Endogenous respiration process analysis between aMBR and UV/O3-aMBR for polluted surface water treatment


 The microbial endogenous respiration process is very important in biological water treatment processes. This study analyzed and compared the endogenous respiration process in an attached growth membrane bioreactor (aMBR) system and a UV/O3 integrated aMBR system (UV/O3-aMBR) in treating polluted surface water with CODMn around 10 mg/L. The endogenous respiration activity of heterotrophic microbes and autotrophic nitrifiers activity in both systems was analyzed and compared. Results show that heterotrophic bacteria and autotrophic nitrifiers enter endogenous respiration at 6 h aeration in an aMBR and 0 h in a UV/O3-aMBR system. Biomass amount on PVA-gel in aMBR was higher than in UV/O3-aMBR in terms of specific respiration rates SOURt, SOURH, and SOURA. Substrate remained on PVA-gel in the aMBR system, but no substrate remained on PVA-gel in the UV/O3-aMBR system. Higher species of microbes, including recoverable and irrecoverable components, existed in the aMBR system as compared to the UV/O3-aMBR system. The UV/O3-aMBR system could make full use of the advanced oxidation process (AOP) and biological process, leading to a higher treatment performance, and has the potential to mitigate total energy demand. Thus, the UV/O3-aMBR system can be used as a new technology for treating polluted surface water with the co-contribution of biological process and AOP treatment.


INTRODUCTION
Surface water quality is facing increasing organic contamination due to human activities worldwide (Ma et al. ; Utaaker et al. ). Overload of organic matter pollution in surface water leads to challenges during treatment using conventional drinking water treatment plants. These challenges include higher consumption of coagulants and higher disinfection by-product formation potential. The water treatment technology should be able to treat the polluted surface water and reach the drinking water quality standard, which is a big challenge for the world's drinking water supply. Table 1 demonstrates the drinking water national standards of some selected countries and institutions. Improving current wastewater treatment technology or developing high efficiency water treatment technology can be the solution to the polluted surface water issue. A previous study has reported that in an attached growth membrane bioreactor (aMBR) for treating polluted surface water with COD Mn around 10 mg/L, over 50% COD Mn removal was achieved (Li et al. ). The aMBR system works as an enhanced natural purification process, and polyvinyl alcohol gel (PVA-gel) was used as a bio-carrier to immobilize bacteria for the degradation of organic pollutants from polluted surface water. The aMBR system was operated without adding activated sludge and, as a result, membrane fouling was largely mitigated, and PVA-gel was used as the main contributor to organic removal in the system. In order to improve the recalcitrant organic compounds' removal, advanced oxidation processes (AOPs) were integrated into the aMBR system as a polishing step (UV/O 3 -aMBR), to partially degrade recalcitrant pollutant and improve its biodegradability.
The organic loading rate in the aMBR and UV/O 3 -aMBR system was much lower than in conventional wastewater treatment plants, because the COD Mn concentration was only around 10 mg/L. The endogenous respiration of bioprocess with higher organic loading rate, such as wastewater, was reported by earlier studies (Fall et al. ; Li et al. , a; Ordaz et al. ; Park et al. ). The bioprocess in the aMBR system is expected to be quite different from conventional activated sludge processes or other biological treatment processes. The AOPs combined with an aMBR system could also lead to changes in substrate utilization and microbe activities. In addition, the bioprocess between aMBR and UV/O 3 -aMBR is expected to be quite different and thus needs to be compared and analyzed.
Bioprocess aerobic respiration commonly includes exogenous respiration and endogenous respiration (Hao et al. ). Endogenous respiration always occurs once no substrate remains for the microbes to use, and the active cell starts utilizing intracellular substances (Gujer et al. ). Endogenous respiration has been commonly regarded as an evaluation method for active microorganisms, and can be tested quickly and simply with oxygen uptake rate methods (Spanjers & Vanrolleghem ). The widely used oxygen uptake rate analysis method is one type of respirometry method, which detects the dissolved oxygen (DO) concentration with a DO meter providing real-time data, to intuitively reflect the substrate utilization by microbes There has been limited research on the membrane bioreactor (MBR) in treating polluted surface water, let alone the biological process in the MBR in this sector. This study analyzed and compared the endogenous respiration process in aMBR and UV/O 3 -aMBR systems in treating polluted surface water by the respirometry method. The microbe activities, including heterotrophic and autotrophic entry time of endogenous respiration, and ability in antidecay, were also compared and discussed. This study could provide useful information for the understanding and practical application of aMBR and UV/O 3 -aMBR processes in treating polluted surface water for drinking purposes in a more environmentally friendly way.

