Silica litters, while non-toxic, are nonorganic and will not biodegrade (break down). However, they do contribute less landfill pollution than traditional clumping litters which can help to reduce carbon footprint.
A 6lbs bag of PrettyLitter, for example, should last 1 month for a single cat household compared to traditional clumping or clay litters that average 28 to 40lbs per month for a single cat household.