Recycled Paper Mill Research Articles

Energy saving potential in steam systems: A techno-economic analysis of a recycling pulp and paper mill industry in Morocco

The industrial sector represents nearly 40 % of global energy consumption, with steam systems accounting for 30 % of energy use in manufacturing. As the need for cleaner processes intensifies within African and global energy transitions, adopting energy efficiency practices becomes imperative. Governments are focusing on reducing energy losses in fossil fuel-dominated manufacturing industries, which are major producers of greenhouse gas emissions. Despite their significant energy consumption, industrial heat processes are often overlooked in efforts to reduce costs and emissions. To address this, the study examines thermal energy processes and identifies energy-saving opportunities within steam systems through a techno-economic analysis of a recycling pulp and paper mill in Morocco. This is the first study focusing specifically on steam systems within a particular industry in the country. The research uses a system approach covering steam generation, distribution, end-use, and recovery. The current system is benchmarked through a rating questionnaire, and proposed energy-saving actions include leak repair, pipe insulation, blowdown management, and improved boiler combustion efficiency. An economic study evaluates the cost-effectiveness of these opportunities. Findings reveal a 6 % reduction in thermal energy consumption, annual savings of $417 899 and nearly 482 TJ, a 6 % decrease in GHG emissions (2158.5 tons of CO2 per year), and water savings of approximately 5300 m³ per year (13 %). The total investment of $8750 has negligible payback periods. Despite diverse industrial activities, this work is useful for other industries since the fundamental components of the steam system are widely shared. Moreover, it supports the African Union's Agenda 2063 and United Nations Sustainable Development Goals by providing practical insights and recommendations for enhancing industrial energy efficiency.

Ozone/biological aerated filter integrated process for recycled paper mill wastewater: A pilot-scale study

In this study, the effluent from recycled paper mill was treated using a combined ozone (O3) and biological aerated filter (BAF) process. Key operational parameters such as ozone dosage, pH, hydraulic retention time (HRT), volume load and gas-to-water ratio were optimized. Under optimal conditions, with a total ozone dosage of 100 g/m, a gas-to-water ratio of 4:1, and an HRT of 3.0 hours in the BAF, the chemical oxygen demand (COD) and chroma of the treated wastewater were reduced to 44–55 mg/L and 2–4 PCU, achieving removal efficiencies of 70 % and 95 %, respectively. The discharge effluent not only satisfy the new discharge standard of China (GB3544–2008), but also can be used as recycling water. Additionally, the treatment cost of wastewater was ca. 1.3 ¥/m3 in pilot-scale test, significantly decreasing the cost. Ozone pretreatment has a significant effect on wastewater decolorization by disrupting the molecular chemical structure of pollutants, which increase the biochemical properties of biofilm and is beneficial to the sequential BAF treatment. The sludge in the O3/BAF system exhibited increased biomass with minimal filamentous bacteria and higher dehydrogenase activity, confirming stable and robust bacterial growth. GC-MS analysis revealed substantial reduction in pollutant content and diversity post-treatment, although the recalcitrant compound (Z)-13-docosenamide remained relatively high, decreasing from 27.37 % to 21.14 %. The mechanism of the O3/BAF process for the pollutant degradation were also proposed. This study demonstrated that a combination of ozone and fixed biofilm treatment is an efficient and cost-effective treatment, providing the theory and practical applicability for the industrial wastewater.

Utilizing de‐inked paper sludge for sustainable production of medium‐density fiberboard: A comprehensive study

