Vermistabilization of primary sewage sludge

Bioresource Technology 102 (2011) 2812–2820

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Vermistabilization of primary sewage sludge Subrata Hait, Vinod Tare ⇑ Environmental Engineering and Management Programme, Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur 208 016, India

a r t i c l e

i n f o

Article history: Received 15 July 2010 Received in revised form 4 October 2010 Accepted 6 October 2010 Available online 12 October 2010 Keywords: Activated composting Vermicomposting Primary sewage sludge Eisenia fetida Compost recycling

a b s t r a c t An integrated composting–vermicomposting process has been developed for utilization of primary sewage sludge (PSS). Matured vermicompost was used as bulking material and a source of active microbial culture during aerobic activated composting (AAC). AAC resulted in sufficient enrichment of bulking material with organic matter after 20 cycles of recycling and mixing with PSS and produced materials acceptable for vermicomposting. Vermicomposting caused significant reduction in pH, volatile solids (VS), specific oxygen uptake rate (SOUR), total organic carbon (TOC), C/N ratio and pathogens and substantial increase in electrical conductivity (EC), total nitrogen (TN) and total phosphorous (TP) as compared to compost. Environmental conditions and stocking density have profound effects on vermicomposting. Temperature of 20 °C with high humidity is favorable environmental condition for vermicomposting employing Eisenia fetida. Favorable stocking density range for vermiculture is 0.5–2.0 kg m 2 (optimum: 0.5 kg m 2) and for vermicomposting is 2.0–4.0 kg m 2 (optimum: 3.0 kg m 2), respectively. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction In India, primary sewage sludge (PSS) is generated in huge quantities and poses challenges in safe disposal due to the presence of certain soil contaminants, such as organic compounds, heavy metals, and human pathogens. Most of the conventional means of sewage sludge disposal like open dumping, sanitary land-filling, aerobic and anaerobic digestion and incineration have created more serious problems like soil and plant toxicity, surface and ground waters contamination and air pollution. Furthermore, the ever increasing cost and unavailability of land near urban areas, more stringent waste disposal regulations and public awareness have made open dumping and land-filling increasingly expensive and impractical (Ndegwa and Thompson, 2001). This frightening situation of sludge disposal and management is of no exception in other developing countries and probably prevails in other parts of the world too (Abbasi and Ramasamy, 2001). More recently, alkaline extraction for production of surface active value-added agents (García Becerra et al., 2010) and production of hydrogen as alternative energy (Massanet-Nicolau et al., 2010) have been employed to utilize sewage sludge, although these techniques are economically expensive. Hence, there is an utmost need for ecologically as well as economically sustainable technologies which encourage possible recovery and recycling of nutrients and capable of sanitizing the human pathogens present in sewage sludge.

⇑ Corresponding author. Tel.: +91 512 259 7792; fax: +91 512 259 7797. E-mail address: [email protected] (V. Tare). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.10.031

Sludge treatment wetlands, also known as sludge drying reed beds, have been used in many regions of the world as a new sustainable technology based on constructed wetlands for sludge treatment (Peruzzi et al., 2009; Uggetti et al., 2009, 2010; Melidis et al., 2010). The major limitations of sludge treatment wetlands involving dewatering processes (draining and evapotranspiration) are requirements of large land area, favorable climatic conditions and further treatment to improve sludge hygienisation (Zwara and Obarska-Pempkowiak, 2000). Composting, another such ecologically and economically sustainable technology has been widely used over the years for stabilizing and sanitizing the sewage sludge with bulking agents like green wastes, spent activated clay (bentonite), etc. (Tandy et al., 2009; Ho et al., 2010). The major drawbacks associated with traditional thermophilic composting are the long duration of the process, the requirement of frequent turning of the material, the necessity of reduced size of materials to provide required surface area, loss of nutrients during the prolonged composting process, and heterogeneous nature of the product (Ndegwa and Thompson, 2001). Vermicomposting is emerging as a most appropriate alternative to conventional aerobic composting. Vermicomposting is not only rapid, easily controllable, cost effective, energy saving, and zero discharge process, but also accomplishes most efficient recycling of organics and nutrients (Eastman et al., 2001). In contrast to traditional composting, vermicomposting results in homogeneous product (vermicompost) with better quality in terms of desirable aesthetics, reduced levels of contaminants and more soluble and available plant nutrients (Ndegwa and Thompson, 2001). The use of earthworms in sludge processing and management has been

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termed as vermistabilization (Neuhauser et al., 1988). In the recent past, vermicomposting has proven highly useful and competitive for stabilization of sewage sludge (Ndegwa et al., 2000; Ndegwa and Thompson, 2001; Contreras-Ramos et al., 2005; Gupta and Garg, 2008; Khwairakpam and Bhargava, 2009). Literatures on vermistabilization of sewage sludge reveal that most of the studies are concerned with vermicomposting of wasted activated sludge (i.e. a byproduct of secondary wastewater treatment), but sparse literatures are available on the vermistabilization of PSS. Gupta and Garg (2008) have attempted the vermistabilization of PSS amended with cow dung by using the epigeic earthworm Eisenia fetida. It is also evident from the literatures that sewage sludge was vermicomposted either in dewatered or dried state rather than its fresh semi-liquid state in most of the studies. Organic-rich external supplemental materials were used as bulking/amendment material in most of the studies. There is a paucity of data available on the effects of varying environmental conditions on the vermicomposting of sewage sludge. Also, sparse literatures are available on the vermistabilization of sewage sludge under controlled environmental conditions. In these contexts, the present study was set to assess the suitability of a combined system approach comprising of aerobic activated composting (AAC) through recycling of matured vermicompost eliminating the need for external bulking/amendment material and subsequent vermicomposting employing epigeic earthworm E. fetida for processing of fresh semi-liquid PSS for bioconversion and recycling. The effects of environmental condition (i.e. temperature and relative humidity) and earthworm density on vermicomposting are also examined. 2. Methods 2.1. Earthworms (E. fetida) Exotic earthworm species E. fetida of different age groups were cultured and developed outside the laboratory on partially degraded animal dung as feed. Separate cultures that fed on the mixture of partially degraded animal dung and sewage sludge were also maintained by keeping optimum moisture levels. The earthworms (both juvenile and adult) were randomly picked from the separately maintained cultures and used for the purpose of this investigation. 2.2. Primary sewage sludge (PSS) Fresh PSS was procured on periodical basis whenever required from primary clarifier of 130 mld (million liters per day) activated sludge process (ASP)-based sewage treatment plant (STP) at Jajmau, Kanpur, India. The physico-chemical and microbiological characteristics of PSS are given in Table 1. 2.3. Bulking material Bulking material is needed for enrichment of the diversity of microbial population and enzymatic activities in addition to improving the physical environment for the sustenance in the experimental reactors. In the present study, fresh and matured vermicompost, collected from stock culture reactors, was used as bulking material in the start-up operation. The physico-chemical and microbiological characteristics of bulking material are also reported in Table 1. 2.4. Experimental design 2.4.1. Step I: aerobic activated composting (AAC) The aerobic activated composting (AAC) experiments were performed in duplicate heaps in the laboratory backyard under a shed

