Nitrogen Removal by Sulfur-Based Carriers in a Membrane Bioreactor (MBR)

membranes

Article Nitrogen Removal by Sulfur-Based Carriers in a Membrane Bioreactor (MBR)

Thi-Kim-Quyen Vo, Jeong-Jun Lee, Joon-Seok Kang, Seogyeong Park and Han-Seung Kim *

Department of Environmental Engineering and Energy, Myongji University, 116 Myongji-ro, Cheoin-gu, Yongin-si, Gyeonggi-do 17058, Korea; [email protected] (T.-K.-Q.V.); [email protected] (J.-J.L.); [email protected] (J.-S.K.); [email protected] (S.P.) * Correspondence: [email protected]; Tel.: +82-31-330-6695

Received: 14 October 2018; Accepted: 19 November 2018; Published: 22 November 2018

Abstract: Sulfur-based carriers were examined to enhance the nitrogen removal efficiency in a mixed anoxic–anaerobic-membrane bioreactor system, in which sulfur from the carrier acts as an electron donor for the conversion of nitrate to nitrogen gas through the autotrophic denitrification process. A total nitrogen removal efficiency of 63% was observed in the system with carriers, which showed an increase in the removal efficiency of around 20%, compared to the system without carriers. The results also indicated that the carriers had no adverse effect on biological treatment for the organic matter and total phosphorus. The removal efficiencies for chemical oxygen demand (COD) and total phosphorus (TP) were 98% and 37% in both systems, respectively. The generation of sulfate ions was a major disadvantage of using sulfur-based carriers, and resulted in pH drop. The ratio of sulfate in the 2− − effluent to nitrate removed in the system ranged from 0.86 to 1.97 mgSO4 /mgNO3 -N, which was lower than the theoretical value and could be regarded as due to the occurrence of simultaneous heterotrophic and autotrophic denitrification.

Keywords: autotrophic denitriﬁcation; heterotrophic denitriﬁcation; elemental sulfur; membrane bioreactor

1. Introduction The increase of nutrient contamination in water sources has been raising a serious problem, due to significant impacts on the aquatic environment, human and animal health. Of the nutrients, nitrogen and phosphorus compounds are the most important, causing eutrophication and serious damage to the ecology of water sources [1,2]. Nitrate in particular is well-known to be able to cause methemoglobinemia, or blue baby syndrome, for infants, and carcinoma, malformation, and mutation when a high concentration is ingested [3–6]. In the near future, the standards for nutrient discharge in the effluent of wastewater treatment plants will become more stringent in many developed and developing countries. There are various methods for nitrogen removal, including a physico-chemical treatment such as ion-exchange, electrodialysis, precipitation, distillation and reverse osmosis, as well as biological treatment. However, the physical or chemical methods only shift concentrated nitrogen compounds (such as ammonia, nitrite and nitrate) from one liquid phase to another, so a post-treatment is required. On the other hand, the biological method has several advantages over the physical or chemical methods, such as cost-saving, stable and continuous removal of nutrient due to less chemical use, lower energy consumption, and less production of waste solids [7,8]. Nitrogen is removed generally by nitrification and subsequent denitrification in the biological treatment process. First, ammonia is oxidized to nitrite as an intermediate product, and then nitrite is converted to nitrate by autotrophic bacteria (autotrophs) under aerobic conditions. The reduction of nitrate to harmless nitrogen gas takes place under anoxic conditions by heterotrophic bacteria

Membranes 2018, 8, 115; doi:10.3390/membranes8040115 www.mdpi.com/journal/membranes Membranes 2018, 8, 115 2 of 10

(heterotrophs) or autotrophs [9]. The main advantages of heterotrophic denitrification are high denitrification rate and treatment capacity [10]. When a heterotrophic denitrification supplemented with ethanol was conducted in a membrane bioreactor for drinking water treatment, the removal efficiency for nitrate was up to 92% and nitrite generation was lower (0.35 mg/L) [11]. Since the heterotrophs use organic carbon compounds as an electron donor for their growth and development, the denitrification occurs incompletely when the organic carbon source is insufficient. Most of the wastewaters contained high nitrogen and low carbon concentration (low C/N ratio), such as wastewater from the leather industry, fertilizer factory, landfill leachates and livestock farming, which resulted in the requirement to provide an external carbon source. Because it is very difficult practically to add a correct amount of organic carbon to the denitrification system, a post-treatment is needed to remove residual organic carbon compounds [12–14]. In the autotrophic denitrification process, autotrophs such as Thiobacillus denitrificans utilize inorganic − carbon compounds (e.g., CO2, HCO3 ) as their carbon source, and the energy source is derived from 2− oxidation-reduction reactions with hydrogen or inorganic reduced sulfur compounds (H2S, S, S2O3 , 2− 2− S4O6 , SO3 ) as the electron donor [15]. In the sulfur-based autotrophic denitrification, elemental sulfur and nitrate react as an electron donor and an acceptor, respectively [16].

