Applicability of different textile materials in filtration process integrated with SBR for wastewater reuse in small settlements

Increased water needs depending on the increases in world population, industrial activities, agricultural irrigation, etc. have globally appeared as a very critical environmental issue regarding the greater stress on water resources because of water scarcity and climate change effects. In recent years, research activities on wastewater reuse have rapidly increased to provide sustainable water management and efficient usage of natural water resources. Therefore, reuse applications of domestic/urban wastewaters are very important in Turkey as well as all over the world. Turkey has approximately 900 wastewater treatment plants (WWTPs) treating more than 8 Mm³ domestic/urban wastewater in urban areas (TUBITAK-KAMAG 108G167 Project Report 2013). However, the population of the small settlements in Turkey, having about 1/3 of the population, is below 2,000 population-equivalent (PE). In small settlements, mostly septic tanks, packaged biological treatment systems and conventional activated sludge process have been applied. However, the treatment technologies providing better treated effluents are considered for reuse of the wastewaters regarding the negative effects of climate change and other socio-economic conditions in small settlements of Turkey as well as in the world.

Although most of the treatment processes including advanced treatment techniques have been applied for the treatment and reuse of wastewater in urban areas, their applications in rural and/or small settlements may be limited. The development of effective and low-cost treatment and reuse techniques considering the decentralized wastewater treatment systems is a great challenge in terms of investment and operating costs (Taouraout et al. 2019). When comparing central and decentralized WWTPs having activated sludge systems as widely applied processes (Dauknys 2019), small-scale activated sludge systems were found to be more responsive to fluctuations in wastewater flow and level of contamination. Especially, temperature decreases corresponded to lower treatment efficiencies in nitrogen and phosphorus removal from the wastewater (Mažeikienė & Grubliauskas 2021). Molinos-Senante et al. (2015) studied the determination of the best treatment system among seven different technologies including constructed wetland (CW), long aerated activated sludge, membrane bioreactor (MBR), pond, rotating biological contactor (RBC), sequencing batch reactor (SBR) and trickling filter applied in rural areas. They found that wetland and pond systems were the best systems for operation and their costs. De Feo & Ferrara (2017) also confirmed this result, mentioning that activated sludge systems and CWs were the most environmentally friendly options for small settlements. Yusoff et al. (2019) indicated that SBR systems could be used in small settlements due to their advantages like not requiring qualified personnel for the operation, easily managed, integration into the filtration system, physicochemical stability of activated sludge, and high treatment efficiency. Lohani et al. (2020) have pointed out that a possible way to improve SBR performance was to combine it with a post-filtration unit; however, it should be a low-cost option.

One common and important process used in the water and wastewater treatment field is the filtration capturing of particles through a porous medium rather than removal of solids. In gravity filtration, although different filtration media have been used, granular media like sand and anthracite materials are invariably used for this purpose (Kumar & Thakur 2017). However, wool, cotton and the felts made from them, known as natural fibers also having absorbent property, used as filter media, (Sutherland 2008) have limited use in wastewater treatment. These textile fibers can be produced from either natural or synthetic sources. Among the materials, wool and nonwoven fabrics, natural or synthetic fibers have been often preferred in filtration systems depending on their performance and physicochemical properties (Kumar & Thakur 2017); on the other hand, their applicability for wastewater filtration is a complex process for different reasons including wastewater properties, availability of suspended solids, particle size distribution, and the operation mode. The usage of activated carbon embedded textile fabrics for wastewater filtration is very important; however, the thermodynamic compatibility of activated carbon with cotton textiles in terms of the interaction parameter is very critical. Rana et al. (2000) have pointed out that miscible systems were commonly observed with copolymer–copolymer or in copolymer–homopolymer blends even in the absence of specific interactions. Although there are some studies showing the thermodynamic interaction of the blends and composites, for most of them, it has been considered that for such mixtures, like and unlike contact energies were nearly equivalent so that the van der Waals interaction energy could be negligible (Rana et al. 1996). As an advanced filtration technology, membranes are used for wastewater treatment and resource recovery, providing higher permeate quality for water reclamation as well as nutrient, energy, and chemical recovery (Hube et al. 2020). Ayol et al. (2021) showed that textile fabrics such as cotton and metal braided textiles could be effectively used as low-cost support layer materials in dynamic MBR systems. However, the application of membrane filtration systems has some economic limitations for small settlements. As part of a research project on innovative treatment systems for wastewater reuse in small settlements, this study focused on the development of an integrated SBR and textile filtration system. The specific objectives of the paper are therefore:

• to investigate and compare the performances of different textile materials for domestic wastewater treatment,

• to evaluate SBR integration in to the textile filtration for enhancing the treated wastewater quality,

• to debug the applicability of the treatment system in small communities.

