In Search of Pure Water ... (Part 2)

 In Part 1, I discussed the type of trace contaminants found in potable water and, specifically, Hereford tap water processed by the Broomy Hill Water Treatment Works. I also reported on some analytical tests performed at home that confirmed the concentration of some of these trace materials.

In this post, I want to look at a simple option for purifying tap water. In the third and final part. I will discuss the economics of this process along with the environmental impacts.

Potable water has already been through a number of purification processes before it reaches your tap (or faucet for our American friends). These may include some or all of the following: filtration, flocculation, sedimentation, coagulation, adsorption, aeration, precipitation, biodegradation, and disinfection (e.g. chlorination).

Here, we are talking about the final 'polishing' step(s) needed to produce highly purified water, as would typically be used in laboratory/scientific applications, from potable water . The three most widely used forms of purification are distillation, de-ionization and reverse osmosis. The oldest method, distillation, has largely been replaced with reverse osmosis (RO) because it is less energy-intensive and more convenient. The three processes may be used individually or in combination to produce high purity water (sometimes in conjunction with an activated carbon filter to remove organic contaminants).

Since I do not have a RO or ion-exchange system at home, choosing distillation was easy. My distillation kit is shown in Photo 1; a Beko Heat Pump Tumble Dryer with an A++ energy rating.

Photo 1: Distillation Equipment

The distilled water is collected in the tray (upper left) - approximately one litre per drying load. The water is stored in 5L repurposed plastic containers previously used for cleaning products. Residual cleaning materials (e.g. laundry liquids, handsoap, fabric conditioner, toilet cleaner) are rinsed out with hot water (10 times), cold water (5 times) and distilled water (5 times) prior to use as storage vessels.

The next thing to do is test the purity of the home-distilled water using a sample taken directly from the collector tray of the tumble dryer. Table 1 compares the test results for Hereford tap water before and after distillation.

Table 1: Analytical Data on Hereford Tap Water and 'Distilled' Hereford Tap Water

Analytical Test	Hereford Tap Water	‘Distilled’ Hereford Tap Water
Orthophosphate	1.94 mg/L (ppm)	0.00 mg/L
Nitrite	0 mg/L (ppm)	0 mg/L
Nitrate	2 mg/L (ppm)	0 mg/L
NH3-N (ammoniacal nitrogen)	0.00 mg/L (ppm)	2.31 mg/L
pH	7.8	8.0
Conductivity	207 μS/cm (25 oC)	20 μS/cm (25 oC)

With the exception of ammoniacal nitrogen (NH3-N), the reuslts are as expected. The inorganic anions (orthophosphate, nitrite and nitrate) have been reduced to below detectable levels, presumably, along with any associated cations. A 10-fold reduction in conductivity confirms the reduction in inorganic ions.

Surprisingly, ammoniacal nitrogen that was absent in tap water is now present at relatively high concentrations. Where has that come from?

My first and, let's face it, only idea was that it must have come from the laundry detergent (Ecover Wool & Silk Laundry Liquid). The only other laundry product I currently use is an oxygen-based bleach based on sodium percarbonate. Ammonium lauryl sulfate (ALS) seemed a possible candidate as this anionic surfactant is widely used in detergents. However, only sodium lauryl sulfate is mentioned in the list of ingredients. Ethylene diamine tetraacetatic acid (EDTA), usually as the sodium salt, is another common ingredient in detergents where it acts as a water softener by complexing with calcium and magnesium. Although not mentioned in the list of ingredients, I have requested an MSDS from the manufacturers as well as asking the direct question - does your product contain EDTA? I am awaiting their reply.

The Nesslerization test method used to quantify ammoniacal nitrogen (NH3-N) requires the sample solution to be made alkaline (basic) to convert any ammonium ions to free ammonia which then reacts with a tetraiodomercuric salt to form a coloured complex. I have searched for possible interferents such as other amino componuds (e.g. EDTA) but have not found anything, yet. As noted in Table 1, the pH of the distilled tap water has increased to 8.0; i.e. is more alkaline (basic) than the original tap water. The presence of compounds such as ALS and EDTA would make the water more acidic. Ammonia would increase the pH (more basic) of the water though at the concentrations indicated in Table 1, the pH would be expected to be nearer 10, unless something is buffering it at pH 8. The low conductivity of the distilled water suggests not much in the way of impurites.

In the meantime, I thought I'd try a few more experiments. Could I remove the ammonia/ammonium in the distilled tap water by (i) bubbling air through the solution, or (ii) boiling the water. My sample of distilled tap water was taken directly from the tumble dryer water collector tray. Some of the sample was used to rinse all the 'equipment', i.e. a mug and a pan, prior to the experiment. Test 1: the sample was placed in a mug and air bubbled through for 5 minutes using an aquarium air pump on maximum setting (Photo 2).

Photo 2: Sample Degassing with an Air Pump

Test 2: the sample was placed in a small non-stick pan brought to a rolling boil (Video 1) maintained for 5 minutes and then allowed to cool for 2 hours.

Video 1: Sample Degassing by Boiling

Sample analysis results for Test 1 & 2 are summarised in Table 2. Degassing with air (Test 1) has increased the electrical conductivity of the distilled water (lower dissolved oxygen content?)) and reduced its pH (increased carbonation?) but had no effect on the concentration of ammoniacal nitrogen. Boiling the distilled water (Test 2) has reduced the ammoniacal nitrogen content with little or no change in pH and conductivity - loss of ammonium as ammonia? A better experiment would be to repeat Test 2 after increasing the pH (addition of alkali) of the distilled water to 'convert' ammonium to ammonia in order to drive off the ammonia gas more easily. Now where can I get some sodium hydroxide?

Table 2: Analytical Data for Test 1 and Test 2

Analytical Test	Distilled Hereford Tap Water	‘Distilled’ Hereford Tap Water (Air degassed - Test 1)	‘Distilled’ Hereford Tap Water (Boiled - Test 2)
Nitrite	0 ppm	0 ppm	0 ppm
Nitrate	0 ppm	0 ppm	0 ppm
NH3-N	1.53 ppm	1.52 ppm	1.00 ppm
pH	8.3	7.8	8.2
Conductivity	15 μS/cm (25 oC)	91 μS/cm (25 oC)	24 μS/cm (25 oC)

Time to bring this post to an end. The home-distilled water is free of phosphate, nitrate and nitrite and can be used for cleaning and storing the cuvettes used for phosphate analysis. It is also suitable for diluting high phosphate samples prior to analysis in order to bring them within the 0 - 3 ppm range of the Hanna Low Range Phosphate checker. I am also happy to continuing to water my indoor plants with the distilled water but I will not be using it to top up my goldfish tank (Video 2). Ammonia (NH3) is toxic to fish and could present a danger at these concentrations, pH values and temperatures. 

Video 2: Hoover the Goldfish at Feeding Time*

In the third and final instalment, I will look at the economics and environmental impacts of producing distilled water using a tumble dryer.

* Hoover's nickname is Lucky - he is well past the 20 year old mark (actual age unknown), has a bent tail fin and is blind in one eye.