Comparing the Imhoff/ Faustformel minimum slope curve to that of Macke / ATV-A 110 (1988), it is clear that large diameter pipes (> DN 800 (32'')) as per Imhoff require significantly greater slope than that of ATV-A 110 (1988). The results of Macke were further refined by Pecher et al. In 1991, in the form of differentiated calculations based on a variation of the sediment transport equation with partial filling (h_t / d = 0.05 to 0.3). It is worth noting that part of the fill factor has a significant influence on the size of the minimum slope. The lower the partial filling degree, the greater the slope needs to be in order to allow, under otherwise identical conditions, a deposit-free operation ATVA110:2001 [Peche1991c].

Similar conclusions by Sander came in 1994 on the basis of new findings on sedimentation. It therefore recommended to dimension pipes with a diameter of d ≤ 1000 mm (40 in) with a larger specified minimum slope as in ATV-A 110 (1988). He also confirmed the mean particle diameter of the sediment as d₅₀ = 0.35 mm [Sande1994].

All the above analyses and suggestions were reflected in the new edition of the ATV-A 110 in 2001. Thus, there are now formulas and tables for determining the limits I_C, v_c, τ_min for circular pipes DN 150-4000 with a partial filling h_t/d >0,1 , for both combined and storm sewers [ATVDVWKA 110:2001].

The picture shows the limit curves thus determined for the required minimum slope for a deposit-free operation for h_t / d ≥ 0.1 and h_t / d ≥ 0.5. From this it can be deduced that the required minimum slope is significantly influenced by:

• the degree of capacity utilization (filling height),
• the circular pipe diameter of the sewers d,
• the dewatering process and thus indirectly on the characteristics of the transported solids within the wastewater.

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