0 − + 2− + 55S + 20CO2 + 50NO3 + 38H2O + 4NH4 →4C5H7O2N + 55SO4 + 25N2↑ + 64H (1)

Some advantages of autotrophic denitrification include that: (a) there is no external organic carbon source, like ethanol or methanol, which can reduce the cost treatment and poisoning effect of some organic carbon; (b) less biomass is produced, which means sludge production in autotrophic denitrification is two- to threefold less than in heterotrophic denitrification, resulting in the minimization of sludge handling [17–19]; (c) elemental sulfur is cheap and readily available, non-toxic, insoluble in water, and stable under normal conditions [20]; and (d) the autotrophic denitrification produces less N2O (a greenhouse gas) than heterotrophic denitrification [21]. However, the major disadvantages of this process are sulfate and acid generation, and alkalinity consumption [22]. According to Equation (1), when nitrate is reduced to nitrogen gas, sulfur is oxidized to sulfate and an H+ proton is released. As sulfate generation causes the decrease of the pH in the effluent, thus, alkalinity should be supplied in order to maintain a neutral pH in the system. In addition, the specific denitrification rate of autotrophs is lower than that of heterotrophs [18]. Oh et al. reported that a lab-scale sulfur packed column had a complete autotrophic denitrification with the influent − concentration of 100 mgNO3 -N/L and hydraulic retention time of 1.5 h [23]. The combination of sulfur-based denitrification with membrane technology was suggested because autotrophic bacteria have a low specific growth rate, and it is hard to maintain an appropriate amount of biomass within a reactor [24]. In recent years, the membrane bioreactor (MBR) process, which combines biological activated sludge and membrane filtration, has drawn significant attention due to various advantages over conventional activated sludge treatment, such as stable performance, high effluent quality, ease of operation, space saving, high mixed liquor suspended solids (MLSS) concentration, and high sludge retention time (SRT) to maintain biomass concentration at a much higher level. On the other hand, MBR has some drawbacks, such as membrane fouling, and need for membrane cleaning and replacement [25]. A combined process of sulfur-based denitrification and separation by membrane was proposed by Kimura et al., in which the process was operated at a flux of 0.5 m3/m2/day, hydraulic retention time (HRT) of 160 min, and a biomass concentration of about − 1000 mg protein/L, and achieved a complete removal of 25 mgNO3 -N/L in groundwater [18]. The denitrification performance of a combination of sulfur-utilizing autotrophic denitrification and separation by membrane was evaluated in the study. By installing a membrane, autotrophic denitrifiers, in which growth rates are considerably slow, are kept inside the reactor to increase the nitrogen elimination. The other studies focused only on the autotrophic denitrification in drinking water to remove nitrate, none of which has been applied to nitrogen treatment in wastewater. In addition, granular sulfur is widely Membranes 2018, 8, 115 3 of 10

Membranes 2018, 8, x FOR PEER REVIEW 3 of 10 used in the autotrophic denitrification processes. The objectives of this study were to compare the nitrogen denitrification, and demonstrate the feasibility of a sulfur-based carrier application to increase removal performance of mixotrophic denitrification to heterotrophic denitrification, and demonstrate the nitrogen removal effectiveness. feasibility of a sulfur-based carrier application to increase nitrogen removal effectiveness.

2. Materials and Methods

2.1. Experimental Set-Up Experiments werewere performedperformed in in the the apparatus apparatus illustrated illustrated in Figurein Figure1. Two 1. lab-scaleTwo lab-scale systems systems were operatedwere operated in parallel in parallel with workingwith working volume volume of anoxic, of anoxic, anaerobic anaerobic and MBRand MBR compartments compartments of 2, of 3 and2, 3 6.9and L, 6.9 respectively. L, respectively. Two Two ﬂat sheetflat sheet membrane membrane modules modules (width (width× height × height = 120 = 120 mm mm× 120× 120 mm) mm) with with a surfacea surface area area of of 0.06 0.06 m 2mand2 and pore pore size size of of 0.4 0.4µ m,µm, from from Pure Pure Envitech, Envitech, South South Korea, Korea, were were submerged submerged in eachin each MBR. MBR. Two Two systems systems (S1and (S1 S2)and were S2) were automatically automatically operated operated by using by programmed using programmed timers, andtimers, the trans-membraneand the trans-membrane pressure (TMP)pressure was (TMP) recorded was by digitalrecorded pressure by digital gauges. pressure Synthetic gauges. wastewater Synthetic was pumpedwastewater directly was pumped into the directly anoxic compartmentinto the anoxic by co usingmpartment a peristaltic by using pump a peristaltic to control pump the to feed control rate. MBR’sthe feed permeate rate. MBR’s pumps permeate were pumps maintained were withmaintained intermittent with intermittent suction (9 mins suction in operation (9 mins inand operation 1 min standby).and 1 min Two standby). membrane Two bioreactorsmembrane werebioreactors operated were at ﬂuxoperated of 8 L/m at flux2/h of (LMH). 8 L/m Nitrate2/h (LMH). in the Nitrate internal in recyclethe internal stream recycle from stream the MBR from fed the to MBR the anoxic fed to compartment.the anoxic compartment. Aeration was Aeration served was by served air diffusers by air installeddiffusers atinstalled the bottom at the of bottom MBRs inof theMBRs rear in end the of re membranear end of membrane modules, inmodules, order to in supply order dissolvedto supply oxygendissolved for oxygen aerobic for conditions, aerobic andconditions, to control and the to membrane control the fouling membrane by air scouring.fouling by Stirring air scouring. motors wereStirring installed motors inwere the installed anoxic and in the anaerobic anoxic and compartments anaerobic compartments to keep biomass to keep suspended. biomass suspended. Dissolved oxygenDissolved (DO) oxygen concentration (DO) concentration was maintained was maintained in anoxic, in anaerobic, anoxic, anaerobic, and MBR and compartments MBR compartments at values ofat values 0.4, 0.1 of and 0.4, 4 0.1 mg/L, and 4 respectively. mg/L, respectively. When TMP When was TMP higher was higher than 20 than kPa, 20 membrane kPa, membrane modules modules were externallywere externally cleaned cleaned with 0.5%with of0.5% NaOCl of NaOCl for 6 hfor prior 6 h toprior use. to use.

