Environ Eng Res > Volume 19(1); 2014 > Article
Pinwisat, Phoolphundh, Buddhawong, and Vinitnantharat: Effect of Surfactant-Coated Charcoal Amendment on the Composting Process and Nutrient Retention

### Abstract

This research investigates the quality changes during composting of bagasse and pig manure amended with 30% of surfactant-coated charcoal (SC). Two treatments, 30% uncoated charcoal (UC) amendment and no charcoal (NC) amendment, were done as control. Charcoal was coated with 0.37 mM tetradecyltrimethylammonium bromide (TDMA), a cationic surfactant, at the dosage of 10 g/L. At the end of the composting period, the carbon to nitrogen (C/N) ratio of SC amendment was 9.7; whereas, the C/N ratios of UC and NC amendment were 12.6 and 21.4, respectively. Plant nutrients contents of the compost produced from SC amendment were 20.7 mg NH4+-N/g, 42.8 mg NO3-N/g, and 41.7 mg P/g. High nitrate and phosphate concentrations in SC amendment were due to the adsorption of these anions on the positive charge of TDMA. Desorption of plant nutrients retained in the compost pellets was also investigated. It was predicted that nitrate was fully desorbed from a pellet at 23 days for SC amendment, which was later than UC (14 days) and NC (10 days) amendment. A slow release of nitrate from the compost pellet will reduce the nitrate leaching into the environment. Thus, the adding of SC in the compost pile is one of the alternative methods to improve the quality of compost and plant nutrient retention.

### 1. Introduction

Bagasse is one of the crop residues from the sugar cane industry. Large quantities of bagasse of about 20 Mt are produced annually in Thailand. It has high organic carbon and can be used as fuel and for paper products and ethanol production. In addition, it consists of nitrogen, potassium and phosphorus, which are the essential nutrients for plants. Thus, composting of bagasse is an alternative waste management, because it is low-technology and offers an incentive to the farmer. Nitrogen sources are normally derived from animal waste, such as pig manure. The aerobic condition is normally kept in the compost piles to reduce nitrogen loss to nitrogen gas and nitrous oxide. However, nitrogen loss can be found under aerobic condition in the form of nitrate, which has the potential to leach to groundwater. In addition, nitrogen loss in the form of ammonia occurs at high pH. To reduce this effect, charcoal amendment is interesting, because it is inexpensive. Charcoal also acts as a bulking agent, which improves soil structure after application. It is also reported that the adding of charcoal into inorganic fertilizer also improves plant growth and increases grain yield [1]. In addition, fertilizer impregnated charcoal was reported as a slow-release type fertilizer [2]. However, charcoal is normally made from wood, which contains high cellulose content and exhibits negative charge. In order to enhance the adsorption of nitrate and phosphate for nutrient retention during composting, the chemical surfaces of charcoal should be modified to positively charged exchange sites. Previous researches used the cationic surfactant loaded onto zeolite, which resulted in controlling the ammonium and nitrate release from fertilizer [3, 4]. None of the previous studies reported the use of surfactant coated-charcoal in the compost pile to restore the ammonia and nitrate. This method offers not only an improvement of the compost quality, but also protection of the environment. Thus, this research aims to investigate the effect of adding charcoal and surfactant-coated charcoal on the compost quality and nutrient retention in the compost.

### 2.1. Composting Materials

Bagasse from sugar cane industry was cut to yield 5 cm in size. Pig manure from pig farm was air dried and then mixed with bagasse to obtain a carbon to nitrogen (C/N) ratio of 30–40/1 (total 20 kg, with a dimension of 0.7 × 0.7 × 1 m3). The chemical properties of bagasse and pig manure are shown in Table 1. Wood charcoals from mango and rain trees were ground and passed through a 16 size mesh. Then, they were divided into 2 portions, which were coated and uncoated with cationic surfactant. Coated charcoal was prepared by shaking 1 g of charcoal in 100 mL of 0.037 mM tetradecyltrimethylammonium bromide (TDMA) for 24 hr. The characteristics of the surfactant-coated and uncoated charcoal are shown in Table 2.

### 2.2. Composting Piles and Monitoring

The mixtures of composting materials were employed at laboratory scale to determine the effect of charcoal amendment on compost quality. The selected mixtures were:
1. No charcoal amendment (NC): pig manure (7.8 kg DW) + bagasse (13 kg DW)

2. 30% uncoated charcoal amendment (UC): pig manure (7.8 kg DW) + bagasse (13 kg DW) + charcoal (6.24 kg DW)

3. 30% of surfactant-coated charcoal (SC): pig manure (7.8 kg DW) + bagasse (13 kg DW) + surfactant-coated charcoal (6.24 kg DW)

The mixtures were placed in containers made from bamboo. The moisture in each treatment was controlled at 50%–60% over the entire period of experiment. Hollow pipes of 2.5-cm diameter were put into the pile for air ventilation. The composts were observed by measuring temperature, pH, total organic carbon, total organic matter, total Kjeldahl nitrogen, phosphorus, and potassium, following methods by the AOAC [5]. The weight loss of composting piles was also determined in terms of dry weight (DW) at the first day and at the end of the composting period of 70 days. In addition, the total viable plate count was used to identify mesophilic and thermophilic bacteria [6].

