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Frontiers in Environmental Engineering ·

Volume 5 - 2026 | https://doi.org/10.3389/fenve.2026.1722898

Waste plastic bottle strips as biofilter media for a sustainable onsite sanitation system

  • Ugyen Dorji1,2
  • Abdulaziz Almuntashiri3
  • Yeshi Choden1
  • Hokyong Shon1
  • Sherub Phuntsho1*corresponding author
  1. 1University of Technology Sydney, Ultimo, NSW, Australia
  2. 2College of Science and Technology, Royal University of Bhutan, Phuentsholing, Bhutan
  3. 3Department of Environmental Engineering, Umm Al-Qura University, Makkah, Saudi Arabia
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Abstract

This study explores the use of waste plastic bottle strips as biofilter media for improved urban on-site sanitation systems. Initially, Synthetic wastewater was treated in column reactors packed with plastic strips of varying sizes (3 mm, 5 mm, and 7 mm) as biofilter (BF) media. The biofilter system demonstrated a reduction in hydrolysed and acidified fractions, alongside elevated oxidation-reduction potential values (exceeding −250 mV) under both winter and summer conditions. The non-woven plastic matrices provided effective support for biofilm formation and biosolid retention, while simultaneously addressing plastic waste problems. Over a 150-day ambient laboratory-scale study, the striped BF achieved average chemical oxygen demand (COD) removal rates of 72%, 71%, and 70% for 3 mm, 5 mm, and 7 mm sizes, respectively, under an optimum hydraulic retention time (HRT) of 12 h. The UASB + BF onsite treatment system can produce a much-improved effluent quality compared to a conventional onsite sanitation system using a septic tank. These findings underscore the potential of waste plastic bottle strips as a cost-effective, sustainable biofilter medium for decentralised sanitation, offering dual benefits of wastewater treatment and plastic waste reuse.

Keywords

1 Introduction

As most developing countries struggle to meet the rapidly increasing demand for sewerage infrastructure in the urban areas, onsite sanitation system is expected to play a major role for many years to come (Sainati et al., 2020). Most onsite sanitation system in the unsewered urban and peri-urban areas consist of a traditional two-chambered septic tank followed by a soak pit. Such onsite sanitation systems function well only if the land area is adequate and the soil properties are ideal for effluent disposal, not generally available in urban areas (Dorji et al., 2019). A recent study in Nairobi’s peri-urban, Utawala observed that combined septic tanks and soak pits achieved only 44% overall treatment efficiency, with nearly 78% of systems failing primarily due to poor soil characteristics, maintenance lapses, and inadequate design (Maeda et al., 2024). Several studies have confirmed that septic tanks and pit latrines frequently release nitrates, phosphates, pathogens, and heavy metals into soil and water (Shivendra and Ramaraju, 2015; Mbae et al., 2024). While the septic-soak pit systems offer basic onsite containment in urban and peri-urban areas in the absence of the public sewer system, their inconsistent treatment performance, high contamination risks, and vulnerability to poor design, soil constraints, and neglect limit their effectiveness.

This often results in inadequate onsite sanitation systems, risking the health of the environment and the public (Shaw and Dorea, 2021) and contamination of groundwater if the water table is shallow (Shivendra and Ramaraju, 2015; Mbae et al., 2024). For example, more than 80% of Bhutan’s urban residents use on-site sanitation systems to manage household wastewater and 40.5% of buildings in the towns in Bhutan were found to have septic tanks without soak pits, indicating that septic tank effluent is being discharged to the environment without proper soil treatment (Dorji et al., 2019). On-site sanitation systems demonstrate high efficiency in domestic wastewater treatment, with consistent pollutant removal rates across multiple technologies (Sharma et al., 2019). Besides, most conventional onsite sanitation system is designed to treat only black water from toilets and greywater from kitchens, bathrooms and laundry is discharged openly into the surface drains, impacting the environment and the water quality (Helmer and Hespanhol, 1997; NSBB, 2017a). Consequently, there is a need for an improved on-site sanitation system that is affordable and effective in the absence of a public sewerage system.

