A new hybrid treatment system of bioreactors and electrocoagulation for superior removal of organic and nutrient pollutants from municipal wastewater

This article appeared in a journal published by Elsevier The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited In most cases authors are permitted to post their version of the article (e.g in Word or Tex form) to their personal website or institutional repository Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/authorsrights Author's personal copy Bioresource Technology 153 (2014) 116–125 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech A new hybrid treatment system of bioreactors and electrocoagulation for superior removal of organic and nutrient pollutants from municipal wastewater Dinh Duc Nguyen a, Huu Hao Ngo b, Yong Soo Yoon a,⇑ a b Department of Chemical Engineering, Dankook University, Republic of Korea School of Civil and Environmental Engineering, University of Technology, Broadway, Sydney, NSW 2007, Australia h i g h l i g h t s  A new hybrid system consisting of RHMBR, MBR and EC was developed  Complete nitrification was achieved by the combination explored  T-N concentration in treated effluent of this system was low (3.81 ± 0.9 mg/L)  The system effectively eliminated phosphorus (0.03 ± 0.024 mg/L in treated effluent) a r t i c l e i n f o Article history: Received October 2013 Received in revised form 16 November 2013 Accepted 19 November 2013 Available online 27 November 2013 Keywords: Integrated hybrid system Municipal wastewater Phosphorus Nitrogen Internal recycling ratio a b s t r a c t This paper evaluated a novel pilot scale hybrid treatment system which combines rotating hanging media bioreactor (RHMBR), submerged membrane bioreactor (SMBR) along with electrocoagulation (EC) as post treatment to treat organic and nutrient pollutants from municipal wastewater The results indicated that the highest removal efficiency was achieved at the internal recycling ratio as 400% of the influent flow rate which produced a superior effluent quality with 0.26 mgBOD5 L1, 11.46 mgCODCr L1, 0.00 mgNHỵ -N LÀ1, and 3.81 mgT-N LÀ1, 0.03 mgT-P LÀ1 During 16 months of operation, NHỵ -N was 4 completely eliminated and T-P removal efficiency was also up to 100% It was found that increasing in internal recycling ratio could improve the nitrate and nitrogen removal efficiencies Moreover, the TSS and coliform bacteria concentration after treatment was less than mg LÀ1 and 30 MPN mLÀ1, respectively, regardless of internal recycling ratios and its influent concentration Ó 2013 Elsevier Ltd All rights reserved Introduction The increase of inorganic nutrients in naturally receiving nonpoint sources, especially nitrogen and phosphorus, can induce eutrophication, causing negative effects on water resource quality (Yang et al., 2010) As a major strategy to control the unintended nutrient enrichment of surface waters, a number of wastewater treatment plants have adopted various treatment systems that can highly and simultaneously remove nitrogen and phosphorus from wastewater Among those, biological nutrient removal processes, such as suspended and attached growth biofilm techniques, have been developed and widely applied due to their economic advantages over other chemical treatment processes (Fan et al., 2009) ⇑ Corresponding author Tel.: +82 31 8005 3539; fax: +82 31 8021 7216 E-mail addresses: nguyensyduc@gmail.com (D.D Nguyen), chemyoon@unitel co.kr (Y.S Yoon) 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd All rights reserved http://dx.doi.org/10.1016/j.biortech.2013.11.048 Compared to the common activated sludge process, biofilm processes are increasingly being employed in wastewater treatment because of their advantages due to smaller facility operating areas footprints, ease of operation, short hydraulic retention time (HRT), insensitivity to organic and hydraulic shock loading, and higher biomass concentration (Jou and Huang, 2003; Cresson et al., 2006; Nguyen et al., 2010) They also minimize biomass drift to other work units Nowadays, several new biofilm technologies, based on the modification of existing processes, have attracted a great deal of attention from those responsible for treating wastewater For example, Tandukar et al (2007) evaluated the performance of the down-flow hanging sponge (DHS), which was preceded by an up-flow anaerobic sludge blanket (UASB) for treating sewage and showed removal rates of 94.3%, 89.7% and 55.9% for total BOD, total COD, and T-N, respectively Kim et al (2010) reported that the integrated fixed-film activated sludge (IFAS) with a media of extruded high density polyethylene demonstrated Author's personal copy D.D Nguyen et al / Bioresource Technology 153 (2014) 116–125 higher removal efficiencies of 90%, 90% and 85% for COD, TP and ammonia, respectively, with a solids residence time (SRT) of days Recently, Di Trapani et al (2011) Performance of a hybrid activated sludge/biofilm process for wastewater treatment in a cold climate region: Influence of operating conditions The results showed that the average removal efficiencies of total COD and ammonium were higher than 76% and 70–99% for HRT of 3.5 h and 4.5 h, respectively One of the biofilm support media is made of plastic They use various forms/types of plastic, such as polyethylene (PE) and polypropylene (PP) (Khoshfetrat et al., 2011) Hence, when selecting a suitable biofilm carrier media for use in the RHMBR studies, certain parameters were set (Orantes and