Investigation of mechanisms of an upflow-downflow siliceous sand filtration system for surfactants bathroom grey water treatment

ARE

Average removal efficiency

BET

Brunauer, emmett and teller

BJH

Barrett-joyner-halenda

BOD₅

Biochemical oxygen demand

COD

Chemical oxygen demand

DEWATS

Sustainable decentralized wastewater treatment systems

Dp

Pore diameter

FTIR

Fourier transform infrared

HLR

Hydraulic load rate

P

Phosphorus

pH_pzc

pH of the point of zero charge

(p/p°)

relative pressure

S_BET

Specific Surface Area

SDG

Sustainable Development Goal

SNK

Student-Newman-Keuls

SS

Siliceous sand

TSS

Total suspended solids

V_p

Total pore volume

WASH

Water sanitation and hygiene

XRD

X-Ray diffraction

XRF

X-Ray fluorescence

Billions of people around the world lack safe water, sanitation and hygiene facilities. In addition, ecosystems and water resources are becoming more contaminated, funding for water, sanitation and hygiene (WASH) services is inadequate and delivery systems are weak and fragmented (UN Water 2018). Water resources in Tunisia are subjected to many pressures related to urbanization and anthropogenic activities which will be exacerbated by climate change (Bahri et al. 2016). These pressures reached the UN-Sustainable Development Goal 6 and Goal 13 to strengthen rural clean water and sanitation management. The largest pressure on water resources occurs in Tunisia due to the importance of irrigation in their economy. As one of the most arid countries in northern Africa, Tunisia suffers from high water scarcity with an important national blue water footprint of crop production amounting to 31% of the country's renewable blue water resources (Soula et al. 2020).

As pressures on population and freshwater resources grow around the world, water is becoming more scarce forcing planners to consider developing non-conventional water resources, including treated grey water reuse to satisfy increasing non-potable, agricultural, and industrial applications. Grey water makes up about 60–70% of the total wastewater generated in households in developing countries (Edwin et al. 2014). Educational buildings generate a considerable volume of grey water due to the vast number of floors. The effort for collection and purification of grey water, to make it appropriate for reuse, is considered as such an innovative strategy that simultaneously alleviates the environmental and economic concerns of water usage and helps reduce the alarming global water scarcity (Dhiman et al. 2022). Many study cases in literature reported environmental, health and socioeconomic benefit analysis of grey water treatment and reuse in residential schools that are substantially higher than the internal and external costs through a life cycle perspective (Hourlier et al. 2010; Mourad et al. 2011; Nnaji et al. 2013; Patil et al. 2016; Platzer et al. 2016; Chaabane et al. 2017; Rodríguez et al. 2020; Xu et al. 2021). However, grey water contains high levels of surfactants, which can pose a serious risk to human health and the environment. The application of various low cost sustainable materials such as zeolite, activated carbon, shells, fly ash, blast furnace slag, pine bark, sawdust, bricks, sand and silica gel for the treatment and removal of surfactants has attracted interest as these granular materials are abundant and cheap commodities (Parjane & Sane 2011; Dalahmeh et al. 2012; James & Ifelebuegu 2018; Shreya et al. 2021).

The selection of suitable granular media filtration for the treatment of grey water with compromising the quality of treated grey water, effect of type of filter media, media size and media depth along with the effect of operating conditions significantly influence the filter performance (Shaikh & Ahammed 2022). Less attention has been given, however, to mechanisms. Studies of granular material-surfactants interactions are strongly suggested to identify functional groups involved in mesoporous constructed micelle organizing for surfactants removal.

