Bleach. It is natural. If not then try a pressure washer or super powerful steam blaster and a brush.
Below is from wikipedia:
No emissions of sodium hypochlorite from normal household or institutional use find their way directly to the environment. Sodium hypochlorite degrades quickly, primarily to sodium chloride, during use or in sewage systems. It also decomposes in soil, primarily to salt. Typical use was found to be not harmful to sewage treatment or septic tanks
Sodium hypochlorite is practically non-toxic to birds and mammals
While highly toxic to fish and invertebrates in confined spaces, fish will swim away from the source if possible. In addition, sodium hypochlorite readily disperses and degrades mostly to salt in surface waters, limiting impact
Very low levels absorbable organic halides (AOX) can be found during reaction of sodium hypochlorite and soils, including carbon tetrachloride, trihalomethanes (THM, such as chloroform), and trihaloacetic acid (THAA). Most AOX go into the sewer with wash water; amounts emitted to air well below safe limits. Most AOX degrades in sewage treatment like starting soil; wastewater genotoxicity not increased. Remnants are not harmful at levels detected (acute and chronic); no persistent or lipophilic chlorinated compounds were detected. Limited amounts of AOX have been detectable on fabrics below significant effect levels
