A comprehensive review on the coagulant recovery and reuse from drinking water treatment sludge

Electrochemical activation of alum sludge for the adsorption of lead (Pb(II)) and arsenic (As): Mechanistic insights and machine learning (ML) analysis

Alum sludge (AlS) has emerged as an effective adsorbent for anionic contaminants, with traditional activation methods like acid/base treatments and calcination employed to enhance its adsorption capacity. However, these approaches encounter significant drawbacks, including excessive waste generation, structural degradation, and limited efficacy for cationic contaminants. To overcome these challenges, this study proposes electrochemical activation as a sustainable method to enhance alum sludge adsorption performance by generating oxygen-containing functional groups (O-FGs) on its surface. In particular, cathodic activated AlS (E-AlS) leads to the formation of hydroxyl (-OH) and carboxyl (-COOH) groups, which served as key active sites for Pb(II) adsorption through complexation mechanisms. E-AlS effectively removed both Pb(II) and As within 4 h, showcasing its dual functionality for cationic and anionic contaminants. While HCl- and KOH-activated AlS also achieved 100 % Pb(II) removal, they caused substantial aluminum (Al) leaching, exceeding 1,000 mg/L, due to structural instability. In contrast, E-AlS minimized Al leaching, preserved structural integrity, and exhibited a 6.5-fold higher Pb(II) adsorption capacity than raw AlS. X-ray photoelectron spectroscopy (XPS) and machine learning (ML) validated the enhanced adsorption performance of E-AlS. These findings highlight electrochemical activation as a cost-effective and environmentally friendly remediation.

Recovery of coagulants via acid treatment in potabilization sludges and their reuse in raw and urban wastewaters

Conventional water treatment plants have used aluminum-based coagulant solutions to remove colloidal substances and other suspended particles from raw water. During this process, a byproduct known as water treatment sludge (WTS) is generated that is typically discharged without prior treatment, causing serious environmental problems for surface waters and nearby ecosystems. Studies have been conducted to evaluate its potential reuse in various processes such as agriculture, construction material manufacturing, pollutant absorption, and its reuse as a coagulant. This study evaluated the recovery of aluminum via acid treatment in WTS from a drinking water treatment plant (DWTP) that supplies water for a population of 413,484 inhabitants, using different pH levels and mixing speeds. The efficiency of the recovered coagulants was evaluated for the removal of color and turbidity in raw and wastewater. The main results showed a maximum aluminum recovery at a pH of 1.5 with values up to 810.5 mg Al/L, with pH being the most influential factor in the process. The removal of color and turbidity of 95.84 and 97.06% were achieved in wastewater and 69.78 and 69.73% in raw water, respectively. The recovered coagulant could be used in DWTPs and in chemically enhanced primary treatment for sewage treatment.

Environmental impact assessment of tannin-based coagulants production from chestnut shells

The increasing demand for freshwater faces challenges and assessing the environmental sustainability of wastewater treatment is crucial. The coagulation-flocculation-sedimentation process is common in water treatment plants, with chemical coagulants playing a pivotal role. However, conventional coagulants like aluminum and iron salts present challenges, prompting a search for environmentally friendly alternatives. Natural coagulants from renewable resources are gaining attention for their biodegradable and non-toxic properties. In Portugal, chestnut shell waste provides a sustainable source for tannin-based coagulant production. This study evaluates the environmental impacts caused by producing a natural coagulant derived from chestnut shells using Life Cycle Assessment (LCA). Two functional units were examined. The first (FU1) focuses on the extraction phase, comparing Solid-Liquid (SLE) and Microwave-Assisted (MAE) extraction methods to produce 100 g of condensed tannins. MAE was identified as the more environmentally friendly option and was used for tannin extraction. The second (FU2) evaluates the environmental impacts of tannin-based coagulants in treating 1 m3 of river water with spiked kaolin to achieve residual turbidity <1 NTU, comparing its effectiveness with conventional coagulants. LCA scrutinizes critical phases, including sample preparation, tannin extraction, drying, and cationization, identifying freeze-drying as a primary hotspot due to electricity consumption. Natural coagulants offer benefits such as not affecting pH/alkalinity, better cost-benefit ratios, and being non-toxic and non-corrosive.

