Environmental Technology & Innovation 21 (2021) 101273 Contents lists available at ScienceDirect Environmental Technology & Innovation journal homepage: www.elsevier.com/locate/eti Influence of hydraulic loading rate on performance and energy-efficient of a pilot-scale down-flow hanging sponge reactor treating domestic wastewater ∗ Van-Tung Tra a , Bao-Trong Dang b , , Quach An Binh c , Quy-Hao Nguyen b , Phuong-Thao Nguyen b , Hong-Hai Nguyen b , Thanh-Tin Nguyen b , ∗∗ Thanh-Hai Le a , Duc-Trung Le a , Tomoaki Itayama e , Xuan-Thanh Bui b,d , a Institute for Environment and Natural Resources, Vietnam National University Ho Chi Minh, Ho Chi Minh City, Viet Nam Key Laboratory of Advanced Waste Treatment Technology, Vietnam National University Ho Chi Minh (VNU-HCM), Linh Trung ward, Thu Duc district, Viet Nam c Faculty of Applied Sciences-Health, Dong Nai Technology University, Dong Nai 810000, Viet Nam d Faculty of Environment and Natural Resources, Ho Chi Minh City University of Technology (HCMUT), Ho Chi Minh City 700000, Viet Nam e Graduate School of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan b article info Article history: Received 18 September 2020 Received in revised form 25 November 2020 Accepted 25 November 2020 Available online 27 November 2020 Keywords: Down-flow hanging media reactor Hydraulic loading rate Sponge Nitrification Denitrification a b s t r a c t The hydraulic loading rate (HLR) is a vital factor affecting the biological attached growth processes Herein, a pilot-scale down-flow hanging sponge reactor (DHS) with a capacity of 0.5 m3 /d was investigated under different HLRs (5.56 and 11.12 m3 /m2 d) The DHS reactor was operated at an organic loading rate of 1.2 kgCOD/m3sponge d The results showed that COD removal attained 60.4 ± 11.4% at HRL of 5.56 m3 /m2 d while this removal increased to 78% during the steady stage of operating at HLR of 11.12 m3 /m2 d Total suspended solid (TSS) was moderately removed with the efficiencies of 69.7%– 75.6% High NH+ -N removal of 80% was attained during operation, indicated that the nitrification process was mildly sensitive to change of HLR This proposed technology brings a beneficial economic in decreasing energy consumption This study suggested a viable treatment solution of the DHS using natural air ventilation, which favors low-income areas © 2020 Elsevier B.V All rights reserved Introduction Ho Chi Minh City (HCMC) of Vietnam is the grandest city, an outstanding financial capital with a population of 8,993,082 in 2019 Due to the rapid urbanization, the water supply is projected to increase by 4.75 million m3 /d in 2025 (Dan et al., 2011) The anthropogenic activity generated 2,303,000 and 780,000 m3 /d of domestic wastewater from an urban and suburban area, respectively (Dan et al., 2011; Babut et al., 2019; Le and Aramaki, 2019) However, up to now, only 10% amount of wastewater is treated due to limited wastewater treatment plants (van Leeuwen et al., 2016) Ever-growing scarce land funds and a narrow financial framework govern a delay in setting up appropriate facilities As a result, the ∗ Corresponding author ∗∗ Corresponding author at: Key Laboratory of Advanced Waste Treatment Technology, Ho Chi Minh City University of Technology (HCMUT), Vietnam National University Ho Chi Minh (VNU-HCM), Viet Nam E-mail addresses: dbaotrong0701@gmail.com (B.T Dang), bxthanh@hcmut.edu.vn (X.T Bui) https://doi.org/10.1016/j.eti.2020.101273 2352-1864/© 2020 Elsevier B.V All rights reserved V.T Tra, B.T Dang, Q.A Binh et al Environmental Technology & Innovation 21 (2021) 101273 water environment needs to deal with 243 tons BOD/d (van Leeuwen et al., 2016), a wide range of persistent compounds (Strady et al., 2017; van Emmerik et al., 2018; Babut et al., 2019) The pollutant remarkably impacts water intake quality, eutrophication, and oxygen depletion (Nguyen et al., 2019a,b) Consequently, this problem raised a critical need to establish more plants to gather up 90% of untreated wastewater Several technologies have been proposed for domestic wastewater treatment such as wetland roof systems (Thanh et al., 2014), low-cost spiral membrane (Cao Ngoc Dan