Article pubs.acs.org/JAFC
Treatment and Biotransformation of Highly Polluted Agro-industrial Wastewater from a Palm Oil Mill into Vermicompost Using Earthworms Su Lin Lim,† Ta Yeong Wu,*,† and Charles Clarke‡ †
Chemical Engineering Discipline, School of Engineering and ‡School of Science, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150, Selangor Darul Ehsan, Malaysia ABSTRACT: In this laboratory-scale study, earthworms were introduced as biodegraders of palm oil mill effluent (POME), which is a wastewater produced from the wet process of palm oil milling. POME was absorbed into amendments (soil or rice straw) in different ratios as feedstocks for the earthworm, Eudrilus eugeniae. The presence of earthworms led to significant increases in pH, electrical conductivity, and nutrient content but decreases in the C/N ratio (0.687−75.8%), soluble chemical oxygen demand (19.7−87.9%), and volatile solids (0.687−52.7%). However, earthworm growth was reduced in all treatments by the end of the treatment process. Rice straw was a better amendment/absorbent relative to soil, with a higher nutrient content and greater reduction in soluble chemical oxygen demand with a lower C/N ratio in the vermicompost. Among all treatments investigated, the treatment with 1 part rice straw and 3 parts POME (w/v) (RS1:3) produced the best quality vermicompost with high nutritional status. KEYWORDS: cleaner production, rice straw, soil, wastewater treatment, organic fertilizer, Eudrilus eugeniae, palm oil mill effluent (POME), vermicomposting
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INTRODUCTION Agro-industries play a major role in the global economy, especially in developing countries. Developing nations often rely on a combination of subsistence farming and formal and/or informal agro-industries for income generation, yet associated health risks and environmental pollution can have a great impact on these nations. Due to the unsafe disposal of untreated agroindustry wastes in the form of solid wastes or effluents, clean water supplies are polluted, productive potential is exhausted, and health risks around the region are gravely increased. Malaysia is one of the leading global producers and exporters of palm oil. The Malaysian palm oil industry is growing rapidly and is quickly becoming a significant agricultural industry in this country. However, the production of large amounts of crude palm oil leads to the generation of a greater amount of agro-industrial wastewater or palm oil mill effluent (POME). The wet process of palm oil milling consumes a large amount of water. It is estimated that POME is generated at nearly 3 times the rate of crude palm oil.1 In Malaysia, approximately 44 million tonnes of POME was generated during oil palm processing in 2008. As the Malaysian palm oil industry continues to grow, the amount of POME generated is expected to increase annually.2 POME is a brown slurry, containing 95−96% water, 0.6−0.7% residual oil, and 4−5% solids (mainly organic). POME is characterized by a high biochemical oxygen demand (BOD) (25000 mg/L) and chemical oxygen demand (COD) (53630 mg/L) and contains 8370 mg/L oil and grease, 43635 mg/L total solids, 19020 mg/L suspended solids, and a high concentration of organic nitrogen.3 One of the disposal options for POME is a ponding system, which is used by >85% of palm oil mills in Malaysia. This treatment method is proven to be economically viable, but it accumulates large amounts of sludge, causes uncontrollable release of greenhouse gases, and requires © 2013 American Chemical Society
large areas of land. Furthermore, it was reported that this disposal option allows the lowest utilization of renewable resources.2 Other conventional POME treatment processes are aerobic and anaerobic digestions, physiochemical treatments, and membrane treatments. However, biological processes such as aerobic and anaerobic digestion are inefficient ways to treat POME due to its colloidal nature (suspended solids), low pH, and high BOD and COD loadings.3 In many countries, waste management systems are undergoing changes due to the threat of global climate change and other environmental issues.4 Waste management systems are also influenced by socio-economic, political, and environmental factors, including population growth, consumption patterns, and technological development of waste systems.5 Recently, numerous studies have examined the “zero waste” concept. Waste treatment technologies such as composting give rise to opportunities for resource recovery and production of nutrientrich organic fertilizer and soil conditioner from the waste.6 Thus, vermicomposting could be used as an alternative treatment method for managing POME. In this process, earthworms and micro-organisms convert organic wastes into a useful product known as vermicompost.7 Earthworms affect the soil biological activity through organic matter decomposition and nutrient mineralization. They are the important drivers for substrate conditioning, whereas microbes are responsible for the biochemical degradation of organic matter. Among all of the different species of earthworms, epigeic earthworms are the most suitable for the vermicomposting process. Epigeic earthworms Received: Revised: Accepted: Published: 691
September 24, 2013 December 25, 2013 December 27, 2013 December 27, 2013 dx.doi.org/10.1021/jf404265f | J. Agric. Food Chem. 2014, 62, 691−698
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Malaysia, and kept in the refrigerator (at 4 °C) before being used as a feedstock in vermicomposting. As mentioned earlier, rice straw and soil were used as amendments. The rice straw was obtained from a paddy field in Sekinchan, Selangor, Malaysia, and experimental soil was a locally mixed peat moss soil obtained from Peat Organic (M) Sdn Bhd, Selangor, Malaysia. The initial chemical characteristics of POME, rice straw, and soil are given in Table 1.
