Flow state

Where does our tap water come from, and where is it going? (9 minute read)

More than 2700 years ago, the ancient city of Nineveh was the capital of the Assyrian Empire, which stretched from the east of modern-day Iraq to the Mediterranean Sea. The city had a population of more than 30,000, covered 700 hectares, was surrounded by a wall with fifteen gates, and was crowned by a magnificent palace. What really set it apart from other ancient cities, though, was its water supply. Nineveh was supplied with abundant water from hills at least forty kilometres away. Today, the remains of one of the aqueducts which supplied the city can still be seen at Jerwan, north of Mosul in Iraq. It’s not a place I ever plan to visit, but I’m still impressed that it's there.

The Assyrians were the among the first to build aqueducts, providing a reliable supply of water from a source far away. Five centuries later, the ancient Romans would master the skill, and supply water to a city of half a million people. It wasn’t only drinking water that the Roman aqueducts supplied. The Romans were obsessed with bathing, and aqueducts supplied the many public baths in Rome and other cities. At around the same time, the city of Chang’an, in Han Dynasty China, was supplied with water from a large reservoir. The Nabateans, in modern day Jordan, grew crops in the desert and supplied water to Petra from multiple sources. Later, the Aztecs supplied water to Tenochtitlan and the Khmer supplied water to cities on the Angkor Plains.

Baths of Caracalla, Rome (image credit: Melanie Newfield)

However, the Persians may have surpassed them all. Around 2500 years ago, they began constructing underground tunnels called qanats which tapped and transported groundwater. What makes these tunnels remarkable is their durability. Rome’s aqueducts are now mostly ancient ruins. Many of Persia’s qanats are still in use today. (Sometimes, the things I learn writing this newsletter truly blow my mind. Learning about Persia’s qanats certainly did that.)

The quality of water supplied by these ancient systems varied and depended on a number of different factors. First, it depended on the quality of the source. Some of ancient Rome’s aqueducts supplied water from lakes and rivers, which could easily be contaminated. On the other hand, since Persia’s qanats tapped groundwater, the source water was probably safe. Secondly, there was a risk that the water would be contaminated in transit. At least some parts of the aqueduct which supplied Nineveh were open, so even if the source water was pure, it might not be when it reached the city. Ancient Rome used lead, a toxic metal, for many of its pipes. However, this may not have been quite the disaster it appears. The Romans did recognise the toxicity of lead, and the danger of lead pipes. However, many of the lead pipes carried water with a high mineral content, and the minerals were deposited inside the pipes, creating a barrier which prevented lead from getting into the water. While there is evidence that the use of lead pipes did increase the amount of lead in the water, it’s unclear whether it reached dangerous levels.

Although ancient people understood that some sources of water were better than others, there is relatively little evidence suggesting that they had much knowledge about treating water to make it safe. The bible is largely silent on the subject, for example, and the Egyptians believed that bronze could purify water. The exception seems to be the ancient Indians, who recorded water purification by boiling, and filtering through charcoal, sand or gravel in texts which may be 4000 years old. Around 2000 years ago, the Egyptians had some understanding of filtration, but in Rome, the public water supply was largely treated by simply keeping the water in settling tanks, which allowed for more visible contamination to fall to the bottom, but didn’t necessarily deal with disease-causing microbes. Because there was a well-developed water supply infrastructure in China at around the same period, they may have also had some methods of treatment, but my access to information is limitedby my ability to read only what is available in English.

Until the germ theory of disease, people didn’t necessarily understand why some water was better than other water. However, they recognised that some water tasted or smelled bad, and they didn’t like it if their water was muddy or full of insects. By the 18^th and 19^th centuries, the use of filters (usually sand), boiling and chemical treatments with acids such as oil of vitriol (sulfuric acid) were known in Europe. Paisley in Scotland was perhaps the first place to filter an entire town’s water supply in the early 1800s. However, the motivation was not to make the water safe to drink, but to render muddy water more suitable for use in bleaching cotton.

Late in the 19^th century, when people finally understood the connection between water, microbes and disease, chlorine was first used to treat water. Because chlorine was difficult to handle, its use was limited at first, but advances in transportation and storage led to its use increasing over the 20^th century. It is now the cornerstone of water treatment systems around the world. However, the water which comes out of our taps has undergone a range of treatments, not just chlorine.