While there are ranges of advanced and modular onsite sanitation systems in the international market; however, anaerobic treatment processes are generally more affordable and attractive for small communities in developing countries due to their low-cost construction and O&M costs (Almansa et al., 2023). The upflow anaerobic sludge blanket (UASB) is a low-cost and low-energy technology widely adopted for domestic wastewater treatment in tropical and sub-tropical regions (Santiago-Díaz et al., 2019). The UASB treats wastewater using an upward flow that enhances contact between influent and anaerobic biomass (sludge granules). UASB includes a three-phase separator for gas, solids, and liquid, enabling sedimentation, digestion, and polishing across compartments. Organic matter (COD, BOD, and suspended solids) is biologically decomposed, forming biogas (methane and CO2) and other by-products in the process, removing 60%–90% of the organic matter (Santiago-Díaz et al., 2019). UASB is a form of primary treatment, sometimes used as a septic tank, and hence its effluents require further treatment to meet effluent discharge quality using low-cost post-treatment processes (El-Khateeb et al., 2009; Khan et al., 2016). In the earlier studies by this group, UASB was suggested as an advanced substitute to a septic tank for onsite sanitation to treat blackwater (Dorji et al., 2021).

Tricking filters and biofilters have been used for further treatment of UASB effluent and produce much improved effluent quality by removing BOD/COD and some faecal coliform (Chernicharo et al., 2015). BF provide a large surface area for diverse microorganism communities to thrive, facilitating biosolids retention and biological degradation of dissolved organic loads (Wąsik and Chmielowski, 2017). While several low-cost packing materials have been used for anaerobic effluent post-treatment (Kaetzl et al., 2018), the use of shredded waste plastic bottles in Bhutan was recently investigated (Dorji et al., 2022). In that study, a shredded plastic flakes were found highly effective as a biofilter media for secondary treatment of UASB effluent and greywater. Several other studies also demonstrated the effectiveness of waste plastic bottles as carriers for domestic wastewater treatment (Fauzi et al., 2023; Anh et al., 2024). The specific surface area of the biofilter media (m2/m3) can play a major role in the effectiveness of the wastewater treatment. Plastic bottles, when cut into long strips and rolled, is expected to create biofilter media with a significantly large specific surface area and high porosity providing an ideal environment for active biomass to attach, grow, and form colonies, enabling efficient microbiological processes for wastewater treatment. To our knowledge, such biofilters have not been reported in the literature and this concept was inspired by our earlier work using plastic bottle flakes as biofilter media. Compared to plastic flake media, the use of smaller strips is expected to significantly increase the surface area per unit volume of packed media, thereby enhancing treatment performance.

This research exemplifies circular economy principles by transforming post-consumer plastic bottles—traditionally considered waste—into a long-lasting functional biofilter media for wastewater treatment. Instead of entering landfills or polluting ecosystems, these plastics are repurposed to enhance effluent polishing from UASB reactors, creating value from the plastic waste. The approach closes material loops by extending the lifecycle of plastics while reducing reliance on virgin materials for treatment infrastructure. By integrating waste recovery with environmental protection, this innovation promotes resource efficiency, mitigates plastic pollution, and supports sustainable sanitation systems in line with global circular economy goals.

This study aims to investigate the feasibility of employing nonwoven waste plastic strips as a low-cost biofilter (BF) media for improved on-site sanitation in Bhutan’s urban areas. The study evaluated the influence of packing density of BF, hydraulic retention time (HRT), strip size, etc., under ambient local conditions.

2 Materials and methods

2.1 Wastewater characteristics

The initial performance study of all the BF media was conducted using synthetic wastewater simulating a typical UASB effluent and greywater (mixed in a 1:4 ratio), and the recipe for the synthetic wastewater is described elsewhere (Dorji et al., 2021). The wastewater characteristics presented in Table 1 reflect the mixture of UASB effluent and greywater, which was used as the influent for the ABF reactor columns according to the experimental design. Greywater was combined with UASB effluent in both synthetic and real wastewater trials, as greywater is often discharged untreated into stormwater drains in Bhutan, contributing significantly to overall pollution—accounting for up to 40% of total COD and over 70% of domestic wastewater volume (Dorji et al., 2021).

TABLE 1. Characteristics of synthetic wastewater and real wastewater used as influent for the biofilter treatment, and they represent the mixture of UASB blackwater effluent and greywater in the ratio 1:4.

ParametersTemp (°C)CODt (mg/L)CODs (mg/L)TN (mg/L)TKN (mg/L)NH4 + (mg/L)PO4 3− (mg/L)
Synthetic wastewater influent (mixed 1:4)(pH: 6.90–7.65)21 (±3.49)429 (±39.12)215 (±3.12)51 (±3.09)48 (±2.68)20 (±1.01)43 (±0.91)
Real domestic wastewater influent (mixed 1:4)(pH: 6.52–7.23)26 (±0.81)240.07 (±43.43)77.77 (±19.95)20.13 (±3.90)17.52 (±3.96)12.85 (±3.14)4.42 (±0.78)

CODt: total COD; CODs: soluble COD; values between brackets are standard deviations.

