Reuse of Treated Wastewater in the Manufacture of Concrete: Major Challenge of Environmental Preservation

This work concerns the reuse of treated wastewater from Er-Rachidia wastewater treatment plant (WWTP) in the mixing of ordinary B25 concrete, in order to reduce the overexploitation of groundwater, avoid its discharge into watercourses and reduce the risk of environmental pollution due to its mineral and organic matter load. In this respect, Tree types of mixing water were used in this study: Drinking Water (DW), Groundwater (GW) and Treated Wastewater (TW). The results recorded for each type of mixing water, in the fresh and hardened state of concretes, are then compared with the requirements of the standards. The obtained results show that the treated wastewater does not have any adverse effect upon the quality of the concrete; it has shown an improvement of the mechanical resistance from the rst stage, a similar density, setting time and porosity and a slight decrease of the workability compared with the control concrete. A Oneway analysis of variance (ANOVA) at the 5% signicance level indicated no signicant difference between concrete samples produced and cured with treated wastewater and control samples at ages 7, 14, 28 and 90 days. Throughout this study the substitution of drinking water by treated wastewater will help to minimize the need for its use. Additionally, it saves drinking water for consumption and makes wastewater treatment plants more economically attractive, together with other similar goals for sustainable development.


Introduction
The scarcity of water resources and the degradation of their quality is a major challenge for the twenty-rst century. Drought and lack of clean water have become common concerns in many arid and semi-arid regions of the world. The United Nations (UN) World Water Development Report (WWDR) predicted that nearly 6 billion peoples would suffer from clean water scarcity by 2050; while at present, slightly less than one half of the global population, 3.6 billion people or 47%, live in areas where freshwater is not enough (WWAP 2018). Therefore, in some applications, it appears important to preserve natural water resources that can be substituted by alternative water resources; treated wastewater and rainwater are one of these resources. In terms of volume, concrete is the second most consumed product in the world after water with 10 billion m 3 per year, an average consumption of 1.5 m 3 per person (Sarrazin and Lafarage. 2011). According to studies carried out by the National O ce of Drinking Water of Morocco, the demand of drinking water of a Moroccan city in a semi-arid region, like Er-Rachidia, will evolve from 12.  Kucche et al. 2015), and the huge quantities of fresh water used for washing mixer trucks, concrete pumps, equipment, aggregates, curing concrete and ready-mix concrete. To cope with this situation, various lines of research have been developed to protect this drinking water and natural water resources by proposing other alternatives. One of those alternatives is to recycle treated wastewater for a partial or total substitution of drinking water in the manufacture of concrete (Kosmatka et al. 2002;Renaud et al. 2008). This research has been developed to address the issue of the conservation of drinking water by recycling treated wastewater of Er-Rachidia wastewater treatment plant (WWTP) in concrete. The experimental program is based on a comparative study of the physico-mechanical properties of ordinary B25 concretes using three types of mixing water: drinking water (DW), groundwater (GW) and treated wastewater (TW) in order to qualify the use of the treated wastewater from Er-Rachidia WWTP in concrete.

Materials And Methods
The mixing waters used in this investigation were sampled from the Moroccan city Er-Rachidia. The city of Er-Rachidia located at the gateway to the great Moroccan Sahara, 320 km south of Meknes. It is characterized by a pre-Saharan climate in the north and desert to the south, where water has become one of the few resources in this region.

