Philippine Journal of Science

149 (1): 179-188, March 2020

ISSN 0031 - 7683
Date Received: 13 Aug 2019

Efficiency of Combined Co-composting, Vermicomposting,

and Drying in the Treatment of Cadmium, Mercury, Helminths,
and Coliforms in Sludge from Wastewater Facilities
for Potential Agricultural Applications

Maria Aileen Leah G. Guzman1*, May Ann A. Udtojan1, Marylle F. Del Castillo1,
Emilyn Q. Espiritu1, Jude Anthony N. Estiva2, Jewel Racquel S. Unson1,
Joan Ruby E. Dumo1, and Jay Roy E. Espinas1

1Department of Environmental Science, Ateneo de Manila University,

Loyola Heights 1108 Quezon City, Philippines
2Aparri Engineering LLC, 131 Main St., Suite 180, Hackensack NJ 07601 USA

Sludge generated from wastewater treatment facilities has been applied in agriculture as soil
conditioners. However, the incomplete and/or inappropriate treatment of wastewater may
result in sludge that may still contain heavy metals, helminth ova, and coliforms posing a risk
to both humans and the environment. This study assessed various pretreatment techniques
such as co-composting, vermicomposting, and a combination of these on sludge samples to
remove heavy metals (cadmium and mercury), helminth ova, and coliforms. Physico-chemical
and biological analyses were used to compare untreated (i.e. raw) and treated sludge samples.
The results showed that for the raw sludge, mercury (4.02 +/– 0.17 mg/kg) and cadmium (6.30
+/– 0.48 mg/kg) exceeded the limits specified under the Philippine National Standard (PNS)
for Organic Soil Amendments of 2 mg/kg and 5 mg/kg, respectively. Laboratory examinations
also revealed the presence of helminth ova (5 ova/g) and coliforms (10 CFU/g) in the samples.
Sludge samples subjected to a combination of co-composting and vermicomposting resulted in
the elimination of mercury and a significant reduction in cadmium concentration from 6.30 mg/
kg to 1.12 mg/kg. No helminth ova were observed in the samples after further drying. However,
both treated and untreated sludge samples had low nutrient content. The study highlights the
need for raising public awareness and educating farmers on the potential risks associated with
the use of raw sludge for agriculture.

Keywords: composting, heavy metals, helminths, sludge, total coliform, vermicomposting

INTRODUCTION Sinha et al. 2009). The production of massive quantities

of sludge has led to the development of various disposal
The management of sludge produced from wastewater methods including the use of landfills, incinerators, and
treatment facilities is one of the most expensive challenges land application. Among these, the use of sludge for land
faced by the wastewater industry that engineers and application is proposed to be the most resourceful and
regulators are trying to solve (Metcalf and Eddy 2003; economical alternative method for disposal. Sludge is
*Corresponding Author: mguzman@ateneo.edu
a potential source of nutrients that can be used as a soil

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Vol. 149 No. 1, March 2020 Vermicomposting, and Drying

