Figure 3 illustrated the common process flow for sludge reduction treatment. TSR methods primarily encompass incineration, landfilling, and anaerobic digestion, all of which entail substantial fixed-asset expenditures. Incineration is a method that can efficiently diminish sludge volume through the complete oxidation of organic matter. However, to minimize air pollution, it necessitates investment in state-of-the-art emission control technologies (Liu et al., 2023). Landfilling has faced limitations in its application due to its long-term adverse environmental impacts on the soil. The leachate generated from landfilled sludge can contaminate groundwater, while the decomposition of organic matter in landfills leads to significant greenhouse gas emissions (Zhao et al., 2019). Anaerobic digestion, on the other hand, demands a considerable amount of thermal energy to maintain the ideal temperature for the anaerobic microorganisms to function effectively. Even after the digestion process, the residual sludge still has a high moisture content, which requires additional treatment steps (Wang et al., 2022). Overall, each of these TSR methods has its own drawbacks in terms of cost, environmental impact, and post-treatment requirements, highlighting the need for more sustainable and efficient sludge management strategies.

The advent of ISR methods presents a promising solution to the issues mentioned above, which has consequently captured global attention (Cheng et al., 2021; Wang et al., 2019; Zhou et al., 2023). ISR methods are designed to curtail sludge production at its source during the generation process, thereby effectively alleviating the burden and associated costs of subsequent sludge handling. In the treatment process, the volume of WAS has been minimized, and the removal efficiency was significantly enhanced by meticulously adjusting biological, chemical, or physical conditions (Li Y.L. et al., 2022; Li C. et al., 2022; Li W. et al., 2022). Specifically, traditional ISR methods encompass multiple strategies. One approach involves precisely controlling the microbial oxygen consumption. By regulating the oxygen supply, the metabolic activities of microorganisms can be optimized, which in turn affects the growth and decomposition of sludge. Another important measure is to reduce the usage of chemical coagulants. Excessive use of chemical coagulants can lead to increased sludge production and potential environmental risks. Minimizing their use not only cuts down on costs but also promotes a more sustainable treatment process. Furthermore, advanced oxidation techniques, such as ozone or ultrasonic treatment, have been employed. These techniques are capable of breaking down complex organic substances, enhancing the biodegradability of the sludge (Apollo et al., 2023). The improvement in biodegradability facilitates the subsequent treatment steps and ultimately contributes to a more efficient sludge reduction and removal process.

The implementation of ISR method demonstrated profound environmental and economic implications. By leveraging advanced biological and physical–chemical processes, ISR can effectively curtail sludge production, mitigate the potential for secondary pollution and offer notable economic advantages. The adoption of ISR can lead to a remarkable improvement in the operational efficiency of WWTPs and align seamlessly with the overarching goals of sustainable development (Wang L. et al., 2024).

In comparison to ISRB, physicochemical methods exhibit pronounced drawbacks. Physical treatment techniques such as ultrasonic and microwave treatment primarily rely on mechanical shear, cavitation effects, or localized high temperatures and pressures to disrupt sludge flocs and lyse microbial cells (Parvathy Eswari et al., 2022). These processes can effectively reduce sludge volume or enhance subsequent anaerobic digestion efficiency. However, the substantial energy input required for these treatments can significantly increase the operational costs of WWTPs and considerably amplify their carbon footprint (Shen et al., 2022). Chemical methods, for instance, the addition of coagulants, flocculants, or oxidizing agents, achieve rapid sludge thickening and volume reduction by altering the surface charge, structural stability, and water-binding characteristics of sludge particles. While effective in the short term, these methods often result in the accumulation of chemical residues (Guo et al., 2024). These residues may enter aquatic ecosystems through treated water discharge, posing potential threats to the ecological balance, biodiversity, and overall health of these ecosystems (Kong et al., 2023). In addition, when evaluating the economic viability of physicochemical methods, one must take into account both the initial investment and subsequent maintenance costs. The high-cost equipment, continuous energy consumption, and the need for a steady supply of chemicals could offset the overall economic benefits of these methods, making them less attractive from a long-term financial perspective (Liu et al., 2022).

Conversely, ISRB could offer a more sustainable alternative. Strategies such as the addition of enzymes, microbial agents and bacteriophage cocktails can effectively reduce WAS production on-site. These approaches harness specific biological processes to achieve sludge reduction (Yin et al., 2020). By artificially enhancing the biodegradation capacity, ISRB could minimize the reliance on chemical inputs and energy-intensive operations, leading to a reduction in greenhouse gas emissions and aligning with the principles of green and sustainable wastewater treatment (Keb-Fonseca et al., 2021). Table 1 summarized ISR methods, highlighting the mechanisms, advantages, disadvantages, and sludge reduction rates of various methods.