System setup
A schematic and flowchart of aMBR and UV/O 3 -aMBR parallel operation system setup are shown in Figure 1   UV light 8 W with a wavelength of 254 nm was used and an O 3 generator (Nippon Ozone Co., Ltd, Japan) with concentration of 1.5 mg/L was used.

Experimental materials
PVA-gel used as a bio-carrier in both systems was taken out for endogenous respiration analysis from the stable operated aMBR and UV/O 3 -aMBR system, respectively. The removal performance of basic parameters including COD Mn , DOC, UV 254 , specific UV absorbance (SUVA), and total nitrogen (TN) was tested before taking out the PVA-gel from both systems. The suspended solids concentration in the carrier side of both systems was less than 100 mg/L and was not taken into consideration for endogenous analysis.
A respirometer was set up with an automatically recording DO meter (HQ40d, HACH, USA) and in a room at constant temperature; the schematic of the system is as shown in Figure 2. During the oxygen uptake rate test, 50 mg/L NH 4 Cl was used as the nitrogen source and 100 mg/L CH 3 COONa was used as the carbon source. DO values were automatically recorded every 30 seconds for 15 min in all conditions tested. The temperature in all tests was maintained at 25.7 ± 0.2 C. The pH was 7.79 ± 0.04 in the aMBR system and 7.90 ± 0.03 in the UV/O 3 -aMBR system.

Respirometric analysis calculation
A respirometric method with a DO meter recording the DO concentration changes in the aqueous system was used.
Oxygen uptake rate was calculated from the DO values.
The PVA-gel bio-carrier amount in each condition was counted after the test, and the specific oxygen uptake rate was calculated accordingly. The respiration rate was composed of exogenous respiration and endogenous respiration, among which, exogenous respiration included autotrophic respiration and heterotrophic respiration as is shown in Equation (1): Equation (1) divided with OUR T can produce Equation (2): where the OUR T is theoretic total respiration rate, OUR en is real tested endogenous respiration rate, OUR A is real tested autotrophic respiration rate, OUR H is real tested heterotrophic respiration rate, and OUR t is real tested total respiration rate. OUR en /OUR T represents the ratio of endogenous respiration to the theoretical total respiration rate, while OUR A /OUR T represents the ratio of autotrophic respiration to the theoretical total respiration rate, and OUR H /OUR T represents the ratio of heterotrophic respiration to the theoretical total respiration rate. SOUR T is specific total respiration rate, SOUR en is specific endogenous respiration rate, SOUR A is specific autotrophic respiration rate, and SOUR H is specific heterotrophic respiration rate.

