Keywords

1 Introduction

Increase in global population, urbanization and industrialization has resulted in environmental pollution and degradation including diminished water quality (Verma et al. 2012). Disposal of untreated sewage and industrial effluents into the surface water bodies leads to water pollution (Goel 2006). Wastewater carrying organics like biochemical oxygen demand (BOD), chemical oxygen demand (COD) and nutrients like nitrogen and phosphorus results in the problems like depletion of dissolved oxygen (DO) and eutrophication (Metcalf et al. 1991; Zheng et al. 2013). In addition, exposure to the water contaminated by the release of pathogens from sewage into the surface water leads to water-borne diseases (Reddy and Smith 1987). Thus, deterioration of river ecology along with the loss of freshwater sources creates an unhealthy environment for humans (Wang et al. 2012). Furthermore, the per capita available water is becoming less with an increase in the population pertaining to the limitation of freshwater sources (Pimentel et al. 2004). Therefore, it becomes necessary to reuse wastewater generated from households and other places after giving a certain level of treatment. Owing to the water scarcity and water pollution due to anthropogenic activities, there is an urgent need to treat and reuse the treated effluent in industrial, agricultural and non-potable purposes.

For wastewater treatment, anaerobic and aerobic processes are being used worldwide (Speece 1983). In the anaerobic process, microbes convert organic matters into methane and carbon dioxide, whereas in the aerobic process, aerobic microbes convert organic matters into biomass and carbon dioxide (Metcalf et al. 1991). The anaerobic process is effective for high COD wastewater, requires less energy, and produces less sludge in comparison to aerobic process. However, it has been documented that the aerobic process is comparatively better than the anaerobic process in terms of acclimatizing the variation in pH, temperature and organic loading rates (OLR) (Degremont 1991). Further, the aerobic process requires less time to restart and can work between a range of temperature from 25 to 35 °C as compared to the optimum temperature for the anaerobic process is 30 °C (Singh et al. 2019b). However, both conventional wastewater treatment techniques required high capital cost, recurring expenditures, skilled manpower, more time to restart after complete shutdown and mechanized and energy-intensive operations (Noumsi et al. 2005). In addition, the sludge generated from conventional processes requires further treatment before getting disposed into the environment. Other than the biological treatment process, physical and chemical processes are also being used in some part of the world (Adin and Asano 1998). However, physical and chemical processes are not efficient organic and nutrient removal from wastewater (Ra et al. 2000). Thus, in the present scenario, an economical and sustainable process is required to treat wastewater with less capital and operation and maintenance cost and ease of operation process.

Integration of earthworms in wastewater filtration process has evolved as an eco-friendly and economical alternative to conventional wastewater process, collectively known as vermifiltration (Tomar and Suthar 2011). Wastewater passing through the initial layer, where the organic matter is converted into humus by earthworm, is followed by the filtration through filter media which supports microorganism’s growth and subsequently secondary treatment occurs. Recent studies have shown that the vermifiltration technique can emerge as a suitable and sustainable alternative for wastewater treatment and recycling. Thus, the chapter presents an overview of the vermifiltration technique with various filter design configurations, applicability and performance evaluation of the technique for the treatment and recycling of sewage and industrial effluents, explaining the mechanisms involved. Additionally, the performance of an integrated macrophyte-vermifiltration system for wastewater treatment and recycling has also been presented. Further, the effects of different filter design and operational parameters on the system performance have been summarized. Moreover, future research perspectives have been provided towards the improvement of the efficacy of the system for wastewater treatment and recycling.

2 Overview of Vermifiltration Technique

Vermifiltration system comprises an earthworm active zone along with filter media bed which supports microbial community for domestic and industrial wastewater treatment. The species of earthworms employed in vermifiltration include Eisenia fetida, Lumbricus rubellus, Eudrilus eugeniae and Eisenia andrei with filter bed consisting of soil, compost and cow dung which are available for pollutant degradation in earthworm active zone (Singh et al. 2019b; Xing et al. 2011). In filter media design, different materials like sand, gravel, cobblestone and quartz sand are commonly used (Singh et al. 2019b). In vermifiltration system, wastewater is firstly passed through earthworm active zone followed by filter media bed. Depending on the wastewater flow direction, vermifiltration system, in general, can be of two types: horizontal flow system (HFS) and vertical flow system (VFS). In HFS, wastewater flows horizontally through the bed while in VFS wastewater is fed vertically through the bed as shown in Figs. 1.1 and 1.2, respectively. A hybrid system combining both horizontal and vertical flow systems in the sequence is used for the treatment of wastewater. The flow of wastewater in the hybrid system is either from a horizontal system followed by a vertical one or vice-versa as schematically presented in Fig. 1.3a, b, respectively.

