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Article

Performance of Textile-Based Water-Storage Mats in Treating Municipal Wastewater on Urban Rooftops for Climate-Resilient Cities

by
Khaja Zillur Rahman
1,*,
Jens Mählmann
2,
Michael Blumberg
3,
Katy Bernhard
1,
Roland A. Müller
1 and
Lucie Moeller
1
1
Department Systemic Environmental Biotechnology-SUBT, Helmholtz Centre for Environmental Research—UFZ, Permoserstrasse 15, 04318 Leipzig, Germany
2
Sächsisches Textilforschungsinstitut e.V. (STFI), Annaberger Straße 240, 09125 Chemnitz, Germany
3
Blumberg Engineers, Gänsemarkt 10, 37120 Bovenden, Germany
*
Author to whom correspondence should be addressed.
Clean Technol. 2025, 7(3), 75; https://doi.org/10.3390/cleantechnol7030075
Submission received: 10 July 2025 / Revised: 21 August 2025 / Accepted: 26 August 2025 / Published: 1 September 2025
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Abstract

The aim of this study was to evaluate the treatment efficiency and applicability of using textile-based mats as roof biofilters on urban buildings for purifying preliminary treated wastewater (PTW) collected from a three-chamber septic tank. Therefore, a pilot plant with a 15° pitched wooden roof and two tracks for laying two mats made of different materials—polypropylene (PP), designated as Mat 1, and polyethylene terephthalate (PET), designated as Mat 2—was constructed at ground level under outdoor conditions. The plant was operated in parallel for a period of 455 days. Significant differences (p < 0.05) were observed in the results of the mass removal efficiencies between the two mats, with Mat 1 achieving mean removals of five-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), ammonium-nitrogen (NH4-N), and total nitrogen (TN) of 85%, 73%, 75%, and 38%, respectively, and Mat 2 achieving comparatively higher removals of 97%, 84%, 90%, and 57%, respectively. The mean concentrations of BOD5 and COD at the outflow of both mats met the minimum water quality requirements for discharge and successfully met the minimum water quality class B for agricultural reuse. However, the comparatively low mean E. coli removal efficiencies of 2.0 and 2.4 log-units in Mat 1 and Mat 2, respectively, demonstrate the need for an effluent disinfection system. Highly efficient mass removal efficiencies were observed in the presence of dense vegetation on the mats, which may lead to a potential improvement in the urban climate through high daily evapotranspiration. Overall, this study demonstrates the potential for using lightweight, textile-based mats on rooftops to efficiently treat PTW from urban buildings, offering a promising decentralized wastewater management approach for climate-resilient cities.

Graphical Abstract

1. Introduction

Water scarcity due to climate change and increasing water demand is a challenging issue worldwide, and therefore, the conservation of all water resources is very essential nowadays. Currently, more than half of the world’s population lives in urban areas, and due to rapid population growth, this number is predicted to increase globally [1]. A United Nations report [2] estimated that by the year 2050, over 70% of the global population will reside in urban areas. However, cities are severely affected by rapid urbanization, as well as by rising temperatures, prolonged droughts, and heatwaves in recent years [3]. This phenomenon of increasing the magnitude of the urban heat island effect may potentially have undesirable effects on the urban environment and thermal comfort in outdoor spaces and puts pressure on urban resilience [4,5,6,7]. In this context, non-conventional water resources such as the reclamation and reuse of treated wastewater are becoming increasingly important for sustainable water management in water-scarce regions worldwide [8].
However, domestic wastewater contains various consistent pollutants and is characterized by an elevated load of organic matter, solid particles, microorganisms, and nutrients. If not properly treated before discharge or reuse, these pollutants collectively pose a significant risk and substantial threat to human health and aquatic ecosystems. Therefore, effective wastewater treatment is necessary and increasingly important to produce a clear and safe effluent for reuse in agriculture, horticulture, and landscaping or for safe discharge into nearby water bodies without causing any environmental harm [9].
The most common approach to treating sewage in urban areas is the centralized wastewater treatment, whereby sewage from toilets and greywater from bathrooms, kitchens, and laundries are transported via sewer pipelines to a centralized treatment plant [10]. Decentralized wastewater management systems can be more efficient and cost-effective than traditional centralized wastewater treatment systems due to the on-site collection and treatment of rainwater and wastewater (greywater or blackwater). However, space can be very limited in heavily populated urban communities, making the application of these decentralized systems very challenging.
Over the last few years, the use of green roof systems has been recognized as a functional approach to purifying urban wastewater. This is achieved by utilizing various substrates, vegetation, and other mechanisms to filter and absorb conventional wastewater macro-pollutants, and it offers many ecological and economic benefits for urban landscapes [11,12,13,14,15]. These advantages include cover temperature regulation [16], improved air quality [17], mitigation of the urban heat island effect [18], and enhanced biodiversity [13]. Yan et al. [19] provide an overview of the implementation of green roofs for wastewater treatment and rainwater management in a review of recent studies.
The number of retention roofs being built is currently increasing. These roofs store rainwater, which is then evaporated through the activity of plants or vegetation (evapotranspiration, ET) [20,21]. However, the load-bearing capability of the building structure must be carefully considered before constructing green roof systems. Yan et al. [19] have reviewed the commonly used main substrate components and various thicknesses for different green roofs. To prevent potential damage, the total weight of the green roof should be minimized, either by restricting the substrate depth or by using lightweight materials [22]. Consequently, there is an urgent need for research into developing lightweight, low-density materials for wastewater treatment technologies that focus on providing more efficient, sustainable solutions for existing buildings with low load-bearing capacity in growing cities.
Recently, integrated vegetative and water-storage mats, cocopeats, etc., with a lower weight and volume than conventional solid substrate media, such as soil, sand, gravel, and expanded clay aggregates, were implemented as biofilters (biological filters) for performing the biofiltration process. In this process, attached growth microorganisms adhere to the filter material and begin to colonize within the developed biofilms [20,23]. Previous studies under realistic operating conditions have demonstrated the suitability of helophyte mats (wetland roofs with swamp plants) for treating greywater, where the roots of the plants grow into the structure of the textile fleece materials that help to further strengthen the entire helophyte mat [20,21,24].
Engineered textiles produced from different materials or by using different textile construction methods and mats made from these textiles may have a high specific surface depending on their structure. Water-purifying sessile microorganisms attached to the growing biofilms, i.e., microbial interaction, as well as mechanisms such as adsorption, filtration, sedimentation, oxidation/reduction, etc., contribute significantly to the ability of these materials to treat wastewater, thereby removing various constituents that are present in domestic wastewater, such as organic matter, nutrients, suspended solids, microorganisms, etc. A previous study [25] evaluated the influence of the roof slope as well as the length on the retention and detention capacity of green roof drainage mats. However, no scientific data regarding the wastewater treatment performance and engineering applications of such textile-based mats as rooftop biofilters in urban areas have been found to date.
This research paper presents the results of a study conducted at a pilot plant to investigate the treatment performance of two different textile-based mats in treating PTW under outside conditions. The main objectives of this pilot study were as follows: (i) to investigate the treatment efficiency of different textile-based water-storage mats treating PTW and to compare the quality of the treated effluent with allowable limit values for discharge to the environment or reuse in agriculture, gardening, horticulture, etc.; (ii) to evaluate the influence of naturally growing vegetation on the water quality and their potential role in ensuring stable treatment performance; and (iii) to inspect the effect of weather conditions and water loss from the mats due to ET. This study revealed effective processes for the removal of conventional macro-pollutants from municipal wastewater using lightweight biofilter mats on urban rooftops and led to recommendations for developing these mats with innovative designs as an attractive, decentralized wastewater management system within urban infrastructure.

