Regarding the estimate of ...

Regarding the estimate of wastewater strength and generation rates, just do an internet search on that topic.

Also consider that phosphates and nitrates are plant nutrients.  When they are found in wastewater, they are a resource that is simply out of place.  Depending on your location and other factors, land treatment, artificial wetlands and lagoon treatment options are low technology and simple to operate.  Land treatment, that is using the wastewater to irrigate crops, beneficially uses the nitrates and phosphates to fertilize and water crops which can be harvested and sold.  The proceeds from crop sales further defray the cost of treatment.  Water quality effluent from land treatment systems is equivalent to advanced waste treatment technologies.  There are many sustainable, low energy options available using natural systems to treat wastewater containing nitrates and phosphates.  I encourage you to look into them.