Synthetic wastewater simulating domestic wastewater was used during the experimental studies. The synthetic wastewater mainly comprised CH₃COONa, NH₄Cl, KH₂PO₄, FeSO₄.7H₂O, CaCl₂, and MgSO₄.7H₂O, and the trace element solution was daily prepared according to the recipe detailed elsewhere (Li et al. 2007). It was fed to either the filtration unit alone or an SBR integrated filtration system. Synthetic wastewater characteristics determined according to Standard Methods (APHA 2005) are given in Table 1. All analyses were in triplicate, and values are expressed as means. The synthetic wastewater exhibited high-strength domestic wastewater properties as given in Metcalf & Eddy (2003).

Inoculum activated sludge (AS) used for SBR operation start-up was taken from the recycling line of a municipal WWTP having advanced biological nutrient removal units with a treatment capacity of 7.2 m³/s, located in Izmir, Turkey. AS was used in the start-up of SBR and also kept alive in aerated conditions in a separate tank. However, the inoculum sludge was also taken a second time due to the Covid-19 pandemic lockdown period in Turkey. The sludge characterization studies were done based on the pH, electrical conductivity (EC), salinity, chemical oxygen demand (COD), suspended solids (SS), volatile suspended solids (VSS), capillary suction time (CST), and sludge volume index (SVI). CST analysis was done by using a Triton M 304 type CST meter (Triton Electronics Ltd., Essex, UK). Table 2 shows the characterization results of AS samples. A significant difference was observed on EC, salinity, and SVI parameters of both sludges. However, COD, SS and VSS results were found to be very close to each other.

A laboratory-scale filtration unit was constructed from a plexiglas system in two parts. The first part of the unit was a rectangular reactor having 24 L volume with proper arrangement of ports for sampling, filling and withdrawal of water; the second part of the system was a rectangular filtration cassette having 18 cm×20 cm×5 cm (L×W×H) dimensions. The filtration system had an active filtration area of 0.056 m². The schematic diagram of the experimental setup is given in Figure 1. Daily prepared synthetic wastewater was fed to the filtration unit from a feeding tank having 40 L volume. Also, in the case of SBR integration, a second tank with aeration system was used to feed the SBR effluent to the filtration unit. Active aeration was provided in the bottom part of the reactor by using an aquarium pump Resun Air Pump LP-40 (Resun Shenzhen Xing Resheng Industrial Co., Ltd., Shenzhen, China). The synthetic wastewater was pumped by using a diaphragm pump to the filtration system. A flowmeter having five different stages was used to control the flowrate as 13, 37, 61, 80 and 90 mL/min of synthetic wastewater, corresponding the flux values as 13.93, 39.64, 65.36, 85.71 and 96.43 LMH (L/m².h), respectively. The filtration materials were placed at both sides of the filtration cassette made from delrin material as single layer or multilayers depending on the experimental trial. Treated wastewater was collected in a clean water tank. pH, salinity, EC, alkalinity, BOD₅, COD, Sulphate, TDS, SS, VSS, Chlorine, Fluoride, NH₄⁺-N, TN, TP, and particle size distribution analysis were done to determine the quality of the treated wastewater. Particle size distribution analysis was done by using Malvern Mastersizer 2000 analyzer (Malvern Panalytical Ltd., UK) with HydroQM unit.

The filtration unit was fed with synthetic wastewater or SBR effluent. For SBR operation, startup of the reactor was done by using inoculum sludge and then synthetic wastewater was fed to the reactor. The amount of the wastewater was steadily increased and mixed liquor suspended solids (MLSS) concentration of the reactor was kept as 5,000 mg/L. The operating conditions of SBR are summarized in Table 3. Total cycle time (t_t) of SBR operation was adjusted to 5.5 hours.

Throughout the study, five different textile materials were used in the cassette system. These filter materials were cotton textile fabric (TF), metal braided cotton-polyester fabric (MBTF), 1% activated carbon coated cotton textile fabric (S1), 3% activated carbon coated cotton textile fabric (S3), and activated carbon woven fabric (AC), respectively. The photographs of the materials are given in Figure 2.

TF and MBTF were produced from cotton and MBTF was woven with 50-micron stainless steel wire. Both woven fabrics were developed under R&D studies as explained in Ayol et al. (2021). Textile filter was coated with 1% (S1) and 3% (S3) by weight of powdered activated carbon purchased from Merck by using sol-gel method in Department of Textile Engineering at Dokuz Eylul University. Activated carbon woven textile filter (AC) having 2,100 m²/g of specific surface area was supplied from an R&D company located in Turkey. For the first time, this material was used for wastewater treatment purposes. The fabric materials may carry a heavier pollutant load per unit area because of their thickness.