conditioner or fertilizer (Stadelmann et al. 2001; Metcalf German standards on heavy metals. Furthermore, the
and Eddy 2003). Several studies have shown that sludge author used irradiation, specifically gamma radiation, for
contains compounds that provide a rich source of organic pathogen control. The study also suggests that biological
matter, nitrogen, phosphorus, potassium, and other plant composting and alkaline stabilization using a combination
nutrients that may be of agricultural value (EC 2001; of heat, high pH (12), and drying can be used as an
Usman et al. 2012). The high organic matter in sludge alternative procedure for treating sludge.
can improve the physical characteristics of soils such as
its structure, water retention, and porosity (Hossain et al. In response to this need, this study aimed to investigate the
2017). Hence, it is environmentally friendly and also an potential of sludge as a safe and cost-effective alternative soil
economically efficient method of solid waste disposal. conditioner. Specifically, the research aimed to: (a) determine
the physicochemical characteristics and composition of the
However, the land application of sludge has been restricted sludge obtained from various sources; and (b) describe the
due to the reported presence of toxic contaminants, such effectiveness of various sludge treatment processes for heavy
as heavy metals and microbial pathogens of which pose metal, helminth ova, and coliform removal.
environmental and human health risks (USEPA 2000).
Heavy metals are of major concern due to their non-
biodegradable (Duruibe et al. 2007) and accumulative
nature that can lead to a decline in soil fertility (Singh MATERIALS AND METHODS
and Kalamdhad 2011). For instance, some crops such
as tomatoes or lettuce grown in soils with high metal Description of Sites
concentrations could directly transfer the heavy metals The sludge samples used in the study were obtained from
to humans if the crops are eaten raw (Cappon 1981). two wastewater treatment facilities on the island of Luzon
Similarly, pathogenic organisms such as helminths and in the Philippines. One is a septage treatment plant while
coliforms can persist in soils and in plants for months to the other is a sewage treatment plant.
years (USEPA 1993, 2003). As such, these contaminants
may expose humans to various diseases (e.g. long-term The septage treatment plant is a local government-
cadmium exposure can cause kidney damage while owned and -controlled corporation with a 30 m3 daily
helminth ova can cause gastrointestinal infections) as septage capacity. It only provides septage service to its
they enter the food chain through crop consumption domestic water customers. The desludging service cycle
(WHO 1989; Schonning and Stenstrom 2004; Lesmana is every five years on a regular basis (BWD 2012). The
et al. 2009). These issues highlight the need for a more septage treatment process separates the solid (sludge)
thorough understanding of the nature of sludge and to and the liquid (effluent) components of the raw septage.
search for possible remediation measures to ensure its The sludge is disposed of as earth fill and soil enhancer
safe application in agriculture. whereas the effluent is used in irrigation and re-used
within the wastewater treatment facility (e.g. for cleaning,
Various treatments have been suggested from the United landscaping, etc.) according to the treatment facility staff.
States of America, the United Kingdom, Netherlands,
France, etc. (USEPA 1994; EC 2001). However, when The sewage treatment plant is originally designed as an
considering an economical and sustainable approach, oxidation ditch system with a treatment volume capacity
composting technology is the most practical option of 8,600 m3 of wastewater per day. Only 20% of its design
(Nair et al. 2006). Most studies on sludge treatments capacity, however, was being treated (JICA 1991). It was
were conducted in Pakistan, Ghana, and Malaysia – initially constructed to service domestic areas connected to
specifically on co-composting (Bazrafshan et al. 2006; the sewage system but later included commercial areas that
Cofie et al. 2009; Hock et al. 2009). On the other were constructed within the vicinity. The plant also accepts
hand, vermicomposting studies have been conducted septage from areas that are not connected to the sewage line.
in Malaysia, Florida, and Australia (VermiCo 2013; Sewage is directly loaded into the oxidation ditches. Part
Eastman et al. 2001; Sinha et al. 2009) while integrated of its full function is the inclusion of sludge thickeners and
co-composting and vermicomposting studies have sludge drying beds to process sludge as a by-product of the
been conducted in Iran (Ndegwa and Thompson 2001; wastewater treatment process. The dewatered sludge is dried
Alidadi et al. 2007). In the Philippines, there has been in drying beds and then sold as an alternative agricultural
little to no studies about the characterization and toxic fertilizer/ soil conditioner (Robinson 2003; CEPMO 2014).
effects of sludge and its treatment. Only the study by
Manguiat (1997) was found wherein the author studied Sample Collection and Preparation
sewage sludge from agro-industrial wastewater treatment The sample size and the method of analysis were
plants and showed that the sludge passed both US and determined using the Publicly-Owned Treatment Works

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Vol. 149 No. 1, March 2020 Vermicomposting, and Drying

