Valorization of water treatment sludge for applications in the construction industry: a review

Citation: Mattoso, A.P.; Cunha, S.; Aguiar, J.; Duarte, A.; Lemos, H. Valorization of Water Treatment Sludge for Applications in the Construction Industry: A Review. Materials 2024,17, 1824. https:// doi.org/10.3390/ma17081824 Academic Editor: Roberta G. Toro Received: 19 March 2024 Revised: 8 April 2024 Accepted: 12 April 2024 Published: 16 April 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Review Valorization of Water Treatment Sludge for Applications in the Construction Industry: A Review Ana Paula Mattoso 1, Sandra Cunha 1, JoséAguiar 1,* , António Duarte 1and Helena Lemos 2 1CTAC—Centre for Territory, Environment and Construction, University of Minho, Campus of Azurém, 4800-058 Guimarães, Portugal; [email protected] (A.P.M.); [email protected] (S.C.); [email protected] (A.D.) 2Águas do Norte, S.A., Dom Pedro de Castro, 5000-669 Vila Real, Portugal; [email protected] *Correspondence: [email protected] Abstract: To address the growing global water demand, it is imperative to implement advanced treatment systems and sustainable alternatives for managing the large amount of waste generated during the water purification process, known as water treatment sludge (WTS). Worldwide, researchers and companies are exploring alternatives and methods for the valorization of WTS as a raw material in other processes. It is urgent that all productive sectors, which contribute significantly to greenhouse gas emissions, adopt this management principle to ensure more sustainable production, contributing to the global goal of climate neutrality. Notably, in civil construction, incorporating WTS as a supplementary cementitious material (SCM) shows great promise, considering that the industrial waste currently used for this purpose is increasingly restricted. The use of WTS as a raw material in the cement industry not only contributes to the reduction of the carbon footprint, but also reduces the high waste load still disposed of in landfills. The emerging applications for WTP sludge are reviewed, with emphasis on its valorization in the civil construction as an SCM. The main characteristics of this waste and their impacts on the environment are also addressed. Keywords: water treatment sludge; waste recovery; circular economy; sustainable constructions 1. Introduction 1.1. General Considerations As the world’s population grows, water consumption is expected to double by 2050 [ 1 ] and to meet the new increasing demand, water treatment plants (WTPs) will need to increase their production of drinking and industrial water, which will mean generating a greater amount of waste from treatment process, known as water treatment sludge (WTS). On a global scale, it is estimated that more than 10,000 tons of sludge are generated per day [ 2 ]. Management practices for this waste generally differ between countries, depending on local regulations, but the most common destination is landfills, which has a negative impact on the environment. For this reason, the search for sustainable alternatives for the treatment and reuse of WTS has become an object of study for researchers all over the world [ 3 ] and one of the options is to use it as a raw material in other production processes, applying the principle of the circular economy. This practice will help to minimize the problems of natural resource scarcity faced nowadays. On the other hand, to guarantee sustainable production, decarbonizing production chains has become a global challenge. One of the strategies adopted by the European Union (EU) to overcome this challenge and achieve the goal of zero net greenhouse gas (GHG) emissions by 2050 is to encourage more efficient use of natural resources, mobilizing industry towards a circular and clean economy [ 4 ]. As a large part of greenhouse gas emissions come from the extraction and processing of natural resources and considering that the amount of waste generated in the world is tending toward increasing significantly, Materials 2024,17, 1824. https://doi.org/10.3390/ma17081824 https://www.mdpi.com/journal/materials Materials 2024,17, 1824 2 of 20 expanding the implementation of a circular economy among the various productive sectors is an essential measure for achieving the global goal of climate neutrality. It is in this context that the waste generated in water treatment plants appears to be a promising alternative to be valorized as a raw material in various sectors, since WTS has added value; it comes from an essential activity and is therefore always available; and it can be recycled. In this respect, the construction sector is one of the biggest consumers of natural resources, with a high environmental impact, mainly associated with the production of cement [ 5 – 7 ]. Some alternatives to reduce the impact of production have already been proposed, such as alternative raw materials, alternative fuels, energy recovery and new clinker compositions and geopolymer binders [ 8 ]. However, these options often become unfeasible due to technical, operational and regulatory issues [ 9 ]. The use in the