Potential use of sludge cake from paper mill wastewater treatment as degradable flower pot

Abstract

Objectives

Sludge cake produced in paper mill industries is disposed into a landfill and may cause the environmental and health problems. Now many researchers have tried to recycle rigid materials from it for the purpose of decreasing its volume. The aims of this study were to clarify three hypotheses: (1) whether a flower pot would be economically made from sludge cake, (2) whether it would be safe for environment, and (3) when vegetables would grow enough in it, whether they would be safe for human consumption.

Methods

Sludge cake was mixed with soil (soil texture: heavy clay). The circular plaster mold was used as a fixed mold. As the toxicological testing, leaching test and seed germination test were used. Heavy metal concentrations in vegetables grown in the flower pot were measured.

Results

The flower pot was sufficiently formed by drying in natural open air. The results of leaching test showed three heavy metals, lead, nickel and copper, were lower than the standard in Thailand. The seed germination test suggested no negative effects of the flower pot on the germination of Chinese kale. Lead concentrations in the Chinese kale were higher than the recommended maximum level in leafy vegetables.

Conclusion

The new flower pot can be made from sludge cake with soil. It has the possibility to have no negative effect on the environment. Although the vegetables grown in this flower pot are not suitable to eat, this flower pot has the possibility to solve the environmental and health problems.

Introduction

Paper mill industries in Thailand generally produce sludge cake after wastewater treatment. It is usually disposed into an industrial open landfill [1, 2]. Since this sludge cake contains some heavy metals, especially lead [3, 4], it is one of the sources of pollutants to cause environmental degradation [2, 5] and health problems, which significantly contribute to public health risk. Management of the ecological risks, such as soil erosion and water pollution, is necessary [6].

Now many researchers have tried to recycle rigid materials from sludge cake for the purpose of decreasing its volume disposed into a landfill [7]. In the previous study [8], the researchers have tried to make a disposable bin to reduce pollution emission economically. Researchers also have clarified that the bin can be made from the cleaned sludge cake with latex or plaster and white cloth or newspaper. This bin has the possibility to have no negative effect on the environment at disposing the sludge cake. However, it is still unclear that plants growing in the soil containing the sludge cake would be eatable.

The researchers have further idea on the other object that looks close to the bin but smaller than it. It was a flower pot designated to be used in the area under roof protecting rain in an industrial site. This flower pot would be applied for sprout growth, and be finally buried underground. The aims of this study were to clarify three hypotheses: (1) whether a flower pot would be economically made from sludge cake or not, (2) whether it would be safe for environment or not, and (3) when vegetables would grow enough in the soil in it, whether they would be safe or not for human consumption.

Methods

Cooperative industry

The cooperative industry is a medium-sized (220 workers) paper mill industry in Thailand. The wastewater is approximately 3,800–4,500 m³/day. The wastewater is treated chemically using aluminum sulfate. Sludge settled in the first sedimentation tank is transferred to sludge collector. Treated wastewater is sent to treat biologically using urea, ferric chloride and phosphoric acid. The effluent is discharged into the river. Excess sludge from the second sedimentation tank is condensed by the sludge thickener. The thickened sludge is transferred to the sludge collector to mix with sludge in the first sedimentation tank (Total amount of sludge is about 150 m³/day). After mixed with polyacrylamide (about 15 kg/day) the mixture is transferred to screw press. After this treatment the sludge becomes sludge cake (Eurochem, Activated System). The sludge cake (about 5 m³/day) is disposed into the industrial landfill.

Urea, ferric chloride and phosphoric acid are non-toxic. Polyacrylamide used in agriculture may contaminate food with acrylamide, but it itself is relatively non-toxic [9]. Aluminum sulfate, however, is toxic to aquatic organism and is recommended not to release into the environment.

Shape forming

Sludge cake was analyzed in Central Laboratory, Faculty of Public Health, Mahidol University. The researchers measured concentrations of 8 elements regulated in Thailand and copper at the beginning of this study series (in the different year). The concentrations were shown in the Table 1. The concentrations of all elements except copper were quite lower (less than 1/100) than Thai standard [10]. The concentrations of cadmium, copper, mercury and nickel were in the range of 1.3 (Mercury) to 7.0 % (Copper) compared with Canadian standard [11], and that Canadian standard values of cadmium and mercury were less than Japanese standard [12], researchers selected 2 metals, copper and nickel to be measured, although there is no standard for copper in Thailand. Researchers also selected lead because earlier reports pointed out high concentration of lead in the sludge cake [3, 4] and because its concentration varies depending different types of paper corresponding to the request of costumers. The sludge cake used in this study contained 0.556 mg/kg lead, 0.113 mg/kg nickel, 1.006 mg/kg copper, 1557 mg/kg organic carbon, 378.5 mg/kg nitrogen and 50.97 mg/kg phosphorus, and the pH was 8.2 [13].

