I have silt build up in the cistern. S/E Water suggest it is a leak under my house. One plumber told me that was normal while another said due to galvanised pipes, deteriation caused the silt. Who is correct? House was built in 1946 with renovations done over the years but do not know if pipes were updated.
It is hard to tell without looking at the material and the cistern, as well as how long it has taken to accumulate.
My guess is that if it has accumulated over some time, it is likely to be deposits from galvanised pipes and sediment from water entering the property. As you house was built in 1946, it is likely to have galvanised pipes unless they have been full replaced by one of the renovations. Galvanised pipes over time rust and rust deposits are mobilised from the pipes and end up in the cistern where it is allowed to settle out.
Reticulated water contains small amounts of sediment (usually clay sized fractions) and if the water sits in a cistern for some time (say overnight or longer without being flushed), the sediment can settle out on the base of the cistern. Sediment is more noticeable over a long period of time (many years to decades), depending on the quality of water entering the property. Sediment is harmless to the consumer, as almost everything consumed contains some form of sediment from soils.
If the sediment has accumulated quickly (days or weeks), then a pipe leak or sediment from nearby works on the reticulated water system is potentially the most likely explanation. It can enter from works where the pipes aren’t fully flushed out (back flushing) after the works are completed. This will cause a rapid increase in sediment within a cistern.
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