Figure 1. Schematic diagram of a lab-scale system. 2.2. Synthetic Wastewater, Seed Sludge and Sulfur-Based Carriers 2.2. Synthetic Wastewater, Seed Sludge and Sulfur-Based Carriers The synthetic wastewater was made with 137.5 mg/L C6H12O6, 282.14 mg/L NH4HCO3, The synthetic wastewater was made with 137.5 mg/L C6H12O6, 282.14 mg/L NH4HCO3, 21.94 21.94 mg/L KH2PO4, 15 mg/L MgSO4·7H2O, 0.09 mg/L MnSO4·H2O, 0.3 mg/L ZnSO4·7H2O, mg/L KH2PO4, 15 mg/L MgSO4.7H2O, 0.09 mg/L MnSO4.H2O, 0.3 mg/L ZnSO4.7H2O, 55 mg/L 55 mg/L CaCl2·2H2O, 3 mg/L FeCl2·2H2O, 300 mg/L NaHCO3. The concentration of synthetic CaCl2.2H2O, 3 mg/L FeCl2.2H2O, 300 mg/L NaHCO3. The concentration of synthetic wastewater is wastewater is chemical oxygen demand (COD) of 141 ± 2.21 mg/L, total nitrogen (TN) of + chemical oxygen demand+ (COD) of 141 ± 2.21 mg/L, total nitrogen (TN) of 51 ± 0.90 mg/L, NH4 -N of 51 ± 0.90 mg/L, NH4 -N of 49.24 ± 0.85 mg/L and total phosphorus (TP) of 5.10 ± 0.10 mg/L. 49.24 ± 0.85 mg/L and total phosphorus (TP) of 5.10 ± 0.10 mg/L. The seed activated sludge was collected from a wastewater treatment plant located in Yongin, The seed activated sludge was collected from a wastewater treatment plant located in Yongin, South Korea. The sludge retention time was maintained at 40 days during operation periods in South Korea. The sludge retention time was maintained at 40 days during operation periods in both both systems. To control SRT, the volume of suspended sludge was withdrawn daily from reactors. systems. To control SRT, the volume of suspended sludge was withdrawn daily from reactors. MLSS MLSS concentration in reactors was maintained at 4526 ± 196 mg/L during operation periods. concentration in reactors was maintained at 4526 ± 196 mg/L during operation periods. The carriers were composed of sulfur (S), calcium carbonate (CaCO3) and powder activated The carriers were composed of sulfur (S), calcium carbonate (CaCO3) and powder activated carbon (PAC). The elemental sulfur is cheap, readily available, insoluble in water, stable under normal carbon (PAC). The elemental sulfur is cheap, readily available, insoluble in water, stable under normal conditions, and easy to handle and store. For these reasons, elemental sulfur was chosen as an electron donor for an autotrophic denitrification in this study. The autotrophic denitrification

Membranes 2018, 8, 115 4 of 10 conditions, and easy to handle and store. For these reasons, elemental sulfur was chosen as an electron donor for an autotrophic denitrification in this study. The autotrophic denitrification decreases pH value of the reactor, thus, CaCO3 served as pH buffer and PAC was used to improve the biofilm formation on carriers. Sulfur, CaCO3, and PAC were mixed with a suitable ratio, and sodium alginate was added to the mixture to bond the components together. The mixture was dried at 105 ◦C for 24 h, and then broken into pieces. Carriers were added in the anoxic compartment of the first system (S1) at a rate of 1 g/day while the second system (S2) was operated without carriers. In this study, four types of carrier with different percentages of sulfur (C1, C2, C3, C4 with 22%, 27%, 32%, and 47% of sulfur, respectively) were used in order to indicate the nitrogen removal capacity. The percentage of PAC in carriers was kept unchanged at 10%.

2.3. Analytical Methods

+ − − 2− Parameters such as COD, TN, NH4 -N, NO2 -N, NO3 -N, TP, and SO4 were determined according to standard methods [26]. TMP values were daily recorded by digital pressure gauges. Ultraviolet absorbance (UVA254) value was measured by an Ultraviolet photometer (UV-2800, Shimadzu, Kyoto, Japan).

3. Results and Discussion

3.1. Removal of Nitrogen Table1 summarizes the average concentrations of ammonia, nitrite, nitrate and total nitrogen in the effluents. Combining anoxic and oxic units was studied for nitrogen removal in wastewater + treatment. In the MBR zone (an oxic compartment), ammonia (NH4 -N) was oxidized to nitrite − − (NO2 -N), and then converted to nitrate (NO3 -N), a process known as nitrification. The average + concentration of NH4 -N in influent was 49.24 ± 0.85 mg/L, while it was almost 0 mg/L in two + effluents (Table1). This indicated that ammonia was converted completely, and that the NH 4 -N removal efficiency was 100% in both systems. The specific nitrification rates (SNRs) were approximately + 0.018 mgNH4 -N/mgMLSS/day in both systems, and there was no significant change in SNR value in the first system (S1) with different types of carrier. These results indicated that carriers did not affect the nitrification process.

Table 1. Nitrogen species in the efﬂuents.

