Valorization of undervalued aluminum-based waterworks sludge waste for the science of “The 5 Rs’ criteria”

Abstract

Alum sludge (AS) is an underrated by-product waste resulted from treating raw water through conventional water treatment plants. Water is attained from various reservoirs such as rivers and aquifers, and it may contain a wide variety of contaminants as drinking water processing systems produce “waste” sludge as a residual material that causes significant environmental issues. Hence, numerous efforts aimed to sustainable reuses of such sludge. This article illustrates the beneficial reuses of the aluminum-based sludge to close the loop between the sludge waste generation and the sustainable environment with providing past, current and updated knowledge with the introduced uses with their advantages and challenges. Alum sludge as a resource not as a “waste” for reuse facilities is considered a value-added alternative for management achieving the legislation requirements and proposing “end-of-waste” concept. 5Rs, including “Reduce, Reprocess, Reuse, Recycle and Recover,” are providing sustainable solution to fulfill present and future aspects for green environment and safe sludge disposal. In this regard, now, it is essential to focus and adopt the 5Rs criteria for the concept of circular economy that replaces the “end-of-life” principal with restoration.

Introduction

The undervalued materials that arise as a result of numerous human activities (i.e., domestic agricultural, industrial etc.) are unwanted solid waste materials. They may cause a direct deterioration to the environment, and they could be a reason for a land pollution as well as water contamination (Tony and Ali 2021). Therefore, in the modern societies, for managing such unwanted solid wastes, the 5Rs “Reduce, Reprocess, Reuse, Recycle and Recover” principle is introduced. Such criteria are applied to utilize the waste materials into support sustainable environment, minimize various environmental pollution types, diminish the consumption of unnecessary substances and promote the maximal lifetime utilization of the products. Reduce includes minimizing the unnecessary use solid waste. To add up, reduce could be attained by reclaiming the waste by repressing and reusing it for different purposes. Thus, it increases the lifetime of waste and reduces its production. Hence, this preserves the environmental resources. However, recovering the previously used waste materials and introducing a new life from it is termed as recycle. Processing and modifying the waste into another form is so-called recycling the waste. This could be applied in such a case that the waste could not possess the potential of reuse and reduce. Recover is characterized as processing the waste in a way that saves it from being destroyed. With the aim to achieve economical savings of waste disposal for materials and maintain the environment sustainably option, 5 Rs’ criterion is a suitable alternative for waste management (Zhao et al. 2021; Zhu et al. 2021). Thus, alum sludge as a solid waste is introduced to the 5Rs criteria.

Worldwide, potable water is a prerequisite as well as its quality is cardinal as a result of urbanization and booming population. Therefore, a water-related technology for the production of reliable and safe drinking water treatment plants is a must (Ashour et al. 2014; Thabet et al. 2020; Zhang et al. 2021). Therefore, utmost priority has been attained globally to receive a clean drinking water. In this regard, integrated numerous methodologies have been applied in the typical drinking water treatment plants (Thomas et al. 2016; Yu et al. 2021). Thus, different unit operations are used, in which coagulation and flocculation operations chemicals are used as primary coagulants or as coagulant aids. The result is a proportional amount of by-product residual sludge is generated that is essentially required to be subjected through a dewatering process before its final disposal in order to its thickening (Gregory and Duan 2001; Carty et al. 2002; Ashour and Tony 2017).

In some countries, such as Egypt, the disposal technique of alum sludge is the direct disposal into nearby drains that meets the water sources intake (Tony and Tayeb 2016). In other places, it could be discharged into open lands areas which is considered a simple and cost-efficient of sludge disposal technology (Vasudevan; Thabet et al. 2021). However, this is not considered ideal solutions since such techniques cause contamination of water streams and soil from the chemicals disposed off with the sludge. Hence, efficient alum sludge management in an eco-friendly way remains an issue for researchers. The answer is the 5 Rs’ “Reduce, Reprocess, Reuse, Recycle and Recover” of the sludge which could offer a sustainable management solution of such waste.

In the available literature, abundant research is cited dealing with sewage sludge utilization (Fytili and Zabaniotou 2008; Kelessidis and Stasinakis 2012) although the review studies regarding waterworks sludge utilization and reuse and applications are not many and recent (Babatunde and Zhao 2007; Dassanayake et al. 2015). Therefore, in order to expand such previous investigations and to generate a comprehensive outlines of the alum sludge production and possible options for end possibilities of reuse, the main target behind this work is to present a comprehensive review of sustainable approaches and prospective trends in waterworks sludge handling and management for its beneficial reuses. So, this study also deals with highlighting the particular focus on numerous studies that are dealing with the alum sludge applications such as land applications (Dassanayake et al. 2015), construction materials enhancement and wastewater treatment applications. The barriers stands over the widespread of such reuses will be also explored. Understanding and identification of the main gaps and issues surrounding the potential environmental impacts are included. Likewise, the bibliometric data analysis were performed through this investigation, via VOSviewer software, the key terms, title and abstract were incorporated to the software in order to create a term co-occurrence mapping and density visualization.

In the line of the 5Rs’ principle for sustainable development, alum sludge has been intensively studied in the last decades as a value-added material. On that basis, converting undervalued alum sludge waste into useful product improved alum sludge into more expansive and improved alum sludge reuse routes.

Alum sludge production and quantities

Incorporated coagulation along with filtration (Konieczny et al. 2009; Yin 2010) and sedimentation (De Sena et al. 2009; Amaral Filho et al. 2016) where the water quality is a superiority (WHO 2011) is applied in the drinking water treatment facilities. Chemical purification methodology that is signified as coagulation process is categorized to be an expensive technology since the costive chemicals applied (Teh et al. 2016; Yu et al. 2017; Sillanpää et al. 2018; Tayeb et al. 2019). In such technique, coagulant aids that is usually depends on metal-based coagulants, i.e., Al and Fe, are used to precipitate the soluble metal ions in water (schematic representation of the drinking water treatment steps is displayed in Fig. 1). But, special concerns have been gained scientists attention since massive amounts of sludge including heavy metals residuals are generated from such processes. Such sludges are categorized as potentially toxic to the aquatic environment (Tetteh and Rathilal 2019; Thabet et al., 2021a).

