Sodium (Na)
While salinity is the current topic of media and government attention, the real demon in the mixture of cations (+ve) and anions (-ve) that gives rise to salinity, the sodium cation (Na+) is the one that is the most hazardous because of its effect upon the osmotic pressure in plant cells and its ability to affect soil stability through the effects of dispersion.
The effects of sodium on plants and soil are not new scientific revelations but ones that are well understood in agricultural and horticultural enterprises and certainly well examined by soil scientists.
The sodicity of liquid laundry detergents was measured by mixing the range of liquids, purchased for this research, at the manufacturer's recommended dose for a normal wash in either a front loader or a top loader washing machine. The liquids were shaken in deionised water for 30 minutes prior to measurement.
The impact of sodium on plants is relative to its concentration, with plant production decreasing with increasing sodicity, until such concentrations in the soil permit only the most tolerant species to survive. At what level of sodicity of the greywater (used washing machine water or other effluent) is a question that can only be considered in relation to soil properties and the vegetation type and is beyond the scope of this research. Suffice to consider that the lower the sodicity, the less risk there is to detrimental plant or soil effects. Refer back to discussion of sodium in the Laundry Products Research page.
As with other components measured in water of a quality equivalent to very clean rainwater, the effect of adding these laundry products is cumulative, that is, add the concentration of sodium in the graphs below to the concentration of the sodium in the town water supply. This will vary from location to location as municipal water treatment plants add chemical to adjust for hardness, pH and add total alkalinity to reduce corrosive effects of the water on the distribution system.
It has been explained in previous sections that an arbitrary level of sodium has been chosen at 200 mg/L /wash because that equates to about one tonne of sodium chloride equivalent per hectare, a level at which agriculturalists would be alerted to a potential sodium problem. However, this level will vary depending upon existing soil type, soil conditions and rainfall (to wash salts deeper into soil profile).
Table LF3-Na is a ranking of the sodium concentration for the liquids when mixed at the rate for front loading washing machines (75 L per full wash).
Table LT3-Na is a ranking of the sodium concentration for the liquids when mixed at the rate for top loading washing machines (150 L per full wash).
Assessment
The simple assessment of the liquids when compared with the powders is that liquids present less of a sodium hazard than the powders. The average value for the powders was 283 mg/L for the front loader (median 250 mg/L) and average 210 mg/L (median 173 mg/L) for the top loader while for the liquids the values were 24 mg/L average (median 19 mg/L) for the front loader and 21 mg/L average (median 13 mg/L) for the top loader. These values when converted to annual loadings on the disposal area are significantly different. For front loaders the average sodium load is 0.7 kg from liquids compared to 15.5 kg from powders, while for the top loaders the average was 0.6 kg compared to 11.5 kg from powders.
When examined as the differences between liquids and powders for their sodium contribution to the wastewater, the average sodium load from liquid detergents used in front loaders was 1.8 g/wash and for liquids in top loaders at 3.1 g/wash. However, when powders were used, the average sodium load from front loaders was 21.3 g/wash but was 30.2 g/wash for top loaders.
It is, therefore, possible to choose either a powder or a liquid with very low sodium, but the average liquids have significantly less sodium than the average powder. It would be equally correct to say that there are more hazardous laundry powders than liquids.
In some locations sodium concentrations are unimportant because municipal sewage treatment works dump into the ocean. For the residents that are responsible for on-site treatment and disposal of their own wastewater, sodium can be a significantly important consideration. For those wishing to use greywater from the laundry for beneficial purposes, the issue of sodium in the wastewater is critical for both soil and plants.