Creating a stink

We need to talk about sewage (10 minute read)

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In 1879, prominent Wellington resident John Plimmer was creating a stink. Wellington’s city councillors were considering a loan to fund a sewerage system for the city. Plimmer deemed the idea "sentimental nonsense". Disease, he argued, emanated from stagnant water and rotting vegetation, harking back to the disproven “miasma” theory of disease. All Wellington needed to solve its sanitary problems was a clean supply of water to the city, better surface drainage and more cleanliness at home.

Plimmer’s view prevailed. Disposal of waste remained largely a matter of personal choice – or what people could afford. The wealthy had water closets, which flushed waste directly into drains and then into nearby streams. The less well-off paid a night-soil collector to take their waste away to dump it in a nearby valley. Those who couldn’t afford to pay the night soil collector dumped their waste wherever they could. Waste from Wellington’s residents continued to flow from the city, to local streams and into the harbour, entrenching the city’s reputation for unsanitary conditions. Hundreds more people died of diseases such as typhoid before a sewerage system was finally completed in 1899.

The new system was an improvement, but only just. Sewage flowed through pipes under the city and to the south coast at Moa Point, where it flowed into Cook Strait. The strong ocean currents dispersed the waste, and Wellingtonians proceeded to forget about the problem for nearly a century. It was only in the 1980s that the raw sewage was screened to remove solid material, and in the 1990s that there was any secondary treatment (more on what that means later).

Wherever groups of people gather together, the distasteful problem of human waste arises. Ancient toilet habits are difficult to study, but usual practices included collecting waste for use as fertiliser, burying it in pits or washing it into a nearby waterway. As well as recognising its value for growing crops, people found other uses for waste too. In Ancient Rome, pots for collecting urine were placed around the city, so it could be used for bleaching clothes.

Humans and other animals produce two types of waste, urine and faeces. Among mammals, the obvious difference between the two is that one is liquid while the other is (usually) solid, but there are other important differences that are useful for understanding the problems of waste disposal. Urine is mostly water, with various compounds dissolved in it, particularly urea, which is high in nitrogen. It used to be considered sterile, but it actually contains a range of bacteria even in healthy people. Faeces also contain a lot of water (around 75%), but the remainder is largely organic matter, such as indigestible plant matter. And faeces are absolutely teeming with bacteria and viruses. There are billions and billions of them, including the well-known E. coli, which I wrote about a couple of weeks ago.

After that unpleasant discussion, here’s a near-sunset from Antarctica (image credit: Melanie Newfield)

These billions of bacteria and viruses are the main reason sewage treatment is important. While most are harmless, some of them can cause disease. Polio, typhoid, cholera, giardia and campylobacter are just a few of the diseases which can be spread by faeces. Urine is much less of a problem for disease transmission, but some diseases, such as typhoid, can be spread by urine.

But there are other reasons to worry about sewage. Most obvious is the smell. If you’ve ever used an old-style long-drop you’ll appreciate the way that modern plumbing largely keeps the odours at bay. If you lived on Auckland’s North Shore during the 1980s and 1990s, you’ll also remember how malodorous the Rosedale wastewater treatment plant was at the time. Our waste creates an awful stink.

Much less obvious is the fact that both urine and faeces are packed with nutrients. They aren’t nutrients which are much good to us directly, but they’re very good for growing plants, algae and bacteria such as cyanobacteria. Urea, for example, can be used directly as a fertiliser. But in waterways, and to a lesser extent on land, excess nutrients can cause problems. I wrote about some of these problems two weeks ago when I reviewed the main problems with New Zealand’s waterways, and last year when I looked at nitrogen fertilisers. The short version of the story is that excess nutrients cause excessive growth of plants, algae and cyanobacteria. The excessive growth can end up sucking all the oxygen in the water, killing aquatic animals like fish and encouraging the growth of bacteria which cause botulism in birds. Cyanobacteria can poison livestock which drink the water and dogs which eat mats which wash up on river banks. Altogether, they can turn waterways, especially lakes, into a foul, toxic soup.

When Wellingtonians began pumping their sewage out to Moa Point, they were relying on the diluting effect of the ocean to deal with the smell, the nutrients and the disease-causing microbes. Since then, we’ve developed better ways of dealing with our sewage, but we’ve still got some distance to go if we want to fix the problems we create with our waste.

There are a number of different ways to treat sewage, but a good modern plant uses three stages – primary, secondary and tertiary treatment. The primary stage is about removing solid material. It uses a mixture of screens and settling ponds or tanks. The solid material includes things which should never have been in the sewage in the first place, such as wet wipes, as well as some of the “suspended solids”, such as biological material from faeces.