A multicriteria decision-making model for the selection of conventional/hybrid coagulants in water treatment

The coagulation process is a well-known process that has been used in water treatment for many years. The coagulation process shows treatment efficiency in terms of turbidity and suspended solids removal. However, despite good results, coagulant selection is not an easy task. A coagulant can effectively remove suspended solids but, at the same time, increase conductivity; coagulants can also significantly increase chemical sludge production. The final selection of coagulants depends on the importance attributed to the parameter desired to be removed. In this study, the use of multicriteria decision-making is proposed to help select organic coagulants and hybrid coagulants, which have emerged as an alternative to inorganic coagulants in recent years due to the disadvantages of chemical coagulants and whose use and variety are increasing. Therefore, starting from the parameters determined by coagulation process results, these techniques allow weighting these parameters according to the judgments of drinking water treatment plant professionals and determining priorities among coagulants. The agreement between the obtained results and the literature shows that the analytical hierarchy process (AHP) method is a useful tool for selecting coagulants. According to the results of the AHP matrix created with four criteria and nine alternatives, it was found that hybrid coagulants with similar performance to inorganic coagulants have not yet been adopted by drinking water treatment professionals. This may have been due to the higher price of hybrid coagulants and the fact that they are not yet widely used in the drinking water industry.

Transforming drinking water treatment residuals into efficient adsorbents: A review of activation and modification methods

The management of drinking water treatment residuals (DWTRs) poses significant environmental and economic challenges for water treatment facilities; however, these residues have considerable potential as effective adsorbents for pollutant removal. The objectives of this review are to evaluate research conducted from 2015 to 2024 on treatment and modification techniques aimed at enhancing DWTRs' efficacy as adsorbents, analyze the influence of preparation methods on DWTRs performance, evaluate DWTRs adsorbents for different pollutants, and discuss the limitations and challenges in DWTRs applications. The review addresses the knowledge gap by detailed analysis of these advanced modification methods, which have not been extensively reviewed before, and their direct impact on the physicochemical properties and adsorption performance of DWTRs. The review explores various methods including thermal treatment, chemical activation, granulation, pelletization, and the development of composite materials. Key findings indicate that thermal treatment significantly increases surface area and porosity, while chemical activation introduces functional groups that enhance adsorption capacity. Composite DWTRs, incorporating metals, organic compounds, or magnetic properties, demonstrate superior performance in adsorbing diverse contaminants such as dyes and heavy metals. Despite these advancements, challenges remain, particularly in reporting the life cycles and costs of the treated and modified DWTRs and the regeneration of spent adsorbents. The review highlights the importance of optimizing preparation techniques to enhance the physicochemical properties and adsorption performance of DWTRs. By synthesizing existing knowledge and identifying key areas for future research, this review aims to advance sustainable practices in water treatment and resource recovery, aligning with global sustainability goals.

Persulfate catalyst synthesized with waterworks sludge for degrading Safranine T in the presence of boron

Energy conservation and emission reduction are the general trend of the present world. In this study, a catalyst of 3WSH based on the waste recycle of waterworks sludge (WS) and Chinese herbs was prepared using one-step calcination treatment and then characterized by SEM, XRD, XPS, FTIR and BET. The catalytic performance of 3WSHB for activating potassium persulfate (PDS) was evaluated through the degradation of Safranine T (ST) in the presence of boron powder (B). The effects of vital parameters on ST removal were systematically studied, including PDS concentration, 3WSHB dosage, initial solution pH, B dosage, temperature and coexisting cations. The highest efficiency of ST removal was up to 93.0% under the optimal condition with 1.85 mM of PDS, 0.3 g/L of 3WSHB, 0.35g/L of B, 7 of pH. EPR and free radical quenching experiments demonstrated that •OH was the dominant reactive oxygen species for ST degradation in the PDS/3WSHB/B system. Moreover, the intermediates determined by HPLC-MS indicated that the oxidization of benzene ring substituents in ST and a hydrogen abstraction by electron transfer might occur during ST degradation. The dissatisfied reuse performance of 3WSHB might be attributed to its low Fe content and simple reusing way. The results demonstrate the effectiveness of WS recycling and reuse in the field of pollutant remediation.