Respirometric test setup
One hundred and forty milliliters of PVA-gel were sampled from the aMBR and UV/O 3 -aMBR systems, of which 20 mL from each system was taken out for testing at an The respiration test for one set of aeration time was carried out as follows.
(1) 20 mL carrier was put into 400 mL RO water in a respirometer, and OUR en was tested in this situation.
(2) The solution in the respirometer was then discharged and the respirometer filled with 400 mL RO water.
Ten mL of 2,000 mg/L NH 4 Cl was then added to obtain a concentration of 50 mg/L, and OUR N was then tested. (3) The solution in the respirometer was then discharged and the respirometer filled with 400 mL RO water. Ten mL of 4,000 mg/L CH 3 COONa was added to obtain 100 mg/L of a CH 3 COONa concentration, after which OUR c was then tested. (4) The solution in the respirometer was discharged and it was filled with 400 mL RO water, 10 mL of 4,000 mg/L CH 3 COONa and 10 mL of 2,000 mg/L NH 4 Cl was added to obtain 100 mg/L CH 3 COONa concentration and a NH 4 Cl concentration of 50 mg/L, then OUR t was tested. The flowchart of this process is shown in Figure 3.
OUR en was obtained when no substrates were added, OUR N was obtained when only nitrogen source was added, OUR c was obtained when only carbon source was added, and OUR t was obtained when both nitrogen source and carbon source were added:     This also could suggest that sufficient ammonia oxidation bacteria existed on the PVA-gel in the aMBR system.
According to Figure 4, 6 h is a turning point, where SOUR H starts to decrease after increasing, thus, 6 h can be regarded as the point where heterotrophic bacteria enters endogenous respiration in the aMBR system. As shown in results also suggest that in the UV/O 3 -aMBR system, substrate biodegradability was higher than in the aMBR system.
As shown in Figure 4,  in aMBR and UV/O 3 -aMBR was increased with increasing aeration time, which indicated that the endogenous respiration will increase when facing adverse environmental conditions such as lacking substrate. The higher endogenous respiration rate implied a higher oxygen consumption required for microbe fundamental metabolism, which also presents a higher operation cost demand of the system.
The SOUR en /SOUR t value in aMBR was lower than in UV/O 3 -aMBR, which indicated that the microbe activity was higher in the aMBR system as compared with the advanced oxidation process (AOP) integrated system. Calculated from the theoretical SOUR T , heterotrophic bacteria occupied around 60% and autotrophic bacteria 40% in the aMBR system. As shown in Figure 4, it can also be seen from the system that the SOUR H was always higher than SOUR A throughout the aeration time tested. Also, as shown in Figure 4, there is no absolute stability of SOUR H , SOUR A , and SOUR t during 120 h aeration and this indicated that the microbes' activity in the aMBR system is tough and active enough. Thus, the addition of any substrates can motivate their respiration processes within 120 h aeration.
The respiration map of bacteria growth on PVA-gel in the UV/O 3 -aMBR system is shown in Figure 5. Compared with Figure 4, the values obtained for SOUR t , SOUR H , and SOUR A were lower than the values obtained from the aMBR system in all samples tested during 120 h aeration.
This shows that the biomass amount on PVA-gel in the UV/O 3 -aMBR system was much lower than in the aMBR and 5 that the SOUR t in the two reactors was nearly the same, and this indicated that, even with higher biomass in the aMBR system, it provides a similar degree of bioactivity as the UV/O 3 -aMBR system when sufficient nitrogen source and carbon source were provided.

Practical applications and future perspectives
The membrane-based technology could guarantee the permeate water quality in drinking water treatment. The biological process is an environmentally friendly treatment technology for dealing with organic matter in polluted surface water. With the deterioration of surface water quality, the combination of membrane technology and biological technology-membrane bioreactor system has the potential to be developed as an effective alternative technology in drinking water supply. However, very limited work has been done in illustrating the MBR related system in the treatment of polluted surface water. Polluted surface water was defined as water quality with COD Mn around 10 mg/L and NH 3 -N around 3 mg/L. This is much lower as compared with the municipal wastewater, while being higher than the common source water that is used for drinking water supply. The increase of organic matter in surface water is an inevitable trend, which has gradually brought challenges to the conventional coagulation and flocculation drinking water treatment process. The integration of an AOP into the aMBR system as a polishing step to improve the permeate water biodegradability and then to be treated by biological processes could largely improve the recalcitrant organic matter removal. More work has to be focused on the investigation of the MBR technology application in the treatment of polluted surface water.
The biological process has played an important role in the MBR system for treating polluted surface water. The illustration and comparison of the biological process in the aMBR and UV/O 3 -aMBR systems could provide detailed information for a future pilot-scale system design. The endogenous respiration map method used in this study is a simple and accurate one. The use of an endogenous respiration map could provide information about microbes in the system, and provide guidance in practical application.
It is also recommend to build up a small program to generate a corresponding respiration map simply with the input from the necessary parameters.

CONCLUSIONS
This study investigated the endogenous respiration process in aMBR and UV/O 3 -aMBR in treating polluted surface water. Results indicated that microbes enter endogenous respiration at different times in the two systems. With the integration of the UV/O 3 process into the aMBR system, the biomass on the bio-carrier was changed. Higher abundance of microbes including recoverable and irrecoverable components existed in the aMBR system compared to the UV/O 3 -aMBR system. The use of AOPs in aMBR for treating polluted surface water could improve recalcitrant removal, while the design of AOP conditions also has to be carefully involved to guarantee the contribution of the biological process.