The present study examines the application of an upflow-downflow filtration system using available low-cost siliceous sand (SS) media filter for surfactant grey water treatment of bathroom grey water discharged from the student residential complex. The objective of this study is to achieve the average efficient removal of surfactants in grey water using an upflow-downflow SS filtration system under operating factors and conditions: hydraulic loading rate and SS amount. In the present paper, mechanisms of surfactant removal from grey water has been investigated using micro-pore distribution and FTIR analysis of SS filter media before and after upflow-down flow filtration maturation cycle. Maintenance practices of upflow-down flow SS filtration system have been also listed in order to reduce many problems in grey water treatment and reuse system without supplementary costs.

At the beginning of the entire sampling operation, samples of grey water were taken at the equalization unit and finally treated. The equalization unit regulates raw grey water inflows and outflows to the treatment system and equalizes the quality and temperature of the raw grey water. This unit receives raw grey water directly from the bathrooms of the student residential complex at the Higher School of Engineering of Medjez El Bab (Tunisia). Concentrations of surfactants were analyzed calorimetrically by using Hach Lange cuvette tests. Measurements were made in triplicate for the analysis of surfactants and data were recorded when the variations between two readings were less than 5% (P < 0.05).

SS, which is used for building purposes, derived from ‘Ermil-Bouarada’ (North of Tunisia) with abundant amounts, was first sieved to get 1.25–2 mm and 0.4–0.63 mm sand particle sizes and washed with distilled water several times followed by settling. After removing the dust particles the residual wet sand particles were dried at 40 °C for 24 h. The clean dried SS particles were used for experimental purposes.

In order to get a better understanding of the mechanisms involved in the targeted matter removal onto filter media particle size distribution, X-ray diffraction (XRD), X-ray fluorescence (XRF), and FTIR spectroscopy analysis were performed on the surface of these solid matrixes before and after upflow-downflow SS filtration. The micro-pore structure, the distribution of the pore size, pore volume, and specific surface area (S_BET) of these mineral materials are measured by N₂ adsorption-desorption isotherms gained at liquid nitrogen temperature with a Micromeritics ASAP 2020 gas sorption analyzer with high vacuum, adds 10-mmHg, micropore adds 1-mmHg and pore size range: 3.5 to 5,000 Å. The Barrett-Joyner-Halenda (BJH) method was used to evaluate the average adsorption-desorption surface area, pore volume, and pore diameter. The X-ray powder diffraction pattern of SS was obtained using a Philips PW1710 X-ray powder diffraction system where the tube anode was Cu with Kα radiatio, Si external standard. The voltage and filament current were maintained at 40 kV and 40 mA respectively. The scan was obtained over a 2θ range from 2 to 70° at the scan speed of 0.02. The powder pattern is prepared by using a quasi-random preparation method and the diffractograms were analyzed with the computer programs X'Pert Highscore software for phase identification, semi-quantitative phase analysis of siliceous sand. Scattering angles, d-spacings and mineral phases are presented in Figure 2.

XRF is a quick and inexpensive non-destructive technique for identifying the bulk geochemical composition of a variety of materials in the form of compressed powder. Infrared absorption spectra of SS before and after upflow-downflow filtration were obtained using an EQUINOX FTIR 55 spectrometer with frequency range: 370–25,000 cm⁻¹, resolution better than 0.5 cm⁻¹ and variable temperature cell: 100–400°K. The samples were ground with 200 mg of KBr (spectroscopic grade) in a mortar and pressed into 10 mm diameter disks with less than 10 tonnes of pressure and a high vacuum for FTIR analysis. The conditions used were 64 scans at a resolution of 4 cm⁻¹ measured between 4,000 and 400 cm⁻¹.

Data were analyzed using the General Linear Model procedure of SPSS software for all the parameters used in this study. The purpose of this experiment was to determine if the design and operational factors (HLR and SS media filters amount) had a significant effect on the ARE of surfactants. For the variables where the F test was significant, the General Linear Model procedure was followed by a multiple range test with a probability level of α ≤ 5%. This was done by post-hoc comparisons according to the Student-Newman-Keuls (SNK) test.