are litter dwellers and live in or near the soil surface. These species feed on decaying organic matter and have high reproduction rates. Examples of epigeic earthworms are Eisenia fetida, Eisenia andrei, Eudrilus eugeniae, and Perionyx excavatus.8,9 Epigeic earthworms are also less susceptible to toxicity than endogeic or anecic earthworms.10 In the reported study, E. eugeniae was used because it is able to decompose organic wastes rapidly and it is a suitable species for vermicomposting in tropical countries.8 Compared to compost, vermicompost is produced in less time and has greater fertilizer value with a higher humus content and less phytotoxicity.7 A study conducted by CoriaCayupán et al.11 showed that vermicompost application increased the yield of lettuce without significant loss in food nutritional value. Additionally, vermicompost can be used for soil remediation purposes, such as degradation of pesticides.12 According to Hait and Tare,13 vermicomposting is a costeffective, energy-saving, and zero-discharge process. However, several factors such as moisture content, pH, temperature, and aeration need to be monitored closely during the vermicomposting process because earthworms are involved. The surrounding conditions need to be favorable for the earthworms to break down organic wastes.9 Nevertheless, the utilization of earthworms to stabilize solid wastes has been well documented. Vermicomposting of wastes such as animal manure, sewage sludge, agricultural wastes, and industrial wastes showed that it is an efficient technology for waste management.9,14 This process can also be performed at any scale, from small-scale household vermicomposting to community or pilot-scale vermicomposting.9 Pilot-scale vermicomposting has been attempted on paper mill and dairy industry sludges and on pineapple wastes by Elvira et al.15 and Mainoo et al.,16 respectively. Both studies showed that the pilot-scale vermicomposting process is feasible. However, there were differences between the end products produced at the laboratory scale and the pilot scale. For example, Elvira et al. found that the pilot-scale trials yielded better quality vermicompost than the laboratory-scale trials.15 Thus far, no studies have been conducted on the vermicomposting of POME because vermicomposting is generally used in solid waste management. Wastewaters (for example, swine and urban wastewaters) are usually treated using a vermifiltration process, which is adapted from a traditional vermicomposting system.17,18 However, the present study shows that it is possible to treat POME if it is mixed with an appropriate ratio of amendments, which act as absorbents of wastewater. In this study, rice straw and soil were used as amendments to achieve a conducive physical and chemical environment in which the earthworms could thrive. POME was absorbed into the amendments to produce a suitable solid feedstock for the growth and reproduction of the earthworms. The addition of amendments also improved the C/N ratio of the initial organic wastes, making the POME more suitable for decomposition by the earthworms.19 Therefore, the aim of this study was to assess the potential of using E. eugeniae to treat and biotransform POME into organic fertilizer after absorption of wastewater into amendments.
