Investigation of treatment performances and energy recoveries from a real textile wastewater in conventional and high-rate MBR processes

Abstract

Textile industry wastewater, which is among the most water-consuming sectors worldwide, is extremely hazardous for receiving water bodies due to its toxic and complex structure. Many studies have been conducted involving physical, chemical, biological and combined processes for the textile wastewater treatment, but none of them alone is sufficient to meet discharge standards, and each process requires higher investment and operating costs. Additionally, energy-neutral plants should be expanded to ensure circular economy during the textile wastewater treatment, and the treated wastewater should be appropriate to be used for further reuse processes. Aerobic membrane bioreactors (MBRs) have been widely preferred in the textile wastewater treatment due to their ease of operation, lower volume requirement, and providing higher quality effluent compared to traditional biological methods. However, aerobic MBRs require higher energy input to both supply the oxygen requirement of microorganisms and minimize membrane fouling. In the treatment of textile industry wastewater, a single aerobic process is not sufficient to remove organic matters, azo and reactive dyes (anaerobic/aerobic combined system are needed) and nitrogen-based pollutants (nitrification and denitrification processes are needed). In addition, processes selected should be less-energy consuming due to high volume of textile industry wastewater. Considering that water resources are decreasing globally today, it is not enough to treat textile industry wastewater and their recycling is also extremely important. Therefore, sustainable and practical treatment strategies should be developed for textile industry. This thesis aims to investigate the treatment potential of two different biological processes with different advantages for real textile wastewater. First of all, the treatment performance of the real textile wastewater and the recovery of organic matter that can be preferred as a raw material source for biomethane generation were investigated in an aerobic high-rate MBR including a hollow fiber (HF) ultrafiltration membrane (UF) with a pore size of 0.04 µm. In the high-rate MBR process, it was aimed to recover organic matters rather than oxidation at short (0.5-5 days) sludge retention times (SRTs) and hydraulic retention times (HRTs). Additionally, the effects of SRT/HRT ratios on membrane filtration performance, sludge characterization, sludge production, and the energy requirements for aeration were examined, and all parameters were compared with a conventional long SRT aerobic MBR system operated in parallel. In another MBR process equipped with an HF-UF membrane, the feasibility of intermittent aeration strategy for simultaneous nitrification and denitrification to remove nitrogen-based pollutants and for energy minimization were investigated. Additionally, the effects of different aeration patterns on membrane filtration performance, sludge characterization, sludge production and energy requirements for aeration, and specific removal rates were studied. Within the scope of this thesis, all studies were carried out in four stages. In the first stage, textile wastewater treatment performances of conventional MBRs were investigated at SRTs of 30, 20 and 10 d. During this stage, two identical MBR systems were run, one at SRT for 30 days (MBR-1), and the other at SRT for 20 and 10 days (MBR-2), respectively. The average total chemical oxygen demand (COD), color and total dissolved nitrogen concentrations of textile wastewater used in the study averaged 927±277 mg/L, 910±287 Pt–Co and 39±10 mg/L, respectively. While COD removal performance was above 90% in all SRTs, color removal reached its lowest value at SRT of 10 d and did not show any correlation with SRT. In both MBRs, transmembrane pressure (TMP) was below 10 mbar throughout the study and no membrane fouling was observed. Supernatant filterability (SF) and specific filtration resistance (SRF) values increased at 10 d of SRT. A decrease in viscosity values was also observed due to the decrease in suspended solids (SS) concentration as SRT decreased. While SMP concentrations were similar at all tested SRT values, an increase in EPS concentration was observed at 10 d SRT. Additionally, reducing SRT resulted in an increase in waste sludge generation and observed biomass yield (Yobs), and a decrease in the energy