Potential of different biobed–biopurification systems in the treatment of domestic sewage

Domestic wastewater is one of the major threats to water resources in India (CPCB 2015). As reported by Central Pollution Control Board (CPCB) India, there is a notable difference between the total quantity of wastewater produced, which is 61,754 million litre per day (m³/d)(106 L d⁻¹), and the total volume of wastewater discharged after treatment, which is 2,2963 m³/d. Consequently, a substantial volume of untreated sewage (38,791 m³/d) is discharged into aquatic environments. Considering a low probability of thorough treatment for wastewater, it is predicted that by 2051, rural as well as urban India will produce 50,000 m³/d and 120,000 of sewage, respectively (Shukla et al. 2021). India's rural population lacks wastewater treatment facilities, and hence, in the current environment, developing a sewage treatment plant for all of the country's rural population is economically unsustainable. As a result, sewage is immediately released into the environment without any processing, which leads to water pollution, thus decreasing the amount of freshwater (CPCB 2015).

On the other hand, as farming has grown tremendously over the past few years in order to meet the need for food and as a means of subsistence, for a growing number of people, enormous quantities of organic materials have been produced. Globally, rice husk material, a by-product of rice milling operations, is a major agricultural waste generated nowadays (Tu Nguyen et al. 2022). India as an agricultural country generates a significant amount of rice husk annually, and burning is the easiest preferred way to get rid of this kind of waste. Because rice husk burning releases a lot of carbon dioxide and carbon monoxide, it is quite dangerous. As per Mor et al. (2016), rice husk accounts for one-fifth of the world's rice production, or around 545 million metric tons, and India generates between 18 and 22 million tons of rice husk annually (Patel et al. 2022). Consequently, as a way to decrease this waste's impact on the natural environment, proper management is required. Because of its lack of economic value, rice husk reduction during rice processing poses problems with waste disposal. Similarly, because of its low density, managing and transferring rice husk ash is also challenging. When rice is processed, a large portion of it undergoes burning or is dumped as waste. As an alternative, rice husk can be burned in brick kilns to provide fuel and as building materials (Pushpakumara & Mendis 2022). Several investigators use different forms of rice husk for the treatment of contaminants including textile dyes and other organic chemicals. Numerous dyes, such as Methylene Blue (Alver et al. 2020), Crystal Violet (Homagai et al. 2022), Brilliant Vital Red (Rehman et al. 2011), Direct Red-31 and Direct Orange-26, Congo Red (Shaban et al. 2017) and Safranin-T (Gun et al. 2022) were adsorbed by rice husk, rice bran, and rice ash. Few studies have been focused now on the use of agricultural waste, i.e., rice husk as an adsorbent in combination with other filter media in the treatment of domestic sewage (Jóźwiakowski et al. 2018). While many media have been employed to detoxify wastewater, agricultural waste has received very little attention, especially rice husk, soil and vermicompost, in combination with constructed wetland media, i.e., gravel and sand (Cizeikiene et al. 2018) in biobed–biopurification systems (BBPSs).

Adsorption by various media (biomixture, gravel, and sand), decomposition by microbial activity, and phytoaccumulation by plant uptake are the main methods of treatment in BBPSs (Singh et al. 2021, 2022; Pérez-Villanueva et al. 2022). Thus, management of solid waste and treatment of sewage water in BBPSs is a potential growth field regarding simultaneous management of waste generation (Shukla et al. 2021; Yadav et al. 2021; Nzengung & Gugolz 2022; Singh et al. 2023). This technology may also overcome the drawbacks of other advanced techniques of wastewater management such as ozonation (Rekhate & Srivastava 2020), advanced oxidation (Hassaan & El Nemr 2017), UV radiation in wastewater management due to their high maintenance and operation costs, periodic electrode replacement, and limited reduction in elevated biochemical oxygen demand (BOD) and chemical oxygen demand (COD) levels. Other methods are also effective in treating wastewater like non-ionic polyacrylamide and poly acrylamide-co-acrylic acids (Hamza et al. 2024) and applications of metal-organic frameworks (MOFs) (Manzoor et al. 2024), but their production process is more complex and needs more professionalism for their operation, thus making them non-sustainable and non-preferable solutions. Therefore, more environmentally friendly alternative technologies are recommended. Among them, constructed wetlands (CWs) and BBPSs, constitute an effective natural treatment technology, with low operational and maintenance costs, simple and easy operation and less environmental impact for wastewater management. Furthermore, the primary goal of creating BBPSs is to let locals manage their sewage on their own without needing help from the municipality that is responsible for the sewage system. Furthermore, the installation of the aforementioned facilities promotes blue water conservation by repurposing the treated water for activities such as vehicle washing, backyard gardening, borewell replenishment, and toilet flushing.