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Table 1. Chemical Characteristics of POME, Rice Straw, and Soil (Means ± SD, n = 3) parameter
POME
pH 4.51 ± 0.01 electrical conductivity 1396 ± 21 (μS/cm) C/N 15.94 ± 1.17 Ca (g/kg dry wt) 0.51 ± 0.00 K (g/kg dry wt) 1.73 ± 0.42 Mg (g/kg dry wt) 0.61 ± 0.01 P (g/kg dry wt) 0.26 ± 0.00 volatile solids (g/kg dry wt) COD (mg/L) 65667 ± 1803
rice straw
soil
7.17 ± 0.03 908 ± 6
4.65 ± 0.04 1645 ± 7
47.36 ± 2.11 4.97 ± 0.35 13.19 ± 0.04 1.45 ± 0.14 1.34 ± 0.09 751.6 ± 13.5
52.59 ± 2.67 9.90 ± 0.15 5.73 ± 0.85 3.77 ± 0.74 0.42 ± 0.04 806.8 ± 23.8
Experimental Design. Rice straw or soil amendments (200 g) were mixed with POME in three different ratios, namely, 1:1, 1:2, and 1:3 w/v (amendment:POME) (Table 2). The initial chemical
Table 2. Description of the Different Treatments Used for the Vermicomposting Experiment treatment in w/v RS1:1 RS1:2 RS1:3 S1:1 S1:2 S1:3
treatment description 1 1 1 1 1 1
part part part part part part
rice straw:1 part POME rice straw:2 parts POME rice straw:3 parts POME soil:1 part POME soil:2 parts POME soil:3 parts POME
characteristics of all treatments are given in Table 3. The amendments with different volumes of POME were then placed in rectangular plastic containers (17 cm × 14 cm × 12 cm). A 1:4 (w/v) treatment ratio was not attempted because earthworm mortality was observed at the initial stage of vermicomposting. Furthermore, the POME was not absorbed fully into the amendments (especially soil) in the 1:4 (w/v) treatment. Control bins (without earthworms) were also set up for all ratios investigated. Waste mixtures were turned periodically for 2 weeks to eliminate anaerobic conditions and remove any volatile gases potentially toxic to earthworms.20 After 2 weeks, 10 nonclitellate E. eugeniae (∼0.60 g each) were selected from the stock culture and released into each experimental container. The experimental containers were kept in triplicate for each treatment. Periodic sprinkling with water maintained moisture content at approximately 75% throughout the vermicomposting process. Experimental containers were placed in a dark laboratory at a temperature of 25 ± 2 °C. The vermicomposting process was conducted for 6 weeks. In each experimental container, the changes in earthworm biomass were measured at the beginning and the end of vermicomposting process to reduce the stress on the earthworms due to changing environmental conditions.21 Earthworms were hand-sorted and then washed with distilled water to remove any adhering materials from their bodies. The live weights of the earthworms were measured, and no correction for gut content was applied to any of the data. Approximately 10 g of homogenized wet substrates (free from earthworms, hatchlings, and cocoons) was collected for analysis on the first and final days of vermicomposting. The samples were oven-dried at 60 °C for 48 h, ground in a blender, and stored in polythene bags (at 4 °C) for further chemical analysis.
MATERIALS AND METHODS
Epigeic Earthworms and Collection of Organic Waste. E. eugeniae worms were obtained from ESI Agrotech, Malaysia. Stock earthworms were cultured and developed in the laboratory under ambient temperature on partially decomposed papaya and cow dung. POME was obtained from Hulu Langat Palm Oil Mill, Selangor, 692
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Table 3. Initial Chemical Characteristics of the Different Treatments (Means ± SD, n = 3)a treatment in w/v RS1:1 RS1:2 RS1:3 S1:1 S1:2 S1:3 a
pH 6.77 7.17 6.96 5.19 5.17 5.50
± ± ± ± ± ±
EC (μS/cm)
0.21 0.08 0.04 0.02 0.03 0.01
a b ab c c d
719 827 898 853 1055 1313
± ± ± ± ± ±
P (g/kg dry wt)
86 a 18 b 48 b 24 b 9c 45 d
1.42 2.18 1.70 0.91 1.40 1.03
± ± ± ± ± ±
0.25 0.13 0.27 0.21 0.10 0.04
ab c bc a ab ab
K (g/kg dry wt) 6.19 9.03 12.38 5.93 6.27 6.07
± ± ± ± ± ±
1.38 0.09 0.07 1.25 1.33 1.24
ab b c a ab ab
Ca (g/kg dry wt) 6.26 10.80 11.15 17.51 15.09 22.46
± ± ± ± ± ±
1.15 1.60 1.20 1.85 0.86 0.82
a b b c c d
Mg (g/kg dry wt) 2.40 3.20 3.48 4.36 4.66 6.49
± ± ± ± ± ±
0.05 0.26 0.42 1.08 0.87 0.05
a ab ab b b c
C/N ratio 46.90 35.67 31.61 50.07 46.60 43.37
± ± ± ± ± ±
2.15 1.90 4.36 0.98 1.46 7.30
a bc c a a ab
Mean values followed by different letters are statistically different (ANOVA; Tukey’s test, P < 0.05).
