ECOLOGIA BALKANICA2019, Special Edition 2 pp. 207-215

Synopsis

Modelling of Water Systems in a Convenient Way

Manfred Schütze*

ifak - Institut für Automation und Kommunikation e.V. Magdeburg,Department of Water and Energy, Werner-Heisenberg-Str. 1, 39106 Magdeburg, GERMANY

*Corresponding author: manfred.schuetze@ifak.eu

Abstract. Protection of water resources requires many decisions, which, in turn, can be aided by a modelof the respective water system. Such a model then can be used, for example, for the analysis of variousscenarios (of external forces) and various action options (which can be influenced). On the other hand,modelling often is perceived as something hard or laborious to do (and often, it is). This paper presentssome examples of modelling concepts and of their implementation in an easy-to-use simulator, focusing onthe combination of dynamic process models with water-balance models, assisting in the management ofwater resources. The contribution presents examples from various countries.

Key words: biochemical processes, modelling, river water quality, Water-Energy-Food Nexus,water systems, wastewater treatment plant.

Introduction Management of water resources and

water systems becomes ever more important.This holds true in particular in times of climatechange and increased pollution of waterbodies. Among the core tools supportingsustainable water management, mathematicalmodels have emerged and are used for a widerange of tasks (BACH et al., 2014). Models areconsidered as simplified representations ofreality, often in a simplified way, aiming atassisting in certain tasks. Such tasks include notonly design and operation of water systems,but also the visualisation (also to the non-expert) of relevant processes and interactions –thus models can play an important role instakeholder participation and decision makingprocesses. Furthermore models are useful foreducation and system understanding.

On the other hand, also a certain reluctanceabout the use and application of models can be

observed. Notwithstanding the fact thatmodelling indeed involves a number ofchallenges to be addressed in any givenmodelling task, this paper aims to motivate theapplication of modelling in water management,illustrating the use of models for a number ofexamples. Model application is also greatlyenhanced when a user-friendly and easy-to-usesimulation system can be applied. The presentpaper illustrates modelling applications usingthe simulation framework Simba#, whichemerged from 25 years’ experience fromresearch and application projects and now formsone of the key simulation systems in watermanagement applied in research and practice allover the world.

Why modelling water systems?When buying a car, it is generally accepted

to be prudent to do a test drive before taking thefinal purchase decision. Such a test drive can

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Modelling of Water Systems in a convenient way

assist to identify strong and weak characteristicsof the car and, together with a large number ofcriteria usually considered (e.g. purchase price,operational costs, demand on petrol, space,colour, …), forms an important base for thedecision of whether to buy the car or not. Whendecisions on water systems, for example,construction or extensions of wastewatertreatment plants are to be made – wouldn’t italso be desirable to do a “test drive” first. Buthow to do that, before the plant has been built?Here, modelling can assist – models, even if notcalibrated and validated against data, canprovide an estimate on the behaviour of theplant to be built. Design and operational optionscan be tested using the model, often leading to abetter design of the plant and/or its betteroperation, resulting in considerable cost savingsor reduction of energy consumption.

Obstacles to modelling of water systemsWhilst dynamic modelling of the physical,

biological and chemical processes in wastewatertreatment is quite widespread, using, forexample the Activated Sludge Models of theInternational Water Association (HENZE et al.,2000) and their implementation in variouscomputer simulation programs, and, whilst alsoriver water quality modelling is advocated,using, for example, the River Water QualityModel of the International Water Association(SHANAHAN et al., 2001), or less complexmodels, such as those of the QUAL2 family,there is also some reluctance in uptake and useof modelling when addressing practical issuesof water management.

Some obstacles to application of modellingare based on the perception that always adetailed calibration-validation process isnecessary prior to any model application. Formany applications this is true, and any steptaken to increase confidence that the model isable to represent reality to a sufficiently accuratedegree for the study under question isappreciated; however, for many applicationsmodelling approaches these days have beenestablished to such a degree that, at least fortypical conditions (e.g. wastewater treatmentmodelling for typical domestic wastewater and

common plant layouts), models can well beapplied using default parameters. For example,DZUBUR et al. (2019) discuss a methodology forestimating wastewater treatment plant influentcharacteristics under data-scarce conditions.

Another obstacle to practical application ofmodels is seen in the lack of user-friendliness ofsimulation software. This indeed can be quite alimiting constraint, as in daily practice – such asoften in engineering consultancies and waterauthorities – not everyone is a modellingspecialist as many other tasks are also to becarried out in daily routine. Based on thisexperience and on over two decades experiencein cooperation with consultancies and waterauthorities and companies, the dynamicsimulation environment Simba# has emerged,which meets the needs of practice – easy-to-useand quick-to-comprehend simulator, but alsoprovides sufficient flexibility to the researcherand experienced modeller (see, for example, thefeature to modify the pre-defined biochemicalprocess models, such as those by HENZE et al.,2000, or to define and solve the user’s own setsof differential equations for biochemicaltransformations in a user-friendly way) (ALEX etal., 2013, IFAK, 2018). The present paperillustrates some examples of modellingapplications of the Simba# simulator and itsextensions for different areas of watermanagement.