S1 with Carriers Nitrogen Species S2 without Carriers C1 C2 C3 C4 NH4+-N (mg/L) 0 0 0 0 0 NO2−-N (mg/L) 0.02 ± 0.01 0.04 ± 0.02 0.05 ± 0.01 0.14 ± 0.02 0.06 ± 0.02 NO3−-N (mg/L) 23.5 ± 2.3 22.3 ± 1.1 22.2 ± 1.0 19.5 ± 1.5 27.3 ± 1.7 TN (Total Nitrogen) (mg/L) 24 ± 2 23 ± 1 22 ± 1 19 ± 1 28 ± 2

The denitrification in which nitrate is converted to nitrogen gas takes place in the anoxic compartment. In the second system without carriers (S2), nitrogen in wastewater was removed only by the heterotrophic denitrification process. Heterotrophic denitrifying bacteria derive the required electrons from an organic carbon source that expressed as COD [8], due to the rapid decrease of COD concentrations in anoxic compartments. However, not all nitrate can be removed by the heterotrophic denitrification process if the wastewater has a deficient COD concentration. According to Fu et al., the biological nitrogen removal efficiency was highly affected by the influent COD/N ratio, so that the percentage of nitrogen removal was around 90% at COD/N ratio of 9.3, while it decreased from 71% to 69% when COD/N ratio decreased from 7 to 5.3, respectively [27]. The influent COD/N ratio was only 2.8 in this study, which means that an insufficient organic carbon source − was provided for full denitrification to take place, and the NO3 -N concentration of the permeate was high of 27.3±1.7 mg/L. However, in the first system (S1) with carriers in anoxic compartment, Membranes 2018, 8, 115 5 of 10 Membranes 2018, 8, x FOR PEER REVIEW 5 of 10

consideredthe elemental to sulfuroccur ofin theS1. carrierThe effluent acted asnitrate an electron concentration donor for of theS1 was conversion reduced of by nitrate stimulating to nitrogen the simultaneousgas (as Equation heterotrophic (1)). Therefore, and autotrophic not only andenitrification. autotrophic denitrificationThe specific denitrification but also a heterotrophic rate (SDNR) ofdenitrification S1 was 0.011 could mgNO be considered3−-N/mgMLSS/day, to occur which in S1. was The higher effluent than nitrate that concentration of S2 (0.007 ofmgNO S1 was3−- N/mgMLSS/day).reduced by stimulating The SDNRs the simultaneous of S1 with C1, heterotrophic C2, C3, C4 andwere autotrophic 0.009, 0.010, denitrification. 0.010 and 0.011 The mgNO specific3−- − N/mgMLSS/day,denitrification rate respectively. (SDNR) of S1 was 0.011 mgNO3 -N/mgMLSS/day, which was higher than that of − S2 (0.007 mgNO3 -N/mgMLSS/day). The SDNRs of S1 with C1, C2, C3, C4 were 0.009, 0.010, 0.010 − and 0.011 mgNO3 -N/mgMLSS/day, respectively. As a result of conversion from nitrate to nitrogen gas, nitrogen in wastewater is removed. Consequently, the effluent total nitrogen concentrations of both systems were decreased (Figure2). The first, second, third and fourth periods correspond to carrier C1, C2, C3, C4 added to the anoxic compartment of the first system (S1), while the second system (S2) was without carriers during operation periods. TN removal efficiencies of S1 at various types of carriers from C1 to C4 were 54 ± 4%, 56 ± 2%; 58 ± 2%; 63 ± 3%, in that order. In addition, the specific nitrogen removal rates in the first system were 0.009, 0.010, 0.010 and 0.011 mgTN/mgMLSS.day at C1, C2, C3, C4, Membranesrespectively. 2018, 8 There, x FOR wasPEER aREVIEW difference in total nitrogen removal efficiency between types of carriers,5 of 10 because of increasing the percentage of sulfur in carriers (from 22% to 47%). When sulfur content consideredincreased, moreto occur elemental in S1. sulfurThe effluent reacted nitrate with nitrate concentration to carry outof S1 the was denitrification. reduced by Thestimulating TN removal the simultaneousefficiency increased heterotrophicFigure 2. by Variation around and of 20%,autotrophic totaland nitrogen the denitrification. specificconcentration TN removal at The different specific rate operation alsodenitrification increased periods. fromrate (SDNR) 0.008 to − − of0.011 S1 mgTN/mgMLSS/day,was 0.011 mgNO3 -N/mgMLSS/day, compared to the which second was system higher without than that carriers. of S2 It (0.007 can be mgNO said that3 - − N/mgMLSS/day).heterotrophicAs a result coupling ofThe conversion SDNRs with autotrophicof from S1 with nitrate C1, denitrification C2,to nitrC3,ogen C4 were bygas, adding 0.009,nitrogen 0.010, carriers in 0.010wastewater enhanced and 0.011 theis removed.mgNO nitrogen3 - N/mgMLSS/day,Consequently,removal in wastewater the respectively. effluent treatment total nitrogen (Figure3 concentrat). ions of both systems were decreased (Figure 2). The first, second, third and fourth periods correspond to carrier C1, C2, C3, C4 added to the anoxic compartment of the first system (S1), while the second system (S2) was without carriers during operation periods. TN removal efficiencies of S1 at various types of carriers from C1 to C4 were 54 ± 4%, 56 ± 2%; 58 ± 2%; 63 ± 3%, in that order. In addition, the specific nitrogen removal rates in the first system were 0.009, 0.010, 0.010 and 0.011 mgTN/mgMLSS.day at C1, C2, C3, C4, respectively. There was a difference in total nitrogen removal efficiency between types of carriers, because of increasing the percentage of sulfur in carriers (from 22% to 47%). When sulfur content increased, more elemental sulfur reacted with nitrate to carry out the denitrification. The TN removal efficiency increased by around 20%, and the specific TN removal rate also increased from 0.008 to 0.011 mgTN/mgMLSS/day, compared to the second system without carriers. It can be said that heterotrophic coupling with autotrophic denitrification by adding carriers enhanced the nitrogen removal in wastewater

treatment (FigureFigure 3). 2. Variation of total nitrogen concentration at different operation periods. Figure 2. Variation of total nitrogen concentration at different operation periods.

