The average person is responsible for approximately 1 gram of phosphorus per day. About 3/4 gram is from liquid waste (urine) in the form of inorganic orthophosphate. About 1/4 gram is from solid waste (feces) and is in the form of organic poly phosphates. Most all dish-washing and clothes washing detergents are phosphate free. In many states, phosphorus containing fertilizer is prohibited from use by homeowners. Other than food scraps, most of the phosphorus in the septic tank is human generated as part of our normal living activity.
There are other sources of phosphorus in nature. All living plants through decay release phosphorus into the ground where it is recycled to be absorbed into their root systems to provide essential growth. Phosphorus can become mobile during storm events and can travel into waterways and lakes. Once phosphorus is in a water body, like terrestrial recycling, phosphorus recirculates between the root systems of plants to stimulate the foliage and finally to the decaying foliage to form organic silt.