Dirty or stained grout can make any tile surface look dingy. Since grout is porous, dirt and stains get trapped and won't usually come off with most basic all-purpose cleaners. Oxygen bleach is a non-toxic alternative to chlorine bleach, safe to use around pets and kids, and won't create harmful fumes. Oxygen bleach whitens grout by penetrating and breaking down stains with oxygen ions. It brightens and disinfects without discoloring most surfaces, but requires a longer dwelling time than chlorine bleach.
Mix 1/4 cup of powdered oxygen bleach per gallon of warm water in a spray bottle or bucket until it dissolves.
Spray the grout thoroughly with the cleaning solution. You can also apply it with a mop or pour it over the grout. The grout should be completely saturated. Allow it to sit for 30 minutes to 1 hour, depending on the stain level. Apply more solution as needed to keep it saturated.
Scrub the grout with a grout brush or scrub brush to remove any remaining stains.
Rinse the grout with clean water and a cloth, sponge or mop. Allow it to dry. The grout will appear lighter when it dries.