manufacture of clinker, for example, comes up against the problem of material availability, since these minerals are industrial by-products and are increasingly restricted to supply cement demand [ 10 – 13 ]. In the case of fly ash, the industry that supplies the waste is gradually being phased out. Around 40% of coal-fired power stations in the US have been decommissioned in the last five years; the UK plans to decommission all its coal power stations by 2025 [ 14 ], while in the Netherlands, coal-fired power stations will be decommissioned by 2030 [ 15 ]. Another option pointed out by researchers in the field is the use of more efficient cements, such as low-heat portland (LHP) cement, which has a wide range of applications and is considered to have satisfactory performance [16]. In addition, construction materials use large quantities of cement, the production of which not only requires the extraction of non-renewable minerals, but is also responsible for high emissions of carbon dioxide (CO 2 ). In this way, introducing WTS into the construction industry chain to partially replace cement, as a supplementary cementitious material (SCM), will reduce the use of this binder and positively impact the global goal of zero net greenhouse gas emissions. Research carried out by a working group supported by the United Nations Environment Program Sustainable Building and Climate Initiative (UNEP-SBCI) also identified the use of MCS to partially replace clinker or as a partial substitute for cement in construction materials as the most favorable carbon reduction alternative for the industry [10]. Therefore, considering the scenario of the global plan for sustainable development defined in the 2030 Agenda of the United Nations (UN), the subject of this research review is inserted in the following Sustainable Development Goals (SDGs): SDG 6: Ensure sustainable drinking water for all; SDG 9: Sustainable industries and infrastructures; SDG 11: Building sustainable and resilient cities; SDG 12: Efficient use of natural resources and environmentally sound waste management. The aim of this work is to present an overview of the alternatives for recovering WTP sludge in the context of the circular economy, with an emphasis on applications for this waste in the construction industry. The main characteristics of WTS and its environmental impacts will also be addressed in this research. 1.2. Scope of the Review Paper The methodology employed in this review prioritized papers published in the last 15 years to ensure the review remains up to date and presents a comprehensive perspective on the research conducted in this area. The review is divided into four sections. Section 1presents a brief description of the potential for recovering the waste generated in water treatment plants as a raw material for other industries, with special attention to the construction industry, highlighting the challenges faced by the sector today and the context in which the subject of this review is inserted. Section 2presents the main physicochemical characteristics of WTS and the negative environmental impacts that its improper disposal can cause. In the same section, some properties of cement and other pozzolanic materials are also presented, for comparison purposes and to prove the viability of using WTS. Section 3portrays the applications for Materials 2024,17, 1824 3 of 20 WTS in different sectors, with a more detailed approach leading to its valorization in the construction industry, presenting different subsections that include cement manufacture, mortar and concrete production, the manufacture of tiles and ceramic products and the application of sludge in geotechnical works. This highlights the huge potential for using this waste as a raw material for industry. Finally, Section 4highlights the conclusions of this review and Section 5includes some recommendations regarding new approaches for future studies in the area. 2. Main Characteristics of Sludge Generated in Water Treatment Plants Ensuring safe drinking water for all is among the Sustainable Development Goals put forward by the UN as one of the targets needed to ensure the health and well-being of humanity. Therefore, to achieve the goal of universalization of this service by 2030, it will be necessary to implement more producer systems and from this perspective, sanitation companies and governments will also need to implement sustainable alternatives for the treatment and disposal of the large amount of sludge that will be generated in the process. WTS is waste generated during the process of treating drinking water, consisting of water and suspended solids originally contained in the source of supply, plus products resulting from the reagents applied in the treatment. Some of the chemical products commonly used in WTPs as coagulants/flocculants include aluminum sulphate, ferric chloride, chlorinated ferrous sulphate, ferric sulphate, aluminum hydroxy-chloride, synthetic polymers (cationic, anionic and non-ionic) and natural polymers (cassava and potato starch) [2,17]. The qualitative and quantitative characteristics of this waste depend on the quantity