For convenience and uniformity, soluble COD (CODs) is referred to as “COD” throughout.

The synthetic wastewater was formulated to closely resemble real domestic wastewater, ensuring a realistic carbon-nitrogen balance for biological treatment processes (Dorji et al., 2021). By incorporating carbohydrates, proteins, and nutrients, the synthetic mix simulates typical household waste, enabling controlled and reproducible performance testing. While synthetic wastewater offers advantages such as consistent composition and pathogen avoidance, it does not fully capture the variability and complexity of real wastewater encountered under local conditions.

All chemicals used for the preparation of synthetic wastewater, such as carbon source, major nutrients (nitrogen and phosphorus), and micronutrients, were of analytical grade (Sigma Aldrich, Australia). Later, the performance study of the biofilter media was validated using a real wastewater obtained from a pilot-scale UASB reactor that treats domestic wastewater within the College of Science and Technology in Bhutan, mixed with real greywater (Table 1 shows the composition of synthetic wastewater and the real wastewater).

The differences in wastewater characteristics between synthetic and real samples arise because synthetic wastewater is prepared with fixed amounts of chemicals for consistency, while real wastewater is a complex, variable mixture influenced by household activities, product use, and seasonal changes.

2.2 Biofilter media

The PET (Polyethene Terephthalate) waste plastic bottles (soft drinks, water, food containers) collected from the CST (College of Science and Technology, Phuentsholing, Bhutan) campus were washed, dried, and subsequently striped into 3 mm, 5 mm, and 7 mm plastic strips with a length up to 5 m long using an improvised sharp blade attached to a wooden block. All plastic strips were bundled into spherical rolls, weighed, and counted before being placed in the three BF columns (bioreactors) with a freeboard space left under the column’s flat lid for biogas collection. The porosity/density of the packed biofilter media was determined by measuring the volume of water required to fill the bioreactor column of known media height to a specific level, assuming water fills all voids within and between the media. Refer to Table 2 for detailed information on the filter media.

TABLE 2. Waste plastic strips biofilter media used and their basic characteristics.

Biofilter media types and their propertiesStriped BF7Striped BF5Striped BF3Commercial BF
Strip media size5,000 mm × 7 mm5,000 mm × 5 mm5,000 mm × 3 mm10.5 mm
Thickness (mm)0.20–0.401
Surface area per unit volume of packed media (m2/m3)777.78798.61833.33827.46
Packed media porosity (%)77808268
Pore volume (L)1.111.161.180.93
Numbers & weight (g)40 nos. & 345 g23 nos. & 237 g16 nos. & 209 g1,202 nos & 195 g
Images of the biofilter media made from balls of plastic stripsTangled translucent plastic strips resembling packing or strapping material are loosely gathered into an irregular bundle on a textured dark surface.Coiled translucent plastic ribbon or strip arranged loosely on a dark surface, exhibiting a shiny and smooth texture with overlapping and interwoven segments creating a tangled appearance.White rubber bands are bunched and intertwined together to form a roughly spherical shape, resting on a dark, solid surface.A pile of small, white, cylindrical plastic rings with internal cross-shaped structures scattered on a wooden surface, resembling bio-media filters typically used in water or aquarium filtration systems.

2.3 Biofilter columns (bioreactors)

Four bioreactors were constructed using transparent acrylic tubes, each with an internal diameter of 45 mm and a water column height of 1 m, as illustrated in Figure 1. Each column has a total volume of 1.59 L and an effective working volume of 1.44 L, filled with different BF media (three reactors filled with striped BF and one with a commercial biofilter). Designed as up-flow vertical bioreactors, the system promotes optimal contact between biofilm and influent wastewater while preventing sludge washout. To enable convenient sampling and monitoring, three sampling ports were installed at 300 mm intervals along each column. Due to the compact design and lightweight nature of the striped BF media, cuboidal marbles were placed on top of the packing material to prevent media flotation and maintain structural integrity. Wastewater was introduced from the bottom of each column, ensuring uniform flow distribution and enhanced biofilm interaction.

Schematic layout of the lab-scale bioreactor using striped BFs and commerical BF for the post-treatment of a wastewater effluent containing combined UASB effluent and greywater (1:4 v/v).
Figure 1. Schematic layout of the lab-scale bioreactor using striped BFs and commerical BF for the post-treatment of a wastewater effluent containing combined UASB effluent and greywater (1:4 v/v).