Substrates used in all the BBPS structures were rice husk, soil, vermicompost, gravel and sand as adsorbents in different proportions according to the design and treatment needs. Before being used in this study, rice husk was collected from a neighbouring rice mill and sieved (IS 460:1962, 710 μm). After thoroughly cleaning the rice husk in distilled water, it was dried in an oven set at 100 °C for a few hours. The soil used in the BBPS structure was collected with a spade from the upper layer (0–25 cm) from the field of IESD, BHU, Varanasi, and was dried for some days, and ground and sieved through a 0.5-mm sieve to obtain uniformity. The soil provides sufficient organic material, especially in the early stages of the system, to support microbial activity, modify rice husk composition, and support plant development. Vermicompost was procured from a nearby agricultural facility in the Varanasi region; vermicompost is used to increase porosity, and thus, the adsorption of contaminants in the media. All the substrates were selected according to their ease of availability. Biomixture was prepared by mixing a definite proportion of rice husk (50%), soil (25%), and vermicompost (25%) together. Gravel and sand used in the treatment process were also washed thoroughly 5–7 times to dissolve any adhered impurity and dried properly before use. The maximum hydraulic loading rate of all the treatment setups is similar, i.e., 0.006 L/(m², cycle). The maximum flow rate of the setups was calculated by multiplying the hydraulic loading rate by the surface area of the setups which was 4.8 m³/cycle. Empty bed contact time (EBCT) is equal to the volume of the empty bed divided by the flow rate, for these setups per cycle EBCT equals 3,666.6 min (for 72 h cycle). The Brauner–Emmet–Teller (BET) method is a commonly used technique to calculate the surface area of porous materials, including sand and gravel, based on nitrogen adsorption isotherms. However, the BET method itself does not directly calculate porosity. Instead, it helps in estimating the surface area, which can be related to porosity when combined with other parameters such as the material's bulk and particle densities. BET analysis of gravel and sand reveals that the surface area of gravel was 3.079 m²/g and sand was 1.2897 m²/g, and the pore radius of gravel was 23.462 and sand was 17.285 A°.

The effluent collected at different HRTs was examined using standard techniques for various physico-chemical parameters, namely pH, EC, TDS, BOD, COD, phosphates, nitrates, chloride, alkalinity and hardness; after sample collection, unstable parameters like pH, temperature, EC and TDS, were assessed immediately by using a portable multi-parameter (Hanna HI98194), to reduce the inaccuracies that occur over time as a result of the chemical and biological interactions between the sample and the environment (Hutton 1983). Sample collection, storage, transfer and other testing criteria were followed in compliance with the American Public Health Association guidelines (2017). The samples were analysed for BOD by Winkler's azide modification method, whereas nitrate and phosphate were estimated by the UV spectrophotometric method (Shimadzu UV1900 series). The minima, maxima, average and standard deviation values of the studied parameters have been determined based on the test results, along with the standard deviations (Table 1).

Excel 2019 was used to calculate the minima, maxima, mean and standard deviation for each parameter. A 3D trend was created using Origin Pro 2023. Since the majority of information collected was not distributed according to a normal distribution, a correlation matrix plot among each of the variables was created using the Spearman correlation test.

Samples of sewage water were taken from the primary settling tank at the sewage treatment facility at Bhagwanpur and were initially examined for several physico-chemical parameters. Table 1 displays the min–max and average–standard deviation of all the physico-chemical properties of the effluent, indicating its alkaline character; the pH of sewage water was neutral to slightly alkaline. The slightly alkaline nature, i.e., 8.05 indicates that the wastewater may be alkaline due to the washing area of the residential town. EC, which corresponds to the dominating ions that are the result of ion exchange and solubilisation in water, is a key factor in determining whether it is suitable for irrigation. The EC of effluent varies with an average mean of 1110.11 μs/cm. The effluent's EC has a strong positive correlation with TDS, having an average range of 555.05 mg/L. Sewage effluent had an average total alkalinity of 761 mg/L. The data showed that pH, EC and TDS were within the limits described in the revisions made to CPCB 2017 for its safe release into the in-land surface water and on-land irrigation. Nitrate levels in the effluent samples with min–max were 21.87–196.15 mg/L. The nitrate concentration in sewage effluent corresponds to the household stuff, runoffs from agricultural fields, cattle yards, etc. According to various reports, sewage discharge is a significant source of phosphate from a variety of sources, which causes eutrophication of aquatic bodies (Vymazal & Kröpfelová 2008; Gizińska-Górna et al. 2020). The phosphate content of the effluent ranged from 0.04 to 0.34 mg/L with an average concentration of 0.24 mg/L. BOD is typically employed as an index to quantify the level of organic contamination in wastewater, which can be decomposed by bacteria under anaerobic conditions. Variations in the BOD of the effluent ranged from 50.89 to 83.84 mg/L with an average of 66.22 mg/L, whereas the values of COD varied from 220 to 297 mg/L. Other parameters are described more elaborately in Table 1.