et al., 2016), sponge-membrane bioreactor (sponge-MBR) (Nguyen et al., 2020, 2019a,b) While the activated sludge process performed sufficient treatment on pollutants, their inherent obstacles are high capital and operating cost, which covered mechanical aeration, external carbonaceous, alkaline chemicals, and excessive sludge production (Mahmoud et al., 2011) Recently, a down-flow hanging sponge reactor (DHS) has been introduced and exploited its feasibility for wastewater treatment in developing countries This system exhibits a low-cost treatment, effortless operation, and less sludge disposal (Fleifle et al., 2013; Okubo et al., 2015; Hatamoto et al., 2018; Jong et al., 2018; Hasan et al., 2019) The system is designed based on the concept of a conventional trickling filter However, instead of using rock, stones, or plastic particles, Sponges carriers with high porosity from 250 to 2400 m2 /m3 are used The ideal media provides efficient contact of air–liquid for accelerating the bio-reaction rate (Nomoto et al., 2018), and nitrification occurred inside the sponges (Araki et al., 1999) In a DHS system, the passive diffusion of oxygen from ventilation could help to cut off energetic aeration cost and the sponge carriers facilitate anoxic zones in the reactor (Araki et al., 1999; Uemura et al., 2010; Watari et al., 2017a; Aoki et al., 2018; Hatamoto et al., 2018; Nomoto et al., 2018) This unique process has also attracted more attention to industrial wastewater treatment (i.e., dyeing and textile, rubber wastewater) which possess a high organic pollutant from 505 to 2250 mg COD/L (Tawfik et al., 2014; Watari et al., 2017a) Numerous DHS pilot/full-scales have been implemented for treating UASB effluent, septic tank effluent, or agricultural drainage water and at Japan (Tawfik et al., 2006; Takahashi et al., 2011), Thailand (Yoochatchaval et al., 2014), Indonesia (Machdar et al., 2018a), Egypt (Fleifle et al., 2013) and India (Okubo et al., 2015) Although the system might possess cost-benefit advantages, boosting the nitrogen and inert organic removal is still being noticed (Chuang et al., 2007; Wichitsathian and Racho, 2010) It was realized that the hydraulic loading rate (HLR) is momentous for organic remediation and nitrification–denitrification validity This factor is regulated by adjusting either the feed flow rate or internal recirculation (IR) The first method is used to increase treatment capacity (Mahmoud et al., 2011) However, rising to the HLR 12 m3 /m3 Sponge d resulted in decreasing in COD removal by 1.6 times (Mahmoud et al., 2011) The second method, the HLR change based on internal recirculation (IR) is a route to tolerably improve denitrification (Bundy et al., 2017; Ikeda et al., 2013) A past study reported that the efficiency of denitrification enhanced by 50% as IR was increased to 1.5 (Ikeda et al., 2013) Their findings indicated that IR of 1.5 could be a critical value to control the denitrification sufficiently as high strength wastewater (COD of 1440–2800 mg/L) was employed In contrast, wastewater possessing a low carbon (C/N < 3) is suspected to hampering the denitrification process under IR mode In other words, a high hydraulic force and low C/N ratio could together impair the filtration, biodegradation function by undermining attached growth processes Since the effect of IR on DHS treating low strength wastewater has not been explored yet, this is a critical need for an investigation This study aimed to find an optimal condition of IR and propose a guided operation in a pilot-scale DHS system treating domestic wastewater for the low-outcome areas Not only does the current study evaluate treatment performance under different HLRs (5.56 and 11.12 m3 /m2 d), but also it covers initial assessment on energy consumption Materials and method 2.1 Pilot DHS configuration The schematic diagram of the pilot-scale DHS is shown in Fig The reactor was made from polymethyl methacrylate with dimensions of the cylindrical reactor are 2.5 m x 0.3 m x 0.3 m (height x width x length) with a total volume 225 L The reactor was divided into four small segments; three identical segments (0.7 m height) in