Making waves: Breaking the bottleneck of recycling drinking water treatment residue for practical engineering applications in water pollution control

Drinking water treatment residue (DWTR), an inevitable byproduct of water treatment plants, is typically recycled to control water pollution. DWTR poses a low environmental risk and has the potential to function as a functional material for various applications. However, the practical engineering applications of DWTR are limited. These limitations arise from a disconnect between fundamental research and the practical needs of engineering applications, creating a bottleneck for the effective recycling of DWTR. Previous studies have primarily focused on exploring potential DWTR recycling methods that reuse Al, Fe, Mn, Ca, Si, and organic C. However, the varying properties of DWTR obtained from different water treatment plants tend to differ with respect to potential recycling methods, confusing managers and engineers in using relevant knowledge to guide practical engineering applications. To address this challenge, the author advocates for a shift in research toward establishing guidelines that provide direct guidance for practical engineering applications of DWTR. The key components of these guidelines should include risk assessment, capability evaluation, and environmental application procedures with sustainability assessment to break the bottleneck associated with the recycling of DWTR.

Recycling aluminum from polyaluminum chloride sludge through acid dissolution and cation resin separation/purification

To recycle aluminum (Al) from waterworks sludge resulting from polyaluminum chloride (PAC) used as coagulants, this study proposed an innovative strong acidic cation (SAC) exchange resin treatment strategy for Al separation from coexisting fulvic acid (FA) and heavy metals (HMs) in the H2SO4 leachate of PAC sludge. Fluorescence titration confirmed the breakdown of the Al-FA complex at pH 2.0, which facilitated Al separation from FA in the acidic leachate. The species distribution of the dissociated Al (i.e. Ala, Alb, and Alc) significantly influenced the adsorption of Al onto the cation exchange resin. The continuous release of H+ during the cation exchange reaction greatly promoted the transformation of dissociated Alc and Alb into Ala, thereby improving the adsorption of total Al. Moreover, the SAC resin column successfully separated the codissolved HMs from the Al in the leachate even at an influent pH of 2.8, which was attributed to the greater selectivity of the sulfonate groups on the cation exchange resin for free Al3+. The Al eluted from the exhausted resin with 1.1 M H2SO4 was collected as the recycled coagulant after proper pH adjustment. The Al adsorption capacity of the SAC resin decreased by approximately 5 % with each operation cycle and was regained by complete regeneration with 1.8 M H2SO4 after 5 cycles. Overall, the integrated efficiency of Al recovery from PAC sludge by H2SO4 acidification and SAC resin separation/purification reached 70.10 %. The recycled Al from sludge has a water treatment performance comparable to that of fresh PAC coagulant.

Physical-chemical characterisation of an alum-based water treatment sludge in different raw water turbidity scenarios

Characterisation of the water treatment sludge (WTS) generated in drinking water treatment plants (DWTPs) is crucial to define alternatives for its adequate management, including potential reuse options. To define these alternatives, it is necessary to evaluate rainfall seasonality effect on WTS production and its physical and chemical characteristics. This study assessed the production and characterisation of four types of alum-based WTS. The WTS was generated in a pilot-scale system from different raw water turbidities (i.e., low: <5 NTU, medium: 5-10 NTU, high: ≥10 NTU, and very high turbidity: ∼300 NTU) and coagulant doses. To estimate WTS production, mathematical models based on variables such as raw water turbidity, coagulant dosage, and organic matter removed were used. The WTS characterisations included physical (solids and particle size distribution), chemical (metallic oxides, pH, mineral phases), and surface properties (functional groups and zero-charge point pH). The modified Kawamura model presented the best fit (R2 = 1.0, RMSE = 0.1062 and the lower Akaike Information Criterion) for the estimation of WTS production, indicating that at the DWTPs, it is possible to make sludge production projections using only two simple variables: coagulant dose and the raw water turbidity. The four types of WTS consist mainly of amorphous materials (45-65 %), featuring some mineral phases and exhibiting high contents of Al (Al2O3: 30-34 %), Si (SiO2: 21-26 %) and Fe (Fe2O3: 11-13 %). Nevertheless, very high turbidity WTS shows variations in its characteristics, notably a heightened content of clays. As a result of the high concentrations of Al and Fe, the WTS has the potential to be used as coagulants or for the recovery of coagulants, especially low turbidity WTS, which is produced from water with low turbidity and organic matter. The presence of aluminium-silicate clays and the surface functional groups of the silica network suggest that WTS, particularly very high turbidity WTS, also has the potential to be raw materials for generating adsorbents. The potential applications of WTS in coagulation and adsorption can be leveraged in wastewater treatment, promoting the circular economy in the water sector.