AbstractThis study investigated converting abundant paper mill waste, de‐inked paper sludge (DPS), into value‐added medium‐density fiberboard (MDF) panels. The objective was to repurpose this solid waste into a valuable material that aligns with the principles of a circular economy. Previous research has examined the addition of paper sludge to wood composites, but there is limited information on the specific incorporation of DPS into MDF production. This investigation offers new insights into this application. MDF panels were manufactured using hardwood fibers, with varying levels of urea‐formaldehyde resin (10% and 12% by weight) and different loadings of DPS (ranging from 0% to 50% by weight). X‐ray diffraction analysis revealed the presence of defective DPS fiber crystals. Contact angle measurements confirmed that DPS had poor wettability, corresponding to increased water absorption. As more DPS was incorporated, the mechanical properties of the panels progressively weakened due to defective fibers and poor adhesion between the fibers and the matrix. This was evident in scanning electron microscopy micrographs, which showed an escalation in interfacial flaws. Increasing the resin dosage noticeably improved the internal bond strength and reduced moisture uptake across all levels of DPS. However, at a DPS loading of 50%, the properties of the panels deteriorated by 36%–57% compared to the control MDF. Additionally, thickness swelling and water absorption increased significantly with higher DPS content, primarily due to the hydrophilic nature of the fibers. Based on optimization, it was determined that a DPS loading of 20% with a resin content of 12% provided the best balance between performance, waste utilization, cost, and moisture resistance. The reuse of abundant paper sludge supports the goals of a circular economy. However, strategies are required to tailor the processing methods and enable higher levels of recycled material without excessively compromising the key attributes of the product. Further research should focus on enhancing the quality of DPS and its compatibility with wood fibers and resin to enhance the performance of the composite material.Highlights DPS from recycled paper mill assessed for MDF production with wood fibers and resin Increasing DPS loading caused reductions in strength and adhesion of panels Water absorption and thickness swelling markedly rose with more DPS addition It was attributed to hydrophilic fibers and disrupted fiber‐matrix bonding Optimal formulation was 20% DPS loading and 12% resin content, balancing performance, waste utilization, and cost.

Morphology analysis and microbial diversity in novel anaerobic baffled reactor treating recycled paper mill wastewater

Morphology analysis and microbial diversity in novel anaerobic baffled reactor treating recycled paper mill wastewater

Sustainable ceramic tiles incorporated with waste fly ash from recycled paper production

Waste incineration is an effective solid waste disposal method; however, its by-product pollutants hinder its widespread adoption. This research aims to develop novel unglazed ceramic tiles by recycling industrial solid waste incineration fly ash (IWA) from recycled paper mills. Experimental investigations were conducted on ceramic tiles with IWA additions (0–30 wt%) and firing temperatures (1100 °C, 1200 °C, and 1300 °C). Various scientific tests, including XRF, XRD, strength, water absorption, thermal properties, heavy metal leaching, SEM analysis, and micro-CT analysis, were performed. The results suggest that the addition of IWA may deteriorate the technical performance of ceramic tiles; however, satisfactory performance can still be achieved by tailoring the firing temperature due to the existence of iron and calcium in the IWA, which narrows and displaces the densification temperature range. Remarkably, tiles with 30 wt% IWA, which completely replaced the flux materials and fired at 1300 °C, achieved the highest flexural strength (exceeding 30 MPa). These tiles exhibited promising attributes such as water absorption of 5.9%, an estimated energy saving of 33.86%, and compliance with heavy metal leaching limits. This study proposes a sustainable solution for IWA disposal by utilising recycled paper mill fly ash as a substitute for flux material in ceramic tile manufacturing. The findings contribute to minimising environmental impact and conserving non-renewable natural resources, supporting sustainable development.

Phytoremediation Using Different Numbers of Individual Kayu Apu Plant (Pistia stratiotes) in Reducing Lead (Pb) Levels in Paper Recycling Factory Waste

Recycled paper mill waste contains inorganic materials, one of heavy metal lead. Metal lead is non-essential and non-degradable that harmful to living things. Reduce lead levels by using Kayu Apu plants (Pistia stratiotes) is needed. The aims, to determine number of Kayu Apu plants that effective in reducing lead levels, temperature and pH of optimum waste according to quality standards, increase in plant's wet weight, and morphological condition of plants. Method is experiment with completely randomized design. Treatment consisted of number of Kayu Apu plants A: control, B: 2 individuals, C: 5 individuals, and D: 9 individuals with 3 replications. Acclimatization of Kayu Apu plant was carried out, measuring physical, chemical and biological parameters, then data were analyzed using one-way ANOVA and paired sample t-test. Results showed that lead levels of treatment using 9 individuals decreased while in control, 2 individuals and 5 individuals increased. Average temperature and pH of waste are still optimum according to quality standards. Average fresh weight of plants increased in treatments B and C, while treatment D decreased. Number of Kayu Apu plants that effective in reducing lead was treatment using 9 individuals. Average temperature and pH of waste were optimum according to quality standards. Wet weight of plants increased in treatments B and C while D decreased. Morphology of Kayu Apu leaves in all treatments turned yellow, dried and fall off but the buds grow.Keywords: Kayu Apu (Pistia stratiotes), lead (Pb), Paper mil waste