Fig. 1.1
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Schematic of a typical horizontal flow vermifiltration system

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Fig. 1.2
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Schematic of a typical vertical flow vermifiltration system

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Fig. 1.3
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Schematic of hybrid vermifiltration systems based on the wastewater flow direction: (a) VFS followed by HFS and (b) HFS followed by VFS

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Nowadays, researchers are focusing on the integrated macrophyte-vermifiltration system to improve the wastewater treatment efficiency. In macrophyte-assisted vermifiltration system, the concept of wetlands using different plant species like Canna indica, Phragmites australis, Typha angustifolia, Saccharum spontaneum, Cyperus rotundus, etc. is integrated with vermifiltration system for wastewater treatment (Chen et al. 2016; Nuengjamnong et al. 2011; Samal et al. 2017a; Tomar and Suthar 2011; Wang et al. 2010b). Removal from wastewater takes place when macrophyte uptakes significant amount of nutrients for their growth. A macrophyte-assisted vermifiltration system has been schematically shown in Fig. 1.4. The root or rhizospheric zone of plants helps to provide a favourable environment for the growth of the diverse microbial community to degrade organic contaminants (Bezbaruah and Zhang 2005). Further, researchers have found that macrophyte transfers oxygen from the atmosphere to the rhizosphere which is further consumed by the microbial community (Bezbaruah and Zhang 2005; Brix 1994). Increased oxygen is responsible for maintaining aerobic condition for the microbes as well as for earthworms which is useful to accelerate the degradation of organic contaminants.

Fig. 1.4
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Schematic of a macrophyte-assisted vermifiltration system

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3 Performance Evaluation of Vermifiltration System

3.1 Applicability of Vermifiltration for Sewage Treatment

It has been reported that the vermifiltration technique is an efficient and eco-friendly process to treat wastewater originating from households (Kumar et al. 2016; Li et al. 2009). Vermifiltration technique has been applied to domestic wastewater treatment and has shown a significant reduction of COD and NH3+-N (Sinha et al. 2008; Wang et al. 2010a; Xing et al. 2011). Applicability of vermifiltration technique with associated process parameters for wastewater treatment is summarized in Table 1.1. Earthworms consume retained suspended particles in the filter during ingestion and significantly reduce BOD by more than 90% and COD in the range of 80–90% and a significant reduction in nutrients concentration (Li et al. 2009; Wang et al. 2011). According to Kumar et al. (2016), application of vermifiltration employing earthworm species Eisenia fetida and Eudrilus eugeniae to treat wastewater generated from domestic activities has shown the reduction of about 88% and 70% BOD, 78% and 67% TSS and 75% and 66% TDS, respectively, whereas a laboratory-scale study has revealed the removal of contaminants like BOD5, COD and TSS from domestic wastewater in the range of 55–66%, 47–65% and 57–78%, respectively (Xing et al. 2010). The earthworm species Eisenia fetida is one of the most common species employed to treat domestic wastewater (Gunadi et al. 2002; Hughes et al. 2009; Sinha et al. 2008). In another study on domestic wastewater treatment, employing Eisenia fetida as an earthworm species has shown removal of 78% BOD5, 68% COD and 90% TSS (Liu et al. 2013). A study has been conducted by Zhao et al. (2014) to treat synthetic wastewater through macrophyte-assisted vermifiltration using different combinations of vertical sub-surface flow constructed wetlands platned with macrophyte Acorus calamus and earthworm Eisenia fetida. Results of the study revealed the removal of up to 87% COD, 86% total nitrogen (TN) and 83% total phosphorus (TP).

Table 1.1 Application of vermifiltration technique with associated process parameters for the treatment of sewage and industrial effluents
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Nitrogen removal from wastewater is mainly responsible for the nitrifiers and denitrifiers microbes present in the earthworm’s intestinal guts (Ihssen et al. 2003). Earthworms are able to aerate the system through its borrowing action which enhances the nitrification process and creates a favourable microenvironment for the growth of aerobic nitrobacteria (Samal et al. 2017a). Wang et al. (2010b) have combined macrophyte Phragmites australis and earthworm species Eisenia fetida to treat domestic sewage with an OLR of approximately 192 g/m2/d and hydraulic loading rate (HLR) of 1 m3/m2/d, and the results showed an average reduction of about 90% COD, 93% SS and 92% NH4+-N. Wang et al. (2013) reported 63–66% removal efficiency of TN and 72–78% removal of NH3−N from synthetic domestic wastewater. Liu et al. (2013) also reported about 92% NH4+-N removal from domestic wastewater. Further, the removal of phosphorus depends upon the sorption capacity, surface area and size of vermifilter bed material along with chemical reaction like ligand exchange reaction, complexation and precipitation (Samal et al. 2017a). Vermifiltration system combined with macrophytes Perionyx sansibaricus and Cyperus rotundus reported the reduction of wastewater pollutants like COD, total suspended solids (TSS), total dissolved solids (TDS) and NO3− by more than 85% (Tomar and Suthar 2011). Wang et al. (2013) reported 80–82% removal of TP using bedding material which consists of cobblestone, detritus, silver sand and earthworm bed while removal of 87% of TP using cobblestone, soil and sawdust. Furlong et al. (2014) obtained a removal efficiency of TP in the range of 56–59% in human faeces.