Table 4. Initial and Final Soluble Chemical Oxygen Demand (sCOD) and Volatile Solids (VS) of Different Treatments (Means ± SD, n = 3)a sCOD (mg/L)
VS (g/kg dry wt) final
treatment in w/v RS1:1 RS1:2 RS1:3 S1:1 S1:2 S1:3
initial 46400 50000 52800 37333 42667 52800
± ± ± ± ± ±
4525 1697 2263 2013 1665 2884
final
vermicompost bc bc c a ab c
20000 18800 6400 27200 33600 42400
± ± ± ± ± ±
1131 2828 1131 3394 4525 2263
a a b ac cd d
control 127967 130000 140000 44800 64800 45600
± ± ± ± ± ±
1168 a 5091 a 18102 a 2263 b 4525 b 2263 b
initial 790.0 808.4 809.0 838.3 844.4 864.6
± ± ± ± ± ±
17.1 a 9.5 ab 45.7 ab 13.6 ab 11.5 ab 25.6 b
vermicompost 373.4 434.7 449.7 818.3 838.6 829.3
± ± ± ± ± ±
71.0 a 22.5 a 10.6 a 3.3 b 12.2 b 9.0 b
control 570.7 506.0 628.8 885.0 844.6 826.7
± ± ± ± ± ±
86.2 18.2 34.1 49.5 15.9 33.0
a a a b b b
a Mean value followed by different letters is statistically different (ANOVA; Tukey’s test, P < 0.05). Statistical analysis was carried out separately for initial, vermicompost and control values.
Vermicompost Analysis. The electrical conductivity (EC) and pH of the vermicompost in 1:10 (w/v) aqueous solution (deionized water) were measured using digital conductivity and pH meters, respectively.22 Total organic carbon (TOC) was determined using the partial oxidation method.23 Total Kjeldahl nitrogen (TKN) was measured using the micro-Kjeldahl method.24 Total calcium, magnesium, and potassium were measured with the ignition method using an atomic absorption spectrophotometer following sample digestion with concentrated H2SO4 and concentrated H2O2.25 Total phosphorus was determined using the colorimetric method following sample digestion with concentrated HNO3 and concentrated HCl.26 The soluble COD (sCOD) was analyzed in 1:10 (w/v) aqueous solution using the reactor digestion method (Hach method 8000).1 The volatile solids (VS) content (loss on ignition) was obtained by combusting the dried samples in a furnace at 550 °C for 2 h.13 The microstructures of the vermicompost were determined using a variable-pressure scanning electron microscope (VP-SEM), Hitachi S3400N-II, from Japan. Statistical Analysis. One-way ANOVA was used to analyze the differences within vermicomposting and control treatments. Tukey’s b test was also performed to identify the homogeneous type of treatments for the various parameters. IBM SPSS Statistics (version 20) was used for data analysis. All results reported in this study are significant at the P < 0.05 level.
