earthworms, whereas high carbon/nitrogen ratio is suitable for the growth of small or
young earthworms (Aira et al. 2006). Accordingly, the ratio of carbon to nitrogen of
25:1 is beneficial for the growth of earthworms and microorganisms, and their
composting products had a high stability used as fertilizer (Ndegwa and Thompson
2000). Given that the carbon to nitrogen ratio of municipal sludge is often lower than
25:1, many scholars promoted carbon/nitrogen ratio using mixed materials such as
straw and rice husk. However, vermicomposting for dewatered sludge without the
addition of other carbon source was also completed by previous studies (Fu et al.
2015a, b; Huang et al. 2018). As a result, the optimal carbon to nitrogen rate in initial
sludge should be taken into account for vermicomposting of dewatered sludge.
4.4.4 Density and Retention Time of Earthworms
The density and retention time of earthworms are important parameters for
vermicomposting of sludge, which are associated with time, cost, and fertilizer
performance. Undoubtedly, the optimal conditions for vermicomposting and vermiculture are different (Ndegwa et al. 2000). Based on the survey, the density of
earthworms was 0.5–5.0 kg/m
2 or 10–375 worms/kg sludge for vermicomposting
(Table 4.2). In addition, the retention time of earthworm ranged from 14 to 120 days
in vermicomposting system (Table 4.2). As a consequence, there are different
densities and retention times of earthworms in the previous vermicomposting,
which may cause the treatment efficiency of sludge and properties of vermicompost
are difficult to compare. For this, a uniform standard for optimal condition is highly
expected for vermicomposting of sludge. However, due to the variable in the
influents of wastewater treatment plant in every day, the optimal density and
retention time of earthworm are strongly with the properties of dewatered sludge
used in vermicomposting.
4.4.5 Pretreatment Methods
Fresh dewatered sludge easily generates poisonous gas and gives rise to an anaerobic
environment where earthworms cannot survive (Gupta and Garg 2008; Suthar 2010;
Hait and Tare 2012). Therefore, suitable pretreatment method of dewatered sludge is
deemed to be of essence for earthworms. Pretreatment of sludge mainly includes
mixing with other materials, dry, pelletized, and vermi-bedding used, as displayed in
Table 4.2. The dry method such as sunshine and oven can reduce the water and
toxicant gas content in sludge (Xing et al. 2012; Azizi et al. 2013; Cui et al. 2018).
The blending bulking materials such as cow dung (Gupta and Garg 2008; Suthar and
Singh 2008; Domínguez-Crespo et al. 2012; Ludibeth et al. 2012; Reddy et al. 2012;
Lv et al. 2015; Das et al. 2015; Xie et al. 2016; Zhao et al. 2018; Lv et al. 2018a, b),
sugarcane trash (Suthar 2010; Biruntha et al. 2019), and composting products (Hait
64
K. Huang et al.
young earthworms (Aira et al. 2006). Accordingly, the ratio of carbon to nitrogen of
25:1 is beneficial for the growth of earthworms and microorganisms, and their
composting products had a high stability used as fertilizer (Ndegwa and Thompson
2000). Given that the carbon to nitrogen ratio of municipal sludge is often lower than
25:1, many scholars promoted carbon/nitrogen ratio using mixed materials such as
straw and rice husk. However, vermicomposting for dewatered sludge without the
addition of other carbon source was also completed by previous studies (Fu et al.
2015a, b; Huang et al. 2018). As a result, the optimal carbon to nitrogen rate in initial
sludge should be taken into account for vermicomposting of dewatered sludge.
4.4.4 Density and Retention Time of Earthworms
The density and retention time of earthworms are important parameters for
vermicomposting of sludge, which are associated with time, cost, and fertilizer
performance. Undoubtedly, the optimal conditions for vermicomposting and vermiculture are different (Ndegwa et al. 2000). Based on the survey, the density of
earthworms was 0.5–5.0 kg/m
2 or 10–375 worms/kg sludge for vermicomposting
(Table 4.2). In addition, the retention time of earthworm ranged from 14 to 120 days
in vermicomposting system (Table 4.2). As a consequence, there are different
densities and retention times of earthworms in the previous vermicomposting,
which may cause the treatment efficiency of sludge and properties of vermicompost
are difficult to compare. For this, a uniform standard for optimal condition is highly
expected for vermicomposting of sludge. However, due to the variable in the
influents of wastewater treatment plant in every day, the optimal density and
retention time of earthworm are strongly with the properties of dewatered sludge
used in vermicomposting.
4.4.5 Pretreatment Methods
Fresh dewatered sludge easily generates poisonous gas and gives rise to an anaerobic
environment where earthworms cannot survive (Gupta and Garg 2008; Suthar 2010;
Hait and Tare 2012). Therefore, suitable pretreatment method of dewatered sludge is
deemed to be of essence for earthworms. Pretreatment of sludge mainly includes
mixing with other materials, dry, pelletized, and vermi-bedding used, as displayed in
Table 4.2. The dry method such as sunshine and oven can reduce the water and
toxicant gas content in sludge (Xing et al. 2012; Azizi et al. 2013; Cui et al. 2018).
The blending bulking materials such as cow dung (Gupta and Garg 2008; Suthar and
Singh 2008; Domínguez-Crespo et al. 2012; Ludibeth et al. 2012; Reddy et al. 2012;
Lv et al. 2015; Das et al. 2015; Xie et al. 2016; Zhao et al. 2018; Lv et al. 2018a, b),
sugarcane trash (Suthar 2010; Biruntha et al. 2019), and composting products (Hait
64
K. Huang et al.