The most crucial parameter in the sewage treatment from the human health point of view is pathogen removal. In this context, a comprehensive review of available literature by Swati and Hait (2018) underscores that earthworms are capable of pathogen reduction from various wastes. Arora et al. (2014) reported around 99% removal of Escherichia coli (E. coli), total coliform (TC), faecal coliform (FC) and faecal streptococci (FS) from synthetic wastewater spiked with sewage in a vermifiltration system. Further, Kumar et al. (2016) have treated domestic wastewater with vermifiltration and achieved a reduction of FC by 99%. An experimental run of 365 days of vermifiltration showed the reduction of COD by more than 87 and 99% thermotolerant coliforms using domestic wastewater (Furlong et al. 2014).

3.2 Applicability of Vermifiltration for Industrial Effluents

Initially limited to the treatment of the domestic wastewater, the vermifiltration technique has gradually evolved to be studied for the treatment of the industrial effluents. However, very few studies (Table 1.1) are being carried out on the vermifiltration of industrial wastewater because of the sensitive nature of earthworms towards parameters like pH, heavy metals, pesticides and salinity. Regardless of this, vermifiltration applied to industrial effluent from the food and beverage sector has shown encouraging pollutant removal efficiency and can pave way for application for many other industrial effluents that have low or no toxicity (Singh et al. 2019a). Additionally, vermifiltration system has been applied to other industrial effluents, such as petroleum industry and pharmaceutical industry (Dhadse et al. 2010; Sinha et al. 2012). Sinha et al. (2007) have successfully applied vermiltration system to treat effluent from dairy industries which mainly consist of organics like proteins, carbohydrates and fats. According to the study, earthworm species Eisenia fetida has resulted in the removal of about 99% BOD5 and COD in the range of 80–90%. It also leads to the removal of TDS and TSS in the range of 90–92% and 90–95%, respectively. Another study conducted by Sinha et al. (2012) on petroleum industry wastewater has shown 99% removal of C10–C14, C15–C28 and C26–C36. Further, cheese whey waste has been treated by using vermifiltration and achieved about 76% BOD, 82% COD and 77% TSS removal efficiency (Merlin and Cottin 2009). Ghatnekar et al. (2010) reported the removal of COD and BOD by 89 and 90%, respectively, from gelatine industry wastewater employing earthworm species Lumbricus rubellus. Dhadse et al. (2010) studied application of vermifiltration on herbal pharmaceutical effluents using earthworm Eudrilus eugeniae at different organic loading rates (OLR) of 0.8, 1.6, 2.4 and 3.2 kg COD/m3/d with 3.2 kg COD/m3/d as the optimum with COD and BOD removal efficiencies in the range of 85–94% and 90–96%, respectively. Macrophyte-assisted vermifiltration was applied to treat synthetic dairy wastewater by employing macrophyte species Canna indica and reported removal of BOD, COD, TSS, TDS and TN by 81%, 76%, 85%, 23% and 43%, respectively (Samal et al. 2017b).

