Engineering firms and developers often ask which set of rules actually governs a water supply or sewerage design. The honest answer is that there are two layers, and they do different jobs.
The framework layer sets outcomes, not design details
Broader regulatory frameworks covering drinking water quality and wastewater treatment define what a finished system has to achieve: water quality parameters, treatment performance, environmental discharge limits. They do not tell a designer what pipe diameter to use or how to size a pumping station. That gap is intentional: outcomes are set at a policy level, and the technical path to those outcomes is left to national or industry bodies closer to the ground.
The technical layer sets the design rules
This is where national and utility-level technical standards come in: minimum pressure requirements, material specifications, pipe sizing methodology, testing procedures. These rules vary by jurisdiction and by the authority operating the infrastructure, even where the underlying engineering problem is the same everywhere.
What this means in practice for a design
For any given project, the practical question is not "which framework applies in general" but "which authority will approve this and which utility will operate it." That authority's technical rules are the ones the design has to satisfy directly. Our water supply and sewerage and drainage design work is produced against the applicable technical standard for each project, so the documentation set matches what the approving authority actually expects to see.
If you are unsure which rules apply to a specific project, that is a reasonable starting point for a consulting engagement before design work begins.