2. Materials and Methods

2.1. Site Description

The experiment was conducted at the Centre for Research, Training and Demonstration for Decentralized Wastewater Treatment (BDZ e.V.) in Leipzig, Germany. The site is described in detail by Rahman et al. [26,27]. Thirteen (13) different decentralized wastewater treatment technologies, each with a design capacity of 4 to 8 PE (population equivalents) are loaded with the same local municipal wastewater and operated under the same climatic conditions. A pipe network collected municipal wastewater from the nearby pumping station and loaded it into a ring line of the circulatory system (wastewater pressure line), following a daily routine. Treated wastewater from all the test facilities, as well as excess water, is collected in a control shaft. From there, it is conveyed back to the original discharge shaft and transported to the centralized wastewater treatment plant for further treatment.

2.2. Experimental Design: Pilot Plant Description

To simplify the operation and maintenance of our experiment, this study was not carried out on an actual household roof. Instead, a pitched roof segment of a wooden structure with a 15° slope was constructed at ground level as a pilot-scale plant, and this was close to realistic conditions. The pilot test rig corresponded to the design specifications already tested in practice with rainwater by Rhizotech (Rosdorf, Germany). A 0.1 m high raised edge on both sides and two tracks (floor area of each track: 440 cm length × 55 cm width) were constructed on the pilot test rig wooden structure and used for parallel testing of various water storage mats treating PTW. The design of both tracks secured an even distribution of the PTW flow across the entire width of the mats, from the inlet (the upper part of the pitched roof) to the outlet (the lower part). A three-chamber concrete septic tank with a volume of 3.6 m3 was used for the preliminary treatment of municipal wastewater by allowing it to settle in different chambers. This septic tank was connected to a storage tank (polyethylene, PE) and timer-controlled peristaltic pumps, which enabled the periodic distribution (the pumps were activated every 2 h for 40 min at a flow rate of around 0.2 L/min) of the PTW as influent to the pilot plant.
Figure 1 shows the schematic illustration of the pilot plant and the detailed flow direction from the septic tank inlet to the pilot plant outlet.
A geotextile (300 g/m2, Bausep GmbH, Limbach, Germany) was laid over both tracks of the pitched roof, followed by a 1 mm thick PVC liner (Heissner, Lauterbach, Germany), to protect the wooden structure. A commercially available, non-rotting water storage mat (WSM 150, ZinCo GmbH, Nürtingen, Germany), made of recycled synthetic fibers (recycled polypropylene, PP; mean thickness: 17 mm; dry mass per unit area: 1500 g/m2; water-storage capacity: 12 L/m2) was laid over the surface of the first track and designated as Mat 1 throughout this study. On the second track, a nonwoven textile mat, which was developed by the Saxon Textile Research Institute (Sächsisches Textilforschungsinstitut e. V., STFI, Chemnitz, Germany) and was a warp-knitted fabric with a polyethylene terephthalate (PET) nonwoven as weft insertion (mean thickness: 26 mm; mass per unit area: 2438 g/m2; water-storage capacity: 19.8 L/m2), was laid over the surface and designated as Mat 2. These two mats (Mat 1 and Mat 2) of the same size with an overall surface area of 2.42 m2 each (440 cm long × 55 m wide) were placed on the two tracks upon the PVC liner of the pitched roof and along the flow direction. In this system, the mats are the sole substrate.
The advantage of using synthetic materials and production techniques lies in the controllability and predictability of the targeted performance parameters, such as the water-storage capacity, pore sizes, and drying behavior. Apart from being lightweight, using these synthetic materials was aimed to be more efficient and a sustainable solution for domestic wastewater treatment on existing urban buildings. However, these two commercially available mats (PP and PET) were selected as contrast materials due to their differences in other specifications, such as the water-storage capacity, mass per unit area, and evaporation rate, and to compare their treatment performance when treating PTW under outside conditions.
There was no intention to grow plants or to add plant seeds on both mats at the beginning, but many different plant species started to grow naturally and rooted on both mats during the course of the experiment. At one stage, dense vegetation covered almost the entire surface of the mats under outdoor condition at the site (Figure 2). The role of naturally growing plants on the mats in terms of enhancing PTW treatment has been discussed in the following section of this paper.
The moist conditions due to loading with the PTW, which was also the source of valuable plant nutrients for the vegetation growth on both mats [20], promoted natural greening by seeds dispersed by the wind.
On the outlet side, a freely hanging metallic strainer with a 3 mm perforation diameter was placed over the outlet hole of each track to avoid blockages due to coarse particles [20] and thereby prevented any potential clogging. A 100 mL tipping counter (Umweltanalytische Produkte GmbH, Ibbenbüren, Germany) with a data-logger was used as a flowmeter at each outlet to record the amount of water flowing out of the mats. The treated water from the mats was collected in a storage tank (PE) located near the pilot plant and afterwards was conveyed to the centralized wastewater treatment plant.

2.3. Experimental Conditions and Operation of the Pilot Plant

During the whole investigation, both mats were operated with the same municipal PTW (mean BOD5 concentration ~210 mg/L) from a storage tank at the site. It was loaded intermittently from this storage tank and distributed evenly across the full width of the mats. Two peristaltic pumps (Ecoline VC280, ISMATEC, Glattbrugg-Zurich, Switzerland) in combination with a modified irrigation system made of thin stainless-steel plate with V-notch weir distributed the PTW on both mats with the same hydraulic loading rate (HLR) of around 40 L/(m2 × d), with an interval by using a timer. Prior to sampling, the inflow rate to each mat was estimated manually, and the outflow rate was calculated using the data collected from the data-logger connected to each tipping counter.
This pilot study was conducted from May 2022 to August 2023 under outdoor conditions at the site. However, to avoid freezing of the electro-mechanical equipment connected to the pilot plant, the whole investigation and operation was paused for 5 months (from December 2022 to April 2023) during the winter months. The mats remained on the pilot roof, without being loaded with PTW during the winter break. Together with the winter break, the duration of the entire investigation lasted almost 16 months (455 days) and the measured mean air temperature ranged from 8 to 26 °C, with an average of 17 ± 5 °C, across the various sampling campaigns at the study site.