the top-down direction used to accommodate the sponge media with air ventilation placed upward each segment The bottom unit (0.4 m height) contains the effluent A spraying distributor with the perforated disk was installed on the top to ensure the even distribution of raw wastewater to the media Sponge unit (5 cm height x 3.5 cm diameter) made from polyurethane with a specific surface area of 250 m2 /m3 , the porosity of 90%, the density of 30 kg/m3 The total volume of media was 50.4 L, approximately 26.7% volume of three segments Three hundred fifty sponges (cylindrical shape) were packed into each segment accounted for 0.5 m height which was randomly distributed, protection with a round net made from polypropylene plastic SRT was not controlled throughout the experimental period It depends on the natural worn out of the formed biofilm The sponge carriers were soaked with activated sludge taken from local WW treatment plants to shorten acclimatization time and enhance microbial consortia 2.2 Characteristics of wastewater and operating conditions The system was operated outdoors at ambient temperature (24–34 ◦ C) Feed wastewater was collected every day The wastewater was taken from a sewage drain Briefly, the influent parameters are given in Fig (n = 21) The total influent V.T Tra, B.T Dang, Q.A Binh et al Environmental Technology & Innovation 21 (2021) 101273 Fig Schematic diagram of pilot-scale down-flow hanging sponge reactor (DHS) legend, the reader is referred to the web version of this article.) (For interpretation of the references to color in this figure COD contained 116 ± 30 mg/L while soluble COD (sCOD) after filtered by membrane 0.45 µm in the range of 92 ± 26 mg/L; most of the nitrogen was in the form of ammonia of 35 ± 11 mgN/L, TP 5.6 ± 2.1 mg/L and pH at 8.2 ± 0.41 The change in HLRs was adjusted at 5.56 and 11.12 m3 /m2 day (Eq (1)) or 9.92 and 19.84 m3 /m3 Sponge d (Eq (2)) corresponding to the hydraulic retention times (HRT) of 5.83 h and 2.91 h, respectively The organic loading rate (OLR) was estimated at 1.2 kg COD/m3 sponge d (CODinf = 116 mg/L) (Eq (3)) Note that the change in HLR based on IR does not affect the OLR The parameters were calculated based on equations: HLR (m3 /m2 d) = Qinf + Qrecirculation (1) Cross-sectional area Qinf + Qrecirculation 3 or HLR (m /m sponge d) = VSponge (Q × CCOD inf ) inf OLR (kg COD/m3 sponge d) = VSponge (2) (3) Wastewater was collected and transported from the sewage drain to a feed tank with a volume of 500 L Here, there will be two centrifugal pumps working 12 h each alternately, and the wastewater was pumped into a flow meter to adjust the water flow at 20.8 and 41.6 L/h before feeding to the DHS, respectively The air was passively diffused via a spray distributor on top and eight windows located at different reactor levels Wastewater was recirculated at IR = (500 L/d) by a centrifugal pump from the bottom segment to the influent pipe (blue) of DHS through a recirculation line (green) V.T Tra, B.T Dang, Q.A Binh et al Environmental Technology & Innovation 21 (2021) 101273 Fig Characteristics of domestic wastewater during the operation period (Fig 1) Treated wastewater went through the settling tank volume of 180 L; there was a bottom discharge valve, while the upper clean water will automatically flow out of the system The water sample will be collected from the collection pipe, with a valve to collect samples for analysis 2.3 Sampling and analysis Analyses were adopted according to ‘‘Standard Method for Examination of Water and Wastewater’’ by American Public Health Association (APHA 1999) Physico-chemical parameters including COD (5220 C), TSS (2540 D), NH4 + -N (4500-NH3 B and C), NO2 − -N (4500-NO2 − B), NO3 − -N (4500-NO3 − B), and TP (4500-P D), and alkalinity (2320 B) were analyzed biweekly from the influent and the effluent of the reactor The COD was assumed as organic particulate (pCOD), and soluble matter (sCOD) The pCOD was calculated by the difference between total COD and sCOD A 0.45 µm glass fiber filter was used to assess TSS and separate component COD parameters simultaneously The removal efficiency of monitoring parameters