The characteristics of SS filter media were investigated using a combination of characterization techniques, including particle size distribution measured by N₂ adsorption-desorption isotherms, pH of the point of zero charge (pH_pzc), XRD, and XRF (Table 1 and Figure 2). XRD and XRF analysis indicate that SS is composed of quartz with relatively high contents of SiO₂ (Average percent of SiO₂ ≈ 92%). The pH_pzc of SS was determined at 7.6. As a consequence, under the experimental conditions (initial pH = 7.35 for raw grey water), the SS particles surface should be positively charged. Similar filter media of size ranging from 0.4 mm to 0.63 mm used in this study was adopted with an effective size less than 0.65 mm in most of the studies on grey water filtration (Shaikh & Ahammed 2022).

The average settled grey water production from bathrooms of the student residential complex at the Higher School of Engineering of Medjez El Bab (Tunisia) was 275 L/c.d. This value is very high compared with the water requirement for students of Ashram Schools India (12–18 L/c.d) (UNICEF 2007) and similar to the highest total grey water generation rates estimated for European communities (274 L/c.d) (Palmquist & Hanaeus 2005). Average concentrations of different physicochemical parameters and coliforms found in bathroom grey water are presented in Table 2. The range of pH values in bathroom grey water was similar to other literature values where pH values ranged from 6.5 to 8.5. Compared to previous studies, raw grey water is characterized by low concentrations of chemical oxygen demand (COD) with average values of between 95 and 178 mg/L. The lowest concentration of this parameter is apparently due to the very high per capita water consumption. The bathroom grey water showed a concentration of the Total Suspended Solids (TSS) with reported values of 86 to 456 mg/L higher than the range of 48 to 120 mg/L reported in the literature (Ledin et al. 2001). The use of solid soaps may cause a higher amount of TSS colour and turbidity (Mohamed et al. 2014). The BOD₅/COD ratio ranged from 0.27 to 0.32 indicates that it is not easily biodegradable. The low BOD₅/COD ratio of grey water from urban households ranging from 0.2 to 0.4 has also been reported in previous studies (Katukiza et al. 2014). The P/BOD₅ ratio ranged from 0.16 to 0.29 indicates that it is not suitable for the biological treatment of phosphorous (Grady Jr et al. 2011). The range of surfactants values in bathroom grey water, which varied from 12 to 16 mg/L, was similar to other literature values where anionic surfactants values average was 15 mg/L (Kaminska & Marszałek 2020). A high faecal coliform count detected in bathroom grey water is undesirable and indicates a greater chance of human illness and infections developing through contact with the wastewater. Consequently, an equalization tank is an important component of the bathroom grey water pretreatment system. It is required to balance temporal fluctuation of hydraulic flow rate, to take into account that higher flow of grey water generated during morning hours due to bathroom use, to enjoy the spontaneous partial neutralization between components, to ensure continuous supply to the filtration system even if the influent is discontinuously and to distribute hydraulic loads creating a more uniform system.