and quality of the water that feeds the plant, the treatment technology used and the chemicals and dosages applied in the process. [ 18 ]. Thus, colloidal particles and suspended materials such as silt, clays, humic substances and metals, among other impurities originally present in the raw water, and chemicals in the form of aluminum or iron hydroxide, using salts of these metals as coagulants, will make up the solid fraction of these sludges. Silica (SiO 2 ), alumina (Al 2 O 3 ) and hematite (Fe 2 O 3 ) generally account for a significant proportion of these solids [ 19 ]. However, other oxides such as calcium oxide (CaO), magnesium oxide (MgO), sodium oxide (Na 2 O), potassium oxide (K 2 O), phosphorus pentoxide (P 2 O 5 ) and sulphur trioxide (SO 3 ), as well as traces of other metals, chlorides (Cl − ), sulphates (SO 42− ) and other organic and inorganic compounds, removed from the raw water or added as impurities contained in the chemical products, will also be present in the composition of the sludge. The moisture content of this waste is usually over 80% by weight and the organic matter content is around 25% [20]. When aluminum salt coagulants are used, the waste generated is known as “alum sludge”, but if the coagulant used is based on iron salts, the waste obtained is called “ferric sludge” [ 2 ]. Alum sludge is more common, because aluminum sulphate and aluminum chloride are the most used coagulants in water treatment [ 21 , 22 ] and the aluminum content in these sludges generally represents 16% by weight of their chemical composition [ 23 ]. Whatever its origin, the characterization of the waste is essential to define the best way to handle and use it, given that its composition varies greatly [ 3 ]. Table 1shows the elemental chemical composition (main oxides) of WTS generated in some countries. These chemical characteristics affect the options for reuse and final disposal more than the ability to handle and dewater. WTS has high humidity levels and to facilitate its handling, it is extremely necessary to dewater it. Both alum sludge and ferric sludge are considered difficult to dewater when they have specific resistance to filtration values between 5 × 10 12 m/kg and 50 ×1012 m/kg [ 24 ] and to facilitate the dehydration of this waste, polyelectrolytes (synthetic polymers) are generally used as chemical conditioning agents. In addition to specific strength, other physical characteristics that significantly affect the sludge’s ability to be handled, compacted and dewatered are solids concentration, compressibility and particle size distribution [ 25 ]. Materials 2024,17, 1824 4 of 20 Table 1. Chemical composition of WTS (Data from: Ahmed et al. [ 26 ], Abo-El-Enein et al. [ 27 ], Shamaki et al. [28], Altheman et al. [29], He et al. [30] and Liu et al. [31]). Chemical Composition (%) Countries Iraq [26] Egypt [27] United Kingdom [28] Brazil [29]China [30] Australia [31] SiO236.29 36.51 10.28 42.00 43.75 26.43 Al2O327.92 22.21 44.24 35.00 36.57 28.27 Fe2O35.33 5.65 2.51 18.00 6.00 6.66 CaO 3.77 2.66 2.50 0.41 1.00 5.36 MgO 1.12 1.34 0.34 1.13 0.60 1.11 Na2O 1.31 1.35 0.15 0.04 - - K2O 1.81 0.49 0.43 0.95 2.00 1.23 SO30.55 0.08 1.24 0.86 2.04 0.48 P2O50.43 - 0.44 0.47 0.62 - WTS can be dewatered naturally, in drying beds or sludge lagoons, or using equipment to speed up the process, such as filter presses, vacuum filters, centrifuges and bags [ 25 ]. The most suitable method varies depending on several factors, such as area availability, climatic conditions, equipment costs and the operation and maintenance of the treatment system. In addition to these factors, one must also take into account the requirements defined by the control bodies in relation to the final concentration of solids. Generally, final disposal in landfills requires the sludge to be dewatered to a minimum solids content of 20% [ 32 ]. These values can be achieved using centrifuges or filter presses. After the mechanical dehydration process, in order to obtain a product with physical and chemical characteristics that increase its opportunities for valorization in the industry, many companies have started to use solarpowered greenhouse to carry out the final drying stage, ensuring greater efficiency and lower costs [ 33 , 34 ]. Figure 1shows an image of the solar drying greenhouse installed at the Areias de Vilar WTP in Portugal when it received its first load of sludge from a centrifuge. Materials 2024, 17, x FOR PEER REVIEW 4 of 19 compacted and dewatered are solids concentration, compressibility and particle size distribution [25]. Table 1. Chemical composition of WTS (Data from: Ahmed et al. [26], Abo-El-Enein et al. [27], Shamaki et al. [28], Altheman et al. [29], He et al. [30] and Liu et al. [31]). Chemical Composition (%) Countries Iraq [26] Egypt [27] United Kingdom [28] Brazil [29] China [30] Australia [31] SiO2 36.29 36.51 10.28 42.00 43.75 26.43 Al2O3 27.92 22.21 44.24 35.00 36.57 28.27 Fe2O3 5.33 5.65 2.51 18.00 6.00 6.66 CaO 3.77 2.66 2.50 0.41 1.00 5.36 MgO 1.12 1.34 0.34 1.13 0.60 1.11 Na2O 1.31 1.35 0.15 0.04 - - K2O 1.81 0.49 0.43 0.95 2.00 1.23 SO3 0.55 0.08 1.24 0.86 2.04 0.48 