Fig. 1

adapted from: EPA, 2002; Gregory and Dillon, 2002; Mazari et al. 2018; Zhao et al. 2021)

Schematic representation flowchart of watercourses treatment plant and sludge stream in water works plant (

On the other hand, a technical generation of a proportional amount of by-product sludge is produced which is brought through a dewatering process. It is passed through treatment steps before the final disposal to reduce its amounts (Zhao et al. 2018; Tony and Lin 2020a). Such generated by-product is aluminum-rich-based sludge which is so-called alum sludge. Alum sludge is generated as a result of adding aluminum sulfate [A1₂(SO₄)₃.14H₂O] as a flocculating agent in the drinking water treatment process (Eq. 1) (Ahmad et al. 2016). The resultant alum sludge is a two-phase mixture of solids and water, and its water content is generally in the level between 99% (before thickening) and 95% (after thickening). Such sludges are often referred as ‘difficult-to-dewater’(Goldman and Watson 1975; Zhao and Bache 2001; Simpson et al. 2002; Zhao et al. 2018).

$$2{\text{Al}}^{3 + } \; + \;6{\text{HCO}}_{3}^{ - } \to 2{\text{Al}}\left( {{\text{OH}}} \right)_{3} \; + \;6{\text{CO}}_{2}$$

(1)

Virtually, alum sludge that is produced from drinking water treatment plants signifies up to 3% the volume of the raw water that is subjected for treatment (Hidalgo et al. 2017). However, it is difficulty to attain an exact estimation to the alum sludge produced in a region or a specific country. Also, for the European union (EU) countries, the alum sludge is categorized as Code 19’s list of waste, so it is difficult to attain a published data about the waste statistics according to the local authorities (Renou-Wilson et al. 2019). Therefore, it is unsurprisingly that it could not attain a specific on regional and consequently the global alum sludge waste data.

Annually, the generated alum sludge in numerous countries is estimated and illustrated in Fig. 2, which summarizes the data available in the two decades (Foss and O'Connell 1996; Babatunde et al. 2007; Xiong and Mahmood 2010; Hu et al. 2012; Dassanayake et al. 2015; Ahmad et al. 2016; Abd El-Razek et al. 2019).

Fig. 2

Estimated alum sludge production quantities in some selected countries

Bibliometric analysis

Recently, bibliometric analyses were emerged as a useful tool for analyzing and identifying a certain subject and mapping key aspects. The most cited items related to alum sludge and their related terms are investigated via bibliometric mapping. Therefore, to attain a state of the art outline of the alum sludge, the literature survey was performed on the Web of Science platform. The search term was “Alum sludge OR Aluminium sludge OR Waterworks sludge”. Then, the information revealed from “Web of Science Core Collection” database was extracted through the period of 2000 to August 2021.

The relatively limited articles in the last two decades attained from the Google Scholar search platform are given in Fig. 3 to illustrate a profile of the current statues of the studies conducted and published related alum sludge. Overall, articles were extracted and displayed as annually cumulative publication numbers jointly with the alum sludge application profiles. The number of research studies according to Fig. 3 is in increase which gives the opportunity of the research and development (R&D) of the alum sludge to its maximal level of reuse facilities to reach to a satisfied reasonable sustainable disposal. A brief overview of the studied research investigations represents popularity of this technology application as referred in the sub-plotting in Fig. 3. A wide range of application include wastewater treatments with varies reuse purpose comprising adsorption (Liu et al. 2021a, b), coagulation and wastewater treatment facilities. Also, alum sludge could be used as a building material in cement industry enhancement or brick making. Moreover, numerous studies as well dealt with the land applications such as the agriculture uses and as a substrate in constructed wetland (CW).

Fig. 3

Joint plotting of number of publications during the last two decades regarding alum sludge and the cumulative publication of the type of alum sludge applications (data based on Google Scholar Search on August 2021)

Web of Science platform and the VOSviewer software (version 1.6.16.0, accessed on 15 August 2021) were applied for this design purpose and to analyze the keywords of the papers. In brief, the designed attained maps are incorporating network, and overlay and density visualization mapping are designed through the software according to the general procedure incorporating: (1) downloading the search data attained from “Web of Science” platform; (2) the attained data from Web of Science are saved in the required format and inserted to VOSviewer software; (3) analysis is conducting through the required method. In the current article, the co-occurrence associated with authors’ keywords was selected to be the minimum number of occurrences. Based on the program manual, mapping was subsequently designed after adjusting the parameters in the software. Overlay visualization is one of the most common features of the VOSviewer to display density visualization over time periods. Alum sludge research is conducted that extracted from the cited research articles, and the bibliometric mapping of clusters can be identified as seen of Fig. 4. The hotspots clusters could illustrate the intensive research studies based on 1509 results obtained through data from “Web of Science” core collection from the search terms “TI is (alum sludge OR aluminium sludge OR waterworks sludge)”.

Fig. 4

Keyword density visualization of alum sludge research work in the last two decades based on “Web of Science” platform to signify the research clusters using 1509 results obtained through data from “Web of Science” core collection from the search terms “TI (alum sludge OR aluminium sludge OR waterworks sludge)”

To add up, extra data analysis was conducted via software (Fig. 5) displaying overlay visualization of the bibliometric mapping. The big dot areas signify hotspots of analysis. The size of clusters is related to the research importance and significance in the area. As seen in the legend in Fig. 5, in the 2000s, focus was placed on adsorption studies of phosphorus and other elements, while in the 2010s, adsorption mechanisms and the development of reuse strategies were the main focus. In the 2020s, reuse in wastewater treatment technology became more accepted in AS studies.

Fig. 5

Bibliometric research mapping through the last two decades based on overlay density visualization

Composition and physicochemical characterization

Recent lifestyles are requiring more water consumption per capita, so fresh water demand is in increase. Further, the urgent need for clean water is a prerequisite. In such context, drinking water treatment plants produce massive amounts of waste by-product, which is so-called waterworks sludge. Aluminum sulfate is usually used as a primary coagulant in drinking water treatment plants, the result is that aluminum ions were hydrolyzed into aluminum hydroxide that is precipitated, and the so-called alum sludge (AS) is generated (Basri et al. 2019). Insoluble water impurities and organic substances are absorbed by aluminum hydroxide through raw water processing. Typically, the solid content of the thickened sludge is about 2–4%; however, the solids could be reached to 17–23% after mechanical dewatering through centrifugation.