was calculated as the following equations: Removal (%) = (Cinf − Ceff ) × 100% (4) Cinf The results were processed via the R platform to visualize data (R studio IDE, http://www.rstudio.com/) The Student’s t-tests were used to compare mean values of parameters and the null hypothesis as no significant difference existed between the two data sets A significance level (p-value) of α = 0.05 was used Results and discussion 3.1 TSS removal Table presents a comparison of the TSS characteristics obtained from HLR conditions, and its time course is shown in Fig 3A The influent TSS concentration was quite stable at HLRs of 5.56 and 11.12 m3 /m2 d (t.test, 38.4 ± 4.1 mg/L vs 34.1 ± 5.8 mg/L, p > 0.06), respectively This concentration was like domestic wastewater study from Thailand (Onodera et al., 2014b) but lower from Japan (Tawfik et al., 2006), Indonesia (Machdar et al., 2018a), Egypt (Mahmoud et al., 2011), and India (Onodera et al., 2016) that frequently observed in the range of 155–270 mg/L TSS removal is mainly dependent on the biological conversion of organic matter and the physical retention of suspended particles (Mahmoud et al., 2010) The removal efficiency achieved 69.7 ± 6.1% at HLRs of 5.56 m3 /m2 d, agreed with findings for TSS removal around (51%–64%) from the previous study (Onodera et al., 2014b; Okubo et al., 2015) It indicated that DHS performance approached stability conditions Interestingly, this study found that increasing HLR to 11.12 m3 /m2 d does not impact the TSS performance (75.6 ± 6.9%), although its HRT condition has been reduced from 5.83 to 2.91 h It was reported that shifted HRT from 11.7, 5.8 to 2.9 h or to h could trigger a decrease in TSS removal (Mahmoud et al., 2011; Tawfik et al., 2011) Because the higher hydraulic force or shortage HRT could affect the ability accumulation of organic matter through changing physical retention With this study, the contrast results could be due to the existence of the settling tank, which has been added after DHS unit (Fig 1) This tank itself has an HRT of 4.3 h, V.T Tra, B.T Dang, Q.A Binh et al Table Effect of HLR condition on the performance of DHS Parameters (mg/L, except pH) TSS COD pCOD sCOD NH+ -N TP pH NO− -N NO− -N Alkalinity (mgCaCO3 /L) HLR = 11.12 m3 /m2 d QCVN 14-MT: 2015/BTNMT GB 18918–2002 Grade 1-B Pop >100 k Non-coastal areas Influent Effluent Removal (%) Influent Effluent Removal (%) Vietnam Aa Chinab EUc Japand 38.4 ± 4.1 118.2 ± 37.5 49.0 ± 27.3 69.2 ± 20.2 44 ± 5.1 7.9 ± 1.2 7.9 ± 0.4 ND ND 367.3 ± 18.3 11.6 ± 2.2 47.6 ± 19.6 14.4 ± 10.5 33.2 ± 17.4 5.1 ± 0.4 ± 1.0 7.5 ± 0.2 0.2 ± 0.1 29.3 ± 9.2 139 ± 28.8 69.7 60.0 45.3 54.4 88.4 11.7 – – – 61.8 34.1 ± 5.8 114.0 ± 24.6 34.0 ± 21.3 80 ± 16.7 28.2 ± 8.9 4.5 ± 1.4 8.4 ± 0.2 ND ND 309.8 ± 58.8 8.3 ± 2.7 53.5 ± 22.9 23.3 ± 18.8 30.2 ± 7.3 5.1 ± 3.6 3.4 ± 0.9 7.6 ± 0.1 0.2 ± 0.1 23.3 ± 3.6 138.6 ± 25.2 75.6 ± 6.9 51.5 ± 20.4 6.5 ± 110.9 59.6 ± 16.0 80.7 ± 15.4 23.3 ± 5.8 – – – 54.3 ± 9.3 50 75 – – 10 6–9 – – – 20 60 – – 6–9 – – – 35 125 – – – – – – – 150 120 – – – 5.8–8.6 – – – ± ± ± ± ± ± 6.1 11.4 77.1 19.5 0.9 2.2 ± 8.9 Vietnam National technical regulation on domestic wastewater, QCVN 14-MT:2015/BTNMT Discharge standard of pollutants for municipal wastewater treatment plant (National Standard GB 18 918–2002) c Secondary Treatment Standards, Council Directive of 21 May 1991 Concerning Urban Waste Water Treatment (91/271/EEC) for population >100.000 (https://www.epa.gov/npdes/npdes-permit-writersmanual) d National Effluent Standards of Japan (http://www.env.go.jp/en/water/wq/nes.html, last updated: October 21, 2015 b Environmental Technology & Innovation 21 (2021) 101273 a HLR = 5.56 m3 /m2 d V.T Tra, B.T Dang, Q.A Binh et al Environmental Technology & Innovation 21 (2021) 101273 Fig Time course of COD removal of the system: (A) TSS; (B) influent COD fractionation of pCOD and sCOD and (C) the effluent of pCOD and sCOD given sufficient time for gravity settling Such addition is a safe and straightforward approach whether IR mode has been employed, a surprisingly low effluent concentration (4–16 mg/L) could be received The improved TSS removal efficiency V.T Tra, B.T Dang, Q.A