Enhanced performance and mechanism of adsorption pretreatment for alleviating membrane fouling in AGMBR: Impact of structural variations in carbon adsorbents

The structural variances of adsorbents play a crucial role in determining the number of effective adsorption sites and pretreatment performance. However, there is still a gap in comprehending the impact of different carbon structural adsorbents on membrane fouling. Therefore, this study aimed to compare the efficacy of granular activated carbon (GAC), powdered activated carbon (PAC), and activated carbon fiber (ACF) in mitigating membrane fouling during municipal sewage reclamation using an aerobic granular sludge membrane bioreactor (AGMBR). The results demonstrated that the utilization of PAC significantly enhanced the normalized flux and reduced fouling resistance in comparison to GAC and ACF systems. PAC effectively adsorbed low and medium-molecular-weight pollutants present in raw sewage, resulting in an increase in average particle size and a decrease in foulant content on the membrane surface. The Hermia model indicated that adsorption pretreatment minimized standard blocking while promoting the formation of a sparse and porous cake layer. Moreover, according to the extended Derjaguin-Landau-Verwey-Overbeek theory, PAC has been demonstrated as the optimal antifouling system owing to its enhanced repulsion between membrane-foulant and foulant-foulant interactions. Correlation analysis revealed that the exceptional antifouling performance of the PAC system was due to its high removal rates of chemical oxygen demand (~78 %) and suspended solids (~97 %). This research offers valuable insights into the mitigation of membrane fouling through the utilization of adsorbents featuring diverse carbon structures.

Use of wastewater alum-coagulation sludge as a phosphorus fertiliser - a mini review

The use of aluminium (Al) salts, particularly alum, in coagulation is a widespread and conventional treatment method for eliminating pollutants, including phosphorus (P) which can cause eutrophication, from wastewater. However, a significant challenge of this process is the substantial amount of sludge generated, necessitating proper disposal. Historically, land disposal has been a common practice, but it poses potential issues for plant life on these lands. Despite the associated drawbacks, sludge contains elevated concentrations of vital plant nutrients like P and nitrogen, presenting an opportunity for beneficial use in agriculture. Given the imminent scarcity of P fertilizers due to the eventual depletion of high-grade P ores, this review explores the potential advantages and challenges of utilizing Al sludge as a P source for plants and proposes measures for its beneficial application. One primary concern with land application of Al sludge is its high levels of soluble Al, known to be toxic to plants, particularly in acidic soils. Another issue arises from the elevated Al concentration is P fixation and subsequently reducing P uptake by plants. To address these issues, soil treatment options such as lime, gypsum, and organic matter can be employed. Additionally, modifying the coagulation process by substituting part of the Al salts with cationic organic polymers proves effective in reducing the Al content of the sludge. The gradual release of P from sludge into the soil over time proves beneficial for plants with extended growth periods.

Potential of ash from agricultural waste as substitute of commercial FeCl3 in primary treatment of landfill leachate

Due to the ever increasing global population, higher volumes of industrial waste discharges to landfill have caused major problems for the environment. This study investigated the performance of rice straw ash (RSA) as a natural coagulant under different conditions for the treatment of landfill leachates by coagulation-flocculation and microfiltration, with and without addition of FeCl3. The highest performing treatment conditions (RSA = 2.48 g/L, FeCl3 = 4.98 g/L, settling time = 54.75min) were achieved with the combined use of RSA and FeCl3 as coagulant and led to a sludge volume index of 41.65 mL/g, 51.27% COD removal and 76.48% total suspended solid removal. In contrast, FeCl3 alone achieved slightly better COD and total suspended solid removal rates, however it resulted in higher sludge volume index and sludge production. The combined use of RSA and FeCl3 reduced the consumption of these two coagulants by 78.76% and 46.69% respectively. Functional groups and thermal stability of the flocs showed that RSA + FeCl3 synergistically enhance the mechanisms of the coagulation-flocculation process, including adsorption by particle's bridging, charge neutralization and size of flocs. Combining the coagulants resulted in increased van der Waals forces and lower attractive forces of the inter-colloidal energy barrier in the leachate. Additionally, the highest and lowest heavy metals removal rates for treatment by microfiltration were found for Fe (92.15%) and Mg (7.63%), with a total heavy metals removal efficiency in the range of 6.08-90.78%. The findings of this study show that RSA can serve as a natural eco-friendly coagulant both alone and in combination with FeCl3 in the leachate treatment.