As a result of conversion from nitrate to nitrogen gas, nitrogen in wastewater is removed. Consequently, the effluent total nitrogen concentrations of both systems were decreased (Figure 2). The first, second, third and fourth periods correspond to carrier C1, C2, C3, C4 added to the anoxic compartment of the first system (S1), while the second system (S2) was without carriers during operation periods. TN removal efficiencies of S1 at various types of carriers from C1 to C4 were 54 ± 4%, 56 ± 2%; 58 ± 2%; 63 ± 3%, in that order. In addition, the specific nitrogen removal rates in the first system were 0.009, 0.010, 0.010 and 0.011 mgTN/mgMLSS.day at C1, C2, C3, C4, respectively. There was a difference in total nitrogen removal efficiency between types of carriers, because of increasing the percentage of sulfur in carriers (from 22% to 47%). When sulfur content increased, more elemental sulfur reacted with nitrate to carry out the denitrification. The TN removal efficiency increased by around 20%, andFigure the specific 3. TN (TotalTN removal Nitrogen) rate removal also increased efficiency from of different 0.008 to types 0.011 of carrier.mgTN/mgMLSS/day, compared to theFigure second 3. TN system (Total withoutNitrogen) carriers. removal efficiIt canency be of said different that typesheterotrophic of carrier. coupling with autotrophicNguyen denitrification et al. indicated by that adding the TN carriers removal enhanced efficiency the of Sponge-MBR,nitrogen removal where in heterotrophic wastewater Nguyen et al. indicated that the TN removal efficiency of Sponge-MBR, where heterotrophic treatmentdenitrification (Figure occurred, 3). was 51 ± 18% with hydraulic retention time (HRT) of 10 h, SRT of 20 days, denitrification occurred, was 51 ± 18% with hydraulic retention time (HRT) of 10h, SRT of 20 days, and a specific TN removal rate of 0.011 mgTN/mgMLSS/day [28]. Another study by Kim et al. and a specific TN removal rate of 0.011 mgTN/mgMLSS/day [28]. Another study by Kim et al. showed showed the nitrate treatment performance of sequential heterotrophic and autotrophic denitrification the nitrate treatment performance of sequential heterotrophic and autotrophic denitrification processes, in which two columns connected together were used as reactors for each process to treat 500 mg/L of nitrate in synthetic wastewater. The experimental results indicated that the total nitrate

Figure 3. TN (Total Nitrogen) removal efficiency of different types of carrier.

Nguyen et al. indicated that the TN removal efficiency of Sponge-MBR, where heterotrophic denitrification occurred, was 51 ± 18% with hydraulic retention time (HRT) of 10h, SRT of 20 days, and a specific TN removal rate of 0.011 mgTN/mgMLSS/day [28]. Another study by Kim et al. showed the nitrate treatment performance of sequential heterotrophic and autotrophic denitrification processes, in which two columns connected together were used as reactors for each process to treat 500 mg/L of nitrate in synthetic wastewater. The experimental results indicated that the total nitrate

Membranes 2018, 8, 115 6 of 10

Membranes 2018, 8, x FOR PEER REVIEW 6 of 10 processes, in which two columns connected together were used as reactors for each process to removaltreat 500 efficiency mg/L of nitratewas 96 in± 5%, synthetic of which wastewater. 49 ± 12% was The of experimental heterotrophic results denitrification indicated and that 46 the ± total12% wasnitrate of autotrophic removal efficiency denitrifi wascation 96 ±[29].5%, Additionally, of which 49 ±methanol12% was wa ofs heterotrophicadded as an organic denitrification carbon in and a packed-bed46 ± 12% was bioreactor of autotrophic filled with denitrification sulfur, limestone, [29]. Additionally,and activated methanolcarbon particles was added so that as autotrophic an organic andcarbon heterotrophic in a packed-bed denitrification bioreactor occurred filled with simultan sulfur, limestone,eously and and the activated complete carbon denitrification particles so of that 75 mgNOautotrophic3−-N/L and was heterotrophic achieved [10]. denitrification occurred simultaneously and the complete denitrification − of 75 mgNO3 -N/L was achieved [10]. 3.2. Sulfate Generation 3.2. Sulfate Generation Sulfate (SO42−) is one of the end products of a sulfur-based autotrophic denitrification. The 2− variationSulfate of (SOsulfate4 ) isconcentrations one of the end for products different of a sulfur-basedtypes of carrier autotrophic is given denitrification. in Figure 4. The variationaverage influentof sulfate sulfate concentrations concentration for different was 17 types ± 2of mg/L. carrier The is given second in Figuresystem,4. Thewith average only a influent heterotrophic sulfate ± denitrification,concentration was did 17 not 2produce mg/L. The sulfate, second so system,the effluent with onlysulfate a heterotrophic concentration denitrification, was similar didto the not influent,produce while sulfate, the so sulfate the effluent concentrations sulfate concentration increased in the was first similar system to theas the influent, percentage while of the sulfur sulfate in carriersconcentrations increased increased (Figure in 4). the The first average system effluent as the percentagesulfate concentrations of sulfur in of carriers S1 for increasedcarriers C1, (Figure C2, C3,4). ± ± ± C4The were average 44 ± effluent 1, 48 ± sulfate 2, 55 ± concentrations 2 and 75 ± 7 mg/L, of S1 for respectively. carriers C1, C2,More C3, sulfate C4 were production 44 1, 48 indicated2, 55 2 thatand 75 ± 7 mg/L, respectively. More sulfate production indicated that more nitrate was removed. According more nitrate was removed. According to the stoichiometric equation (Equation (1)), 7.54 mg SO42− is 2− − to the stoichiometric equation (Equation (1)), 7.54 mg SO4 is produced when 1 mg NO3 -N is removed. produced when 1 mg NO3−-N is removed. The ratios of the effluent sulfate to the removed nitrate in The ratios of the effluent sulfate to the removed nitrate in the first system for various carriers C1, C2, C3, the first system for various carriers C1, C2, C3, C4, were 0.86, 0.89, 1.35 and 1.97 mgSO42−/mgNO3−-N, 2− − respectively.C4, were 0.86, The 0.89, ratios 1.35 from and1.97 the experiments mgSO4 /mgNO were 3lower-N, respectively.than the theoretical The ratios value from above-mentioned, the experiments whichwere lower means than that the heterotrophic theoretical value and above-mentioned, autotrophic denit whichrification means occurred that heterotrophic simultaneously and autotrophic[23]. Oh et denitrification occurred simultaneously [23]. Oh et al. also reported that under mixotrophic conditions, al. also reported that under mixotrophic conditions, this ratio was 3–4 mgSO42−/mgNO3−-N for 2− − this ratio was 3–4 mgSO4 /mgNO3 -N for denitrification of 100 mg/L nitrate [23]. The lower ratio denitrification of 100 mg/L nitrate [23]. The lower ratio indicates that less elemental− sulfur was indicates that less elemental sulfur was− utilized to remove unit amount of NO3 -N, resulting in the utilized to remove unit amount of NO3 -N, resulting in the reduction of the treatment cost. reduction of the treatment cost.