2.4 Operations of bioreactors for wastewater treatment

Bioreactors were inoculated using seed sludge obtained from a pilot UASB plant located at the College of Science and Technology Campus, Phuentsholing, Bhutan. After seed sludge was added to the bioreactors (filling about 50% of the BF media), they were left to inoculate for 2 days to initiate microbial adaptation. The bioreactors were then filled with synthetic wastewater and left for an additional day to support microbial growth and adaptation.

Synthetic wastewater was then recirculated in the reactors under a 1-day HRT to promote the growth of biofilm on the BF media by maintaining pH at around 7 ± 0.5 using NaHCO3 dosing. Based on the earlier study (Dorji et al., 2021), an optimal HRT of 20 h was selected for COD removal. After 1 month of inoculation, the BF columns were operated continuously at this HRT for 7 weeks to stabilise performance and prevent VFA accumulation.

Subsequently, HRT was then gradually reduced to 12 h, 8 h, and 6 h over 5–6 weeks intervals to allow microbial adaptation to increased organic loading rates. After 150 days of operation with synthetic wastewater, real domestic wastewater (composed of a 1:4 v/v mix of UASB effluent after treating black water and greywater) was introduced at a 12 h HRT. At this stage, the BF columns were considered stabilised under anaerobic conditions and are capable of handling fluctuating organic loads. This HRT was selected based on optimal COD removal during the treatment of synthetic wastewater. See Table 3 for the operational details of the BF columns.

TABLE 3. Operational conditions of the bioreactor columns using synthetic and real wastewater.

Operational parametersUsing synthetic wastewaterUsing real domestic wastewater
HRT (h)2012861286
OLR* (kg COD/m3.d)0.260.430.650.860.180.280.36
V up (m/h)0.010.020.030.050.020.030.05

OLR*: organic loading rate; V up: up-flow velocity.

Although this study was conducted under ambient conditions in a treatment shed, it is important to note that wastewater treatment efficiency in real-world applications can be affected by weather, location, and seasonal variations. Therefore, field-scale systems should incorporate flexible and adaptive operational strategies to ensure reliable performance under diverse climatic conditions.

2.5 Wastewater sampling and analysis

Effluent water quality monitoring from the bioreactor was conducted twice weekly using grab samples. Parameters measured included pH, temperature, electrical conductivity (EC), turbidity (NTU or Nephelometric Turbidity Units), and oxidation-reduction potential (ORP). COD and nutrient concentrations (total nitrogen - TN, total Kjeldahl nitrogen -TKN, ammonia nitrogen - NH3-N, and phosphate -PO4 3-). Due to low total suspended solids (TSS), turbidity was used as a surrogate indicator. All samples were filtered through 0.45 μm Nylon filters (Agilent) before analysis. Standard methods from APHA (1998) guided COD, TKN, and PO4 3- assessments. COD, PO4 3-, TKN, NH3-N, and total volatile acids (TVA) were measured using HACH test kits and a DR3900 spectrophotometer. pH and ORP were recorded using ion-specific probes (Lab Quest, Vernier), while temperature and EC were measured with HANNA Instruments. Turbidity was assessed using a 2100P Turbidimeter (HACH, Australia). Biogas composition was analysed using a BIOGAS 5000 analyser (Geotech, UK) after collection in 5 L gas sampling bags (Sigma-Aldrich). All water samples were analysed in triplicate to ensure data accuracy and reliability.

3 Results and discussions

The results are presented separately and comparatively. First, the treatment or analysis results from synthetic wastewater are presented and interpreted in corroboration with the literature, emphasising controlled conditions, specific parameter changes (e.g., COD concentrations and turbidity levels), and biodegradability indicators. Second, the real wastewater results are presented and interpreted in alignment with the literature, noting variability, unexpected behaviour, or performance deviations due to the complex biofilter matrix effects. The results are discussed to show how synthetic wastewater results relate to or differ from real wastewater behaviour, focusing on practical implications for scaling up or environmental relevance.

3.1 Start-up operation of bioreactors

Following 7 weeks of inoculation, a steady-state operating condition was achieved for all the bioreactors as indicated by reliable COD effluent removal under a 20-h HRT (see Figure 2). This dependable performance confirms that organic matter was not accumulating in the biofilter columns, marking the completion of the start-up phase of the bioreactors. Effluent characteristics remained consistent under the applied organic loading, supported by controlled operational parameters, particularly pH (see Table 4).

