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Embedding sustainability in the design of water supply and drainage systems for buildings L B Jack J A Swaffi eld School of the Built Environment Heriot Watt University Edinburgh Scotland EH14 4AS UK a r t i c l e i n f o Article history Available online 3 April 2009 Keywords Water Drainage Buildings Numerical modelling Sustainability a b s t r a c t In addressing sustainability issues for the built environment focus is often directed towards minimising energy consumption and material use Often forgotten however is the potential for the integration of sustainable solutions when designing water and waste management systems for buildings The funda mental functions of such systems are clearly recognised but traditional design principles often constrain opportunities for performance enhancement and for water and pipework economies To an extent this is unsurprising given the basic premise that steady state analysis of fl ows underpins many of the codes and guidelines used worldwide However advances in simulation methods mean that system perfor mance resulting from the use of new techniques and from the integration of innovative and more sustainable design approaches can now be fully assessed This paper provides an overview of the water supply and drainage systems for buildings whose performance has been assessed through the development at Heriot Watt University of a suite of numerical simulation models These models accurately predict using appropriate forms of the St Venant equations the pressure and fl ow regime within such systems by applying the Method of Characteristics fi nite difference technique The paper provides three different examples of application where the focus of each is on embedding sustainability in design 2009 Published by Elsevier Ltd 1 Introduction In providing water supply and waste management systems for buildings it is essential that performance is assured Key functions encompass the provision of potable water and that required for basic hygiene the removal of water that has been contaminated with waste products and the provision of a physical barrier between the potentially harmful miasma present in drainpipes and sewers and the habitable space It is also important that the building uses to best benefi t any impinging rainwater as well as any resultant wastewater thus reducing unnecessary wastage and limiting the loading on sewer and drainage networks and or collection systems Sustainability should underpin design theory in eachoftheseaspectsthroughlimitingwatersupplyand consumption and through reducing material use cost and envi ronmental impact Water supply and drainage systems for build ingsthereforeprovideanumberofopportunitiesforthe integration of sustainable solutions however these must be ach ieved without compromising performance and thus the response of systems during use must be fully understood Often the approach adopted for the design of water and wastewater systems is based upon the application of steady state principles in order to determine for example fl ow loading or pressure response Although such methods facilitate system spec ifi cation in a somewhat deterministic fashion they seldom provide the opportunity to assess the time dependent response of systems information that can readily inform key design decisions The following text will therefore illustrate howan understanding of the dynamic response of systems coupled with the development at Heriot Watt of a suite of numerical simulation models has facili tated the effective and effi cient design and analysis of water supply and drainage for buildings thereby enabling a comprehensive assessment of the potential for integration of innovative and sustainable design solutions It is worth noting at this point that throughout this paper the term water supply will be presented within the context of water use within the building that indirectly dictates supply from large scale networks Each component model contributing to the suite developed at Heriot Watt utilises the Method of Characteristics technique This technique was fi rst used by Massau in 1900 to analyse open channel fl ow and then by Lamoen in 1947 to analyse water hammer and transforms the appropriate forms of the St Venant equations of continuity and momentum into a pair of total differ ential equations solvable by fi nite difference methods These Corresponding author Tel 44 0 131 451 4646 fax 44 0 131 451 4617 E mail address l b jack hw ac uk L B Jack Contents lists available at ScienceDirect Renewable Energy journal homepage 0960 1481 see front matter 2009 Published by Elsevier Ltd doi 10 1016 j renene 2009 02 009 Renewable Energy 34 2009 2061 2066 equations are termed the C and C characteristics and defi ne the conditions at a node one time step in the future in terms of current conditions at adjacent upstream and downstream nodes The fi nite difference grid is defi ned using the independent variables distance x and time t linked with dependent variables either u and c fl uid velocity and propagation wave speed for air or u and h fl uid velocity and depth for free surface water It will be appre ciated that at system boundaries an additional equation is required to complete the fi nite difference solution Equations are therefore defi ned at these locations and provide information on the static or dynamic behaviour as appropriate of the boundary The theoretical and empirical defi nition of these boundary condition equations has formed