Binh et al Environmental Technology & Innovation 21 (2021) 101273 at HLR 11.12 m3 /m2 d could overcome the effects of HLR and render effluent concentration below the standard limit (100 mg/L) 3.2 Organic removal The influent total COD is not a significant difference between HLRs (t.test, p > 0.75) retained at 118.2 ± 37.5 mg/L and 114.0 ± 24.6 mg/L (Table 1) At HLR of 5.56 m3 /m2 d, the average removal of total COD was 60.4 ± 11.4% These results are consistent with previous studies when COD removal in the range of 60%–70% in the case of applying OLR from 1–2 kg COD/m3 d and IR = (Onodera et al., 2014b; Yoochatchaval et al., 2014; Watari et al., 2017b) (Table 2) However, this fact was reduced by approximately 10% as operated with increasing HLR at 11.12 m3 /m2 d To better understand, COD removal has been further analyzed and visualized as based pCOD and sCOD (Fig 3B, C) Therein, there was a declined removal efficiency of pCOD (59.2 ± 15.6% vs 6.5 ± 110.9%) when IR mode being employed, raising pCOD concentration in the effluent (14.4 ± 10.5 vs 23.3 ± 18.8 mg/L) Meaning that high HLR seems to reduce performance in eliminating particulate COD, but it could not impact the removal of soluble COD (54.4 ± 19.5% and 59.6 ± 16.0%, p > 0.5) Generally, particulate COD is mainly obtained from the decomposition of particles and colloids underlying mostly slow or non-biodegradable organic form (Wichitsathian and Racho, 2010) Less biodegradation of particulate COD could be caused by the loss of large quantities of mixed culture due to the biofilm detachment under a high hydraulic loading rate The ecosystems with extremely long food web were reported to be responsible for the degradation of organic particles by heterotrophic bacteria (Uemura et al., 2010; Furukawa et al., 2016; Hatamoto et al., 2018) Heterotrophic phylotype such as Flavobacteriales, Saprospiraceae, Gordonia, Chloroflexi, and Cytophaga plays a major role in responsible for the degradation of particulate organic matters (Furukawa et al., 2016) and soluble microbial products (SMP) (Kim et al., 2016) The food web was possibly lost during the transition period of HLR 11.12 m3 /m2 d, thus reducing the removal of particulate COD (Fig 3C, Table 1) As reported, operating conditions (i.e., HRL, IR) remarkably affected the change in abundance of Alphaproteobacteria and Gammaproteobacteria groups (Watari et al., 2020) These bacterial groups played a vital role in the degradation of organic matters in the DHS reactor (Watari et al., 2020) The growth of heterotrophic bacteria, which can metabolize complex molecules, is dependent on carbonaceous loading in the feed The reduction in feeding carbonaceous material at the transition period might reduce heterotrophic bacteria development Evidently, after day 63, the removal of total COD was gradually improved from 37.5% to 78% (day 75), corresponding to the increase of pCOD removal efficiency from 28.5 to 83% In this study, the system has quickly regulated patterns adapting to double the increase in HLR condition (11.12 m3 /m2 d) The past works have proposed the moving bed biofilm reactor coupled with multimedia filters for domestic wastewater treatment (Liao et al., 2003; Loupasaki and Diamadopoulos, 2013) Their findings indicated that the SS and total COD removal efficiency achieved 71.7–81.5% and 72%–75% respectively In this study, our DHS system performed a relatively similar treatment efficiency at an HRL of 11.12 m3 /m2 d Operating at higher HRL facilitates to shorten of a HRT to 2.9 h and thus results in reducing the footprint (i.e., decreasing capital cost) 3.3 Nitrogen removal Fig shows the changes in species of nitrogen The predictive area that included a shaded zone covers a mean line using statistics transformation (95% confidence interval) The average feed ammonia nitrogen concentration was naturally affected by raw influent that shifted from 44 ± 5.1 to 28.2 ± 8.9 mg-N/L corresponding with HLR 5.56 and 11.12 m3 /m2 d (Table 1) Although feed ammonia nitrogen tends to decrease by 36%, removal through DHS is stable The effluent was 5.1 ± 0.4 mg-N/L and 5.1 ± 3.6 mg-N/L, respectively Alkalinity consumption was 