Sludge Sampling and Analysis Guidance Document distilled water (here referred to as “reconstituted” sludge
(USEPA 1989). The sampling of sludge from the two or RS). The other sludge samples with 72% moisture were
wastewater facilities was conducted once during the wet used as is in the succeeding stage.
season. The sludge was dewatered using a dewatering
machine and drying beds resulting in 72% and 10% Co-composting. For the RS, co-composting (CC)
moisture content, respectively. Composite sludge samples experiments were performed in plastic bins containing
were randomly collected from each treatment facility a 2:1:1 ratio of vegetable scraps, RS, and soil – with a
and placed in bags. Seven (7) bags each weighing final weight of 2 kg. Holes were drilled at the bottom
approximately 10 kg were obtained from the septage of the bins to allow adequate drainage. The interior of
treatment plant and one bag weighing approximately 20 the bins was lined with aluminum foil so as to reach
kg of sludge samples from the sewage treatment plant an internal temperature of 35–65 °C. The internal
was collected. These were immediately transported to the temperature was checked daily using a thermometer.
laboratory for processing and analyses. The daily moisture content of the co-composting
materials was monitored and maintained at 60%. The
manual turning of co-composting materials was done
Physico-Chemical Characterization of the Sludge once a week to aerate and homogenize the mixture. The
Samples sludge samples with 72% moisture were mixed with
The raw sludge samples (i.e. with 72% and 10% vegetable scraps in a 1:2 sludge-to-vegetable-scraps
moisture content) were analyzed for various parameters ratio, with a total weight of 3 kg. The mixtures were
using standard methods. The heavy metals arsenic, composted for 30 d, similar to the procedure followed
cadmium, chromium, and lead were analyzed using with the RS.
USEPA Method 3050B; while mercury was analyzed
using the cold-vapor technique. These were quantified Vermicomposting. For the RS, vermicomposting (VC)
using atomic absorption spectrophotometry. In compliance experiments were performed using a set-up similar to that
with the PNS for Organic Soil Amendments (PNS/BAFS of CC as described previously, with a final weight of 2 kg
183:2016), the carbon-nitrogen ratio was computed, total before the addition of 50 g of Eudrilus eugeniae (African
nitrogen was analyzed using the Kjeldahl method, total Nightcrawler) earthworms to each bin. To prevent the
phosphorus using USEPA Method 265.3, organic matter earthworms from escaping, a plastic net was placed over
and moisture content using the loss-on-ignition method, each bin. At the end of the treatment, earthworms were
pH using a portable soil pH meter, temperature using sieved to separate them from the vermicompost. After
a thermometer, and color consistency and odor using weighing the worms, the worms were subjected to heavy
sensory observations. metal analysis.
For the sludge with a 72% moisture level, the final weight
Microbiological Characterization of the Sludge was 3 kg. Vermicomposting was performed following the
Samples same procedures done to the RS with the addition of 60 g
The raw sludge samples were subjected to microbiological African Nightcrawler earthworms to each bin instead of
characterization based on the total helminths (expressed as 50. The difference in the number of worms used was based
ova/g) following the Standard Methods for the Recovery on the weight of the composting materials. Vermicastings
and Enumeration of Helminth Ova in Wastewater, Sludge, were manually harvested after 30 d.
Compost, and Urine-diversion Waste in South Africa
Combined co-composting and vermicomposting. For the
(Moodley et al. 2008). Total coliform was determined
RS, combined co-composting-vermicomposting (CV)
using the pour plate method of the Philippine Coconut
experiments were performed by first co-composting
Authority laboratory.
the sludge with the vegetable scraps as described
previously for 28 d, followed by vermicomposting as
Optimization of the Sludge Treatment described previously. For the sludge with 72% moisture,
In order to comply with local standards for the use vermicomposting of sludge was mixed with market
of sludge in agriculture (PNS/BAFS 183:2016), prior waste on a 1:2 sludge-to-scraps ratio. The mixtures were
treatment is necessary to remove pathogens and heavy composted for 20 d in bins with holes and mixed for proper
metals that may be present in sludge. Several treatment aeration. After 20 d of co-composting, 60 g of earthworms
strategies were employed to determine which of these were added and the vermicasts were manually harvested
would produce the desired results. Each treatment was after 10 d.
prepared in triplicates along with a control set-up of pure
sludge. The sludge samples with 10% moisture content Combined co-composting and vermicomposting
were hydrated to raise the moisture level to 60% using with subsequent oven-drying. For the sludge with
72% moisture, co-composting and vermicomposting

