In this study, sequencing batch reactor (SBR) using anaerobic/aerobic/anoxic process was coupled to a solar photocatalytic reactor (SPCR) for greywater treatment. The greywater effluent from SBR (operated at the optimal condition: 6.8 h hydraulic retention time (HRT), 0.7 Volumetric exchange ratio (VER) and 7.94 d solids retention time (SRT) with optimal corn cob adsorbent dosage (0.5 g/L)) was fed to the SPCR (operated at optimal conditions: pH – 3, H2O2 dosage – 1 g/L, catalyst dosage – 5 g/L). Chemical oxygen demand (COD) removal of 92.8±0.5% and ∼100% were achieved in SBR and SBR-SPCR, respectively. Similarly, total organic carbon (TOC) removal of 91±0.9% and ∼100% were observed in SBR and SBR-SPCR, respectively. After SBR treatment, average total nitrogen (TN) removal of 84% was found and this TN removal increased to 93% after combined SBR-SPCR treatment. The maximum PO43−_P reduction of 80±1.5% % was achieved with SBR-adsorption system. In addition, a maximum of 87±0.9% of net PO43−_P removal was reached after SBR-SPCR treatment. 58.9±2.3% BP (benzophenone-3) removal was obtained in the SBR while the integration of SBR and SPCR treatment was resulted in 100% BP removal. An effective anionic surfactant (AS) removal rate (80.1±2.2%) was observed in the SBR phase, which further improved to 94.9±1% at the end of 4 h SPCR treatment.

  • Biological – photochemical system successfully treated greywater.

  • The coupled system efficiently mineralizes COD, TOC and BP.

  • Significant removal of TN (93%), PO43−_P (87%) and AS (94.9%) was achieved.

  • Solar photocatalysis was able to remove emerging contaminants.

  • Treated SBR-SPCR greywater could meet reuse standards (toilet flushing and others).

Graphical Abstract

Graphical Abstract
Graphical Abstract

Treatment and reuse of greywater could be a sustainable solution to meet an ever-increasing water demand. Greywater is all wastewater, i.e., discharged from a house, excluding blackwater (toilet water). Generally, greywater constitutes of organic matter, nutrients, emerging contaminants (ECs) and pathogens. The primary sources for the pollutants are waste food particles, personal care products, cloth cleaning products, traces of urine and faeces (Eriksson et al. 2002; Noutsopoulos et al. 2018). The organic matter reduces the dissolved oxygen (DO) of receiving surface water bodies to a dangerous level that deteriorates aquatic life. Similar manner, the prolonged release of nutrients such as nitrate and phosphate into lakes can cause eutrophication, which can lead to the extinction of aquatic life. Furthermore, surfactants have the potential to harm fish gills and reduce the lifespan of heterotrophic nanoflagellates and ciliates (Venhuis & Mehrvar 2004). Besides, ECs, such as UV filters, have shown potential adverse effects on the marine ecosystem, such as coral bleaching and the loss of marine organisms (Downs et al. 2016).

So far, several methods have been used for the treatment of greywater, including physical, chemical, biological and physicochemical processes in the past decades to remove various greywater contaminants. Physical systems, namely, filtrations using nylon sock filters, ultrafiltration membrane and slow sand filters with slate waste, could achieve high solids removal but are inefficient in removing the organics, nutrients and pathogens and could also result in membrane fouling (Ramon et al. 2004; Zipf et al. 2016). Chemical approaches using electrocoagulation and ferrate (IV) salt are operated with short retention time and simple operation, but these systems utilize a vast amount of chemicals (Lin et al. 2005; Song et al. 2017). On the other hand, biological systems such as upflow anaerobic sludge blanket (UASB) reactor, membrane bioreactors and constructed wetlands could achieve higher organics, nutrients (N, P) removal. However, they have disadvantages such as longer retention times, membrane fouling, and large installation area with low efficiency in emerging contaminants removal (Elmitwalli & Otterpohl 2007; Saumya et al. 2015; Atanasova et al. 2017). Various photochemical systems (UV and ozone-based AOPs) utilized for greywater treatment showed good removal of ECs, but these systems reported high energy consumption (Chin et al. 2009; Alrousan & Dunlop 2020).