Effluent COD from striped BFs, under HRTs of 20 h, 12 h, 8 h and 6 h, with a steady influent COD (215 mg/L).
Figure 2. Effluent COD from striped BFs, under HRTs of 20 h, 12 h, 8 h and 6 h, with a steady influent COD (215 mg/L).

TABLE 4. pH, ORP and effluent temperature in the striped BF and commercial BF treating synthetic wastewater and real wastewater under decreasing HRTs.

​Using synthetic wastewaterUsing real wastewater
ParametersStriped BFCommercial BFStriped BFCommercial BF
HRT (h) →2012612868612861286
pH6.90–7.656.89–7.486.52–7.236.52–7.07
ORP (−mV)(−)332 – (−)185(−)308 – (−)230(−)289 – (−)173(−)286 – (−)208
Effluent temperature (°C)16.1–27.516.2–27.524.3–28.224–27.8
Room temperature (°C)16.7–27.624.3–28.3

After 150 days of bioreactor operations using synthetic wastewater, similar steady-state conditions were attained using real wastewater (Table 4). The pH values remained within the optimal range for anaerobic digestion (6.3–7.8), maintained using NaHCO3 dosing during synthetic feed and naturally stabilised by the alkalinity of domestic wastewater.

Oxidation-reduction potential (ORP) of the bioreactor effluents also aligned well with anaerobic conditions of the wastewater. Optimal ORP for acidogenesis and biological nitrification ranges from −220 mV to −60 mV (Noike et al., 2004). As HRT decreased, ORP values became more negative, reflecting enhanced microbial activity and oxidising capacity (Fan et al., 2019). Despite fluctuations, ORP remained below −150 mV, supporting anaerobic digestion, with values approaching −300 mV indicating full anaerobic conditions (Gerardi, 2003).

3.2 Hydrolytic activity and acidification in the bioreactors

Anaerobic digestion in the bioreactors was supported by stable operational conditions: pH, ORP, and temperature, facilitating the production of volatile fatty acids (VFAs), measured as total volatile acids (TVA). VFAs such as acetate, propionate, and butyrate are key indicators of hydrolytic and acidogenic activity, and their accumulation often signals process instability (Ahring et al., 1995). TVA generation results from glucose fermentation and subsequent degradation of VFAs (Alimahmoodi and Mulligan, 2008; Ososanya and Faniyi, 2015). Although BF media size had minimal impact on TVA production, performance differences were observed between striped BF and commercial BF under varying HRTs.

Unexpectedly, reducing HRT from 20 h to 6 h led to decreased COD and TVA levels (Table 5). This suggests enhanced methanogenic activity, likely supported by stable pH (7.2–7.4) maintained via NaHCO3 dosing (Lettinga and Van Haandel, 1994). At 20 h and 12h HRTs, elevated TVA levels indicated limited methanogenesis or dominant acidogenesis. At 8 h HRT, TVA increased further, reflecting microbial imbalance and adaptation stress (Hoehler et al., 2019). This is likely attributed to longer HRT may allow for more acidogenic bacteria activity producing volatile acids faster than methanogens can consume them, leading to their accumulation, as indicated by the performance differences between single-stage and two-phase anaerobic digestion systems at varying HRTs (Maspolim et al., 2015). However, at 6 h HRT, TVA declined, suggesting microbial adaptation and VFA consumption by methanogens. Lower TVA in commercial BF compared to striped BFs suggests improved anaerobic performance due to better media design in terms of matrix effects. While differences in TVA between striped BFs and commercial BF were incremental, media-specific surface area and orientation had a minor influence on overall performance (Young, 1991).

TABLE 5. Effluent TVA for striped BF and commercial BF using synthetic wastewater under decreasing HRTs.

