pH values are within the limit set for irrigation water according to the Chilean Standard NCh 1333.Of 78 (pH 5.5-9.0). Therefore the pH values of the leachates in the soil columns amended with sludge do not constitute an environmental risk. Similar behavior was reported by Santibanez et al. (2007) using sewage sludge for mine tailing re-vegetation.

Figure 1. pH values of leachates collected after four applications of sludge (0 to 30 Mg ha-1) during a year. Bars indicate standard error of the mean (n=3)

3.3. Analysis of nitrate, ammonium and phosphorus in leachates

The descent of water through soil can result in leaching of N, usually in the form of NO3--N. This can have adverse effects on the environment, such as groundwate contamination. However, the use of organic residues in agriculture, with appropriate management, can help to lower NO3-- N leaching as compared with application of N in mineral form (Carneiro et al., 2012).We evaluated the effect of successive applications of pulp mill sludge to soil on NO3-- N leaching after one year of treatment. The results are shown in Figure 2.

Figure 2. Nitrate concentration in leachates collected after four applications of sludge (0 to 30 Mg ha-1) during a year. Bars indicate standard error of the mean (n=3). Means with different letters differ significantly by Tukey test (p< 0.05)

In general, the NO3-- N content rose with the increase of both the dose and the number of applications. There was no significant difference (p<0.05) in NO3-- N content between treatments for the first application of sludge, with average of about 15 mg L-1 of NO3-- N in the leachates. However, the NO3-- N concentration gradually increased from 15 to 26 mg L-1 when higher levels of sludge application were used and after the fourth application. According to FAO guidelines, a concentration between 5 mg L-1 and 30 mg L-1 NO3--N is permitted for irrigation water; this means that a fourth application of sludge at a rate of 30 Mg ha-1 could represent some risk whenever the NO3-- N concentration in the leachate approaches the upper limit. NO3-- N is readily dissolved in soil water, therefore it will move with water. The high available N content in sludge (586 mg kg-1), the high irrigation rate applied to the columns, and the negative charge of NO3-- N (preventing its adsorption on soil adsorption sites), are all factors that could contribute to NO3--N leaching after the addition of sludge to soil. Although the level of NH4+-N in the leachates of the various treatments presented significant differences (p<0.05), it was low in all treatments (<1.8 mg L-1) (Figure 3). When sludge was added at rate of 10 Mg ha-1, the concentration of NH4+-N was the highest after the first application, whereas no differences were observed after the second, third and fourth applications. When sludge was added at rate of 20 Mg ha-1 however, a high NH4+-N concentration in leachate was observed after the second application (1.6 mg L-1); the values for NH4+-N decreased to approx. 0.9 mg L-1 after the third and fourth applications. When sludge was added at rate of 30 Mg ha-1, the concentration of NH4+-N was higher after the first, second and fourth applications than the third application. The variations in NH4+-N values in leachates are associated with the nitrification process, as we can confirm by the increase of NO3-- N in the leachates (Figure 2). Similar results have been obtained by other researchers who have investigated the effect on N of repeated applications of sludge (Burton et al., 1990; Egiarte et al., 2005; Santibanez et al., 2007). In our study, the highest NH4+-N concentrations (1.7 mg L-1) were observed for the treatments that received 20 Mg ha-1 and 30 Mg ha-1 sludge. Finally, in the control column (without sludge), the NH4+-N concentration diminished strongly from 0.9 to 0.1 mg L-1, due to nitrification resulting from the high biological activity generally detected in Andisols, and the high irrigation rate (1200 mm per year). Nitrogen mineralization and nitrification increased with irrigation.

Figure 3. Ammonium concentration in leachates collected after four applications of sludge (0 to 30 Mg ha-1) during a year. Bars indicate standard error of the mean (n=3). Means with different letters differ significantly by Tukey test (p< 0.05)

The presence of P in the leachates was also analyzed; however it was not detected after four sludge applications at dosage of 0 to 30 Mg ha-1 over one year of treatment. The P is extremely immobile in soils, especially those with acidic pH such as Andisols where the bulk of P accumulates as macromolecular-P complexes closely associated with OM, possibly through Al and Fe bridges (Redel et al., 2007).

3.4. Analyses of soil in fractionated columns

The positive effect of the application of pulp mill sludge to soils increases with the amount of sludge applied (Gallardo et al., 2007, Nunes et al., 2008; Ribeiro et al., 2010; Gallardo et al, 2012). However, to ensure that repeated sludge applications are managed sustainably, the cumulative effect of nutrients and contaminants on the soil must be taken into account.