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Table 1. Summary of initial sludge characteristics obtained from the two wastewater treatment facilities.
Physico-chemical and Sewage treatment plant Septage treatment plant PNS-BAFS standards (2016)
microbiological properties
Color Brown Black Brown to black
Odor No foul odor No foul odor No foul odor
Consistency Not friable Friable Friable
pH at 30 °C 6.84 ± 0.06 6.61 ± 0.04 Not regulated
Moisture content 10.62 ± 0.04 72.3 ± 0.01 10–35%
Organic matter 25.52 ± 0.60 83.5 ± 0.10 > 20%
Total N-P2O5-K2O 1.28 ± 0.08 (N) 0.04 ± 0.01 (N) 2.5 – < 5%
0.04 ± 3.01E–03 (P2O5) 5.2 ± 0.01E06 (P2O5)
0.35 ± 0.03 (K2O) 0.001 ± 0.6 (K2O)
Arsenic 1.18 ± 0.17 0.118 ± 0.10 20 (mg/kg)
Cadmium 6.30 ± 0.48 0.0163 ± 0.40 5 (mg/kg)
Chromium 20.19 ± 1.09 Not analyzed 150 (mg/kg)
Lead 23.37 ± 2.16 0.0204 ± 0.01 50 (mg/kg)
Mercury 4.02 ± 0.17 Not analyzed 2 (mg/kg)
Total coliform < 10 ± 0 < 10 ± 0 < 10 (cfu/g)
Total helminths 1±0 5±1 Not regulated (ova/g)
Each value represents the mean ± SD (n = 3).
Sewage treatment plant – sludge samples with 10% moisture content
Septage treatment plant – sludge samples with 72% moisture content
PNS/BAFS 183:2016 – Philippine National Standard / Bureau of Agriculture and Fisheries Standards for Organic Soil Amendments

Table 2. Summary of the characteristics of sludge subjected to various treatment strategies.

Co-composting Vermi-composting Co-composting and vermi-
Control
(CC) (VC) composting (CV)
PNS-BAFS
Parameter standards
Septage Sewage Septage
Sewage treatment Septage Sewage Sewage Septage (2016)
treatment treatment treatment
plant treatment plant treatment plant treatment plant treatment plant
plant plant plant

Brown to
Color Brown Black Brown Black Brown Black Brown Black
black

No foul No foul
Odor No foul odor No foul odor No foul odor No foul odor No foul odor No foul odor No foul odor
odor odor

Consistency Not friable Slimy Friable Friable Friable Friable Friable Friable Friable

Temperature 40 °C –
30 °C 28 °C 37 °C – 30 °C 41 °C – 28 °C 28 °C – 25 °C 28 °C 33 °C – 29 °C –
28 °C

Not
pH 6.07 ± 0.12 6.61 ± 0.04 6.80 ± 0.07 6.91 ± 0.13 7.11 ± 0.10 6.66 ± 0.4 7.02 ± 0.01 6.80 ± 0.11
regulated

Moisture content
7.93 ± 0.29 22.3 ± 21.6 8.85 ± 0.19 20.6 ± 12 4.20 ± 0.14 27.9 ± 0.01 4.21 ± 0.37 26.7 ± 6 10–35
(100%)

Organic matter (%) 24.70 ± 0.79a 47.2 ± 10 31.50 ± 1.69a 13.3 ± 3.8 17.68 ± 0.57b 23.8 ± 0.03 17.13 ± 0.68b 13.9 ± 1.6 > 20

0.0001 ±
1.29 ± 0.15a 0.04 ± 0.01 1.12 ± 0.13a 0.016 ± 0.004 0.51 ± 0.007b 0.025 ± 0.001 0.49 ± 0.05b
0.00

0.000047 ± 0.000057 ± 0.017 ± 0.000028 ±

Total N-P2O5-K2O 0.057 ± 0.02ac 5.2E06 ± 0.0 0.086 ± 0.043cd 0.0056 ± 0.003b 2.5 – <5%
4.2E06 7.8E05 0.005ab 3.9E05

Not
0.354 ± 0.007b 0.001 ± 0.6 0.356 ± 0.004b 0.003 ± 0.004 0.37 ± 0.003a 0.003 ± 0.01 0.36 ± 0.009b
detected

Total coliform < 10 ± 0 < 10 ± 0 4.3E3 ± 2.5E02 < 10 ± 0 1.5E04 ± 5.0E03 < 10 ± 0 < 10 ± 0 < 10 ± 0 < 10cfu/g

Not
Total helminths Not detected 13 ± 3 1±0 18 ± 5 1±0 19 ± 4 Not detected 26 ± 16
regulated

Each value represents the mean ± SD (n = 3).

