Urban Metabolism: filling gaps in analysis

Urban metabolism, Ecological Footprints and Material Flow Analysis

Town planning is often a collection of disparate disciplines which are grouped together as much by the expedient of cost, as for any recognition that each section is actually related to the other.

For instance water supply and waste water may or may not be under the same management in a town or city. Sometimes they are even given out to entirely separate organisations by sub-contract. If you haven't thought they are related then at the very least there is a cause and effect relationship as every time you turn on a tap almost all of that water will end up nearly directly into waste water. Especially if you're:

• having a shower
• washing your hands
• doing the dishes (even in a dish washer)
• flusing your toilet (yes, it fills up from the tap)
• preparing or cooking food some pasta or noodles for instance

If you have trouble imagining this, put the plug into your sink, don't flush your toilet and block up your shower hole for a day. Imagine how it would be after a week. I think you can quickly see how important these things are.

Looking around in nature you'll find animals and plants all living together. Forests don't look dirty even though there's no taps, no one takes away the garbage or has a toilet. Mainly this is because the population density of the creatures which live in the forest is within the 'carrying capacity' of the environment. Carrying capacity is part of the method called "Ecological foot printing" which works out how much land is needed to provide for and assimilate the waste products of a city (or the urban footprint). Its a really interesting and revealing look out how much space our city really takes up.

We may think that our city sprawls over some dozens of square kilometers and we may be concerned about the impact of that on the environment. Wackernagel & Rees however in their book "Our Ecological Footprint: Reducing Human Impact on the Earth" show us that it extends to require much more than we actually see.

Knowing and understanding this is important in grasping our influence on the environment, but it doesn't really help us find ways to reduce it ... other than just saying "gosh, maybe we should use less"

What has this got to do with metabolism?

Consider for a moment two things one is your own body and the other is a large group of single cell algae living in a lake (say, here in Finland), both humans and algae are independent organisms which are alive. People however are made up of many different cells, while the algae are just clusters of exactly the same cell.

But the density of cells that makes up your body is enormous compared to what we would consider to be the most dense algal bloom in the lake.

In the small area of your body are millions of cells all living in just the same manner as the bunch of Algae on the lake. The algae could not ever hope to have the same density of population (a measure of success in some ways) because they have individual metabolisms confined to their own cells, your body on the other hand has an integrated metabolism which exists over and above the metabolism of any given cell in your body.

Key in this is that you have a circulatory system to carry nutrients and oxygen to the most inner parts of your body, and also to carry out waste.

Since the Algae do not have this they would start to die in their own waste products If one single system (such as your kidneys) were to stop working. I'm sure you would find your own body producing wastes well above what the cells can tolerate. If you've ever experience a power outage in your city, or had some water break down in your building you see pretty quickly that the same things seem to apply there too.

However it is exactly how our early cities, in medieval periods were built and operated. The European cities were quit filthy, often riddled with disease and the stink of garbage because there were no utilities such as waste disposal. In fact its not hard to imagine what it would be like to live in this manner now, as all you have to do is visit any large city in the developing world where budget is not spent on this sort of infrastructure.

So it is not just a loose metaphor to compare the metabolism of a city to that of your own body. At a different scale a city is indeed a collection of cells, each household can be thought of as a cell. Each apartment in a block, each home, each building is connected together by a network of pipes and wires which bring in water, energy, communication and dispose of wastes generated by those cells.

Without that we would be living in the wastes we produce ... sort of like these places.

Urban metabolism vs Material Flow

Material Flow analysis is a tool for understanding and tracking the flow of stuff and energy into and out of an ecosystem / city / environment. This is handy stuff, but in many ways its just inventory. It can be surprising to learn just how much and of what goes in and out.

Consider a basic analysis of water in the house, some 140 liters of water is needed by each person per day in the house, and you then need to dispose of about the same amount (yes, we keep very little of what water we put into our mouths). A curious researcher may extend this to wonder about energy, and wonder how much energy is used by our community. What Materials Flow doesn't provide is a basis for logically relating these informations.

For example if we consider the metabolism of water But it doesn't just end there, getting water to you requires energy, as does removing waste water. Processing waste water is even more costly than the process of carting it away. In fact in a typical urban situation the cost of waste water removal and processing is about 75% of the costs.

Yes that's right. To give a concrete example from a large Australian east coast city in 2005:

• disposal of sewage accounted for 78% of the electricity usage of
• while supply and treatment only required 22%.

With only 67KWh consumed per property in water supply yet 255KWh per property in waste water treatment per year.
Part of the reason for this is the linear flow of water, and part of it is related to our historical use of gravity to supply water, thus assisting the costs. Dams are a major supply of water in countries like Australia.

This may be different in countries like Finland which rely more on ground water (which must be pumped up from the ground). Perhaps there the water supply costs would be higher as gravity is not really being employed to assist.

Either way it follows the pattern in the figure to the left of drawing water from the source, treating it and providing it for use. Then after use waste water is treated (thank god we've started doing that, many places still dispose of sewage raw) and then disposed of into the environment again.



Putting a metabolism spin onto this view, lets consider the city as something like a tree. A collection of cells, depending on water from the roots and a system of pipes (xylem) to transport the water to where its needed like the leaves (households and properties).

In this way we get a model rather like this one on the left.

It seems similar until you realize that plants go to great lengths to stop water loss at the leaves. They have mechanisms to prevent it (such as closing the stomata). As well water is reused in their metabolism as a normal practice. Plants are actually so effective with their metabolism of water that if they have CAM metabolism they can get by on almost no water inputs at all.

This is where making using a model for analysis which makes comparisons to natural systems brings its other advantages over simple scientific analysis. It facilitates comparison with already working and proven successful systems.

Putting metabolism to use

Could we reuse water?

Sure, what would stop you from saving the shower water to water your plants (why put good high quality treated drinking water onto the plants), or your could save it to use to flush your toilet.

If you did more than one load of washing at a time then rinse water could be reused to mix with detergent in the next wash cycle ...

all this stuff has been and is done by people who live in places where water is in short supply. Heck if you've ever stayed in a cottage or gone camping where you have to draw water from a well I'm sure you've learned to be frugal with it.

As well municipalities are now starting to treat waste water to higher qualities and using it themselves and distributing it to other users for non-drinking purposes (like watering local parks / golf courses / washing roads and dust control on dirt roads.

Metabolically on site water reuse is different to the reuse of water collected by the town and redistributed.

Water reused on site like the washing water requires far less energy, remember the costs of waste water disposal above. But rather than simply dispose of it we can still put it to use.

I've represented this in the diagram to the left.

This not only makes greater use of the water resources we have, but reduces greatly the impacts on the environment of disposing of the wastes.

Following practices like this we can transform our cities into much cleaner places for us just like our kidneys transform the concentrations of waste products in our blood increasing our bodys ability to tolerate the dense cellular population.