TF and MBTF textile materials' filtration performances based on COD and BOD₅ removals were first evaluated as one layer and four layers application in the filtration unit. For these trials, synthetic wastewater was directly fed to the filtration unit without SBR operation. In the filtrate samples, pH was found between 7.3 and 8.5 while salinity was about 2.5 mS/cm. The lowest and highest temperature values were measured as 17.3 °C and 20.4 °C in four layers MBTF and TF filtration applications, respectively. BOD₅, COD results, and removal efficiencies for synthetic wastewater and filtrate samples as a function of flux are plotted in Figure 3. Based on BOD₅ and COD removals, one layer and four layers TF applications had lower removal efficiencies: 8% and 15% for BOD₅, and 10% and 14% for COD, respectively. MTBF trials as either one or four layers gave best removal performances. Maximum BOD₅ removals were obtained as 30% and 50% for one and four layers MTBF, respectively, while COD removals were found about 16% and 36% for the same applications. Due to the achievement of low removals, it was decided to add SBR integration to the filtration unit. The filtration materials were chosen as four layer MBTF alone and four layer MBTF plus activated carbon coated TF materials (S1 and S3) to enhance the effluent quality in SBR integrated trials.

Figure 4 shows the COD values in SBR effluents and filtrate samples and removal efficiencies as a function of flux applied in the filtration unit. Following the SBR effluent, four layers MBTF alone and four layers MBTF with S1, S3, and AC as a outer layer, respectively, were used as filtration material. COD values ranged between 95 to 290 mg/L in SBR effluents for the first trial, where the first inoculum sludge was used in the system as given in Figure 4(a). However, the second inoculum sludge taken from the WWTP after the Covid-19 pandemic lockdown was used for the other trials. At the beginning of the experiments, COD values of the SBR effluents as shown in Figure 4(b) were found to be slightly higher than those of the other trials. SBR treatment performance improved and COD values in SBR effluents varied between 80 and 160 mg/L as depicted in Figure 4(c) and 4(d). Depending on the sludge adaptation, the quality of SBR effluents drastically increased. Also, the temperature increases in the location might enhance the microbial activity.

In the case of four layers MBTF filtration application following SBR, the COD removal efficiency of the filtration unit reached up to 90%, corresponding to a total removal rate of 96–98%, as can be seen from Figure 4(a). In the SBR operation, good nutrient removal efficiencies were obtained depending on the SBR operating conditions. TP and TN removal efficiencies were found to be 80% and 53.8%, respectively, although a full anaerobic cycle was not applied for phosphorus release. Total BOD₅ removal efficiency in the system after SBR improvement reached up to 98.3%.

After mounting S1 textile fabric as an outer layer to the filtration cassette system, total COD removal efficiencies for SBR+filtration system were found to be up to 90%, as depicted in Figure 4(b). It should be noted here that the studies had to be suspended for 2.5 months due to the Covid-19 pandemic, and this elapsed time had negative effects on the SBR's performance. The decrease in COD removal can be explained by a release of excess biomass and partly degraded organic particles (Kaetzl et al. 2020). COD removal efficiencies in the filtration unit reached treatment levels of over 57%. Removal efficiencies continued at high rates throughout the entire operation in terms of NH4⁺-N removal at approximately 96%. TP and TN removal efficiencies were about 29% and 31%, respectively. This decrease may be attributable to the dependence of nutrient removal on SBR operating conditions and temperature; there are still some barriers that limit the use of anaerobic processes in SBR, including the process instability at temperatures below 20 °C (Lijó et al. 2017). Sulphate and chloride removals were found to be around 6%. Influent alkalinity concentration was generally higher than effluent alkalinity concentration. This situation indicated that the removal efficiency for COD increased in slightly acidic rather than alkaline conditions, as shown in the study by Abood et al. (2014).

Due to the better performance of S1 addition as an outer layer because of activated carbon coating, the S3 combination was studied. Powdered activated carbon in suspended activated sludge reactors has been applied because of its beneficial features: microorganism protection from load peaks and the degradation and adsorption of refractory organic compounds (Munz et al. 2007). In our study, activated carbon coated cloth as a filtration material enhanced the treatment performance regarding many parameters. In this trial, removal efficiencies continued at high rates in terms of COD and BOD₅ removals, which reached 92.7% and 93.3%, respectively. In addition, only the S3 material led to 49.4% additional removal on COD parameter as depicted in Figure 4(c). There was an additional increase in TN removal to 74.8%; however, TP removal efficiency was about 12.4%. Sulphate removal was found to be around 5%. There was almost no chloride removal, with 1.6% of removal efficiency.