Table 1 Physico-chemical and microbiological characteristics of primary sewage sludge and bulking material. S. No.

Parameter

Primary sewage sludge (PSS)

Bulking material (VC)

1

Water holding capacity (%) pH EC (dS m 1) Total solids (%) Volatile solids (%) SOUR (mg-O2 g-VS 1 h 1) TC (%) TOC (%) TN (%) C/N ratio TP (%) TK (mg g 1) Total coliforms (MPN g-dw 1) Fecal coliforms (MPN g-dw 1) Salmonella (MPN g-dw 1) Enterococcus (MPN g-dw 1) Viable Helminths ova (ova 4 g-dw 1)

NM

74.30 ± 0.86

7.47 ± 0.27 2.05 ± 0.29 7.6 ± 0.8 51.8 ± 2.6 42.7 ± 5.9

7.82 ± 0.21 1.46 ± 0.07 50.24 ± 0.68 35.40 ± 0.78 0.17 ± 0.09

22.79 ± 1.30 21.53 ± 1.11 1.70 ± 0.16 13.51 ± 1.16 0.72 ± 0.27 5.50 ± 0.58 3.05  106 ± 9.98  105

24.15 ± 0.60 22.95 ± 0.44 1.58 ± 0.07 15.34 ± 0.37 1.03 ± 0.23 10.17 ± 0.61 <3.60

2.65  105 ± 5.26  104

<3.60

1.35  104 ± 2.08  103

<3.60

2.68  104 ± 6.74  103

<3.60

4.1 ± 0.4

ND

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

ND: not detected, NM: not measured. Values indicate mean ± standard deviation based on 20 samples with triplicates (n = 60).

under ambient conditions. In order to cater high water content of fresh semi-liquid PSS, the mixing of bulking material was necessary depending on the water holding capacity (WHC) of the bulking material and the percentage of total solids in the PSS. Based on the observation from preliminary trial, bulking material was mixed with the PSS at the ratio of 1.5:1 (weight basis). Five liters of fresh PSS was mixed with 7.5 kg fresh bulking material (vermicompost) and a composting heap was prepared. The composting heaps were manually turned once in a day to ensure aerobic degradation. A total of 20 cycles of recycling and mixing (loading) of bulking material with PSS were performed during AAC till the mixture material tends to deteriorate in terms of WHC. Mixing and recycling duration of bulking material depends upon the loss of water content in the material based on the ambient conditions like temperature and relative humidity (RH). The moisture content of the heaps was always maintained above 40% during the entire activated composting period to preserve the microbial activity within the heaps. Changes in pH and electrical conductivity (EC) of the heaps were also monitored daily. The mixture material was analyzed for pH, EC, WHC, moisture content and organic matter before and after each mixing. The composted material was analyzed for various physico-chemical and microbiological parameters prior to vermicomposting process. 2.4.2. Step II: vermicomposting Vermicomposting of composted material was performed in triplicate in transparent PVC reactors under controlled environmental conditions using a temperature–RH controlled chamber. The reactors were covered using perforated lids to provide proper air ventilation. The detailed design of the experimental conditions is presented in Table 2. The treatments or the factors of interest were the environmental conditions and the stocking density, at the six levels and the eight levels, respectively. This experimental design fits the classical two-way factorial design approach and helps to save on resources by efficiently using one reactor (bin) to simultaneously provide as a replicate for an environmental condition and a stocking density variant. Varying temperature (10–30 °C) and RH (50% and 90%) combinations were examined

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2.6. Statistical analysis

Table 2 Details of the experimental design for vermicomposting. S. No.

Experimental condition

Nomenclature

Reactor

Stocking density (SD) (kg m 2)

1

RH: 50%, Temp.: 10 °C RH: 90%, Temp.: 10 °C RH: 50%, Temp.: 20 °C RH: 90%, Temp.: 20 °C RH: 50%, Temp.: 30 °C RH: 90%, Temp.: 30 °C

C1

PVC Reactor (Bin) Dia.: 180 mm Depth: 90 mm Surface area: 0.025 m2

0–5.0 (SD0, SD0.5, SD1.0, SD1.5, SD2.0, SD3.0, SD4.0, SD5.0)