2.4. Sampling Campaigns and Analysis

Sampling campaigns were conducted to determine the operational characteristics and treatment performance of each mat during the whole study. Samples were collected from the inlet and the outlet of both mats on a weekly basis as a part of a routine. All grab samples were analyzed in the laboratory to measure the concentrations of the following parameters: 5-day biochemical oxygen demand (BOD5) (DIN 38,409 H52, OxiTop, manufactured by WTW OxiTOP®, Weilheim, Germany), chemical oxygen demand (COD) by using TNTplusTM 821/822 with HR 20–1500 mg/L (manufactured by HACH®, Düsseldorf, Germany), ammonium–nitrogen (NH4-N) according to DIN EN ISO 11,732 [28], nitrate–nitrogen (NO3-N) according to DIN EN ISO 10304-1 [29], total nitrogen (TN) content by using the TNTplus™ Vial Test TNT828 with UHR 20-100 mg/L N (manufactured by HACH®, Düsseldorf, Germany), total phosphorous (TP) according to DIN EN ISO 15681-1 [30], and total suspended solids (TSS) by using vacuum filtration unit according to DIN 38409-1 [31]. The concentrations of TN and COD were analyzed using the spectrophotometer DR 2800 (HACH®, Düsseldorf, Germany) according to the standard method specified by the manufacturer. Dissolved oxygen (DO) (ConOx®, WTW, Weilheim, Germany), pH (SenTix® pH, WTW, Weilheim, Germany), redox potential (Eh) (SenTix® ORP, WTW, Weilheim, Germany), and electrical conductivity (EC) (Cond 330i, WTW, Weilheim, Germany) were measured using a handheld meter (Multi 350i®, WTW, Weilheim, Germany). Turbidity (TU) was determined by a portable turbidimeter (HACH 2100Q IS, Shanghai, China) in the inflow and outflow samples. The Escherichia coli (E. coli) content was quantified using the IDEXX Colilert-18 Quanti-TrayTM method (IDEXX, Westbrook, ME, USA) according to ISO 9308-2 [32].
All the samples were analyzed in the laboratory within 24 h of being collected from the site. To confirm the accuracy of the measurements, all analytical instruments were calibrated on a regular basis according to the instructions from the manufacturers.
Atmospheric air temperature (T °C), precipitation in the form of rainfall (in mm), and humidity (in %) were recorded by using a weather station (ClimaVUE™50, Campbell Scientific, Logan, UT, USA) installed near the pitched roof pilot plant at the site.
The removal efficiency of macro-pollutants was estimated in terms of concentration reduction and mass removal (in %) by using the following Equations (1) and (2), respectively:
Concentration   reduction   ( % )   =   C i n − C o u t C i n × 100
where Cin is the influent concentration [mg/L] and Cout is the effluent concentration [mg/L].
Mass   removal   efficiency   ( % ) = C i n · F i n − C · F o u t C i n · F i n × 100
where Fin is the inflow rate [L/d] and Fout is the outflow rate [L/d]. The mass removal rate [g/(m2 × d)] of the macro-pollutants was calculated as the difference between the mass loading rate [g/(m2 × d)] in the influent and effluent.
The removal of E. coli was calculated as the base-10 logarithmic difference between the concentrations of the inflow and outflow.
Water loss due to ET, i.e., evaporation from the mats and transpiration from the natural vegetation, was calculated by applying the following Equations (3) and (4):
ET (L/d) = Fin + Rin − Fout
ET   ( % ) = E T F i n + R i n × 100
where Fin represents the daily inflow rate [L/d] of the PTW, Rin is the daily amount of rainfall [L/d], and Fout represents the daily outflow rate [L/d] of the treated wastewater.

2.5. Data Analysis and Statistical Methods

All data on the different physicochemical and biological parameters obtained from the inflow and outflow samples are provided in the format of mean ± standard deviation (SD) using the Microsoft Excel 2024 (Microsoft Office LTSC Standard 2024; Version: 2408; Build 17932.20328 Click-to-Run) package in this study. Graphical representations of the obtained data were also generated by using OriginPro® 2024b (64-bit; 10.1.5.132; 1991-2024 OriginLab Corporation; Northampton, MA, USA).
Statistical analyses were carried out, and the achieved experimental results were statistically evaluated using a one-way analysis of variance (ANOVA) test at a 95% significance level in the Microsoft Excel 2024 Software Package to compare several datasets in terms of the differences in inflow, outflow, and mean removal efficiencies, as well as the differences in treatment performance between the mats with dense vegetation and those without any vegetation at all in this study. The statistical differences and test results were considered to be statistically significant when p-values were less than 0.05 (p < 0.05).

3. Results and Discussion

The results described here are based on evaluating the treatment performance of the two textile-based mats as a roof biofilter for treating PTW. Furthermore, the following sections also summarize the effects of water loss due to evaporation and/or ET, as well as the presence of naturally growing vegetation, and the potential applications of such lightweight mats on rooftops in growing urban residential areas.

3.1. Overall Treatment Performance of the Mats

Table 1 presents the mean concentrations of conventional water quality parameters from the inflow and outflow samples, alongside a comparison of the outflow data with the discharge water quality limit values according to the German Standard DWA-A 221 [33], and the European Union’s [34] recommended reclaimed water quality requirements for agricultural irrigation.

3.1.1. Variations in pH, Eh, EC, DO, TU, and TSS

Statistical analysis revealed significant differences (p < 0.05) in pH, Eh, EC, DO concentrations, and turbidity values between the influent and effluent of both mats in this study. Compared to the inlet, the pH values increased overall at the outlet of the mats, ranging from 6.8 to 8.8 in Mat 1 and from 6.6 to 8.7 in Mat 2 during the experiment. However, no significant differences (p > 0.05) in pH values were noticed between the outlet samples of both mats.
Similar trends of increasing Eh were also observed in the outlet of both the mats. Compared to the low Eh value between −307 and −12 mV in the PTW inflow samples, both mats showed significantly higher Eh values (p < 0.05), with mean values of 137 ± 34 mV and 129 ± 56 mV in the Mat 1 and Mat 2 outflows, respectively. This suggests that aerobic conditions prevailed in both mats during this study. The increased redox potential at the outlet of both mats was most likely caused by a general decrease in organic load.
The EC results, which are commonly used as an indicator of salinity, decreased down to mean values of 1303 ± 241 and 1340 ± 237 μS/cm in the outflow, compared to the mean inflow values of 1506 ± 242 and 1512 ± 238 μS/cm in Mat 1 and Mat 2, respectively (see Table 1). The decreased EC in the effluents indicated that the textile mats and the naturally growing vegetation potentially contributed to reducing the concentrations of dissolved ions such as salts and other inorganic substances within the treated water.
The DO concentrations increased significantly (p < 0.05) at the outlet of both the mats compared to the inlet (0.6 ± 0.3 mg/L), ranging from 5.7 to 11.4 mg/L with a mean concentration of 8.3 ± 1.7 mg/L at Mat 1’s outlet and from 5.8 to 10.8 mg/L with a mean concentration of 8.8 ± 2.5 at Mat 2’s outlet. The higher DO concentrations at the outlet suggest that there are abundant levels of oxygen within the mats, as well as favorable redox conditions mentioned above, which are necessary for the oxidation of the macro-pollutants within the PTW. The increased DO concentrations were caused by the increased contact of water with the atmospheric air, which was potentially due to the porous structures of the textile mats in this study.
Compared with the mean influent TU of 167 ± 57 NTU, the TU in the effluents from both the mats declined significantly (p < 0.05) to mean values of 10.7 ± 8.5 and 5.3 ± 4.4 NTU in Mat 1 and Mat 2, respectively. Turbidity refers to the haziness of water caused by suspended particles, and the recorded mean values in the effluents clearly indicated a high quality of treated water achieved from the mats when treating PTW in this study. The TU value of the Mat 2 effluent (5.3 ± 4.4 NTU) almost met the EU’s recommended Class A reclaimed water quality standard for agricultural irrigation (≤5 NTU) (Table 1).
There was no significant difference (p > 0.05) in the TSS concentration between the outflows of the two mats. The mean inflow TSS concentrations of 164 ± 82 and 57 ± 33 mg/L were decreased to mean TSS concentrations of 6.6 ± 1.4 and 10.5 ± 9 mg/L at the outflow of Mat 1 and Mat 2, respectively. This is attributed to a highly efficient mean TSS concentration reduction of 90% in Mat 1, compared to a comparatively low reduction of 74% in Mat 2. However, both mats were highly effective at removing suspended solids from PTW, and the outflow of Mat 1 complied with the EU standard for Class A water quality (<10 mg/L of TSS) for agricultural reuse (Table 1). The mean TSS at the outflow of Mat 2 (10.5 ± 9 mg/L) almost met the EU Class A standard for reuse. However, of the four measurements taken, one set of data with TSS > 20 mg/L in the Mat 2 outflow was above the Class A water quality standard of <10 mg/L due to a clogging issue at the outlet but met the Class B standard of <100 mg/L according to the EU regulations. No guideline for TSS concentration at the effluent is yet to be provided for discharge into the environment. The macro-pollutant mass loading rate and removal efficiency showed a highly efficient TSS removal with a mean removal efficiency of 88 and 96% from Mat 1 and Mat 2, respectively (Table 2). However, the low number of samples taken from both mats for TSS analysis suggested that no definitive statement can be made regarding the TSS removal performance in this study.