Therefore, sequential treatment with a biological process followed by a photochemical process could be an effective strategy for greywater treatment. The combination of biological and photochemical systems appears to be very attractive solution (full mineralization, comparatively economical) for greywater treatment. Biological process such as sequencing batch reactor (SBR) has been widely investigated treatment method with various advantages such as operational flexibility, smaller footprint, low installation and maintenance and controlled sludge production (Farabegoli et al. 2010; Azizi et al. 2015). Combining anaerobic, aerobic and anoxic processes along with adsorption in SBR could improve nitrogen and phosphorus removal efficiencies. Further, AOP such as solar photocatalysis (or solar photocatalytic reactor (SPCR)) for treating greywater could remove the limitation of expensive treatment. Previous solar photocatalytic studies mainly employed slurry-based catalyst rather than supported catalyst, which makes the post-treatment separation difficult (Chin et al. 2009; Alrousan & Dunlop 2020). The SPCR with supported catalyst could be a more sustainable option for greywater treatment. The aim of the study is to evaluate the performance of sequential SBR-SPCR for removing greywater contaminants (organics, nutrients and ECs). Treatment performance for SPCR as post treatment method (after biological treatment) was assessed with photocatalyst (nitrogen-doped TiO2) immobilized on natural support material such as graphite. The mechanism behind the sequential biological and photochemical system for treating greywater was studied.

Synthetic greywater

The composition of synthetic greywater was made up of analytical-grade chemicals (obtained from Avra Synthesis Private Ltd, Hyderabad) such as sucrose 0.16 g/L, soluble starch 0.09 g/L, urea 0.05 g/L, ammonium chloride 0.05 g/L, potassium dihydrogen phosphate 0.05 g/L, toothpaste 0.01 g/L, dishwashing gel 0.1 mL/L and bathing gel 0.044 mL/L (Scheumann & Kraume 2009; Kraume et al. 2010). Further, the stock solution of trace element solution (TES) composition (added as 1 mL TES/L greywater) was prepared by mixing the chemicals including zinc sulfate – 0.18 g, ferrous sulfate – 0.37 g, cobalt chloride – 0.55 g, manganese sulfate – 0.30 g and sodium molybdate – 0.12 g with tap water (Suwannasing et al. 2015). The stock solution was prepared by adding all the above chemicals to tap water and simultaneously mixed by magnetic stirrer for 30 min at 1,000 rpm and heated at 70 °C. The physicochemical properties of synthetic greywater are shown in Table 1.

Table 1

Characteristics of greywater

ParametersUnitConcentration
pH  7.3±1.4 
Chemical oxygen demand (COD) mg/L 285±10 
Total organic carbon (TOC) mg/L 122±12 
Phosphate (PO43−-P) mg/L 19.5±3 
Nitrate (NO3-N) mg/L 2.2±0.8 
Nitrite (NO2-N) mg/L 0.8±0.3 
Ammonia (NH4-N) mg/L 22.3±2 
Total Kjeldahl nitrogen (TKN) mg/L 51.4±6 
Anionic surfactants (AS) mg/L 22±4 
Benzophenone-3 (BP) mg/L 11 
ParametersUnitConcentration
pH  7.3±1.4 
Chemical oxygen demand (COD) mg/L 285±10 
Total organic carbon (TOC) mg/L 122±12 
Phosphate (PO43−-P) mg/L 19.5±3 
Nitrate (NO3-N) mg/L 2.2±0.8 
Nitrite (NO2-N) mg/L 0.8±0.3 
Ammonia (NH4-N) mg/L 22.3±2 
Total Kjeldahl nitrogen (TKN) mg/L 51.4±6 
Anionic surfactants (AS) mg/L 22±4 
Benzophenone-3 (BP) mg/L 11 

The chemicals used in this study included model emerging pollutant (benzophenone-3, 98% purity) (BP), acetic acid glacial (>99.99%), hydrogen peroxide (H2O2, 30%), ethylene diaminetetraacetic acid (EDTA, C10H16N2O8, >99.9%), dodecyl benzenesulfonic acid (C12H25C6H4.SO3H) (anionic surfactant), high-performance liquid chromatography (HPLC) water and acetonitrile (analytical grade).