Gonzalez-Martinez, 2003; Levstek and Plazl, 2009; Nacheva and Chavez, 2010) as follows: (i) the media must have a high specific surface area to support the high-density presence of active microorganism; (ii) it must have a low apparent specific weight per square centimeter, yet be strong enough to support the added weight of the cultured biomass; and (iii) the material used must be durable and highly resistant to environmental conditions, for effectiveness and longevity As PE & PP media met these conditions well, they were chosen as the carrier media to be used for the RHMBR in this study In recent years, membrane bioreactor (MBR) processes have been widely used to reduce or eliminate nutrients due to their advantages over other conventional activated sludge systems These advantages include a smaller footprint, less sludge production, high organic loading rate, highly improved effluent quality, water reuse and potential for removal of pathogenic microorganisms (Defrance et al., 2000; Le-Clech et al., 2006; Judd Simon and Claire, 2010) However, the nitrogen removal efficiency of the conventional submerged MBR is limited because its configuration does not compensate for anaerobic or anoxic conditions that hinder biological denitrification process (Yang et al., 2009) Phosphorus discharge standards for municipal wastewater in all developed and developing countries have become increasingly stringent, while the phosphorus concentrations in final effluent from Biological Wastewater Treatment Systems has been difficult to manage These limitations have caused levels to exceed more than mgT-P LÀ1, creating an urgent need for a better treatment technology Thus, there is a need to explore novel and applied advanced technologies to create high efficiency in phosphorus removal The criterion for these technologies is restrictive They must use less space, lower capital investment; lower installation 117 cost; have lower operating and maintenance costs, and eliminate the need for additional, frequent, and expensive chemical use (Markus et al., 2011; Wahab et al., 2011; Oleszkiewicz and Barnard, 2006; Bektas et al., 2004) For these reasons and others, this EC proß cess study was applied as a post treatment add-on, with potential for reasonably easy retrofitting to existing facilities In this study, a hybrid system consisting of RHMBR – SMBR with EC as post treatment was developed and implemented as a pilot scale unit to treat municipal wastewater The objectives of this study were: (1) to investigate the performance of an integrated hybrid system to remove organics, nitrogen, and phosphorus with respect to the nitrogen and phosphorus loading rate as a function of operation time or hydraulic residence times; and biological and non-biological phosphorus removals in the hybrid system were also studied; (2) to determine the efficiency of T-N, and T-P removal at different initial concentrations; and (3) to evaluate the efficiency of denitrification and nitrification toward total nitrogen removal at different internal recycle ratios of a long-term, real-world operation Methods 2.1 Experimental set-up and description Experiments were conducted using a large pilot-scale hybrid RHMBR MBR and EC located at the municipal wastewater treatment plant (WWTP) of Y City, Korea, for 475 days of continuous operation as shown in Fig The pilot system was constructed using an external steel framework and pre-fabricated PDF wall panel tank system, with a lining made of high-density polyethylene (HDPE) inside (Gentrol Co., LTD., Korea) The equalizing reactor (EQ) with functioned to reduce variation in influent flow, influent pollutant concentrations/loads, and reduced oxygen concentration in the internal recycle flows, and was divided into three compartments: EQ1, EQ2, and EQ3 They were constructed with a working volume of 1.226 m3, 1.197 m3, and 1.989 m3, respectively, followed sequentially by a RHMBR (9.922 m3), a MBR (9.44 m3) and then EC with electrolysis time of approximately The influent wastewater was pumped continuously from WWTP using two submerged pumps (Wilo Pump, Korea) to the pilot system, which has a working volume of approximately 53 m3 dayÀ1 The influent was passed through a fine screen (FS), with mm openings, to remove the larger materials and avoid damage to the work units beyond, especially the membrane, prior to the wastewater flow Fig Schematic diagram of the pilot scale hybrid treatment system used Author's personal copy 118 D.D Nguyen et al / Bioresource Technology 153 (2014) 116–125 entering the EQ and then the RHMBR The primary-function of the RHMBR is denitrification Secondly, it partially removes phosphorus, and, thirdly, it enhances contact between biomass with the carbon source and nutrients in the wastewater The RHMBR effluent was treated using the MBR under aerobic conditions before being discharged alternately through two automatic suction pumps (P3, P4) Wastewater level in the MBR reactor was controlled using level sensors The wastewater was allowed to flow naturally from the EQ through the RHMBR to the MBR via gravity flow to save capital costs Both the equalizing reactor and RHMBR were agitated at 120 rpm and 0.16 rpm, respectively by a commercial agitator (Hyup Dong Co., LTD., Korea) In RHMBR, fiber polypropylene media was on a mount and turned around the axis of the agitator at the center of the reactor (Fig 1) The packing ratio of the fibrous was 60 ± 5% based on the volume of the reactor for the attached growth biomass The picture