FigureFigure 4. 4. VariationVariation of of sulfate sulfate concentratio concentrationn at at different types of carrier. 3.3. COD and TP Removal 3.3. COD and TP Removal The COD concentration of feed and permeates are shown in Figure5. The average influent CODThe concentration COD concentration was maintained of feed and at permeates 140 ± 2 mg/L, are shown and thein Figure COD 5. concentration The average influent in permeates COD concentrationwas almost below was maintained 5 mg/L even at at140 different ± 2 mg/L, operation and the periods.COD concentration The average in removalpermeates efficiency was almost and below 5 mg/L even at different operation periods. The average removal efficiency and specific specific removal rate were 98 ± 2% and 0.049 ± 0.003 mgCOD/mgMLSS/day in both systems during removal rate were 98 ± 2% and 0.049 ± 0.003 mgCOD/mgMLSS/day in both systems during operation operation periods, respectively. There was not a significant difference in term of COD removal for periods, respectively. There was not a significant difference in term of COD removal for both systems both systems with different types of carriers. These results indicated that carriers have no effect on with different types of carriers. These results indicated that carriers have no effect on biological biological treatment of organic matters. In comparison to the results of Wen et al., the COD removal treatment of organic matters. In comparison to the results of Wen et al., the COD removal efficiency efficiency of a conventional MBR achieved was only 80%, and the COD permeate concentration was of a conventional MBR achieved was only 80%, and the COD permeate concentration was lower than lower than 30 mg/L [30]. Nguyen et al. also reported that the COD in the permeate of a conventional 30 mg/L [30]. Nguyen et al. also reported that the COD in the permeate of a conventional MBR MBR operated at flux of 6 LMH was always 11–16 mg/L, with 84 ± 10% of COD removal [28]. It was operated at flux of 6 LMH was always 11–16 mg/L, with 84 ± 10% of COD removal [28]. It was noticeable that most of COD was biodegraded in anoxic tanks. COD acts as an organic carbon source noticeable that most of COD was biodegraded in anoxic tanks. COD acts as an organic carbon source for heterotrophic denitrification processes occurred in anoxic compartments as shown in Figure6. As a for heterotrophic denitrification processes occurred in anoxic compartments as shown in Figure 6. As a result, the COD concentration of both anoxic compartments decreased rapidly. It was clear from these data that both systems could provide a significant removal of COD in this study.

Membranes 2018,, 8,, 115x FOR PEER REVIEW 77 of of 10 result, the COD concentration of both anoxic compartments decreased rapidly. It was clear from these dataMembranesMembranes that both2018 2018, 8 systems, ,8 x, xFOR FOR PEER PEER could REVIEW REVIEW provide a signiﬁcant removal of COD in this study. 7 of7 10of 10

Figure 5. Variation of COD (Chemical Oxygen Demand) concentration at different operation periods.

Figure 5. Variation of COD (Chemical Oxygen Demand) concentration at different operation periods. Figure 5. Figure 5. VariationVariationof of CODCOD (Chemical(Chemical OxygenOxygen Demand)Demand) concentrationconcentration atat differentdifferent operationoperation periods.periods.

Figure 6. Variation of COD concentration in compartments.