Reactor set-up

Greywater was added into 5 L SBR reactors (made up of acrylic sheets, dimension: 20 cm×20 cm×20 cm), and then about 500 mL seed sludge (volatile suspended solids (VSS) -19.54 g/L) was added to the SBR reactor that obtained from the sediment of an anaerobic pond located at the Indian Institute of Technology Bhubaneswar. The SBR reactor was made air-tight with an opening provision for sample collection.

For SPCR part in sequential SBR-SPCR experiments, the required amount of effluent from SBR was fed to the SPCR reactor (tray-type, dimension: 25 cm×35 cm×8 cm). SPCR reactor was filled with nitrogen-doped titanium dioxide (N-TiO2) nanoparticles immobilized on graphite (as one layer uniformly). N-TiO2 was prepared with the precursor titanium isopropoxide (TTIP) using sol-gel method and a molar ratio of 2:1 (N/Ti) for urea and TTIP was kept. The solution with distilled water was continuously stirred for 30 min at 1,000 rpm, until the entire contents were dissolved. Further, the resulting solution was ultrasonicated for 30 min and the precipitate was dried in a hot air oven. The dried samples were ground and then calcined in the muffle furnace for 3 hours at 300 °C. The dip-coating method was used to prepare the N-TiO2 coating over the support. Catalyst coating of 0.8–1 g/100 g graphite was observed with gravimetric analysis before and after coating. In leaching studies, the coated photocatalysts were washed five times with distilled water and the effluent from washed water was checked using filter paper (0.45 μm) to confirm the particles of catalyst on it. No photocatalyst particles were found after the last washing step. Then, the coated photocatalysts were dried in an oven at 70 °C overnight. A tungsten-halogen lamp (Heber scientific) of 150 W was employed as a visible light source with a wavelength varied from 400 to 700 nm. Peristaltic pump (PP-20-EX, MICLINS) and submersible water pump (WP3200, Sobo) were used for regulating flow rate and circulating cooling water, respectively. For sequential SBR-SPCR treatment, the greywater effluent from SBR was fed into the SPCR and samples were collected after each hour from both the reactors and refrigerated (at 4 °C) until the analysis (maximum kept for 48 h). The schematic diagram of the experimental set-up for the sequential SBR-SPCR system is presented in Figure 1.
Figure 1

Schematic diagram of sequential SBR-SPCR system.

Figure 1

Schematic diagram of sequential SBR-SPCR system.

Close modal

Reactor operation

SBR cycles were operated in simultaneous nitrification and denitrification (SND) with the following phases: filling (0.25 h), anaerobic (1.77 h)/aerobic (2.77 h)/anoxic (2.27 h) reaction, settling (1 h) and decanting (0.25 h). The reactor was supplied with an air diffuser pump connected to the programmable switch timers (EURO, socket type timer) for the aerobic phase. During the anoxic phase, the reactor contents were stirred using an overhead stirrer at a rate of 100 rpm, while no stirring was provided during the anaerobic phase. Greywater effluents were collected after each SBR phase and then analysed immediately or stored at 4 °C for a maximum of 48 h. The SBR reactors were kept for 16 days acclimatization for the start-up period. For sequential SBR-SPCR experiments, the SBR reactors were operated in previously obtained optimized conditions (23 full factorial design) with hydraulic retention time (HRT) of reaction phase – 6.8 h, volumetric exchange ratio (VER) – 0.7 and solids retention time (SRT) – 7.94 d) and the experiments were conducted in triplicates. The detailed data of optimization of SBR parameters is not included in this study. For the study, VER and SRT were calculated by the following Equations (1) and (2):

VER:
formula
(1)
where Vf is filled greywater volume for a cycle and Vt is the total working volume of the reactor.
SRT:
formula
(2)
where X is the mixed liquor volatile suspended solids (MLVSS) in the SBR reactor with full filled (mg/L), Xw is the MLVSS in the waste stream (mg/L), tc is the total cycle time and Vw is the waste sludge volume (L).