of polypropylene fiber media is shown in Fig 1c A bundle of the media consisted of thousands of fibers having a total specific surface area of 560–725 m2 mÀ3, and a specific weight of 0.530 ± 0.027 kg mÀ3 Two flat-sheet modules of submerged membrane in the MBR were microfiltration membranes (model TC10A05, Yuasa Corporation, Korea) with outline dimensions of 1.3 m in length, 0.75 m in width, and 1.52 m in height The number of membrane elements was 75 per module The effective filtration area per membrane element was 0.8 m2 with an average pore size of 0.25 lm (ranging from 0.1 to 0.45 lm) and total surface area of 120 m2 The designed operational trans-membrane pressure (TMP), in the range of À0.05 to 0.0 MPa, and the phenomenon of bio-fouling in the MBR were monitored for changes in TMP via the vacuum gauge The primary-function of MBR is to maintain a high biomass density under aerobic conditions and separation of particles larger than the membrane pore size Two air blowers (3 phase Ring blower, model HRB-402S, Hwang Hae Electric Co., Ltd.) were operated alternately, maintaining an uninterrupted air supply through an air diffuser system It was installed beneath the membrane modules to provide coarse bubble aeration (15.49–17.98 m3 air hÀ1) for enhanced organic carbon oxidation and nitrification while helping to reduce the membrane fouling and increase the sludge mixing All pumps, agitators, air blowers, electric valves, sensors, membrane backwashing system and other equipment were automatically controlled by a programmable logic controller (PLC) There was also a manual operating mode A flow diagram of the EC process used in this study is shown in Fig 1b A small portion of wastewater from the effluent of the hybrid pilot plant was contained in a 200 L polypropylene tank From this tank, wastewater was continuously pumped through the flow meter in an upward axial flow through an annular region between two coaxial cylinders of radius cm and 9.5 cm in the EC reactor as shown in Fig 1d Electrodes were connected to a Dual DC Power Supply (Sunchang Electronic Co., LTD., South Korea) which includes: voltage and current monitor, an on-off switch, and a rheostat used to vary the desired output voltage In each channel of the DC power supply, there are digital voltage meters with a voltage response (0–30 V) monitor and current meter to set the applied potential and current level Table Characteristics of raw municipal wastewater Parameters Units Range Average pH SS BOD5 CODCr T-N NO -N NHỵ -N Unitless mg LÀ1 mg LÀ1 mg LÀ1 mg LÀ1 mg LÀ1 mg LÀ1 mg LÀ1 7.0–8.0 175–460 166.73–222.32 187.7–334.9 30.83–63.08 0.00–1.06 18.89–43.54 2.51–6.95 7.70 281.90 205.76 238.83 41.16 0.20 29.85 4.46 mg LÀ1 mgCaCO3 LÀ1 MPNÁ(100 mL)À1 3.00–8.39 90–220 1.5E + 6–2.0E + 43.85:7.55:1.00 5.45 163.43 7.56E + 06 PO3À -P T-P Alkalinity Coliform bacterial COD:N:P ratio Raw water The MBR was operated for a 10 cycle-filtration consisting of of filtration and of relaxation An internal recycle flow (R) rate from the MBR to the EQ (like a buffer tank) was performed to carry out reduced oxygen concentration in the internal recycle, denitrification and phosphorus removal The recirculation flow rate was adjusted into the compartments (EQ1, EQ2, and EQ3) of EQ as shown in Table The R value remained at 1.0, 2.0, 3.0 and 4.0, based on the influent flow rate, corresponding to Run 1, Run 2, Run and Run 4, respectively The primary-function of MBR is to maintain a high biomass density under aerobic conditions and separation of particles larger than the membrane pore size Throughout the study period, MLSS in the RHMBR and MBR was kept at around 4155–7810 mg LÀ1 and 4565–8690 mg LÀ1, respectively, depending on the internal recycling ratio Excess sludge was discharged from the MBR tank to keep the MLSS concentrations at the designated values with an amount of 120–180 L dayÀ1 Specific HRTs of the individual reactors, recirculation ratios and other parameters are summarized in Table The EC process consists of a pair of aluminum electrodes cylindrically shaped and placed in concentric cylinders together The inside electrode has dimensions of cm ID Â 45.5 cm H with a geometric area of 714.71 cm2 while outside electrode has dimensions of 9.5 cm OD Â 50 cm H with a geometric area of 1492.26 cm2 and a working surface area of 1357.95 cm2 The gap between the electrodes was 2.25 cm (Fig 1d) During operation, a constant electric potential of 10 volts (V) was applied with a hydraulic retention time of (more details are summarized in Table 6) 2.3 Analytical methods The influent, EQ, RHMBR, MBR and the effluent samples were collected 1–3 times per week to monitor the performance and kept in a refrigerator prior to analyses The water quality parameters including biological oxygen demand (BOD5), total coliform, mixed liquor volatile suspended solids (MLVSS), mixed liquor suspended solids (MLSS), total suspended solids (TSS) and alkalinity were analyzed according to standard methods (APHA, 2005) Chemical oxygen demand (CODCr), total nitrogen (T-N), ammonia nitrogen (NHỵ -N), nitrate (NO -N), total phosphorus (T-P), phosphate 2.2 Operating conditions Characteristics of raw and treated municipal wastewater used for the hybrid pilot plant are shown in Table The DO concentrations in the RHMBR and MBR were controlled and sustained under 0.05 mg LÀ1 and over 1.7 mg LÀ1, respectively, during the study period In addition, pH in each tank ranged from 6.6 to 8.0 Water flux and TMP were maintained in a range of 