The agency estimates that over a fifth of homes not connected to public water and sewage systems do not meet requirements such as having a sludge well, mandatory in Sweden since 1969.
According to the agency's estimate, those households let out as much phosphorus as all houses connected to the public sewage systems in Sweden combined.
A tax on homes with sub-par sewage systems would make homeowners upgrade, argues the agency, but the Homeowners' Association says a tax is a blunt tool that will punish even households with modern systems in place.
"Most homeowners take care of their sewage systems. A tax would be unfair," says Daniel Liljeberg, chief economist at the association.