Dynamic modelling of wastewatertreatment plants

As an example, Fig. 1 illustrates a simpledynamic model for wastewater treatmentplants, which also can be used to model (andto optimise) operation of the plant. Thevarious components of the model (clarificationand aeration tanks) simulate the physical andbiochemical transformation processes, using,in this example, the Activated Sludge ModelNo. 3 (HENZE et al., 2000).

Assessing of impacts of urban dischargeson river water quality

River water quality in urban areas is oftenaffected by discharges from sewer systems (e.g.combined sewer overflows), from wastewater

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treatment plants (treated effluents) andindustrial discharges. These discharges are ofquite different nature (e.g. treatment planteffluents have usually lower contents ofreadily biodegradable organic matter (SSfraction of Chemical Oxygen Demand) thancombined sewer overflows) and their impactsare overlapping – thus making simple mixingcalculations inappropriate. In order to assessthe impacts of such discharges on river waterquality – after cooperation projects betweenbiologists and engineers - criteria related toduration and frequency of exposure to criticalconcentrations of Dissolved Oxygen andammonium/ammonia (with their balanceaffected by temperature and pH of the waterbody) have been established in the BritishUrban Pollution Management Manual (FWR,1998) and, in a similar manner, in the BWK-

M3/M7 guidelines in Germany (BWK, 2004).Driven by the request of the Ministry of theEnvironment of the German Federal State ofHesse to develop an easy-to-apply rivermodel, the Simple Water Quality Model(SWQM), considering the main processesrelated to Dissolved Oxygen and ammoniacalnitrogen (reaeration, decay, nitrification, …)has been developed (SCHÜTZE et al., 2011,HMUELV, 2012) and implemented in theSimba# simulation framework.

The treatment plant model shown aboveis combined, within the same simulationenvironment, with a river model using theSimple Water Quality Model and with ahydrological sewer system model, thusforming an integrated wastewater model of atypical city in Germany (Astlingen, seeSCHÜTZE et al., 2017) (Fig. 2).

Fig. 1. Example model of an activated sludge treatment plant.

Fig. 2. Integrated model of the Astlingen wastewater system.

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Such an integrated modelling setupthen allows to analyse the impact ofdischarges and of operational measures onthe river. For example, Fig. 3 and Fig. 4compare the effects on Dissolved Oxygen of(a) limiting the influent to the wastewatertreatment plant to its standard setting of 330l/s and (b) permitting the inflow to theWWTP to be increased for one hour.MUSCHALLA et al. (2009) provide usefulguidance to carrying out such integratedmodelling studies.

Visualisation of resource fluxes insanitation systems

Currently, there is a significant amountof discussion in Germany – and elsewhere –about new and alternative sanitationsystems. Attempts are called for to reducewater demand for flushing toilets and tomake better use of the resource“wastewater”, calling also for increasedrecovery of its nutrients contents. In order toaid such discussions, a simple simulator(“SAmpSONS”), modelling and visualisingresource fluxes of user-defined sanitationsystems has been developed (SCHÜTZE et al.,2019a), based on earlier work byORMANDZHIEVA et al. (2014). Fig. 5 illustratesa simple example of a sanitation system withseparate greywater treatment and co-digestion of kitchen waste and blackwater.

Combining system modelling withLife Cycle Analysis

When discussing upgrades or changesto (waste)water systems, a number ofdifferent criteria are to be considered. Theseoften include, besides capital andoperational expenditure, criteria such asenergy consumption/generation, emissionsof greenhouse gases, eutrophicationpotential, among others. Traditionally theseare estimated in separate calculations,sometimes involving additional, often cost-intensive, software. Within the SAmpSONSsimulator (a freely available subset of theSimba# simulation system, ifak.eu), thesecalculations – often forming part of separate

Life Cycle Assessment (LCA) studies – areintegrated with the process simulationmodules. Therefore the mentioned criteriaare calculated – once input data, such as unitprocess costs, have been provided – as a by-product of the process simulation.