2 3 4 5 6

C2 C3 C4 C5 C6

in order to study the seasonal effects of varying environmental conditions on the vermicomposting of sewage sludge. Stocking density of earthworms was also varied in the range of 0–5.0 kg m 2 to observe the effects of varying stocking density on the vermistabilization of sewage sludge. The reactors with earthworm densities below 1.0, in the range 1.0–2.0, and above 2.0 kg m 2 were considered as low, medium and high stocking density reactors, respectively. All the replicates were fed in a single batch with enough composted material for the entire four weeks duration of vermicomposting study. Acclimated earthworms (E. fetida) were introduced into each of the respective similar reactors as per the target stocking density. The moisture level was maintained through out the study period at 70 ± 5% of WHC of the feeding material by periodic spraying/sprinkling of an adequate quantity of tap (potable) water using a spray can. The vermicomposting process was terminated at the end of the fourth week for every treatment combination after which each reactor was manually examined for survival and growth of earthworms and offspring production. Total weight of cast was estimated and the daily cast production rate was calculated based on the weekly cast production. The vermicompost was analyzed for various physico-chemical and microbiological parameters. 2.5. Physico-chemical and microbiological analysis All the parameters of PSS were analyzed according to the procedures outlined in the Standard Methods (APHA, AWWA, WEF, 1995). The analysis of pH, EC, WHC, moisture content, VS and various microbiological parameters like total and fecal coliforms, Salmonella, Enterococcus and viable helminths (Ascaris) ova for compost were performed by methods as mentioned in USDA and USCC (2002). Moisture content was determined upon drying the samples to constant weight at 70 ± 2 °C for 24–72 h by using hot air oven. The WHC was determined by submerging the material for 24 h in water and draining the excess water for 30 min. For determination of pH and EC, 1:10 (w/v) suspension in de-ionized water was mixed at 230 rpm for 30 min. Organic matter content (loss on ignition) was obtained by combusting the dried samples in muffle furnace at 550 ± 5 °C for 2 h. The SOUR was determined as per the procedures outlined by Lasaridi and Stentiford (1998). The TC and TN were determined by using Elemental Analyzer (Model: CE440, Leeman Labs Inc., USA) on dried samples. The TOC was determined on dried samples using Total Organic Carbon Analyzer (Model: TOC-VCPN, Shimadzu, Japan). TP was analyzed by acid digesting the samples and subsequently using ammonium molybdate method (APHA, AWWA, WEF, 1995; USDA, USCC, 2002). TK analysis was done using atomic absorption spectrophotometer (Model: 220 FS, Varian, Australia) by digesting the dried samples (USEPA 3051 method modified for compost) in microwave digester (Model: V-800, Varshal Inc., USA).

Since the experimental design fits the classical two-way factorial design, a two-way analysis of variance (ANOVA) was therefore used to analyze the significant difference between different experimental conditions. Tukey’s HSD (honestly significant differences) test was also performed as a post hoc analysis to identify the homogeneous type of the experimental conditions. The probability levels used for statistical significance were P < 0.05 for the tests. The SPSS Package (Version 16) was used for the statistical analysis.

3. Results and discussion 3.1. Physico-chemical and microbiological changes during aerobic activated composting The AAC stage can be considered as the main processing unit since the major utilization and treatment of PSS were performed during this stage. It was observed that the organics content of mixture material was slowly increasing and building-up with successive mixing (loading) cycle. The AAC stage through recycling and several cycles of mixing of bulking material with PSS served dual role of concentration and simultaneously natural degradation of organics. The changes in some typical parameters like pH, EC, VS and WHC of mixture material for successive mixing cycle during AAC are presented in Table 3. The changes i.e. difference in added and measured value after each mixing can be attributed to natural degradation of organics during composting. The mixture material was observed to be deteriorated in terms of WHC and may become hard for earthworms to breakdown after 20 cycles. The WHC of the mixture material was decreased from an initial value of 74.30 ± 0.86% to 49.80 ± 0.47% after 20 cycles of mixing and activated composting. Both pH and EC of the mixture material were found to increase from an initial value of 7.82 ± 0.21 and 1.46 ± 0.07 dS m 1 to 8.72 ± 0.06 and 5.16 ± 0.04 dS m 1, respectively, after 20 cycles of mixing and activated composting. A net average increase of 59.4% (from 35.40 ± 0.78% to 56.44 ± 0.16%) in VS content of the mixture material was observed after 20 cycles of mixing (i.e. addition of organics) and activated composting (i.e.

Table 3 Changes in some typical parameters of mixture material for successive mixing cycle during aerobic activated composting. Mixing cycle

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

EC (dS m

1

Volatile solids (%)

pH

PSS

M

PSS

M

PSS

M

)

52.06 56.23 49.53 50.74 48.14 50.93 53.98 55.34 52.19 50.02 47.26 48.93 51.47 48.24 52.36 54.31 51.67 52.69 56.05 53.91

37.59 39.21 40.96 42.42 43.45 44.58 45.81 47.13 48.06 48.83 49.79 50.70 51.53 52.12 52.98 53.77 54.57 55.29 55.86 56.44

7.75 7.87 7.23 7.29 7.11 7.16 7.06 7.40 7.47 7.42 7.48 7.47 7.89 7.70 7.45 7.61 7.90 7.77 7.23 7.12

8.03 8.12 8.23 8.31 8.36 8.43 8.51 8.56 8.64 8.73 8.62 8.57 8.70 8.76 8.82 8.95 9.05 8.91 8.86 8.72

2.10 2.21 2.09 2.38 2.06 1.89 2.49 2.43 1.99 2.16 1.50 1.67 2.04 1.73 1.60 1.88 2.18 2.59 2.06 1.95

1.71 1.97 2.18 2.35 2.57 2.62 2.77 2.96 3.15 3.34 3.50 3.72 3.94 4.15 4.36 4.55 4.73 4.87 5.03 5.16

PSS: primary sewage sludge, M: mixture of PSS and bulking material. Values indicate mean of two samples with triplicates (n = 6).

WHC (%)

73.7 72.9 72.2 71.4 70.8 69.9 69.2 68.1 67.1 65.9 64.8 63.5 62.4 61.1 59.6 58.1 56.2 54.0 51.9 49.8

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S. Hait, V. Tare / Bioresource Technology 102 (2011) 2812–2820 Table 4 Physico-chemical and microbiological characteristics of compost from aerobic activated composting.

Table 5 Summary of two-way ANOVA results of different physico-chemical and earthworm parameters.