To describe a modern water treatment system, I’ve used the example of the Te Mārua treatment plant, in Wellington. This treatment plant supplies 40% of Wellington’s water, and it’s the one which supplies water to my house, so it seemed as good an example as any. The water begins in a relatively remote area of forest in the south of the Tararua Ranges, imaginatively known as the Hutt Water Collection Area. Although access into the area is permitted, there’s little in the way of marked tracks and no huts (overnight stays are not allowed). The water is drawn from the Hutt River at Kaitoke, just south of the water collection area, where it is filtered through sand and strained to remove material such as leaves and twigs, then piped to the treatment plant.

The water drawn from the Hutt River is ‘soft’, which means it is low in chemicals such as calcium and iron, and usually slightly acidic. Soft water can be corrosive to pipes (as can very ‘hard’ water, although for different reasons), and so carbon dioxide and lime are added to make it less corrosive. Further chemicals are added to react with small particles in the water – these chemicals cause the particles to form clumps, which are then settled and filtered out. The process is known as flocculation (excuse the big word, but I really like it) and I’ve linked to a video showing the process here. Once the water has been filtered, chlorine is added to disinfect the water, along with fluroride for dental protection. Another chemical, sodium hydroxide, is also added to make the water less acidic and protect pipes against corrosion.

All of this sounds less appealing than pure water from a remote mountain stream, but it meets New Zealand’s standards for drinking water. It’s also worth noting that it’s very low in nitrates – a good deal lower than some brands of bottled water. The problem with Wellington’s water is not its quality. It’s not the quantity either – our abundant rainfall means that we have more than enough, or we should have.

The problem with Wellington’s water supply is what happens between the treatment plants and our taps. That precious, clean, safe water flows into the pipes and then escapes – leaking into the ground or bubbling up like springs through the road and washing away to the sea. At least 40% of the water which enters the system is wasted in this way, a staggering 67 million litres per day. Newspapers usually describe this volume as ‘Olympic-sized swimming pools’, reporting that Wellington loses 30 Olympic-sized swimming pools per day (the figure of 27 swimming pools also appears sometimes, but that was last year’s estimate).

Since Te Mārua supplies 40% of Wellington’s water, this means that, in effect, the entire output is wasted. This figure, though, is approximate, because there’s currently no way to measure it. Wellington’s water company knows how much water it supplies, but there are no household water meters, unlike Auckland and Christchurch.

However, those cities have no reason for complacency. Christchurch loses 27% of its water supply to leaky pipes (38 million litres or 15 swimming pools a day). Auckland is losing only around 13%, but that is 50 million litres or 20 swimming pools.

The problem is mostly with our pipe network. Decades of spending money on other things means that pipes are long past due for replacement, with some of Wellington's pipes up to 100 years old (unfortunately, our water network wasn’t built with the durability of Persia’s qanats). Pipes which supply water are much more vulnerable to leaks than wastewater and stormwater pipes, because the water is under pressure – if it wasn’t, it wouldn’t flow up through the pipes and out of the tap.

This water pressure is important for more than just ensuring we can have a decent shower. Pressure is the reason that the water which comes out of our taps is still safe to drink even though there are 2700 known leaks in Wellington. The pressure pushes clean water out of the crack or gap in the pipe, so dirty water can’t get in.

Since in New Zealand we have enough water to keep a constant supply to everyone (even if there are sometimes restrictions), the leaks aren’t a risk to our health. But that works only as long as there is pressure in the pipes.

In some cities, such as Nairobi, there simply isn’t enough water to keep water in all the pipes all the time. Water supply companies then resort to what is known as ‘intermittent supply’. In Nairobi, this means that most places in the city only have water flowing one day a week. There’s a detailed timetable explaining which parts of the city get water on which days and at which times. Nairobi is far from alone. Around a third of the world’s population has water from an intermittent supply. This doesn’t mean they can’t use their taps – many homes have water tanks which fill automatically when the water is flowing – but it does create other problems.

Without the constant pressure of water in the pipes, dirty water can potentially get into any crack or hole. It doesn’t matter if the water is treated at the treatment plant – by the time it reaches the tap it won’t be safe any more. There’s a clear difference in the levels of disease-causing microbes in water depending on whether the supply of water is continuous or intermittent. That is why there are warnings about drinking tap water in many countries – even if it has been treated, it may have been contaminated on its way to the tap.

Wellington is a long way from resorting to intermittent supply, but it’s a reminder of what we could face in future years if we don’t fix our antiquated pipes and we continue to waste water.