the focus of both past and present research at Heriot Watt and has facilitated the development of the three component models referred to in this text DRAINET AIRNET and ROOFNET All three are based on the Method of Characteristics technique described and each has been successfully used to enhance the design approach for relevant systems DRAINET deals with the transient analysis of partially fi lled i e free surface pipe fl ow predominantly addressing the performance of internal building drainage systems Its application has recently been extended to encompass local external drainage systems where the fl ow regime maystill be characterised by wave attenuation AIRNET examines the transient response of drainage ventilation systems by predicting the pressure and airfl ow that infl uences the integrity of the water based appliance trap seal whereas ROOFNET assesses the performance of both conventional and siphonic rainwater drainage systems for buildings It will be appreciated that to an extent ROOFNET and DRAINET can be operated jointly to facilitate the prediction of rainwater conveyance from roof surfaces through to local drainage systems This paper will illustrate through the use of examples how these model components may be applied to integrate and embed sustainability in the design of water supply and drainage systems for buildings 2 Potable water use and the impact of reducing w c fl ush volumes The defi nition of sustainability may and often does differ depending upon the context within which it is set For many developed countries sustainability focuses on reducing or opti mising the use of for example energy or materials whereas in other regions sustainability is more about the stable provision of basic needs Within the latter context and set against the UN s Millennium Development Goals one keyaim conveyed in the UN s Task Force on Water and Sanitation is to halve by 2015 the proportion of people without sustainable access to safe drinking water and basic sanitation 1 It therefore seems counterintuitive that in many countries a signifi cant proportion of the potable water supply to buildings is used for w c fl ushing The direct cost savings associated with any reduction in w c fl ush volume that arise from treatment processes alone are clearly signifi cant and when coupled with indirect savings facilitated by a reduction in pipe size for both supply and drainage systems increase yet further Proposals to introduce any signifi cant reduction in w c fl ush volume however are often met with concerns over the effi ciency of the removal of waste and other products from sanitary appliances and of their conveyance through associated drainage networks In the UK a fl ush volume of 40 l had been recognised as excessive as early as around 1900 however following a dramatic reduction to 9 1 l 2 gallons it then tooka number of decades before any further signifi cantreductionswereimposed Statutoryregulations implementedby2001 2 nowstipulate forinstallation a maximum fl ush volume of 6 l and a reduced fl ush volume not greater than two thirds of the maximum thereby targeting the seemingly disproportionate level of around one third of domestic water supply currently used for w c fl ushing 3 Assuming any waste products are either organic or that they comply with accepted fl ushabilitycriteria the focus therefore shifts to the performance of the pipework that conveys this waste to a downstream drain or sewer The fl ow regime in the pipework serving the sanitary appliance is inherently unsteady and there has been a substantial body of work undertaken at Heriot Watt and elsewhere with the aim of predicting the impact of design changes and or changes in water consumption upon the drainline carry of discretesolids Being able topredict the location of solid deposition and being able to take preventative action clearly avoids the propensity for blockage The following text presents a straightforward example of how the performance of pipework when subjected to variable w c discharge volumes can be assessed using DRAINET In this case the discharge volume from the appliance shown connected to Pipe 2 Fig 1 has been varied between 9 6 4 5 and 3 l Each of the four fl ush volumes selected was represented using a profi le of the type also shown in Fig 1 In this example the time at which the solid leaves the appliance was varied appropriately to ensure that in all cases this preceded the point at which peak fl ow discharge occurs This is important as it is well known that in addition to discharge profi le solid parameters and pipe slope diameter roughness and base fl ow the solid discharge time relative to the overall fl ush duration i e thereby defi ning the trailing volume has a signifi cant infl uence on drainline carry where early solid removal ensures a greater travel distance 4 In this simulation a second w c with a fl ush volume of 3 l is connected 5 m downstream but initially was not operated All downstream pipework was specifi ed as 100 mm diameter set at a slope of 1 in 100 Through the use of DRAINET it is possible to simulate both the free surface attenuation of the discharge fl ow from the w c and the location at which any deposition of a discrete solid will occur This requires the inclusion of pre defi ned boundary condition equations that link fl ow depth fl ow rate and or time and that determine conditions at the locations of components of the physical system for example pipe junctions and hydraulic jumps Equations defi ning the fl ow conditions