Field Measurement and Dispersion Modelling of Hydrogen Sulphide for Recycled Paper Mill in Malaysia

Field Measurement and Dispersion Modelling of Hydrogen Sulphide for Recycled Paper Mill in Malaysia

Fuel properties and combustion performance of hydrochars prepared by hydrothermal carbonization of different recycling paper mill wastes

AbstractThe incineration of high‐moisture solid residues generated at the recycling paper mills represents an energetically unfavourable method of resource utilization. Alternatively, hydrothermal pre‐treatment is considered. In this study, low‐value paper sludges from three different recycling streams were hydrothermally carbonized at 205, 225, and 245°C for 3 h. The raw feedstocks and derived hydrochars were analyzed for energy properties, chemical characteristics, surface morphology, functional groups, and combustion performance employing energy densification and mass yield quantification, scanning electron microscopy, elemental analyzer, Fourier‐transform infrared spectroscopy, and thermogravimetry. The increase in reaction temperature was reported to cause a decrease in mass yield and an increase in energy densification and calorific values corresponding to 5.98%–49.35% and 10.10%–58.51% for raw fibre rejects and final sludge‐derived hydrochar, respectively. Hydrothermal carbonization (HTC) had a non‐significant influence on the energy densification of primary clarifier sludge‐derived hydrochar. Higher reaction temperatures favoured the rate of dehydration and decarboxylation, leading to hydrochars with lower H/C and O/C ratios, thereby enhancing the overall fuel properties. The low‐nitrogen and low‐sulphur fuels obtained validated the effectiveness of HTC treatment to produce high‐quality cleaner solid fuel. The burnout temperature was mostly reduced with an increase in HTC temperature. At HTC‐205 and 225°C, the ignition temperature and the combustion performance increased as a result of the HTC reaction mechanisms. HTC effectively recovered hydrochar with increased carbon content, improved energy densification, and good combustion adequacy. Hydrochar derived from recycling mills may play a role in the energy sector as a substitute for coal or in co‐combustion at coal‐fired power plants.

Indigenous bacteria as an alternative for promoting recycled paper and cardboard mill wastewater treatment

The present study aimed to investigate indigenous bacteria possibility in recycled paper and cardboard mill (RPCM) wastewater treatment through the isolation and identification of full-scale RPCM indigenous bacteria. The molecular characterization of the isolated bacteria was performed by 16S rRNA gene sequencing. Klebsiella pneumoniae AT-1 (MZ599583), Citrobacter freundii AT-4 (OK178569), and Bacillus subtilis AT-5 (MZ323975) were dominant strains used for RPCM wastewater bioremediation experiments. Under optimal conditions, the maximum values of chemical oxygen demand (COD) and color biodegradation by C. freundii AT-4 were 79.54% and 43.81% after 10 days of incubation, respectively. In the case of B. subtilis strain AT-5 and K. pneumoniae AT-1, the maximum values of COD and color biodegradation were 70.08%, 45.96%, 71.26%, and 32.06%, respectively. The results from optimal conditions regarding efficiency were higher in comparison with the efficiency obtained from the oxidation ditch treatment unit in full-scale RPCM-WWTP. Therefore, the present study introduces the isolated indigenous bacteria strains as a promising candidate for improving the RPCM-WWTP efficiency using bioremediation.

Kinetic Study of the Anaerobic Digestion of Recycled Paper Mill Effluent (RPME) by Using a Novel Modified Anaerobic Hybrid Baffled (MAHB) Reactor

The process kinetics of an anaerobic digestion process for treating recycled paper mill effluent (RPME) was investigated. A laboratory-scale modified anaerobic hybrid baffled reactor (MAHB) was operated at hydraulic retention times of 1, 3, 5, and 7 days, and the results were analyzed for the kinetic models. A kinetic study was conducted by examining the phase kinetics of the anaerobic digestion process, which were divided into three main stages: hydrolysis kinetics, acetogenesis kinetics, and methane production kinetics. The study demonstrated that hydrolysis was the rate-limiting step. The applied Monod and Contois kinetic models showed satisfactory prediction with μmax values of 1.476 and 0.6796 L day−1, respectively.