For adsorption experiments in the SBR, the adsorbent preparation was carried out with washing of corn cobs (agricultural waste by-products) and then heat-drying at 70 °C in a hot air oven. Further, the dried corn cobs were activated in a muffle furnace at 550 °C for 30 min. For the integration of the SBR-adsorption system, a corn cob adsorbent dosage of 5 g/L was added at the start of the aerobic phase in SBR reactors and stirred at 100 rpm using an overhead stirrer. The SBR reactor contents were slowly mixed at 100 rpm for two successive phases (aerobic and anoxic). Following each adsorption-SBR experiment, the SBR with adsorbent was run for 5 days at the SBR optimized condition without adding any additional adsorbent. Meanwhile, another similar laboratory-scale SBR was used for other adsorption-SBR experiments. No separation of added adsorbent from SBR sludge was carried out. The corn cob adsorbent was used to enhance the phosphate removal in SBR system.

For sequential SBR-SPCR study, the SPCR was operated at previously obtained optimal conditions (pH–3, H2O2 dosage – 1 g/L, catalyst dosage – 5 g/L) (Priyanka et al. 2020). Samples were taken every hour from the reactor and refrigerated (at 4 °C) until the analysis (maximum kept for 48 h). All the SPCR experiments were carried out up to 6 h of treatment period.

Sampling and analysis

The greywater characteristics, such as COD, phosphate (PO43−-P), nitrate (NO2-N) and NH₄+-N, were obtained from the laboratory tests as per standard methods (APHA/AWWA/WEF 2012). Nitrate (NO3-N) measurement was done by a nitrate meter (Thermo Scientific, EUTECH ION 2700). Dissolved oxygen (DO) meter (EUTECH instruments, DO600) and pH meters (ADWA Bench meter, AD800) were used to monitor the DO and pH, respectively. The DO and pH were determined at 30 min intervals. The concentration of TOC in the solution was measured using a TOC analyzer (TOC-L, Shimadzu). Anionic surfactant (AS) concentration was measured using a developed spectrophotometric method as described in Jurado et al. (2006). Samples (filtered with 0.22 μm) were analyzed for BP by high-performance liquid chromatography (HPLC) system (Ultimate 3000, DIONEX, USA) coupled with pump and column C-18 (HYPERSIL GOLD 5UM: 250 mm×4.6 mm, Thermo scientific). The mobile phase was acetonitrile, and 0.1% acetic acid (70:30, v/v) and a 1 mL/min flow rate were maintained. The retention time for BP was detected at 6.71 min, and HPLC chromatograms were determined using Chromeleon 7 software. A digester equipped with a distillation unit was used to perform the total Kjeldahl nitrogen (TKN) analysis. Following distillation, the collected sample was titrated with methyl orange and bromocresol green as indicators and 0.02 N H2SO4 as a burette solution using the titration method. The toxicity test was performed using a Microtox M500 analyzer. In Microtox assay, the inhibition of the cellular activity of Aliivibrio fischeri (NRRL number B-11177) is characterized based on luminescence measurements at a wavelength of 490 nm. In this assay, the light generated by A. fischeri during exposure was compared to the sample (greywater effluent) with respect to a blank (without toxic sample). For the experiment, a stock culture (commercially available) of freeze-dried A. fischeri strain was kept at −20 °C to −25 °C was procured. For diluting the sample, 2% NaCl diluents were prepared with distilled water (without any toxicity) and the osmotic pressure was maintained with 22% NaCl solution similar to the bacterial habitat (marine waters). In this study, toxicity assessment was conducted with the standardized Microtox protocols using Microtox analyzer and Microtox OmniTM software version 4.3.0.1 after 30 min incubation (basic test at 81.9% concentration and screening test at 81.9% concentration) (Turek et al. 2020).

Organics removal

COD removal for synthetic greywater was achieved up to 58.1±0.7% during the anaerobic phase in SBR, and the removal efficiency was increased to 88.8±1.6% in the aerobic phase (Figure 2). Further, the COD removal of 92.8±0.5% was achieved at the end of SBR treatment (after the anoxic phase). The SBR effluent was added to the SPCR reactor that operated at optimum condition. Full mineralization (99.5±1.2%) was achieved within 2 h of SPCR treatment.
Figure 2

Performance of sequential SBR-SPCR treatment system on COD and TOC removal.