Table 1 Concentrations of the measured heavy metals in the sludge cake at the beginning of this study series and standard values in Thailand, Japan and Canada

The analysis with a fiber-length analyzer (Kajaani, FS-200) at a laboratory in Asian Institute of Technology (AIT Laboratory) showed a lot of short fibers in it [14].

The components of shape forming were a binder and a fixed mold. In the first trial, the researchers made the slurry in the manner same as making the disposable bin [8]. The latex as the binder was mixed with cleaned sludge cake (latex: sludge cake = 1:20). The fixed mold was made of two acrylic sheets placed 10 mm apart. The slurry was poured into the space between two sheets. The slurry in the fixed mold was very difficult to be dried under the natural sunshine. The fixed mold of the mixture was heated in an oven at 35–85 °C for 24 h to make it solid. The product was distorted and looked ugly. It was not uniform in shape and impossible to stack each other for transportation. Consequently the researchers tried a different approach.

In the second trial, the clay in the soil was used as the binder. The soil was brought from Bang Len, Nakhon Pathom Province, Thailand. This contained sand 20.68 %, silt 28.30 % and clay 51.02 %, and was classified as heavy clay. Its pH was 4.72 and cation exchange capacity was 23.68 cmol(+)/kg [15]. Sludge cake without cleaning was mixed with the soil at the set ratio by a paddle. The set ratio of sludge cake: soil by weight was 1:1.5, 1:2 and 1:3. Some water was added. The mixture was mixed again by the paddle.

In this time, two machines were used to make the flower pot. They were mixing machine, in which there were two screw parts to mix homogeneously, and shape forming (jigger) machine. The material mixed by the paddle was poured into the mixing machine. The thoroughly mixed material by the mixing machine (clay slurry) was put into the circular plaster mold as a fixed mold. This plaster mold looked like hollow cone (12.0 cm height, inner diameter 15.6 cm at top and 9.6 cm at bottom). Researchers poured the clay slurry into the plaster mold until about half of its volume. The plaster mold of the clay slurry was set under the arm of jigger machine (Fig. 1). As the clay slurry came into contact with the vertically rotating head-piece, it was pressed and spread until it finally became the shape of the plaster mold.

Fig. 1

Plaster mold and arm of jigger machine

Formations of flower pot were dried in the room for 1 day before removing from the plaster mold. They were dried for one more day in a shadow area, and drilled (about 15 mm hole in the bottom) for root outlet and water drainage. After that, they were left out of doors for 14 days.

When heavy rain had attacked them, all of them had completely collapsed within half to one hour. Thus, the flower pots seemed to be unable to resist heavy rain.

At the end of manufacturing process, the size of flower pot was measured using eight samples of each set ratio. Its durability was also studied. Soil was loosely filled up to the top of each flower pot, and then 150 ml water was sprinkled from about 15–20 cm height above the top in 30 min every 3 days (about 26 mm/h: strongish rainfall). When the flower pot broke out, it was judged as degradation.

Toxicological testing

Leaching test and seed germination test

The flower pots made from the mixture of sludge cake: soil 1:2 were used for toxicological testing. Leaching test and seed germination test were used to confirm negative effect to the groundwater and the germination.

Procedures to determine the concentrations of three heavy metals, lead, nickel and copper, in the sample were total element analysis (wet digestion) [16] and leaching test under USEPA SSW 846 [17]. Three samples were used for these testing.

In the seed germination test, Chinese kale (Brassica oleracea var. alboglabra) was selected as the seed. A rectangular plastic tray (12.5 cm wide, 28.5 cm long, and 6.0 cm deep) was used. Its four wall sides had the screen with air and water permeability. Chuan Chom ^TH soil from Lopburi Province, Thailand was used for the seed germination. The characteristics of its soil used in this experiment are as follows: its pH was 8.0, and it contained the enriched soil materials, such as 8280 mg/kg organic carbon, 550 mg/kg nitrogen and 96.5 mg/kg phosphorus [13]. Crushed material of flower pot was mixed at appropriate ratio. Three groups were run. They were:

• 100 % Soil: 1000 g of the soil

• 50 % Soil and 50 % FP: 500 g of the soil mixed with 500 g of crushed flower pot

• 100 % FP: 1000 g of crushed flower pot

In each tray, 50 seeds of Chinese kale were planted. Watering was done twice a day (morning and evening). Observation period for germination was set 30 days after planting. Three samples in each group were used for this testing.