Modeling and optimization of process parametric interaction during high-rate anaerobic digestion of recycled paper mill wastewater using the response surface methodology

Abstract This study carried out the anaerobic digestion of recycled paper mill wastewater (RPMW) in a high-rate novel anaerobic baffled reactor. The parametric interaction between influent chemical oxygen demand (CODin) and hydraulic retention time (HRT) was modeled, and process responses were optimized by the response surface methodology (RSM) using a three-level factorial design. The results showed that the optimal condition was determined at CODin of 4,000 mg/L and HRT of 2 days and predicted values for COD removal, biochemical oxygen demand (BOD) removal, lignin removal, CH4 content, and CH4 production were found to be 94%, 98%, 68%, 85%, and 20.8 L CH4/d, respectively. According to the statistical analysis of the RSM, all models were significant with very low probability values (from 0.0045 to <0.0001). The parametric interaction showed that increasing the CODin positively influenced the COD, BOD, and lignin removal efficiencies, effluent alkalinity, and methane content and production but was unfavorable for pH and effluent volatile fatty acid (VFA). Shortening the HRT negatively affected the COD, BOD, and lignin removal efficiencies, pH level, alkalinity, and methane content and production, and increased the VFA effluent concentration. The optimal conditions were established at 4,000 mg/L COD and HRT of 2 days, corresponding to the predicted COD, BOD, and lignin removal efficiencies of 91, 98, and 71%, respectively, whereas 28 mg/L of VFA and 0.125 L of CH4/g CODremoved were generated.

Characterization of hydrophobic-treated recycled paper mill sludge in bituminous materials

The experimental investigation of the performance of hydrophobic-treated recycled paper mill sludge (RPMS) incorporated into asphalt mixtures is presented in this paper. This research implements RPMS as a solid waste additive to partially replace the mineral filler in the asphalt mixture while practicing green asphalt technology. The raw RPMS required mechanical pre-treatments and its hydrophilic property was modified chemically. The hydrophobicity was assessed by Hydrophilic Coefficient, Water Contact Angle (WCA) and Scanning Electron Microscopy (SEM). The ethanol method, which involved the esterification of ethyl esters that utilized 7ml of waste cooking oil (WCO) and 50ml of ethanol, was adopted. In the Marshall mix design, RPMS was incorporated at 0.5% and 1.0% of the weight of aggregates. Conventional 60/70 PEN bitumen and granite aggregates were used. The optimum binder content (OBC) was evaluated and justified by its adsorption strength. The mechanical properties of asphalt mixtures were determined and compared with the Public Work Department (PWD) specifications. All the volumetric properties satisfied the standard specification by PWD for 0.5% modified RPMS asphalt mixture, and thus it is preferable as it also involved lower binder cost due to the lower OBC achieved.

Recycled paper mill process water pre-treatment using ultrafiltration for water system closure

The use of membranes in integrated water systems has garnered increasing attention in paper mills operating with virgin fibres, with documented closed water loops in industry. However, limited research exists on the performance of ultrafiltration (UF) in mills operating with 100% recycled fibres (RCF) and closed water systems, as these result in the introduction and accumulation of insoluble contaminants in the effluent. This study investigated the effect of transmembrane pressure (TMP) and feed organic load on the flux, types of fouling, and organic separation with UF membranes in a recycled paper mill using 100% RCF. The high feed organic load caused inevitable fouling, limiting the highest achievable flux. However, the TMP could be manipulated to improve on or exacerbate the process performance, within the range that was set as a result of the feed's characteristics. Low TMP (1.35 bar) generated the highest permeability and chemical oxygen demand removal, while higher TMPs produced higher fluxes and recoveries. The process was characterised by high initial fluxes, followed by cake formation, which dominated, and pore blockage. The excess cake formation induced cake filtration. This study shows the potential of UF membranes to be used as pre-treatment in filtering the process water in a recycled paper mill operating with 100% RCF, a critical step forward in implementing an integrated water system, and eventually closing the water loop.

