Wastewater is collected in ducts that gradually get bigger. An amazing 320 different types of cross-sections have been adopted over the years to built the Milanese sewer ducts; these may have the circular (small and mid-sized), ovoid (mid-sized) or polycentric cross-section (large).
The network structure as of 2009 consisted mostly (about 45 percent) of English ovoid cross-section pipes (Ω = 0.80 x 1.20 m or 2.6 x 3.9 ft). Designers chose this particular section as early as 1868 because it can keep high flow rates which limit the deposits settling at the bottom of the pipe, the result of fairly small slopes and small quantities during low-water periods.
A combined sewer system (CSS) is used, with the sewage and stormwater runoff collected in a single pipe gravity-powered system which exploits the slightly sloping Milan Municipality ground, to the order of 0.27 percent in the NW-to-SE direction. In few situations pumping systems are required to overcome uphill sections, with a moderate energy cost impact.
Wastewater runs in open ducts, as opposed to the drinking water piping through which the water is pumped.
The lack of receptive capacity of the watercourses into which the treated wastewater is discharged led to the choice of a network consisting of oversized and interconnected collector piping forming a closed grid that follows the layout of the City.
This enables automatic overflow drainage from the areas under heavy rainfall to adjacent areas, creating a considerable basin capacity of about 1,650,000 m³ (363 million imp gal) capable of disposing quickly of the huge volume of water and prevent flooding.
All sewage pipelines have manholes for inspection and maintenance, providing access to underground chambers whose size varies with piping sections, yet ensures safe and relatively comfortable work environments.
In some cases, particularly along the larger pipes, or at the main hydraulic nodes, manholes are quite large and their particularly interesting shapes make them relevant from the architectural point of view.
The collected water is ducted into the three water treatment plants of Milano Nosedo, Milano San Rocco and Peschiera Borromeo, following the natural division of the terrain.
Ordinary maintenance of the network is structured in the following activities:
- control of the sewage system;
- operation and management of the relative systems;
- internal duct inspection to check their integrity;
- Emergency Service;
- purging of the sewage pipes.
The last activity is important for the overall performance of the wastewater collection system. Pipes are purged to removing sludge which builds up in them. On average, more than 200 km (124 mi) of piping are purged each year. Each pipe is therefore cleaned on average every seven years, exception made for specific pipe types which require purging several times a year.
Extraordinary maintenance of the sewer system has the main goal of preventing duct section from collapsing, reduction and elimination of damage to the system at weak points identified during inspection.
The main types of operation consist of:
- reconstruction or consolidation of deteriorated or damaged pipes, amounting to about 90% of the total work;
- laying new piping;
- improving the management efficiency of the system;