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' I Author-produced version of the article published in Water and Environment Journal,2015,29(3),360-364. The original publication is available at:http://onlinelibrary.wiley.com/dol/10.1111/wej.12114/pdf doi:10.1111/wej.12114 1 Simple deterministic model of the hydraulic buffer effect in septic 2 tanks 3 Nicolas Forquetl and Matthieu Dufresne2 4 Abstract 5 Septic tanks are widely used in on-site wastewater treatment systems. In addition to anaerobic pre- 6 treatment,hydraulic buffering is one of the roles attributed to septic tanks.However there is still no 7 tool for assessing it,especially in dynamic conditions.For gravity fed-system,it could help both 8 researchers and system designers.This technical note reports a simple mechanistic model based on the 9 assumption of flow transition between the septic tank and the outflow pipe.The only parameter of this 10 model was calibrated using CFD modeling for a wide range of discharge rates.The resulting model 11 highlights that a septic tank plays a hydraulic buffer role when faced with sudden and large discharge 12 flow but this role tends to disappear when input hydrographs are smoother. In those cases there is an 13 observable lag between the input hydrograph and outflow hydrograph. 14 Key words: on-site wastewater treatment—septic tank—buffer effect—mechanistic modeling 15 Introduction 16 On-site wastewater systems usually consist of a septic tank followed by a treatment unit. In many 17 countries(including the USA and France),treatment units are commonly gravity-fed from the outlet of 18 the septic tank(e.g.a drainfield trench or vertical flow sand filter).Because flow at the outlet of a 19 septic tank is not constant,the distribution over the surface of the treatment unit is rather uneven, 20 especially in early filter operation(Bridson-Pateman et al.,2013;Gill et al.,2009).Flow variability 21 stems mainly from household water usage patterns,but the septic tank also induces some flow 22 modulation.The hydraulic buffer effect of septic tanks is often cited but to our knowledge there is still 23 no method for quantifying it.It is often assumed that for the purposes of studying of long-term 24 phenomena(like clogging of the treatment unit at month-long or year-long scale),the outflow can be 25 considered constant(Winstanley&Fowler,2013).However,as we gain progressively more 26 knowledge on the actual hydrograph produced by a household, it may be interesting to quantify how 27 effectively the septic tank can buffer large inflow rates.This may be of particular interest for 28 estimating the efficiency of gravity-driven distribution on secondary treatment unit and to eventually 29 optimize it. 1 Irstea,UR MALY,centre de Lyon-Villeurbanne,5 rue de la Doua—CS 70077,69626 Villeurbanne Cedex, France.nicolas.forquet@irstea.fr 1 Ecole Nationale du Genie de I'Eau et de I'Environnement de Strasbourg, Laboratoire(Cube(Universite de Strasbourg,CNRS,INSA de Strasbourg,ENGEES),(Cube,2 rue Boussigault 67000 Strasbourg,France.