3.1.2. BOD5 and COD

Figure 3 shows the results of the BOD5 and COD concentrations between the inflow and outflow, as well as the concentration reduction (%) in both mats over the course of this study.
In comparison to the respective inflows, significant reductions (p < 0.05) in the mean BOD5 concentrations at the outlets of Mat 1 and Mat 2 contributed to an efficient concentration reduction of 82 and 92%, respectively. In general, the BOD5 concentration in the inflow exhibited fluctuating tendencies, whereas a consistently stable concentration was observed in the outflow of both mats (Figure 3a). The mean BOD5 concentration in the inflow was 209 ± 141 mg/L, which was reduced to mean BOD5 concentrations of 23 ± 14 and 11 ± 10 mg/L in the outflows of Mat 1 and Mat 2, respectively. These concentrations successfully met the minimum requirements for discharge water quality according to the German standard and were in compliance with Class B water quality (<25 mg/L) for agricultural reuse according to EU regulations (Table 1). The BOD5 reduction (%) dynamics in Mat 1 showed a relatively low reduction at the beginning as compared to the performance of Mat 2 and afterwards showed a stable concentration reduction until the end of the experiment. With regard to mass removal, there were significant statistical differences (p < 0.05) in the mean BOD5 mass removal efficiency between the two mats, with a mean value of 85% in Mat 1 and 97% in Mat 2 (Table 2).
The COD concentrations in the outflow of both mats fluctuated during the study period (Figure 3c). However, the mean COD concentrations of 119 ± 60 and 114 ± 93 mg/L in Mat 1 and Mat 2, respectively, met the minimum water quality requirements for discharge (i.e., <150 mg/L) according to the German ordinance for domestic wastewater. At the start of the experiment, quite a few outflow COD concentrations in both mats were higher than the allowable limit value for discharge. Afterwards, the outflow concentrations met the limit value and remained stable. However, two measurements taken at the outflow of both mats on days 378 and 391 were recorded as very high (415 mg/L in Mat 2 on day 378 and 204 mg/L in Mat 1 on day 391), exceeding the allowable limit value for discharge into water bodies. This might be due to the fact that the inflow COD concentrations were also analyzed as very high on these two specific days (1100 mg/L on day 378 and 1317 mg/L on day 391). However, both effluent COD concentrations subsequently stabilized again. The dynamics of COD reduction also showed a fluctuating trend in both mats (Figure 3d). However, a significant difference (p < 0.05) in the COD mass removal efficiency was observed between Mat 1 and Mat 2, with mean values of 73% and 84%, respectively (Table 2). From the inlet mean COD load of 11.2 g/(m2 × d), the COD mass removal with a mean value of 9.8 g/(m2 × d) in Mat 2 contributed to a COD mass removal mean efficiency of 84%, compared to only 73% COD mass removal in Mat 1 (Table 2).
The BOD5 and COD levels in wastewater can be reduced by bacteria (both aerobic and anaerobic) that grow within the mats. These microorganisms are essential for the decomposition of organic matter through aerobic respiration using oxygen under high DO contents or through anaerobic digestion without oxygen [35]. During wastewater treatment, microorganisms significantly reduce the COD by consuming and breaking down organic matter. However, the limited oxygen supply restricts the microbial degradation of the COD [36]. The relatively high DO concentrations in the outlet of both mats, ranging from 5.7 to 11.4 mg/L, were potentially responsible for the efficient removal of BOD5 and COD.

3.1.3. NH4-N, NO3-N, TN, and TP

Figure 4 presents the dynamic behavior of the different forms of nitrogen in the effluents of both mats during the whole experimental period of 455 days. The concentration reductions as well as the removal efficiencies of NH4-N and TN are summarized in Table 1 and Table 2.
Significant reductions (p < 0.05) in the concentrations of NH4-N and TN in the outflow of Mat 1 and Mat 2 were observed in comparison with the respective inflow. Additionally, significant differences (p < 0.05) in the mass removal efficiencies of NH4-N and TN were observed between the mats. Higher fluctuations in the NH4-N concentration in the effluent of Mat 1 as compared to Mat 2 were observed (Figure 4a).
However, the mean NH4-N removal efficiencies of 75% and 90% in Mat 1 and Mat 2, respectively (see Table 2), suggest that aerobic conditions for nitrification prevailed within the mats during operation. A comparatively higher NH4-N removal efficiency of 90% in Mat 2 was observed, resulting in a mean NH4-N removal rate of 1.62 g/(m2 × d).
The removal of NH4-N is related to the nitrification, i.e., NO3-N production, but no significant differences (p > 0.05) in the NO3-N concentrations between the outflows of the two mats were observed in this study. The outflow dynamics of the NO3-N concentrations (Figure 4b) did not show any particular trend of NO3-N production within the two mats. A high nitrification process was consistent and also suggested an oxidative environment within both mats where oxygen-based nitrification was strongly prevailing.
The mean inflow TN concentration of 77.3 ± 8.5 mg/L was reduced to 61.4 ± 15 and 55.1 ± 15 mg/L in the Mat 1 and Mat 2 outflows, respectively, resulting in mean TN concentration reductions of only 21% and 32% (Table 1). With regard to mass removal, the mean TN removal was also low in both mats and showed a mean removal efficiency of only 38% in Mat 1 and 57% in Mat 2 (Table 2). High fluctuations in the outflow TN concentration dynamics with no clear trend were observed (Figure 4c). However, a comparatively low TN with a mean concentration of 55.1 ± 15 mg/L was observed in the Mat 2 outflow, which was attributed to a TN mass removal rate with a mean value of 1.41 g/(m2 × d) and a removal efficiency of 57%. Based on the NH4-N and TN removal, it can be summarized that Mat 2 outperformed Mat 1 in this study. TN removal is based on the efficiency of the nitrification process, i.e., the oxidation of NH4-N to NO3-N, and denitrification, i.e., the transformation of NO3-N into nitrogen gas such as N2O or N2. Microorganisms play a crucial role in ammonium conversion through various processes such as ammonification and nitrification, and denitrifying bacteria are important for nitrate conversion through a series of biochemical reactions under anaerobic conditions into gaseous nitrogen that has been permanently removed from the system [37]. The lack of an available carbon source as well as high DO contents can negatively affect TN removal [38], and hence, a low TN removal efficiency was observed within both mats in this study. However, a high DO content is specifically important for the efficient removal of organic carbon and elimination of NH4-N via nitrification [39]. The mean Eh values in the outflow of Mat 1 (137 ± 34 mV) and Mat 2 (129 ± 56 mV) were significantly higher (p < 0.05) than the mean inflow PTW values (−248 ± 85 mV in Mat 1 and −238 ± 97 mV in Mat 2), which clearly did not favor nitrate reduction [40] and potentially inhibited denitrification and TN removal in this study.
In terms of the removal of TP, clearly no significant variations (p > 0.05) were observed when comparing the outflow concentrations and TP removal efficiencies between Mat 1 and Mat 2 in this study. Both mats exhibited almost similar TP mass removal efficiencies with a mean value of nearly 50% (Table 2). A potential mechanism for phosphorous removal can be adsorption within the mats and biological processes, such as uptake by microorganisms and plants.