with rice straw and soil were 56.9−87.9 and 19.7−27.1%, respectively. The higher percentages of sCOD removal in treatments amended with rice straw can be explained by the higher decomposition rate of organic matter by the earthworms and microorganisms.17 Khwairakpam and Bhargava22 reported that COD was reduced by 29−73% during vermicomposting of sewage sludge. In comparison, conventional POME treatment using aerobic and anaerobic digestion is usually able to reduce the COD of POME by >95%.2 Although the vermicomposting process could not remove the same amount of COD as the conventional biological method, it managed to transform POME into an organic fertilizer for agricultural purposes (Table 5). The sCOD removal was highest in the RS1:3 treatment, which was statistically different from the other treatments (P < 0.05). Figure 1 shows a linear relationship between sCOD and VS for treatments amended with rice straw and soil. The sCOD and VS of the initial feedstocks and final vermicomposts were represented well by linear equations, with R2 values of 0.8826 and 0.7414 for treatments amended with rice straw and soil, respectively. As shown in Figure 1, the slope of the equation demonstrates large differences between the two amendments. The slope for treatments amended with soil was higher, as there was a minimal decrease in VS content relative to the treatments amended with rice straw during vermicomposting process. Similar to sCOD removal, higher VS reduction was observed in treatments amended with rice straw relative to those amended with soil. In general, more VS reduction was also observed in the vermicomposts relative to controls for all treatments (Table 4). The decrease in VS is an indication of decomposition, substrate mineralization, and compost maturity.13 VS reduction in the vermicomposts in treatments amended with rice straw (44.4−52.7%) and soil (0.7−4.1%) was also statistically different (P < 0.05). Hait and Tare29 also showed decreases in volatile solids in waste-activated sludge during the vermicomposting process. The higher rate of VS reduction in POME treatments amended with rice straw showed that the
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RESULTS AND DISCUSSION sCOD and VS of Vermicompost. sCOD is an important indicator of wastewater organic load. More sCOD was removed in treatments inoculated with earthworms than in those without earthworms (controls) (Table 4). The results suggest that, by feeding on the solid fractions of the POME, earthworms were responsible for the removal of the organic load in wastewater.17 Earthworms secrete gut enzymes that help to reduce the chemical compounds in wastewater that cannot be decomposed by microbes alone.27 In fact, Zhao et al.28 suggested that, with the help of microorganisms, earthworms could accelerate the decomposition of organic wastes by favoring the breakdown of excreted polysaccharides. There was also a significant difference in sCOD removal when different amendments were used. The ranges of sCOD removal for earthworm treatments amended 693
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Mean values followed by different letters are statistically different for each parameter (ANOVA; Tukey’s test, P < 0.05). Statistical analysis was carried out separately for vermicompost and control values.
Article
Figure 1. sCOD content (mg/L) against volatile solids (g/kg dry wt) of the treatments amended with rice straw and soil.
earthworms were consuming and breaking down these wastes at a much higher rate.22 pH of Vermicompost. The pH shifted toward alkaline conditions except in treatment S1:1, which showed no significant difference from the initial sampling (Tables 3 and 5). The shift in pH could be attributed to degradation of shortchain fatty acids and intensive mineralization of nitrogen by microbes.25 A similar increase in pH was also observed by Shak et al.30 and Tognetti et al.31 Final pH ranged between 5.08 and 5.79 for treatments using soil as an amendment, but the final pH in the rice straw treatments ranged between 7.98 and 8.46. There was a significant difference between the rice straw and the soil treatments (P < 0.05). According to Singh and Suthar,32 the difference in pH when different amendments were used was mainly due to variation in amendment quality, which affected the mineralization process and the intermediate species produced during the vermicomposting process. Electrical Conductivity of Vermicompost. EC describes the soluble salt