Mixing water
To produce B25 ordinary concrete, three types of mixing water were used: DW: Drinking water from the city of Er-Rachidia.
TW: Wastewater Processed from the Er-Rachidia wastewater treatment plant.
GW: Groundwaters of the Aïn El Atti artesian aquifer, 25 kilometers north of the city of Erfoud.
All waters were characterized by the measurement of the physico-chemical parameters (hydrogen potential, pH, conductivity γ, temperature T °, chemical oxygen demand COD, biological oxygen demand after 5 days BOD5, Nitrates NO3 − , Nitrites NO2 − , Suspended matter MES), the trace elements (Potassium (K), Zinc (Zn), Phosphorus (P), Calcium (Ca), Magnesium (Mg), Sodium (Na), Iron (Fe), Lead (Pb)) and the bacteriological parameters (CF Faecal Coliforms, CT totals and the material charge organic biodegradable); then compared to the allowable limits of these parameters for the water used in the production of concrete.
The temperature, the conductivity and the pH were carried out on sampling sites. The pH measurements, the conductivity and the BOD5 are analyzed respectively by a HACH model Sension2 type pH meter and a HACH model Sension5 type conductivity meter, and a VELP SCIENTIFICA type 6D range pressure sensor 2000 PV. COD and SS are performed by colorimetric (NF EN ISO 7393-2 2000) and gravimetric methods respectively (NF T 90 − 009 1986). The heavy metals are measured by Inductively Coupled Spectrometry (ICP) type Ultima 2-Jobinyvon. As for the fecal load of all the waters studied, it is almost nil. However, we note a low presence of Escherichia coli for treated wastewater that could be removed by simple puri cation before use. As for the groundwater, it is rich in iron but does not exceed the limit detrimental to the strength of the concrete (40 000 Figure 1 gives the results of the particle size analysis performed on the materials used. These results indicate that Wadi sand is composed of a majority of ne grains (98.4%) with a diameter of less than 0.63 mm, and that of crushing sand does not exceed 32.4%. In order to adjust the neness module to the quality standards of a good formulation, we have calculated the mixing proportions of  Table 5.

Methods
Concrete formulation was carried out using the Dreux Gorisse method (Dreux and Festa. 2002) which appears to be the most practical and most exploited method because it is based on experimental results.
Thus, this method makes it possible to obtain coherent and exploitable results in a simple, e cient and fast manner. The granular mixture has been optimized so as to have a granulometric curve of the mixture as continuous as possible: improve the performance and the implementation of the concrete.
The water quantity is determined to approach the ratio E / C = 0.5. Table 6

Characteristics of fresh concretes
Setting time Fig. 3 shows a comparison of the setting times (initial and nal) for three mixtures of mortars which have the same formulation and differ only by the mixing water. The results presented in Fig. 3 allows to deduce a slight increase in the initial setting time of cement pastes mixed with puri ed waste water (about 3%) in comparison with those spoiled with drinking water while respecting the different standards (EN 1008 1990 and ASTM C94 1996) that limit the gap in less than one hour to 25%.
The type of mixing water in uences the setting time. Water from the source reduces the start time to about 2.7%, which is acceptable according to the same standards (EN 1008 1990 and ASTM C94 1996). These results could be explained by the presence of nitrates that could react with sodium at the pH of the medium and slightly accelerate the hydration reaction; Sodium and nitrate has a promotion effect on the hydration of cement matrices at the pre-induction period. The same behavior has been reported in earlier work (Zheng et al. 2020). In addition, the slowing of the hydration of concrete mixed with the treated waste water appears to . The setting delay obtained by the puri ed water is very useful for concreting, especially in hot weather in the city of Er-Rachidia, for the transport of long-distance concrete and concreting in large volume. We note that the beginnings of catch were reached between 7h18 mn and 7h 36 mn whereas those of the ends were between 10h 24 mn and 11h 02 mn.

Measurement of slump
The slump test of the low-uid fresh concrete makes it possible to determine its consistency according to the type of water used. Substitution of drinking water by treated wastewater and groundwater reduces subsidence. The workability of concrete mixed with these waters was rm and marked a slump between 2.

Density of fresh concrete
The different types of mixing water had a slight impact on the density of the fresh concrete, as indicated by the density variation of less than 1% between the concretes manufactured with GW (2.00) and the TW (1.99 ) and that made with the DW (1.98) as control concrete which are all slightly lower than that of the traditional concrete B25 which must have a density around 2,35, this slight difference of density for the concrete tempered with the water from the tablecloth could be due to the presence of iron. It can act as a ller of the microstructure of cement pastes by re ning the pore structure and reducing the total porosity, thus increasing the density of the composite; due to the lling, up of pores by secondary C-S-H formation (Sikora et al. 2016).