The same behaviorFigure Figurein the 6. 6.systemVariation Variation performance ofof CODCOD concentration could be in inobserved compartments. compartments. for TP removal. Variation of TP concentrationThe same behavior duringFigure in operation the 6. system Variation periods performance of CODis given concentration couldin Figure be observedin 7. compartments. The influent for TP removal.TP concentration Variation was of maintainedThe same at 5 mg/L.behavior The in average the system TP performanceconcentrations could in permeatesbe observed of for S1, TP S2 removal. were obtained Variation at 3.2of ± TPTP concentration concentration during during operation operation periods periods isis givengiven inin Figure 77.. The The influent influent TP TP concentration concentration was was 0.3 andThe 3.1 same ± 0.3 behaviormg/L, respectively. in the system The performanceTP removal efficiency could be andobserved specific for removal TP removal. rate were Variation around of maintainedmaintained at 5at mg/L.5 mg/L. The The average average TP TP concentrations concentrations in in permeates permeates of of S1, S1, S2 S2 were were obtained obtained at at3.2 3.2± ±0.3 37TP ± concentration 4% and 0.0004 during mgTP/mgMLSS/day operation periods in both is given system in s,Figure respectively. 7. The influentAs in terms TP concentration of COD removal, was and0.33.1 and± 3.10.3 ± mg/L 0.3 mg/L,, respectively. respectively. The The TP TP removal removal efficiency efficiency and specific specific removal removal rate rate were were around around themaintained carriers could at 5 mg/L. not improve The average the total TP phosphorusconcentrations removal. in permeates The TP removalof S1, S2 efwereficiency obtained of this at study 3.2 ± 37 37± 4%± 4%and and 0.0004 0.0004 mgTP/mgMLSS/day mgTP/mgMLSS/day in in both both system systems,s, respectively. respectively. As As in interms terms of ofCOD COD removal, removal, was0.3 and higher 3.1 ± than0.3 mg/L, the respectively.result of Nguyen The TP et removal al., who efficiency reported and that specific TP removal removal rateefficiencies were around of a thethe carriers carriers could could not not improve improve the the total total phosphorus phosphorus removal. removal. The The TP TP removal removal efficiency efficiency of of this this study study was conventional MBR and a Sponge-MBR operated at flux of 6 LMH, HRT of 8 h were 20 ± 15% and 26 higher37was ± 4% thanhigher and the 0.0004 than result mgTP/mgMLSS/daythe of result Nguyen of etNguyen al., who in et both reportedal., systemwho thatreporteds, respectively. TP removal that TP efficienciesAs removal in terms efficiencies ofof aCOD conventional removal, of a MBR±the 11%,conventional carriers and respectively a could Sponge-MBR MBR not [28]. and improve This a operatedSponge-MBR was the caused total atflux operatedphosphorus by ofan 6an LMH,ataerobic flux removal. of HRT compartment 6 LMH, ofThe 8 HRTTP h were removal ofinstalled 8 20 h were± ef 15%ficiencyin 20the and± system.15% of 26 thisand± In study2611%, the respectivelyanaerobicwas± 11%, higher respectively tank, [ 28than]. Thispoly-phosphorus the [28]. was result This caused wasof byNguyen caused anwas anaerobic by hydrolyzedet an al., an compartmentaerobicwho toreported compartment release installed thatortho-phosphorus, installedTP in theremoval system. in the efficienciessystem. In thus the anaerobic Inthe the ofTP a tank,concentrationconventionalanaerobic poly-phosphorus tank, MBRin the poly-phosphorusand wasanaerobic a Sponge-MBR hydrolyzed compartment was tooperated releasehydrolyzed waat ortho-phosphorus, fluxs higher,to of release 6 LMH, while ortho-phosphorus, HRT thus ortho-phosphorus of the 8 TPh were concentration 20thus ± 15% isthe usually and inTP the 26 anaerobicaccumulated± 11%,concentration respectively compartment within in [28].the bacterial was anaerobicThis higher, was cells caused whilecompartment and ortho-phosphorusreleased by an an waaerobicoust ofhigher, system compartment is usually while by accumulatedwastingortho-phosphorus installed sludge withinin the [8]. system. bacterialis Suspended usually In cells the andsludgeanaerobicaccumulated released was tank, outwithdrawn within of poly-phosphorus system bacterial from by wasting the cells membrane and sludgewas releasedhydrolyzed [8 ].bioreact Suspended out oforto system daily sludgerelease toby was maintainortho-phosphorus,wasting withdrawn sludgeSRT fromat [8]. 40 the Suspendedthusdays membrane theand TPto bioreactoreliminateconcentrationsludge wastotal daily withdrawnphosphorus.in to maintainthe anaerobic from SRT the at 40 compartmentmembrane days and tobioreact eliminatewas orhigher, daily total to phosphorus.while maintain ortho-phosphorus SRT at 40 days is and usually to accumulatedeliminate total within phosphorus. bacterial cells and released out of system by wasting sludge [8]. Suspended sludge was withdrawn from the membrane bioreactor daily to maintain SRT at 40 days and to eliminate total phosphorus.

Figure 7. VariationVariation of TP (Total Phosphorus) concentrationconcentration atat differentdifferent operationoperation periods.periods. Figure 7. Variation of TP (Total Phosphorus) concentration at different operation periods.

Figure 7. Variation of TP (Total Phosphorus) concentration at different operation periods.