Alum sludge feature is subjected to the raw water quality and the chemicals grades used specially the alum purity. Consequently, the physicochemical characteristics and geotechnical properties of the AS may be widely varied. Based on the available literature cited, the physicochemical characteristics and geotechnical data range of the alum sludge are displayed in Table 1. Such data reveal a wide range in most characteristics this is mainly due to the different sources of alum sludge waste, which is based on the source of watercourse water (Babatunde et al. 2007; Ippolito et al. 2011; Ren et al. 2020). Moreover, alum sludge properties are highly characterized by the variation in the coagulant applied in the water treatment plant. The solid content in the alum sludge varies from one water treatment plant to another, as the type of the raw water treated is varies in the solid concentration and the turbidity. The solid content of the sludge from ordinary different waterworks steps are varied. The sludge outlines the sludge blanket clarifier containing 0.1 to 0.5% solid content. However, in the gravity thickening tank (within 3 to 4 months) contains 3–4% solid content. This could be increased to 4 to 6% when polymer is added. Also, if the sludge dried, the solid reached 45% (Ahmad et al. 2016; Rebosura Jr et al. 2020; Zhao et al. 2021). In contrary, the resultant sludge from the sedimentation/coagulation process has a low solids concentration which makes it difficult to dewater (Tony 2020b).

Table 1 Physicochemical characteristics and geotechnical properties of alum sludge*

Assessment of alum sludge toxicity

According to the available literature cited, there is insufficient information regarding the potential toxicity of drinking water plants residual sludge. Also, the available information is limited and supplies some conflict. Hence, critical research studies assess alum sludge toxicity from several decades. Aluminum is very toxic, both acute and chronic terms, to aquatic habitat in water, especially in low pH levels. This paper provides a description of ecological risk assessment that is based on the framework recommended by United States Environmental Protection Agency (USEPA), which includes problem formulation, characterization of exposure and its effects and risk characterization.

George et al. (1995) stated the alum sludges discharged from the several waterworks plants in North America into the waters might affect algal growth. In contrary, Skene et al. (1995) revealed that the Al-toxic effect is not evident since alum sludges were applied as growth media. Likewise, other study by Sotero-Santos et al. (2005) did not report an exact toxicity evidence of alum sludge on short-term exposure; however, low toxicity is attained through the long-term contact, while the wide range of sludge characteristics is not related to the degree of toxicity. Further research was conducted by Dayton and Basta (2001) concluded that heavy metal levels in the waste alum sludge are less than the regulatory limits. Similar data are highlighted by Elliot and Dempsey (1991). However, Förstner and Haase (1998) investigation reported that the metals from alum sludge are a pH-dependent process. According to the ecological risk assessment provided by United States Environmental Protection Agency (USEPA), aluminum is very toxic to the aquatic habitats in ecological system (Mortula et al. 2009). Wang and He 2010 reported that aluminum concentration in the Zaohe River in USA and Canada and could do harm to human health.

Hence, lack of reasonable qualitative and quantitative assessment available and the magnitude of the potential toxicity effect of alum sludge is alarming to do more research to investigate a rational data.

Evaluating the sludge dewaterability

CST: Capillary suction time, CST, and specific resistance for filtration, SRF, are the most known methods to evaluate the sludge dewaterability as quantitative indexes for the evaluation of the dewatering performance. CST test has been used since the 1970's as a reliable, inexpensive, easy and a rapid method for characterizing filterability and dewaterability of a given sludge; hence, it is considered a pragmatic index. CST stands for the time required for sludge to complete its filtration (Yin et al. 2004; Scholz 2005). Although the CST is a good index for the solid concentration of the sludge, it cannot estimate directly the bound water for the sludge (Yin et al. 2004). However, CST can be used as an index for evaluating of the average SRF of the evaluated sludge (Ma et al. 2007). Investigators results reported that alum sludge from the water works is slightly more difficult to dewater than some wastewater sludge since waterworks sludge ranged CST value ranged from 57.3 to 84 s (Zhao and Bache 2001; Zhao 2004); however, biological sludge showed about 30 s for CST (Mikkelsen 2001).

SRF: Also, specific resentence for filtration, SRF, is a sludge dewaterability index through evaluating the extent of water yield via filtration procedure. SRF is a useful method to evaluate the dewaterability of the sludge and to optimize the coagulant dose. SRF is established on the relation between sludge viscosity and the decrease of pressure over a certain distance. The standard SRF test was performed using simple lab test technique. Generally, the sludge type governs the SRF values since numerous studies recorded various sludge values according to the sludge type used. For instance, Zhao and Bache, (2001) represented that the raw alum sludge from the drinking water treatment plant ranges from 63.2 to 133.4 × 10¹² m/kg. Ma et al., (2007) recorded it as 7.3 × 10¹² m/kg. Lu et al. 2001 reported that the original activated sludge is 1.17 × 10¹³ m/kg, while (Buyukkamaci 2004) Buyukkamaci, (2004) reported 9.162 × 10¹⁰ m/kg is the SRF value of the sludge used through such study.

Christensen et al. (1993) correlated a relation between CST and SRF (Eq. 2):

$${\text{CST}} = {\text{SRF}} \times c_{1} \times \mu + w \times c_{2} \times \mu$$

(2)

where c₁ and c₂ are coefficients related to CST, ${\upmu }$ is the viscosity of the filtrate, and w is the solid content in unit volume of the filtrate.

Although CST and SRF are the most widely reliable used techniques to evaluate sludge dewaterability, there are other methods available for measuring sludge dewaterability.

DS: dry solids (DS) content (Pan et al. 2004; Dewil et al. 2005): DS technology evaluates and measures the DS content in the sludge cake after evaporation (at 105 °C).

Bound Water: Generally, the aqueous phase in the sludge is classified into two main categories, free water and bound water, whereas according to (Yin et al. 2004), the water in sludge is classified according to their bonding to the solids in the sludge. The classification falls into free water and bound water (Yin et al. 2004). Free water is not attached to sludge solids and could be separated by simple gravitational settling or mechanical dewatering. However, bound water can be broken down into: (i) interstitial water, water that is trapped inside the floc structure by capillary forces or within a cell; (ii) surface water, water held onto the surface of solid particles by adsorption and adhesive forces; and (iii) chemically bound water (intracellular water): water that is chemically bound to the particles and it cannot be released easily by mechanical means; and it can be released only by applying heat to help in the thermal chemical destruction of the particles. Centrifugation method, dilatometric measurement and differential scanning calorimetry could measure the bound water concentration in sludge (Yin et al. 2004).

Zeta Potential: this is a physical measure technique of the charge on a colloidal particle and presents the surface characters of sludge (Yin et al. 2004). Such measurement is dependent on the repulsive force between sludge particles and the distance over the particles that may repel each other and thus prevent coagulation. The high positive or negative values of zeta potential mean that the particles are stable and hard to coagulate. In contrary, the low positive or negative zeta potential means that the particles are unstable and easy to coagulate (Yin et al. 2004). − 17.44 to − 37.4 mV is reported to be the possible zeta potential of the alum sludge (Guan et al. 2005).

