Analysis of potential of Napier grass, Vetiver and Equisetum plants for the treatment of domestic greywater using box-type constructed wetlands

Water is one of the inevitable products for the existence of life on earth. Water demand, pollution, climate change are giving extra pressure leading to water scarcity in different parts of the world. It is estimated that around 80% of people in the world are encountering the threat of water scarcity (Pulla et al. 2018). Water is unavoidable in domestic, agricultural, industrial, and other allied activities. Among these, only 20% of water consumed is used for potable purposes and the rest 80% is employed for other activities, where there is only a need for treated water (Bhattacharyya & Prasad 2020), and can help a lot in saving fresh potable water. In the case of domestic usage, two-thirds is used for bathing, showering, hand-washing, and toilet flushing (Dixon et al. 1999). The wastewaters released from washing, laundry, bath, showers, kitchen, etc are known as greywater and those from toilet usage are named blackwater and both together comprise 55–75% of wastewater (Avery et al. 2007; Kraume et al. 2010; Shaikh & Younus 2015; Parameswara Murthy et al. 2016). If we can treat and reuse the domestic greywater, it can curb the problems related to water demand and water scarcity to a major extent (Bakheet 2020). A vast variety of greywater treatment techniques are available, mainly of physical, chemical, and biological treatment techniques, comprising various processes like adsorption, coagulation, filtration, precipitation, aeration, disinfection, and biodegradation (Revitt et al. 2011; Parameshwara Murthy et al. 2016). Among these, the most advantageous one is biological treatment (Yoonus & Al-Ghamdi 2020).

Constructed wetlands – CWs (also known as artificial wetlands or planted soil filters) are engineered systems employing ecological processes found in natural wetland ecosystems, utilizing wetland vegetation, soil, and their microbial populations, thereby helping to stabilize, sequester, accumulate, degrade and metabolize/mineralize contaminants (USEPA 2000; Mueller et al. 2003; Yadav & Ghaitidak 2013; Nelson 2014). Constructed wetlands differ in mode of flow, soil surface, and mode of feeding and can be classified as Surface Flow, Subsurface Flow, Vertical Flow, Horizontal Flow, and Continuous/Batch Feeding constructed wetlands (Stottmeister et al. 2003; Borkar & Mahatme 2013; Nelson 2014; Abdelhakeem et al. 2016). Constructed wetlands are comprised of media (soil, gravel, sand, etc), plants (emergent macrophytes, submerged macrophytes, freely floating macrophytes, bamboo, reed, etc), microbes, and so on (Brix et al. 2001; Vymazal 2010; Zidan et al. 2013; Arunbabu et al. 2015; Masi et al. 2017). Various physical, chemical, and biological processes like filtration, flocculation, settling, sedimentation, precipitation, oxidation, ligand exchange reactions, absorption, decomposition, nitrification and denitrification, biochemical transportation, and so on are involved in the treatment of greywater in a series of steps (Nelson 2014; Zhang et al. 2015; Arden & Ma 2018). Surface nature, particle size, bulk porosity, pore space of growth media, and so on determine the purification efficiency of CWs (Amos & Younger 2003).

Different plants have diverse root structures and thus play the role of aerators (Avery et al. 2007). Microbes are attached to the root surface area and help in the effective degradation of organic materials (Haberl et al. 2003). Various plants are widely used in constructed wetlands. For instance, Axonopus compressus (Arunbabu et al. 2015), Phragmites australis (Avery et al. 2007), Coix lacryma-jobi (Dallas et al. 2004), Juncus effuses, Typha latifolia (Shtemenko et al. 2005), etc. In the present study, three plants were analyzed for the potential of greywater treatment in constructed wetlands, that is, Vetiver (Vetiveria zizanioides), Equisetum (Equisetum hyemale), and Napier grass (Pennisetum purpureum). Vetiver plants can grow easily in various environmental conditions. These are hyperaccumulators and the roots can penetrate even a thick layer of soil and are capable of holding soil particles through fibrous roots (Dewi et al. 2019). Equisetum grows very fast and well (Wahyudianto et al. 2019a). Napier grass is a perennial forage crop, which can be easily established and has high biomass, and is drought tolerant (Klomjek 2016). Napier grass have a life period of more than two years, whereas vetiver and equisetum have more than one year of life period (Orodho 2006). The major aim of the study was to assess the potential of the plants – vetiver, equisetum, and napier grass – to be used in box-type constructed wetlands for the treatment of domestic greywater. As these plants are easily available, cheap, and grow well in all climatic conditions, they can be used in all regions for the treatment of greywater.