at the location of any discrete solids are also required to facilitate the simulation of both single and multiple solid deposition Fig 2 shows how for a fl ush volume of 9 and 6 l the point of depositionofthesolidliesbeyondthenetworkmodelledi e beyond 9 m thereby indicating that the solid has successfully been conveyed to an appropriate downstream connection For fl ush volumes of 4 5 and 3 l the travel distances are 7 9 m and 5 9 m Total volume Peak flowrate Time seconds flowrate l s 0 Flush profile 9 3 litre w c Pipe connections Pipe 2 5m Pipe 1 2m 3 litre w c Pipe 3 4m flow Fig 1 w c fl ush profi le and schematic showing 2 pipe connection L B Jack J A Swaffi eld Renewable Energy 34 2009 2061 20662062 respectively thus indicating inbothcases depositioninPipe3 This scenario is often perceived as a failure of the system to be remedied by an increase in fl ush volume however by adjusting the pipe diameter to 75 mm it can be shown Fig 2 that the drainline carry can be extended to beyond 9 m and 7 8 m for the 4 5 and 3 l fl ush respectively It will be appreciated that for a downstream connectionpointlocated9 mfromtheappliance thedischargefrom the 3 l fl ush remains insuffi cient in terms of conveyance However through the simulation of a subsequent 3 l fl ush from Pipe 1 rep resenting for example discharge from an adjacent property or room at a simulation time of 30 s fl ow conditions can extend the travel distance for this solid beyond the minimum required This example illustrates how a reduction in fl ush volume need not be accompanied by a reduction in the drainline carry perfor mance of the network In this case conveyance was facilitated in the main by a reduction in pipe diameter A similar improvement can be implemented through an adjustment of pipe slope or by assessing combined or sequential discharge fl ows It will be appreciated that although the example presented herein is based upon the use of only three pipes DRAINET is clearly capable of simulating any number of pipes representative of a typical building or a small cluster of buildings and can therefore readily provide information on the best approach when imple menting water conservation policies or when pursuing sustain ability in the design of water supply and drainage systems 3 Maintaining a physical separation between the habitable space and the drainage pipework It will be appreciated that any unsteady fl ow from a discharging appliance will naturally generate pressure changes within a pipe network This is particularly true when vertical pipes are subjected to discharge fl ow that forms a water annulus and where an asso ciated airfl owis entrained fromsystemvent locations Any pressure change within the drainage pipe network will clearly have an effect upon the overall response of the system however it is predomi nantly the transient nature of pressure excursions that induces the potential for depletion of trap seal water Typically the water based trap seal provides the physical barrier between the habitable space and the miasma present in the pipework that serves the building and provides a conduit to the sewer system and it is therefore important that any pressure changes that might displace this water thereby compromising the integrity of the barrier are minimised Air pressure transients are most commonly generated within drainage and ventilation pipework when there is a relatively rapid change in the rate of the discharge fl ow from one or more appli ances Fig 3 illustrates how the formation of the annulus within the vertical or stack pipe entrains though the principle of no slip an airfl ow that in most cases is drawn from the upper stack termi nation Fig 3 also shows how a change in the discharge fl ow rate at a given point is communicated through the system by means of a change in air pressure and how when ventilation is provided by the upper stack termination results in an imposed change in pressure at all connected traps encountered en route The Heriot Watt developed numerical model AIRNET similarly uses the Method of Characteristics technique to facilitate the predictionofwholesystempressureand airfl owresponse Boundary conditions again require defi nition to enable system simulation and a signifi cant component of the work undertaken at Heriot Watt has focussed on the characterisation of appropriate theoretical and empirically derived descriptive algorithms repre senting both the system drivers and components 5 The model also encompasses an advanced approach to the simulation of the water air interface shown in Fig 4 that through the integration of dimensionless velocity differential terms releases the model from the constraints of single discharge fl ow simulation to enable an analysis of multiple branch inlet fl ows 6 Flexibility of system specifi cation as input data coupled with appliance discharge patterns thus allows the prediction of transient airfl ow and pres sures and trap seal retention levels thereby providing an impor tant step forward in the ability to assess system performance in response to changes initiated in the pursuit of sustainability Water conservation can clearly have an impact upon the fl ow regime within this drainage pipework Generally the effect of a reduction in fl ow volume is characterised by an overall reduction in the terminal water velocity within the stack hence resulting in 0 1 2 3 4 5 6 7 8 9 10 05101520253035404550 Simulation time secs Travel distance m Indicates conveyance to appropriate downstream connection 4 5