Integrated treatment of black wastewater using sedimentation, constructed wetland, and nanoparticles coagulation

Approximately two-thirds of our Earth's surface is, certainly, covered by water, making it one of the most plentiful natural resources. However, many nations around the globe, particularly those countries in the Middle East and North Africa (MENA) are facing a serious lack of access to clean and/or safe drinking water (Emenike et al. 2017). Water scarcity has been identified as the greatest global threat facing humanity (Deshpande et al. 2020). In addition, water contamination is now a major global concern (Jhansi & Mishra 2013). Conversely, world population growth, industrialization, and inadequate wastewater treatment (WWT) are all to be blamed for the global concern (Chowdhary et al. 2020; Maurya et al. 2020; Zhongming et al. 2020). The primary sources of pollution in water resources worldwide are the discharge of different wastewaters (WWs) including industrial activities, municipal effluents, hospital wastes, and/or urban stormwater runoff (Wang et al. 2014; Zhongming et al. 2020). Therefore, immediate action should be taken to manage such a problem correctly. In this respect, it has been predicted that the quality of diverse water resources in the MENA countries will continue to worsen as the most threatening problem (Zhongming et al. 2020). Due to the global rise in water scarcity today, effective WWT and safe reuse through a circular economy (CE) are urgently needed (Jhansi & Mishra 2013).

Conversely, black WW is generally composed of flushing water, faecal matter, and urine. It accounts for 27–30% of domestic sewage. Furthermore, grey WW accounts for 70–73% of such domestic sewage. The latter originates from the daily showers, washing basins, laundry, and so on. (Zhou et al. 2020). Black WW is a major potential source of pollution in the developing countries due to lack of sanitation facilities. The direct discharge of untreated black WW can lead to the contamination of the environment including soil and water ways. This in turn creates problems with drinking water and food. Thus, high-efficiency and economical technologies are urgently needed for the effective treatment of such blackwater to reduce environmental pollution and avert the impending water crisis (Xu et al. 2023). Therefore, appropriate WWT and safe recycling are strongly required owing to the rising demand for clean, inexpensive water worldwide (Kumar & Pal 2018; Leader & Wijnen 2018). When WW is properly handled, it can be safely reused for different purposes according to the degree of treatment. To solve the water crisis, countries have adopted rules and regulations for WW assessment and remediation (Abdel-Shafy et al. 2017; Miarov et al. 2020).

Nevertheless, adequate WWT and/or upgrading/upscaling are all great challenges and costly due to their capital expense. It is a reliable alternative system to sewage treatment (Abdel-Shafy & Dewedar 2012; Abdel-Shafy & Mansour 2018; Abdel-Shafy & Mansour 2019; Abdel-Shafy et al. 2022a, b). In this context, it has been confirmed that constructed wetlands (CWs) have been successfully employed for the treatment of several domestic WW sources. The CWs have proved to be simple, and low-cost in construction, operation, maintenance, and management (Abdel-Shafy et al. 2017, 2022a, b, 2023; El-Khateeb & Abdel-Shafy 2022). Consequently, the application of CWs for WWT has been recently proposed as a reliable alternative to other techniques and water reuse measures (Abdel-Shafy & Mansour 2020; Mainardis et al. 2022).

It was reported that black WW was treated efficiently in Egypt by employing sedimentation, followed by horizontal then vertical wetlands treatment (Abdel-Shafy et al. 2022a, b). However, the final treated effluent could only cope as a secondary treated effluent.

Conversely, nanoparticles (NPs) have been extensively applied in environmental remediation due to their high surface area and efficiency in WWT (Li et al. 2022). An appropriate amount of NPs in WWT proved to upgrade the quality of the final treated effluent from 69 to 92% according to different researchers (Fouda et al. 2021; Khilji et al. 2022).

The present study aimed to evaluate the efficiency of combining the sedimentation process, CWs, and subsequent chemical coagulation using NPs for the treatment of blackwater. The final treated effluent was evaluated for the purpose of safe reuse.

The source of black wastewater was obtained by a piping separation system from a house across the street to a pilot plant area in the National Research Centre (NRC), Cairo, Egypt. The piping systems separate black, grey, and yellow WW. Each type of piping system was connected to a separate manhole. The black and grey waters were raised separately to the sedimentation tanks (STs) using electric pumps. The STs were erected at a height of 4 m each.