3.1.4. E. coli

Figure 5 shows the dynamic profile of the E. coli counts in the inflow and outflow samples from both mats.
Figure 5a shows the behavior of E. coli as a pathogen indicator in the inflow and two outflows of Mat 1 and Mat 2 as well as the log reduction dynamics (Figure 5b) during the whole operational period. The concentrations of E. coli in the inflow decreased significantly (p < 0.05) in the outflows, with an average reduction of E. coli of between 0.2 and 4.6 log reduction in Mat 1 and 0.4 and 4.0 log reduction in Mat 2 (Table 1). Based on the mass loading and removal rate, a mean E. coli removal efficiency of 2.0 log removal in Mat 1 and 2.4 log removal in Mat 2 was observed (Table 2). The results for the removal of E. coli were almost identical, with no significant differences (p > 0.05) between the two mats’ outflows in this study. However, according to EU regulations [34], the E. coli concentrations in the Mat 1 and Mat 2 outflows were very high and consistently exceeded the Class D reclaimed water quality requirements for agricultural reuse (<10,000 MPN/100 mL). A combination of various physical mechanisms, such as filtration, sedimentation, screens, etc., can potentially separate E. coli within the mats in this study. Moreover, chemical disinfection mechanisms, such as ozonation, chlorination, UV radiation, etc., can potentially inactivate E. coli and prevent it from replicating during the wastewater treatment process. Pradhan et al. [41] suggested that the pathogens pose the greatest risk in greywater treatment using green roofs. Therefore, the preferred reuse option should be toilet flushing, as the water quality requirements are lower and the risk of pathogens is lower. For the reuse of treated water, where monitoring of microbial pollution or pathogen risk is crucial, additional treatment steps may potentially need to be applied after secondary wastewater treatment using such biofilter mats on rooftops. Tondera et al. [42] proposed a few methods as effective ways such as ozonation or ultraviolet (UV) disinfection at the outlet to remove E. coli, viruses, and parasites from treated wastewater.

3.2. Weather Conditions and Water Loss

Figure 6 shows the weather conditions in terms of air temperature and amount of precipitation together with the volume of inflow, outflow, and water loss that were observed on a daily basis within both mats when treating PTW in this study.
Weather conditions, such as temperature, humidity, and precipitation, can potentially impact the quality of the outflow water from green roofs or other substrates, such as biofilter mats, on rooftops. Temperature and humidity play an important role in the growth of natural vegetation and other physiological processes, such as photosynthesis, as well as in microbial activity. This subsequently affects the quality of treated water. Both the outflow volume and water quality of green roofs are influenced by the intensity and frequency of rainfall, as well as seasonal fluctuations. Higher intensities and frequencies may potentially contribute to the increased macro-pollutant mass transport from the substrate [19]. The average air temperature changed over time and ranged from 2.5 to 29.3 °C with a mean value of 17.1 ± 4.6 °C, and the relative humidity ranged from 41.3 to 98.7% with a mean value of 73.6 ± 12.6% at the site during this study (Figure 6). Based on the daily mean precipitation, the total amount (volume) of precipitation on each wooden track surface with the mat was calculated as 436 L.
In Mat 1, the daily mean inflow and outflow rate during the experiment were recorded as 62 ± 28 and 44 ± 24 L/d, respectively, in this study. Considering the amount of daily precipitation together with the daily inflow rate, the amount of mean daily water loss from Mat 1 was calculated as 19.8 L/d (31%). In Mat 2, the daily mean inflow and outflow rate were recorded as 63 ± 28 and 33 ± 25 L/d, respectively, and by adding the amount of precipitation with the daily inflow, the daily mean water loss from Mat 2 was calculated as 31.5 L/d (49%).
As compared to Mat 1 with a daily mean water loss of 8.2 L/(m2 × d), a relatively higher water loss of 13.0 L/(m2 × d) was observed in Mat 2. This was potentially due to less runoff at the outlet of Mat 2 as more intense and dense vegetation was growing on the mat surface during the experiment. In Mat 1, a relatively low water-storage capacity and lack of intense vegetation on the mat surface may have resulted in a higher runoff at the outflow, and therefore, low water loss due to ET was observed. However, water was always present in the outlet of both the mats, and the vegetation never dried out during hot summer days with relatively high temperatures.
The results showed that weather conditions, such as rainfall and evaporation due to sunlight, have a significant impact (p < 0.05) on the mass removal (%) of conventional macro-pollutants (BOD5, COD, NH4-N, TN, and TP) as well as on E. coli removal in both mats. Based on the results of this study, it can be concluded that these environmental parameters may influence the quality of treated water, i.e., they may affect the performance of wastewater treatment systems.
A higher daily water loss rate through ET may generate greater cooling effects for urban buildings, as more heat is dissipated from the green roof or wetland roof systems in the form of water vapor [21]. Zehnsdorf et al. [20] and Rahman et al. [43] showed that helophytes (marsh plants) exhibit greater ET capabilities than terrestrial plants on green roofs, making them more effective for passive air conditioning applications. Based on this concept, it is indicated that Mat 2 with dense vegetation and higher ET can potentially provide more benefits such as a greater cooling effect than Mat 1 used in this study.