level in an organic amendment and is a good indicator of the applicability of vermicompost for agricultural purposes.33 In this study, EC increased significantly in all vermicomposts. Control treatments also had higher final EC values, except in the RS1:1 treatment as compared to initial feedstocks (Tables 3 and 5). The loss of organic matter and release of minerals in the form of cations during vermicomposting are the main reasons for the EC increase.20,34 Cations such as potassium, magnesium, and calcium are contributors to EC in vermicomposts,35 which explains the higher EC in all treatments. The percentage of increase in EC also depends on the extent of organic matter mineralization under favorable experimental conditions.29 The EC of the final vermicomposts did not exceed the threshold value of 3000 μS/cm, indicating that they can be safely applied as agronomic fertilizers.33 C/N Ratio of Vermicompost. The C/N ratio is used as an indicator of stabilization and mineralization of organic waste during the vermicomposting process.36 The C/N ratio decreased for both the controls and the vermicomposts when POME was absorbed into rice straw. The treatments with earthworms showed a larger reduction in the C/N ratio relative to the treatments without earthworms (control) (Tables 3 and 5). Decreases in the C/N ratios in vermicomposts and controls using rice straw as an amendment ranged from 54.1 to 75.8% and from 35.6 to 55.4%, respectively. The greater reduction in the vermicompost C/N ratio relative to the control was due to the mineralization process caused by both the earthworms and microorganisms.19 This is because earthworms release carbon as CO2
a
1.24 2.06 3.07 2.62 3.97 3.15 ± ± ± ± ± ± 20.92 22.15 20.36 50.54 54.96 55.77 a a a b bc c 0.90 0.60 0.20 2.30 4.51 1.29 ± ± ± ± ± ±
vermicompost
11.34 16.37 9.64 49.60 46.28 41.86 0.38 0.82 0.00 0.76 0.08 0.20 ± ± ± ± ± ±
control
3.43 4.60 6.19 5.86 5.66 5.83 ab ab ab a b ab 0.77 0.53 0.77 0.65 0.85 0.93 ± ± ± ± ± ±
vermicompost
6.26 5.32 6.88 4.76 7.63 7.04 0.20 1.17 2.76 1.87 0.20 1.02 ± ± ± ± ± ±
control
19.90 13.73 17.25 27.83 27.04 32.15 a b ab b b a 1.00 1.82 2.34 0.56 1.19 4.19 ± ± ± ± ± ±
vermicompost
27.70 19.18 24.25 20.68 21.39 29.39 0.42 1.80 1.25 1.00 0.45 0.43 ± ± ± ± ± ±
control
11.77 11.63 10.07 4.71 6.60 6.15 a a a bc b c 1.63 1.68 0.45 2.03 0.03 0.18 ± ± ± ± ± ±
vermicompost
20.04 20.27 21.35 8.10 10.70 5.38 0.26 0.35 0.31 0.04 0.13 0.32 ± ± ± ± ± ± 2.54 3.00 3.55 0.34 1.63 3.40 a a a b b b 1.37 0.61 0.74 0.44 0.30 0.12 ± ± ± ± ± ±
vermicompost
5.79 5.15 6.29 1.83 1.24 1.48 140 a 57 b 151 c 132 d 185 b 122 b ± ± ± ± ± ±
control
597 2130 1141 1564 2009 2050 132 a 17 b 64 a 65 a 74 a 4b
vermicompost
± ± ± ± ± ± 0.28 0.25 0.20 0.40 0.14 0.37 ± ± ± ± ± ± 8.44 6.54 9.09 5.77 5.45 6.05 a a a b c c 0.08 0.36 0.10 0.08 0.32 0.05 ± ± ± ± ± ± 8.26 7.98 8.46 5.08 5.63 5.79 RS1:1 RS1:2 RS1:3 S1:1 S1:2 S1:3
control
a b a c c bc
1189 1734 1437 1240 1359 1682
control
a ab b d c ab
K (g/kg dry wt) P (g/kg dry wt) EC (μS/cm) pH
treatment in w/v vermicompost
Table 5. Final Chemical Characteristics of Different Treatments (Means ± SD, n = 3)a
a a a b b b
Ca (g/kg dry wt)
a b ab cd c d
Mg (g/kg dry wt)
a ab c bc bc bc
C/N ratio
control
a a a b b b
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prominent in vermicomposts than controls (Tables 3 and 5). The maximum increase in magnesium content in vermicomposts occurred in the treatments RS1:1 (161%), RS1:3 (97.7%), RS1:2 (66.3%), S1:2 (63.7%), S1:1 (9.17%), and S1:3 (8.47%). Other researchers have reported an increase in magnesium content during the vermicomposting process. For example, Raphael and Velmourougane41 reported an increase in magnesium (210%) during vermicomposting of coffee pulp amended with farmyard manure. Similarly, Liu and Price42 reported a 50% increase in magnesium during vermicomposting of spent coffee grounds. Currently, there is no known contribution of earthworms to magnesium metabolism. Therefore, it is postulated that fungal and microalgal hyphae, which easily colonize freshly deposited earthworm casts, contribute to magnesium content in the vermicomposts.43 Statistically, only treatment S1:1 was significantly different (P < 0.05) from the S1:2 treatment. In general, the overall increase in nutrient contents (phosphorus, potassium, calcium, and magnesium) in the vermicompost can be explained by the reduction in weight of the feedstock material during the vermicomposting process.21,44 According to Garg et al.,34 the concentration effect was caused by the degradation of organic matter, which reduced the volume of feedstock due to the release of CO2. In addition, the total nutrient contents in the vermicompost were generally higher than in the control, implying that the earthworms accelerated the decomposition of organic matter. Earthworm Biomass. The biomass of earthworms in all treatments had decreased by the end of the vermicomposting process (Figure 2). There were no significant differences