### 2.3. Determination of Nutrient Retention of Compost Pellet

In order to imitate the real used fertilizer in the field, the mature compost was extruded to make a pellet. The release of nutrients (NO3, NH4+, PO43−) from the compost pellet was done in static condition adapted from Liang et al. [7]. The 2.5 g compost pellet was placed in 2 L of distilled water and the pH was adjusted to 5.0–6.0. It was kept in a controlled room at 35ºC. The released nutrients in the solution were monitored every day, following the standard method for examination of water and wastewater [8].

### 3.1. Compost Process and Quality

The chemical property of each compost pile during composting process is shown in Table 3. The temperature dynamics of each pile showed a similar trend, which was at the highest after 2 weeks of composting period for UC and SC amendment. As for NC amendment, the highest temperature was found after 3 weeks of composting period (data not shown). The high temperature of the compost pile promotes a high amount of thermophilic bacteria (Fig. 1). It also found that the pH increased during the thermophile process, particularly for SC amendment. This means that the decomposition of organic nitrogen occurred and that ammonia was produced. Ammonia was then biodegraded to nitrate and hydrogen ion leading to lowering of the pH. Monitoring the microbial population confirmed that the numbers of thermophilic bacteria depended on the temperature. As the decomposition progressed, the number of bacteria gradually increased both thermophilic and mesophilic bacteria to the maximum at 14 days of composting period. Then, they decreased and were not changed after 35 days of composting period. However, SC amendment gave the highest number of bacteria among the other compost piles. Intense microbial activity led to organic matter mineralization. Degradation of organic matter followed the zero-order kinetics, as follows:
##### (1)
$Ct=C0-K0t$
where C0 and Ct are the organic matter content at initial and at time t; t is time (day) and K0 is the reaction rate. The rate of organic matter mineralization is the slope of the plot of C0 and t, as shown in Fig. 2. It was found that SC amendment could enhance the organic matter degradation rate to 4.60 mg/g·d, compared to 2.46 and 1.91 mg/g·d for UC and NC amendment, respectively. The high rate of SC amendment occurred, because charcoal provided a suitable condition for microbial growth. Charcoal from wood species usually contains large and small vessels, and fiber cells act like capsules for retention of nutrient [2]
Observation of the nitrogen variation during the composting period showed that SC and UC amendment could retain more nitrogen than UC amendment. The C/N ratio sharply decreased during the initial stage and gradually decreased until stable (Fig. 3). It was recommended that C/N should be less than 20 to ensure the nitrogen available for plant [9].
The C/N ratios of this study are 20.8, 14.8, and 10.6 for NC, UC, and SC amendment, respectively. Although the C/N ratio in this study is less than 12 for the UC and SC amendment, the nitrogen in the compost is still higher than for the NC amendment. This finding also corresponded with the use of ash mixing in the compost [10]. Thus, the nitrification index (NH4+/NO3) can be applied to determine the ripeness of compost, which should be less than 1 [11]. The NH4+/NO3 ratios of SC and UC amendment were 0.5 and 0.7, respectively. This was 16.5 for NC amendment, since the nitrate leaching may have occurred during bioconversion. The SC amendment yielded the highest nutrients, as shown in Table 3. The negative charges of nitrate and phosphate are favorable to cationic charge of TDMA.

### 3.2. Nutrient Release from Compost Pellet

Nitrogen (NH4+/NO3) and phosphorus releases from the compost pellets in the distilled water were observed in the term of accumulated desorption (Fig. 4).
It can be seen that the pattern of nitrogen and phosphorus is similar. SC and UC amendments could gradually desorb in solution. The observed data of the accumulated desorption experiment were fitted to the polynomial equation, and thus the day for full desorption of nutrients can be predicted. Nitrate was fully desorbed from SC amendment at 23 days, which was later than UC (14 days) and NC (10 days) amendment. Charcoal retains the nutrients in its structure and can release nutrients, which will protect the nitrogen loss by leaching and volatilization during the composting process. The adsorption and desorption of nutrients from surfactant coated-charcoal were attributed to surface anion exchange [3]. The surfactant is aerobic biodegradable in soil environment, so the risk of soil biota is very small [12].