Sludge conditioning/dewatering for disposal practices

The whole sludge management trained is commonly comprising of a series of generally divided into successive five treatment steps. Such successive steps, namely thickening, stabilization, conditioning/dewatering and the final disposal or reuse facilities. Among them, water is removed from sludge solids through thickening and dewatering procedures in which the volume of water is deduced and hence the sludge volume is reduced to be ready for treatment in the further processes. Conditioning is subjected for the object of improving the sludge dewaterability. Conditioning could be achieved through chemical aids such as flocculants, acid, ferric chloride and lime addition or through physical disruption. Finally, before the sludge disposal it is subjected to mechanical dewatering in which press filter, centrifuges or dryers are applied to reduce the water content in the final sludge to about 80% including a range of (20–25%) of dry solids, DS in the dewatered sludge cake (Yin et al. 2004).

Conditioning and dewatering techniques gained scientists and researchers great attention due to the challenges of the massive sludge produced. Extensive research has been conducted that are dealing with sludge conditioning and dewatering to satisfy the more stringent rules and restrictions appeared by the authorities and legitimations. Thus, reasonable conditioning and dewatering performances to attain higher sludge disposal and reuse facilities in order to reach to a minimal environmental impacts are ever-increasing (Tony et al. 2008; Zhou et al. 2014).

Numerous methodologies incorporating both physical, i.e., heat, freezing and mechanical treatments, and/or chemical treatment amenities are extensively applied to such sludge handling and conditioning to improve its dewaterability. However, chemical treatments still more pronounced effect and involve energy-saving advantages which include adding the flocculating agents, acids and alkaline (Zhou et al. 2014). In the recent decades, advanced oxidization conditioning technologies comprising Fenton’s reagent have been applied as a non-polymer conditioning techniques. Not only energy-saving of the chemical conditioning in compassion to the physical conditioning its merits, but also, the oxidization processes are potentially removing recalcitrant compounds in such sludge. Thus, the final sludge disposal to the environment is more environmentally benign (Ma et al. 2007).

Chemical conditioners that are based on advanced oxidation processes are gaining a great scientist’ attention. Fenton’s oxidation is a promising superior dewatering technique possessing numerous advantages (Tony et al. 2008). Although the Fenton oxidation is a superior conditioning process compared to the other chemical conditioners, a few research cited are dealing with such investigation (Zhao et al. 2009). Therefore, there are still significant research gaps at present and more research is required to examine the relationship between the needed sludge oxidants and dewaterability and optimization of the operational parameters.

The data tabulated in Table 2 reveal that the scattered limited studies dealt with alum sludge collected from different regions of water treatment plants (WTP) as a by-product residual from waterworks. Sludge dewatering showed that polymer dewatering displayed higher treatment efficiencies than other conditioners. However, it is noteworthy to mention that, the toxicity related to polymers stands as a problem. Therefore, scientists are dealing with searching for alternatives. Generally, low toxicity is attained for the normally applied anionic and non-ionic polymers. But, cationic polymers types found to be more hazards to the eco-system and aquatic organisms. Hence, there are strict limits on their use in the drinking water treatment plants for preventing their existence over the permissible limits to stop the environmental damage. Notably, monomers used in the polymer production showed toxicity more than the polymer itself; nonetheless, severe quantities on their levels are sustained, especially with regard to acrylamide. In this regard of rigorous regulations on the polymer uses, polymer toxicity could not seem to be a big problem (Bolto and Gregory 2007).

Table 2 Summary of relevant studies on alum sludge conditioning via various chemical conditioners capabilities

Additionally, it is significant to mention that not only the environmental aspects are stands behind searching for polymer alternatives, but also the economic prospective. Even the most cost-efficient polymer types are not considered from the industrial potential of a lowest cost (Tony et al. 2008). Notably, it is recorded by Ghebremichael and Hultm (2004), 65% of dewatering is achieved through SRF and CST reduction recording via using Moringa oleifera (MO). It is a significant research which concluded that MO could be effectively used and replaced alum or polyelectrolytes for chemical conditioning of water works sludge to prepare it for dewatering. It is recommended as a green alternative option. Moreover, the study introduced by Pan et al. (2004) explored that minimal effect could be attained when surfactant is applied as conditioner in comparison to polymer; however, no effective distinguish is achieved with types of the surfactant charge.

Moreover, it is significant to illustrate that dual polymer conditioning does not demonstrate a considerable improvement and further reduction in the sludge dewaterability and filterability assessed via CST and SRF comparing it with the solo polymer conditioning as seen the results in Table 2 by Ma et al. (2007). Hence, considering dual conditioning achievement should notably consider both cost and administration, which is not a recommended strategy for the real industrial scale applications.

As a further bonus, the oxidation conditioners (i.e., Fenton reaction) are less aggressive to the environment since their reactions are environmental benign than organic polymers, resulting in a green and sustainable ecosystem. Although alum sludge conditioning by advanced oxidation process specially Fenton’s oxidation is not superior as polymer conditioning for improvement of sludge dewaterability, its environmentally benign compared to polymeric addition should be considered. Moreover, till now, most of the researches conducted are mainly focused on the application of classical Fenton peroxidation reaction according to the data displayed in Table 2. Additionally, only limited pilot-scale tests had been introduced. Hence, data are still lacking for large-scale applications results, in which optimization and pilot-scale tests are required. Also, there is limited research conducted on alternative oxidization processes such as different iron sources or other transition metals oxidization process and ozonation process. Furthermore, basic mechanisms of alternative advanced oxidation processes should be clarified and studied in more detailed. The influence of more effective or enhancing the catalytic oxidation conditioners performance still requires more study. Moreover, detailed research is needed dealing with the mechanism on their chemical performance that has as yet only been investigated superficially.

Sustainable reuse of waterworks sludge residues

Research into a sustainable disposal opportunity is the scientists and researchers job to reach a green ecosystem. Hence, alternative waterworks sludge disposal option to maximize their value and improve their benefits and reach these massive amounts from underrated material to a value added product is crucial research work to satisfy the restrictions and limitations. In this regard, alum sludge is practiced to numerous applications according to the following trails: (1) building material; (2) agriculture applications; (3) adsorbent material; (4) coagulant material; (5) co-conditioner and (6) constructed wetland applications. Figure 6 summarizes varies beneficial reusing and reprocessing applications of AS for sustainable endpoint solution.

Fig. 6

Summary of the sustainable AS applications

Building materials

Soil stabilization before road infrastructure

Ever though the aforementioned difficulties associated with alum sludge handling as a waterworks residue, just limited research articles have tried to valorize it as a building and construction substance. However, utilization of alum sludge residues as a building material in the large scale cannot only work as raw material for soil stabilizer, brick manufacturing and cement production, but also could help in its associated waste management problems. For infrastructure and construction, soil stabilization is a vital stage for development.

