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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/doi/10.1111/wej.12114/pdf doi:10.1111/wej.12114 121 domain(Roland et al.,2009).Here,we used the one presented in Butler and Gatt(1996)as an input in 122 our model.The daily hydrograph was pre-normalized so that daily hydraulic load is the same as in 123 case 2.For all three test cases,the characteristics of the septic tank are the same,i.e.a 4 m2 area at the 124 invert of the outflow pipe(100 mm in diameter)that corresponds to a commercial standard. 125 Figure 4 presents the results.A single event, like a bath emptying(case a),is considerably smoothed 126 by the septic tank.The outflow discharge reaches only 57%of the inflow rate at the end of loading. 127 Many treatment systems(e.g.sand filters)downstream of the septic tank have gravity-driven 128 distribution systems and therefore depend on inflow velocity to ensure an even distribution. If the 129 septic tank significantly smoothes its outflow discharge,this could affect the quality of hydraulic 130 distribution in the treatment system.When applied to more averaged hydrographs such as those 131 obtained synthetically based on daily hydraulic load and typical household water usage(AFNOR, 132 2013;case b)or those experimentally observed at sewer level(Butler&Gatt, 1996;case c),the 133 smoothing effect tends to be less significant:mean outflow discharges are only 0.6%and 3.75%lower 134 than mean inflow rates for the second and third test cases,respectively. In these cases,the septic tank 135 only induces a lag in the propagation of the hydrograph.In addition, case b shows that the septic tank 136 may help ensure a better spreading of hydraulic load over the day. 137 Evaluation of septic tank buffering on effluent distribution over the secondary treatment unit would 138 requires better measurements of the inlet hydrograph with a small time resolution.The hydrograph 139 suggested by(AFNOR,2013)is too averaged and actually presents a shape close to the observed in 140 sewers.Patel et al.(2008)measured outflow at the inlet of gravity distribution devices(after the septic 141 tank)and concluded that the most common flow rates were between 0.0016-2.0.03 1 h-1 with peak 142 values up to 0.2 1 h-1.For secondary treatments that are not fed by gravity,the septic tank buffer effect 143 may help to ensure a better spreading of the influent over time.Furthermore,the current model could 144 be easily applied to septic tank alternative designs such as tank in series and tank in parallels as they 145 are already widespread in USA and tend to develop in Europe. A sensitivity analysis on parameters S 146 and D could also indicate which one influence the most buffer effect and lag time. 147 Finally,we would like to stress that not only septic tank induces a buffer effect.Pipes,conducting 148 flow to and out of the septic tank,may also be of importance regarding their diameter(typically 100 149 mm).