Fig. 1

Layout of the pilot gravitational sewers and the sectional view. A well-controlled pilot system was set up to simulate the practical operation of sewers. Seven PVC (Poly Vinyl Chloride) sewers with identical size (with 8.0 m length, 57.0 mm inner diameter and I = 8‰) were placed and operated in parallel under the wall-shear stresses of 0.5, 0.8, 1.12, 1.29, 1.45, 2.0 and 2.5 Pa. Synthetic sewage that added into the seven sewers was prepared in an elevated PVC tank (20.0 L) with the water temperature maintained in the range of 20.0–25.0 °C, which was similar to temperature in real sewers. According to our actual monitoring, the pH was maintained within 7.0–7.5, similar to which was in the real sewer in this study. The outflows were then collected by seven PVC tanks and pumped back to the elevated tank operated 24 h/day. PVC chips (3.0 × 3.0 cm) were installed on the sewer inner-wall below water and they can be removed for regular measurement of thickness of the developed biofilms