Figure 2

Performance of sequential SBR-SPCR treatment system on COD and TOC removal.

Close modal

A low TOC removal of 48.5±2% was obtained at the end of the anaerobic phase in SBR (Figure 2). The TOC removal efficiency was drastically improved to 86.2±1.9% after the aerobic phase. A maximum of 91±0.9% TOC removal was achieved in the SBR system. Moreover, a net total efficiency of 99.3±1.5% was obtained after treatment in SPCR.

Nutrients removal

Different forms of nitrogen such as TKN, NH3-N, NO3-N and NO2-N removal were studied in SBR-SPCR system. Overall good removal efficiencies of total nitrogen (i.e., sum of TKN, NO3-N, NO2-N and organic nitrogen) were obtained after SBR (84%) and SBR-SPCR (93%) treatment. Additionally, TKN (sum of NH3-N and organic nitrogen) removal of 89.6% and. 95% was achieved after SBR and SBR-SPCR treatment. The NH4+-N removal efficiency was found to be 2.4±1.9% in the anaerobic SBR phase (Figure 3(a)). A drastic increment in the removal efficiency (55.4±2.1%) was observed in the aerobic phase due to the oxidation of NH4+-N to NO2-N (by ammonia-oxidizing bacteria, AOB) and then to NO3-N (by nitrite-oxidizing bacteria, NOB) (Figure 4) (Kumwimba & Meng 2019).
Figure 3

The variation profile for (a) removal of ammonia and nitrate (b) DO concentration and pH during SBR-SPCR operation.

Figure 3

The variation profile for (a) removal of ammonia and nitrate (b) DO concentration and pH during SBR-SPCR operation.

Close modal
Figure 4

Nitrogen removal mechanism in the SBR-SPCR system.

Figure 4

Nitrogen removal mechanism in the SBR-SPCR system.

Close modal

The anoxic denitrification (conversion of NO3-N to N2 gas) resulted in NH4+-N removal efficiency of 81.6±1.3%. In addition, greywater effluent from SBR showed further removal of NH4+-N in the SPCR system (84.6±0.8% in 1 h treatment). The main contribution could be photocatalytic oxidation in the presence of hydroxyl radicals (OH) at acidic pH (initial pH to be adjusted to 3) (Figure 3(b)). Moreover, the NH4+-N removal efficiency of 90.2±0.6% was obtained at the end of 3 h SPCR treatment. After 4 h SPCR treatment, no NH4+-N concentration was measured in the greywater effluent. Mohammadi et al. (2016) reported that high initial ammonia concentration (above 400 mg/L) could decline the possibility of OH and ammonia molecules interaction on the catalyst surface as most active sites occupied by ammonium ions. However, greywater constitutes low ammonia concentration (22.3±2 mg/L measured in study), favouring the formation of OH on the catalyst surface. Further, the NO3-N removal was removed progressively in the SPCR process, which suggests the NH4+-N removal could have followed the pathway with N2 gas generation (Tugaoen et al. 2017) (Figure 4). In addition, the removal efficiency of 95.4% of NO3-N was achieved after SBR-SPCR treatment. At last, the maximum NO3-N removal efficiency of 66% was observed after combined treatment.

The variation of DO concentration and pH at the different operation time of SBR-SPCR system was presented in Figure 3(b). The pH was increased from initial 7.3±0.4 to 8.1±0.1 during SBR treatment. However, pH was slightly decreased during aerobic phase due to alkalinity consumption, whereas there was increase of pH value during anoxic phase due to denitrification process (Rodríguez et al. 2011). Further, a pH range of 7.7–7.9 provided favourable environment for AOB growth in the aerobic phase. Jones et al. (2007) reported that AOB activity or growth rate could decline at pH values over 8.2. For SPCR part, the system pH of 3 was maintained before the start of the operation period. The pH value in SPCR system increased considerably from initial 3 to 6.9±0.1 at the end of treatment period. DO levels decreased from 3.9±0.6 mg/L to 0.52±0.1 mg/L during the anaerobic phase of SBR treatment, indicating a favourable environment for denitrifiers (Figure 3(b)). The DO was maintained above 2 mg/L (up to 4 mg/L) in the aerobic phase to achieve high nitrification efficiency. During the anoxic phase, DO decreased to 0.8±0.2 mg/L. In addition to this, pH of 0.5±0.2 was observed throughout the anaerobic period in SBR system. Lastly, in SPCR system, DO concentration was significantly raised to 4.7±1.5 in 1 h treatment period because of H2O2 dissociation that added (1 g/L) to SPCR initially. Eventually, the DO was decreased in subsequent periods of SPCR treatment during photocatalytic treatment of greywater contaminants.