Results sharing the same letter are not significantly different at p < 0.05.
C – control (untreated soil); N – nitrogen; P – phosphorus; K – potassium;
Sewage treatment plant – sludge samples with 10% moisture content
Septage treatment plant – sludge samples with 72% moisture content

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Vol. 149 No. 1, March 2020 Vermicomposting, and Drying

were performed as previously described for 30 d. The production of organic acids from microbial metabolism
vermicasts were then subjected to further oven-drying at as well as the production of humic and fulvic acids during
57 °C for 1 h (USEPA 1994). decomposition (Ndegwa and Thompson 2000; Dominguez
and Edwards 2004; Suthar and Singh 2008).
At the end of each treatment, physicochemical and
microbiological analyses were conducted to compare the For the sludge samples from the sewage treatment plant,
sludge characteristics before and after the treatments,
as well as the effectivity of each treatment in removing
the helminths and heavy metals. Furthermore, after
the experiments, the earthworms that were used in the
treatments were disposed of following the guidelines
described in the Department of Environment and Natural
Resources (DENR) Administrative Order 36, Series of
2004 (DAO 2004-36) for disposing of hazardous waste.

RESULTS AND DISCUSSIONS

Figure 1. Average heavy metal concentration (mg/kg), expressed
Physico-chemical and Microbiological as mean ± SD, in the treated sludge samples after
Characterization of the Sludge treatment. Each value represents the mean ± SD (n =
3). Results sharing the same letter are not significantly
The initial visual examination of the sludge collected from different at p < 0.05. C – control (untreated soil); CC
the two wastewater treatment facilities during the wet – co-composting; VC – vermicomposting; CV – co-
season showed that both samples met the required color composting + vermicomposting.
and odor (Table 1). However, the sludge sample from the
sewage treatment plant was not friable while the moisture
content of the sludge sample from the septage treatment a 27.5% significant increase (p < 0.05) in organic matter
plant was too high (72.3 ± 0.01% moisture). Hence, content was found after CC (31.50 ± 1.69%) whereas
both sludge samples failed to meet the requirement for a 28.4% and a 17.13% significant loss (p < 0.05) was
consistency and moisture content, respectively. detected after VC (17.68 ± 0.57%) and CV (17.13 ±
0.68%), respectively (Table 2). Results for the sludge
The results of the initial chemical analyses, expressed as samples from the septage treatment plant showed a
mean + standard deviation (SD), showed that the average significant loss (p < 0.05) in organic matter content of
concentrations of the organic matter (sewage treatment 67.5%, 41.8%, and 66% after CC (13.3 ± 3.8%), VC
plant, 25.52 ± 0.60%; septage treatment plant, 83.5 ± (23.8 ± 0.03%), and CV (13.9% ± 1.6), respectively
0.10%) of both samples were within the required level (Table 2). Partial mineralization and humification are
set by the PNS for Organic Soil Amendments (i.e. > chemical changes that occur because of organic matter
20%). However, the nitrogen and potassium level of both bio-oxidation (Fornes et al. 2012). Greater reduction in
sludge samples failed to meet the established limit for soil organic matter content indicates better degradation and
conditioner (2.5 – <5%), while the total concentration mineralization, giving more stable products (Ndegwa and
of Hg (4.02 ± 0.17 mg/kg and Cd (6.30 ± 0.48 mg/kg) Thompson 2001). Hence, VC and CV treatments can be
from the sewage treatment plant exceeded the prescribed used to efficiently stabilize sludge.
standard. Microbiological results showed that the total
coliform count of both samples met the standard, but The total N content of the final product of each treatment
helminth ova were detected in both sludge samples. The found in the sludge samples from the sewage treatment
results of this study highlight the need for additional plant ranged from 0.49–1.12%. Significant loss (p <
treatment to remove heavy metals, and helminths. 0.05) was found after VC (0.51 ± 0.01%) and CV (0.49
± 0.05%) but no significant reduction was measured after
After the 8-wk treatment process, changes in the CC (1.12 ± 0.13%). On the other hand, the total N found
physicochemical properties of the initial samples were in the sludge samples from the septage treatment plant
observed. The end product of each treatment was observed ranged from 0.01–0.02%. Significant loss was found after
to be much darker in color, odor-free and visually CC (0.016 ± 0.004%), VC (0.025 ± 0.001%), and CV
homogenous as compared to their initial condition. (0.0001 ± 0.004%). The significant reduction in total N
The resulting pH values (6.02–7.11) were found to be content after treatment could be due to mineral N leaching
slightly acidic to neutral. This could be attributed to the because of frequent application of water (Cogger et al.