In this study, we used an Andisol characterized by high P adsorption capacity, low base saturation percentage, high levels of exchangeable Al, and moderate acidity (Gallardo et al., 1999; Mora et al., 2004). Phosphorus in these soils is usually stored in a non available form for plants, and P fertilization is required to obtain a higher crop yield (Mora et al., 2004). Taking into account the characteristics of the sludge (rich in OM, N and P), it could be an appropriate option for improving the soil characteristics and productivity of Andisols while reducing the use of inorganic fertilizers.

Values of pH, OM, total N and available P in the fractionated soil columns after four successive applications of sludge to soil during one year are shown in Table 2. The soil pH increased significantly (p<0.05) as the depth in the soil column increased from 0 to 60 cm. The average pH values were 5.26, 5.52 and 5.72 at 0-20, 20-40 and 40-60 cm depth respectively. Significant differences (p<0.05) were observed with the increasing dose of sludge at depth 0-20 cm; however, no significant differences (p<0.05) were observed due to the increasing sludge dose applied at depths of 20-40 and 40-60 cm. In general, the use of inorganic NH4+-N fertilizers increases acidification in Chilean Andisols, due to nitrification by free H+ release (Mora et al., 2004, Mora et al., 2007). In our study, Freire soil was not acidified after four successive applications of sludge. On the contrary, a slight increase in pH was observed in the upper soil fractions; the production of H+ through nitrification was probably compensated by the large amounts of bases added with the sludge, almost ten times higher than in soil (Table 1).

Table 2. Organic matter (OM), pH, total nitrogen (N), available phosphorus (P), nitrate (NO3--N) and ammonium (NH4+-N) concentrations in fractionated soil columns (0-20, 20-40 and 40-60 cm depth) with pulp mill sludge (0 to 30 Mg ha-1) after a year of leaching experiment

Same letters in the same column indicate no significant difference at p<0.05.

Values are mean ± standard error of the mean (n= 3)

Organic matter content was highest at the top of the columns, decreasing as the column depth increased. However, an increment of OM content in the first fraction of the soil columns (0-20 cm) was observed as the sludge application increased, and significant differences (p<0.05) were found between the control (without sludge) and the highest sludge dose applied (30 Mg ha-1). Soil analysis in the 20-40 and 40-60 cm fractions showed no significant differences when different doses of sludge were applied compared with the control at each depth. The OM content changed from 12.46% average at 0-20 cm depth to 3.24% average at 40-60 cm depth.

A similar tendency was found for total nitrogen values in sludge-amended soil columns. The highest values of N were found at the top of the columns and the lowest values at the bottom. Total N in fractionated soil columns was not modified significantly by the addition of sludge. At 0-20 cm depth the total N concentration was in the range of 0.7to 0.8% when sludge was applied at a rate of 0 to 30 Mg ha-1, whereas the total N concentrations at 20-40 and 40-60 cm depth were about 0.4% and 0.24% respectively. The phosphorus content in the 0-20 cm fraction increased from 17.60 to 24.60 mg kg-1 with the different sludge doses (addition of sludge at 10 to 30 Mg ha-1 respectively). The concentration of P was more than 20 times higher in the upper part of the soil column and the addition of sludge did not influence the P content below 20 cm depth. P accumulated markedly in the first 20 cm of soil,due to the high P content (313 mg kg-1) in the sludge and the low mobility of this element, especially in acidic soils. Phosphorus accumulation is an important aspect in Andisols, since they have a strong capacity for fixing P associated with acidic pH and high aluminum saturation percentage (Gallardo et al., 1999). In general, there is an inverse relationship between pH and Al saturation as well as between P availability and Al saturation percentage in Andisols (Mora and Demanet, 1999). Small increments in pH due to the addition of pulp mill sludge to Andisols cause important reductions in the aluminum saturation percentage (Gallardo et al., 2007). On the other hand, Nunes et al. (2008) reported that the addition of secondary paper mill sludge produced a linear increase in available P in the soils used in their study. The reduced P adsorption and increased P availability following applications of organic residues to soil can be a consequence of several mechanisms: release of inorganic P from decaying residues, blockage of P adsorption sites by organic molecules released from the residues, a rise in soil pH during decomposition and the complexation of soluble Al and Fe by organic molecules (Haynes and Mokolobate, 2001; Pypers et al., 2005).