The PO43−_P removal rate in the integrated SBR-SPCR system is presented in Figure 5. Initially, the PO43−_P concentration was increased from 19.5±3 mg/L to 33.9±5.8 mg/L. This could be due to the release of PO43−_P into the bulk solution increased PO43−_P concentration during the anaerobic phase of the SBR process. Further, the average PO43−_P removal rate in the aerobic phase was 60.8±1.5% for synthetic greywater. In the aerobic phase, the PO43−_P reduction from greywater occurs due to phosphate accumulating organisms (PAO) uptakes PO43−_P from bulk solution. Moreover, the SBR system was operated in SRT of 7.9 d, which is favourable for a higher abundance of PAOs and other phosphate degrading organisms (Onnis-Hayden et al. 2020). The corn cob adsorbent used in the SBR system also improved the PO43−_P removal efficiency. The overall PO43−_P removal was 80±1.5% after the anoxic phase of SBR treatment with adsorbent. This removal efficiency was decreased to as low as 74.2% without adsorbent addition. Further, the PO43−_P removal efficiency of 87±0.9% was achieved in SPCR system (within 3 h). The PO43−_P reduction in this system could have been occurred due to the adsorption of H3PO4 (dominating form in acidic pH) (Yi et al. 2013).
Figure 5

Performance of sequential SBR-SPCR treatment system on phosphate removal.

Figure 5

Performance of sequential SBR-SPCR treatment system on phosphate removal.

Close modal

Emerging contaminants removal

The biological degradation of EC, such as BP in the SBR system, was found to be 58.9±2.3% (Figure 6). As reported in Wei et al. (2018), BP showed good biodegradable properties with the electron-withdrawing group, namely, -CO- in BP. Mao et al. (2020) reported that low concentration of BP (<5%) was adsorbed onto sludge compared to other ECs (e.g. parabens, benzotriazoles and triclosan), which could be due to neutral charge of BP. The BP removal rate reached 98.6±0.5% within 1 h SPCR treatment period and achieved mineralization (100%) in 5 h of the treatment period.
Figure 6

Performance of sequential SBR-SPCR treatment system on BP and AS removal.

Figure 6

Performance of sequential SBR-SPCR treatment system on BP and AS removal.

Close modal

The performance of the sequential SBR-SPCR process for the removal of AS was presented in Figure 6. SBR anaerobic period showed a minimal reduction (15.7±3.9%) in AS, which was followed by the next phase (aerobic) that progressively improved the removal rate (36.7±5.8%) and finally, the anoxic phase displayed a tremendous increment of AS removal (80.1±2.2%). The overall AS removal of 94.9±1% was achieved at the end of 4 h treatment time and further in the next hour of SPCR treatment, no surfactant was detected in the greywater effluent. In the study of Khosravi et al. (2020), it was observed that the stability of AS by-products enhanced during the anaerobic period and did not transform to the previous complex form. Further, this stable structure of AS was easily degraded in the next aeration period. Andrade et al. (2017) studied the removal of AS by anoxic reactors and observed the presence of nitrates in moderate concentration (88 mg/L), improve the surfactant removal efficiency. Since the SBR system was performed in the anaerobic-aerobic-anoxic phase in this study, there would be an oxidized nitrate concentration in the anoxic phase. This outcome could be the reason behind the higher degradation in the anoxic period. The photocatalytic degradation by the SPCR process could be due to OH radical oxidation and adsorption. The AS molecules adsorb the light from the source that affects the formation rate of OH radical on the catalyst surface (Aoudjit et al. 2019). The overall removal efficiencies of different greywater contaminants achieved after SBR and combined SBR-SPCR treatment were provided in Table 2.

