The operational ef ﬁ ciency of a novel AnMBR treating antibiotic solvent wastewater in start-up stage

The performance of a novel anaerobic membrane bioreactor (AnMBR) for treating antibiotic solvent wastewater was investigated in the start-up stage. The removal ef ﬁ ciencies of the four tested antibiotics were over 90%, mainly attributed to the biological process. Volatile fatty acid increased along with anaerobic sludge acclimation. pH (mean value 7.5) and a (mean value 0.12) remained stable. Mixed liquid suspended solids and mixed liquor volatile suspended solids increased along with the sludge acclimation as well. The protein and polysaccharide in suspended sludge decreased, while the protein/polysaccharide in exopolysaccharides increased. Microbial community analysis showed the abundance of Methanosarcina spp. ﬂ uctuated over time and was ﬁ nally stable at 17%. The abundance of Methanosaeta spp. increased signi ﬁ cantly. There are two kinds of hydrogen producing methane producing microorganisms ( Methanobacteriales and Methanomicrobiales ) in AnMBR. Methanobacteriales was the dominant methanogenus. These results indicate that an AnMBR can effectively treat antibiotic solvent wastewater in the start-up period.

Additionally, the shorter hydraulic retention time (SHRT) and the higher COD volumetric loading rate (HOLR) are two very important directly controllable operating parameters in an AnMBR. This is because SHRT-HOLR not only relates to the AnMBR operating efficiency and reactor size, but also relates to the microbial community characteristics and membrane fouling control in an AnMBR (Ramos et al. ; Pretel et al. ). Research on the operational efficiency of AnMBRs treating antibiotic solvent wastewater also is scarce under SHRT-HOLR conditions. Therefore, the purposes of this work are: (a) to explore the feasibility of an AnMBR for treatment of antibiotic solvent wastewater containing M-cresol (MC), isopropanol (IPA), tetrahydrofuran (THF), and N,N-Dimethylformamide (DMF); and (b) to understand the removal efficiency for organic matter, the removal efficiency for characteristic pollution, the membrane efficiency and the structure change of the microbial community in the start-up stage of an AnMBR system.

Test apparatus
A new split type AnMBR was designed as shown in Figure 1.
The main body anaerobic fermentation tank in the AnMBR adopted continuous stirring and complete mixing. A threephase separator was set for separating biological gas, wastewater and anaerobic sludge. The biogas was collected from the top of the reactor. The anaerobic sludge automatically slid down and settled to the bottom of the reactor. Wastewater flowed out from the clear zone. This not only Figure 1 | Experimental set-up of AnMBR. 1. pH/ORP/temperature monitor; 2. gas meter; 3. back pressure valve; 4. liquid flow; 5. pressure meter; 6. rotor flowmeter; 7. UF tubular membrane; 8. frequency converter; 9. electromagnetic valve; 10. sampling port; 11. check valve; 12. anaerobic digester.
reduced the loss of anaerobic sludge, increasing the anaerobic sludge concentration in the reactor, but also reduced the sludge content and sludge viscosity, thus effectively reducing membrane fouling.

Synthetic wastewater
In order to explore the optimized experimental conditions in the AnMBR, the experimental antibiotic solvent wastewater was artificially prepared. The main organic pollutants in the wastewater were MC, IPA, THF, and DMF. The specific water quality indicators are shown in Table 1.
The content of ammonia nitrogen in the simulated wastewater was low. The nitrogen, which is used by microorganisms, was insufficient. It was necessary to add the appropriate amount of ammonium chloride and potassium dihydrogen phosphate into the influent to supplement the nitrogen and phosphorus (C:N:P ¼ 250:5:1) in this work.
Glucose was added to the wastewater simultaneously as the co-substrate (1.032 g glucose COD is 1.0 g), forming the glucose and antibiotic solvent (MC, IPA, THF, DMF) mixed wastewater. The glucose concentration in the wastewater was gradually reduced, while the concentration of antibiotic solvent was increased in the experiment. Thus, the anaerobic sludge was domesticated, making it suitable for the degradation of antibiotic solvent. The influent eventually became a pure solvent of antibiotic wastewater. The dosages of glucose, buffer, nutrients and trace elements in the synthetic wastewater are shown in Table 2.  Biochemical oxygen demand (BOD) (mg/L) 200-1,000 400-2,000 2,000-5,000 2,000-6,500

