Phosphorus is a non-renewable resource, yet essential to all life and severely limited over much of the earth's food production areas. However, phosphorus is also a valuable builder in laundry detergents and useful in keeping our fabrics 'clean'. The problem then becomes one of disposal of the wastewater. Dumping the phosphorus loaded wastewater in the ocean removes that resource from future generations. Reusing wastewater for agricultural production (large scale municipal treatment works) or for greywater reuse for the home garden is an excellent opportunity to utilise the phosphorus. If you don't put the phosphorus out of your laundry water, then you are going to put it on from another source - chemical fertiliser.
While replacement of phosphorus in laundry detergents is seen, by some, as environmentally "friendly", it is then a matter of what is the environmental effect of the replacement chemical and and economic one of "how well does it work?". The best option is for reuse of wastewater (greywater).
The graphs below are presented in grams of phosphorus per wash, based upon 75 L for front loaders and 150 L for top loaders which accounts for the differences in water use and dosing rate.
Figure F4-P Loads of phosphorus per wash when recommended dose for front loading washing machine's full wash (75 L)
Figure T4-P Loads of phosphorus per wash when recommended dose for top loading washing machine's full wash (150 L)
Assessment
A phosphorus load of 1 g/wash, for seven washes per week (average family use) and 52 weeks a year, an effluent reuse area of 100 m2 for the dispersal of all the laundry water only, equates to a fertiliser application rate of 330 kg of single super per hectare, much more than the agricultural producers are returned to their pastures (about 150 kg/ha). At higher loading rates, the area across which the laundry water is applied needs to increase. It is not just a simple matter of tipping the laundry water out the back door. On sandy soils where there is no inherent ability of the soil to store phosphorus, it simply leaches to groundwater or runs-off with stormwater and enters the nearly drainage line.
The beneficial use of greywater, or reclaimed water is a legitimate use of a non-renewable resource. However, its application needs to be considered in light of the crop being produced (lawn or pasture, vegetables or horticulture) and the soils to which it is applied.
It is generally accepted that pastures and lawns uptake about 30 kg P/ha each year (that about 300 g over a 100 m2 application area) provided the grasses or pastures are removed. In the agricultural setting, grazing animal move the phosphorus around the paddocks and into 'camps'. In the home garden, lawn clipping and vegetation thinnings need to be removed for composting and use in other areas of the property that do not have wastewater reuse. So, if you were using a laundry detergent that had 2 g/wash (730 g P/year), you would need just over 240 m2 of application area to effective disperse the phosphorus. In many soils, there is an inherent ability to lock up phosphorus, but that is beyond the scope of this discussion.
Analysis of phosphorus labelling
There are two common labels used by manufacturers to identify the amount of phosphorus in their products. The "NP" symbol denotes that there has been no added phosphorus to the product, although minor amounts may be present in the various ingredients they use. The "P" symbol denotes that the product contains less than an industry imposed upper limit for phosphorus (not sure how this value was derived). An analysis of the product labels of the products tested in this range of laundry products is given below.
This section is yet to be completed - please be patient!