The Trieste outfall sewer system is active since 1992, discharging treated sewage at a distance of 7.5 km offshore and average sea depth of 20 m. The submerged diffuser has a Y shape, being constituted of a 500 m long inlet pipe, 2 m in diameter, followed by two pipelines, 6 and 6.5 km in length, each leading to the diffusers, 500 and 1000 m long, respectively. This system is efficient, provided full nozzle venting. This condition, however, is difficult to assess, both for difficulties in continuous monitoring and for possible nozzle occlusions. Moreover, risers are subject to damaging, leading to larger treated sewage fluxes and a lesser dilution efficiency. The aging of the pipe system is assessed in comparison with the “as new” and design conditions. Some site measurements of dilution characteristics give some hints on actual system efficiency.