Treatment of synthetic wastewater by SBR+ four layers MBTF+AC combination gave very good removal efficiencies. This AC textile material was used in wastewater treatment for the first time. COD and BOD₅ removal efficiencies reached up to 92.95% and 95%, respectively. In addition, only AC material achieved a 49.9% ±3.03 additional removal efficiency for COD while operated with SBR effluent, as given in Figure 4(d). TN and TP removals were very high at 67% and 51.75%, respectively. Sulphate removal was about 15.4%, while chloride removal of 11.8% was obtained. The results indicated that AC addition as an outer layer in the filtration unit led to remarkable increases in COD, BOD₅, TN, TP, sulphate, and chloride removal. The chloride removal from the wastewater is very important since it limits the reuse of treated effluents for agricultural irrigation in many countries. Costa & Féris (2020) reported that the increase in chloride removal could be attributed to the higher percentage of activated carbon in textile material accelerating initial adsorption kinetics.

The proper selection of filter material is the most important factor in achieving efficient filtration. As expected, retaining the particles in wastewaters and high amount of filtrate are very important results determining the applicability of the materials in full-scale plants. Furthermore, the performance of the backwashing process is a critical issue for cleaning the filter material, resulting in low resistance to flow and chemicals. Investment and operational costs, especially for small settlements, become more important compared with urban areas. However, the reuse options for treated wastewaters are seen as the most significant applications for agricultural activities providing food security. The cost of membrane technologies is still considered high and not economically feasible for small settlements (Pronk et al. 2019). Therefore, in this study, low-cost filter materials with the integration of SBR system were studied based on the treatment performance. Regarding certain treatment parameters such as COD, BOD₅, TN, and TP, the usage of four different textile materials with SBR integration led to COD removal efficiencies higher than 90%. Also, the activated carbon coated materials (S1, S3) and AC textile fabric itself achieved higher than 50% removal of COD. Kaetzl et al. (2020) reported that COD removal efficiencies for biochar and sand filters were 74±18% and 61±12%. Table 4 shows the removal efficiencies of the pollutants for each experimental trial. Angelakis (2017) presented some research results comparing the activated sludge processes and three-layer textile filter (UC Davis) in terms of BOD₅ and TSS parameters and concluded that the textile filtration had almost ten times better quality.

The need for removal and replacement of sand-like materials by new filter materials is an operational challenge. In this study, the construction of a textile filter system permitting the backwashing to remove physical blockages was done; however, the clogging problems for all textile materials were not observed even when the filtration unit was operated more than 5 hours for each trial. Particle size distributions are necessary for determining the effectiveness of filtration, as in all solid/liquid separation processes. As is well known, the particle size of solids is a critical parameter for deposition on the filter material. While larger particles lead to an increased level of formation in short filtration times, small ones can penetrate the filter material in the early formation stage, resulting in high permeate concentrations (Darby et al. 1991). The particle size distribution of treated samples measured by the Malvern Mastersizer 2000 is depicted in Figure 5. Surface weighted mean D [3,2] values for four layers MBTF, S1, S3, and AC trials after SBR integration were determined as 5.739, 33.702, 10.332, and 4.019 μm, respectively. Surprisingly, the results for S1 and S3 applications were found to have higher values than the four layers MBTF and AC trials. It might be that very small particles passing the filter material were coagulated in the effluent water tank.

Although there were fluctuations in SBR effluent, AC addition led to high treatment efficiencies in many parameters when testing four layers MBTF plus S1, S3, and AC materials, respectively. Sutherland (2008) reported that coating the filter medium with a fine powder – the precoat – to build up a thin cake layer on which the main filtration took place was a solution to achieve high filtration efficiencies at low particle sizes. The activated carbon textiles and AC led to better performance than the uncoated textile material. Color and turbidity reductions in effluent samples occurred in all filter materials; but, the AC filter had best performance. Figure 6 presents some photographs taken from the effluent samples for each trial.

In this study, the applicability of different low-cost textile filtration materials by integration of SBR in treating wastewater generated by small settlements was investigated. The study demonstrated the use of textile materials with their high treatment performance. The main concluding remarks from the research study could be given as follows:

• SBR integration in to the filtration unit greatly improved the treatment performance.

• Nutrient removal was obtained by adjusting the operational conditions of SBR. In addition, applying an anaerobic cycle can enhance the phosphorus removal. However, this case can be discussed if the treated effluent will be used for agricultural irrigation.

• No clogging problem was observed in all textile materials, and the backwashing was not required during the operation. This situation can be interpreted positively, as it requires little or no backwashing in scale-up studies.

• Even though all textile filtration applications gave better COD and BOD reduction, the coating the textile fabrics with activated carbon and also activated carbon fabric itself showed good removal, especially for sulphate and chloride reduction.

In small settlements, by expanding the use of textile materials in wastewater treatment, the treated water can be reused for irrigation instead of being discharged in to water bodies. Ongoing work in this project is focused on the application of this system for real wastewater treatment.

This study was supported by DEU-BAP under grant #2019.K.B.FEN018 ‘Urban Wastewater Management for Small Communities in Turkey: Development of An Innovative and Onsite Treatment System’ Research Project.

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