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Vol. 149 No. 1, March 2020 Vermicomposting, and Drying

Table 3. Average change in accumulated metal concentration in earthworm tissues, expressed as mean ± SD (n = 3).
Heavy metal (mg/ % accumulated metals % accumulated metals
Initial concentration After VC After CV
kg) after VC after VC
Cd 0.79 ± 0.08a 1.89 ± 0.87ab 20.40 3.0 ± 0.24b 40.84
Hg 0.02 ± 0a 1.31 ± 0.80ab 39.31 2.94 ± 0.80b 86.98
Results sharing the same letter are not significantly different at p < 0.5.
VC – vermicomposting; CV – co-composting + vermicomposting

2000) during vermicomposting to maintain optimum Hg level showed a decrease after CC. The combined
moisture content. A similar result was reported by Fornes composting also demonstrated a significant (p < 0.05)
et al. (2012) in which heavy irrigation resulted to greater and 100% removal of Hg among all the treatments (see
leaching of the total N content and most macronutrients Figure 1).
in vermicomposting and integrated composting systems
as compared to composting alone. Several studies have shown that earthworms can
accumulate heavy metals from organic waste through their
An increase in available P concentration was detected skin and gut (Suthar and Singh 2008; Sinha et al. 2009;
after CC (0.086 ± 0.043%) of sludge from the sewage Suthar et al. 2014; Soobhany et al. 2015). However, only
treatment plant and after VC (0.000057 ± 7.8E05%) of a few studies focused on the use of Eudrilus eugeniae as
sludge from the septage treatment plant. While decreases vermiworms for the removal of toxic heavy metals from
in the available P were noticed after VC (0.0056 ± 0.003%) specific waste (Graft 1982; Iwai et al. 2013; Soobhany
and CV (0.017 ± 0.005%) from the samples taken from et al. 2015) due to their narrow temperature tolerance
the sewage treatment plant and after CC (0.000047 ± compared to Eisenia fetida (Reinecke et al. 1992). In
4.2E06%) and CV (0.000028 ± 3.9E05%) from the this study, a large increase in the concentration of heavy
samples taken from the septage treatment plant. The result metals was measured from the body tissues of Eudrilus
of the VC treatment showed a significant decrease (p < eugeniae relative to the initial concentration. For Cd, the
0.05) in P concentration. This may be linked to phosphate concentration increased by 1.1 mg/kg after VC and 2.21
leaching brought about by frequent watering during mg/kg after CV. For Hg, the concentration increased by
VC. The decrease in available P was consistent with the 1.29 mg/kg after VC and 2.92 mg/kg after CV (Table 3).
findings of Ndegwa and Thompson (2001). Specifically, a significant increase (p < 0.05) in heavy
metal concentrations was measured from the earthworm
The total K concentration following all composting
tissues after the CV treatment. Earthworms subjected to
treatments generally increased as compared to the
the CV treatment accumulated 40.84% and 86.98% of the
control, with the VC treatment showing a statistically
total Cd and Hg in their system, respectively. This could be
significant increase (p < 0.05) in K concentration. This
due to the palatability of waste mixtures in the combined
could be attributed to the addition of soil, which was
treatment as evidenced by the remarkable increase in
found to contain 0.37% K. The increase in the total K
earthworm biomass. It is worth mentioning that the CV
concentration seen after the VC (sewage treatment plant,
system was the treatment that also exhibited a significant
0.37 ± 0.003%; septage treatment plant, 0.003 ± 0.01%)
decrease in organic matter decomposition.
treatment might have been due to the acid production
by the microorganisms present in the gut of earthworms
which could solubilize the insoluble potassium (Kaviraj
and Sharma 2003). Similar findings were reported by
Tripathi and Bhardwaj (2004), Gupta and Garg (2008),
and Suthar and Singh (2008).

