Chau, Henry K. M.

Choi, David K. W.

Lee, Joseph H. W.

Qiao, Q. S.

English

Hydraulic Engineering Repository

Article, Published VersionLee, Joseph H. W.; Qiao, Q. S.; Choi, David K. W.; Chau, Henry K. M.Fluid mechanics for carbon reduction in wastewatertreatment plantsHydrolinkVerfügbar unter/Available at: https://hdl.handle.net/20.500.11970/110522Vorgeschlagene Zitierweise/Suggested citation:Lee, Joseph H. W.; Qiao, Q. S.; Choi, David K. W.; Chau, Henry K. M. (2022): Fluidmechanics for carbon reduction in wastewater treatment plants. In: Hydrolink 2022/3. Madrid:International Association for Hydro-Environment Engineering and Research (IAHR). S.84-87. https://www.iahr.org/library/hydrolink?hid=472.Standardnutzungsbedingungen/Terms of Use:Die Dokumente in HENRY stehen unter der Creative Commons Lizenz CC BY 4.0, sofern keine abweichendenNutzungsbedingungen getroffen wurden. Damit ist sowohl die kommerzielle Nutzung als auch das Teilen, dieWeiterbearbeitung und Speicherung erlaubt. Das Verwenden und das Bearbeiten stehen unter der Bedingung derNamensnennung. Im Einzelfall kann eine restriktivere Lizenz gelten; dann gelten abweichend von den obigenNutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte.Documents in HENRY are made available under the Creative Commons License CC BY 4.0, if no other license isapplicable. Under CC BY 4.0 commercial use and sharing, remixing, transforming, and building upon the materialof the work is permitted. In some cases a different, more restrictive license may apply; if applicable the terms ofthe restrictive license will be binding.84  |  #HydrolinkMagazine IAHR.orgIN DEPTH >  CLIMATE CHANGE3 | 2022Fluid mechanics for carbon reductionin wastewater treatment plantsBy Joseph H W Lee, Q S Qiao, David K W Choi and Henry K M ChauIntroductionTurbulent mixing is at the core of hydrodynamic transport cal-culations required to predict the impact of man-made processeson the environment. Two- and three-dimensional (2D and 3D)hydrodynamic and water quality models are routinely used indrainage and environmental impact assessment (EIA) studiesfor many urban and coastal developments. Over the past twodecades, the water and environmental industry has benefittedgreatly from the progress in turbulence modeling and the deve-lopment of powerful numerical methods for solving the Navier-Stokes equations or their variants. Coupled with rapidly increa-sing computing power, affordable and user-friendly CFD modelshave played an important role in enhancing the sustainabilityof the environment.While CFD models have been extensively used in EIAstudies (e.g., setting of water quality standards), emergencyresponse (accidental pollution spills), and detailed engineeringdesign of infrastructure works, their use has mainly been limitedto predictions in the receiving water. On the other hand, thereis great potential in the use of fluid mechanics in reducing car-bon emissions and improving operations in many of the energy-and resource-intensive urban water and wastewater treatmentplants – beyond just showing beautiful colorful graphics ofcomplex flow simulations from CFD models. The use of CFDto substantially enhance cost effectiveness for waste-watertreatment plants (WWTP) has increasingly been recognized 1.Mixing is an important aspect in any reactor processinside a WWTP. However, when modeling for design or operation,simplified plug flow or complete mix stirred reactors are oftenassumed with the chemical kinetics based on bench scale jartests. The simplifications may lead to design flaws resultingin sub-optimal performance of WWTP. This article presents anoverview of a case study of optimization of sewage disinfectionin the Hong Kong Harbour Area Treatment Scheme (HATS) -the largest Chemically Enhanced Primary Treatment (CEPT)Fluid mechanics can play an important role in carbon reduction in waste-water treatment plants operations.Full scale in-plant experiments in the Hong Kong Harbour Area Treatment Scheme (serving populationof 5.7 million) demonstrated that significant savings in chlorine dosage in CEPT sewage disinfection canbe achieved by optimal jet diffuser design and placement. At nominal applied chlorine dosages of 12mg/L(20 mg/L) in winter (summer) respectively, the chlorine demand can be reduced by up to 30% as comparedto the conventional dosing design. As