S. No.

Parameter

Compost

Source of variation

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Water holding capacity (%) pH EC (dS m 1) Volatile solids (%) SOUR (mg O2 g-VS 1 h 1) TC (%) TOC (%) TN (%) C/N ratio TP (%) TK (mg g 1) Total coliforms (MPN g-dw 1) Fecal coliforms (MPN g-dw 1) Salmonella (MPN g-dw 1) Enterococcus (MPN g-dw 1) Viable Helminths ova (ova 4 g-dw

49.80 ± 0.52 8.72 ± 0.07 5.16 ± 0.04 56.44 ± 0.18 22.70 ± 0.25 32.43 ± 0.11 30.89 ± 0.28 2.27 ± 0.16 14.27 ± 0.62 1.42 ± 0.05 13.10 ± 0.42 1.56  106 ± 2.13  105 1.31  105 ± 1.78  104 1.43  103 ± 1.53  102 8.01  103 ± 9.62  102 2.7 ± 0.4

pH Environmental conditions Stocking densities Error

1

)

Values indicate mean ± standard deviation based on two samples with triplicates (n = 6).

degradation of organics). Also, Kaushik and Garg (2003) pointed out that pre-composting is essential to prevent earthworm mortality in vermicomposting by releasing volatile gases potentially toxic to earthworms. The complete physico-chemical and microbiological characteristics of compost are shown in Table 4. 3.2. Vermicomposting 3.2.1. Physico-chemical and microbiological changes during vermicomposting Vermicomposting showed significant impact in changing the physico-chemical and microbiological properties of the compost material. Both the environmental condition (i.e. temperature and humidity) and earthworm stocking density exerted statistically significant (P < 0.05) effect on the changes in various physicochemical parameters during vermicomposting (Table 5). Only the environmental condition rather than stocking density had significant (P < 0.05) effect on the changes in potassium content and all the microbiological parameters. As compared to the control (SD0), the vermicompost showed significant difference (P < 0.05) for all the physico-chemical and microbiological parameters of the substrate under all experimental conditions. Vermicompost showed a great improvement in terms of WHC as compared to the compost and had a value 72.70 ± 0.62%. A slight decrease in pH in vermicompost relative to the initial composted material under all the experimental conditions was observed. The maximum reduction was in experimental reactors C4 and SD3.0, whereas C5 and SD0.5 showed the lowest reduction for pH. The pH of the vermicompost was in the range of 7.88–8.48 (Fig. 1). The decreasing trend in pH during vermicomposting corroborates with the findings of other researchers (Ndegwa et al., 2000; Khwairakpam and Bhargava, 2009; Gupta and Garg, 2008). The decrease in pH during vermicomposting may be due to CO2 and organic acids produced by microbial metabolism (Elvira et al., 1998). A significant increase in EC in vermicompost was observed in all the reactors as compared to the initial compost material. The EC was increased in the range of 27.6–66.1% for vermicompost and 10.4–32.0% for control. The maximum increase in EC was observed in experimental reactors C4 and SD3.0 (66.1%) while C5 and SD0.5 (27.6%) showed the minimum increase for EC. The final EC of the vermicompost varied from 6.65 to 8.52 dS m 1 (Fig. 1). Some researchers (Kaviraj and Sharma, 2003) postulated that the increase in EC might have been due to loss of organic matter and release of different mineral salts in available forms such as phosphate, ammonium, potassium, etc.

EC Environmental conditions Stocking densities Error VS Environmental conditions Stocking densities Error SOUR Environmental conditions Stocking densities Error TC Environmental conditions Stocking densities Error TOC Environmental conditions Stocking densities Error TN Environmental conditions Stocking densities Error C/N ratio Environmental conditions Stocking densities Error TP Environmental conditions Stocking densities Error Total biomass (final) Environmental conditions Stocking densities Error Individual weight (final) Environmental conditions Stocking densities Error Earthworm growth rate Environmental conditions Stocking densities Error Cocoons/worm Environmental conditions Stocking densities Error Hatchlings/worm Environmental conditions

SS

df

MS

F

P

0.561

5

0.112

86.170

0.000

2.856 0.125

7 96

0.408 0.001

313.388

0.000

3.484

5

0.697

23.117

0.000

42.097 2.894

7 96

6.014 0.030

199.520

0.000

48.192

5

9.638

186.286

0.000

2217.318 4.967

7 96

316.760 0.052

6122.192

0.000

2.147

5

0.429

12.764

0.000

1818.386 3.230

7 96

259.769 0.034

7721.180

0.000

21.766

5

4.353

136.533

0.000

343.381 3.061

7 96

49.054 0.032

1538.559

0.000

20.073

5

4.015

144.574

0.000

316.178 2.666

7 96

45.168 0.028

1626.586

0.000

3.087

5

0.617

29.858

0.000

6.504 1.985

7 96

0.929 0.021

44.938

0.000

27.430

5

5.486

54.327

0.000

145.214 9.694

7 96

20.745 0.101

205.439

0.000

0.918

5

0.184

32.065

0.000

6.076 0.550

7 96

0.868 0.006

151.553

0.000

2979.753

5

595.951

21959.930

0.000

58363.656 2.280

6 84

9727.276 0.027

358436.209

0.000

0.050

5

0.010

390.833

0.000

0.938 0.002

6 84

0.156 0.000

6132.715

0.000

61.962

5

12.392

9519.469

0.000

1613.057 0.109

6 84

268.843 0.001

206518.561

0.000

32.164

5

6.433

2245.891

0.000

82.084 0.241

6 84

13.681 0.003

4776.298

0.000

5.619

5

1.124

4396.895

0.000

(continued on next page)

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S. Hait, V. Tare / Bioresource Technology 102 (2011) 2812–2820

Table 5 (continued) Source of variation

SS

Stocking densities Error Cast production rate Environmental conditions Stocking densities Error

df

MS

F

P

16.516 0.021

6 84

2.753 0.000

10770.810

0.000

0.280

5

0.056

117.423

0.000

0.158 0.040

6 84

0.026 0.000

55.255

0.000

SS = sum of square, df = degrees of freedom, MS = mean of squares, F = likelihood ratio, P = probability.