Application of sequencing batch biofilm reactor (SBBR) to recycled paper mill effluent treatment

Abstract Paper mill effluent has been characterized as recalcitrant because of containing lignin and its derivatives. Since biofilm system exhibits a notable potential for the removal of recalcitrant contaminants, a sequencing batch biofilm reactor (SBBR) was employed to treat coagulated recycled paper mill effluent in this study. The results indicated that the SBBR removed 91.3 % of chemical oxygen demand (COD), whilst total suspended solid (TSS) and color removal reached 83.1 % and 71.0 %, respectively. The microbial analysis suggested that three typical heterotrophic phyla, Proteobacteria, Bacteroidetes and Acidobacteria are dominant bacteria and reflected the removal of recalcitrant contaminants. The COD removal rate of SBBR is evidently superior to conventional activated sludge process due to high sludge concentration as well as long sludge retention time (SRT). Whilst the problem of sludge bulking can be successfully avoided, the blockage of reactor caused by TSS accumulation and microbial growth deserve further investigation.

Performance of Modified Anaerobic Hybrid Baffled (MAHB) bioreactor treating recycled paper mill effluent: Effects of organic loading rates

The performance of modified anaerobic hybrid baffled (MAHB) bioreactor treating recycled paper mill effluent (RPME) was investigated at various organic loading rates (OLR) of 1, 2, 3 and 4 g COD/ L.day. The bioreactor was operated continuously at constant hydraulic retention time (HRT) of a day without effluent recycled and chemicals adjustment/addition. Throughout 70 days of operation, a maximum removal efficiency up to 97% of chemical oxygen demand (COD) and 98% of volatile fatty acid, biogas production of 12.51 L/day equivalent to methane (CH4) yield of 0.108L CH4/ g COD and a stable pH system between 6.6 to 7.2 were achieved. Additionally, alkalinity of the bioreactor system shows a stable profile that indicates the whole system was well buffered with a quit high degradation of volatile solid (VS) up to 18%. These results indicated that MAHB bioreactor has been successfully treated RPME at various OLR.

A hybrid treatment system for water contaminated with pentachlorophenol: Removal performance and bacterial community composition

A hybrid treatment system of multimedia–sequencing batch biofilm reactor (MM-SBBR; B) and three control bioreactors (C, D, E) processing synthetic wastewater containing pentachlorophenol (PCP), were used to study the effectiveness of PCP removal and the impact of high-levels of PCP (100 mg L−1) on bacterial community compositions. Another MM-SBBR (A) treating real effluent of the recycled paper mill industry containing PCP was also operated and compared. The MM-SBBR (B) achieved a high chemical oxygen demand (COD), ammoniacal nitrogen (NH3-N), and PCP removal efficiencies, reaching 97%, 93% and 99%, respectively, which were attributed to the enrichment of high PCP resistant bacteria in this reactor. It was found that the number of bacterial species was the highest (10 species) in Bioreactor B when compared to the other bioreactors (Bioreactor A, C, D, and E). Proteobacteria phylum (9 species) was found dominated throughout the study, suggesting that the combination MM with granular activated carbon (GAC) and plastic media encouraged the growth of various bacterial species that were resistant to high-levels of PCP and thus enhancing the removal of PCP. It can be concluded that the hybrid treatment system of multimedia (MM) and SBBR is highly capable of removing PCP from water contaminated with PCP due to the diversity presence of PCP resistant bacteria in this bioreactor. MM-SBBR could be the effective treatment method for wastewater containing PCP to improve water quality and ensure sustainability.

Temperature analysis of a novel MAIB reactor during the treatment of wastewater from recycled paper mill

Abstract Anaerobic digestion (AD) is an essential technology for wastewater management, resource recovery and biogas production, and it is considered as an efficient and reliable treatment method for many wastewaters. Operating parameters have been shown to directly affect the stability and treatment performance of AD, especially temperature. For 180 days, the AD of recycled paper mill wastewater (RPMW) was carried out in a modified anaerobic inclining-baffled (MAIB) reactor under various temperature conditions, i.e. 29 °C (low mesophilic), 37 °C (mesophilic) and 55 °C (thermophilic). It was found that total COD removal of 94, 96 and 76%, and methane yields of 0.125, 0.196 and 0.256 L CH4/g CODremoved were attained at temperatures of 29, 37 and 55 °C, respectively. Throughout the three transition periods, the pH level in the MAIB reactor fluctuated slightly within the range of 5.8–6.5 without affecting the system stability. The results concluded that thermophilic condition strongly influenced the MAIB reactor performance, leading to lower COD removal, higher methane yield and gradually recovered pH level.

