Comment on BBC uncovers 6,000 possible illegal sewage spills in one year

LNRDrone@sopuli.xyz ⁨2⁩ ⁨weeks⁩ ago

• tal@lemmy.today ⁨2⁩ ⁨weeks⁩ ago

  Having to ever discharge raw sewage at all sounds like there’s something clearly wrong with the infrastructure

  This is very common, unless you live in a very young city. It’s due to the use of combined sewers, which were around until comparatively recently due to the evolution of cities.

  en.wikipedia.org/wiki/Combined_sewer

  A combined sewer is a type of gravity sewer with a system of pipes, tunnels, pump stations etc. to transport sewage and urban runoff together to a sewage treatment plant or disposal site. This means that during rain events, the sewage gets diluted, resulting in higher flowrates at the treatment site. Uncontaminated stormwater simply dilutes sewage, but runoff may dissolve or suspend virtually anything it contacts on roofs, streets, and storage yards.[1]: 296  As rainfall travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediment, heavy metals, organic compounds, animal waste, and oil and grease. Combined sewers may also receive dry weather drainage from landscape irrigation, construction dewatering, and washing buildings and sidewalks.

  In the US, unless you’re west of the Mississippi, there’s good odds that your city may still have combined sewers. If you’re in Europe, there’s very good odds that you have combined sewers.

  The earliest sewers were designed to carry street runoff away from inhabited areas and into surface waterways without treatment. Before the 19th century, it was commonplace to empty human waste receptacles, e.g., chamber pots, into town and city streets and slaughter animals in open street “shambles”. The use of draft animals such as horses and herding of livestock through city streets meant that most contained large amounts of excrement. Before the development of macadam as a paving material in the 19th century, paving systems were mostly porous, so that precipitation could soak away and not run off, and urban rooftop rainwater was often saved in rainwater tanks. Open sewers, consisting of gutters and urban streambeds, were common worldwide before the 20th century.

  In the majority of developed countries, large efforts were made during the late 19th and early 20th centuries to cover the formerly open sewers, converting them to closed systems with cast iron, steel, or concrete pipes, masonry, and concrete arches, while streets and footpaths were increasingly covered with impermeable paving systems. Most sewage collection systems of the 19th and early to mid-20th century used single-pipe systems that collect both sewage and urban runoff from streets and roofs (to the extent that relatively clean rooftop rainwater was not saved in butts and cisterns for drinking and washing.) This type of collection system is referred to as a “combined sewer system”. The rationale for combining the two was that it would be cheaper to build just a single system.[4]: 8  Most cities at that time did not have sewage treatment plants, so there was no perceived public health advantage in constructing a separate “surface water sewerage” (UK terminology) or “storm sewer” (US terminology) system.[2]: pp. 2–3  Moreover, before the automobile era, runoff was likely to be typically highly contaminated with animal waste. Further, until the mid-late 19th century the frequent use of shambles contributed more waste. The widespread replacement of horses with automotive propulsion, paving of city streets and surfaces, construction of municipal slaughterhouses, and provision of mains water in the 20th century changed the nature and volume of urban runoff to be initially cleaner, include water that formerly soaked away and previously saved rooftop rainwater after combined sewers were already widely adopted.

  When constructed, combined sewer systems were typically sized to carry three[2]: pp. 2–4  to 160 times the average dry weather sewage flows.[5]: 136  It is generally infeasible to treat the volume of mixed sewage and surface runoff flowing in a combined sewer during peak runoff events caused by snowmelt or convective precipitation. As cities built sewage treatment plants, those plants were typically built to treat only the volume of sewage flowing during dry weather. Relief structures were installed in the collection system to bypass untreated sewage mixed with surface runoff during wet weather, protecting sewage treatment plants from damage caused if peak flows reached the headworks.[6]

  Cities are very slowly moving to sewers that are separate from rainwater systems, but it’s costly, since two intermingled systems have to be untangled and it’s under a whole city.

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