WWTPs are significant contributors to carbon emissions, accounting for approximately 2% of the total societal carbon footprint. If all WASs were treated via incineration, this could lead to the generation of over 7,023 tons of carbon dioxide equivalent (tCO₂-eq) (Zhang et al., 2022). ISRB could offer an effective approach to indirectly mitigate these carbon emissions by curbing the volume of sludge produced. When calculated based on an 80% sludge reduction rate achieved through ISRB, subsequent sludge disposal methods such as anaerobic digestion result in a substantially lower carbon emission of merely 576 tCO₂-eq, as shown in Table 2.

Compared with physicochemical methods, ISRB can significantly reduce carbon emissions through the following “3R” strategies (Guo et al., 2013):

Energy consumption reduction: It is well-established that sludge transportation, incineration, and landfilling are energy-intensive processes. By minimizing the amount of sludge, ISRB reduces the energy demand associated with these processes (Zhao et al., 2019).

Enzymes could break down complex organic molecules, thereby facilitating the reduction of WAS and enhancing its biodegradability (Sharma and Leung, 2021). The biotechnological mechanism underlying enzyme-mediated ISR primarily encompasses the following key aspects, as shown in Figure 4:

Hydrolysis of extracellular polymeric substances (EPS): EPS serve as a protective shield for diverse microorganisms, acting as the initial line of defense during the process of excess sludge lysis (Flemming, 2016). Recent investigations have revealed that alpha-amylase derived from Streptomyces griseus could effectively disrupt the EPS of multidrug-resistant bacteria, with a particular efficacy against Pseudomonas aeruginosa. In the case of P. aeruginosa, the total carbohydrate content within the EPS matrix was notably reduced to 74.09% (Lakshmi et al., 2022). This hydrolysis of EPS has far-reaching implications for the rheological properties of the sludge and, consequently, the stability of biofilms (Kanwar et al., 2019).

Microbial inoculants could modulate and optimize the microbial community structure within sludge treatment systems, which serves multiple crucial functions, including the reduction of biomass production, enhancement of system stability, and improvement of removal efficiency, ultimately leading to the achievement of sludge reduction goals. However, to ensure the long-term effectiveness and sustainability of this process, precise microbial selection and well-thought-out system management strategies are indispensable.

Introduction of slow-growing microorganisms as dominant species: The enrichment of ammonia-oxidizing bacteria and anaerobic ammonia oxidizing bacteria has been effectively implemented in the SNADF system, where they were characterized by a low growth rate and high efficiency in converting ammonia nitrogen, consequently leading to a reduction in the total amount of sludge (Ma L. et al., 2023). Remarkably, the denitrification efficiency of the system can reach up to 94.6%, with the observed yield coefficient as low as 0.05 kgMLSS/kgCOD (Zhang Y. et al., 2023). Additionally, optimizing operational parameters, such as employing aerobic granular sludge process, anaerobic feeding, periodic feast-famine cycling, and shortened settling durations, can enhance sludge granulation. Meanwhile, the selection of slow-growing microorganisms with unique metabolic traits further advances this granulation process (Nancharaiah and Sarvajith, 2019).

Collectively, these strategies could significantly elevate the wastewater treatment system’s efficiency and minimize sludge production (Zhou et al., 2024), as shown in Figure 5.

Protozoa and metazoa exert a sophisticated and effective influence on the sludge reduction via biodegradation and predation processes. These organisms are pivotal in regulating the microbial structure and the associated metabolic functions (Huang et al., 2023). The following are the key biotechnological mechanisms at play:

Microbial predation: Protozoa, including ciliates, flagellates, and amoebae, along with certain small metazoa like rotifers, contribute to the regulation of microbial communities by preying on excessive bacteria (Hailei et al., 2017). A study reported that nematodes could have a significant impact on the microbial community in biological reactors, where grazing by nematodes was observed to reduce biomass growth by 45% in a biotrickling filter (Klein et al., 2016).