FiltersStriped BFCommercial BF
Size →3 mm5 mm7 mm10.5 mm
HRT ↓Effluent TVA (mg/L) under a constant influent TVA of 19 mg/L
20 h29.90 ± 2.1035.70 ± 2.2230.00 ± 2.5322.80 ± 1.12
12 h30.90 ± 2.5338.55 ± 2.5534.90 ± 2.6226.30 ± 1.86
8 h44.55 ± 3.4042.10 ± 3.3339.75 ± 2.8136.25 ± 2.64
6 h20.55 ± 1.6625.55 ± 1.8526.50 ± 1.3220.30 ± 1.35

3.3 Performance of striped biofilters using synthetic wastewater

3.3.1 COD removal

The COD removal efficiencies of bioreactors were evaluated using synthetic wastewater with a consistent influent COD concentration of 215 mg/L into the bioreactors. Under HRT of 20 h, all BF media configurations demonstrated effective organic matter removal, with commercial BF achieving an average of 74% COD reduction. The striped BF showed slightly lower performance (∼71%), likely due to reduced up-flow velocity and less uniform substrate distribution within the bundled media. The higher performance of commercial BF at longer HRTs is attributed to their engineered surface area and geometry, which promote better biofilm development and substrate contact. In contrast, striped BFs likely experienced channelling (short-circuiting effects) and dead zones, limiting microbial activity and substrate accessibility.

As HRT was gradually reduced to 12 h over several weeks to simulate increased organic loading, the 3 mm striped BF outperformed other striped BFs and commercial BFs, likely due to its higher surface area and improved biomass retention (Table 2). At 8 h HRT, COD removal efficiency declined in striped BFs, possibly due to increased up-flow velocity dislodging biofilm and reducing contact time. However, the commercial BF maintained more stable performance, suggesting better hydraulic design and mixing characteristics.

At 6 h HRT, all media configurations met effluent COD discharge standards of 125 mg/L as per the National Environment Commission of Bhutan (NEC, 2020), as their effluent COD values were 50.5 mg/L (3 mm striped BF), 51.5 mg/L (5 mm striped BF), and 47.5 mg/L (commercial BF). These results demonstrate that striped BF systems can effectively treat domestic wastewater under short HRTs, provided media design supports adequate biomass retention and flow distribution. Interestingly, striped BFs achieved higher COD removal compared to shredded plastic flakes in the earlier study (Dorji et al., 2021), suggesting that media structure and biosolid entrapment are more critical than surface area alone (Young, 1991). The narrowed pore-packed volume in striped BFs likely enhanced suspended biomass retention, contributing to improved organic matter degradation. However, biogas production was negligible throughout the study, likely due to the low influent COD concentration and small reactor scale.

COD removal performance varied with media sizes across different HRTs. At a longer HRT of 20 h, striped BFs with larger media and higher porosity showed better COD removal due to enhanced flow distribution and substrate contact. However, under shorter HRTs (8 h and 6 h), the 3 mm PF strips–with the highest specific surface area and lowest porosity–achieved superior COD reduction, indicating that surface area and porosity are key factors influencing treatment efficiency. This trend aligns with findings from upflow aerobic biofilters, where media with smaller particle size, lower porosity, and higher surface area improved COD removal (Srinikethan et al., 2008). The results suggest that in anaerobic biofilters, media design optimising surface area and biomass retention is more critical than porosity alone, especially under reduced contact times.

Using synthetic wastewater and showing that optimising surface area and biomass retention is more important than just porosity under short contact times, these results show which design features truly drive treatment efficiency. These mechanistic insights signal biofilter improvements by focusing on how microbes attach and grow on the media, rather than just how easily water flows through it. Thus, the use of synthetic wastewater serves as a controlled baseline that enables understanding the core factors affecting the performance of anaerobic biofilters, without the complexity of real wastewater’s variability. Hence, the use and testing of synthetic wastewater provides a simpler “model system” to refine biofilter design and better predict how they will perform in real and more complex conditions.

3.3.2 Turbidity removal

While Bhutan’s effluent standards specify a TSS limit of 100 mg/L (NEC, 2020), turbidity was used as a surrogate due to negligible TSS in the BF effluents. The influent synthetic wastewater had a turbidity of 116 NTU. Although the national drinking water standard is 5 NTU (NEC, 2016b), effluent below 10 NTU is considered safe for discharge (WHO, 2006).

As shown in Figure 3, there were no significant differences in effluent turbidity among the various biofilter media across different HRTs. Effluent turbidity values for all biofilters were consistently within the range of 10 NTU – 20 NTU, suggesting stable performance across media types. Interestingly, a reduction in HRT from 20 h to 12 h resulted in a slight improvement in turbidity removal, contrary to typical expectations where longer retention times are presumed to enhance treatment performance. However, further reductions in HRT to 8 h and 6 h did not yield additional improvements and, in some cases, led to a slight increase in effluent turbidity, likely due to increased turbulence and reduced particle settling time. Among the media types, the striped biofilters demonstrated marginally better turbidity removal compared to the commercial biofilter, particularly at lower HRTs. The minimum effluent turbidity values recorded were 11.7 NTU for 3 mm striped BF, 11.4 NTU for 5 mm striped BF, 9.8 NTU for 7 mm striped BF, compared to 12.1 NTU for the commercial BF at 12 h HRT. The improved performance of the striped biofilters may be attributed to their larger specific surface area, which enhances particle capture and retention, as supported by findings from Lapointe and Barbeau (2018).