P2O5 0.43 - 0.44 0.47 0.62 - WTS can be dewatered naturally, in drying beds or sludge lagoons, or using equipment to speed up the process, such as filter presses, vacuum filters, centrifuges and bags [25]. The most suitable method varies depending on several factors, such as area availability, climatic conditions, equipment costs and the operation and maintenance of the treatment system. In addition to these factors, one must also take into account the requirements defined by the control bodies in relation to the final concentration of solids. Generally, final disposal in landfills requires the sludge to be dewatered to a minimum solids content of 20% [32]. These values can be achieved using centrifuges or filter presses. After the mechanical dehydration process, in order to obtain a product with physical and chemical characteristics that increase its opportunities for valorization in the industry, many companies have started to use solar-powered greenhouse to carry out the final drying stage, ensuring greater efficiency and lower costs [33,34]. Figure 1 shows an image of the solar drying greenhouse installed at the Areias de Vilar WTP in Portugal when it received its first load of sludge from a centrifuge. Figure 1. Solar drying greenhouse of the Areias de Vilar WTP. The geotechnical properties of the sludge must also be assessed to define alternative applications for this waste. The geotechnical analysis of alum sludge subjected only to thickening characterized the residue as a clay with high plasticity, high compressibility and very low permeability. The results were attributed to the large amount of water bound to the coagulant, the high affinity of the coagulant metal for water and the high organic content of the sludge [35], showing that untreated sludge is sometimes unsuitable for use Figure 1. Solar drying greenhouse of the Areias de Vilar WTP. The geotechnical properties of the sludge must also be assessed to define alternative applications for this waste. The geotechnical analysis of alum sludge subjected only to thickening characterized the residue as a clay with high plasticity, high compressibility and very low permeability. The results were attributed to the large amount of water bound to the coagulant, the high affinity of the coagulant metal for water and the high organic content of the sludge [ 35 ], showing that untreated sludge is sometimes unsuitable for use in construction. However, by treating this sludge or incorporating it into other materials, the negative impact of these characteristics can be mitigated [ 36 ]. Other studies have also showed the similarity of WTS to clayey soil, based on the USCS (Unified Soil Classification System) [ 37 – 39 ]. However, despite this similarity, the concentration of organic matter and chemicals in WTS is higher than that in clay soils [40]. Materials 2024,17, 1824 5 of 20 Due to its physicochemical characteristics, the use of WTS as a raw material for other processes is increasingly widespread. One promising application is its use as a supplementary cementitious material in civil construction, specifically to partially replace cement. This occurs because sludge contains high levels of SiO 2 e Al 2 O 3 , giving them pozzolanic characteristics. Pozzolans are defined as a fine material, rich in silica or amorphous silica– alumina, capable of reacting with calcium oxide or hydroxide and forming compounds with cementing properties [ 41 ]. This property is enhanced in heat-treated WTS [ 42 ], where the crystalline content of silica and alumina is totally or partially broken down, forming a highly reactive transition phase. Heat-activated alum sludge has been classified as a Class N pozzolan, based on its chemical composition. In addition to the chemical composition, the research also compared some physical properties of WTS, dried in an oven (105 ◦ C for 24 h) and then calcined (800 ◦ C for 2 h) with cement and another pozzolanic material [ 43 ] (Table 2). Heat treatment at temperatures between 600 ◦ C and 800 ◦ C has already been used successfully by other researchers to activate WTP sludge and make it viable for use as a pozzolan [ 42 , 44 , 45 ]. As the sludge is generally coarse-grained once it has been dried and calcined, it is necessary to crush and grind the waste to reduce its particle size and thus increase its pozzolanic potential [46]. Table 2. Chemical composition and physical properties of cement, WTS and blast furnace slag (data from: Owaid et al. [43]). Characteristics Cement Dry Sludge (105 ◦C for 24 h) Calcined Sludge (800 ◦C for 2 h) Blast Furnace Slag Chemical composition (%) SiO220.18 42.38 47.00 32.6 Al2O35.23 35.03 41.94 12.57 Fe2O33.34 4.94 4.86 0.24 CaO 64.40 0.13 0.41 41.0 MgO 1.80 0.29 0.40 6.04 Na2O 0.07 0.10 0.09 0.39 K2O 0.44 1.87 0.99 0.35 SO32.98 0.14 0.10 1.31 P2O50.26 0.28 - Loss on ignition 2.17 11.4 2.64 1.48 Physical properties Specific gravity 3.12 2.34 2.53 2.83 Specific surface (m2/kg) 338 1110 1160 739 Average particle size (µm) (d50)16.9 11.0 10.1 16.8 Resistance activity index 7 days (%) 100 78 84 84 Resistance activity index 28 days (%) 100 86 93 101.4 A new treatment method applied