Bacteriophages, as viruses capable of infecting and eradicating specific bacteria, exert a substantial influence on the dynamics of microbial communities and the composition of sludge through intricate infection processes (Carroll-Portillo et al., 2023). Phages can be primarily classified into two distinct types, as shown in Figure 6b. Lytic phages, the first type, are responsible for the lysis of bacteria. They hold significant potential in regulating bacteria with high-growth rates. If competing bacterial cells are lysed by these phages, the intracellular nutrients will be released. This nutrient release serves as a supplementary resource for the growth of neighboring WAS-degrading microbes (Chan et al., 2021). In a particular study, a lytic phage targeting H. hydrossis was isolated from the mixed liquor at a wastewater treatment facility. After the addition of the phage at an optimal virus-to-host ratio of 1:1,000, the SVI remarkably decreased from 155 to 105. Interestingly, H. hydrossis-infecting phages showed no cross infectivity with other bacteria in activated sludge systems (Kotay et al., 2011).

The second type is the lysogenic phage, which has a symbiotic relationship with slow-growing bacteria and proliferate with the host genome replication accordingly. Under certain conditions, slow-growing bacteria can selectively retain prophage-derived genomic elements, especially under the circumstance that these genes could confer some survival advantages (Gebhart et al., 2012). For example, R-type bacteriocins derived from phage tail-like particles, could eliminate neighboring competitor bacteria to provide slow-growing Bacillus subtilis with a competitive edge in the microbial community (Howard-Varona et al., 2017).

In summary, bacteriophages, through their diverse interaction mechanisms with bacteria, play a crucial and multi-faceted role in shaping the ecological balance and functional characteristics of microbial communities in sludge systems, offering new perspectives for the optimization of sludge reduction processes.

Biofilms represent highly organized microbial communities where microorganisms adhere to one another and to surfaces, ensconced within a self-synthesized EPS matrix (Wang X.B. et al., 2021). These biofilms exert a multifaceted influence on the sludge reduction process in several ways, where Figure 7 illustrated the cycling process during biofilm formation.

Enhanced microbial metabolism: In the biofilm environment, microbial cells are in close proximity, which greatly promotes the exchange of genetic materials and metabolic by-products (Xia et al., 2025). Wei et al. (2023) reported that biofilm formation could lead to a remarkable increase in microbial richness. Specifically, the Chao index reached 1,700 on the second day of cultivation. This heightened microbial activity results in more comprehensive degradation of organic matter, which in turn significantly contributes to sludge reduction (Li Y.L. et al., 2022; Li C. et al., 2022; Li W. et al., 2022).

EPS, primarily composed of proteins, carbohydrates, humic substances, nucleic acids, uronic acids, and lipids secreted by microorganisms or released through cell lysis, form a complex polymeric network (Pan M. et al., 2023; To et al., 2020). Approximately 80% of the mass of WAS can be attributed to EPS, which have a profound influence on the viscosity and water-retention capacity of the sludge, thereby exerting a substantial impact on its dewatering performance (Li et al., 2024; Frølund et al., 1996).

Water molecules interact with the EPS structure predominantly via electrostatic forces associated with hydroxyl groups and hydrogen bonds, which could contribute significantly to the overall binding energy of EPS. Consequently, the hydrolysis of EPS can enhance the dewaterability of sludge by reducing the polymer concentration (Neyens et al., 2004). A high content of EPS has been shown to undermine the flocculation and sedimentation performance of sludge, posing challenges for conventional mechanical dewatering processes to attain optimal efficiency (Li Y.L. et al., 2022; Li C. et al., 2022; Li W. et al., 2022). To et al. (2020) discovered that the WAS dewatering performance in the majority of WWTPs is suboptimal (averaging 20 wt%), with TB-EPS playing a decisive role in dewatering efficiency (Li Y.L. et al., 2022; Li C. et al., 2022; Li W. et al., 2022).

Additionally, microbial EPS are well-known for providing a multitude of protective functions, such as cryoprotection, resistance to desiccation, buffering of salinity and pH, which could facilitate the formation of aggregates and biofilms (Nagar et al., 2021). The protective layer formed by EPS can hinder the degradation of organic matter in sludge by microorganisms, thus affecting sludge reduction. Wang S. et al. (2023a) summarized that three major families of biofilm dispersal enzymes, namely glycosidases, proteases, and deoxyribonucleases, which target exopolysaccharides, extracellular proteins, and extracellular DNA respectively, can significantly reduce sludge viscosity, enhance dewatering efficiency, and disrupt the protective functions of EPS by regulating the synthesis and degradation processes. It has also been reported that EPS influence the degradation pathways of pollutants in WWTPs. Shapley additive explanations indicated that soluble EPS, S-EPS_PS and S-EPS_PN, were the most critical factors affecting methane productivity and membrane contamination, respectively. Consequently, these factors impact the “gasification” reaction process of organic carbon (Niu et al., 2024).