One may ascertain the function of plants and biomixture in eliminating organic and biogenic materials by contrasting the values shown in Table 3 (the approximate concentrations of each naturally occurring component that plants and biobed medium absorb). The MRR, which compares the quantity of an element eliminated to the unit of surface area of a BBPS system, is a metric that may be utilised for this purpose. Table 3 compares the theoretical MRR values (calculated using Formula (2)) for the main contaminants, i.e., nitrate, phosphate, BOD and COD for each bed. The obtained value shows the removal of all the biogenic and organic parameters, but the MRR value for BOD was the maximum. Accordingly, BBPS1 and BBPS2 performed much more efficiently in comparison to others for the removal of nitrate phosphate and BOD.

Rice husk's inclusion in biomixture facilitates easy growth and adhesion of microorganisms due to its larger surface area and presence of labile carbon. As a result, any biological reactions occurring in a reactor using rice husk as the carbon source can be proved faster than the other methods. As a result of the experiment, BBPS2 shows a fast removal effect at lower HRT. Thus, in this experiment, to enhance the reaction rate of BBPS1 which corresponds to the conventional constructed wetland structure, rice husk as a biomixture was added as an additional media in combination with gravel and sand (BBPS2), which results in higher removal efficiency for most of the parameters. Likewise, this research also shows that the effectiveness of contaminant removal in BBPS1 and BBPS2 is very high in comparison to BBPS3 and BBPS4 because the filter material, i.e., only biomixture, has lost its ability to sorb substances, and there is a possibility that the reactor might chock after an interval. Thus, BBPS3 and BBPS4 need additional media to filter larger particles, as the presence of sand and gravel can also act as filter media that enhance the interaction among iron (Fe), calcium (Ca) and aluminium (Al) in the sand or gravel substrate through adsorption and precipitation (Arias et al. 2001). The capacity of a BBPS to remove contaminants like phosphorus may, therefore, be dependent on the contents of minerals present in the substrates, thus lacking in them, and reducing the adsorption capacity of BBPS3 and BBPS4. The remedial capacity for total nitrogen by the tested BBPS1 and BBPS2 was very high, as the elimination of nitrogen from the denitrification and nitrification processes changes compounds that contain nitrogen, making sewage reusable. For this reason, researchers of the majority of research suggest that single-stage systems, i.e., either single media or media without plant, cannot accomplish a high efficiency of pollutant treatment since many processes cannot take place in one bed at the same time. The current findings support this observation. The most obvious finding from this experiment suggests that the hybrid setup, i.e., BBPS2, with more beds, offering the ability to produce both aerobic and anaerobic conditions, as well as filtration capacity and phytoaccumulation properties to the rhizosphere effect, showed an improved efficiency in reducing contaminants that were lacking in BBPS3 and BBPS4.

Additionally, the adsorbent used in a reactor must possess some qualities like quick and easier separation, favourable transportation, chemical, mechanical and heat stability, fouling resistance, regenerating ability, insoluble to the liquid in contact, a large area of pores, excellent efficiency, appropriate pore size and volume, uniformity, availability, easy to regenerate and must cost effective to use. Being ecologically sustainable, yet using straightforward processing techniques, this study fulfils all the criteria. The primary advantage of this research is that C. indica and biomixture along with gravel and sand as adsorbent and filter media can be good candidates for removing organic and biogenic compounds faster at low HRTs and can be used for wastewater treatment generated at the grassroot level and for the treatment at own level without integration of municipal corporations. Additionally, it can also be used in impoverished nations with significant effluent discharge of millions of litres per day because this eco-technology is particularly advantageous because of its minimal maintenance requirements.

The authors are thankful to the Central Instrument Laboratory of IESD for providing their laboratory. This research was not funded by any project grant or funding agency. The University Grant Commission, Government of India provided fellowship to the first and other authors. The corresponding author acknowledges IOE for partial support in conducting this research work.

A.S. contributed to the idea of the article, literature search, and original draft making; G.S. and P.S. contributed to writing and editing, V.K.M. contributed to supervision, critical review of the work, and contribution towards editing of the manuscript.

This research was not funded by any project grant or funding agency.

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

The authors declare there is no conflict.