Traditionally, cement is the most comment material used for such job as an additive in a ration of generally used of 12% (Owaid et al. 2014). However, searching for a sustainable environment and waste management opportunities is the scientists continued role. Since the pozzolanic properties of alum are suitable to be a builder material, it could be used as a soil stabilizer as cement or lime (Lin et al. 2005; Bray 2014). In addition, its production is in bulk amounts from the waterworks plants introduces them for such opportunity. Thus, recent literature (Aamir et al. 2019; Henry et al. 2020) is cited dealing with the application of alum sludge as the waterworks residue for soil stabilization as a novel technology. Such application could attain environmentally benign, cost-efficient and sustainable stabilization chance. The data recorded by Aamir et al. (2019) showed an enhancement and improvement in the soil strength–bearing ratio from 6.53 to 16.86% with an optimal alum sludge addition of 8%. However, more research is still required since there is a lack in the literature in such studies. Hence, in the future to studies, it is required to illustrate compaction properties and energy needed to improve soil strength with the merely utilization of the alum sludge waste material.

Cement-based materials

Alum sludge, generally comprised of clay minerals, could be dehydroxylated if heated in the range of 700–850 °C (Tony 2020b). Moreover, the raw waterworks sludge is primarily aluminum-based material with an amorphous aluminum hydroxide precipitate with minimal quantities of crystalline quartz and montmorillonite (Shamaki et al. 2021). The resultant materials are a reactive alumina and silica. Therefore, such materials enhance the pozzolanic reactions when it is supplied to cement improving it characteristics that are related to strength and durability (Fernandez et al. 2011). Thus, calcined alum sludge samples might be signified as a pozzolan since the temperature of calcination has a major influence on the pozzolanic activity (Shamaki et al. 2021). Thus, in cement industry such sludge comprised in past a resultant alite hydration is enhanced since the undersulfated C₃A reactions are prevented; hence, the cement performance is superior.

Sludge recycle is an incinerator in that the cement kiln as a fuel in cement manufacturing is not the only merit and practical alternative, but also it could be as a raw material for cement (Pan et al. 2004). Rodríguez et al. (2010) studied the replacement of 30% of cement with dried alum sludge resulting in a promising reduction in the compressive strength reached 70% in 28 days. Moreover, they reported in their study that dehydration is deduced that is affecting also the setting times of standard mortars. Pan et al. (2004) replaced the clay in cement with clay from fresh alum sludge, which revealed reasonable setting times and the addition did not alter the f-CaO content of the cement. The compressive strength increased with the alum sludge increase and met the standard of the Chinese National Standard of first degree Portland cement. The ternary blends from silica fume, ground-granulated blast furnace slag besides palm oil fuel ash were applied by Owaid and co-workers (2014) with the calcined water works alum sludge at 800 °C. Higher compressive strength was recorded. To add up, Gastaldini et al. (2015) investigated the optimal alum sludge addition in cement industry and their investigation is mainly focused on the optimal calcination temperature which is revealed at 700 °C for one hour of calcination time. In Shamaki and his group (2021) the UK waterworks alum sludge could be utilized with a performance improvement through the addition of gypsum and limestone powder which accelerator effect of calcined sludge dehydration on cement specimen. Thus, the potential use of alum sludge in cement industry has a clear economic driver. Its use as a building material reduces the cost of production.

Brick manufacturing

To meet the bricks demand as building material, clay could be substituted by alum sludge in brick manufacturing. Water works sludge residues based on aluminum waste are mature enough that could be used as a partial replacement of clay in clay brick manufacturing.

Several researchers in many countries used alum sludge in different types of brick making and the compressive strength and shrinkage of the new made brick are explored. For instance, Patricia et al. used waterworks sludge to prepare ceramic brick (Li et al. 2018). Elangovan and Subramanian (2011) introduced the alum sludge with the commercial local clay that is blended in various quantities and sintered at different calcined temperatures to produce clay sludge brick. Also, Tony and Ashour (2014) and Zhao et al. (2016) revealed alum sludge a suitable candidate in clay brick making. Other researchers used water treatment residues as a clay replacement and colorant in facing bricks manufacturing as both colorant material and clay replacement (Andrew and Evaggelia 2007).

Thus, it is supposed from numerous studies that varies types of bricks can be introduced from different countries of studies that meets the regulations of each country by controlling the quantities of incorporated and the sintering temperature. Such research introduces underrated dumped waste alum sludge material in those cases that offer a favorable future for a cost-efficient, economic and environmentally sustainable green options as a promising building materials additive. However, it is notably that the research conducted in using the underrated aluminum-based sludge by-product in construction materials is limited in comparison to other potential applications. This may be related to its organic and water content which makes its chemical composition variable. However, as aforementioned some supplement and treatments to such sludge reveal a suitable geotechnical characteristics and thus geo-environmental applications due to the alteration on its shear strength, mechanical stability and permeability.

Table 3 summarizes the various application of alum sludge (AS) as a building material amendment reported in the previous literature cited. Multiple trials have explored incorporating AS into clay brick (Pan et al. 2004; Ramadan et al. 2008; Santos et al. 2015; Zhao et al. 2016). Initial investigations revealed numerous additions into the brick as well as “AS” such as a combined addition of explored an enhancement in the attained clay brick incinerated sewage sludge ash (Anderson et al. 2003), excavation waste soil (Huang et al. 2005), silica fume and rice husk ash (Hegazy et al. 2012). AS added to Portland cement (Shamsudin et al. 2017) to produce ecological cement brick. Moreover, incorporated limestone (Tony and Ashour 2014) or sand and soil (Shamsudin et al. 2017) with AS into cement to generate sustainable cement brick showed good compressive strength results.

Table 3 Concluding research studies on alum sludge application as a construction material*

Preliminary results (displayed in Table 3) of research studies reported the suggested the optimum addition of sludge varied from 5 to 75% to comply the regulations and standard test limits including those in the places of study in UK, India, Egypt, Malaysia, Portugal, Taiwan and Ireland. Investigated outcomes and experimental results from those studies could facilitate the rollout of full-scale factory trials for producing a sustainable brick from AS waste to introduce a green environment opportunity.

It is noteworthy to mention in order to manufacture bricks containing higher amounts of alum sludge, more energy is required. More sintering temperature of the bricks is needed, and the sintering temperature is increased must be increased to ~ 1050–1100 °C. This could be due to the lower silica content and higher water content of alum sludge, which makes the need for more heat. Thus, more intensive energy is required and the process in such case became less economic environmentally appealing (Turner et al. 2019).