Box type Horizontal Flow Subsurface CWs were designed and set up at the Department of Environment Sciences, University of Kerala, and three such CWs were set up at a time for the present study. The CWs consisted of three units – one filtration/sedimentation unit and two Constructed wetland units (Figure 1). The filtration unit had 80 L capacity and CW units were made of crates having 65 × 45 × 35 cm length, width, and height respectively. The filtration unit worked in a backflow mechanism. The input pipe was directed to the bottom and then greywater was filtered to the top and passed to the first CW, and then to the second CW. Finally, treated greywater was collected through an outlet pipe from the second CW. Three quarters of all the units were filled with media comprising gravel1 (1/4inch), gravel 2 (3/4 inch), gravel 3 (1½ inch), and sand in the ratio of 1:1:1:1. Apart from the media, CW units had a thin top layer of soil-coir pith-sand mixture, upon which plants were grown. Plants were acclimatized in the CW for two weeks. Among the three sets of CWs, Set-1 employed Napier grass (NP); Set 2 had Vetiver (VT) and Set 3 had Equisetum (EQ). Later, domestic greywater was collected from different houses (kitchen and bathroom) and mixed, and then added to CWs in batches, for 5 weeks. Hydraulic Retention time (HRT) provided was 24 hrs. Effluent samples were collected and analyzed for pH, temperature, Electrical Conductivity (EC), turbidity, Total Dissolved Solids (TDS), Total Alkalinity, acidity, Dissolved Oxygen (DO), Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), hardness, calcium, magnesium, chloride, and salinity.

pH, temperature, turbidity, EC, TDS, salinity, and DO were analyzed using their corresponding probes. Other parameters were detected using a standard procedure (APHA 2012).

where

• Co = Initial Concentration of wastewater;

• Ct = Concentration at time t (Dewi et al. 2019);

• The significance of pollutant reduction is analyzed using two-way ANOVA.

Physico-chemical analysis of input raw samples and output treated samples collected from constructed wetlands were done and the results are given in Tables 1–3 (Supplementary material Figures 3–17).

Among the analyzed parameters, significant pollutant reduction (p < 0.05) was noticed in EC (44.1%), BOD (81.4%), COD (91.1%), acidity (92.8%), total alkalinity (32.3%), calcium (53.6%), chloride (37.6%), magnesium (59.5%), salinity (40%), total hardness (60.4%) and turbidity (92.4%). A significant increase in DO (92.6%) was observed (Table 4) with an average output of 6.2 mg/L (Table 1). pH and temperature had no significant variation with an average output of 7.3 and 33.3 °C respectively (Table 1). Influent greywater comes under the category of very hard water, whereas effluent water is moderately hard in nature (Table 5). The average turbidity of influent is 61.8 NTU, whereas the effluent turbidity is 4.7 NTU, which is less than 20NTU, the standard for water used in irrigation (Table 7). The average BOD of effluent is reduced to 42.3, which renders it suitable for land irrigation. The average COD of influent is outside the limit of 250 whereas effluent COD is within the standard limit for irrigation (Table 7). Influent greywater is slightly saline (1,328.3 ppm), whereas effluent water is non-saline (847.2 ppm) (Table 6). Magnesium, calcium, chloride, alkalinity, total hardness, and so on fall inside the standard values for drinking water (Table 1).

Pollutant reduction efficiency of vetiver is significant for BOD (81.9%), COD (92.9%), acidity (87.9%), alkalinity (34.2%), calcium (54.5%), chloride (34.8%), magnesium (69.1%), salinity (37.5%), total hardness (65.3%) and turbidity (82.6%) (Table 4). Also, there was a significant increase in DO samples (47.1%). Average hardness categorizes influent water as very hard and effluent as moderately hard (Table 5). Influent water is slightly saline; on the other hand, effluent water is non-saline (Table 6). Average output pH shows a slight increase on input which is insignificant (Table 2). The turbidity of the input sample (61.9NTU) is outside the standard limit of irrigation, whereas the average output turbidity is within the standard limit of irrigation quality (10.8 NTU). The average BOD (41.0) of output fits for use in land irrigation and this is the same with COD (62.1) too. Output values of magnesium, hardness, calcium, magnesium, and chloride even fall within the permissible limit of drinking water (Table 2).

Pollutant reduction efficiency of Equisetum showed significance (p < 0.05) in BOD (80.0%), COD (88.2%), EC (51.0%), acidity (91.5%), alkalinity (51%), calcium (42.7%), chloride (46.8%), magnesium (52.7%), salinity (45.8%), total hardness (49.3%) and turbidity (94.7%) (Table 4). Like other plants, Equisetum also had a significant (p < 0.05) increase in DO (97.2%). pH and temperature have no significant variation (Table 3). The average input TDS is slightly saline whereas the output is non-saline (Table 5). Effluent water comes under drinking water for the following parameters: turbidity, pH, calcium, chloride, magnesium, total alkalinity, and total hardness (Table 7). Input BOD and COD are outside the limit for usage in irrigation and inland surface water use; on the other hand, output BOD and COD comply with standards for usage for land irrigation (Table 7).

TSS removal in the constructed wetlands is mainly by means of sedimentation and filtration, particularly by trapping the solids in between the media particles (Vymazal et al. 1998; Wahyudianto et al. 2019b). COD is removed by the microbes that are concentrated around the vast root structure of the plants (Ujang et al. 2018). The selected plants have the ability to increase the development of microbial communities around the roots (Vymazal 2013). Other soluble organics are removed by means of aerobic and anaerobic microbial degradation (Chandekar & Godboley 2017). All the three plants possess the properties of contaminant uptake, translocation, transformation, compartmentalization, rhizofiltration, mineralization and biodegradation which render them capable of reducing the pollutants to a great extent (Chandekar & Godboley 2017; Ujang et al. 2018).

