Table 4.

Selected studies on microplastics removal from sewage sludge through advanced sludge treatment technologies

Technology	Operating conditions	Removal (%)	Mechanism	Advantages	Limitations	References
Pyrolysis	Pyrolysis of MPs embedded SS at different temperatures, i.e., 150–500 °C, with a holding time: 30 min and heating rate: 10 °C/min. (MPs: PE, PP, PS, PA)	99.1–99.4% at 350 °C and 99.8% at 450 °C	Random/end chain scission of plastics followed by generation of chain terminus radicals, and production of short chain liquid products and ashes with increased temperature	No release of PAHs in biochar, dioxins, furans, and PCBs at 450 °C with effective removal	Pretreatment of sludge is required to dry it before utilizing pyrolysis technology	[16, 41•]
Hyperthermophilic composting	SS after 45 days of composting treatment sludge-based MPs at high temperature, which they demonstrated in full scale (200t)	43.70%	In situ biodegradation of sludge-based MPs	Efficient MPs removal with resource recycling, easy operation, short processing time, little residue, and odor emission	Organic mineralization increases with a decrease in organic content leading to less fertilizer efficiency; oily, lipid, or salinity-rich solid waste can cause system instability; human safety of hyperthermophilic composting microbes must be addressed before large-scale application	[29•, 111]
Hydrothermal carbonization	Different temperatures, i.e., 180–260 °C at a heating rate of 10 °C/min for 3 h	79.71% at 260 °C	Depends on the chemical type of MPs: PET-MPs: hydrolysis of PET-MPs dur to presence of hydrophilic group PP-MPs: polymer random chain scission	Efficient degradation of MPs in sludge with reduced environmental risk of sludge as an important carrier of MPs, less harmful emission compared to pyrolysis	Complete degradation of MPs in the sludge cannot be achieved in a single HTC, and the residual MPs (NPs) remain a potential threat to the natural environment	[16]
Hydrothermal liquefaction	Continuous HTL process operated at supercritical conditions (Temp: 400 °C; pressure: 30 MPa)	97% as plastic mass and 76% as particle number	• Dilution of polymer phase while shifting the pyrolysis selectivity from bimolecular hydrogen abstraction and addition to unimolecular β-scission • Initiation of proton transfer reactions between polymer and supercritical water through hydrogen bonding facilitating the formation of carbocations serving as an initiator to activate C–C cleavage in the polymeric chains	Involves the use of wet biomass, no excess energy for drying feedstock; carbon- and cost-efficient way; efficient removal of micropollutants	Decreasing particle size increases the threat of a high concentration of nanoplastics that are more harmful than MPs	[42]
Vermi-wetlands	The concentrations of 1 μm, 100 μm, and 500 μm PMMA MPs were added to the excess sludge and treated through constructed Vermi-wetland. (Earthworm species: E. fetida; wetland plant species of A. calamus)	100% (500 μm), 95.44–99.52% (100 μm), and 86.62–95.69% (1 μm MPs)	Synergistic action among earthworms, microorganisms, and plants; an interception effect responsible for MPs elimination in excess sludge in vermi-wetlands	Effective in MPs removal, economical, eco-friendly, and greater ability to stabilize and reduce sludge	High MPs concentration can hinder degradation, and can lead to oxidative stress and neurotoxicity in earthworms	[43, 112]
Enzymatic degradation	Degradation of HDPE beads by three hydrolytic enzymes lipase (Rhizopus oryzae), cellulase (Trichoderma reesei), and protease (Aspergillus oryzae) at various temperatures	4% initial bead mass was removed with protease (88 mg/L) at 55 °C in a 3-day batch experiment. Moreover, degradation up to 95% of HDPE beads in 20 days of AD retention time (mesophilic conditions) (through simulations)	Adsorption of enzymes on the polymeric surface, followed by hydrolysis/hydroperoxidation of bonds	Efficient MPs removal at lab-scale experiments	Enzymes utilization in large-scale reactions is not economical because the enzymes are hard to be recovered or reused with relatively short lifetimes; further improvement is needed on parameters such as recyclability, efficiency, and robustness	[44, 113]
Biodegradation	Biodegradation of 100 mg of PS-MPs through co-culture of PS-MPs and conventional thermophilic and hyperthermophilic composting inoculum in 50 mL of liquid carbon-free basal medium and 10 mL cell suspension. (Conventional composting: 40 °C; hyperthermophilic composting: 70 °C; incubation period: 8 weeks; shaking at 180 rpm)	Hyperthermophilic composting inoculum 7.3% degradation at 70 °C, which is 6.6 times more than conventional composting	Facilitation of more microbial deposition fostering biodegradation due to changes in operating conditions such as high composting temperature accountable for decreasing the hydrophobicity of MPs by introducing C-O and C = O groups	Low investment, inexpensive, low operating cost, easy operation, no secondary pollutant generation, and flexible to handle	Controlled environmental conditions and nutrient requirement, difficult to scale up batch and pilot scale studies to large scale, time consuming as compared to other techniques, aggregation of microbial assemblage on MPs surface, lack of reproducibility, difficult to find suitable microbial community	[29•, 114–116]