to sludge consists of subjecting the waste to the rapid calcination process, in which the dried and ground sludge is calcined at high temperatures (800 to 900 ◦ C) for a short period of time (0.5–1 s) and immediately cooled. The result is a more reactive material with potential for application in construction materials, obtained from a more sustainable method [47]. The sludge generated in WTPs is produced mainly in settling tanks, to a lesser extent in rapid filtration units and, in even smaller proportions, in preparation tanks for the chemical products used in the process. The settling tanks accumulate the largest portion of solid waste, which represents 60 to 95% of the total sludge generated at the WTP, and the average volume of sludge generated daily in the decanting units of a full-cycle WTP can reach 3% of the volume of water treated by the plant [ 25 ]. The amount of sludge produced is directly related to the content of suspended solids presents in the raw water, removed during treatment process and the chemicals’ dosages applied in the unit processes can vary seasonally (rainy or dry-weather periods) or due to changes in raw water quality [ 2 ]. The types of treatment units where sludge are generated and the techniques used to remove this waste also affect the final quantity obtained. Materials 2024,17, 1824 6 of 20 Quantifying the production of sludge from a WTP is an essential step in planning sustainable alternatives for using this waste. This quantity can be estimated at the design stage through tests in pilot plants or laboratories, with the raw water to be treated [ 25 ] or, in WTPs in operation, by carrying out the system’s mass balance or using empirical formulas that relate parameters such as affluent flow rate, dosage (D) of coagulant and other products applied in the treatment, as well as the concentration of suspended solids in the raw water [ 48 ]. Katayama et al. [ 49 ] used the formula proposed by the American Water Association (AWWA) [ 50 ] to estimate the production of sludge in full-cycle WTPs that used aluminum sulphate or ferric chloride as a coagulant (Equation (1)) and compared the results with estimates made using the mass balance method. For the author, empirical formulas are widely used because of their practicality, but the mass balance method offers more precision and representativeness. W = 86.4·Q·(D + SST + Dp + Dcap + 0.1Dcal)·10−3(1) where: W—Dry solid production, (kg/day); Q—Flow rate of water to be treated (L/s); D—4.89·DAl or 2.9·DFe; DAl—Aluminum sulphate dosage (mg/L); DFe—Ferric chloride dosage (mg/L); Dp—Polymer dosage (mg/L); SST—Suspended solids in raw water (mg/L); Dcap—Calcium hardness removed (mg/L CaCO3); Dcal—Lime dosage (mg/L). Precise information on the amount of WTS produced by each country is limited in the literature. However, there are records showing that in the USA, more than two million tons of dried sludge are produced annually [ 51 ]; in Italy, 750,000 tons of dewatered sludge are generated in one year, with an estimated transport cost of around 50 million euros/year [ 52 ]. The annual production of liquid sludge in Morocco has been estimated at one million tons, with the prospect of an increase of around 20% by 2030 [ 34 ]. There are reports that in Australia the annual generation of sludge from a WTP can reach 43,500 tons [ 20 ]. In India, the annual production of a WTP was estimated at 29,700 tons [ 2 ], while in Portugal, the WTPs managed by the Águas de Portugal Group (AdP) produced 18,076 tons of sludge in 2022, representing a per capita production of 26.2 kg/hab.year [ 53 ]. In this country, the cost associated with handling and transporting the sludge generated in a WTP reached 400,000 euros/year [54]. Impacts of WTP Sludge on the Environment The European Waste List (EWL) classifies WTS as non-hazardous solid waste [ 55 ]. In Brazil, these sludges have been classified as Class II A—non-inert waste [ 56 ], showing that if they are not properly treated and disposed, they can cause damage to the environment. This has captured the attention of researchers worldwide who are exploring sustainable management alternatives for this waste, particularly in supply systems in large urban centers, where the volume of sludge generated is significant and can lead to pollution problems if not properly disposed of. There is already published evidence in the literature regarding the toxicity of sludge and its negative effects on organisms in both soil and aquatic environments, predominantly due to the high concentrations of metals and organic compounds in WTS composition. Nevertheless, some scholars suggest that more in-depth studies should be carried out to clarify existing gaps and establish standards for use in specific areas [57,58]. In some countries, it is still common for WTS to be dumped directly into the environment. This is the case in Brazil, where most of the WTPs in operation discharge this waste, without proper treatment, directly into the water bodies located near the plants, directly affecting the quality of these water sources. Among the main impacts of this Materials 2024,17, 1824 7 of 20 practice are