Characteristics of concretes in the hardened state
Compressive and tensile strength of hardened concretes Fig. 5 shows the in uence of the mixing water (DW, GW and TW) on the splitting resistance of concretes at the age of 7, 14, 28 and 90 days of curing. The results show a regular increase in the splitting tensile strength with the increase of immersion time for all concretes whatever the type of mixing water used. The replacement of drinking water by wastewater treated in ordinary concrete increases its resistance compared to other concretes mixed with drinking water by 3.5%, 12% and 16% at 7 days, 28 days and 90 days of cure, respectively. On the other hand, the replacement of drinking water by groundwater increased the tensile strength by 3.5% at 7 days and by 7% at 90 days. Fig. 6 and 7 shows the evolution of the compressive strength of the ordinary B25 concretes including the three types of mixing water used (DW, GW and TW) according to the age of preservation in drinking water.
The results show that: The compressive strength of all concretes used increases regularly with age and shows no drop. At the age of 7 days, the test pieces prepared by the treated waste and the drinking water have comparable compressive strengths and are slightly higher than that of the concrete mixed with the water of the water. At the age of 1 month (+ 28 days), the development of the mechanical resistance of concretes mixed with drinking water and groundwater was similar. The compressive strength of B25 concrete mixed with treated waste water is 16.36% higher than that of concrete mixed with drinking water.
The comparative analysis of the concrete results mixed with GW and DW shows that the compressive resistances with GW are improved at the age of 28 days and 90 days. As noted by Waddell (1974), a good measure of concrete quality could be obtained by using the ratio of 28day concrete strengths (f TW28 ) to that of a similar mixture cast with potable water (f d ).
The ratio of f TW28 / f d is 1.16, which indicates that the 28-day compressive strength of concretes prepared with treated wastewater is 16% higher than that of concretes prepared with drinking water. The water of the water, the ratio of f GW28 / f d is 1.036, thus indicating a 3.6 % increase in the resistance to 28 days than that of DW. The results thus obtained favor the use of water treated wastewater from the Er-Rachidia wastewater treatment plant for the production of ordinary concrete as they signi cantly improved the development of B25 concrete resistance in the long term (28 days and more). Table 7 illustrates the compressive strengths and porosity at 60 days of concretes made with different types of mixing water.
As shown in Table 7, the porosity decreases with the increase of the compressive strength. Thus, concretes made with groundwater or treated wastewaters are less porous in comparison with those prepared by drinking water. This result might be explained by the formation of gypsum and ettringite that is more favored in the rst case and that ll the voids left by the mixing water trapped in the pores and the capillaries of the concrete, which evaporates slowly with time. Thus, the effect of the type of mixing water used does not stop in the mechanical properties, it could also in uence the porosity of the concretes and therefore its durability, since the rise of the porosity reduce the resistant of concrete. It shows that the development of concrete with wastewater types studied in this research does not signi cantly affect. Thus, these types of treated wastewater can be used for the development of concrete offering better performance in terms of resistance to fractional traction.

Conclusion
The use of the treated water of the Er-Rachidia wastewater treatment plant, as compared to groundwater and drinking water, has the following conclusions: Given the huge and sustainable availability, treated wastewater can potentially combat the scarcity of water.
The analytical results of the quality of treated wastewater tested and the allowable limits of mixing water for concrete show that the TW is suitable for the production of concrete.
The standard consistency of cement decreases by 4% and 20% for concretes spoiled respectively by puri ed wastewater and groundwater.
The different types of mixing water had a slight impact on the density of the fresh concrete which did not affect its quality in terms of resistance, Concretes tempered with treated wastewater indicate a slight increase in the initial and nal setting times of the cement, which remains within the prescribed limits giving them good mechanical strength. This limits their use in ordinary concrete. However, this retardation will be very useful for hot-weather concreting, long-distance transport of concrete and mass concreting.
The compressive strength shows that the type of mixing water has direct effects on the compressive strength, especially in the long term.
In conclusion, the recycling of treated wastewater in the mixing of ordinary concretes is a promising operation because it does improve their mechanical resistance ages ago. It does not, also, affect much the other parameters. Further research is needed because it would be much telling to study the durability of such materials to know their behavior over 3 months and the stability of all the chemical elements in the concrete. Throughout this study the substitution of drinking water by treated wastewater will help to minimize the need for its use. Moreover, it eliminates the need to expand the drinking water supply for use in the concrete industry. Furthermore, it increases the number of water treatment plants due to population growth.
Additionally, it saves drinking water for consumption and makes wastewater treatment plants more economically attractive by reusing water before nal treatment, together with other similar goals for sustainable development. Declarations