3.3. Role of Plants on Water Quality

Figure 7 shows a comparison of the macro-pollutant mass removal efficiencies between the mats that were observed in those days when the mat surfaces were completely covered with densely grown vegetation and when there was no vegetation at all in this study.
A significant statistical difference (p < 0.05) was observed in the mass removal efficiencies of macro-pollutants in the presence and absence of plants during this study for Mat 1. Highly efficient mass removals of BOD5 (91%), COD (84%), and NH4-N (94%) and a moderate removal of TN (50%) were observed during the time when the mat surface was completely covered with dense vegetation growth. However, comparatively lower mean mass removals of BOD5 (77%), COD (61%), NH4-N (76%), and TN (34%) were observed during the time when the mat surface had no vegetation at all. Only in the case of TP removal, a higher TP mass removal of 42% was observed in the absence of plants on Mat 1, whereas only 17% of TP mass removal was observed in the presence of dense plants (Figure 6a).
For Mat 2, no significant difference (p > 0.05) was found in the macro-pollutant mass removal efficiencies in the presence or absence of plants. Highly efficient mass removals of BOD5 (98%), COD (88%), and NH4-N (94%) and a moderate removal of TN (45%) in the presence of dense vegetation and an almost similar trend of macro-pollutant mass removals with mean values of BOD5 (97%), COD (78%), NH4-N (93%), and TN (46%) were observed in the absence of plants on the surface of Mat 2 (Figure 6b).
Plants are known to take up nutrients for their biomass production, and comparatively higher removals of NH4-N and TN were observed in the presence of plants than in their absence on the mats’ surface. Of course, the differences are affected by weather conditions, plant biomass production, and by differences in the plant species. In general, higher levels of biological removal of nitrogen and organic matter were observed in the presence of plants when treating PTW. In the case of TP removal from Mat 2, the presence of plants or their absence played no role within this study. Terrestrial plants were settled on both mats, and these terrestrial species consume oxygen to generate energy from the assimilates (root respiration) [44]. They extract this oxygen from the non-water-filled pores of the substrate, which comes from the atmospheric air via diffusion [45,46]. A higher NH4-N mass removal indicating an oxidative regime with high DO concentrations, which was dominant in both the mats, facilitating highly efficient BOD5 and COD degradation and nitrification, is therefore obtained exclusively on contact with the atmosphere. The results demonstrated that these textile-based mats can provide an ecological environment for the growth of many different vegetation patterns. NH4-N can be primarily eliminated through microbial nitrification in oxidative environments and can potentially be deposited onto the mats by sedimentation, subsequent degradation, and plant uptake [47]. Figure 8 shows the natural vegetation growth and dense root structure within Mat 1 and Mat 2 at the end of the experiment in this study.
As the experiment progressed, the root zone was fully developed, and thereby, an effective reaction zone for wastewater treatment was created. Both mats were intensively rooted on their bottom side (Figure 8b,c) and thus able to fulfill its function of stabilizing the vegetation on top. Plants utilize a considerable amount of nutrients for growth and reproduction, and can absorb nutrients (such as nitrogen and phosphorous) via their root systems [48]. The secretion of enzymes by well-developed plant roots and the biofilm attached to their surface may enhance the decomposition of organic matter and other wastewater pollutants, thereby accomplishing water treatment processes (the rhizosphere effect) [49]. Plants could enhance TP mass removal efficiencies through the transformation of phosphorous and further promote P deposition as well as uptake a very small proportion by the plants [50], but this process was probably not taking place within the mats, especially in Mat 1, where a low TP mass removal (only 17%) was observed in the presence of dense plants in this study. Potentially all physicochemical and biological processes occur in the active reaction zone, which is known as the rhizosphere [51]. It has been shown that the cycling of organic matter and biologically available N can be enhanced by root exudates [52]. The TN mass removal efficiencies in both mats were lower, but the results indicated that TN removal was significantly increased (p < 0.05) in the presence of plants in this study. However, plant contributions to macro-pollutant removal from wastewater have not yet been well defined, and it is unclear in this experiment as well. Further research with a longer investigation period will be required to determine the full extent of the role of terrestrial plants when treating wastewater using such textile-based mats as roof biofilters.

3.4. Potential Applications, Challenges, and Practical Recommendations

The analysis of the water quality and treatment performance of the two textile-based mats in this study demonstrated their potential for implementation with the aim of treating PTW or greywater on rooftops. Applying these lightweight mats can make use of the free roof space of existing urban buildings. As roof biofilters, these mats can form a sustainable and efficient wastewater treatment system that reduces the macro-pollutants in PTW, which can potentially improve urban sanitation. The treated water collected from these biofilter mats can be reused for different purposes such as irrigation after proper hygienization or discharged safely into nearby water bodies. These treatment systems are environmentally friendly and a practical option with low operational and maintenance requirements. In the future, implementing such mats on rooftops for wastewater purification purposes may potentially result in both ecological and economic benefits.
The results in this study also showed that these textile-based mats can serve as a substrate for the growth of natural vegetation, and the existence of dense plants could potentially enhance the efficiency of wastewater treatment. The mats exhibit high water-storage capacity and a supply of essential plant nutrients from PTW, and the persistent aerobic conditions within the mats facilitated dense plant growth (Figure 8b,c).
The ingredients used for the production of such mats are non-toxic and resistant to UV-radiation. However, there are a few challenges that need to be resolved, such as keeping a better distribution of PTW at the inlet and ensuring that the orifices are free, which need to be checked on a regular basis.
The presence of dense plants on rooftops is limited due to the unique rooftop environment, which includes intense sunlight, strong winds, and fluctuating air temperatures [19]. Vegetation that grows from flying seeds can adapt to irrigation with wastewater containing high levels of organic matter and nutrients, and some plants can grow very tall with a dense root structure. However, the presence of plants on textile-based mat surfaces can potentially insulate the roof, reducing heat loss in winter and mitigating the urban “heat island effect” in summer [53]. Further research is needed to investigate these potential effects of using textile-based mats with dense plant growth for PTW treatment on urban rooftops.
Another advantage is that such lightweight, textile-based mats put less stress on the roof structure as compared to other substrates (such as sand, gravel, plant substrates, etc.) used for water storage and as plant carriers on conventional green roofs [24]. The aim was to construct a treatment system that did not require the use of other substrates and thus saved on weight and additional costs. These lightweight mats also offer a great opportunity for PTW or greywater treatment and for the greening of existing urban buildings where the roof structure cannot potentially support the load of a conventional green roof.
During the winter months, the freezing air temperature may cause the PTW within the system to freeze. During this time, the wastewater can be directed immediately into the sewerage systems that transport it to a centralized wastewater treatment plant. In the case of an off-grid wastewater system, it can be returned to the septic holding tank [54]. If neither of these two options is available, then the climate may limit the implementation of such textile mat-based systems in areas aiming to treat wastewater all year round or stop operating the system when the air temperatures drop below 2 °C to ensure the connected water pipes run empty and do not freeze during the extreme winter months [54]. However, it is not recommended to mow the plants as they lie on the roof like straw during the winter months. This layer of detritus provides good thermal insulation. To prevent wastewater from freezing and to protect the electro-mechanical equipment connected to the system, the entire experimental operation in this study was paused for 5 months during the winter months (winter break) when the outdoor air temperature was low. For this reason, such textile-based roof biofilters may be more viable in warmer climates, where irrigation with wastewater can be applied all year round or for seasonal purposes depending on the surrounding climate.
A research study carried out by Castro et al. [55] investigated the impact of varying green roof slopes of 0° and 15° on runoff water quality, using a lightweight substrate made of organic and synthetic materials, such as nylon and organic matter, for root retention. The study’s findings revealed no significant differences in the concentration of conventional macro-pollutants between the two varying slopes with the vegetation cover [55]. Still, relatively steeper green roofs with a 15° slope demonstrated slightly higher BOD5, COD, and TSS concentrations in the effluent compared to the group treated with a 0° flat slope. This could be due to the fact that a steeper inclination (15° slope) intensifies drainage flow volume and erosion capacity, facilitating the release of more organic matter and solids from the substrate [19,55].
An excessively high hydraulic loading rate (HLR) within the system can have a detrimental impact on the treatment performance [43], while an insufficient retention time reduces the interaction between water, plants, and microbes, as well as the time that the substrates are in contact with them. In this study, maintaining a steady HLR of nearly 40 L/(m2 × d) had no significant impact on the treatment performance of either mat. Furthermore, implementation of an intermittent inflow method has been shown to enhance DO diffusion into the system, promoting nitrification and improving NH4-N removal, as well as considerably increasing the removal efficiencies of both COD and TN [19]. Therefore, when designing such mat-based systems for wastewater treatment purposes on urban rooftops, it is essential to establish appropriate and effective operating conditions. However, further research is needed to investigate the long-term stability and potential reduction of the heat island effect, as well as the cost effectiveness of using such mats for PTW or greywater treatment on urban rooftops.