requirement for aeration. According to gel permeation chromatography (GPC) results, as SRT decreased, organic compounds with low molecular weight had higher signals. In the second stage, aim was to investigate textile wastewater treatment performance and organic matter recovery efficiency at short SRTs, and a laboratory-scale high-rate aerobic MBR was run at SRTs of 0.5 – 5 d and HRTs of 1.2 – 24 h, corresponding to predetermined different SRT/HRT ratios of 5, 10 and 20. The average total COD, color, and soluble nitrogen concentrations in the wastewater were 834±143 mg/L and 1037±407 Pt-Co and 51±11 mg/L, respectively. While COD removal performances ranged between 86 and 92% at SRT of 5, 3, and 2 d (in all SRT/HRT ratios), it decreased to 82 and 77% at SRT 1 and 0.5 d (at SRT/HRT ratio of 10), respectively. There was no correlation between decolourization performance and SRT or HRT as it varied between 26% and 70%. The nitrification performance in the system stopped completely at SRTs ≤ 2 d. In particular, when SRT decreased from 5 days to 1 day, the amount of sludge produced and Yobs values increased. The SRT/HRT ratio played an important role in the energy requirement for aeration. In addition, reducing the SRT in the system resulted in higher SRF values, lower SF values, and rapid membrane fouling. SMP in the supernatant increased especially at SRTs ≤ 2 d. The total EPS concentration increased as SRT decreased, but the it decreased as the SRT/HRT ratio increased at each SRT value. No significant change occurred in the molecular weight distributions of the organic substances in the supernatant and filtrate at SRT of 3, 2, and 1 d. Throughout the study, in the cake layer deposited on membrane, Al, Si, and Fe were detected below 2%. In the third stage, the aim was to investigate optimum operating conditions for the simultaneous nitrification and denitrification processes to remove organic matter, color and nitrogen-based pollutants in the real textile wastewater using an MBR with an intermittent aeration strategy. The system was first operated at different dissolved oxygen values (DO of 6 and 3 mg/L) and then aeration-on/off durations varying between 2 min/2 min and 90 min/360 min. The average total COD, color, and TN concentrations of the wastewater were measured as 793±173 mg/L, 1171±458 Pt–Co and 65±15 mg/L, respectively. COD removal performance ranged from 84 to 91%. In all tested conditions, color removal performance was highly variable and independent of operating conditions, ranging from 40 to 68%. While ≥89% nitrification performance was achieved in the MBR at a minimum aeration-on durations of 30 min, the highest denitrification efficiency was achieved in the cycle with an aeration-off durations of 360 min. With the intermittent aeration process, higher TN removal, less sludge production and less energy requirement for aeration were achieved. However, membrane fouling profiles occurred more quickly at the aeration-off durations of 60 min and longer. Additionally, while SS, VSS, SF and viscosity values decreased under intermittent aeration conditions, SRF values increased. Although SMP concentrations decreased with intermittent aeration, EPS concentrations were quite similar. While no change was observed in the molecular weights of the supernatant and filtrate samples of the MBR, the average particle sizes in the supernatant increased as the aeration-off time increased. Finally, according to SEM-EDS results, inorganic substances such as Ca, Mg, Si, and Na were detected in the cake-deposited membrane surfaces. In the fourth stage, the impacts of different aeration patterns on the specific ammonium oxidation, denitritation and denitrification rates were investigated in batch assays. The batch experiments were conducted using the sludge taken from the MBR operated at various DO concentrations and aeration on/off times. While the specific ammonium oxidation rate was determined by batch tests and respirometric studies, specific denitritation and denitrification rates were determined by parallel batch reactors containing different nitrite and nitrate concentrations. The highest specific ammonium oxidation, denitritation, and denitrification rates were obtained as 5.4, 3.8, and 5.3 mg N/(g VSS.h), respectively, at the aeration-on/off durations of 90/360 min. Specific ammonium oxidation rates increased by 1.8 and 2.1 times in the last period (the aeration-on/off time of 90/360 min), compared to continuous aeration conditions with DO of 6 and 3 mg/L, respectively.