18.45–28.21 L mÀ2 hÀ1 (LMH) and 20.0–51.0 kPa, respectively Table Distribution mechanism of internal recycle flows into the compartments of EQ Recirculation modes (internal recirculation value) Run Run Run Run (1Q) (2Q) (3Q) (4Q) EQ1 1Q 1Q EQ2 EQ3 1Q 1Q 1Q 1Q 1Q 1Q 2Q Author's personal copy 119 D.D Nguyen et al / Bioresource Technology 153 (2014) 116–125 Table Operational conditions of the hybrid pilot plant Factors Internal recirculation rate (R) Run Run Run Run 55.0–65.0 (60.64) 1.83–2.16 (1.97) 1.74–2.06 (1.87) 5.33–8.055 4.960–6.63 120–180 16.15–20.11 (17.04) 18.45–28.21 (22.81) 20.0–51.0 (31.72) 0.00–0.05 (0.03) 1.70–3.21 (2.64) filtration + idle 15.49–17.98 13.2–25.6 (20.35) 7.01–7.95 (7.67) NaOCl solution 0.5–1.2% 0.1–0.21 (0.15) Influent flow rate (m3 dayÀ1) Anoxic/anaerobic HRT (h) Oxic HRT (h) MLSS (g LÀ1) in MBR MLSS (g LÀ1) in RHMBR Sludge waste (L dayÀ1) SRT (day) Flux (LMH) TMP (kPa) Anoxic DO (mg LÀ1) Oxic DO (mg LÀ1) Operation cycle (min) Specific aeration (m3 airÁhÀ1) Temperature pH Chemical cleaning reagents F:M ratio (gCOD/g MLVSS) 46.0-63.0 (57.67) 1.26-1.73 (1.39) 1.20-1.64 (1.32) 6.73-8.69 5.64-7.81 44.6-59.6 (51.43) 1.00-1.33 (1.16) 0.95-1.27 (1.11) 4.565-7.260 4.155-6.40 45.0-66.3 (52.44) 0.72-1.06 (0.92) 0.68-1.01 (0.87) 5.438-6.980 5.06-6.295 0.09–0.16 (0.11) 0.08–0.19 (0.12) 0.09–0.16 (0.11) The average value is show in parentheses Table Nitrogen removal during the operational period of different Runs R parameter Unit process Influent EQ3a RHMBRa MBRa Effluent Run T-N (mg LÀ1) NOÀ -N (mg L1) NHỵ -N (mg L1) pH Alk (mg LÀ1) 42.43 ± 6.65 0.55 ± 0.23 25.45 ± 4.30 7.67 ± 0.27 164.3 ± 12.6 8.39 ± 1.58 1.38 ± 0.82 7.07 ± 0.82 7.56 ± 0.32 – 9.45 ± 0.56 0.00 ± 0.00 7.66 ± 1.25 7.47 ± 0.33 114.8 ± 12.9 12.09 ± 1.41 9.95 ± 1.34 0.00 ± 0.00 7.21 ± 0.32 62.7 ± 8.6 11.17 ± 1.21 9.61 ± 1.27 0.00 ± 0.00 6.93 ± 0.31 – Run T-N (mg LÀ1) NOÀ -N (mg L1) NHỵ -N (mg L1) pH Alk (mg LÀ1) 43.04 ± 8.05 0.48 ± 0.10 25.95 ± 2.24 7.66 ± 0.30 165 ± 12.1 7.96 ± 2.57 2.28 ± 1.80 5.42 ± 1.37 7.48 ± 0.44 – 8.62 ± 1.54 0.05 ± 0.12 5.20 ± 1.55 7.37 ± 0.40 109.64 ± 15.5 10.03 ± 1.61 8.22 ± 0.85 0.06 ± 0.15 7.25 ± 0.27 74.36 ± 16.1 9.55 ± 1.30 8.18 ± 0.86 0.00 ± 0.00 7.08 ± 0.14 – Run T-N (mg LÀ1) NOÀ -N (mg L1) NHỵ -N (mg L1) pH Alk (mg LÀ1) 41.24 ± 7.36 0.11 ± 0.17 29.45 ± 3.86 7.69 ± 0.19 170.33 ± 16.4 7.60 ± 2.25 2.52 ± 1.16 3.75 ± 0.98 7.63 ± 0.13 – 5.75 ± 1.53 0.32 ± 0.39 4.01 ± 0.99 7.66 ± 0.15 99.7 ± 10.1 7.31 ± 1.64 5.42 ± 1.27 0.12 ± 0.15 7.59 ± 0.18 71.48 ± 12.4 6.34 ± 1.39 5.1 ± 1.33 0.00 ± 0.00 7.54 ± 0.20 – Run T-N (mg LÀ1) NOÀ -N (mg L1) NHỵ -N (mg L1) pH Alk (mg LÀ1) 40.48 ± 6.58 0.06 ± 0.14 31.47 ± 4.97 7.41 ± 0.2 152.6 ± 20.2 6.41 ± 1.66 1.86 ± 0.98 2.70 ± 0.79 7.36 ± 0.49 – 3.53 ± 1.24 0.25 ± 0.33 2.36 ± 0.76 7.46 ± 0.12 104.7 ± 13.2 4.44 ± 0.89 3.88 ± 0.77 0.25 ± 0.16 7.45 ± 0.15 72.7 ± 8.61 3.81 ± 0.90 3.4 ± 0.74 0.00 ± 0.00 7.41 ± 0.14 – Alk = Alkalinity (mg CaCo3 LÀ1) a The values measured after filtering through glass microfiber filters 1.2 lm (PO3À -P) were analyzed with analyzer kits using Spectrophotometer HS 3300, and an HS R200 Oven (Humas Co., LTD., Korea) Values for pH, and both dissolved oxygen (Martin & Nerenberg) concentration and temperature were measured online using a XL60 (AccumetÒ XL60, Thermo Fisher Scientific Inc.) and YSI 550A DO Instrument (YSI Environmental, US), respectively All samples from EQ, RHMBR, MBR, and after EC treatment, were filtered using a WhatmanÒ GF/C glass microfiber filters 1.2 lm Results and discussion 3.1 Ammonia nitrogen removal Fig shows the ammonia nitrogen concentration in each reactor both in influent flow and efuent ow, and NHỵ -N loading rate, NHỵ -N/MLVSS ratio based on MLVSS in MBR, and CODCr/ NHỵ -N ratio in the hybrid system during operation It was ob4 served that the nitrogen removal in this system was based on simple denitrification in RHMBR, followed by nitrification in MBR, where nitrifying bacteria convert nitrogen in the form of ammonia into nitrite and nitrate Nitrification is the important primary process in removing total nitrogen from influent wastewater However, its requirements of long SRT and high DO concentration are usually considered as the limiting steps of the nitrogen removal process in wastewater (Tan and Ng, 2008) As shown in Fig 2a, the inuent NHỵ -N concentration ranged from 18.86 to 43.46 mg LÀ1 with an average of 29.79 4.69 mg L1 The nal efuent NHỵ -N concentrations of all runs was nil This means that the nitrification process in this system was fully completed as all NHỵ -N was converted into nitrate throughout the Author's personal copy 120 D.D Nguyen et al / Bioresource Technology 153 (2014) 116–125 Table The influent and effluent T-P, PO3À -P concentrations and various key parameters of ratios versus phosphorus during the operational period of different Runs Parameters Internal recirculation rates T-P of influent (mg LÀ1) 5.72 ± 0.69 4.75 ± 0.89 1.05 ± 0.27 1.02 ± 0.24 1.30 ± 0.32 1.29 ± 0.33 1.49 ± 0.27 1.25 ± 0.34 14.07 ± 3.35 46.45 ± 11.43 8.16 ± 2.52 0.84 ± 0.17 15.25 ± 2.25 41.79 ± 7.15 7.25 ± 1.41 0.84 ± 0.16 1.84 ± 0.45 50.15:8.11:1.0 PO3À -P/T-P ratiob NOÀ -N/T-P ratioc COD:N:P ratio 5.28 ± 1.09 4.33 ± 0.94 0.03 ± 0.024 mg/L (99.33 ± 0.56%) 16.42 ± 3.60 16.57 ± 3.34 51.93 ± 11.90 47.04 ± 6.79 8.60 ± 3.12 7.93 ± 2.30 0.78 ± 0.15 0.82 ± 0.16 PO3À -P of effluent (mg LÀ1) T-P of final effluenta T-P loading rate (g mÀ3 dayÀ1) CODCr/T-P ratiob T-N/T-P ratiob 2.31 ± 0.59 44.60:7.81:1.0 2.16 ± 0.49 40.99:7.07:1.0 2.42 ± 0.34 