large quantities of chlorine (300 tonnes of 10% sodium hypochloritesolution) are used every day, the reduction in chlorine dosage translates to a saving in 1.6 million kWh/yearassociated with chlorine production, and around 1200 tonnes per year in CO2 emission reduction. The rela-tively simple dosing reconfiguration results in significant savings in chlorine and energy, reduces harmfulimpact to the environment, and improves plant operation reliability-thus enhancing the environmentalsustainability of the city as a whole.plant in the world. Through understanding the interaction bet-ween jet mixing and chlorination kinetics, significant savingsof chemicals and energy (up to 30%) can be achieved by opti-mal design and placement of the chlorine dosing.Optimisation of sewage disinfection using densechlorine jetsThe Hong Kong Harbour Area Treatment SchemeChlorine is commonly used in disinfection processes in waste-water treatment plants. The chlorine solution is typically dosedby turbulent mixing; the chlorine reacts with the inorganic andorganic nitrogenous compounds at a fast rate (in the order of0.1 second). A significant portion of the dosed chlorine can berapidly consumed by these reactions instead of destroying thepathogens - a waste of chemicals and energy -. Despite decadesof use of chlorine for sewage disinfection, the interaction ofturbulent mixing with chlorine consumption in the complexsewage environment remains an elusive problem.The Hong Kong Harbour Area Treatment Scheme (HATS)serves a population of 5.7 million on both sides of the VictoriaHarbour (https://www.dsd.gov.hk/others/HATS2A/en/). Sewageis collected from a number of nodes through a deep tunneledsystem to the Stonecutters Island Sewage Treatment Works(SCISTW) for centralized chemically enhanced primary treatment(CEPT) (Figure 1). In order to protect nearby beaches, a concen-trated 10% sodium hypochlorite solution (specific gravity of1.2) is used to disinfect the CEPT effluent. The dense chlorinesolution is injected into the sewage flow through multiport jetdiffusers in a Flow Distribution Chamber (FDC) at the endof the sedimentation tanks (Figure 2). The chlorinated sewageflows over a weir in the FDC and exits into an undergroundtunnel to allow for sufficient contact time. The total residualchlorine (TRC) concentration is monitored both at the end ofthe FDC and at a downstream chamber (Chamber 15A) beforedischarge into the submarine outfall (Figure 3).Since commissioning of disinfection in HATS in 2010, it wasfound that the disinfection dosage based on bench scale testsfailed to work in practice as designed. Operation experiencerevealed mysteriously large fluctuations of chlorine consumptionand hence variable bacterial quality of the HATS effluent intocoastal waters2, 3, 4. It should be noted that excess chlorine inthe SCISTW effluent is also harmful to the environment.Mixing chlorine with a river of sewageAs good mixing is a pre-requisite for acceptable disinfectionperformance, how to mix the concentrated chlorine dosing flow(6 L/s) with a large flow of sewage (1.8 million m3/d, or 20.8 m3/s)is a most challenging engineering problem. The mixing of sewagewith chlorine (and hence the chlorine demand) in a small beakerin the laboratory is very different from the mixing of a chlorinejet in a sewage flow inside the WWTP (Figure 4). For example,if a full mixed TRC concentration of 10 mg/l is desired; in thelaboratory we can mix 0.1ml of 10% sodium hypochlorite solution(100,000 mg/l) with 1l of water almost instantaneously. In thebeaker we can then study the chlorine demand (or kinetics)for this sample of known concentration. On the other hand,in a real chlorination setting, different sewage parcels are‘sucked’ into different parts of the chlorine jets by turbulententrainment – the overall chlorine consumption is the compositeof the chlorine demand of the different sewage parcels at diffe-rent TRC concentrations. It turns out the optimal jet diffuserdesign and placement can make a big difference to the chlorineand energy consumption. On the other hand, the interactionof the turbulent mixing with the complex chlorination kineticsis a most challenging problem.IAHR.org #HydrolinkMagazine  |  85IN DEPTH >  CLIMATE CHANGE 3 | 2022Figure 1 | The Hong Kong Harbour Area Treatment Scheme (HATS) and the nearbybathing