This study investigated the performance of an autohydrogenotrophic membrane biofilm reactor (MBfR) to remove nitrate from water with high sulfate concentrations. The results of simulated running showed that TN removal could be over than 98.8% with the maximum denitrification rate of 134.6 g N/m3 d under the conditions of the influent sulfate concentrations of 300 mg SO42−/l. The distribution ratio of H2 electron donor for nitrate and sulfate was 70.0 : 26.9 at the high influent loading ratio of sulfate/nitrate of 853.3 g SO42−/m3 d : 140.5 g N/m3 d, which indicated that denitrification bacteria (DB) were normally dominated to complete H2 electron with sulfate bacteria (SRB). The results of molecular microbiology analysis showed that the dominated DB were
Nitrate-contaminated river or groundwater occurred everywhere in the world because the fertilizers were utilized extensively and part of the wastewater from industries was discharged randomly, especially in developing countries [
The effective methods to reduce nitrate include ion exchange [
Recently, a new technology of hydrogen- (H2-) based membrane biofilm reactor (MBfR) has developed and got a good effect, which used autohydrotrophic bacteria in the denitrification processes [
The following equations could describe the stoichiometry of hydrogenotrophic denitrification and sulfur-reducing:
While in some sites in the world (e.g., natural mineralogy), the contents of sulfate could be as high as hundreds or thousands micrograms per liter in the groundwater, which is used as a drinking water. Because SO42− is not normally considered a health concern, and no MCL has been established for SO42−, so many references of autohydrogenotrophic denitrification could concern about sulfate reduction, but the concentrations of SO42− were relatively lower in the influents for research [
The aim of this study was to investigate the performance of autohydrogenotrophic denitrification under the high concentrations of sulfate by a hollow fiber membrane bioreactor with polyvinyl chloride (PVC) membrane.
The theory of denitrification using hydrogenotrophic bacteria is shown in Figure
MBfR in the experiment (a) and theoretical views of MBfR (b).
The parameters of the reactor.
Parameters | Unit | Value |
---|---|---|
Numbers of fiber module | 2 | |
Outer diameter of fiber | cm | 0.15 |
Inner diameter of fiber | cm | 0.085 |
Fiber number in the reactor | 96 | |
Length of fiber | mm | 140 |
Volume of fibers | cm3 | 23.74 |
Available surface area | cm2 | 633.34 |
Available volume of reactor | cm3 | 560 |
Void ratio | % | 95.76 |
Specific surface area | m2/m3 | 113.10 |
Height | cm | 22.0 |
Section area of reactor | cm2 | 28.26 |
Diameter of reactor | cm | 6.0 |
Available volume of reactor | cm3 | 560 |
In the study, the influent water was taken from the sulfate- and nitrate-contaminated groundwater in the vegetable land at the suburb of Qingzhou (Weifang, China), where a lot of fertilizer had been used in the lands. The shallow groundwater around the vegetable land had been contaminated by nitrate and sulfate, and the water quality is shown in Table
Water quality parameters of the groundwater.
Total dissolved solids (mg/l) | pH | Alkalinity (mg/l as CaCO3) | Hardness (mg/l as CaCO3) | DO | Nitrate (mg N/l) | Nitrite (mg N/l) | Sulfate (mg/l) |
---|---|---|---|---|---|---|---|
300–400 | 7.2~7.5 | 320~500 | 400~650 | 6.0–6.4 | 35~60 | ND | 250~450 |
ND: not detected.
We stated up the reactor by inoculating the biofilm microorganisms from other MBfRs running for hydrogenotrophic denitrification for years in our lab. For simulating the different concentrations of sulfate in the influent water, some dosage of FeSO4·7H2O was fed in the influent pumped from the actual groundwater. The detailed experimental design of the reactor running could be seen in Table
Experimental design of the reactor running.
Start-up | Run I | Run II | Run III | |
---|---|---|---|---|
Running time (day) | 3 | 1–40 | 41–80 | 81–155 |
H2 pressure in the fiber (MPa) | 0.02 | 0.03 | 0.04 | 0.05 |
Nitrate concentration in the influent (mg N/l) | 10.0 ± 2.0 | 20.0 ± 2.0 | 40.0 ± 4.0 | 50.0 ± 4.0 |
Sulfate concentration in the influent (mg/l) | 100 ± 10.0 | 200 ± 10.0 | 250 ± 10.0 | 300 ± 10.0 |
Flow rate (ml/min) | 1.1 | |||
HRT (h) | 8.5 |
All the fluid samples collected in the experiments were kept at 4°C until the samples were analyzed. The NO3−-N, NO2−-N, and SO42− were measured by the ion chromatography (Dionex ICS 3000). The H2 unutilized by the denitrifiers would go into the headspace of the reactor. A GC 14-B equipped with a TCD detector (Shimadzu Co.) was used to test the H2 gas concentration in the headspace in the reactor by pumping gas from the gas port by a syringe, and the hydrogen content in the liquid could be calculated by Henry’s law.