The following diagram presents the train of the studied black wastewater treatment system.

The dimensions of the HF-CW are 3.0 × 1.5 × 0.9 m (Supplementary Table S1). In addition, Supplementary Table S1 presents the operating conditions of the investigated HF-CW, including the wastewater flow rate, organic loading rate (OLR), surface loading rate (SLR), and the hydraulic retention time (HRT).

Materials: Ferric nitrate [Fe (NO₃)₃·9H₂O] (from Qualikems India) and ethylene glycol (C₂H₆O₂) (from Oxford Lab. Chem, purchased from Mumbai India) – analytical grade (the purity is more than 95%) were used to prepare the magnetite ferric oxide NPs (Fe₃O₄).

Synthesis of magnetite NPs (Fe₃O₄): The procedure of synthesizing magnetite NPs is described as follows:

Prepare 0.2 mol ferric nitrates dissolve in 100 ml ethylene glycol followed by vigorous stirring for 2 h at 40 °C. The obtained solution is heated to 80 °C and kept at this temperature until obtaining a brown gel. The obtained gel is kept at room temperature for about 1 h. The obtained gel is annealed at 200 °C in a furnace under an air atmosphere. Finally, the magnetite NPs are synthesized as (Fe₃O₄), according to Jing Xu et al. (2007).

X-ray diffraction (XRD; PANalytical Empyrean) is employed under the following conditions: scan axis: Gonio, start position 2θ = 5.0129°, end position 2θ = 79.9709°, step size 2θ = 0.0260°, scan step time at 18.8700 Sec, scan type: continuous, PSD length 2θ = 3.35°, measurement temperature at 25 °C, anode material: Cu-Kα1 at 1.54060 Å, Cu-Kα2 at 1.54443 Å, Cu-Kβ at 1.39225 Å, ratio Kα2/Kα1 = 0.50000, generator settings: 30 mA, 45 kV.

The jar test procedure as a bench-scale experiment was employed according to the procedure reported by Abdel-Shafy (2015). The effluent of HF-CW was subjected to different doses of the synthesized Fe₃O₄ NPs, as follows: 5, 10, 15, 20, and 25 mg/l, in five different individual jars containing 1 L each of the HF-CW effluent. All the physicochemical characteristics of the black WW throughout this study were determined according to the Standard Methods (Baird 2017). Therefore, the optimum dose of Fe₃O₄ NPs was determined. Each experiment was conducted six times. The given results are the average of all the studied experiments. The standard deviation of each result was calculated accordingly.

Supplementary Table S2 presents the physical/chemical characteristics of the raw black WW. The levels of COD, BOD, and TSS were 890, 605, and 415 mg/l, respectively. This black WW is considered to be within the category of medium-strength WW.

The efficiency of the ST for the removal of certain pollution parameters is shown in Figure 6.

Different mechanisms supported the performance of HF-CW. These mechanisms include filtration, sedimentation, plant uptake, and biotransformation, respectively. Further reduction of the BOD/COD ratio indicates the biodegradation of the organic loads (Abdel-Shafy & El-Khateeb 2013; Abdel-Shafy et al. 2023).

Chemical coagulation of the HF-CW effluent using Fe₃O₄ NPs at different doses was conducted using the bench-scale jar test apparatus. The purpose is to study the efficiency of Fe₃O₄ NPs for the treatment of CW effluents in terms of removing the pollution parameters. The results are provided in Table 1. These results indicate that increasing the Fe₃O₄ NP dose increases the treatment efficiency. The highest removal rate was obtained by using 25 mg/l Fe₃O₄ NPs. However, it was observed that the iron oxide NPs produced fine flocs that were suspended for more than 30 min. Therefore, it was important to enhance the precipitation of these flocs.

Coagulation is a process of combining small particles into larger and relatively heavier aggregates (flocs). Then precipitates the particulate aggregates consequently these impurities can be removed in subsequent solid/liquid separation processes. The smaller nanoparticles are more efficient than larger particles in terms of coagulation process (Jiang 2015).