Significant reduction in VS is an important indicative parameter for decomposition, substrate mineralization and compost maturity. The final VS of the vermicompost was in the range of 36.04–41.28% (Fig. 1) and was lower when compared to the initial composted material under all the experimental conditions. The loss in VS

8.7

pH

8.5 8.3 8.1

-1

Electrical conductivity (dS m )

7.9 8.6 8.1 7.6 7.1 6.6 6.1 5.6

51.0 49.5 48.0

-1

42.0 40.5 39.0 37.5 36.0 12.0

-1

SOUR (mg-O2 g-VS h )

Volatile solids (%)

52.5

11.0 10.0 1.0 0.8 0.6 0.4 0.2 0.0

0.5

1.0

1.5

2.0

3.0

4.0

5.0

-2

Stocking density (kg m ) C1

C2

C3

C4

C5

C6

Fig. 1. Effect of environmental conditions and stocking densities on pH, EC, VS and SOUR during vermicomposting of primary sewage sludge.

content during vermicomposting was in the range of 26.9–36.1%. The maximum loss was observed in experimental reactors C4 and SD3.0 (36.1%), whereas C5 and SD0.5 (26.9%) showed the minimum loss in VS content. A reduction of 37% in VS content was achieved after 2 months of vermicomposting of green waste (Frederickson et al., 1997). The loss in VS content for control was in the range of 7.3–14.4%. Contreras-Ramos et al. (2005) reported that greater loss in VS content meant the enhanced degradation and mineralization in the presence of earthworms during vermicomposting of organic waste. The SOUR or respiration rate is one of the most reliable indicators for compost maturity (Lasaridi and Stentiford, 1998). The SOUR value in all reactors except control at the end of experimental period complied with the USEPA (1995) regulation of SOUR value of 1.5 mg-O2 g-TS 1 h 1 for aerobically stabilized biosolids as shown in Fig. 1 and thus implied maturation of vermicompost. Yadav et al. (2010) have reported that the SOUR decreased from an initial value of 70 ± 5–0.15 ± 0.09 mg-O2 g-VS 1 h 1 in the fully matured cast during vermicomposting of source-separated human feces. Total carbon (TC) and total organic carbon (TOC) of the final vermicompost were significantly lower compared to the initial composted material under all the experimental conditions. The loss of TC content during vermicomposting was in the range of 13.1–25.2%. The maximum loss was observed in experimental reactors C4 and SD3.0 (25.2%), whereas C5 and SD0.5 (13.1%) showed the minimum loss of TC content. The loss of TOC during vermicomposting followed the same trend as for TC and varied in the range of 13.7–24.8%. The maximum loss was observed in experimental reactors C4 and SD3.0 (24.8%), whereas C5 and SD0.5 (13.7%) showed the minimum loss of TOC content. The TC and TOC contents of the vermicompost were in the range of 24.24–28.10% and 23.21– 26.69%, respectively (Fig. 2). The significant differences in TC and TOC contents of the control (SD0) and vermicompost suggest that the earthworms-mediated rapid organic matter mineralization occurs during vermicomposting. The losses of TC and TOC in the control reactors (SD0) were in the range of 2.7–10.8% and 1.7– 10.2%, respectively. The differences in patterns of loss of carbon between the treatments in the present study were related to the varying environmental conditions (i.e. temperature and RH) and stocking densities (Fig. 2). The observed results in the present study are supported by those of other researchers (Kaviraj and Sharma, 2003; Suthar and Singh, 2008; Khwairakpam and Bhargava, 2009) who have reported 10–45% loss of carbon as CO2 through microbial respiration during vermicomposting of municipal or industrial wastes. A significant increase in TN content in vermicompost was observed in all the reactors as compared to the initial composted material. The TN content of the vermicompost varied from 2.92– 4.15% (Fig. 2). The increase in TN content during vermicomposting was in the range of 31.3–90.0%. The maximum increase was observed in experimental reactors C4 and SD3.0 (90.0%), whereas C1 and SD0.5 (31.3%) showed the minimum increase for TN content. The increase in TN content for control was in the range of 15.2– 49.5%. The increasing trend in TN content during vermicomposting corroborates with the findings of other researchers (Kaushik and Garg, 2004; Suthar and Singh, 2008; Khwairakpam and Bhargava, 2009). The reduction in dry mass i.e. organic carbon in terms of CO2 due to substrate utilization by earthworms and microorganisms and their metabolic activities as well as addition of earthworm N-excrements and evaporative moisture loss might have led to relative increase in TN content (Viel et al., 1987; Suthar and Singh, 2008). Therefore, it is suggested that TN content in vermicompost depends upon the nitrogen content and C/N ratio in the parent substrate, extent of decomposition and evaporative moisture loss due to prevailing environmental condition.

S. Hait, V. Tare / Bioresource Technology 102 (2011) 2812–2820

32.0

TC (%)

30.0 28.0 26.0

TOC (%)

24.0 31.0 29.0 27.0 25.0 23.0 4.2

TN (%)

3.9 3.6 3.3 3.0

C/N ratio

2.7 2.4 13.0 12.0 11.0 10.0 9.0 8.0 7.0 6.0 2.6

TP (%)

2.4 2.2 2.0 1.8 1.6 1.4 0.0

0.5

1.0

1.5

2.0

3.0

4.0

5.0

-2

Stocking density (kg m ) C1

C2

C3

C4

C5

C6

Fig. 2. Effect of environmental conditions and stocking densities on TC, TOC, TN, C/N ratio and TP during vermicomposting of primary sewage sludge.