an increase in the concentration of metals, mainly aluminum (Al) and iron (Fe) and in the concentration of suspended solids; alteration in the nutrient cycle, mainly phosphorus (P); development of anaerobic conditions in stationary or low-velocity waters; an increase in turbidity and color; a change in chemical composition; and siltation of receiving bodies, due to the increase in settleable solids and the possibility of groundwater contamination [ 59 ]. The accumulation of WTS on the benthic layer can inhibit the growth of some species of fish and aquatic organisms [ 60 ]. Studies conducted to assess the toxicity of alum sludge from 10 WTPs concluded that the water-soluble constituents present in the sludge, when discharged into receiving bodies, can affect the growth of algae [ 61 ]. Another study carried out with the aim of comparing the toxicity of iron sludge and alum sludge on Daphnia similis concluded that prolonged exposure to FeCl 3 sludge caused mortality and decreased reproduction of these organisms, while alum sludge only caused reductions in reproduction [ 62 ]. Thus, WTS has the potential to cause negative effects on the soil, such as salinization, accumulation of metals, nitrate leaching, and on water, such as increased turbidity, consequent impairment of photosynthetic processes, increased organic matter, in addition to compromising aquatic flora and fauna [63]. Alum sludge contains high concentrations of Al [ 22 ]. The toxicity of this metal is still little known, but there is already research that shows some concerns about the actions of the element on aquatic organisms. Experiments with trout using different dosages of aluminum in different pH ranges led to the observation of physical changes in the fishes, such as generalized apathy and discouragement, a symptom of inability to keep their balance, changes in coloration and a decrease in perception [ 64 ]. The pH and organic matter content in the water influence the toxicity of this metal, which increases as the pH decreases [ 65 ]. Another study, which aimed to evaluate the oxidizing potential of aluminum sulphate in mouses, showed the role of aluminum in increasing the production and formation of free radicals and in the inflammatory action of the brain tissue of these animals [ 66 ]. Research carried out on individuals to explore the link between exposure to Al in drinking water and Alzheimer’s disease indicated that cognitive decline was more pronounced in individuals with a higher daily intake of Al, confirming that high Al consumption may be a risk factor for Alzheimer’s disease [67]. In any case, the environmental risks associated with WTP sludge are lower when compared to the sludge generated at wastewater treatment plants. This is because the raw water that is used as a source of supply needs to meet the requirements of public health bodies and must therefore be cleaner, in terms of the concentration of heavy metals, organic matter, levels of pathogens and other contaminants [17]. Another concerning factor is the large amount of sludge deposited in landfills, which can overload these units. Due to its high water content, which results in large volumes before being sent to their final destination, the sludge generated in WTPs needs to undergo dehydration treatment [ 59 , 68 ]. Generally, for final disposal in landfills, which is the most common destination in many parts of the world, it is required their dehydration reach a minimum solids content of 20% [ 32 ]. The aim is to reduce their high volume, to make them easier to handle and reduce transportation and storage costs [ 69 ]. In addition to the costs associated with this dehydration and storage stage, the transportation of WTS contributes to an increase in the material’s carbon footprint, due to the use of additional fuels [33]. 3. Emerging Applications for WTS Research points to various alternatives for the application of WTP sludge. The following have already been reported in the literature: its use as a soil improver [ 70 ]; as a waterproofing agent for landfill sites [ 71 ]; in agricultural crops [ 17 ]; for reuse as a coagulant in wastewater treatment [ 19 ] and as a coagulation/flocculation aid in WTP [ 72 ]; as an adsorbent for removing pollutants from soils and bodies of water [ 73 – 75 ]; for cement production, as an supplementary cementitious material [ 76 , 77 ]; and for the production of mortars [ 78 ], concrete [ 3 , 43 , 79 ], bricks, tiles and ceramic materials [ 80 – 82 ]. Recovering the aluminum present in the sludge for later reuse is also pointed out as a sustainable Materials 2024,17, 1824 8 of 20 alternative in recovering this waste [ 83 , 84 ]. Figure 2summarizes the main applications of WTS in different sectors. Materials 2024, 17, x FOR PEER REVIEW 8 of 19 to the costs associated with this dehydration and storage stage, the transportation of WTS contributes to an increase in the material’s carbon footprint, due to the use of additional fuels [33]. 