Effluent turbidity removal from striped BFs under the HRTs of 29 h, 12 h, 8 h and 6 h.
Figure 3. Effluent turbidity removal from striped BFs under the HRTs of 29 h, 12 h, 8 h and 6 h.

3.4 Performance of plastic strip biofilters using domestic wastewater

Following 150 days of testing with synthetic wastewater, 3 mm striped BFs consistently achieved higher COD removal across varying HRTs compared to 5 mm and 7 mm striped BFs, though commercial filters performed slightly better overall (Figure 2). Based on these results, a 12 h HRT was selected for subsequent testing with real domestic wastewater to balance treatment efficiency and hydraulic performance.

As presented in Figure 4, all the BFs achieved consistent effluent COD between 25–40 mg/L, which is well below Bhutan’s discharge standard of 125 mg/L. The best COD removal occurred at 8 h HRT, although differences in the effluent COD values were not significant between the biofilters and the HRTs. Even at the lowest HRT of 6 h tested in this study, all the media maintained effective treatment in terms of effluent COD. Lower effluent COD values compared to synthetic wastewater are likely attributed to the higher proportion of soluble organics in real domestic sewage (Luostarinen and Rintala, 2005). This higher presence of soluble organics in the real domestic wastewater compared to synthetic substrates is attributed to the formation of PHB (polyhydroxy butyrate) from other substrates and the contribution of external SBCOD (Slowly Biodegradable Chemical Oxygen Demand) (Carucci et al., 2001), owing to the higher complexity of real wastewater, mainly attributing to the BW (blackwater) from the toilets.

Effluent COD from striped BFs and commercial BF under the HRTs of 12 h, 8 h and 6 h.
Figure 4. Effluent COD from striped BFs and commercial BF under the HRTs of 12 h, 8 h and 6 h.

Biogas production was not observed, likely due to the low influent COD and limited methanogenic activity, consistent with previous studies reporting minimal biogas generation below 300 mg/L COD (Leitão et al., 2006; Ayaz et al., 2012).

The effluent turbidity fluctuated significantly at 12 h and 6 h HRTs, as shown by the highly scattered data in Figure 5; more than that was observed earlier with the synthetic wastewater. At 8 h HRT, the performances of all the BF media were slightly consistent and, in most instances, the striped BFs outperformed commercial BFs with the turbidity values of 12.2 NTU (3 mm striped BF), 15.6 NTU (5 mm striped BF), and 14.2 NTU (7 mm striped BF), compared to 16.7 NTU for the commercial BF. The lower turbidity removal with real wastewater compared to synthetic wastewater earlier may be due to the presence of detergents and particulates, which are less biodegradable under anaerobic conditions (Mousavi and Khodadoost, 2019). Despite having a lower specific surface area compared to commercial BF, striped BFs achieved better COD removal, likely due to improved flow distribution and less channelling effect (Tay and Show, 1998). Among the striped BFs, the 3 mm striped BF consistently performed best for both COD and turbidity removal, highlighting the importance of surface area and media configuration in enhancing treatment efficiency.

Effluent turbidity from striped BFs and commercial BF under the HRTs of 12 h, 8 h and 6 h.
Figure 5. Effluent turbidity from striped BFs and commercial BF under the HRTs of 12 h, 8 h and 6 h.

These effluent qualities are superior to those from a conventional onsite sanitation system, such as septic tanks, without a soak pit. These indicators, based on the use and results of real domestic wastewater treatment in the nonwoven plastic strips biofilters, show the innovative and novel use of nonwoven plastic strips as biofilters. Given their non-intricate design of plastic biofilters, which makes them easy to install and maintain, this is advantageous for scaling up in regions with limited expertise or resources. When scaling up, considerations should include the availability and cost of nonwoven plastic strips, user acceptance, and integration with existing sanitation infrastructure. Furthermore, the study indicated the suitability of nonwoven plastic strips as biofilters under local environmental conditions, suggesting resilience and effectiveness in diverse settings, enhancing environmental sustainability by improving effluent beyond conventional septic systems. Overall, nonwoven plastic strips biofilters represent a scalable, low-complexity, and environmentally relevant solution for wastewater treatment in onsite sanitation systems.