during respiration and produce mucus and nitrogen excrements, thus increasing the nitrogen content.37 Additionally, during a successful vermicomposting process, organic wastes are generally transformed into more stable, complex organic forms that resemble humic substances through the activity of microbial populations and earthworms.9 However, there were no significant differences between initial and final products in the POME amended with soil treatment (Tables 3 and 5). Soil consumption rates are dependent on the earthworm species, the organic matter content of the feedstock, and the quality of the soil.38 E. eugeniae is an epigeic earthworm that feeds on biodegradable waste; thus, E. eugeniae prefers fresh organic materials to soil.9 Additionally, rice straw is rich in cellulose, which could increase the carbon mineralization rate during vermicomposting due to colonization by cellulose-decomposing fungi, contributing to the lower C/N ratio as compared to soil. The presence of earthworms is known to increase cellulolytic fungal populations and cellulase activity.39 The lowest C/N ratio was observed in RS1:3 (9.64), followed by RS1:1 (11.34) and RS1:2 (16.37). There were no statistical differences between the treatments amended with rice straw. However, the C/N ratios were significantly different between the rice straw and soil amendments (P < 0.05). A final C/N ratio of approximately 10−15 is often taken as an indication of humic material formation and enhanced stability of treated organic wastes.40 Thus, it is possible for the vermicomposts derived from rice straw and POME to be used as organic fertilizers. Phosphorus, Potassium, Calcium, and Magnesium in Vermicompost. After 6 weeks of vermicomposting, all vermicomposts had higher phosphorus contents (43.7−308%), with the exception of the S1:2 treatment. Control treatments (except S1:1) also showed increases in phosphorus content, but the percentage increases were generally lower than those observed in vermicomposts (Tables 3 and 5). Similarly, there was a significant increase in potassium for all vermicomposts except in treatment of S1:3. Potassium content was highest in RS1:3 (21.35 g/kg) and lowest in S1:3 (5.38 g/kg) (Table 5). Higher percentage increases were observed in vermicomposts containing rice straw relative to those containing soil. The phosphorus and potassium contents in the rice straw and the soil treatments were statistically different (P < 0.05). Both Hait and Tare29 and John Paul et al.19 reported similar results during vermicomposting of waste-activated sludge and municipal solid waste, respectively. Increases in calcium in the vermicomposts were observed in RS1:1 (342%), as well as RS1:3 (117%) and RS1:2 (77.6%) (Tables 3 and 5). A significant increase in calcium was also reported by other researchers using the earthworm E. eugeniae in a vermicomposting process.22,41 However, the opposite was found in treatments using soil as an amendment, in which calcium increased more in the control relative to the vermicomposts. There are contradicting reports regarding the final calcium content obtained from different feedstocks. For example, Liu and Price42 also reported a greater increase in calcium content for composts (30.8−171.4%) as compared to vermicomposts (10.3−53.8%) when spent coffee grounds were used as a feedstock. In contrast, Raphael and Velmourougane41 reported a greater increase in calcium content in vermicomposts relative to composts with coffee pulp as a feedstock. The richness of organic wastes used is important in determining the final calcium level in vermicomposts.43 Magnesium content was higher in all vermicomposts and controls than in initial feedstocks, except S1:3 treatment for control. Generally, the increase in magnesium was more
Figure 2. Initial and final earthworm biomass (g) of different treatments. Lower and uppper case letters indicate significant difference (P < 0.05) between initial and final earthworm biomass, respectively.