Table 2

SBR-SPCR treated greywater characteristics

ParametersTreated greywater characteristics (mg/L)ReusesReferences
COD Toilet flushing
Dust control
Landscape irrigation 
CPCB (2012), USEPA (2012)  
TOC 
NH₄+-N <2.5 
NO3-N <0.11 
PO43−-P <3 
BP 
AS <1.5 
pH 6.8   
ParametersTreated greywater characteristics (mg/L)ReusesReferences
COD Toilet flushing
Dust control
Landscape irrigation 
CPCB (2012), USEPA (2012)  
TOC 
NH₄+-N <2.5 
NO3-N <0.11 
PO43−-P <3 
BP 
AS <1.5 
pH 6.8   
Table 3

Removal efficiencies for different greywater pollutants after SBR and SBR-SPCR treatment

Removal efficiency (%)CODTOCTKNNH4+-NNO3-NNO2-NTNPO43−_PASBP
SBR treatment 92.8% 91% 89.6% 85% 41% 25% 84% 80% 80.1% 58.9% 
SBR-SPCR treatment 99.5% 99.3% 95% 90.1% 95.4% 66.7% 93% 87% 94.9% 100% 
Removal efficiency (%)CODTOCTKNNH4+-NNO3-NNO2-NTNPO43−_PASBP
SBR treatment 92.8% 91% 89.6% 85% 41% 25% 84% 80% 80.1% 58.9% 
SBR-SPCR treatment 99.5% 99.3% 95% 90.1% 95.4% 66.7% 93% 87% 94.9% 100% 

Greywater reuse standards for urban reuse: pH 6–9, BOD <10 mg/L, turbidity <2 NTU, faecal coliform – not detectable (USEPA 2012). The treated SBR-SPCR greywater could be reused for different non-potable applications, namely, toilet flushing, dust control and landscape irrigation based on standards provided by CPCB 2012; USEPA 2012 as given in Table 3. To be reused in non-potable applications, the required quality of treated greywater is entirely dependent on its end application.

The toxicity was determined by the inhibition of A. fischeri bioluminescence after 30 minutes of incubation (Ajuzieogu et al. 2018). Under UNI EN ISO 11348-3 standards, samples with less than 20% bioluminescence inhibition are considered non-toxic (Murgolo et al. 2018; Butarewicz et al. 2019). Toxicity tests with A. fischeri revealed that it was non-toxic. The inhibition was 13.6% after SBR-treated greywater was fed to the SPCR, and it decreased to 12.4% after 1 h of SPCR. The toxicity of the collected greywater effluent (30 min incubation) decreased consistently as the solar irradiation time increased (3 h–9.9%, 5 h–6.1%, and 6 h–4%). The sequential SBR-SPCR system is a promising approach for the effective degradation of different greywater contaminants.

A combination strategy of SBR and SPCR was effectively removed pollutants from greywater. The results revealed that the biological treatment (SND-SBR process) caused significant removal of organics (TOC removal of 91±0.9%). The net TOC removal efficiency after SBR-SPCR treatment was 99.3±1.5%. Total nitrogen removal of 84 and 93% was obtained after SBR and SBR-SPCR treatment respectively. The corn cob adsorbent addition after anoxic phase of SBR improved the phosphate removal efficiency from 74.2% to 80%. PO43−_P removal efficiency of 87±0.9% was achieved in SPCR system. The net removal efficiencies of COD, TOC, NH4+-N, PO43−_P, BP and AS were 100%, 100%, 100%, 87±0.9%, 100% and 94.9±1%, respectively. More detailed catalyst reusability or regeneration studies and a detailed economic analysis using Life Cycle Assessment is suggested for sequential SBR-SPCR treatment system before implementing in field-scale application.

The financial support for the study by the Department of Science and Technology (DST), India, under the Water Technology Initiative (WTI) (Project No: DST/TM/WTI/2K16/90) is gratefully acknowledged.

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

The authors declare there is no conflict.

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