Your dosage depends in large part on how hard your water is, since STPP works, in part, by softening the wash water.

If your water comes from a municipal source, the hardness info should be available online.

If it comes from a well, you should be able to test it yourself pretty easily. Check an aquarium supply dealer for water testing materials.

Ours is city water so I just take the data from the annual city water report. Our water hardness is about 145 parts per million (which, I think, is between 8 grains per gallon). This is considered "hard" water.

I use 2 teaspoons STPP per load in a Euro-sized washer, with 3 tablespoons of Persil, or 1 tablespoon of Charlie's Soap.

I don't use STPP with liquid detergents, or when washing wool items.

Hope this helps.

See the section titled "Phosphate Detergent Bans" in the article below. It gives some idea of the percentage of STPP, and percentage of phosphorus, that was used in detergents: 1) When phosphates were first introduced, and 2) When phosphates were reduced but not banned.

Here is a link that might be useful: http://pubs.usgs.gov/wri/wri994007/pdf/wri99-4007.pdf

I'm not sure if it makes sense to use STPP alone in the prewash. STPP isn't a complete detergent, it's an ingredient or a booster.

Suburbanmd is right. Think of STPP of a water conditioner of sorts. It softens the water and conditions it so your detergent can work better. STPP works with all detergent types, liquids, gels, and powders.
With the advent of phosphate free dishwasher detergent, you can even use it in there too.

What's the reason for not using it with liquid detergent?

Thanks for the info everyone!!!!!!

suburbanmd

My theory for using the STPP in the pre-wash wash was more to prep the laundry for the main wash rather than actually do any major cleaning. 95% of the time my clothes don't even need a pre-wash, I do it anyway. They way I see it, I am removing any possible soil, and saturating the laundry with softened water before it ever contacts detergent. However, like said, I do add a little STPP to my detergent to compensate for the drained water from the pre-wash. Maybe I should mix a small amount of detergent into my STPP pre-wash batch, Just to help suspend any soil that may be present. Again thanks for the input everyone.

ohh... and about the liquid detergent. Personally I rarely use it, but when I do, I will use STPP in the pre wash the same way I do with my powdered detergent.

A little STPP goes a long way. Our water has 9 grains and I only use 1 tablespoon per load in a top loader and 1/2 teaspoon in the dishwasher. If you are using a front loading machine, I wouldn't use more than 1 tsp. per load.

Personally, I think using STPP in the prewash without any detergent is wasting your STPP.