Indeed, this is due to the presence of micro and macro particles within the pores. These particles occupy the porous volume, which explains the decrease in V_p and D_p. In addition, the decrease in the S_BET is attributed to the decrease in the surface accessible to the nitrogen molecule, thus grouping together the internal surface conferred by the porosity of the material and the external surface. Pore size distribution analysis suggests that SS filter materials exhibited mesoporous structure (20 < d < 500 Å) during filter maturation. SS filter materials are also characterized by a type II nitrogen adsorption/desorption isotherm in the maturation phase involving multi-layer adsorption on an open surface, as is the case with macroporous materials according to the International Union of Pure and Applied Chemistry classification. The adsorption-desorption isotherms present two steps that result from the pores of the materials clogged by the particles. The first step is related to open mesopores (partially obstructed mesopores) and the second step is due to blocked mesopores (totally obstructed mesopores). A slight hysteresis shift was observed towards low values of relative pressure (p/p°), which increases towards values (p/p°) of 0.2 and 0.5 respectively for the isotherms d adsorption and desorption. The pore distribution curves reveal a mesoporous structure centred around D_p values of approximately 1.83 and 2.36 nm respectively for the differential pore size distribution during nitrogen adsorption and desorption. The decrease of micro-pore parameters is obviously related to the amount of the loaded surfactants, suggesting that these particles are loaded into the pores resulting in shrinkage of the pore size and leading to mesoporous micelle organization. The excess part of loaded particles occupies the intro and the interparticle pores and results in a decrease in V_p and S_BET (Mohamed et al. 2014). According to Bera et al. (2013), sand particle-surfactants interaction takes place from high adsorbent concentration which leads to a decrease in total surface area of the adsorbent and an increase in diffused path length. Clogging refers to the decrease in permeability of the SS porous medium because of physical and chemical processes. In this study, clogging of SS upflow-downflow filter was minimized by the equalization and the sedimentation pretreatment units of raw grey water. The pretreatment of raw grey water should be conducted to prevent clogging of the filter system and sustain its performance (Katukiza et al. 2014).

Good design and maintenance practices will reduce many problems in a grey water treatment system without supplementary costs (chemical addition, odour treatment…). The success of a grey water reuse system will depend on an individual's efforts in maintaining the system. However, the following measures and maintenance are recommended for such purpose at a defined frequency to sustain the upflow-downflow SS grey water filtration performance system for grey water treatment and reuse (Table 5). Grey water systems require regular maintenance, e.g. weekly cleaning or replacing filters, periodic de-sludging and manually diverting grey water back to the sewer, and flushing of drainage lines. Any defect must be rectified as soon as it becomes apparent. The pre-treatment of grey water (equalization and sedimentation units) to remove colloids and total suspended solids is important to prevent filter clogging and reduced performance that would eventually result in bad odours (Healy et al. 2007). An equalization tank is required to balance temporal fluctuation of hydraulic flow rate, to take into account that a higher flow of grey water is generated during morning hours, to enjoy the spontaneous partial neutralization between pollutants in raw grey water and to distribute hydraulic loads in more uniform system. A sedimentation tank is also required to prevent filter clogging by efficient removal of colloids and total suspended solids. The sedimentation tank requires dislodging every month. Filters should be washed with clean water and filter media should be periodically replaced (weekly maintenance). Therefore, the removal of smaller size pathogens such as viruses under similar conditions requires an investigation in case of reuse of the final effluent subject to guidelines such as World Health Organization guidelines for irrigation purposes. Chlorination of final effluent also helps in minimizing pathogens and odour problems. Residence time during storage of potential water saving for the grey water recycling system is recommended for 2 days to maintain the quality of grey water (Liu et al. 2010).

Based on the above results, the following conclusions can be drawn. An equalization tank is required to balance temporal fluctuation of hydraulic flow rate and raw grey water quality. The pretreatment of grey water in the sedimentation tank to remove colloids is also important to prevent upflow-downflow filter clogging and reduced performance that would eventually result in bad odours. In a steady state and under optimal HLR/SS amounts, the overall performance of the upflow-downflow SS media filters reached 93.7% surfactant removal. Micro-pore distribution and FTIR analysis were successfully carried out to get a better understanding of the mechanisms of reactive mineral phases involved in SS upflow-downflow filtration system. The sulfonates, the silanol, and the silane functional groups appear to be involved in mesoporous constructed micelle organizing for surfactants removal. Some measures and maintenance operations are recommended for such purpose at a defined frequency to sustain the upflow-downflow SS filtration system performance for grey water treatment and reuse.

The authors wish to express their gratitude to the researcher's staff from the Department of Geology, Department of Chemistry of the Faculty of Science of Tunis and Cement Bizerte Company for their valuable efforts in supplying this work. Special thanks to Mr Sahbi Bouazizi for the overall English revision of the manuscript.

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

The authors declare there is no conflict.