54%–62% of total natural alkalinity in feed resulting in nitrification achieved 88.4 ± 0.9% and 80.7 ± 15.4%, respectively The decreased feed concentration provoked a slight reduction in nitrification (7.7%) However, the activity of nitrification appears to be less affected by HLR The concentration of nitrate production gradually reached the level of the total feed nitrogen (day 55) Thus, the nitrification process could be preserved at HLR 11.12 m3 /m2 d (Fig 4) It was found that nitrifiers seek to develop within the interior space of porous structure instead of the surface biofilm (Araki et al., 1999) Thus, increased HLR can impact the biofilm on the surface of the Sponge, leading to a decrease in COD removal as mentioned earlier, but it is not an adverse event for nitrification The availability of organic matter is considered an essential factor for denitrification activities A past study has found that shift IR from 1.5 to resulted in decreasing denitrification efficiency from 58.6 ± 6.2% to 50.9 ± 9.3% as the feed COD of 1440–2800 mg/L was employed in DHS system (Ikeda et al., 2013) Their findings suggested that IR < 1.5 is a critical value for enhanced nitrogen removal In this study, the nitrate produced could not be sufficiently taken up by denitrifiers (29.3 ± 9.2 vs 23.3 ± 3.6 mg-N/L, p > 0.05) The low COD/N feed ratio (2.6–4.0) apparently interfered with denitrification performance regardless of whether IR = has been carried out This finding requested an adequately designed DHS reactor because IR mode itself possibly raised DO up to mg/L contributing to the detriment anoxic zone (Ikeda et al., 2013; Bundy et al., 2017) Bundy et al demonstrated that when reactors were equipped with additional influent bypass lines to feed raw wastewater to the anoxic layer, it could improve five-fold nitrogen removal The bypass method could decrease the DO in the anoxic zone for better removal, which likely feasible for the current situation V.T Tra, B.T Dang, Q.A Binh et al Table Current status of DHS application from laboratory scale to full scale No of reactor Wastewater IR HRT (h) HLR (m3 /m2 d) OLR (kg COD/m3 d) COD inf (mg/L) TSS removal (%) COD removal (%) NH+ -N removal (%) TP removal (%) Energy (kWh/m3 ) References Pilot Domestic 1.2 409 ± 59 94 ± 89 ± 99 ± 2.5 43 ± 14 – Pilot Domestic – 1.34 67 ± 18 97 67 98.6 – 0.074 Pilot Pilot Pilot 1 Domestic Agricultural Septic tank 0 – 3.57 – 0.6 3.6 147 ± 66 249 ± 100 449 ± 122 93 ± 91 ± 18 ± 11 85 ± 73 ± 10 18 ± 15 99 ± 85 ± 17 36 ± 25 – – – 0.06 – – Pilot Domestic – 2.03 169 ± 80a 51 ± 33 68 ± 17 82 ± 13 – – Lab Lab Pilot 1 Synthetic Rubber Domestic 1.5 0 12 4.8 – – 12 – 0.97 ± 0.03 1.84 2880 280 ± 100 b 536 ± 162c – 75 ± 22 96 ± > 90 64.2 ± 7.5 90 ± – 60 83 ± – – – – – – Full Lab Pilot Pilot Pilot 1 1 Domestic Domestic Domestic Domestic Domestic 1 1 1.5 28.8 1.33 1.33 2.9 42.2 – – – 11.12 2.84 0.2 – – 1.2 177 173 338 338 114 64 – 94 ± 94 ± 76 ± 79 72.0 89 ± 89 ± 52 ± 20 – 91.9 53 ± 19 70 ± 21 81 ± 15 – – – – 23 ± 0.12 – – – 0.068 Mahmoud et al (2010) Yoochatchaval et al (2014) Miyaoka et al (2017) Fleifle et al (2014) Machdar et al (2018a) Onodera et al (2014a) Ikeda et al (2013) Watari et al (2017a) Mahmoud et al (2011) Okubo et al (2015) Bundy et al (2017) Okubo et al (2016) Okubo et al (2016) This study Remark: a COD influent b COD influent c COD influent d COD influent e COD influent values values values values values are are are are are from from from from from the the the the the effluent effluent effluent effluent effluent of of of of of UASB process the anaerobic UASB process UASB process UASB process in which DHS baffled tank in which DHS in which DHS in which DHS ± ± ± ± ± 44 d 50 31e 31e 25 is used as a post treatment unit is used as a post treatment unit is used as a post treatment unit is used as a post treatment unit Environmental Technology & Innovation 21 (2021) 101273 Scale V.T Tra, B.T Dang, Q.A Binh et al Environmental Technology & Innovation 21 (2021) 101273 Fig Time course concentrations of nitrogen species 3.4 Total phosphorous removal The stability of TP removal reached 11.7 ± 2.2% and 23.3 ± 5.8% at HLR 5.56 and 11.12 m3 /m2 d, respectively