Holistic system modellingExtending the modelling scope of

urban wastewater modelling to a widersystem perspective, also Integrated UrbanWater System models (according to theclassification by BACH et al., 2014) can beimplemented. Such models, covering thewater supply system – includinggroundwater resources, albeit in a rathersimplified way – can assist in overall watermanagement, in particular in water-scarceregions. In a cooperation with researchersfrom Israel, a (simplified) overall model of ageneral urban water system has been set up(see Fig. 6). This is currently being adaptedto selected case study cities in Israel and inGermany. The modelling modulesimplemented here allow the specialconsideration of water demand(communicated “from right to the left”) andwater availability (communicated “from leftto the right”), attempting to balanceavailability with demand of water resources.Stochastic modelling of rainwater tankutilisation allows to find optimum tank sizesand operational strategies of rainwater tankusage (SNIR et al., 2019, SCHÜTZE et al.,2019b). Reuse strategies of (treated)greywater and wastewater streams provideimportant feedback loops in the system andcould be shown to alleviate pressure onscarce water resources.

Modelling the Water-Energy-Food-Waste Nexus

Considering that the water andwastewater system constitutes just oneelement of the urban infrastructure systemor “urban metabolism”, prudent use of theurban infrastructure would also consider theinteractions and potential synergies of thewater, energy, solid waste and urban food

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systems. Whilst some interactions betweenthese subsystems are obvious (e.g.electricity demand for pumping andtreating of water), others might appear,whilst equally important, less obvious. Forexample, energy generation from wasteincineration or from co-digestion ofwastewater sludge and kitchen waste,production of fertilizer from wastecomposting and others) constitute relevanttranssectoral interactions. Modelling andvisualising related resource fluxes and,thus, increasing system awareness amongstakeholders, also might assist in strategicpre-planning of urban infrastructuresystems, considering water, energy, foodand waste in a joint manner.

Within the “Rapid Planning” project(rapid-planning.net), currently models of thecities of Da Nang (Vietnam) and Kigali(Rwanda) are being built, which assist in thispreplanning process (Fig. 7). These modelsassist in the analysis of various scenarios offuture developments of these cities and theirresource and infrastructure needs. Theunderlying modelling principles aredescribed by RAMÍREZ CALDERÓN et al. (2015)and ROBLETO (2019). This work, thus, has awider scope than other Nexus-relatedstudies such as, for example, the study byLANDA CONSIGNO et al. (2019), who focussedmainly on the water system.

Fig. 7 shows the model setup of theinfrastructure systems of the rapidlygrowing city of Da Nang. Fig. 8 provides aconcise summary of the main resource fluxesobtained from the detailed simulation of the“transsectoral” future scenario of the city ofDa Nang. This diagram representing some of

the material flows in the system is a by-product of the detailed simulations carriedout. Whilst it is indeed a challenge to selectand to represent the “most interesting”simulation results for the decision makerand end-user, such visualisations can help toincrease the understanding of theimportance of joint planning ofinfrastructure systems.

ConclusionsThis paper has illustrated some

applications of modelling in water systemmanagement in different contexts. Modellingexamples range from detailed biochemicalprocess kinetics modelling in wastewatertreatment and river water quality overmodelling and visualisation of nutrient fluxesin sanitation systems to modelling of theentire Water-Energy-Food-Waste Nexus oflarge agglomerations. Such modellingapplications appear now to be more feasiblethan ever before as simulation systems havegained considerably in user-friendliness bothfor the novice user as well as for thesimulation expert in research. It is believedthat model building and application could beuseful complements to field-work basedecological studies such as those carried out byIHTIMANSKA et al. (2014). Also novelmodelling concepts, such the linkage ofprocess modelling with LCA-typecalculations within the same modellingframework and the integration of availability-demand balancing with process modellinghave been demonstrated. It is hoped that thispaper stimulates additional modellingapplications, motivated by the statement that“all models are wrong, but some are useful.”

Fig. 3. Dissolved Oxygen results in selected river sections of the Astlingen system –Inflow to WWTP restricted to default.

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Fig. 4. Dissolved Oxygen results in selected river sections of the Astlingen system –Inflow to WWTP temporarily increased.

Fig. 5. Example of a sanitation system modelled in the SAmpSONS simulator.

Fig. 6. Model of a generalised water system, including rainwater harvesting, groundwaterabstraction, greywater and wastewater reuse.

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Fig. 7. Model setup of Da Nang (Vietnam) (SCHÜTZE & ROBLETO, 2019).

Fig. 8. Summary of main resource fluxes in Da Nang (Vietnam) in a transsectoral scenario(SCHÜTZE & ROBLETO, 2019).

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AcknowledgementsFinancial support from various sources for

various projects leading to the results summarisedin this paper is acknowledged (German FederalMinistry of Education and Research-BMBF –projects “Rapid Planning”, “CLUWAL”;SaMuWa”; German Federal Ministry ofEconomics – project “Kommunal4.0”; DeutscheBundesstiftung Umwelt - project “SAmpSONS”).

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Received: 09.08.2019Accepted: 02.12.2019

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