In the experiments, at different running periods for the reactor, the biofilm would be sampled to analyze the changes of the microbial communities. For our study, when the water quality in the effluent was steady, that is, at day 40, day 80, and day 150, the biofilm samples were collected. According to our previous research, DNA extractions, PCR, and DGGE were done; see the detailed methods in [
In the beginning of the experiment, the biofilm established on the out surface of the fiber was only taken 3 days just because of the inoculation of bacteria from the reactors running over than years. Then, the reactor was operated over 155 days to evaluate the performance of MBfR under different conditions. The performance of MBfR over the operation periods was illustrated in Figure
The water quality in the influent and effluent and TN removal.
As shown in Figure
In this experiment, the high sulfate concentrations up to 300 mg/l in the influent were used to investigate the performance of MBfR. Under the conditions of the different contents of sulfate in the influent, the denitrification loadings and sulfate loadings could be seen in Table
The influent loadings and volume reductions for nitrate and sulfate under different influent sulfate concentrations.
Influent sulfate contents (mg/l) | Influent sulfate loading (g/m3 d) | Volume sulfate reduction (g/m3 d) | Nitrate loading (g N/m3 d) | Volume denitrification rate (g N/m3 d) | Sulfate in effluent (mg/l) | Nitrate in effluent (mg N/l) | References |
---|---|---|---|---|---|---|---|
200 | 566.3 | 155.4 | 57.8 | 55.7 | 145.3 | 0.7 | This study |
250 | 707.3 | 166.3 | 112.5 | 111.6 | 191.3 | 0.3 | This study |
300 | 853.3 | 226.7 | 140.5 | 134.6 | 221.5 | 2.1 | This study |
42 | 118.5 | 50.7 | 56.5 | 55.5 | 24 | 0.3 | [ |
92 | 262.6 | 109.6 | 139.5 | 133.8 | 54 | 2 | [ |
78 | 216.8 | 85.3 | 141.7 | 136 | 46.5 | 2 | [ |
The volumetric denitrification rates were changed from 55.7 g N/m3 to 134.6 g N/m3 with a good TN removal over than 94.9%, which was mainly caused by increasing the influent nitrate loadings. The sulfate reduction rate was changed from 155.4 to 266.7 g SO42−/m3, which was not mainly controlled by the influent sulfate loading of 566.3–853.3 g SO42−/m3, and the average sulfate removals were about 23.5–27.4%. It indicated that the nitrate would be utilized preferentially by denitrification bacteria (DB) than sulfate utilized by SRB in completion with H2 in MBfR, and nitrate respiration is energetically more favorable than sulfate respiration [
In the autohydrogenotrophic denitrification in MBfR, the SRB also utilized hydrogen as electron donor to reduce sulfate to sulfide; therefore, there would be a competition for hydrogen between the reductions of nitrate, sulfate, and other electron acceptors. The distributions of hydrogen electron in electron acceptors at different influent sulfate contents in this study and references are shown in Table
Distributions of hydrogen electron in electron acceptors at different influent sulfate contents.
Influent sulfate (mg/l) | Influent nitrate (mg N/l) | Nitrate (%) | Sulfate (%) | Oxygen (%) | Cr (VI) (%) | References |
---|---|---|---|---|---|---|
200 | 20 | 57.9 | 36.1 | 6.0 | This study | |
250 | 40 | 71.8 | 24.4 | 3.8 | This study | |
300 | 50 | 70.0 | 26.9 | 3.1 | This study | |
42 | 20 | 76.0 | 15.9 | 8.1 | [ | |
92 | 50 | 81.2 | 15.2 | 3.6 | [ | |
78 | 50 | 87.5 | 12.5 | [ | ||
78 | 10 | 69.9 | 29.2 | 0.9 | [ | |
78 | 5 | 55.7 | 42.8 | 1.5 | [ |
The effluent H2 concentrations in Runs I–III were very low, from 0.10 to 0.52 mg/l, which indicated that the H2 could be transferred well without bubble from the PVC membrane and be used sufficiently by DB and SRB; meanwhile, the system got an effective removal of nitrate.
The % unutilized hydrogen was calculated according to (
The H2 utility in the MBfR.
Sum of H2 utility (%) | H2 utility for nitrate (%) | H2 utility for sulfate (%) | H2 utility for O2 (%) | |
---|---|---|---|---|
Run I | 97.7 | 61.1 | 36.6 | 9.3 |
Run II | 99.4 | 75.2 | 24.2 | 6.0 |
Run III | 99.5 | 73.0 | 26.6 | 4.9 |
As shown in Table
The microbial communities in each running period of the reactor could be seen in the analyses of the DGGE (Figure
DGGE and on the day 40 (S1), day 80 (S2), and day 150 (S3) (the Arabic numerals meant the different dominated bands in the operation of MBfR).
The study investigated the performance of MBfR to remove nitrate companied with high influent concentrations of sulfate over 155 days. The results indicated that even in high concentration of sulfate in influent, the MBfR also could get a good denitrification effect with nitrate and nitrite under the US standard. The analysis of the molecular microbiology showed that microbial community structures of Runs II and III were similar, simple, and stable. The bacteria species of Betaproteobacteria which include
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This research was supported by the Natural Science Foundation of Shandong Province (no. ZR2018MEE045), the Shandong Provincial Department of Housing and Urban-Rural Construction (no. 2017-K2-002), the 2017 China Scholarship Council foundation (no. 2017-3105), the Foundation of Remediation of Contaminated Sediment in Shandong Province (no. SDHBYF-2012-14), and Shandong Key Scientific and Technical Innovation Project (no. 2018YFJH0902).