Membranes 2018, 8, x FOR PEER REVIEW 8 of 10 Membranes 2018, 8, 115 8 of 10 Membranes 2018, 8, x FOR PEER REVIEW 8 of 10 3.4. Membrane Fouling 3.4. Membrane Fouling 3.4. MembraneAlthough Foulingthe MBR system offers several advantages over the conventional activated sludge process,AlthoughAlthough the membrane the the MBR fouling system is theoffers major several disadvan advant advantagestage.ages Figure over 8 theshows conventional the changes activated in TMP ofsludge both MBRsprocess, during the the membrane membrane the operation fouling fouling periods. is isthe the major majorThere disadvan disadvantage.was notage. significant Figure Figure 8 showsdifference8 shows the thechangesin changesTMP in development TMP in TMP of both of betweenMBRsboth MBRs during both during systems.the operation the operationThe TMP periods. periods.increased There There from was was3.7 no to nosignificant 12 significant kPa in S1difference difference and from in in 2.8TMP TMP to 9.3development kPa in S2 duringbetweenbetween 39 both bothdays systems.(from systems. the The second The TMP TMP to increasedfourth increased period). from from The3.7 3.7 foulingto to 12 12 kPa kPa rates in in ofS1 S1 the and and two from from systems 2.8 2.8 to to were 9.3 9.3 kPaobserved kPa in in S2 toduringduring be 0.21 39 kPa/day days days (from (from for S1the the and second second 0.17 to tokPa/day fourth fourth forperiod). period). S2. The The The carrier fouling fouling debris rates rates attached of of the the two two on systems systemsthe membrane were were observed observed surface couldtoto be be 0.21 0.21be thoughtkPa/day kPa/day tofor forbe S1 S1theand and reason0.17 0.17 kPa/day why kPa/day the for higherS2. for The S2. TMPcarrier The appeared carrier debris debrisattached in S1 attached than on the S2 membrane onwithout the membrane carriers. surface Additionally,couldsurface be could thought ultraviolet be thought to be theabsorbance to bereason the reason whywas whymeasthe higher theured higher for TMP the TMP appearedtwo appeared systems. in inS1 The S1than thanaverage S2 S2 without without UVA254 carriers. carriers.values inAdditionally, the effluents ultraviolet of S1 and absorbanceS2 (0.033 ± 0.008 was meas1/cmured and 0.034for the ± 0.007two systems. 1/cm) were The lower average than UVA those254 valuesof the Additionally, ultraviolet absorbance was measured for the two systems. The average UVA254 values in influentinthe the effluents effluents (Figure of of S19). S1 andThis and S2 indicated S2 (0.033 (0.033± that0.008± 0.008 the 1/cm 1/cm soluble and and organic 0.034 0.034± ± 0.007matters,0.007 1/cm)1/cm) especially were were lower double than bond those linkage of the substances,influentinfluent (Figure (Figure were 9 9).trapped). ThisThis indicated indicatedin membranes thatthat the thecausing solublesoluble the organic organicirreversible matters,matters, fouling. especiallyespecially In comparison doubledouble bondwithbond the linkagelinkage later periods,substances,substances, the were value trapped of UVA in 254 membranes membranes was lower causing at the firstthe irreversible irreversibleperiod. This fouling. means InIn that comparison comparison more soluble with with theorganic the later later compoundsperiods, the were value absorbed of UVA 254on wasthe membrane lower at the surface first period.and could This cause means the higherthat more value soluble of TMP. organic periods, the value of UVA254 was lower at the first period. This means that more soluble organic compoundscompounds were were absorbed on the membrane surface and could cause the higher value of TMP.

Figure 8. Evolution of TMP (Trans-Membrane Pressure) in two systems at different operation periods. FigureFigure 8. 8. EvolutionEvolution of of TMP TMP (Trans-Membrane (Trans-Membrane Pressure) Pressure) in two two systems systems at at different different operation operation periods. periods.

Figure 9. UVA value during operation period. Figure 9. UVA254254 value during operation period. 4. Conclusions Figure 9. UVA254 value during operation period. 4. Conclusions 4. ConclusionsIt was clear that the sulfur-based carrier played a significant role in the nitrogen removal in the It was clear that the sulfur-based carrier played a significant role in the nitrogen removal in the wastewater treatment. Firstly, the carrier induced the occurrence of a simultaneous autotrophic and wastewaterIt was cleartreatment. that the Firstly, sulfur-based the carrier carrier induce playedd the a occurrence significant ofrole a simultaneousin the nitrogen autotrophic removal in and the heterotrophic denitrification to enhance the nitrogen removal efficiency. Secondly, it is favorable to heterotrophicwastewater treatment. denitrification Firstly, to the enhance carrier the induce nitrogend the removal occurrence efficiency. of a simultaneous Secondly, it autotrophic is favorable and to apply the carrier in wastewater treatment systems to treat the influent with a low C/N ratio, as it applyheterotrophic the carrier denitrification in wastewater to treatmentenhance the systems nitrogen to treat removal the influent efficiency. with Secondly, a low C/N it isratio, favorable as it has to has no need of an external carbon source. Thirdly, the actual ratio of sulfate generated to nitrate noapply need the of carrier an external in wastewater carbon source. treatment Thirdly, systems the actual to treat ratio the of influent sulfate withgenerated a low toC/N nitrate ratio, removed as it has removed was lower than that of theoretical ratio. The results also indicated that the carriers containing wasno need lower of thanan external that of carbontheoretical source. ratio. Thirdly, The result the actuals also ratioindicated of sulfate that gethenerated carriers to containing nitrate removed more more elemental sulfur achieved a higher nitrogen removal. Based on the results from this study, elementalwas lower sulfurthan that achieved of theoretical a higher ratio. nitrogen The result remos val.also Basedindicated on thethat resultsthe carriers from containing this study, more the the evaluation of MBR systems using real wastewater will be required in further studies. evaluationelemental sulfurof MBR achieved systems usina higherg real nitrogenwastewater remo willval. be Based required on inthe further results studies. from this study, the evaluation of MBR systems using real wastewater will be required in further studies.

Membranes 2018, 8, 115 9 of 10

Author Contributions: Formal analysis, validation, writing—review and editing, T.-K.-Q.V.; methodology and conceptualization, J.-J.L.; investigation, J.-S.K., S.P.; project administration, supervision, H.-S.K. Funding: This research was supported by a grant (NRF-2015R1D1A1A01058560) from Korea Research Foundation funded by Ministry of Education, Korea. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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