Agricultural applications

The applicability of the waterworks sludge containing aluminum is previously applied to the agricultural soils for fertilizing purposes (Dassanayake et al. 2015). Although, it could be applied as a fertilizer in agriculture, the sludge and soil to fertilize must meet some necessities. The aluminum presence in alum sludge could be a source of toxic aluminum and contaminate the environment.

As demonstrated by previous research cited, aluminum and other heavy metals in the sludge could pose a danger to the environment. However, some other researches (Kluczka et al. 2012); (Central Statistical Office Bochenek 2014; Kluczka et al. 2017) contradict this and concluded the soil are not contaminated with heavy metals. Thus, this gets an optimistic vision through the problem and more research could be done for the use of alum sludge in agriculture purposes.

By applying such sludge for agricultural purposes, this introduces the phytotoxic aluminum material into the soil. Such properties are well recognized as harmful to the trees (Ulrich et al. 1980). Other investigator (Smoliński et al. 2009; Yang et al. 2012; Kluczka et al. 2017) reported that the ever increase in aluminum concentrations in soil is signified as one of the main reasons of forests death in North America and Europe. Aluminum not only affects the trees, but also it does cause a certain damage to the crops such as tomatoes, lettuce and beetroot (Brunner and Sperisen 2013). It noteworthy to mention that the elevation in soil acidity notably raises aluminum and heavy metals mobility in the soil (Kluczka et al. 2012). Therefore, the regulations limited the sludge use in the soil to not more than the pH 5.6 (Kluczka et al. 2017). However, with the ever increasing of the environmental pollution, situation and widespread of the acid rain in the industrial locations recommend monitoring of the bioavailable Al-content is essential (Yang et al. 2012; Kluczka et al. 2017). The summarized results of applying alum sludge as a soil amendment are displayed in Table 4. Dual valuable application of AS into agriculture ameliorant is improving crop production as well as it is an effective sludge management technique. AS can improve soil structure, hydraulic conductivity and nutrient levels. This is related to the presence of organic matter and nutrients in alum sludge. Consequently, alum sludge can be used as a safe soil amendment material to manage and improve soil. However, aluminum toxicity which is present in alum sludge still requires further research to accept AS application in agriculture sector.

Table 4 Summarized results of alum sludge application in agriculture sector*

Adsorption of pollutants

Due to the chemical nature and amorphous structure of the AS from waterworks residuals impart it with a porous and high surface area material, all of such characteristics introduce it to be a superior adsorbent material (Ippolito et al. 2011; Babatunde and Zhao 2007; Ashour and Tony 2020; Li et al. 2021). Aluminum-based metals could be applied in wastewater treatment sector for numerous contaminants elimination. Various studies (Turner et al. 2019; Tony 2020b) have been investigated the application of alum sludge as a low-cost adsorbent material for contaminants removal from aqueous media. Such contaminants including boron, fluorides, pesticides, perchlorate, glyphosate, dyes and lead are listed in Table 5. However, detailed and more published studies are related to phosphorus removal from various wastewater effluents and types, their equilibrium isotherm, kinetic analysis and the effect of system parameters have been studied. The presence of amorphous aluminum nature in the Al-based sludge maximizes its affinity to adsorb phosphorous anions from contaminated streams (Babatunde and Zhao 2007; Tony 2021a). AS sorption capabilities are related to the particle size, surface area and surface charge of the AS particles.

Table 5 Adsorption capacities of numerous uptakes removal from wastewater using AS adsorbents

Research studies conducted the fully characterization of alum sludge via a complete textural characterization and surface chemistry assessment to signify the surface functional groups and to conduct the adsorption kinetics and equilibrium to explain the adsorption mechanisms (Zhao et al. 2021). The entire adsorption mechanisms include surface precipitation, ligand exchange, electrostatic attraction and ion exchange. It should be noted that although a chemical reaction between pollutants and dissolved aluminum has been demonstrated, it is assumed that the chemical reaction plays only a marginal role in the contaminants removal process (Turner et al. 2019; Liu et al. 2021a, b). The adsorption system could be categorized as physisorption or chemisorption process. In physisorption adsorption, the interaction exists between the solid surface and the adsorbed species has a physical nature in which the interaction forces are depending on the weak van der Waal interaction forces, thus making the process irreversible and requiring low energy barrier. However, chemisorption is signified by the formation of chemical bonds between adsorbed molecules and the solid adsorbent surface via electron transfer or pairing. In contrary to the physisorption, chemisorption requires high activation energies and always exists as a monolayer (Tony 2021a, b).

Table 5 summarizes the previous cited studies conducted in the literature using AS as a water works residual material in its nature or modified/treated form for eliminating numerous elements and compounds. As tabulated in the table, the waterworks residuals based on Al-sludge waste could be used and applied for eliminating a wide range of pollutants from wastewater. AS showed a superior effect in phosphorous uptake with high adsorption capabilities is suggested (Gibbons et al. 2009; Gibbons and Gagnon 2010; Wu et al. 2019; Zhao et al. 2021). Their tests with synthetic phosphorous solutions explored that an adsorption capabilities differ according to the operating variables and the initial phosphorous concentration. Additionally, pH has been discovered to have a significant role on the adsorption uptake. The isotherm time could be reached after 24 h (Wu et al. 2019) or 12 days (Gibbons et al. 2009; Gibbons and Gagnon 2011). It could be also applied to remove arsenics from wastewater effluents in a process which is characterized by an increase in the adsorption rate with the increase in the isotherm time which could be reached to 2 days (Makris et al. 2004) at a pH ranged from 3 to 7 according to the type of aqueous effluent (Nagar et al. 2010) (Makris et al. 2009; Gibbons and Gagnon 2010; Nagar et al. 2010). Potentially toxic dye adsorption by AS is well likewise documented in the literature (Tony 2020a, b) for a thermally treated AS samples worked at neutral pH for Procion Blue textile loaded effluents. Scattered studies also showed various pollutants removal from aqueous stream, phenol removal using treated AS has also been investigated (Tony 2019) and Cheng et al. 2016 studied ammonium removal.

Hence, such level of achieved treatment validates the great potential of applying alum sludge (AS) and its environmental benefits, which might be attained through its application especially in developing countries where massive amounts of wastewater from various industries are produced which required to be treated in limited treatment options (Tony and Lin 2021). Such limitations are always related to technical access and cost issues. Otherwise, this AS application is limited to be applied in treating hydrophilic substances. Moreover, the other limitation to AS practical use is the final use of AS after adsorption where they are unsuitable for reuse options and thus need to be landfilled (Wu et al. 2019; Nagar et al. 2010; Turner et al. 2019). It is also noteworthy to mention that the feasibility, cost implications and long-term performances of transforming AS to nanosized particles through milling AS for proper applications could be a reliable option to be a valuable commercially adsorbent for practical applications and reuse facilities (Turner et al. 2019).