### 4. Conclusions

This research study shows that wood charcoal can be co-composted with organic waste like pig manure and bagasse. The structure of charcoal could retain the nutrients during decomposition and enhance the biodegradation rate. The surfactant-coated charcoal amendment in the compost piles could improve the quality of compost and retain the highest nutrients. Compost produced from SC amendment has nitrate of 42.8 mg NO3/g DW, which is 30.6 and 1.2 times the compost produced from the NC amendment. Mature compost can be made as a pellet and developed as a slow release fertilizer.
##### Fig. 1
Changes in bacteria during the composting period. NC: no charcoal, UC: uncoated charcoal, SC: surfactant-coated charcoal.
##### Fig. 2
Changes in organic matter during the composting period. NC: no charcoal, UC: uncoated charcoal, SC: surfactant-coated charcoal.
##### Fig. 3
Changes in nitrogen and carbon to nitrogen (C/N) ratio during the composting period. NC: no charcoal, UC: uncoated charcoal, SC: surfactant-coated charcoal.
##### Fig. 4
Accumulated desorption of various compost pellets: (a) NH4+-N, (b) NO3 -N, and (c) P (phosphorus). NC: no charcoal, UC: uncoated charcoal, SC: surfactant-coated charcoal.
##### Table 1
Chemical analysis of bagasse and pig manure
Component Bagasse Pig manure
Moisture (%) 8.65 ± 1.75 16.29 ± 0.04
pH (solid:water = 1:2) 4.24 ± 0.03 7.57 ± 0.02
Total organic carbon (%) 50.03 ± 0.35 28.97 ± 0.17
Total organic matter (%) 86.26 ± 0.61 49.95 ± 0.20
Total Kjeldahl nitrogen (%) 0.40 ± 0.001 3.68 ± 0.004
C/N ratio 125.02 ± 0.03 7.87 ± 0.01

[i] C/N ratio: carbon to nitrogen ratio.

##### Table 2
Characteristics of surfactant-coated and uncoated charcoal
Component Uncoated charcoal Coated charcoal
Moisture (%) 7.78 ± 0.06 7.64 ± 0.04
pH (charcoal:water = 1:2) 7.76 ± 0.01 7.10 ± 0.02
Iodine no. (mg/g) 178.22 ± 0.12 188.26 ± 0.04
##### Table 3
Chemical properties of composts during the composting process
Type of compost pile

NC UC SC

Decomposting time (day) 0 14 42 70 0 14 42 70 0 14 42 70
pH 6.44 7.37 7.32 7.31 7.45 8.32 7.85 7.40 7.13 8.69 7.62 7.65
Temperature (°C) 28 30 32 31 28 38 32 30 28 41 30 29
Total organic matter (%) 56.94 49.73 46.48 39.79 54.91 50.66 37.50 36.33 52.66 44.61 30.22 28.70
Total organic carbon (%) 33.03 28.84 26.96 23.08 30.34 29.38 21.75 21.07 30.54 25.87 17.53 16.65
Total Kjeldahl nitrogen (%) 0.90 0.94 1.17 1.08 0.83 1.04 1.30 1.67 0.75 1.23 1.68 1.73

[i] NC: no charcoal, UC: uncoated charcoal, SC: surfactant-coated charcoal.

##### Table 4
Compost quality of each treatment
Parameter NC UC SC
pH 7.41 ± 0.15 7.52 ± 0.16 7.52 ± 0.18
EC (dS/m) 1.57 ± 0.21 4.93 ± 0.21 6.93 ± 0.55
Organic matter (%) 39.11 ± 0.95 35.91 ± 0.58 28.43 ± 0.37
C/N ratio (%) 20.82 ± 0.78 14.75 ± 3.01 10.57 ± 1.33
Phosphorus (mg P/g DW) 25.57 ± 0.13 39.29 ± 0.26 41.73 ± 0.11
Potassium (mg K/g DW) 2.86 ± 0.75 2.99 ± 0.79 3.15 ± 0.75
Ammonium (mg NH4+/g DW) 23.07 ± 0.13 24.23 ± 0.03 20.69 ± 0.02
Nitrate (mg NO3/g DW) 1.40 ± 0.03 34.7 ± 0.81 42.8 ± 1.98
Weight loss (%) 58.11 ± 1.46 71.41 ± 0.39 72.87 ± 0.95

[i] NC: no charcoal, UC: uncoated charcoal, SC: surfactant-coated charcoal, EC: electrical conductivity, C/N ratio: carbon to nitrogen ratio, DW: dry weight.

### Acknowledgments

The authors thank the National Research Council of Thailand and the Faculty of Science, King Mongkut’s University of Technology, Thonburi, Thailand for their financial support.

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