3. Emerging Applications for WTS Research points to various alternatives for the application of WTP sludge. The following have already been reported in the literature: its use as a soil improver [70]; as a waterproofing agent for landfill sites [71]; in agricultural crops [17]; for reuse as a coagulant in wastewater treatment [19] and as a coagulation/flocculation aid in WTP [72]; as an adsorbent for removing pollutants from soils and bodies of water [73–75]; for cement production, as an supplementary cementitious material [76,77]; and for the production of mortars [78], concrete [3,43,79], bricks, tiles and ceramic materials [80–82]. Recovering the aluminum present in the sludge for later reuse is also pointed out as a sustainable alternative in recovering this waste [83,84]. Figure 2 summarizes the main applications of WTS in different sectors. Figure 2. Emerging applications for WTP sludge. Due to its composition, WTS has the potential to be applied in different areas. Alum sludge can be applied to the soil to improve its structure, porosity, water retention capacity, nutrient levels and organic compounds, because it contains significant amounts of organic matter and micro and macronutrients [70,85]. A lab-scale study using WTS to correct four types of soil at a rate of 1280 mg/ha proved that the treatment resulted in an increase in hydraulic conductivity and water retention, improving the soil’s physical properties [86]. Alum sludge has been used as an adsorbent to remove phosphorus from wastewater, and the removal capacities ranged from 2 to 43 mg P/g of sludge, depending on the experimental conditions and the characteristics of the sludge [23], and also showed promising results when used to remove emerging pollutants found in water. In this case, sludge used had a high concentration of activated carbon in its composition, which is normally used in treatment when it is desired to remove impurities from the water which affect taste and flavor. The results were promising for the removal of the steroid hormones 17βestradiol and 17 β-ethinylestradiol [87]. Recovering aluminum metal for reuse, or even adapting the alum sludge for uses that require lower levels of aluminum, is a recovery option that offers both environmental and economic benefits. Research carried out to this end, using the alkaline process, managed to recover between 70% and 90% of the metal [83]. Another study used acid washing to Building materials Agriculture and soil recover y WTS •Cement production; •Mortar and concrete production; •Manufacture of bricks, tiles and ceramic materials, Water treatment •As a fertilizer on agricultural crops; •As a soil improver, to recover degraded areas; •As an adsorbent for the removal of pollutants in soils. •As a coagulant in wastewater treatment; •As a coagulation/flocculation aid in WTP; •As an adsorbent for the removal of pollutants in water. Figure 2. Emerging applications for WTP sludge. Due to its composition, WTS has the potential to be applied in different areas. Alum sludge can be applied to the soil to improve its structure, porosity, water retention capacity, nutrient levels and organic compounds, because it contains significant amounts of organic matter and micro and macronutrients [ 70 , 85 ]. A lab-scale study using WTS to correct four types of soil at a rate of 1280 mg/ha proved that the treatment resulted in an increase in hydraulic conductivity and water retention, improving the soil’s physical properties [86]. Alum sludge has been used as an adsorbent to remove phosphorus from wastewater, and the removal capacities ranged from 2 to 43 mg P/g of sludge, depending on the experimental conditions and the characteristics of the sludge [ 23 ], and also showed promising results when used to remove emerging pollutants found in water. In this case, sludge used had a high concentration of activated carbon in its composition, which is normally used in treatment when it is desired to remove impurities from the water which affect taste and flavor. The results were promising for the removal of the steroid hormones 17 β -estradiol and 17 β-ethinylestradiol [87]. Recovering aluminum metal for reuse, or even adapting the alum sludge for uses that require lower levels of aluminum, is a recovery option that offers both environmental and economic benefits. Research carried out to this end, using the alkaline process, managed to recover between 70% and 90% of the metal [ 83 ]. Another study used acid washing to remove aluminum from sludge and apply it to the soil for growing spinach and Japanese mustard. Acid washing (pH = 3) made it possible to reduce the aluminum content of the waste by up to 90%, which when mixed with the soil led to increased phosphate absorption by the plants and increased spinach size [ 65 ]. Acid washing, using a sulphuric acid solution with a molarity of 1.35 M, allowed 98% of the aluminum to be recovered from the sludge [84]. In the water treatment plant itself, the sludge can be used to assist in the coagulation/flocculation process, allowing savings to be made in the use of the coagulant applied in the WTP [ 72 ]. A study carried out in a laboratory setting used material recovered from alum sludge treated with sulphuric acid as a coagulant to treat water collected from a river. Most of the quality parameters of the