beaches in Tsuen Wan.Figure 2 | Chlorine disinfection facilities in the Stonecutters Island SewageTreatment Works (SCISTW) of HATS (top view).Figure 3 | Longitudinal view of chlorine disinfection facilities in HATS (not to scale). Figure 4 | Beaker test vs field dosing of chlorine in SCISTW.Mixing and chemical reaction of chlorine jetsin treated sawageThe importance of mixing in chlorine disinfection has longbeen recognised5, 6; there has been a general recognition ofdosing in points of high turbulence (e.g. hydraulic jumps).However, probably due to the small flows and relatively lowchlorine concentrations no comprehensive field experimentsinvolving the coupling of fluid mechanics and chlorinationkinetics has been attempted.The mixing and chlorine consumption of a chlorine jetdischarging into primary treated effluent are studied for thefirst time at field scale in a 1:2 scale sectional physical modellocated inside the sewage treatment plant, using CEPT treatedsewage and chlorine solution obtained on site (Lee et al 2017).The TRC concentration distribution is measured at differentcross sections of the chlorine mixing chamber. It is confirmedthat at a fully-mixed TRC concentration of 10 to 20 mg/l,approximately 70-80 percent of the chlorine mass flux isconsumed within a very short distance (0.5-1 m, or a matterof several seconds) from the chlorine dosing unit (Figure 5a;first point dosing).The field experiments in the unique physical model laidthe foundation of several theoretical, CFD and laboratory studiesthat unveiled the role of organics and ammonia nitrogen in thechlorination kinetics3, 7. Building on the theory of buoyant jetsin a turbulent crossflow 8, the effect of chlorination kineticsfrom the turbulent mixing can be isolated. Due to breakpointchlorination and oxidation of organics the chlorine consumptiondepends on the source chlorine concentration in a highly non-Professor Joseph Hun-wei LeePresident and Chair Professor of the Macau Universityof Science and Technology. He is an Internationallyrecognized expert in hydraulics with a passion toapply fluid mechanics to solve complex water envi-ronment problems.He is the master mind of the Hong Kong WATERMANcoastal management and forecast system.Dr Q S QiaoPostdoctoral Fellow at the Department of Civil andEnvironmental Engineering of the Hong Kong Universityof Science and Technology. He obtained his doctoraldegree from the China Institute for Water Resourcesand Hydropower Research (IWHR).Member of the Hong Kong WATERMAN project teamand key investigator.Dr David K W ChoiResearch Associate in the Department of Civil andEnvironmental Engineering at the Hong Kong Universityof Science and Technology and is specialist in hydraulicand water quality modelling. He has extensive expe-rience in applied research involving the use of numericalmodels for a range of environmental problems.Ir Henry K M ChauFormer Assistant Director of the Hong Kong DrainageServices Department. He has worked on a wide varietyof drainage and sewerage projects for Hong Kongand was responsible for the design, operation andmaintenance of the Hong Kong Harbour Area Treat-ment Scheme.linear manner: the higher the chlorine concentration, the greaterthe consumption. Hence it is of utmost importance to minimizethe time of exposure of sewage to high TRC concentrations.Various alternatives to optimize disinfection dosage have beeninvestigated, including: (I) use of lower source chlorine concentra-tion; or (II) reduce the TRC concentration as quickly as possible.The optimal solution is to discharge the chlorine jet at a positionof maximum sewage flow velocity (minimize the reaction time)which can result in up to 30% savings in chlorine dosage.In the optimal design, the chlorine jets are placed at a positionof maximum velocity at the weir (Figure 5b). Figure 6 showsthe chlorine jet diffuser tested in open air before installationat the SCISTW plant. The simple and effective design has beenproven in actual operation in both the winter and summer sea-sons. For example, in the summer season, the chlorine dosagerequired for disinfection can be reduced from 98% to 70%.Concluding remarksFluid mechanics can play an important role in carbon reductionin WWTP operations. Full scale in-plant experiments demons-trated that significant savings (up to 30 percent) in