About 6 months ago I commissioned my constructed wetland in our backyard, the idea was to treat water coming out of our washing machine and dishwasher before it went to a couple of fruit trees. I was concerned that since we had bought a top loading washing machine some years ago it had reduced our water usage for clothes washing (good). On the other hand it would also have increased the concentration of salts and other pollutants going onto the soil if I re-used the grey water (bad). The answer was to introduce some method of water treatment, hence the constructed wetland (details here).
So once the wetland was constructed I left it sit for a bit over two months while the plants and beneficial bacteria established. We have now been using it for most of our washing loads (say 5 – 6 loads per week) for the last four months roughly. Was it making a difference? I really had no idea. The water coming out the other end looked pretty good but that meant little and gave no indication as to dissolved salts etc. I was doing it on faith, but I wanted data!
Fortunately a good friend of mine works for a local water utility and has access to a water testing laboratory. He agreed to run some tests for me then help me interpret the results.
Test Conditions
Usually water testing of this type would require multiple “before” and “after” samples to be taken over a period of time and then the results analysed statistically. Due to the access to facilities being limited we took two “grab” samples, the first being from the outlet of the washing machine part way through the effluent discharge from the washing cycle or in other words, worst case scenario. The second sample was taken at the exit from the second part of the constructed wetland, where the water goes to water the fruit trees. The previous loads were washed 3 days ago so the water tested had 3 days residence time in the system before it was tested. Samples were taken about 10:00am in late summer.
So, make of the results what you will, they give an indication of what is happening but are not a comprehensive analysis. I won’t keep you in suspense any longer, this is what we got.
Test 1 – Oxygen levels
Ex washing machine = 5.7mg/litre
Ex constructed wetland = <1 mg/litre
Clearly the oxygen has been consumed in the wetland breaking down pollutants in the water, but sufficient oxygen is being fed into the beds by the plant roots to prevent the beds becoming anaerobic (there is no unpleasant smell).
Test 2 – Electric conductivity (EC)
Ex washing machine = 2170 uS/cm
Ex constructed wetland = 1600uS/cm
The conductivity of the water is a measure of the level of dissolved salts it contains (this was a major concern for me). While the numbers show that the dissolved salt levels have been reduced by 26% the point worth noting is that the absolute upper limit for salts in drinking water is 1600uS/cm so it has gone from definitely brackish to almost drinking water quality.
Test 3 – pH
Ex washing machine = 9.6
Ex constructed wetland = 7.4
pH is of course the measure of acidity or alkalinity of a solution, 14.0 being highly alkaline, 1.0 being highly acid and 7.0 being neutral. As a control, tap water in our area is reported to be typically from 7.6 to 8.2 so water which was clearly alkaline is now almost in our drinking water range.
Test 4 - Oxidation/reduction potential (ORP)
Ex washing machine = 442 mV
Ex constructed wetland = 465 mV
In this case there was no significant difference between the two readings. ORP is a measure of water quality particularly in terms of if it is well disinfected. Generally an ORP of 650+ mV means that it is suitable for use in pools and spas or for drinking. However, a reading in excess of 250 mV indicates that the water is not anaerobic (resulting in foul smells) so in this case the ORP, being between these two limits, shows biological activity which is what we want.
Test 5 – Turbidity
Ex washing machine = 365 NTU
Ex Constructed wetland = 10 NTU
Turbidity is a measure of cloudiness or haziness in the water. It is measured by detecting light scattered at 90⁰ from the incident light beam using (in this case) a nephelometer with the units of the resultant reading expressed as NTU (Nephelometric Turbidity Units). Generally drinking water should be less than 5 NTU. But in this case the wetland has reduced the turbidity by a huge amount, and that works for me.
Conclusion
So, from the test results it appears that the water coming out the exit from the constructed wetland is somewhat improved in several important criteria over the washing machine grey water going in to the wetland.