Apart from removing obvious rubbish, which ends up in a landfill, primary treatment separates the sewage into a solid component, sludge, and a liquid component. These are treated differently at the secondary treatment stage. The liquid component goes to large biological reactors, where bacteria further decompose the waste. These reactors have areas which are high in oxygen and areas where there is no oxygen, and the bacteria in each area are different. In the area with no oxygen, there are bacteria which convert nitrates to nitrogen gas, which can then escape harmlessly into the atmosphere. This is one crucial way that nutrients in sewage are removed. The solid component of the sewage is also decomposed by bacteria to produce a material rather like compost. If this decomposition is done without oxygen, it produces methane, which can either be a problem, if it is released into the atmosphere, or an asset, such as when it is burned to provide power for the sewage treatment plant.

Disease-causing microbes can still survive in the water which comes out of the secondary treatment, so tertiary treatment is used to kill them. Although chemicals like chlorine can be used, many treatment plants these days use ultraviolet light, which is both highly effective and doesn’t release chemicals into the environment. (For an overview of the treatment process, there’s an excellent video from Auckland Watercare. The video explains the process clearly, even though the tone is rather self-congratulatory).

A sewage treatment plant such as Māngere in Auckland can achieve a lot. The 2012 report on water from the Parliamentary Commissioner for the Environment praised the treatment plant, noting that shellfish from the Manukau Estuary were once again safe to eat. The same plant gets nitrate levels much lower than the limits set by the council. In fact, they are at the lower level for groundwater in New Zealand (at least according to the report on the Auckland Watercare website, which covers only 2014/15).

It’s a little-known fact that sewage treatment plants in Antarctica need to be indoors, because otherwise the waste would freeze. This is the plant at McMurdo Station (image credit: Melanie Newfield)

The Māngere plant is a best-case scenario, as far as I can tell, with an effective system for reducing nutrient levels, apart from the fact that it discharges treated water into the sea. I’m not entirely comfortable with wastewater going into the sea, but at least the water from Māngere has been well treated. However, managing sewage is not just about the quality of the treatment plant. The sewage needs to get to the plant in the first place, and there Auckland has a problem.

Auckland is a city of beaches, and Aucklanders love to head to the water in warm weather. However, it’s common for beaches right around the city to be unsafe due to sewage contamination. Over the last summer, I found numerous examples of Auckland beaches being closed, and not just those inside harbours, but also those out on the Hauraki Gulf such as Long Bay.

The problem is not the treatment plants but the state of the pipes. Occasional problems due to blocked pipes are inevitable, but a scan of recent news shows the problem is much bigger than that. The problem stems from stormwater, which is rain running off from the land, into pipes and streams and then straight into the sea, without treatment. There isn’t supposed to be anything harmful in stormwater, but it can, and frequently does, become contaminated with sewage. The problem is most common after heavy rain, and is only going to get worse under climate change, with wet days becoming wetter.

How does the rest of the country compare? My searches indicate that Auckland has the most frequently reported problems, but Wellington often has contaminated beaches too. In Wellington, the problem isn’t only the pipes – there have also been frequent issues with the sewage treatment plants as well.

Auckland and Wellington discharge their treated (or in Wellington’s case, sometimes untreated) sewage into the sea. However, of the over 320 plants around the country, 40% of New Zealand’s wastewater plants discharge directly into freshwater environments. And the different treatment plants aren’t all to the same standard. Less than a third of those which discharge into freshwater are reported to have primary, secondary and tertiary treatment and 16% have only primary treatment.

Discharge onto land is usually considered the best practice for wastewater. Nutrients that are harmful in waterways can be used to fertilise crops such as grass grown for hay and silage. This kind of approach is used by about a third of our wastewater treatment plants. However, 20% of these don’t do any more than primary treatment of the sewage, which means that there is potentially still some risk of disease-causing microbes and nitrates entering waterways.

We also can’t forget that more than 20% of New Zealanders aren’t connected to a council sewerage system. In some parts of New Zealand, such as Northland, the figure is greater than 50%. As with council systems, the quality of home sewage treatment has improved over the years, from the days of the “long-drop” to basic septic tanks and more advanced systems. But, as with council schemes, standards vary. Examples of contamination from septic tanks don’t seem to make the media very often, probably because they are on a smaller scale. However, I did find an article indicating that septic tanks at Muriwai Beach were contaminating a local stream with nitrates. (In case you are wondering, I have no idea why the article I’ve linked to appears to be sponsored by Dairy NZ, I found no other examples).

As far as I can tell, problems with the treatment of human sewage mostly come down to money. Pipe networks can be upgraded. Treatment plants and even advanced home systems can get nitrates and disease-causing microbes down to low levels. There are ways to dispose of treated wastewater so it doesn’t pollute the environment. The problems with human waste are expensive, but they aren’t intractable.

Humans, of course, are not the only animals producing waste in New Zealand. And this is where things get more difficult. We have 10 million cattle and 27 million sheep, and they don’t use toilets, so their waste is spread out over the land. The sheer volume of waste they produce is staggering. Managing their waste is a very different problem, so I will return to that topic in a couple of weeks.

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