The change in C/N ratio reflects the degree of organic waste mineralization and stabilization rate during the process of composting and vermicomposting. The C/N ratio below 20 is indicative of an advanced degree of stabilization and acceptable maturity, while a ratio of 15 or less being preferable for agronomic use of composts as plants cannot assimilate nitrogen unless the ratio is in the order of 20 or less (Senesi, 1989; Morais and Queda, 2003). The C/N ratio of the final vermicompost was decreased considerably as compared to the initial compost material under all the experimental conditions. The C/N ratio of the vermicompost was in the range of 5.97–9.36 (Fig. 2). The reduction in C/N ratio during vermicomposting was in the range of 31.3–59.8%. The maximum reduction was observed in experimental reactors C4 and SD3.0 (59.8%), whereas C5 and SD0.5 (31.3%) showed the minimum reduction for C/N ratio. The decreasing trend of C/N ratio during vermicomposting is in good agreement with other authors (Gupta and Garg, 2008; Suthar and Singh, 2008; Khwairakpam and Bhargava, 2009). The C/N ratio of the control (SD0) varied from 8.59–12.91.

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Therefore, in the present study, a high degree of organic matter stabilization was achieved in all the reactors and certainly indicative of the agronomic potential of vermicompost. The loss of organic carbon as CO2 due to microbial respiration and addition of earthworm N excrements increase the levels of nitrogen and thereby lowers the C/N ratio during vermicomposting (Senapati et al., 1980). The decrease in C/N ratio over time might also be attributed to rapid reduction in organic carbon due to increase in earthworm population. Total phosphorous (TP) content of the final vermicompost was significantly increased as compared to the initial composted material under all the experimental conditions. The TP content of the vermicompost varied from 1.78–2.64% (Fig. 2). The increase in TP content during vermicomposting was in the range of 30.1–86.1%. The maximum increase was observed in experimental reactors C4 and SD3.0 (86.1%), whereas C5 and SD0.5 (30.1%) showed the minimum increase for TP content. The increase in TP content for control was in the range of 8.1–29.7%. The increasing trend in TP content during vermicomposting is consistent with the findings of other researchers (Gupta and Garg, 2008; Suthar and Singh, 2008; Khwairakpam and Bhargava, 2009). It has been postulated that the increase in TP content during vermicomposting is probably through mineralization, the release and mobilization of available P content from organic waste performed partly by earthworm gut phosphates, and further release of P might be due to the P-solubilizing microorganisms present in worm casts (Lee, 1992; Ghosh et al., 1999). Micro-nutrients like potassium (K) are required for assimilation by earthworms during the vermicomposting, although the quantity required is very low as compared to the initial content present in the parent feed material. Potassium was observed to be increasing as compared to the initial compost material in all the reactors by 30.1–51.9% and 7.4–24.0% for vermicompost and control, respectively. A similar increasing trend in TK content of vermicompost was reported by other authors (Gupta and Garg, 2008; Khwairakpam and Bhargava, 2009). Acid production by the microorganisms and enhanced mineralization rate through increased microbial activity during the vermicomposting play major role in solubilizing of insoluble potassium (Kaviraj and Sharma, 2003; Khwairakpam and Bhargava, 2009). The microbiological parameters in terms of total and fecal coliforms, Salmonella, Enterococcus and viable Helminth ova contents in the final vermicompost are presented in Table 6. As compared to the initial composted material, the vermicomposting process caused significant reduction in all the microbiological parameters. The vermicomposting process produced completely sanitized product under all the environmental conditions except C5 and C6. The vermicompost produced at environmental conditions C5 and C6 was not completely sanitized, but the pathogen contents were well below the Class A and B limits of biosolids set by the USEPA (1995) making the vermicompost suitable for agronomic application. It is suggested that oxygen deprivation at higher temperature may have a negative effect on earthworm activity and this may be the reason for not achieving the complete sanitization of vermicompost at environmental conditions C5 and C6. Dominguez and Edwards (2004) reported that high temperatures (above 30 °C) promoted microbiological activity in the vermicomposting system, that tended to consume the available oxygen, and thus had negative effects on earthworm activity. The pathogen reduction during vermicomposting can be attributed to various earthworm actions like intestinal enzymatic action, secretion of coelomic fluids having antibacterial properties and selective grazing (Dominguez and Edwards, 1997; Eastman et al., 2001; Panikkar et al., 2004). It can be inferred from the present study that the combined composting– vermicomposting process can achieve complete sanitization under favorable environmental conditions.

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Table 6 Pathogens contents in vermicompost obtained from primary sewage sludge. Environmental Total coliforms conditions (MPN g-dw-1)

C1 C2 C3 C4 C5 C6 Class A limitsa Class B limitsa

Fecal coliforms (MPN g-dw 1)

Salmonella (MPN g-dw

Enterococcus (MPN g-dw 1)

1

)

Viable Helminth ova (ova 4 g dw 1)

C

VC

C

VC

C

VC

C

VC

C

VC

4.59  103 ± 513a 3.81  103 ± 330b 2.89  103 ± 385c 2.53  103 ± 231 cd 2.25  103 ± 528ce 2.07  103 ± 256de NM

<3.60 <3.60 <3.60 <3.60 20.67 ± 4.04a 15.85 ± 1.68a

1.91  103 ± 253a 1.54  103 ± 224b 1.19  103 ± 156c 9.55  102 ± 168 cd 8.59  102 ± 112ce 7.80  102 ± 175de 1.00  103

<3.60 <3.60 <3.60 <3.60 9.83 ± 1.15a 8.67 ± 1.76a

3.41  102 ± 26a 3.05  102 ± 33ab 2.67  102 ± 44bc 2.16  102 ± 14 cd 2.04  102 ± 22d 1.78  102 ± 23d 3.00

<3.60 <3.60 <3.60 <3.60 6.50 ± 0.50a 5.91 ± 1.19a

1.23  103 ± 93a 1.05  103 ± 132b 9.63  102 ± 26bc 8.22  102 ± 38c 6.48  102 ± 33d 4.67  102 ± 67e NM

<3.60 <3.60 <3.60 <3.60 8.38 ± 1.19a 5.93 ± 1.43a

2.0 ± 0.2a 1.8 ± 0.1ab 1.7 ± 0.2b 1.4 ± 0.2c 1.2 ± 0.2c 0.8 ± 0.1d 1.0

ND ND ND ND ND ND

NM

2.00  106

NM

NM

NM

C: control; VC: vermicompost; ND: not detected; NM: not mentioned. Values followed by the same letter within each column are not significantly different (ANOVA; Tukey’s test, P < 0.05). a USEPA (1995) limits for pathogens for biosolids. A: biosolids of excellent quality; B: biosolids of good quality.