Coagulant

Commonly, aluminum coagulants are widely applicable as appropriate coagulant materials to eliminate colloids and phosphorus combinations from water and wastewater systems. In this regard, the most widely applied aluminum coagulant derivatives are aluminum sulfate, sodium tetrahydroxyaluminate, aluminum chloride and pre-hydrolyzed aluminum (Tony 2021a, b). While they are not cost-efficient in comparison to ferric coagulants, they are widely applied exclusively in drinking water treatment plants since they have no effect on water quality; thus, this gives its publicity. According to the literature cited, there are published data investigating the impact of the composition and structure of such coagulants to residual aluminum content.

However, such work has only referred to the total Al content and research studies do not introduce the analysis of aluminum fractions (Wolborska et al. 1999; Gregory and Duan 2001; Rak and Świderska-Bróż, 2001; Yang et al. 2012; Tony and Lin 2020b). In 2010, the regulation of the Minister of Health of Poland regularized 0.2 mg/L as the limited total Al concentration that could be present in the drinking water. Few research studies related to the Al coagulant with its toxic forms could effect in agricultural applications and may cause a danger to soil or not. Results revealed that a larger solubility of aluminum could be achieved when acidification of alum sludge-treated soils to an unamend control soil. Not only acidification could pose a danger, but also, phytotoxicity of soil is related to the pH of the soil (Codling 2008). However, another results found that a negative effect is appeared to the soil plants as a result of alkalinity (Brautigan et al. 2012). For this purpose, the elevation of alum sludge applicability as a coagulant material requires more studies to examine the post-coagulation of the sludge collected from the water treatment plants. Although such use of alum sludge as a coagulant material make it an ideal reuse option, further study is needed. More data are required for full-scale possibilities. However, the viable is still limited, and its future potential use of technology advance can be streamlined.

The combination of alum sludge with ultrafiltration membrane processes has been explored as a promising approach for tertiary wastewater treatment in industrial applications reuses or as a prior treatment technology before the reverse osmosis membrane (Mazari et al. 2018).

Co-conditioner

Nowadays, a considerable attention is gained toward transforming AS waste into a valuable material, rather than an underrated matter for disposal. In this concept, research academia and engineers attain a superior overwhelming research in the last years since AS is most majority by product from water works plants. Though, actually, limited studies have revealed the use of “AS” as a co-conditioner for sewage sludge dewatering. For example, Lai and Liu (2004) reported that AS acted as a skeleton builder for mixed sludge samples and hence enhanced sludge dewaterability. Yang et al. (2007) and Yang et al. (2009) reported AS application role in condition for improving sewerage sludge dewaterability. Their results demonstrated that polymer dose could be minimized from 120 to 15 mg/L for mixed sludge and thus showed a cost efficient technique. Li et al. (2016) investigated the alum sludge as a drinking water treatment residual which containing polyaluminum chloride and large amount inorganic matters. Chemical conditioner and physical conditioner for improved the sewage sludge dewatering (Tony 2020a). Their results explored that the supplement of AS could reduce the polyacrylamide dosage addition and reduced the moisture content of sewage sludge Ren et al. (2020) demonstrated that alum sludge and sewage sludge mixing in a ratio of 1:1 showed a beneficial AS use as a co-conditioner of sewage sludge and resulted in a good and dewatering results. Realistic economics application of alum sludge co-conditioning opportunity could achieve sustainable reuse development as a conditioner from a waste.

Constructed Wetlands, reed beds and filter beds applications for wastewater treatment

Worldwide, considerable attention has been received in according to the literature published regarding the application of the dewatered AS as a reactive media in constructed wetlands or filter beds. AS is used in the form of AS-cake as the main substrate in constructed wetlands, CW for end of waste lifespan as shown in Fig. 7 (Zhao et al. 2011).

Fig. 7

source to use in constructed wetlands facilities

Lifespan of alum sludge from

Indeed, substrate is an essential constituent in CW; therefore, the best substrate is chosen that possesses multi-functions, i.e., (i) supporter of biofilm formation; (ii) plants growth media; and (iii) adsorbent for pollutants. From the practical point of view, alum sludge is drinking water treatment plants by-product; hence, it is non-toxic not harmful substance in most cases (Zhao et al. 2013; Turner et al. 2019). Further crucially, aluminum is the predominant constituent in alum sludge which posses’ high adsorption affinity to adsorb phosphorus from polluted aqueous streams (Babatunde et al. 2007; Tony 2020b). Hence, aluminum-based sludge cake, as dewatered drinking water treatment plants residuals own a good potential for reuse as raw environmental engineered material as the main substrate in CW for double benefits of reed beds and wastewater treatment.

Extensive studies were explored to assess the capacity and tendency of alum sludge as a low-cost adsorbent material for dyes (Tony 2020a, b), phosphorous (Babatunde et al. 2007; Wu et al. 2019), arsenic (Nagar et al. 2010) and some of heavy metals adsorption (Zhou and Haynes 2011; Castaldi et al. 2014; Silvetti et al. 2015). Hence, it is introduced as a filter bed or constructed wetland medium (Fig. 7). More research is dealing with its advances only in the last decades, and a focus on the research and development is conducted (Babatunde et al. 2007; Zhao et al. 2008; Hu et al. 2012). The major concern that could face the constructed wetland medium over the long-term applications is clogging. Some studies reported that this issue could be partially alleviated by using anti-sized gravel bed ranged from small at the base of the bed and larger at the top (Zhao et al. 2015). However, Oliver et al. (2011) reported low risk to human health could be attained from organic matter release under anaerobic conditions. Not only using AS as a CW is the solo benefit, but also the final sludge cake after the working lifespan end life could Al and P be recovered. Simple precipitation through pH adjustment is achieved by (Zhao et al. 2013) resulted in 97% and 99% for P and Al, respectively. Hence, practical use of Al-sludge as CW substrate is promising; although the presence of Al for instance still needs further research. Moreover, in the long-term life cycle the clogging issues may still require further development for higher performance yield.

Here, it is important to signify that excellent pollutant removal efficiencies have been cited and well accepted by global constructed wetland research studies. Use of dewatered alum sludge as the main constructed wetland substrate to expand its scope for enhanced wastewater treatment especially phosphorous removal displayed a beneficial reuse of alum sludge. Developing new materials as substrates in constructed wetland with high adsorption ability and capacity of P or other contaminants is the attaining a priority in wetland advancement.