The higher removal obtained at HLR 11.12 m3 /m2 d is mainly due to a decrement 1.6 times the concentration of TP in feed (from 7.9 to mg/L, p < 0.05) (Table 1) Another study showed that higher TP removal could be achieved 35% when feed TP is considered low of mg/L and HRT of h (Mahmoud et al., 2010) Generally, phosphorous removal is mainly responsible for activities of phosphorus-accumulating organisms (PAO) via aerobic phosphate uptake and denitrifying PAO capable of utilizing nitrate to take up phosphate under anoxic conditions (Kerrn-Jespersen and Henze, 1993; Bortone et al., 1996; Bunce et al., 2018) In DHS, Sponge could provide both aerobic, anoxic, anaerobic conditions to bacteria that contribute to improving phosphorus removal However, because there is no appropriate method for controlling SRT in the DHS process; therefore, high removal cannot be expected via sludge disposal The important thing is that TP in feed is frequently low, so the remaining TP in the effluent is mostly satisfied with the national standard 3.5 Potential application DHS for domestic wastewater treatment It was reported that high electrical consumption for municipal wastewater treatment reached 30.2 billion kWh (0.8% of total electrical loading) in the United States, wherein aeration was excess of 50% (Electric Power Research Institute, 2013) The normalized energy required was 0.41–0.87 kWh/m3 for U.S, while it could be found in the range of 0.3–2.1 kWh/m3 in EU (Gandiglio et al., 2017; Capodaglio and Olsson, 2020) However, providing a good quality of wastewater treatment based on conventional treatment is a significant challenge for universalizing its application to a low-income area since the social, economic, and political dynamics are not yet warranted Thus, energy saving is a crucial requirement for establishing wastewater treatment stations, particularly emphasized in developing countries For this investigation, the specific energy consumption (SEC) is calculated based on a pump head (10 m), the feed flowrate, the circulating flowrate, the effluent flow rate, and 80% pump efficiency Our study indicated the SEC was approximately 0.068 kWh/m3 for operating the DHS system covering a feed pump and a circulation pump (Table 2) The DHS system showed a relatively same SEC value compared to the previous studies using two reactors in series: 0.074 kWh/m3 (Yoochatchaval et al., 2014) and 0.06 kWh/ m3 (Miyaoka et al., 2017) It is noted that their work showed a slight nitrate removal although a serial configuration of the DHS system was employed only However, it is clearly that using recirculation rather in-series configuration could expected to improve nitrate nitrogen elimination thank to recirculated nitrate uptake by denitrifiers (Ikeda et al., 2013) Thus, if the feed wastewater contains sufficient COD by seasonal variations, denitrification could be enhanced under the recirculation mode Our findings indicated a pivotal role of recirculating flow in enhancing nitrate nitrogen removal Evaluating the DHS process (Table 2), the DHS system mostly satisfied an operation with OLR from 0.5–3.6 kg COD/m3 d, HRT from 1.3–6 h that are sufficient for organics and ammonia removal in domestic wastewater It was also able to operate with HLR of ≥11 m3 /m2 d when operated without IR mode (Mahmoud et al., 2011) and IR mode (this study) However, too high HLR (42.2 m3 /m2 d) might affect TSS removal, and that fact should be considered (Okubo et al., 2015) Since the treated wastewater has well complied with the national discharge standard with the appropriate operating conditions (QCVN 14-MT:2015/BTNMT, class A) Therefore, DHS process is feasible for domestic wastewater treatment in V.T Tra, B.T Dang, Q.A Binh et al Environmental Technology & Innovation 21 (2021) 101273 HCMC and/or other developing cities It should also be pointed out that the treated effluent should be quality enhanced in order to approach stricter regulations based on international standards (Table 1) To enhance the organic and nitrogen removal, there were several findings in favor of using multiple segments or reactors in series (Onodera et al., 2014a), ideal sponge pore