Moreover Napier grass is very effective in removing pollutants. Excessive root systems and a high growth rate aid nutrient removal (Jampeetong et al. 2014). Napier grass was used to treat dairy effluents (Goorahoo et al. 2006), palm oil mill effluent (Ujang et al. 2018), etc successfully. Wastewater treated with napier grass reached USEPA standards for water reuse. Roots of Napier grass act as microbial habitat and may sometimes need a long period to digest organic matters (Wu et al. 2006; Herouvim et al. 2011). In some cases, the yield of the plant can be reduced by limitations of space, water, and nutrients (Klomjek 2016).

Vetiver is an ideal plant for phytoremediation, removing organic and inorganic contaminants from soil and water (Panja et al. 2020a, 2020b). Vetiver is used in more than 100 countries for soil and water conservation, land rehabilitation, pollution control, water quality improvement, and so on (Gupta et al. 2012). Vetiver is characterized by stiff and erect stems, withstands high velocity flows, and is an effective filter for trapping sediment, with a deep, extensive and penetrating root system. These are highly tolerant to adverse climatic conditions, edaphic features, and elevated levels of heavy metals (Truong & Hart 2001). Vetiver has the highest water use rate of more than 7.5 times when compared to other wetland plants (Truong & Hart 2001; Smeal et al. 2003). Vetiver is used to treat and absorb domestic, agricultural, and industrial effluents to degrade antibiotics, brewery waste, etc with BOD and COD removal efficiencies of 80–90% (Truong & Hart 2001; Njau & Mlay 2003; Boonsong & Chansiri 2008; Worku et al. 2018; Panja et al. 2020a, 2020b). Vetiver also treats pH, turbidity, acidity, alkalinity, DO, etc very effectively (Mathew et al. 2016). Roots-associated microbes are core helping agents in remediation (Panja et al. 2020a, 2020b). Vetiver can also be harvested for livestock (Truong & Hart 2001).

Equisetum is easily available, cheap, and has aesthetic value (Apritama et al. 2019). Its survival in extreme to moderate conditions, availability of annual or perennial varieties, and a deep root system render it suitable for phytoremediation (Kurniati et al. 2014). Equisetum is very efficient in removing nutrients from wastewater, laundry waste, leachates, etc (Kurniati et al. 2014; Wahyudianto et al. 2019a, 2019b). On some occasions, the performance of constructed wetlands is affected by the presence of media (Wahyudianto et al. 2020). Though much less utilized, equisetum is a better option for domestic wastewater treatment.

The study has its importance for treating domestic greywater in developing countries. Box-type constructed wetlands need less space and can be managed under shades or terraces of houses, though there is a lack of ground or large area of land. The boxes can be arranged according to the available space and depend on the number of people and the amount of greywater released from the house. The plants and other materials used for construction are easily available and the total cost is also less when compared to other water treatment techniques. These are long-lasting and require less maintenance. Even a common man can understand, utilize and maintain box-type constructed wetlands. This also helps in reducing water bills as freshwater can be replaced with treated water for many requirements such as toilet flushing, gardening, washing of vehicles, and so on. Moreover, CWs are eco-friendly, aesthetic, and employ low technology features.

However, there are certain limitations. More boxes are needed if more greywater is generated and makes the chain a bit hectic and there is a need for more storage and pumping facilities, especially if the CW is placed in terraces, and this in turn can increase the total cost and maintenance. This is suitable only for small domestic areas, canteens, etc, and not for large-scale applications. Rain can also limit its outcome as there is a chance of overflow of water. Despite such small limitations, box-type CWs can be widely used in houses and other small point sources of pollution.

Greywater treatment using Napier grass, Vetiver, and Equisetum in constructed wetlands showed the great potential of all the plants for treating domestic wastewater. Significant pollutant reduction was noticed in almost all the plants, rendering them able to be used in constructed wetlands. As these are easily available and grow well in the climate, these can be further employed for the reclamation of greywater. The majority of the parameters, after treatment, comply with the quality for land irrigation, some even matching drinking water quality. It is sure, napier grass, vetiver, and equisetum have the potential to be used in box-type constructed wetlands. In the context of climate change, water scarcity, depletion of drinking water, and poor domestic greywater treatment facilities in India, the model box-type constructed wetland presented in this study can be utilized effectively for the various small-scale point sources of pollution. Developing countries can employ this technology for reducing the increasing water demand.

The first author greatly acknowledges UGC-DSKPDF for the grants provided to carry out the research. The authors are also grateful to the University of Kerala for providing research fields. We are thankful to Dr Jobin Thomas (CGIST) and Dr Muraleedharan (CET, College of Engineering) for all the support provided.

All relevant data are included in the paper or its Supplementary Information.