3.2.2. Biomass growth and reproduction of earthworms The environmental conditions and stocking densities exerted statistically significant (P < 0.05) effect on earthworm total biomass, individual weight, growth rate and offspring production during vermicomposting of PSS (Table 5). No mortality was observed in any reactor during the entire vermicomposting period as the composting was done prior to vermicomposting and thereby all the toxic gases produced might have been eliminated. It is established that pre-composting is essential to avoid the earthworm mortality (Kaushik and Garg, 2003). The changes in final biomass of E. fetida as a function of environmental conditions and stocking densities are presented in Fig. 3. At environmental conditions C1, C5 and C6, the final earthworm biomass was observed to increase by 50.6–70.9% for stocking densities 0.5–2.0 kg m 2 whereas it was found to decrease by 11.8–41.7% for stocking densities above 2.0 kg m 2 as compared to the initial earthworm biomass. The final earthworm biomass was found to increase by 10.5–84.5% for stocking densities 0.5–3.0 kg m 2 and decrease by 20.6–33.4% for stocking densities above 3.0 kg m 2 as compared to the initial earthworm biomass at environmental conditions C2, C3 and C4. The earthworm biomass varied significantly with the stocking density and the final earthworm biomass was observed to increase by 57.1–78.7% for low stocking density (SD0.5) and by 50.6–84.5% for medium stocking densities (SD1.0– SD2.0) as compared to the initial biomass. At higher stocking densities (SD3.0–SD5.0), the final biomass was observed to increase by 10.5–28.9% for environmental conditions C1, C5 and C6 and decrease by 11.8–41.7% for environmental conditions C2, C3 and C4 as compared to the initial biomass. A maximum increase of 84.5% (from 37.53 to 69.24 g) in earthworm biomass was observed at environmental condition C3 (i.e. Temperature: 20 °C and RH: 50%) and stocking density of 1.5 kg m 2. Neuhauser et al. (1980) studied impact of population density on biomass growth of E. fetida and reported that growth of worms was related to the stocking density. The growth of worms was higher at low densities and low at higher densities. Ndegwa et al. (2000) also reported that biomass increased with stocking densities but sharply decreased after 2.0 kg m 2. The change in individual weight of worms with the environmental conditions and stocking densities followed same trend as for total biomass (Fig. 3). At environmental conditions C1, C5 and C6, a increase in the range of 49.3–76.0% for stocking densities 0.5–2.0 kg m 2 and a decrease by 7.2–45.3% for stocking densities above 2.0 kg m 2 in individual weight of worms were observed as compared to the initial individual weight. At environmental conditions C2, C3 and C4, the final individual weight of worms was found

to increase by 6.7–95.6% for stocking densities 0.5–3.0 kg m 2 and decrease by 20.6–35.9% for stocking densities above 3.0 kg m 2 as compared to the initial individual weight of worms. The individual weight of worms varied significantly with the stocking density and the final individual weight of worms was observed to increase by 52.5–84.7% for low stocking density (SD0.5) and by 49.3–95.6% for medium stocking densities (SD1.0–SD2.0) as compared to the initial individual weight of worms. At higher stocking densities (SD3.0– SD5.0), the final individual weight was found to increase by 6.7– 30.7% for environmental conditions C1, C5 and C6 and decrease by 7.2–45.3% for environmental conditions C2, C3 and C4 as compared to the initial individual weight of worms. A maximum increase of 95.6% (from 0.181 to 0.354 g) in worm individual weight was observed at environmental condition C3 (i.e. Temperature: 20 °C and RH: 50%) and stocking density of 1.5 kg m 2. The findings from the present study in the context of change in individual weight of worms with the stocking density corroborates with the findings of other researchers (Neuhauser et al., 1980; Ndegwa et al., 2000; Monroy et al., 2006). It is suggested that decrease in individual weight of worms at higher stocking densities may be due to the overcrowding of worms and exhaustion of foods below maintenance level in the reactors towards the end of vermicomposting period. The growth rate (mg weight gained worm 1 d 1) has been considered and used as a good comparative index to compare the growth of earthworms in varying conditions (Gupta and Garg, 2008). The effects of environmental conditions and stocking densities on the growth rate of worm are illustrated in Fig. 3. At environmental conditions C1, C5 and C6, the growth rate was estimated to be in the range of 4.05–5.28 mg worm 1 d 1 for stocking densities 0.5–2.0 kg m 2 whereas it varied from 0.94 to 3.59 mg worm 1 d 1 for stocking densities above 2.0 kg m 2. The growth rate was observed to be in the range of 0.86–6.19 mg worm 1 d 1 for stocking densities 0.5–3.0 kg m 2 and varied from 1.61 to 2.95 mg worm 1 d 1 for stocking densities above 3.0 kg m 2 at environmental conditions C2, C3 and C4. The growth rate changed significantly with the stocking density and it was always positive for low and medium stocking densities (SD0.5–SD2.0). The growth rate was estimated to be in the range of 4.20–6.08 mg worm 1 d 1 for low stocking density (SD0.5) and 4.05–6.19 mg worm 1 d 1 for medium stocking densities (SD1.0–SD2.0), respectively. At higher stocking densities (SD3.0–SD5.0), the growth rate was observed to vary from 0.86 to 2.38 mg worm 1 d 1 for environmental conditions C1, C5 and C6 and from 0.94 to 3.59 mg worm 1 d 1 for environmental conditions C2, C3 and C4, respectively. A maximum growth rate of 6.19 mg worm 1 d 1 was observed at environmen-