4 Mechanisms of Vermifiltration Technique

Vermifiltration technique works in combination of earthworms and microbes. Evolving from the basic system, macrophyte-assisted vermifiltration has emerged as an eco-friendly alternative for wastewater treatment and recycling. In order to unravel the treatment mechanisms, the roles of various layers and components of a typical macrophyte-assisted vermifiltration system have been schematically presented in Fig. 1.5. The solids retained on the filter bed are consumed by the earthworms and converted into the humus (Sinha et al. 2008; Singh et al. 2017). A microbial layer formed on the filter bed also contributes to the degradation of the contaminants retained on the filter bed. Generally, vermifiltration system consists of components, i.e. earthworms and filter bed. Filter bed supports the earthworm growth by providing food source by sorption mechanism from the wastewater, and a microbial layer is formed because of low porosity (Liu et al. 2013; Singh et al. 2017; Wang et al. 2010a, b). Further, the earthworm active zone is also known as aerobic zone while filter bed is called anoxic zone in vermifiltration (Samal et al. 2018a). Oxygen level is increased in filter bed by the borrowing action of earthworms. Further, the increase in the surface area of soil particles with an increase in vermibed porosity to retain more organic pollutants and suspended solids facilitates further decomposition by earthworms (Jiang et al. 2016; Sinha et al. 2008; Singh et al. 2018). Earthworms process wastes by actions like ingestion, grinding, digestion and excretion, and these actions have several physical, chemical and biological effects on the internal ecosystem of earthworm active zone (Singh et al. 2017). The ingestion and grinding actions by earthworm result in conversion of feed waste material into small particles (2–4 microns) followed by the digestion due to symbiotic action of microbes and enzymes in intestine (Kumar et al. 2015; Sinha et al. 2010; Singh et al. 2017; Wang et al. 2011). Numerous enzymes like protease, lipase, amylase, cellulase and chitinase are secreted in the gizzard and intestine of the earthworms which lead to biochemical conversion of the cellulosic and the proteinaceous materials present in the wastewater (Sinha et al. 2010). Since earthworm gut hosts diverse microbial communities, ingested food materials are excreted as vermicast into the soil with nutrients. Microbes present in the biofilm for their population growth further degrade nutrients retained on it, and the nutrients present in the vermicast (Sinha et al. 2008). Earthworms secrete mucus (slimy fluid) from their body which is composed of various metabolites to keep their body surface humid, which also helps in absorbing oxygen (Singh et al. 2017). Earthworms are able to convert large organic matter into complex amorphous solids which contains phenolic compounds and this process is called ‘humification’. These humic substances present in vermibed help in metal adsorption and contain those organic compounds which have complex molecular structure as aromatic rings, carbonyl groups, phenolic and alcoholic hydroxyl. This molecular structure binds with different metal ions and thereby helps in metal removal (Singh et al. 2017).

Fig. 1.5
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Schematic representation of the role of different layers and components in macrophyte-assisted vermifiltration system

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In addition, significant pathogen reduction by the vermifiltration technique has been reported (Samal et al. 2017a). Earthworms have the capacity to cull the pathogens present in the ingested materials (Sinha et al. 2010). The pathogen reduction in vermifiltration is caused mainly because of the enzymatic and microbial activities (Alberts et al. 2002; Hartenstein 1978; Monroy et al. 2008, 2009; Swati and Hait 2018). In addition, inhibition in humates in the guts of earthworms is responsible for the pathogen removal (Brown and Mitchell 1981; Hartenstein 1978).

5 Future Perspectives

The potential of vermifiltration to treat domestic as well as industrial wastewater is well documented. An insight of vermifiltration based on the experimental results, design configurations and treatment mechanism has been provided. Vermifiltration integrated with macrophyte is an emerging technique for the wastewater treatment. Most of the studies have demonstrated vertical vermifilter at laboratory-scale level only for synthetic wastewater treatment. For this purpose, vermifiltration studies with real sewage and industrial effluents will be useful to assess the organic, nutrient and pathogen removal efficiency. However, research is warranted to explore the different vermifiltration system design configuration for wastewater treatment. Various process parameters such as earthworm stocking density, flow rate, hydraulic retention time (HRT), OLR and filter bed configuration need to be optimized for scaling-up the process. In addition, most of the studies have employed epigeic earthworm species Eisenia fetida only. In this context, it is pertinent to explore the various other earthworm species as pure and mixed cultures as diverse earthworm species co-exist in nature. Studies are required to be conducted to explore the effect of symbiotic relationships or mixed earthworm species on the removal of contaminants from wastewater.

6 Conclusions

The applicability of vermifiltration technique for the treatment of both sewage and industrial effluents along with the treatment mechanisms involved has been extensively discussed. Additionally, the potential of macrophytes has also been discussed in an integrated vermifiltration system for wastewater treatment. Further, the influence of different filter design and operational parameters on the system performance has been presented. The combined effect of earthworm active zone and filter media in the vermifiltration system has been reported for the effective removal of pollutants from the wastewater. Maximum organic and nutrient removal efficiencies of 99% BOD, 96% COD, 86% nitrogen and 83% phosphorus have been reported in the vermifiltration of wastewater. Pathogen removal of 90–99% for FC and 99% for TC, faecal streptococci and E. coli by the vermifiltration technique has also been reported. Further, the nutrient removal in an integrated macrophyte-vermifiltration system is mainly because of uptake by macrophytes for their growth. Removal of pollutant is highly selective on the components of vermifilter like filter media composition, earthworm species and macrophyte employed in the process. Moreover, it is necessary to assess vermifiltration system for wastewater treatment employing mixed cultures as diverse earthworm species co-exist in nature. The effect of various process parameters like HLR, OLR and earthworm density during vermifiltration is not quite clear. Extensive research is warranted to optimize different process parameters along with an optimized vermifilter design for efficient wastewater treatment and recycling.