Synthetic wastewater consistently achieved higher COD removal than real domestic wastewater across the tested HRTs under ambient conditions. This performance difference is expected because synthetic feeds contain simple, more biodegradable organics, whereas real wastewater contains more complex composition including particulates and slowly degradable fractions that limit biodegradation rates (Manariotis et al., 2006). Using roughened plastic strips as media, the plastic biofilter further improved COD removal for both wastewater types, consistent with previous findings that enhanced surface textures promote biofilm attachment and overall treatment efficiency in anaerobic filters (Nordstedt and Thomas, 1985; Bodík et al., 2002).

Although no prior studies provide a direct comparison of roughened plastic strips for synthetic versus real wastewater, trends from related plastic biofilter research indicate that real wastewater benefits more from increased surface area and roughness because particulates and colloids have greater opportunity to adhere, hydrolyse, and biodegrade on textured media (Nordstedt and Thomas, 1985; Bodík et al., 2002; Manariotis and Grigoropoulos, 2006). This is consistent with the sustained lower effluent COD observed for real wastewater in the present study, even under ambient operational conditions and varying HRTs. The demonstrated effectiveness of roughened plastic strips as low-cost, high-surface-area media highlights their potential for improved treatment of domestic wastewater in developing regions—such as Bhutan—where energy-intensive treatment systems are financially and operationally challenging.

4 Conclusion

This study investigated the use of waste plastic bottle strips as a sustainable biofilter (BF) medium for improving urban on-site sanitation systems in Bhutan. Column bioreactors were packed with plastic strips of varying widths (3 mm, 5 mm, and 7 mm and about 5 m long) and tested initially using synthetic wastewater simulating a mixture of UASB effluent (treated blackwater) and household greywater. Over a 150-day ambient laboratory study, the system demonstrated stable anaerobic performance, with oxidation-reduction potential (ORP) values exceeding −250 mV and effective reduction of hydrolysed and acidified fractions.

The non-woven plastic matrices supported robust biofilm development for wastewater treatment and biosolid retention, while simultaneously addressing plastic waste reuse in small countries like Bhutan. Under a 12-h HRT, the average COD removal efficiencies were 72%, 71%, and 70% for 3 mm, 5 mm, and 7 mm strips, respectively. Optimal COD removal was observed at 12 h HRT, with shorter HRTs yielding less consistent results.

Compared to conventional septic systems, the integrated UASB + striped BF setup can produce treated wastewater effluent with significantly improved effluent quality compared to the conventional onsite sanitation system. These findings highlight the potential of repurposed plastic bottle strips as a low-cost, effective, and environmentally beneficial biofilter medium for decentralised wastewater treatment, offering dual advantages of pollution control and plastic waste valorisation. It must be acknowledged that this study being laboratory-scale study, provided limited datasets for more robust statistical validation of the study.

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Footnotes

Edited by: Shunan Dong, Hohai University, China

Reviewed by: Mansoor Ahammed, Sardar Vallabhbhai National Institute of Technology Surat, India

Jiang Gong, Huazhong University of Science and Technology, China

Matt Pour, Christchurch Polytechnic Institute of Technology, New Zealand

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author SP declared that they were an editorial board member of Frontiers at the time of submission. This had no impact on the peer review process and the final decision.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This study is funded by the Bhutan Trust Fund for Environmental Conservation (BTFEC) (Project Grant No. MB0167Y16), Australian Research Council Industry Research Hub on Nutrient in a Circular Economy (IH210100001), UTS International Research Scholarship (IRS) for UD.

Data availability

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author contributions

UD: Formal Analysis, Visualization, Data curation, Writing – original draft, Validation, Conceptualization, Writing – review and editing, Investigation, Methodology. AA: Writing – review and editing, Data curation. YC: Data curation, Writing – review and editing. HS: Conceptualization, Resources, Funding acquisition, Writing – review and editing, Supervision. SP: Funding acquisition, Supervision, Writing – review and editing, Investigation, Project administration, Resources, Data curation, Conceptualization, Methodology.

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Citation

Dorji, U., et al. (2026). Waste plastic bottle strips as biofilter media for a sustainable onsite sanitation system. Frontiers in Environmental Engineering, Volume 5. https://doi.org/
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5 - 2026

Copyright

Copyright © 2026 Dorji, Almuntashiri, Choden, Shon and Phuntsho.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Sherub Phuntsho

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