46.43:7.78:1.0 Using Electrocoagulation at post-treatment The ratio of influent wastewater The ratio of total nitrate to total phosphorus entering anoxic/anaerobic tank Values in the above table is the average values c entire study (Fig 2a) This complete and constant removal of NHỵ -N was denite, regardless of any internal recycling ratios, or variations in the influent strength and the NHỵ -N loading rate It also corresponded to volumetric NHỵ -N loading rate based on ỵ the volume of MBR from 0.1 to 0.29 kgNH4 -N mÀ3 day1 with an average of 0.17 0.031 kgNHỵ -N mÀ3 dayÀ1 (Fig 2b and Table 4) The results show that in all running modes, the nitricaỵ tion of NH4 -N always occurred completely in MBR but incomplete denitrification in RHMBR was observed (Fig 4c) This can be explained by the process of agitation in the RHMBR was not completely mixed Guo et al (2009) reported that NHỵ -N removal of more than 99% with 10% sponge media at NHỵ -N inuent concentration of 1520 mg L1 It also determined the ratio of NHỵ -N/MLVSS base on the MLVSS in MBR The ratio of CODCr/NHỵ -N in the influent during operation of the hybrid pilot plant was also determined, as shown in Fig 2c and d In addition, during the monitoring period of hybrid system, the average alkalinity concentration in influent, RHMBR and MBR were 163.43 ± 19.06, 103.28 ± 12.65 and 71.89 ± 11.45, respectively The alkalinity in RHMBR was higher than that in MBR as in addition to being available in the inflow of wastewater, alkalinity is also Table Summary of some key operating parameters and results during operation of EC process No Parameters Average (range) Initial T-P (mg/L) T-P of effluent without EC (mg LÀ1) T-P of effluent with EC (mg LÀ1) pH of effluent flow Electrical conductivity (lS cmÀ1) 10 Current (Ampere, A) Current densities (A mÀ2) Specific energy consumption, SEC (kWh mÀ3)a Specific aluminum consumption, SAC (g mÀ3) The mole ratio of Al to T-P 11 Sludge generated (kg mÀ3) 12 13 14 Upflow velocity (m sÀ1) Hydraulic retention time (min) Applied electric potential (Volts, V) 5.45 ± 0.95 (3.00–8.39) 1.28 ± 0.41 (0.04–2.09) 0.03 ± 0.02 (0.00–0.11) 7.67 ± 0.16 (7.10–7.92) 460.02 ± 0.07 (431.00– 548.00) 1.91 ± 0.07 (1.80–2.10) 4.61 ± 0.18 (4.34–5.07) 0.2733 ± 0.0104 (0.2573– 0.3002) 9.1725 ± 0.3489 (8.6349– 10.0741) 8.4416 ± 2.2729 (5.3293– 15.1614) 0.0437 ± 0.0106 (0.022– 0.0437) 0.0038 10 Electric energy consumption of EC process Run Run 100 30 80 + Conc (mgNH4 -N/L) Run Run 40 20 10 Removal efficiency (%) Influent (mg/L) 60 MBR* (mg/L) Effluent (mg/L) 40 RHMBR* (mg/L) 20 (a) + 0.3 A: NH4 -N loading rate (Kg/m day) base on the volume of MBR + NH4 -N loading (kg/m day) A 0.2 (b) 0.1 + 0.010 B: NH4 -N/MLVSS ratio base on the MLVSS in MBR 0.008 0.006 0.004 15 C: COD /NH +-N ratio of the influent wastewater Cr 12 + NH -N/MLVSS ratio (c) + CODCr/NH4 -N ratio C B Conversion efficiency (%) b a 5.53 ± 0.80 4.40 ± 0.47 0.39 ± 0.38 0.39 ± 0.39 PO3À -P of influent (mg LÀ1) T-P of effluent (mg LÀ1) a 5.23 ± 1.05 3.95 ± 0.46 25 50 (d) 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 Operation time (days) Fig Variations of NHỵ -N concentrations, NHỵ -N conversion efciency, NHỵ -N loading rate, NHỵ -N/MLVSS, and CODCr/NHỵ -N ratio in the hybrid system during operation 4 4 Author's personal copy 121 T-N concentration (mg/L) 70 Run Run Run 100 60 80 50 60 40 T-N Removal efficiency (%) 12 Influent (mg/L) Effluent (mg/L) 10 0.20 T-N LR: T-N loading rate (kg/m 3.day) 0.15 (a) 10 (b) 0.20 CODCr/T-N ratio of the influent wastewater T-N loading rate (kg/m day) 0.15 0.10 0.10 0.05 40 20 CODCr/T-N T-N LR Run T-N removal efficiency (%) D.D Nguyen et al / Bioresource Technology 153 (2014) 116–125 (c) 25 50 0.05 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 Operation time (days) Fig Variations of T-N concentrations, T-N removal efficiency, CODCr/T-N ratio, and NLR in the pilot system during operation produced in denitrification under RHMBR condition and then is partially consumed in the nitrification process under MBR condition The results indicated that there was enough buffering available in the wastewater for nitrogen, phosphorus removal process in particular, and a biological process in general throughout all runs 3.2 Total nitrogen removal and mechanism The influent and effluent of T-N concentrations and T-N removal efficiencies; CODCr/T-N ratio in the influent, and volumetric nitrogen loading rater (NLR) based on the total volume of RHMBR and MBR in this system are shown in Fig 3c The average influent T-N concentration, CODCr/T-N ratio in the influent, and NLR were 41.16 ± 7.04 mg LÀ1, 5.96 ± 1.14, 0.11 ± 0.02 kgTN mÀ3 dayÀ1, respectively Fig 3a shows that as the internal recycle ratio (R) increased from 1.0 to 4.0, the T-N removal efficiencies increased from 72.99 ± 5.95% to 90.42 ± 2.43%, which corresponds to the final effluent T-N concentration of 11.17 ± 1.21 mg LÀ1– 3.81 ± 0.9 mg LÀ1, respectively In this study, four recycle ratios 1, 2, 3, and were investigated The higher the recycle ratio (R), the better the nitrogen removal was For example, the T-N removal efficiencies were increased from 72.99 ± 5.59% (R = 1), 77.06 ± 5.99% (R = 2), 84.24 ± 4.09% (R = 3) to 90.42 ± 2.43% (R = 4), respectively The results also indicated that total nitrogen levels could be achieved less than 10 mg LÀ1 with a circulation rate Thus, in this study, the most appropriate circulation rate should be used in Runs or in terms of nutrient removal However, the experimental results also demonstrated that