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0.45

Cast production rate -1 -1 (g-cast g-worm d )

Initial

125 100 75 50 25 0 100 80

Final

Total biomass (g)

S. Hait, V. Tare / Bioresource Technology 102 (2011) 2812–2820

60 40

0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.5

20 0.28

1.0

1.5

2.0

3.0

4.0

5.0

-2

Stocking density (kg m )

Initial

C1

0.24 0.22 0.18 0.42 0.30 0.18

-1 -1

Growth rate

4 2 0 -2 -4 4

(number)

Cocoons/worm

(mg worm d )

0.12 6

3 2 1 0 2

(number)

C3

C4

C5

C6

production was observed for stocking density >3 kg m 2. The cocoon production was in the range of 0.11–3.85 number worm 1. The maximum cocoon production was observed in experimental reactors C4 and SD0.5 (3.85 number worm 1), whereas C5 and SD3.0 (0.11 number worm 1) showed the minimum cocoon production. Similarly, the hatchlings production was in the range of 0.01–1.81 number worm 1. The maximum hatchlings production was observed in experimental reactors C4 and SD0.5 (1.81 number worm 1), whereas C5 and SD3.0 (0.01 number worm 1) showed the minimum hatchlings production. Monroy et al. (2006) reported that production of cocoons depends upon the densities of the earthworms. Kaushik and Garg (2004) found 6.6 cocoons per worm per month and 9.8 hatchlings per worm within 11 weeks in cow dung at 0.075 kg m 2 stocking density. Therefore, it can be suggested that the offspring production is highly variable and depends upon the environmental conditions (i.e. temperature and RH), moisture content, and stocking density. It can be inferred from the present study that the favorable environmental conditions are C1 to C4 with optimum being at C4 (i.e. Temperature: 20 °C and RH: 90%), whereas the favorable earthworm stocking density is in the range of 0.5–2.0 kg m 2 (i.e. low and medium densities) with the optimum being at 0.5 kg m 2 if the prime objective is vermiculture (i.e. production of earthworms).

0.36 0.24

Hatchlings/worm

C2

Fig. 4. Variations in cast production rate with environmental conditions and stocking densities during vermicomposting of primary sewage sludge.

0.20

Final

Individual weight (g)

0.26

1

0 0.5

1.0

1.5

2.0

3.0

4.0

5.0

-2

Stocking density (kg m ) C1

C2

C3

C4

C5

C6

Fig. 3. Effect of environmental conditions and stocking densities on earthworm total biomass, individual weight, growth rate and offspring production during vermicomposting of primary sewage sludge.

tal condition C4 (i.e. Temperature: 20 °C and RH: 90%) and stocking density of 1.5 kg m 2. A minimum growth rate of 3.59 mg worm 1 d 1 was observed at environmental condition C5 (i.e. Temperature: 30 °C and RH: 50%) and stocking density of 5.0 kg m 2. The effects of environmental conditions and stocking densities on the offspring production are presented in Fig. 3. It has been shown that earthworm reproduction and copulation frequency are greatly influenced by the temperature and moisture level (Hand, 1988). The population density also influences the earthworm reproduction. The copulation frequency is high at low population densities and it decreases notably when density approaches the carrying capacity of the system. It was observed in the present study that the offspring production was high at lower densities and decreased with increasing density. No offspring

3.2.3. Cast production rate Vermicompost (cast) production rate is an important aspect as far as vermicomposting (i.e. processing of waste) and treatment capacity of vermicomposting plant are concerned. The cast production rate was found varying in the range of 0.11–0.42 g-cast g-worm 1 d 1 as presented in Fig. 4. The maximum vermicompost production rate was observed in experimental reactors C4 and SD3.0 (0.42 g-cast g-worm 1 d 1), whereas C5 and SD0.5 (0.11 g-cast gworm 1 d 1) showed the minimum cast production rate. Both the environmental conditions as well as stocking densities had significant (P < 0.05) effects on the cast production rate (Table 5). The cast production rate in the present study is in good agreement with the results reported by the other researchers. A cast production rate of 0.23–0.35 g-cast g-worm 1 d 1 with an average value of 0.30 g-cast g-worm 1 d 1 has been reported during vermicomposting of source-separated human feces employing E. foetida (Yadav et al., 2010). It can be suggested that the vermicompost production rate depends on the environmental condition and stocking density apart from the characteristics of waste. It has been also observed that the net cast production at SD5.0 was lower as compared to SD4.0 and thereby indicating that the cast production was reduced with increasing density at stocking densities >4.0 kg m 2. This can be explained by the fact that overcrowding of worms had undesir-

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able effects on vermicomposting process even under favorable environmental conditions (Dominguez and Edwards, 1997). In the context of the present study, it can be inferred that the favorable environmental conditions are C1–C4 with optimum being at C4 (i.e. Temperature: 20 °C and RH: 90%), whereas the favorable earthworm stocking density is in the range of 2.0–4.0 kg m 2 with the optimum being at 3.0 kg m 2 as far as the feed (waste) processing is concerned.

4. Conclusions The combined composting and vermicomposting process can be deliberated as better treatment option for stabilization of PSS. Use of matured vermicompost as bulking material diminished the requirement of organic-rich external supplemental materials. Nutrients-rich vermicompost is highly suitable for agronomic applications due to high degree of stabilization and complete sanitization. Environmental conditions (i.e. temperature and RH) and earthworm density have profound effects on vermicomposting process. Temperature of 20 °C with high RH is favorable environmental condition for vermicomposting process employing E. fetida. Favorable earthworm density range for vermiculture is 0.5–2.0 kg m 2 (optimum: 0.5 kg m 2) and for vermicomposting is 2.0–4.0 kg m 2 (optimum: 3.0 kg m 2), respectively.

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