size (Machdar et al., 2018b), upgrading the shape, distribution of the carriers (Chuang et al., 2007; Aoki et al., 2018; Hatamoto et al., 2018) Regarding this study, although the high hydraulic conditions did not affect the TSS removal, it slightly impacted COD removal during the transition period Therefore, to avoid this problem, several configuration refinement solutions can be accessed, such as increasing the number of trays in the perforated tray units (Roy et al., 2020), select the type of media distribution inside the reactor, control the appropriate HLR value along with bypass method Because of such approaches, the DHS demands less investment cost or modification while it might provide a way to optimize DO level and decrease the HLR effect Therefore, the upgraded configuration is the potential for direct domestic wastewater treatment and those revised parameters need to be more fully identified in future studies Conclusions This work examined the influence of HLR on the performance of the pilot DHS treating domestic wastewater The DHS system could maintain stable performance despite increasing 2-fold HLR 78% COD removal was achieved successfully while the nitrification efficiency was up 88.4 ± 0.9% under the steady stage A low COD/N ratio (2.6–4.0) of domestic wastewater hampered the denitrification process that occurred inside sponge carriers Overall, an HLR of 11.12 was proposed for a guided operation to comply with the Vietnam national discharge standard This proposed condition consumed a low specific energy consumption of 0.068 kWh/m3 , which brings a beneficial energy cost due to using natural ventilation CRediT authorship contribution statement Van-Tung Tra: Funding, Supervision, Writing - review & editing Bao-Trong Dang: Investigation, Software, Writing original draft Quach An Binh: Investigation, Software, Writing - original draft Quy-Hao Nguyen: Data curation, Conceptualization, Methodology Phuong-Thao Nguyen: Data curation, Conceptualization, Methodology Hong-Hai Nguyen: Data curation, Conceptualization, Methodology Thanh-Tin Nguyen: Investigation, Software, Writing - orginal draft Thanh-Hai Le: Funding, Supervision, Writing - review & editing Duc-Trung Le: Funding, Supervision, Writing - review & editing Tomoaki Itayama: Supervision, Writing - review & editing Xuan-Thanh Bui: Funding, Supervision, Writing - review & editing, Editing, Revise the final MS Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgments This research is funded by Vietnam National University Ho Chi Minh City (VNU-HCM) under grant number C2020-2402 We acknowledge the support of time and facilities from Ho Chi Minh City University of Technology, VNU-HCM for this study This study has been conducted under the framework of CARE-RESCIF initiative References American Public Health Association (APHA), 1999 Standard methods for the examination of water and wastewater, twenty first ed American Public Health Association, Washington DC Aoki, M., Noma, T., Yonemitsu, H., Araki, N., Yamaguchi, T., Hayashi, K., 2018 A low-tech bioreactor system for the enrichment and production of ureolytic microbes Polish J Microbiol 67, 59–65 Araki, N., Ohashi, A., Machdar, I., Harada, H., 1999 Behaviors of nitrifiers in a novel biofilm reactor employing hanging sponge-cubes as attachment site Water Sci Technol 39, 23–31 Babut, M., Mourier, B., Desmet, M., Simonnet-Laprade, C., Labadie, P., Budzinski, H., De Alencastro, L.F., Tu, T.A., Strady, E., Gratiot, N., 2019 Where has the pollution gone? 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T., Onodera, T., Uemura, S., Yamaguchi, T., Ohashi, A. , Harada, H., 2015 On- site evaluation of the performance of a full -scale down- flow hanging sponge reactor as a post-treatment process of an... Nagano, A. , Shimozaki, S., Sumino, H., Araki, N., Yamazaki, S., Kawakami, S., Harada, H., 2011 Performance of a pilot- scale sewage treatment: An up -flow anaerobic sludge blanket (UASB) and a down- flow. .. and profile analysis of a down- flow hanging sponge system treating agricultural drainage water Sep Purif Technol 116, 87–94 Furukawa, A. , Matsuura, N., Mori, M., Kawamata, M., Kusaka, J., Hatamoto,