the changes in the CODCr/T-N, CODCr/NHỵ -N and NHỵ -N/MLVSS ratio 4 ranged from 3.52 to 8.82 (Fig 3b), 4.72–13.13 (Fig 2d), and 0.0036–0.01037 (Fig 2c), respectively, but did not signicantly affect T-N, NHỵ -N and CODCr removal On the other hand, changes in the R strongly effected the T-N removal With an increase in the R, nitrogen removal efficiency significantly improved The effect of the R on nitrogen removal was also investigated in previous studies (Baeza et al., 2004; Tan and Ng, 2008) Ahn et al (2005) have shown that the T-N removal efficiency improved to 67% as the internal recycle ratio was 300% of influent flow rate Similarly, Lee et al (2010) observed that T-N removal efficiency was increased from 70 ± 9% to 89 ± 3% in a pre-denitrification membrane process as the internal recycle ratio from aerobic to anoxic zone increased from to Temperature is one of the important factors in the process of nitrification and denitrification During operation, the temperature was varied from 13.2 °C to 25.6 °C Depending on the variations of the internal cycling ratio, the biomass concentrations were controlled from 3.330 to 6.949 gMLVSS LÀ1 (4.155–7.810 gMLSS LÀ1) and from 3.640 to 6.881 gMLSS LÀ1 (4.565–8.690 gMLSS LÀ1) in RHMBR and MBR, respectively (Table 3) During the operating period, the CODCr/T-N ratios in the influent flow rate were between 3.52 and 8.22, with an average 5.96 ± 1.14 (Fig 3b), Alkalinity buffering in the RHMBR and MBR was 103.28 ± 13.16 mgCaCO3 LÀ1 and 71.89 ± 11.65 mgCaCO3 LÀ1, respectively The experimental results also suggested that there was enough carbon available in the municipal wastewater for removal of nitrogen in all runs, without adding an external carbon and energy source The variations of NOÀ -N concentrations during the study are also represented in Fig During the whole operation, the initial À concentrations of NO3 -N in the wastewater were low (0.0– 1.06 mg L1, average of 0.16 mg L1) The NHỵ -N concentration in final effluent were zero, indicating almost NHỵ -N completely nitri4 cation to NO3 -N in the MBR (Fig 2) The average of NOÀ -N con3 centrations in the final effluent were low and significantly decreased, from Run to Run were 9.61 ± 1.27 mg LÀ1, 8.18 ± 0.86 mg LÀ1, 5.10 ± 1.33 mg LÀ1, and 3.4 ± 0.74 mg LÀ1 (Fig 4d), respectively In general, the experimental results demonstrated that the R influenced the nitrification and denitrification The increase in R improved the nitrification rate in MBR conditions (Fig 4c), but gradually reduce the denitrification rate in RHMBR conditions (Fig 4b) However, the nitrification efficiency was high enough to produce low NOÀ -N concentrations of 0.0–1.26 mg LÀ1 (average of 0.25 mg LÀ1) after flowing through the RHMBR, indicating that most of NOÀ -N in the MBR was converted into nitrogen gas in the anoxic/anaerobic conditions of the RHMBR The RHMBR showed its important role in the denitrification process, which can provide media support for microbial growth utilizing excellent material, and agitation to increase contact with denitrifying bacteria In addition, these results also demonstrated that T-N removal efficiency increased with increasing in the internal recycling ratio R The T-N in the final effluent was mainly in the form of NOÀ -N, with concentrations ranging from 1.89 to 11.4 mg LÀ1 with respect to the R However, an increased internal recycle ratio would increase the energy consumption, causing a subsequent increase the operating costs Author's personal copy 122 D.D Nguyen et al / Bioresource Technology 153 (2014) 116–125 Run NO3 /MLVSS - NO3 -N concentration (mg/L) Run Run Run Influent (mg/L) 50 - (c) Effluent (mg/L) 25 (b) MBR* (mg/L) 0.003 NO3 -N/MLVSS ratio base on the MLVSS in RHMBR 0.002 0.001 0.000 (a) RHMBR* (mg/L) 1.0 0.5 0.0 1.5 1.0 0.5 0.0 12 12 (d) (e) NO3 -N/MLVSS ratio - 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 Operation time (days) Fig Variations of NOÀ -N concentrations, removal efficiency and NOÀ -N/MLVSS in the pilot system during operation Consequently, it is suggested that the total recycling ratio can be adjusted according to the effluent nitrogen requirements It is important in choosing the best value internal recirculation by balancing between energy costs, effluent nitrogen quality requirements and a number of other parameters that would be favorable to improving the effluent quality 3.3 Phosphorus removal TN/TP COD/TP T-P LR 3- PO4 -P removal (%) Run Run 100(a) Run Run 80 60 40 20 -20 -40 333PO4 -P influent (mg/L) PO4 -P effluent (mg/L) PO4 -P removal efficiency (%) 100 (b) Starting EC T-P remv _without EC (%) T-P remv _with EC (%) 80 60 40 20 T-P conc eff._with EC (mg/L) T-P conc eff._without EC (mg/L) T-P influent (mg/L) T-P in RHMBR (mg/L)* T-P in MBR (mg/L)* 20 (c) T-P removal (%) T-P conc (mg/L) 3- PO4 -P influent (mg/L) Figs 5(b–e) shows the variations of phosphorus concentrations in influent, effluent, and in each tank’s total phosphorus removal efficiency, T-P loading rate, T-N/T-P ratio, and CODCr/T-P ratio of the influent wastewater in different phases throughout the study The influent T-P concentration fluctuation ranged between 3.00 and 8.39 mg LÀ1 (average 5.45 ± 0.95 mg LÀ1) and generally, the effluent T-P concentration was stable and lower than 2.0 mg LÀ1 with an average of 1.28 ± 0.41 mg LÀ1 regardless of internal recycling ratios (Fig 5b) The influent total nitrogen to total phosphorus ratio was in a range between 4.15 and 18.87 (average 7.87 ± 2.26) (Fig 5e) The influent CODCr to total