Sewage-based compost given out by San Francisco is laced with contaminants such as heavy metals, which exist in comparable levels in commercially available soil amendments, The City found.

Sewage is often treated and dried to create what’s called biosolids, then it’s heaped on agricultural land.

The San Francisco Public Utilities Commission in 2007 took the process further and began providing more-heavily treated biosolids compost to residents for gardens.

The program’s future was thrown into doubt this year after protests at City Hall by food-safety activists who characterized the compost as toxic sludge.

The organizers, some of whom advocate using personal”composting toilets, have long opposed use of biosolids on farms because of contamination found in sewage.

In response, the SFPUC suspended the program and spent $25,000 analyzing its compost and seven commercial soil products.

Using sewage as compost returns nutrients to the soil, which replenishes land and minimizes landfill.

But sewage is not the only waste that ends up in sewers and treatment plants.

Plastic bottles, industrial solvents, agricultural runoff and oils flow into storm drains and mix with sewage or get flushed into sewers.