Results of the Heavy Metal Analyses

Heavy metal contents of the final products showed an
increase of 41.6% in Cd concentration after CC (mean
± SD = 5.39 ± 1.22 mg/kg) but this result did not differ
significantly from the Control (mean ± SD = 3.81 ± 0.74
mg/kg). A 70.5% significant reduction (p < 0.05) on Cd
concentration was found after CV (mean ± SD = 1.12
± 0.02 mg/kg) (see Figure 1). The results for the total

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Vol. 149 No. 1, March 2020 Vermicomposting, and Drying

Percent accumulation of metal in the earthworm tissues

was calculated from the equation:
(Metal in ET After Treatment – Metal in ET Before Treatment)
% Metal Accumulation = x 100 (1)
Metal in the Initial Wastes

Percent removal efficiency is calculated from the equation:

(Metal in the Initial Wastes – Metal in the Compost)

% Removal Efficiency = x 100 (2)
Metal in the Initial Wastes

Therefore, the combined treatment of co-composting and However, a more detailed temperature specification for
vermicomposting (i.e. CV) can be used to effectively the composting system was noted by Jenkins (1999).
remove heavy metals – specifically Cd and Hg from Based on his study, complete pathogen reduction in a
sludge – as shown in Table 4. These results are consistent composting system can be assured if the temperature
with findings from other studies using different metals, is attained at 62 °C for 1 h, 50 °C for 1 d, 46 °C for 1
organic wastes, and earthworm species (Suthar and Singh wk, or 43 °C for 1 mo. Unlike the other treatments, the
2008; Sinha et al. 2009; Iwai et al. 2013). temperature for the CV treatment reached 50 °C on its
first day, thereby resulting in low coliform count and
Table 4. Percent removal efficiency of co-composting,
elimination of helminth ova. These findings indicate that
vermicomposting, and combined co-composting and
vermicomposting. the CV treatment is more effective in terms of reducing
heavy metal concentrations (Cd and Hg), helminth ova
Heavy metal % removal efficiency
and coliforms than a single composting treatment in
CC VC CV sludge with 10% moisture. While a combination of co-
Cd 0.20 29.82 79.07 composting or vermicomposting followed by further
Hg 75.00 89.14 100.00 drying is the best treatment to completely eliminate
helminth ova in sludge with 72% moisture.

In terms of the helminth ova and coliform levels found in

the sludge from the sewage treatment plant, total coliform
after the CV treatment was below 10 CFU/g and no CONCLUSION
helminth eggs were detected. This is in contrast with the
There is a need to characterize sludge prior to agricultural
other treatments (CC and VC) in which helminth eggs
application. This study shows that the raw sludge samples
were detected and a large increase in the concentration of
from either type of treatment plant did not completely meet
total coliforms was measured after CC and VC. Moreover,
the minimum requirements set in PNS/BAFS 183:2016.
sludge from septage treatment plant after CC, VC, and
High concentrations of Cd and Hg in sludge from the
CC exhibited below 10 CFU/g total coliform count but
sewage treatment plant were found to exceed the maximum
the total helminth count exceeded the allowable limits
allowable level. Moreover, the presence of helminth ova in
(Table 2) – hence the need to further dry the treated
sludge from the septage treatment plant indicates that raw
sludge to significantly reduce the helminth ova present
sludge cannot be recommended for land application as a
(USEPA 1994).
soil conditioner. Thus, there is a need for further treatment
For CC, the temperature only reached 37 °C on its 5th day prior to its use in land application to prevent risks to both
and decreased subsequently while the VC temperature the environment and human health. The findings of this
stayed below 30 °C. According to USEPA (2003), study showed that a combination of co-composting and
composting can significantly reduce helminth ova and vermicomposting was significantly more effective than
coliforms provided that the temperature of sewage sludge a single composting treatment in terms of eliminating Hg
is raised to 40 °C or higher for 5 d and the compost piles and significantly reducing the concentration of Cd. For the
exceeds 55 °C within 4 h during the 5-d period. This 72% moisture sludge, further drying was necessary after
suggests that the temperature in both treatments (CC the combined treatment in order to eliminate the pathogens
and VC) was not high enough to kill the helminth ova (coliforms and helminths). Moreover, caution needs to be
and coliforms; thus, raising the possibility of coliform observed with regards to the proper disposal of earthworms
multiplication or re-activation. used in the treatments as stated in DAO 2004-36 to avoid
contamination.

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Vol. 149 No. 1, March 2020 Vermicomposting, and Drying

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