treated water met the desired standards, indicating that the recovered product has the potential to be reused as a coagulant in a WTP [19]. Laboratory-scale experiments have also been reported where sludge (Fe or Al) has been used as an adsorbent to remove heavy metals from contaminated soils and bodies of water. The results obtained showed that small quantities of sludge were necessary for the Materials 2024,17, 1824 9 of 20 adsorption of mercury (Hg) (19 mg/g) [ 73 ], of cadmium (Cd) (25 mg/g) [ 74 ] and of lead (Pb) (21.75 mg/g) [75]. The sludge has also been used as a coagulant in wastewater treatment plants (WWTPs) [ 88 – 90 ]. WTS applied in the post-treatment of upflow anaerobic sludge blanket (UASB) reactor effluents provided high removal efficiencies in terms of biochemical oxygen demand (BOD) (78%), chemical oxygen demand (COD) (74%) and suspended solids (SS) (84%), suggesting that this application is a promising option [ 91 ]. Another way of using WTS in wastewater treatment plants is to use it as a co-conditioning agent for dewatering wastewater biosolids [92,93]. 3.1. Applications in the Construction Industry 3.1.1. Cement Manufacturing Among the main constituents of WTS are SiO 2 , Al 2 O 3 and Fe 2 O 3 , which are also present in Portland cement, and this increases its potential for use as an SCM [ 9 , 94 ]. Supplementary cementitious materials are made up of siliceous, aluminosiliceous or calcium aluminosiliceous powders, used as partial substitutes for clinker in cement or as partial substitutes for Portland cement in concrete mixtures [15]. Iron mud mixed with lime powder in proportions of 1:3, 1:1 and 3:1 by weight and incinerated at 1000 ◦ C for 4 h was used to produce cement. The properties of the cement produced were investigated, and the results confirmed that it could be used for masonry work in general, according to the American Society for Testing and Materials (ASTM). The best results were obtained using the mixture with a 1:1 ratio [95]. In China, alum sludge was used to replace clay in the production of clinker, and the effects of this addition on the sintering condition and cement quality were evaluated. Cement products made with this substitution met the Chinese National Standard for firstgrade Portland cement [ 96 ]. Another study, also carried out in China, proposed replacing the siliceous raw material used in cement production with WTP sludge. The results showed that all samples with a content between 4% and 10% of sludge in their composition exhibited higher strength at 3 and 7 days, compared to the control samples [97]. More recently, other studies confirmed the potential for using WTS as an SCM for cement production. After being calcined, the sludge showed characteristics equivalent to normal pozzolanic material. The use of 14% and 35% slurry calcined at 600 ◦ C met the compressive strength requirements to produce blended Portland cements, equivalent to CEM II/A-M, according to Standard EN 197-1 [76]. The chemical, physical, mineralogical and morphological characterization of the sludge was fundamental to verifying its potential for application as an SCM. 3.1.2. Mortar and Concrete Production Ruviaro et al. [ 77 ] showed it to be possible produce sustainable cement composites by replacing Portland cement with up to 20% WTP sludge, obtaining improved mechanical strength results compared to simple cement composites. The sludge used to prepare the cement paste was previously dried in an oven (105 ◦ C for 24 h) and then calcined at 700 ◦ C for 1 h. The research also confirmed that the CO 2-eq emissions associated with the production of 1 m³ of slurry with incorporated sludge decreased about 42% compared to the reference cement paste. Hemkemeier et al. [ 78 ] used WTP sludge as fine aggregate to produce repair mortar. WTS previously dried and crushed was used to replace the fine sand in the mixture, in a proportion of 3% by mass, resulting in a mixture capable of providing more protection for steel reinforcement in aggressive environments with CO 2 and Cl - , reducing the corrosion rate by around 70%. WTS was tested as a pozzolanic material to partially replace cement in concrete. Sludge calcined at different temperatures (400, 500, 600 and 700 ◦ C) was used, with a calcination time of 1 and 2 h. The study concluded that it is feasible to incorporate up to 30% mud by weight, and the most efficient calcination temperature was 600 ◦ C for 1 h. The substitution Materials 2024,17, 1824 16 of 20 Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors acknowledge the Águas do Norte S.A. Conflicts of Interest: Author Helena Lemos was employed by Águas do Norte, S.A. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. References 1. Baillie, J.; Zhang, Y.P. Space for Nature. Science 2018,361, 1051. [CrossRef] [PubMed] 2. Ahmad, T.; Ahmad, K.; Alam, M. Sludge Quantification at Water Treatment Plant and Its Management Scenario. Environ. Monit. Assess. 2017,189, 453. [CrossRef] [PubMed] 3. Ramirez, K.G.; Possan, E.; Dezen, B.G.d.S.; Colombo, M. Potential Uses of Waste Sludge in Concrete Production. 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