The nitrate content in fractionated soil columns increased with the increasing dose of sludge. The NO-N content also increased with the soil depth (Table 2). In general, the NO3--N concentration at the highest application of sludge was twice that in the control, and was associated with the level of NO3--N found in leachates (Figure 2). At 020 cm depth, the NO3--N concentration increased from about 70 mg kg-1 (control) to 185 mg kg-1 at a rate of 30 Mg ha-1. At the same rate of sludge application, the increase of NO3--N concentration was from 108 mg kg-1 to 235 mg kg-1 at 20-40 cm depth, and from 118 mg kg-1 to 264 mg kg-1at 40-60 cm depth. As it is highly water soluble, nitrate is the most polluting inorganic N compound. In our study, up to 25 mg L-1 were measured in leachate after application of 30 Mg ha-1 sludge (Figure 2).Consequently, the highest concentration of NO3-- N was found in the deepest fraction of the soil columns and is associated with the highest sludge dosage (Table 2). Nitrate was found to have lixiviated from the soil columns due to its high solubility and negative charge, which prevent its adsorption by soil exchange sites; lixiviation was further influenced by the high water regime (1200 mm per year) and by the type of soil used (Andisol). A history of previous sludge application has been shown to affect the potential for nitrate leaching. Some authors have reported that greater nitrate leaching losses have been observed in sites subjected to reapplication of sludge than in sites in which no sludge had previously been applied (Burton et al., 1990; Mitchell et al., 2000).

The concentration of NH4+-N was highest in the upper part of the soil columns and lowest at the bottom, and increased as the sludge application rates rose in each column profile (Table 2). An accumulation of NH4+-N was observed in the 0-20 cm fraction of the soil columns, increasing from 7.9 mg kg-1 in the control column to 12.4 mg kg-1 in the soil column with the addition of sludge at a rate of 30 Mg ha-1. On the other hand, as the soil depth increased, the values for NH4+- N also decreased, ranging from 1 mg kg-1 in the control soil columns to2.47 mg kg-1 in columns with sludge addition at a rate of 30 Mg ha-1.

The fact that the lowest NH4+-N concentration is found in the deepest fraction may be explained by the high affinity of this cation for negatively charged surfaces. This has a strong effect in Chilean Andisols, in which the typical zero point charge is around 4.04.5 (Mora et al., 2004). This cation is adsorbed on soil colloids by cationic exchange, or on soil organic matter. Besides, the existence of a high population of nitrifiers in soils -induced by previous sludge application - promotes the transformation of NH4+-N to NO3--N. Similar results have been obtained by other researchers who have investigated the effect of repeated sludge applications on N processes (Burton et al., 1990).

The concentrations of NO3--N and NH4+-N in soil columns in the same fraction followed an inverse pattern (Table 2). Whereas the NH4+-N concentration was highest in the 0 to 20 cm fraction, the NO3-- N concentration was lowest at the same depth. Pulp mill sludge addition produced a linear increase in the NH4+-N and NO3--N content in fractionated soil columns (Figure 4a and b) at the three depths analyzed. The NH4+-N content in the three column profiles was strongly correlated to sludge concentration, with r2>0.98 in all cases. Asimilar situation was found for the NO3--N content, with r2>0.95 (Figures 4a and b).

Figure 4. Relation of nitrate (a) and ammonium (b) content with the soil depth in the fractioned column after sludge application

4. Conclusions

The results of this study indicated that repeated applications of pulp mill sludge to Andisol soil contributes to an increase in nitrate and ammonium concentrations in leachates, whereas the pH and phosphorus in soil column leachates were not affected by the sludge. Pulp mill sludge improved some properties of the Andisol, increasing the organic matter and phosphorus concentrations in soil columns slightly as the sludge rate increased, mainly in the upper part of the columns. Sludge increased nitrate and ammonium contents. However, these two parameters presented inverse patterns across the depth profile: the ammonium concentration was highest in the 0 to 20 cm fraction, while the nitrate concentration was lowest in the same fraction. Sludge addition produced a linear increase in NH4+-N and NO3--N content at the three depths analyzed (r2>0.95). While sludge addition improved the availability of nutrients in the soil, the risk of nitrate lixiviation is high if the sludge rate and periodicity is not regulated, and may present some threat to water quality.


This study was supported by DIUFRO DI11-1001, INNOVA-CORFO N° 09FCO2-6021 and CONICYT/FONDAP/15130015 projects.


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