For the purpose of enhancing the settling of the coagulated particles, it was important to add FeCl₃, which is well known as an efficient coagulant aid. By adding 10 mg/l FeCl₃ in combination with variable doses of Fe₃O₄ NPs, as presented in Table 2, a notable settling efficiency was recorded. This could be attributed to the fact that FeCl₃ enhances the precipitation of the fine flocs by gathering them together to form larger and heavier ones. Hence, relatively heavier flocs precipitate more efficiently (Liu et al. 2021). Consequently, better removal of TSS was obtained (Table 3). The results (Table 3) reveal that the optimum dose is 25 mg/l Fe₃O₄ NPs in combination with 10 mg/l FeCl₃. The recorded removal efficiency of turbidity, COD, and BOD was 64.3, 89.5, and 88.3%, respectively. The residual concentrations of turbidity, COD, and BOD in the effluent were 5 NTU, 8, and 5 mg/l, respectively (Table 3).

The present study deals with black WW characterized by medium strength according to. The ratio of BOD/COD was 0.68 indicating the biological degradation. By implementing the three successive processes, sedimentation via tanks at 24 h retention time could eliminate the pollution parameters of COD, BOD, TSS, phosphates, TKN, and sulfides at the rate of 56.6, 61.7, 34.9, 54.8, 43.8, and 41.7, respectively. The BOD/COD ratio decrease from 0.68 to 0.60 indicated the biodegradation of the given organic load (Regelsberger et al. 2007; Masi et al. 2010; Capodaglio 2017).

The sedimentation effluent was further treated by the horizontal wetland system at an OLR of 42.83 g BOD/m² day and 71.30 COD/m²day. The obtained removal rates of the COD, BOD, and TSS reached 80.3, 81.6, and 80%, respectively. By employing the successive treatment systems, the studied pollution parameters could be successfully eliminated. Such elimination by use of wetlands could be attributed to several factors including RT, nutrients uptake by plants, and biomass formation (Xu et al. 2007; Abdel-Shafy et al. 2017). The BOD/COD ratio indicated the biodegradation of the organic load in the given treated wastewater.

By employing chemical coagulation to the effluent of CW using different doses of the Fe₃O₄ NPs, the characteristics of the treated effluent improved and the optimum dose proved to be 25 mg/l. However, removal of the suspended solids was not satisfactory. Hence, by adding 10 mg/l of FeCl₃ in combination with 25 mg/l of the Fe₃O₄ NPs, the final treated wastewater was greatly improved. Consequently, no release of any non-dissolved nanoparticles was recorded. This can be attributed to the use of FeCl₃ in terms of settling of the coagulated particles (Jagaba et al. 2018; Deng & Abdel-Shafy 2024).

It was reported that alum and FeCl₃ were employed separately for the treatment of WW (Jagaba et al. 2018; Ettalouia et al. 2021). However, much larger doses of these coagulants were used. Therefore, by using small doses of the employed NPs and coagulant two advantages were achieved, namely, efficiency and cost-effectiveness.

By correlating the final overall treated effluent with the permissible limits of the national and international regulation for non-restricted reuse (Supplementary Table S7), it can be confirmed that the obtained final treated effluent can be safely reused for irrigation without any restriction.

In comparison of the present study with the previous work (Abdel-Shafy et al. 2017), where two types of wetlands were employed. Thus, larger area was necessary to be used, including the cost of labour and maintenance. Besides, the final treated effluent was classified as secondary treated effluent. By comparing the present results with the previous one (Abdel-Shafy et al. 2017), it was observed that the latter could achieve the following: (1) only one CW (horizontal) as land area (i.e. relatively smaller land is required), (2) less labour and maintenance cost, and (3) a tertiary treated effluent could be achieved, inside of secondary treated effluent. Hence, three important advantages were reached.

High-strength WW such as blackwater can be treated efficiently by employing the studied successive processes, namely, sedimentation, CWs, and subsequently chemical coagulation using Fe₃O₄ NPs in combination with small dose of FeCl₃. The final treated effluent can be safely reused in irrigation as a non-restricted source of water. Three different advantages were achieved by employing the present study.

The authors are in debt to the facilities provided by the following project: ‘Towards Innovative and Green Water Reuse with Integrated Constructed Wetlands and Ferrate(VI) Treatment’ – number (42688) – supported in whole or part by NAS and USAID, USA – Egypt Science Technology Joint Fund (Cycle.19), and the (STDF-Egypt).

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

The authors declare there is no conflict.