Evaluation of combined thermo-chemical processes for the treatment of landfill leachate using virgin and recovered FeCl3 coagulants

Sludge resulting from treatment of municipal waste landfill leachate contains suitable cationic substances such as Fe-based recovered coagulants which, if not recovered, can cause environmental problems. The present study aimed to maximise coagulant recoverability and investigate its potential reuse for the treatment of municipal waste landfill leachate. The study focused on establishing (i) the effect of mineral acids on leaching of Fe, (ii) the % of maximum recovery of Fe coagulant, (iii) the impact of ultrasound on recovery, and (iv) effectiveness of recovered coagulant when reused in coagulation-flocculation treatment of landfill leachate. Sulfuric acid outran hydrochloric acid in performance, with the acid leaching process leading to the recovery of 70.12% of Fe (acid concentration = 3.80 M, solid-to-liquid ratio = 8%, and heating time = 5 h). Subsequently, a developed acid leaching process was tested, which results showed that the highest rate of Fe recovery occurred without ultrasound treatment, meaning the use of it could reduce the recovery rate due to the increase in the iron (III) oxide-hydroxide [Fe(OH)3] sedimentation. Comparative experiments were undertaken with the recovered and virgin coagulants. These revealed that Fe-based recovered coagulant led to the 60.21% and 91.40% removal of COD and total suspended solid respectively, while the values of the COD and total suspended solid removal with the virgin FeCl3 were 7.66% and 6.42% lower than that of Fe under optimal conditions (dosage = 9.38 g/L, pH = 8.94, settling time = 52.9 min). The present study established that Fe recovered could be exploited as an eco-friendly coagulant to replace FeCl3 in the landfill leachate treatment.

The ability of drinking water treatment sludge to degrade methylene blue in water through combined adsorption/photo Fenton-like process

In the present study, drinking water treatment sludge (DWTS) was reused as a catalyst in advanced oxidation processes for the removal of methylene blue (MB) from aqueous solutions. After determining their chemical and mineralogical compositions by X-ray Powder Diffraction (XRD), BET surface area, scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), Inductively Coupled Plasma (ICP), and FT-IR spectra. DWTS has been used as a heterogeneous photo Fenton-Like catalyst for the oxidation of MB under different parameters, including pH (3-6), H2O2 concentration (9.79-29.37 mM), and dose (1-2.5 g/L). The results showed that within 180 min and under UV light irradiation, more than 86% of MB having a concentration of 50 mg/L were removed using a catalyst loading of 1.5 g/L, a H2O2 dosage of 23.17 mM and a solution pH of 5. The DWTS has a satisfactory stability as the catalyst is stable and have very less iron leaching property.

Study on the removal effect and mechanism of calcined pyrite powder on Cr(VI)

Pyrite exhibits considerable potential as an adsorbent in wastewater treatment. However, few pyrite adsorbents are directly obtained from natural pyrite, as most are composite materials that require a complex preparation process. To develop a pyrite-based adsorbent with a simple preparation process, pyrite was processed by calcination at 400, 600, and 800 °C for 4 h and ball-milled into a fine powder. The adsorption properties of the pyrite powder were systematically explored. The calcined pyrite powder was characterized by SEM-EDS and XRD. The results revealed that the pyrite calcined at 600 °C exhibited excellent adsorption properties and was primarily composed of Fe7S8. The optimum conditions for Cr(VI) removal were a temperature of 45 °C, an adsorbent dosage of 1 g, an equilibration time of 60 min, and an initial pH of 3. Moreover, the calcined pyrite powder exhibited excellent reusability, and the Cr(VI) removal rate exceeded 65% after three cycles. The Cr(VI) adsorption on pyrite can be well described by the Freundlich model and pseudo-second-order kinetic equation. The calcination temperature is the main factor affecting the adsorption performance of pyrite. Therefore, the calcined pyrite powder is expected to be an excellent adsorbent for Cr(VI) in the wastewater treatment industry.

A review of iron use and recycling in municipal wastewater treatment plants and a novel applicable integrated process

Chemical methods are expected to play an increasingly important role in carbon-neutral municipal wastewater treatment plants. This paper briefly summarises the enhancement effects of using iron salts in wastewater and sludge treatment processes. The costs and environmental concerns associated with the widespread use of iron salts have also been highlighted. Fortunately, the iron recovery from iron-rich sludge provides an opportunity to solve these problems. Existing iron recovery methods, including direct acidification and thermal treatment, are summarised and show that acidification treatment of FeS digestate from the anaerobic digestion-sulfate reduction process can increase the iron and sulphur recycling efficiency. Therefore, a novel applicable integrated process based on iron use and recycling is proposed, and it reduces the iron salts dosage to 4.2 mg/L and sludge amount by 80%. Current experimental research and economic analysis of iron recycling show that this process has broad application prospects in resource recovery and sludge reduction.