phosphorus ratio was in a range between 27.44 and 87.39 (average 45.53 ± 10.29) (Fig 5d) In terms of the specific operating conditions, the average T-P 10 T-P loading (kg/m day) 80 T-P LR: T-P loading rate (kg/m day) 60 40 16 12 50 (d) T-N/T-P ratio 25 CODCr/T-P ratio (e) 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 Operation time (days) Fig Variations of T–P concentrations and PO3À -P concentrations, and removal efficiencies, T-P loading rate, CODCr/T-P ratio, and T-N/T-P ratio in the pilot system during operation Author's personal copy 123 (6) 10 (5) 3 SEC(kWh/m ) 0.3 (6) 0.2 0.1 The mole ratio of Al to T-P (5) 0.0 140 Sludge generated (Kg/m )(7) 0 15 (8) (8) 160 180 200 220 240 260 Operation time (days) 280 300 320 (b) 0.5 0.4 (4) Sludge generated (Kg/m ) 25 Run SAC (kg Al/m ) Run Conductivity (µS/cm) TP removal efficiency (%) 0.5 SEC(kWh/m ) The mole ratio of Al to T-P 30 20 (a) 560 480 400 320 240 160 80 -2 (1) Run Run (3) 100 Removal efficiency (%) (4) (2) 80 (2) 60 Current densities (A/m ) (1) 1.6 40 1.2 (2) Α ΑΑΑΑ Α Current Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α 20 (A) Α Α Α Α Α Α Α Α Α Α Α Α Α Α 0.8 (1) Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α Α Current (A) (3) 0.4 discharge limit, 0.2mg/l (2) 0.0 (2) 140 160 180 200 220 240 260 280 300 320 T-P conc influent (mg/L) Whithout EC treatment (mg/L) Whith EC treatment (mg/L) Conductivity (microS/cm) 0x 0x 0x 0x 0x 10 - 10 - 10 - 10 - 10 3 3 SAC (kg Al/m ) 10 T-P concentration (mg/L) Current (A) or Current densities (A/m D.D Nguyen et al / Bioresource Technology 153 (2014) 116–125 0.4 0.3 0.2 0.1 0.0 (7) Fig Effect of the EC process on phosphorous removal, and variation of specific energy consumption (SEC), specific aluminum consumption (SAC), mole ratio of Al to T-P and sludge generated during operation of EC process removal efficiencies of the hybrid system without an EC process were 92.61 ± 7.57%, 80.78 ± 5.49%, 74.42 ± 8.26%, and 73.44 ± 6.03%, corresponding to Rs 1, 2, and 4, respectively These results showed the influence of the internal recirculation flow on the phosphorus removal performance of the system, and in particular, the effect of dissolved oxygen and nitrate concentrations, caused by changes in the internal recirculation ratios (Chen et al., 2011; Ozgur Yagci et al., 2003) It should be noted that the high T-P removal efficiency in the first phase was attributed to the effect of high biomass production which occurred through assimilation (Monclús et al., 2010) Fig 5b indicated that T-P in the MBR was lower than T-P in the RHMBR as phosphorus was taken up under aerobic conditions in the MBR by poly-phosphate accumulating organisms (PAOs), and released under anoxic/anaerobic conditions in the RHMBR Although these occurred simultaneously under the same conditions of anoxic/ anaerobic conditions, there was also a small portion of phosphorus uptake by denitrifying PAOs (Peng et al., 2006) Excess sludge was discharged from the MBR tank to keep the MLSS concentration at the designated values with an amount of 120–180 L dayÀ1 The average influent and effluent T-P concentrations, the ratios of CODCr/T-P, T-N/T-P, PO3À -P/T-P of influent, and the NOÀ -N/T-P ratio of total nitrate to total phosphorus entering anoxic/anaerobic tank in different runs throughout the study are listed in Table The hybrid pilot plant treated municipal wastewater throughout the study with average COD:N:P ratios of 50.15:8.11:1.00, 46.43:7.78:1.00, 44.60:7.81:1.00, and 40.99:7.07:1.00 corresponding to Runs 1Q, 2Q, 3Q and 4Q, respectively The variation of PO3À -P concentration in influent and effluent, and removal of PO3À -P by the hybrid system are shown in Fig 5a The average PO3À -P concentration in the influent was 4.33 ± 0.85 mg LÀ1 and removal efficiencies of the hybrid system were 90.05 ± 9.91%, 76.48 ± 6.00%, 68.40 ± 12.95% and 70.52 ± 11.33% with the values corresponding to Runs 1, 2, and 4, respectively The initial PO3À -P/T-P ratio of wastewater fed into the system was in ranged between 0.42 and (average of 0.82 ± 0.17) It was found that T-P the most appropriate circulation rate should be used in runs or This system also was successful in reducing the fouling of the membrane, as the membrane was only chemically cleaned in place during its year of operation using sodium hypochlorite (NaOCl) solution 0.5–1.2% (v/v) for h without aeration This stabilized the system operation at a constant membrane permeation flux of 22.77 ± 2.19 LMH under ambient temperature conditions This reduced overall maintenance needs and increased operational efficiency of the system 3.4 Enhanced phosphorus removal by EC The hybrid pilot plant was operated without adding supplemental reactive compounds (carbon sources, chemicals, etc.) to the solution which resulted in relatively good phosphorus removal, with less than mg LÀ1 remaining However, due to more stringent regulations and wastewater reuse strategies, it is necessary to achieve phosphorus concentrations after treatment below 0.2 mg LÀ1 (guideline) Innovation and advanced technology are needed to achieve better efficiency in phosphorus removal Analysis results of the PO3À -P/T-P ratio in the effluent flow through the membrane bioreactor averaged 0.94 ± 0.16, indicating that phosphorus in the effluent exists mainly as orthophosphate (PO3À -P) For these reasons and others the electrocoagulation (EC) process using cylindrical aluminum electrodes, was carried out continuously in the 145th to 316th day in post-treatment