AnMBR start-up
There are two strategies to start up an AnMBR: (1) using a constant HRT and COD of concentration to start-up; (2) using a constant HRT and gradually increasing the COD concentration to start-up. In accordance with the antibiotic solvent wastewater characteristics, this work used the second strategy, and the specific operating conditions are listed in Table 3.

Removal rate calculation
The COD, MC, IPA, THF, DMF biological removal rate ¼    As shown in Figure 3, in the start-up period, THF was not detected in the influent until the 5th day. When the influent COD increased from 1,000 to 3,000 mg/L, the influent THF gradually increased from 20 to 100 mg/L, and the THF start loading rate was 0.01 kg THF/(m 3 ·d), the effluent THF was less than 5 mg/L, the total removal rate of THF was 95%.
The THF concentration in supernatant fluctuated from 13 to 24 mg/L, slightly higher than that in membrane effluent. The biological removal efficiency of THF was stable at about 80% throughout the start of the experiment, while the physical removal efficiency of THF was 11%.
As shown in Figure 3, the influent DMF increased from 0 to 300 mg/L, the corresponding DMF volumetric loading rate increased from 0 to 0.15 kg DMF/(m 3 ·d), the membrane effluent DMF and supernatant DMF were similar to each other with an average concentration of 26 mg/L and 39 mg/L, respectively.
During the start of the experiment, the total removal efficiency of DMF increased from 49% to 92%, the DMF biological removal rate increased from 43% to 75%, and the average physical removal rate of DMF was 7%, which proves that the physical removal rate of DMF is also basically achieved by the physical entrapment of the membrane at start-up initial stage because the membrane surface did not form the two DMs. the total removal rate of IPA increased from 53% to 96%, the IPA biological removal rate increased from 48% to 84%, and the average physical removal rate of IPA was 7.3%.
As shown in Figure

Analysis of VFA and terminal fermentation products
In order to investigate the operational efficiency of the AnMBR treating antibiotic solvent wastewater in the startup stage, the change of VFA and terminal fermentation product over time were measured in the start-up phase ( Figure 4).
As shown in Figure 4, VFA increased from 58 mg/L (acetic acid) to 320 mg/L along with the anaerobic sludge acclimation, but the growth rate was slow, indicating that microorganisms in the reactor were adapting to wastewater, a few syntrophicacetogenic bacteria began to appear and VFA began to accumulate, however other volatile acid concentrations did not significantly change.
Throughout the start-up period, the reactor performance was good, which could also be reflected in the pH (the average value was 7.5) and a value (the average value was 0.12). Methanomicrobiales just use hydrogen as the substrate.

Poggi-Varaldo
Methanosaeta spp. was the predominant archaea (the relative abundance was 22%) in the start-up stage. In 6-9 d, the abundance of Methanosarcina spp. had been reduced (the relative abundance was 16%), however its relative abundance increased (mean value is 30%) in days 10-12, and then decreased in day 13 and remained at around 17%.
Methanosaeta spp. was able to grow in a low concen-   ) and the anaerobic moving bed reactor (Borghei & Hosseini ).
In this work, the granular sludge inoculated in the AnMBR was crushed in the inoculation process. This was because AnMBR conditions could not stimulate the formation of granular sludge. Therefore, the increase of methane bacteria of bristle concentration was probably due to lower acetate concentration at the beginning of the operation.
There were two kinds of hydrogen utilization methanogens in the AnMBR (Methanobacteriales and Methanomicrobiales).
In the initial 23 days of reactor operation, Methanobacteriales dominated the archaea community. The relative average abundance was 16%. During the start-up stage, Methanomicrobiales maintained a low abundance with an average value of 5.5%.