41. Construction standards: A standard sand-filter bed built with a gravity feed system must comply with the construction standards in subparagraphs f, h and h.1 of the first paragraph of section 21, subparagraph a of the first paragraph of section 27, subparagraphs b and e of the first paragraph of section 37, and with the following standards:(a) the sand layer must be at least 75 cm deep and must be thoroughly settled by water-spraying before the installation of the upper pipes;
(b) the upper pipes must be laid in a bed of gravel or crushed stone at least 30 cm deep;
(c) the depth of the gravel or crushed stone under the upper pipes must be at least 15 cm;
(d) the upper layer of gravel or crushed stone must comply with subparagraphs g to g.3 of the first paragraph of section 21 and subparagraph c of the first paragraph of section 27;
(e) (paragraph revoked);
(f) the lower pipes must be laid in a bed of gravel or crushed stone at least 20 cm deep;
(g) the gravel or crushed stone must be at least 5 cm deep under the lower pipes;
(h) the grade of the lower pipes must be at least 0.5%;
(i) the total depth of the sand-filter bed must be at least 1.85 m;
(j) when the standard sand-filter bed is built completely or partially above ground, the grade of impermeable or low permeability earth backfill on each side of the sand-filter must be at least 1:2;
(k) there must always be at least 60 cm of impermeable or low permeability soil between the bedrock and the lower part of the standard sand-filter bed.