During that time, T-P concentration in final effluent showed that excellent T-P removal was achieved in the 145th to 316th investigation days The highest effluent concentration detected during the course of the experiment using the EC process was 0.11 mgTP LÀ1 (Fig 6a) Irrespective of internal recycling ratios, the T-P removal efficiency of the hybrid system combined with EC process at post-treatment has now been shown, in practice, as an excellent method with removal percentages of T-P maintained stably and constantly at a high level of 97.23–100% (average of 99.33 ± 0.56%) The corresponding concentration of T-P in the final effluent remaining was approximately 0.00–0.11 mg LÀ1 (average of 0.03 mg LÀ1) During that period without using EC process, the efficiency was only in the range of 73.30 ± 8.65% Author's personal copy 124 D.D Nguyen et al / Bioresource Technology 153 (2014) 116–125 Fig 6a showed that the effluent quality could stably and significantly be maintained for phosphorus removal when the EC process was used as a combined process, despite obvious fluctuations in the concentration of influent T-P During the course of operating with the EC process in continuous-flow, electrical energy consumption cost, amount of aluminum used, and sludge generated per cubic meter of wastewater were averaged The results achieved were 0.2733 ± 0.0104 kWh/m3, 9.1725 ± 0.3489 g mÀ3, and 0.0437 ± 0.0106 kg mÀ3, respectively, and the corresponding mole ratio of Al to T-P was 8.4416 ± 2.2729 (Fig 6b) Knowing the amount of aluminum electrode used per cubic meter of wastewater treated would enable operators to have a predictable plan for replacement of used electrodes The activity of the anode can decrease over time due to the existence of ions such as Ca2+, Mg2+, NHỵ , HCO , SO2 , etc., in wastewa4 ter This is caused by the precipitation of ions or the formation of insoluble hydroxides, or sludge layers on the surface of the electrodes These layers insulate the surface of the electrodes, consequently reducing amperage and preventing the needed anode electrode dissolution in the electrolytic solution (Bektas et al., ß 2004; Chen, 2004; Martin and Nerenberg, 2012; Nguyen et al., 2013) The EC electrodes used in this study were designed and operated to avoid these above concerns To find and establish the optimum operating parameters for effective EC processing in this experiment, a series of lab-scale experiments were done using both synthetic wastewater and real municipal wastewater In this way, the ideal operating conditions for effective EC processing were determined in advance (Nguyen et al., 2013) By using this predetermined optimum condition for T-P removal with the advanced aluminum electrodes in continuous mode, a hydraulic retention time of min, and application of a constant electric potential of 10 V, some of the highest removal rates ever achieved were recorded Other parameters measured during the course of the experiment with EC are shown in Fig 6, and Table During the EC experiment, the temperature and pH value were not altered much, and remained in the range of 13.2–25.6 °C and 7.10–7.92, respectively In spite of the fact that a hybrid system with RHMBR and a submerged MBR performed well in the biological treatment of wastewater, some cases require stringent quality control of T-P concentration after treatment These initial results show that this method of combined EC processing as post-treatment promises to be essential in meeting those requirements and extant stringent regulations Consequently, further investigation is critically and urgently needed for the broad implementation of this pragmatic and effective methodological tool in the struggle to contain the negative anthropomorphic impacts of phosphorus and related wastewater pollution on surface and groundwater resources worldwide Conclusions An integrated hybrid RHMBR and MBR system together with an advanced EC process as post treatment performed extremely well in removing COD, NHỵ -N, T-N and T-P The internal recycling ratio significantly affected on the nitrogen removal efficiency Due to the completed nitrification, T-N in effluent was mainly in the form of NOÀ -N and its removal rate was better at high recycling ratios The EC process as post treatment proved highly efficient in producing high and stable levels of T-P removal References Ahn, Y.T., Kang, S.T., Chae, S.R., Lim, J.L., Lee, S.H., Shin, H.S., 2005 Effect of internal recycle rate on the high-strength nitrogen wastewater treatment in the combined UBF/MBR system Water Sci Technol 51, 241–247 AWWA, WEF, APHA, 1998 Standard Methods for the Examination of Water and Wastewater, 20th ed American Public Health Association, Washington DC, USA Baeza, J.A., Gabriel, D., Lafuente, J., 2004 Effect of internal recycle on the nitrogen removal efficiency of an anaerobic/anoxic/oxic (A2/O) 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treatment system of bioreactors and electrocoagulation for superior removal of organic and nutrient pollutants from municipal wastewater Dinh Duc Nguyen a, Huu Hao Ngo b, Yong Soo Yoon a, ⇑ a. .. M., Ohashi, A. , Harada, H., 2007 Performance comparison of a pilot-scale UASB and DHS system and activated sludge process for the treatment of municipal wastewater Water Res 41, 2697–2705 Wahab,... Water and Wastewater Treatment Elsevier, Oxford, UK Khoshfetrat, A. B., Nikakhtari, H., Sadeghifar, M., Khatibi, M.S., 2011 Influence of organic loading and aeration rates on performance of a lab-scale