Effect of the association of coagulation/flocculation, hydrodynamic cavitation, ozonation and activated carbon in landfill leachate treatment system

Mature landfill wastewater is a complex effluent due to its low biodegradability and high organic matter content. Currently, mature leachate is treated on-site or transported to wastewater treatment plants (WWTPs). Many WWTPs do not have the capacity to receive mature leachate due to its high organic load leading to an increase in the cost of transportation to treatment plants more adapted to this type of wastewater and the possibility of environmental impacts. Many techniques are used in the treatment of mature leachates, such as coagulation/flocculation, biological reactors, membranes, and advanced oxidative processes. However, the isolated application of these techniques does not achieve efficiency to meet environmental standards. In this regard, this work developed a compact system that combines coagulation and flocculation (1st Stage), hydrodynamic cavitation and ozonation (2nd Stage), and activated carbon polishing (3rd Stage) for the treatment of mature landfill leachate. The synergetic combination of physicochemical and advanced oxidative processes showed a chemical oxygen demand (COD) removal efficiency of over 90% in less than three hours of treatment using the bioflocculant PGα21Ca. Also, the almost absolute removal of apparent color and turbidity was achieved. The remaining CODs of the treated mature leachate were lower when compared to typical domestic sewage of large capitals (COD ~ 600 mg L^-1), which allows the interconnection of the sanitary landfill to the urban sewage collection network after treatment in this proposed system. The results obtained with the compact system can help in the design of landfill leachate treatment plants, as well as in the treatment of urban and industrial effluents which contains different compounds of emerging concern and persistence in the environment.

Recycling of water treatment sludge in concrete: The role of water-binder ratio from a nanoscale perspective

For eutrophic water bodies, potassium permanganate is an effective pre-oxidant to remove algae and its residue in water treatment sludge. Recycling water treatment sludge in concrete is an environmentally friendly and high-value utilization measure. However, little research has been done on the effect of manganese-rich drinking water sludge ash (DWSA) on concrete. The effect of water-binder ratio (w/b) on strength, shrinkage and microstructural characteristics of concrete containing DWSA was investigated, and the structural behavior was explained from a nanoscale perspective. The results show that recycling 10 % DWSA in concrete improved the strength and shrinkage resistance of the samples. Reducing the w/b effectively increased the strength of DWSA-modified concrete and reduced the shrinkage deformation. The paste with high w/b had higher contents of non-evaporated water and calcium hydroxide, as well as higher reaction degree of DWSA. Nanoscale characterization shows that reducing the w/b reduced the volume fraction of pore and unhydrated phases in the matrix and increased the proportion of high-density C-S-H. Meanwhile, reducing the w/b also reduced the interfacial transition zone width of DWSA-modified concrete. Recycling DWSA in concrete effectively reduced the total carbon footprint and cost of the mixture. The combined application of reducing the w/b and incorporating DWSA effectively improved the economic and environmental benefits of concrete material. For the concrete modified with 10 % DWSA (w/b = 0.3), its cost and carbon emissions are reduced by 14 %-21 % and 19 %-25 % compared with the reference sample, respectively. Overall, this study reveals the action mechanism of DWSA in cement system at different w/b from nanoscale perspective, and gives a new insight on determining the optimal w/b in full-scale application of DWSA concrete.

Application of modified water treatment residuals in water and wastewater treatment: A review

Large quantities of sludge known as water treatment residuals (WTRs) are generated from water treatment facilities across the world. Various attempts have been made to reuse these residuals. Among the different applications of WTRs, their reuse in water and wastewater treatment has received more attention. However, direct application of raw WTRs is associated with some limitations. In the last decade, in order to improve their characteristics, numerous investigators have modified WTRs by different methods. This paper reviews the different methods applied to WTRs to enhance their characteristics. The effects of these modifications on their characteristics are explained. The applications of modified WTRs as a filtration/adsorption medium for treating textile/dye wastewater, groundwater containing different anionic and cationic pollutants, storm water runoff, and as a substrate in constructed wetlands are presented in detail. Future research needs are highlighted. The review clearly indicates the potential of different modification methods to improve the removal of a variety of pollutants by WTRs from water and wastewater.