Salinity
Salinity is measured in two ways. The first is the total weight of salt used in the wash. This value is often difficult to derived from the information on the packet, because the usual measures are given in volumetric terms (one scoop, one cup) and not in terms of mass (weight). The greater the amount of detergent used, the higher the overall salinity. Therefore the very bulky detergents (usually the generic brands of normal powders) are likely to be the most saline by this measure.
The second method for determining salinity is to dissolve the recommended dose of detergent in the total wash water and measure the electrical conductivity (EC) using an electronic instrument. The higher the salt content of the water, the greater the ability of the water to conduct electricity which is measured by the EC meter. A conversion factor is then used to allow for poorly ionised salts, to convert from EC (measured in deciSiemens per metre) to grams of salt per litre. The conversion value generally accepted for water and wastewater is 680, that is multiply the EC meter reading in dS/m by 680 and call the answer milligrams per litre (mg/L).
The salinity measured in this research is for detergents dissolved in deionised water (equivalent to high quality rainwater), with a starting EC of less than 0.005 dS/m (salinity of about 3 mg/L). Many coastal town water supplies have salinity values up to about 40 mg/L while inland NSW supplies can have salinity in the clean town water up to 250 mg/L. You need to account for the salinity of the town water supply in any calculations you do of possible salinity damage to your lawns, garden or subsoil drainfields.
Table F2-EC is a ranking of the EC for the powders when mixed at the rate for front loading washing machines (75 L per full wash).
Table T2-EC is a ranking of the EC for the powders when mixed at the rate for front loading washing machines (75 L per full wash).
Assessment
The generally accepted level of
suitable irrigation water is taken as 1 dS/m while higher levels are likely to
induce some loss of plant production because of the effects of the increased
salinity on the physiology of the plant and the effect of soil salts on general
plant health. If that level was applied to the use of laundry detergents,
then 53% of the front loading powders exceed this level and 32% of the top
loading detergents. Figure L-EC shows there are very few detergents in the
lower ranges and many above the guidance level of 1.0 dS/m. It is clear,
however, that there are some choices of low salinity products (as measured by
electrical conductivity) among the products listed above for either front
loaders or top loaders.
It is also noted that for the front loaders, the specific products (block letters) designed for those machines fall about this arbitrary value of 1 dS/m and the use of the greywater is not recommended. Similarly for both front loaders and top loaders, the generic brands appear near the top of the list.
The impact of salinity from laundry water is a concern to all end users, whether the water is discharged from municipal sewage treatment works to rivers or land application, or for those households that discharge the wastewater to on-site treatment (septic tank or aerated wastewater treatment system) and then to land application (above or below ground). Unfortunately because the large metropolitan areas discharge to ocean outfall and "waste" all the valuable components in the water, the effects of salinity from laundry detergents appears not to concern government regulators. Your actions need to consider the consequences of salinity on your land application area, choose products that have low salinity - and there are plenty of choices!
Remember: Electrical conductivity measures all the ionised components from the salts in the detergents. While some elements may be important plant nutrients, in laundry detergents most of the positive ions are from sodium. Therein lies the real problem. Now read on to the section on sodium to see that measurement of total salinity may not be a good indicator of potential or plant problems