4. Conclusions

The key results of this study demonstrated that the textile-based mats can efficiently treat municipal PTW to such an extent that the outflow water successfully meets German legislative requirements for discharge into receiving water bodies in terms of BOD5 and COD removal and nearly meets the EU standard for Class A water quality for agricultural reuse in terms of BOD5 and TSS removal. The findings also suggest that an additional treatment step, such as a UV disinfection unit, may need to be installed after the textile-based system to efficiently remove E. coli from the effluents of the mats. When treating PTW under the same environmental conditions, the nonwoven textile-based mat made of polyethylene terephthalate material (Mat 2) showed higher purification efficiency than the commercially available mat made of recycled polypropylene fiber (Mat 1).
However, the design and installation of such lightweight mats can be conducted based on the minimum requirement for reclaimed water quality as well as the quality of the inflow wastewater. This could be an innovative wastewater management and climate regulation system for urban buildings, depending on their specific purpose and the structural load bearing capacity of the buildings. Overall, this study proposes the use of textile-based mats made of various nonwoven fabrics for treating PTW or greywater in urban spaces as a promising system.
Future research and development activities should focus on selecting more sustainable, long-lasting nonwoven fabrics with superior pollutant removal capacities from wastewater. More research is needed to investigate the higher removal of pathogens, antibiotics, and pharmaceutical compounds that are present in municipal wastewater using such textile-based roof biofilter systems. Additionally, attention should be given to investigating whether these textile-based mats can provide thermal insulation for urban buildings or can cool the surrounding microclimate.
Finally, creating water-resilient cities requires a comprehensive approach that integrates both infrastructure and innovative technologies to promote sustainable urban development. If sufficient quantities of wastewater are available at the location where it is generated, innovative decentralized technologies, such as a textile mat-based roof biofilter system, can be a promising approach to sustainable on-site wastewater management, particularly in urban environments. However, the climate can limit the implementation of such textile-based mat systems and may be more viable in warmer climates or regions, where irrigation with wastewater can be applied all year round.
Our study did not specifically quantify the differences in treatment performance between the unplanted and planted mats, as the plants grew naturally on the mat surfaces due to flying seeds. However, the presence of plants on these mats can facilitate the highly efficient treatment of PTW, as well as the greening of existing urban buildings, and can potentially improve the urban climate through high ET, which leads to increased cooling. Overall, our study shows the potential of using lightweight, textile-based mat systems to efficiently treat PTW on urban buildings, transforming a significant health and environmental threat into a clean, valuable resource.

Author Contributions

Writing—original draft preparation, K.Z.R.; conceptualization, M.B., R.A.M. and L.M.; methodology, M.B., L.M. and J.M.; software, K.Z.R.; formal analysis, K.Z.R. and L.M.; investigation, K.B. and L.M.; resources, K.B. and R.A.M.; data curation, K.B., L.M. and K.Z.R.; writing—review and editing, J.M., M.B., L.M., R.A.M. and K.B.; visualization, L.M. and K.Z.R.; supervision, L.M.; project administration, L.M. and R.A.M.; funding acquisition, L.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partly financially supported by the joint research project “Dachbiofilter” (funding code: KK5081716BA3), which was funded by the German Federal Ministry for Economic Affairs and Climate Action (BMWK) based on a decision by the German Federal Parliament (Bundestag) as part of the funding program “Central Innovation Program for SMEs (ZIM)”.

Data Availability Statement

The corresponding author will provide all the data presented in this study on request, whenever it is required.

Acknowledgments

The authors would be delighted to acknowledge the outstanding cooperation and support received from the Helmholtz Center for Environmental Research (UFZ) in Leipzig in the following areas: construction and operation (Anja Scherber); chemical analysis in the laboratory (Grit Weichert); and data management (Jagdish Manohar Sawlani and Emilia Engelhardt). This study was carried out as a part of the Integrated Platform Project “Technologies for Water and Heat Management in Urban Space (CityTech)” of the Helmholtz Center for Environmental Research-UFZ in Leipzig. The authors would particularly like to thank the joint research project sponsor, AiF-Projekt GmbH, Berlin, for overseeing the “Dachbiofilter” project.