Al-recovery from AS, impacts and concerns

Viable alternative for end-of-life AS waste is another additional profit besides AS use to reduce the waste produced as well as limiting its final disposal cost. The safe and sustainable alum sludge utilization option in various applications, aluminum recovery could be the reply. Aluminum presence in AS stands is an obstacle from widening its applications. Another potential environmental concern is the probability of elevated level of Al to release to the environment and ecosystem.

Hence, once aluminum is recovered from such sludge, commercial potential of alum sludge utilization in different sectors could be further increased. Al present in AS is about less than 40% by weight. Scattered studies have been previously investigated to remove and recover aluminum from Al-sludge (Dayton and Basta 2001; Vaezi and Batebi 2001; Stendahl et al. 2006).

Numerous progressive procedures are employed to recover aluminum from the aluminum based drinking water residue for converting this environmental issue into a sustainable and economically comprehensive solution. For instance, some researchers found that Al can be recovered via acid treatments through the pH ranged 1.0–3.0 could acquire Al recovery could achieve Al recovery which could reach to 90% (Abdo et al. 1993; Vaezi and Batebi 2001). Otherwise other researchers reported alkaline treatments at the pH around 11 sodium hydroxide and calcium hydroxide gave a reasonable recovery (Masschelein et al. 1985). Furthermore, membrane separation technology (Sengupta and Shi 1992) and liquid ion exchange methodologies (Cornwell 1982) were also recorded in literature for Al recovery from alum sludge.

Recovery of Al from alum sludge for the option of recycle, reprocess and reuse in order to reduce waste still needs further work data for the full-scale possibilities of coagulant reuse. It is noteworthy to mention that, presently, potential future technologies are required for recovery technologies (Keeley et al. 2014). Till now, according to the current arrangement of recovery data, alum sludge could be recovered and applied in wastewater treatment in less severe regulations regions. No doubt, alum recovering from water treatment residues is minimizing the environmental impacts that could be caused by Al-sludge and also minimize the cost through purchasing alum since the recovered alum may be applied for numerous times (Sanga et al. 2018).

Merits/demerits of sustainable reuse opportunities of 5Rs

Despite the multiple options of alum sludge utilization, still there are unexplored concerns and potential limitations. For instance, in constructed wetland applications, even the disposal route is viable still the lack of the anti-clogging techniques is lack in the literature. The adsorption capabilities of alum sludge are also associated with the pH of the medium. The applicability of AS in soil is also related to the nature of soil, which requires a careful evaluation. Overall, there is still a disconnection between the business enterprise regarding AS applications and the research conducted and academia. Such a lack in connections could be answered through communication of policy makers and scientific support. Otherwise, disposal route such as landfilling of the sludge will be the routine option. Moreover, it is noteworthy to mention that chemical and physical nature of AS differs from one drinking water works plant to another in different countries and regions; thus, further investigation is required before the specific application for definitive conclusion. The merits, demerits and limitations of the diverse alternative AS disposal choices assessed in this article are summarized in Table 6.

Table 6 Summarized disposal AS alternative of reusing options with their associated merits and impacts*

Through the 5 Rs’ criteria, significant chemical investments and savings are achieved as well as the sludge volume is deduced. Resource recovery and reuse enhance the treatment efficiency and attain a sustainable environment. But, close distance should be the alum sludge application and handling place and the waterworks plants that produce alum sludge as a sustainable option in order to overcome shipping distances and charges. Some conservation of natural resources could be attained through the utilization of such AS in various applications such as the use in various civil engineering applications in construction industry as a raw material for brick making, concrete and soil amendment. Moreover, heavy metals recovery from sludge and reuse it as coagulant materials or reuse the AS sludge as co-conditioner are also saving the natural resources and sustain our planet. Additionally, agriculture and land sector application of AS is a simple and inexpensive opportunity as a low-grade fertilizer or to attain excess nutrients. Wastewater treatment sector showed diverse pollutants remediation via various AS from different WTPs in different regions.

Generally, controlled application of waterworks residuals “AS” in these abovementioned varies application and feasibly many extra unexplored techniques would valorize alum sludge from “underrated” into “value added” material, in order to maximizing benefits with minimal impact in tandem with the theme of ecological environment.

Concluding glance of development, barriers and future perspectives

More economic alternative for the high disposal costs especially of sludge landfilling is that AS sludge beneficial uses are discussed. Happily, as noted in the above-mentioned applications AS showed various commercial and applications in different regions in the world including UK, Ireland, Europe, the USA, Middle East, Asia and Australia. Particular relevance is given to the reuse in soil applications, building materials, constructed wetland and wastewater treatment. However, still further research is required especially for the field of Al recovery and reuse of alum sludge as a co-conditioner. Cost estimations of alum sludge handling from waterworks plants to the numerous applications require a study and regional regulations tend to the increase of heavy metals recovery options for reuse in agricultural, ecological and industrial purposes.

An updated database in most countries especially in the developing regions is required on quantity and quality of alum sludge by-product to conclude a significant assembly in the present and future AS disposal budgets and controls. Moreover, resource recovery such as heavy metals from alum sludge is considered chemical savings and reduction in sludge volume, but still suffers from operational problems that should be overcome. Further research is still required to prevail long-term constructed wetland drawbacks from any toxic release and long life cycle of anti-clogging systems. But, for now particular current situation and the probable future trend, there are required attempts for global beneficial reuses of drinking water residues with a critical evaluation for its negative issues from such applications and reuses. Moreover, with increasing attention worldwide to the sustainable and green ecosystem, arising reuse opportunities may be explored.

It is notable that the common use of alum sludge is introduced in adsorption in a powder form after dried and used in grounded and sieved processes. Thus, such powder form of alum sludge stands in the widening of the applications. Moreover, it hinders alum sludge to be attractive adsorbent material since it is difficult to recover after the adsorption process (Zhao et al. 2021). Therefore, in the line of converting alum sludge from a waste into a useful material it is important to granulate it which needs further study.

It is noteworthy to mention that although various reuse applications are present, more work is still needed in the future since the alum sludge material is undervalued material since landfilling is remaining the general final disposal route. Furthermore, regulations concerning the final discharging of alum sludge and beneficial reuse should be updated. Finally, it is noteworthy to mention that civic belief toward alum sludge applications and reuses and the public anxiety from recycling need urgent believable public education for real applications. Further studies are required for the type of alum sludge introduced for reuse for widening and wiser applications. Hence, a complimentary effort may be necessary for commercial applications in the fields of water, environmental and civil engineering.