chlorinedosage in CEPT sewage disinfection can be achieved by optimaljet diffuser design and placement10. The reduction of chlorinedisinfection dosage has significant implications on carbonneutrality and sustainability. First, the daily usage of the 10percent sodium hypochlorite solution is around 300 m3.86  |  #HydrolinkMagazine IAHR.orgIN DEPTH >  CLIMATE CHANGE3 | 2022Figure 5 | Section view of hydraulic flow conditions at first and second dosingposition.A | Firts point dosing B | Second point dosingFigure 6 | Testing of chlorine dosing unit for the second (optimal) dosing positionin the SCISTW.On the other hand, the power consumption for productionof sodium hypochlorite is around 500 kWh/m3. For 30 percentchemical saving, the corresponding saving in power consumptioncan be estimated to be around 1.6 million kWh/year.The annual savings in life-cycle power consumption is compa-rable to the total renewable energy produced by photovoltaic(PV) solar panels operated by the Drainage Services Depart-ment. In terms of reduction of carbon footprint (assuming1 kWh = 0.71kg CO2-emission), the saving in CO2 emission canbe estimated to be 1170 tonnes/year. Furthermore, as thesupplier of the sodium hypochlorite solution is located 90 kmaway in Guangdong Province, the carbon footprint of the deli-very trucks for such long haul would also be reduced significantly- not to mention the reliability of supply for WWTP operationduring chronic or emergency crisis situations. The understandingof the causes of chlorine demand and dosing control greatlyenhances the certainty and robustness of the sewage treatmentplant operation - thus enhancing the environmental sustainabilityof the world city as a whole.In general, the savings are particularly notable in primaryeffluents with organic solids. In recent years there has beena trend to employ CEPT to enhance carbon redirection forrecovery and substantially reduce the organic load to secondarytreatment processes9.References1 | Nopens, I. and Rehman, U. (2018). “Mixing: new insights and opportunities through computational fluid dynamics, in Advances in Wastewater Treatment,G. Mannina et al. (Eds.), IWA Publishing, pp. 321-339.2 | Lee, J.H.W., Qiao, Q.S., Chan, S.N., Choi, K.W. and Huang, J.C. (2017). “Field experiments of chlorine demand in disinfection of treated primary effluent”,Journal of Environmental Engineering, ASCE, 143, 04017073.3 | Chan, S. N., Qiao, Q. S., Lee, J. H. W., Choi, K. W., and Huang, J. C. (2017). “Modeling of mixing and rapid chlorine demand in sewage disinfection with densechlorine jets”, Journal of Environmental Engineering, ASCE, 143, No. 11, 04017074.4 | Li,W.X., Zhang, X.R., Li,X.Y and Lee, J.H.W. (2018). “Mystery of the high chlorine consumption in disinfecting a chemically enhanced primary saline sewage”,Water Research, 145, 181-189.5 | Longley, K.E. (1978). “Turbulence factors in chlorine disinfection of wastewater”, Water Research, 12, 813-822.6 | Stover, E.L., Hass, C.N., Rakness, K.L., and Scheible, O.K. (1986). Design Manual - Municipal Wastewater Disinfection. U.S. Environmental Protection Agency.7 | Yang, A.J.K., Chan, S.N. and Lee, J.H.W. (2019). “Integral model of a reacting chlorine jet in ammonia nitrogen and treated primary effluent”, Journal ofHydro-environment Research, Vol. 27, 50-64.8 | Lee, J.H.W. and Chu, V.H. 2003. Turbulent Jets and Plumes - A Lagrangian Approach. Springer/Kulwer Academic Publishers.9 | Shewa, W.A. and Dagnew, M. (2020). “Revisiting Chemically Enhanced Primary Treatment (CEPT) of wastewater: a review”, Sustainability, 12, 5928(doi:10.3390/su12155928).10 | Lee, J.H.W., Qiao, Q.S., Chan, S.N., Choi, K.W., and Chau, H.K.M. (2022). “Reduction of chlorine disinfection dosage through optimal jet design in the HongKong Harbour Area Treatment Scheme”, Journal of Environmental Engineering, ASCE (accepted for publication).IN DEPTH >  CLIMATE CHANGE 3 | 2022AcknowledgementThe research reported herein has been supported at various times by the Hong Kong Research Grants Council (RGC) andthe Drainage Services Department (DSD) of the Hong Kong SAR Government.

Fluid mechanics for carbon reduction in wastewater treatment plants

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Fluid mechanics for carbon reduction in wastewater treatment plants

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