in total earthworm biomass among the different treatments (P < 0.05). The number of earthworms also decreased in all treatments. The highest sCOD removal rates (Table 4) and lowest C/N ratios in final vermicomposts (Table 5) were observed in the treatments amended with rice straw, indicating the possibility of food shortage after 6 weeks of the vermicomposting process. Food shortage caused the earthworms to lose weight, leading to the observed reduction in biomass and to earthworm fatality. Moreover, earthworms lost weight at a rate that depended on the quantity and nature of their indigestible substrates.20 Degradation of the feedstock during vermicomposting could also result in environmental changes that would increase earthworm mortality. For example, a breakdown of polysaccharides could alter the structure of the feedstock, reducing the water retention capacity.14 Additionally, Liu and Price42 reported that high EC, low oxygen availability, 695
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and high levels of harmful organic compounds during vermicomposting were the reasons for poor earthworm survival rates. On the other hand, treatments amended with soil showed only a marginal reduction in sCOD and no significant decrease in the C/N ratio in final vermicomposts, indicating low organic matter ingestion rates by the earthworms. This was also corroborated by the higher levels of VS in treatments amended with soil. Thus, the decrease in earthworm biomass in treatments amended with soil was most likely due to the feedstock quality and its palatability.38 In general, different amendments exhibit different chemical profiles that affect earthworm growth patterns differently. Therefore, the use of rice straw as an amendment provides a more conducive environment (i.e., easily metabolized organic matter and lower concentrations of growth-retarding substances for earthworms) relative to soil.14 Shak et al.30 also came to the conclusion that growth of E. eugeniae was closely associated with feed quality. Physical Structure of Waste Mixture and Vermicompost. As a comparison of initial waste mixture (Figure 3a)
Figure 4. Photography (a) and scanning electron microscopy (b) images of vermicompost of RS1:3 treatment after 6 weeks of vermicomposting process.
are shown in Figures 3b and 4b, respectively. The SEM image revealed that the initial waste mixture was characterized by long fibers (Figure 3b). After the vermicomposting process, the vermicompost exhibited a distinct physical appearance that was more fragmented and porous (Figure 4b). The observations indicate that the long fibers of the rice straw present in the initial state were digested and fragmented by the earthworms, leading to a larger surface area in the vermicompost.28 Conclusions. This study demonstrates that vermicomposting enables the treatment and biotransformation of POME (and amendment) into mature organic fertilizer with higher nutrient contents than feedstock materials. Rice straw was proven to be a better absorbent for POME relative to soil because vermicomposts that were produced from the POME amended with rice straw had higher nutrient contents and sCOD removal rates but lower VS contents and C/N ratios. Among all of the waste mixtures, RS1:3 had the greatest reduction in sCOD (87.9%) and produced superior quality vermicompost with the lowest C/N ratio (9.64), but considerably high levels of phosphorus (6.29 g/kg), potassium (21.35 g/kg), calcium (24.25 g/kg), and magnesium (6.88 g/kg). The present results suggest that the vermicomposting process can be considered a viable waste treatment technology for POME that is amended with rice straw. This process utilized organic wastes to produce organic fertilizer or vermicompost, suitable for agronomic application. The vermicompost produced could also be used for soil remediation purposes.
Figure 3. Photography (a) and scanning electron microscopy (b) images of waste mixture of RS1:3 treatment.
and the vermicompost obtained from RS1:3 (Figure 4a), the latter had finer granular structure and was darker in color. The vermicompost was also odorless after 6 weeks of vermicomposting. These observations were similar to those in previous studies conducted by Lim et al.25 and Yadav and Garg.45 However, some rice straw residues could still be observed in the final vermicompost (Figure 4a), suggesting that the initial waste mixture was not completely digested by the earthworms. Thus, the duration of the vermicomposting process could be extended to ensure complete biotransformation of rice straw into organic fertilizer. Scanning electron microscopy (SEM) images of the initial waste mixture and final vermicompost in treatment RS 1:3
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This research was supported by the Department of Higher Education, Malaysia, under Fundamental Research Grant Scheme (FRGS/1/2013/STWN03/MUSM/02/1). In addition, we thank Monash University, Sunway campus, for providing Su Lin Lim with a Ph.D. scholarship. Notes
The authors declare no competing financial interest.
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