Heavy metal accumulation in vegetables

Heavy metal concentrations in vegetables were measured to confirm whether vegetables grown in the flower pot would be safe or not for eating. Shoots and roots of Chinese kale on 45th day after planting were analyzed to measure the heavy metals. They were dried in an oven at 103 °C for 1 h and cooled in a desiccator for 24 h to measure dry weight. Two grams of dry shoot and 0.5 g of dry root of each Chinese kale were digested with 100 ml nitric acid. The concentration of lead, nickel and copper were measured by atomic absorption spectrophotometer [13].

Difference in the average was examined by one-way ANOVA. In the comparison of the weight of flower pot and the number of germination of Chinese kale at the same day in the seed germination test, Bonferroni correction was used for multiple comparisons. In the comparison of the number of germination of Chinese kale with that on the 5th day in the seed germination test, Dunnett correction was used for multiple comparisons.

Data handling and statistical analyses were carried out by using SPSS version 19.0 (SPSS Japan). The difference was considered statistically significant for p < 0.05.

Results

Shape forming

The flower pot was sufficiently formed in all three types of set ratio. The size of flower pot at the end of manufacturing process was 12 cm height, 15 cm diameter at outside top, 9 cm diameter at outside bottom and 1.5 cm of thickness. There was no significant difference in such size among three types of set ratio. Its average of weight was 523 ± 17 (average ± standard deviation) g of the mixture of sludge cake: soil 1:1.5 (FP1.5), 533 ± 13 g of the mixture of sludge cake: soil 1:2 (FP2), and 569 ± 10 g of the mixture of sludge cake: soil 1:3 (FP3). The average of the weight was significantly higher in flower pot of FP3 than in other two types.

Figure 2 shows the number of remaining flower pot at each observation day. The durability was different among three types of set ratio. Flower pot of FP2 showed the highest number from the 12th day to 30th day. On the 30th observation day, 5 (62.5 %) of 8 pots still remained.

Fig. 2

The number of remaining flower pot at each observation day

Judging from the results of shape forming, the researchers selected the FP2 as the best choice. Consequently, it was used for the further experiment in this study.

Toxicological testing

The concentrations of lead and nickel in the total element analysis were lower than the value of the Soil Quality Standard, Thailand [10]. Copper is not regulated in this Act. This concentration of copper was used as the background data (Table 2).

Table 2 Results of total element analysis and leaching test of three heavy metals

The results of leaching test of three heavy metals were much lower than those of total element analysis and lower than the standard of Notification of Ministry of Industry, Thailand [18] (Table 2).

Average number of germination of Chinese kale at various times of observation periods is shown in Table 3. The number of Chinese kale with 100 % Soil reached its peak on 10th day and continued. The one with 50 % Soil and 50 % FP also showed same tendency. The one with 100 % FP reached its peak on 20th day and continued. The one with 100 % FP was significantly higher than the one with 50 % Soil and 50 % FP from 10th to 30th day, but showed no significant difference from the one with 100 % Soil.

Table 3 Average number of germination of Chinese kale at various time of observation period

Two groups, 50 % Soil and 50 % FP, and 100 % FP showed that lead concentrations in both the shoot and root were higher than the recommended maximum level of metal concentration in leafy vegetables [19] (Table 4). Whereas copper concentrations were lower in both the shoot and root. 100 % Soil group showed lead and copper in the shoot and root were lower than the recommended value. Nickel concentrations were from 0.02 (root in 100 % Soil) to 0.51 (root in 100 % FP) mg/kg.

Table 4 Results of heavy metal accumulation in the shoot and root of Chinese kale

Discussions

Shape forming

The clay slurry was composed of sludge cake and soil (sand, silt and clay). Sand provides structure, strength and bulk to the product. Clay is cohesive and binds to the sand and the fiber in the sludge cake. Fiber forms a reinforcing meshwork, and helps to hold the structure. Clay powder mixed with fine fiber of hyacinth succeeded in making flower pot [20]. Instead of the fiber of hyacinth, the short fiber in the sludge cake was effective to make a flower pot strong.