Multimedia-sequencing batch biofilm reactor in treating recycled paper mill effluent containing high level of pentachlorophenol: Long-term performance, mechanism and kinetic studies

Discharging ill-treated wastewater from pulp and paper industry into water bodies causes accumulation of toxic and carcinogenic substances in the watercourse. A study on the removal of pentachlorophenol (PCP) from wastewater using a combination of granular activated carbon (GAC) adsorption and biofilm in multimedia-sequencing batch biofilm reactor (MM-SBBR) system was conducted. The performance, mechanism and kinetic of PCP removal were investigated using two lab-scale MM-SBBRs and three control bioreactors over a 600-day period. Both MM-SBBRs composed of Perspex column packed with plastic media filled with GAC were used with initial chemical oxygen demand (COD) of 1000 mg L−1 at a fixed hydraulic retention time (HRT) of 24 h. All five bioreactors working in parallel were spiked with different PCP concentrations (10 mg L−1, 50 mg L−1 and 100 mg L−1). MM-SBBR treating synthetic wastewater displayed the most effective PCP, COD and ammoniacal nitrogen (NH3-N) removals ranging from 95 to 99 %, 93 to 99 % and 92 to 100 %, respectively. The kinetic of PCP removal in MM-SBBR system was best-fitted to Singh model (R2 > 88 %). The adsorption capacity of GAC present in the MM-SBBR bioreactor remained constant and thus, the adsorption mechanism played a key role in PCP removal. In conclusion, MM-SBBR system has potential for wastewater treatment in terms of performance efficiency and process stability, therefore, deserve a vote for pulp and paper mill effluents treatment.

Flowsheet Analysis of Valourising Mixed Lignocellulose and Plastic Wastes via Fast Pyrolysis at a Paper Mill

A valourising process annexed to a recycling paper mill for converting three plastic-fibre rejects (PFRs) into oil, char and utility energy through pyrolysis provides renewable energy and prevents waste disposal. The economic and technical responses of the process are measured by varying the conversion temperature, feedstock characteristics and energy recovery approach, in order to optimise the processs. Previously measured responses of valourising PFRs were simulated in AspenPlus® according to variations in the fast-pyrolysis temperatures (350–550 °C); the contents of lignocellulosic fibres and plastics; and methods for adding value to excess process heat as steam and/or electricity. Economic models scaled at 540 kg/h were derived to determine the required minimum fuel selling price (MFSP) of the oil product. A base case for heat recovery assumes both steam and electricity. For a PFR stream composing mainly lignocellulose, the MFSP of oils ranged from 341 US$/GJ for pyrolysis at 350 °C to 65 US$/GJ for pyrolysis at 550 °C, while the process at 450 °C was most energy efficient. When only recovering steam from waste energy, the MFSPs were reduced by 6%. Pyrolysis of reject streams with increased plastics contents reduced the MFSP of the oil to 37 US$/GJ, due to the higher calorific value of the products. Increasing the feed rate of PFRs to 2400 kg/h could potentially reduce the MFSP to a market-competitive price of 15.3 US$/GJ, at a scale equivalent to a power-plant of 5MWe. PFRs containing higher proportions of plastics are preferred for valourisation through pyrolysis at 550 °C, while waste energy is best recovered as steam.

Treatment of UASB-treated recycled paper wastewater using SBR and SBBR: A comparison

Anaerobic-oxic (AO) systems have been extensively adopted for the biological treatment of wastewater from recycled paper mills, which is characterized by high chemical oxygen demand (COD) concentrations and contains hundreds of organic compounds. In this study, an up-flow anaerobic sludge blanket (UASB) served as the anaerobic treatment of recycled paper mill wastewater. Then, either a sequential batch reactor (SBR) or a sequential batch biofilm reactor (SBBR) were adopted as aerobic treatment to treat the UASB effluent respectively. Parameters such as COD, BOD5, and TSS were measured to compare the treatment performance of SBR and the SBBR. After 80 days’ operation, COD removal efficiency of SBR and SBBR were 21.79 ± 3.4% and 38.38 ± 2.69% respectively; TSS removal efficiencies were 20.84 ± 5.15% and 47.02 ± 5.84% respectively. The results indicated that SBR was effective for removing residual organic matter in UASB effluent. However, SBBR showed significant advantages for the removal of COD and total suspended solids (TSS), which are ascribed to the effective biomass retention and biofiltration of SBBR.