Conflicts of Interest

Author Michael Blumberg was employed by the company Blumberg Engineers. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Schematic diagram of a pilot plant using textile-based mats as roof biofilters to treat PTW at the BDZ site: (a) plan view, and (b) longitudinal view (Sketch: K.Z.R).
Figure 1. Schematic diagram of a pilot plant using textile-based mats as roof biofilters to treat PTW at the BDZ site: (a) plan view, and (b) longitudinal view (Sketch: K.Z.R).
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Figure 2. Pitched roof wooden structure pilot plant with the mats loaded with PTW at the BDZ site: (a) at the beginning of the experiment on 9 May 2022, (b) vegetation growth starts, (c) dense vegetation on both mats dated 1 August 2023.
Figure 2. Pitched roof wooden structure pilot plant with the mats loaded with PTW at the BDZ site: (a) at the beginning of the experiment on 9 May 2022, (b) vegetation growth starts, (c) dense vegetation on both mats dated 1 August 2023.
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Figure 3. The dynamic fluctuations of BOD5 and COD and their concentration reductions (%) observed in both mats: (a) BOD5 at the inflow and outflow, (b) BOD5 reduction, (c) COD at the inflow and outflow, and (d) COD reduction during the whole study period feeding with PTW.
Figure 3. The dynamic fluctuations of BOD5 and COD and their concentration reductions (%) observed in both mats: (a) BOD5 at the inflow and outflow, (b) BOD5 reduction, (c) COD at the inflow and outflow, and (d) COD reduction during the whole study period feeding with PTW.
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Figure 4. The dynamic behavior in terms of (a) NH4-N concentration, (b) NO3-N concentration, and (c) TN concentration at the inflow and outflow observed within both mats during the entire study period when feeding with PTW.
Figure 4. The dynamic behavior in terms of (a) NH4-N concentration, (b) NO3-N concentration, and (c) TN concentration at the inflow and outflow observed within both mats during the entire study period when feeding with PTW.
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Figure 5. The E. coli concentration with (a) the number of E. coli counts in the inflow and outflow samples, and (b) log reduction in both mats during the entire operation of treating PTW.
Figure 5. The E. coli concentration with (a) the number of E. coli counts in the inflow and outflow samples, and (b) log reduction in both mats during the entire operation of treating PTW.
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Figure 6. The daily rate of inflow, outflow, and water loss (in L/d) as well as the air temperature and daily precipitation rate (L/d) observed in (a) Mat 1 and (b) Mat 2 when treating PTW in this study.
Figure 6. The daily rate of inflow, outflow, and water loss (in L/d) as well as the air temperature and daily precipitation rate (L/d) observed in (a) Mat 1 and (b) Mat 2 when treating PTW in this study.
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Figure 7. Comparison of macro-pollutant mass removal with dense vegetation growth and in times with no vegetation at all in (a) Mat 1 and in (b) Mat 2 during the experiment.
Figure 7. Comparison of macro-pollutant mass removal with dense vegetation growth and in times with no vegetation at all in (a) Mat 1 and in (b) Mat 2 during the experiment.
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Figure 8. Natural plant growth during PTW treatment operation with (a) a side-view of the pilot plants with dense vegetation and their root structure in (b) Mat 1, and (c) Mat 2 that were observed after the end of the experiment.
Figure 8. Natural plant growth during PTW treatment operation with (a) a side-view of the pilot plants with dense vegetation and their root structure in (b) Mat 1, and (c) Mat 2 that were observed after the end of the experiment.
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Table 1. Conventional water quality parameters that were measured in the inflow and the outflow of both mats in comparison with the guidelines for discharge and requirements for agricultural irrigation. Mean values are shown together with the standard deviations in parentheses.
Table 1. Conventional water quality parameters that were measured in the inflow and the outflow of both mats in comparison with the guidelines for discharge and requirements for agricultural irrigation. Mean values are shown together with the standard deviations in parentheses.
ParameterUnitTreatment PerformanceDischarge Water Quality According to DWA-A 221 [33] DReclaimed Water Quality Requirements for Agricultural Irrigation According to the EU [34]
Mat 1Mat 2
InflowOutflowReduction
(%)		N AInflowOutflowReduction
(%)		N A
BOD5mg/L209 (141)23 (14)8217209 (141)11 (10)92164010 E | 25 F | 25 G | 25 H
CODmg/L424 (285)119 (60)6723418 (280)114 (93)7022150-
TSSmg/L164 (82)6.6 (1.4)904 B57 (33)10.5 (9)744 B-10 E | 35 F | 35 G | 35 H
NH4-Nmg/L62.5 (21.7)17.1 (20.9)702160.6 (23.8)8.9 (12.9)8322--
NO3-Nmg/L0.7 (0.6)22.7 (21.5)-230.7 (0.6)23.7 (22.8)-23--
TNmg/L77.3 (8.5)61.4 (15)211877.7 (8.5)55.1 (15)3217--
TPmg/L9.6 (1.8)7.2 (3.4)40229.7 (1.8)8.3 (4.5)2522--
E. coliMPN/100 mL3.7 × 106 (2.8 × 106)3.5 × 105 (9.1 × 105)1.9 C233.6 × 106 (2.8 × 106)2.0 × 105 (4.0 × 105)2.1 C21-10 E | 100 F | 1000 G | 10,000 H
pH-7.3 (0.2)7.8 (0.7)-227.3 (0.2)7.5 (0.5)-21--
DOmg/ L0.6 (0.3)8.3 (1.7)-220.6 (0.3)8.8 (2.2)-21--
EhmV−248 (85)137 (34)-22−238 (97)129 (56)-21--
ECµS/cm1506 (242)1303 (241)-221512 (238)1340 (237)-21--
TUNTU167 (57)10.7 (8.5)9314167 (57)5.3 (4.4)9612-≤5 E
Notes: A Sample size; B Start of sampling from May 2023; C Reduction in log-units; D Water quality class C for discharge; E Minimum requirement for Class A reclaimed water that is permitted for all methods of irrigation for growing all types of food crops, including root crops intended for raw consumption and food crops where the edible part comes into direct contact with the water; F Minimum requirement for Class B reclaimed water that is permitted for all methods of irrigation for growing food crops intended for raw consumption, provided that the edible part is produced above the ground and does not come into direct contact with the water; G Minimum requirement for Class C reclaimed water that is used for drip or other methods of irrigation to grow food crops that are intended for raw consumption, provided that the edible part is produced above the ground and does not come into direct contact with the water; H Minimum requirement for Class D reclaimed water that is permitted for all methods of irrigation and intended for reuse in industries, energy sectors, and for growing seeded crops.
Table 2. The mean mass loading rate of the conventional macro-pollutants as well as the removal efficiencies that were obtained in both mats (Mat 1 and Mat 2) throughout the experiment. Mean values are shown together with the standard deviations in parentheses.
Table 2. The mean mass loading rate of the conventional macro-pollutants as well as the removal efficiencies that were obtained in both mats (Mat 1 and Mat 2) throughout the experiment. Mean values are shown together with the standard deviations in parentheses.
ParameterMacro-Pollutant Mass Loading Rate and Removal Efficiency
Mat 1Mat 2
Loading Rate
[g/(m2 × d)]		Outflow Rate
[g/(m2 × d)]		Removal
Efficiency
(%)		N ALoading Rate
[g/(m2 × d)]		Outflow Rate
[g/(m2 × d)]		Removal
Efficiency
(%)		N A
BOD55.7 (4.3)0.45 (0.37)85175.8 (4.1)0.14 (0.1)9716
COD11.3 (9.1)2.15 (0.88)732211.2 (8.6)1.4 (1.2)8421
TSS4.8 (5.5)0.23 (0.15)884 B1.9 (1.6)0.09 (0.08)964 B
NH4-N1.8 (0.8)0.35 (0.47)75211.75 (0.9)0.13 (0.22)9022
NO3-N0.017 (0.01)0.6 (0.59)-220.017 (0.01)0.37 (0.4)-22
TN2.3 (0.6)1.4 (0.5)38182.3 (0.7)0.9 (0.5)5717
TP0.25 (0.09)0.17 (0.12)49190.26 (0.09)0.11 (0.06)5019
E. coli1.1 × 109 C9.5 × 107 D2.0 E211.0 × 109 C4.1 × 107 D2.4 E19
Notes: A Sample size; B Start of sampling from May 2023; C Loading rate of E. coli in the inflow [in MPN/(m2 × d)]; D E. coli in the outflow [in MPN/(m2 × d)]; E Removal in log-units.
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Rahman, K.Z.; Mählmann, J.; Blumberg, M.; Bernhard, K.; Müller, R.A.; Moeller, L. Performance of Textile-Based Water-Storage Mats in Treating Municipal Wastewater on Urban Rooftops for Climate-Resilient Cities. Clean Technol. 2025, 7, 75. https://doi.org/10.3390/cleantechnol7030075

AMA Style

Rahman KZ, Mählmann J, Blumberg M, Bernhard K, Müller RA, Moeller L. Performance of Textile-Based Water-Storage Mats in Treating Municipal Wastewater on Urban Rooftops for Climate-Resilient Cities. Clean Technologies. 2025; 7(3):75. https://doi.org/10.3390/cleantechnol7030075

Chicago/Turabian Style

Rahman, Khaja Zillur, Jens Mählmann, Michael Blumberg, Katy Bernhard, Roland A. Müller, and Lucie Moeller. 2025. "Performance of Textile-Based Water-Storage Mats in Treating Municipal Wastewater on Urban Rooftops for Climate-Resilient Cities" Clean Technologies 7, no. 3: 75. https://doi.org/10.3390/cleantechnol7030075

APA Style

Rahman, K. Z., Mählmann, J., Blumberg, M., Bernhard, K., Müller, R. A., & Moeller, L. (2025). Performance of Textile-Based Water-Storage Mats in Treating Municipal Wastewater on Urban Rooftops for Climate-Resilient Cities. Clean Technologies, 7(3), 75. https://doi.org/10.3390/cleantechnol7030075

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