When the flower pot was made from the clay slurry using the circular plaster mold as a fixed mold, the formation of flower pot was made by drying in natural open air. Using the same plaster mold and the jigger machine made the product uniform in shape and possible to stack up each other for transportation. The new procedures have solved the problems in making the flower pot of the mixture with latex using the mold made of two acrylic sheets.

This study showed the mixture of sludge cake: soil 1:2 was the best choice for the durability of flower pot. A lot of soil leads the decreased proportion of short fibers in the clay slurry, and thus the decreased durability of the flower pot [21]. The best adobe soil had clay between 15 and 30 % in it to bind the material together [22]. Too much clay will shrink and crack excessively and too little will allow fragmentation. The best mixture in this study had sludge cake: soil 1:2, and then theoretically 34 % (0.5102 × 0.6667 = 0.3402) clay, which is corresponding to the value of the best adobe soil.

Toxicological testing

Concentrations of toxicants (lead, nickel and copper) were less than those reported earlier [2, 23]. The leaching values were also lower than the standard values [18]. These heavy metals would little meld out into the ground, even if the rain would penetrate to the flower pot. Thus, utilizing the flower pot would not seriously affect groundwater or infiltration.

The germination in the 100 % flower pot group was more slowly than in the 100 % soil group. This may shows the disturbance effect of flower pot on the germination. However, the disturbance effect would be small, because there was not significant difference in the number of germination between 100 % flower pot and 100 % soil group and because the paper mill sludge cake possibly supplied both chemical materials as fertilizer or conditioners and good aeration [23].

The number of germination in the 50 % soil and 50 % flower pot group was less than that in the 100 % flower pot group. It is not known exactly why the inhibitory effects occur. The further study is necessary to clarify the reasons.

Copper concentrations in the shoot and the root of Chinese kale were less than the recommended maximum level of metal concentration in leafy vegetables [19]. This shows that Chinese kale grown in the flower pot would be safe for human consumption. However, lead concentrations exceeded the recommended maximum level in both the shoot and root [19]. Nickel concentrations in the root in 100 % flower pot group were more than those in vegetables in Japan, which contained around 0.15 mg/kg [24]. These indicate that Chinese kale grown in the flower pot would not be safe for human consumption. Consequently using this flower pot is limited to orchids, jasmine rose and so on. Eatable leafy vegetable, such as cabbage and spinach are not allowed to grow in this flower pot because of high level of lead concentration inside the leaf.

Converting the sludge cake into flower pot has two advantage points. The flower pot weighs about 540 g and is rather convenient to transport although it is heavier than a plastic flower pot. It enables us to decrease usage of the plastic flower pot or small black color bag for seedling or sprout growth. These plastic materials will not be degradable easily when they are buried underground [25] and cause solids wastes problem against the environment at that place.

However, there are also the disadvantages. Because the flower pot has only one color (grey) and a rather bad smell, it is not suitable for decoration to show beautiful shape. Judging from the durability test in this study, the flower pot will not resist natural sprinkling of water or natural rain. The usage of this flower pot is generally limited less than 1 month in the area under roof protecting rain in an industrial site. However the fact that the flower pots are easily degradable under the rain means that they will easily and completely break out when they are buried underground, which shows no solid waste left without negative effect on the environment. In order to use the flower pot for longer time, it is necessary to coat the surface of flower pot with paraffin and so on.

The sludge cake has the possibility to contain aluminum sulfate toxic to aquatic organism. Thus, the flower pots should be used in the area enough far from the water area, such as canals, streams, rivers and sea. After using it, it should not be disposed into the water area. Further research on the effect of sludge cake on the soil organisms, such as earthworm, is necessary.

Possible limitations of the present study should be mentioned. Because researchers were allowed to use the machine for only a limited time, it was impossible to make enough number of flower pots for durability test. In the results of the total element analysis, the leaching test, and the germination test, the researchers showed the average, but not standard deviations.

In conclusion, sludge cake containing hazardous materials, disposed into the open landfill, could be one of the sources of the environmental and health problems. The new flower pot can be made from sludge cake with clay in the soil. It has the possibility to have no negative effect on the environment. Although the vegetables grown in this flower pot are not suitable to eat, this flower pot has the possibility to solve the environmental and health problems at disposing the sludge cake into the landfill.