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Archive for May 2016

Standing on top of the eggs (a visit to the Hamburg wastewater treatment plant)

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Sewage, like water, flows downhill.  So when sewage arrives at a treatment plant, it needs to be pumped up by a considerable height.  The sprawl of our cities and the quantity of sewage we produce explain in part why water and wastewater treatment consumes about five percent of all the energy produced in the western world.

Of course, consuming energy means altering the climate, and any reductions matter.  That’s why I was so impressed to discover that the Hamburg wastewater treatment plant is unique: the only city-sized one that produces more energy than it consumes.  How does it do it?

Lueder Garleff may be the best person to explain this remarkable performance.  He’s an engineer with the energy management team of Hamburg Wasser, the plant operator, and he has been actively involved in the re-invention of the treatment plant as an energy producer.  I was thrilled to find out that he willing not only to answer my questions, but that he could take me on a private tour.

Grey skies over the harbour in November, from the top of the eggs.

Grey skies over the harbour in November, from the top of the eggs.

The treatment plant is located in the harbourfront, on the south bank of the Elbe river.  The plant is a well-known landmark to the locals, because of the nine large egg-shaped methane digesters.  I took the river ferry from Landungsbrücken; the ferry is an integral part of a smooth, seamless transit system.  The upper deck of these ferries may be the best vantage point from where to discover Hamburg: the ultramodern cranes and containers that make up the skyline on the south shore echoing the ancient church spires of downtown, the cool grey of the November sky reflected in the deep grey of the Elbe, driving home the message that Hamburg, first and foremost, a harbour city.  Mr Garleff met me at the wharf.

The deflecting vanes on the centrifuge

The deflecting vanes on the centrifuge

No-nonsense engineer though he may be, Garleff had a twinkle in his eye as he was describing the plant’s innovations to me.  He is particularly proud of a small modification he designed: a vane that is fitted on to the centrifuge, so that the water ejected contributes to the spin of the centrifuge instead of fighting it.  “This simple adjustment, which costs next to nothing, has saved us 1600 megawatt-hours per year”, he said.  (That’s the amount of energy used by about 60 homes in Canada, to put it into perspective.)  This, in essence, may be the story of the plant’s success: no detail is too small to be ignored.

To get to the whole story, though, it may be useful to review how a sewage plant works.  In a nutshell, impurities are removed from sewage, producing a clean(er) water, as well as a watery sludge that needs to be managed; this is where the centrifuge comes in.  Its role is to dewater the sludge as much as possible, to minimize the residual volume that needs to be handled.  The dewatered sludge is then often spread on land as a soil amendment or fertilizer.  Before it is centrifuged, though, the sludge is digested, that is, it is put into a digester, a sealed vessel where fermentation produces biogas, a mixture of methane and carbon dioxide.  Methane is the main ingredient of natural gas; most treatment plants capture the biogas and use it to produce heat and electrical power, making up for part of the huge amount of power needed to run the plant.  (In Hamburg, the digesters are the famous landmark “eggs”; I was lucky enough to get a unique view of the city from the top of them.)

The energy produced (in blue) recently overtaking the diminsihing energy required (in red)

The energy produced (in blue) recently overtaking the diminsihing energy required (in red)

For most treatment plants, like Vancouver’s, that is the end of the story.  Sure, most look for energy economies with more efficient pumps or better diffusers.  But Hamburg’s has added three extra elements: an incinerator, a set of three windmills, and a biogas upgrader.

The incinerator has been part of the plant for over a decade.  The city decided on using an incinerator to burn the sludge because of the expensive trucking costs for disposal.  There was also a concern that the sludge could contain pollutants (such as heavy metals or pharmaceuticals) and that land disposal may cease to be an option, a prediction that has since become true.  Incinerating the sludge produces a only small amount of ash, which is disposed of in a dedicated landfill.

Of course, an incinerator also produces energy, in the form of steam and electricity.  Most of the heat is used to dry the sludge to make it easy to burn, but the leftover is used for heating the methane digesters.  But before then, the high pressure steam runs a turbine that generates electric power, supplementing the production from the three large windmills.

HH wwtp energy

The energy balance of the plant.

Electricity and heat is also created by the biogas from the digesters.  But this is where the upgrader comes into play: it is used to turn the biogas into commercial-grade natural gas, which can be fed into the natural gas grid.  This gives the operation a lot of flexibility, which is the secret of why the plant is a net energy producer.  Often, burning biogas in order to generate electricity produces more heat than can be used by the plant, so the energy embodied in this extra heat is wasted.  This is where the windmills come: when they generate electricity (and there’s a lot of wind in Hamburg), the biogas is upgraded and sold to the grid, instead of being burnt on site.   This works well because the incinerator usually generates enough heat for the plant.  The system works beautifully, and the result is net energy production.  None of this would be possible without the se three elements working together as a system.

One of the plant's three windmills in the background, with the incinerator on the left.

One of the plant’s three windmills in the background, with the incinerator on the left.

Could a system like this be copied elsewhere, say, in Vancouver?  Sure, within reason.  Is it affordable?  The answer may be the most remarkable of all: this was fully funded within the budget of the plant in Hamburg; there were no state or municipal subsidies.  Sure, the fact that energy is more expensive in Europe helps balancing the books when one sells net energy; nonetheless, this is remarkable.  The better question, then, may be: can we afford not to? Hamburg’s experience shows that such an investment is profitable, within the confines of the operating budget of the plant; and Mr Garleff told me that one key advantage is fiscal predictability; the plant operators no longer worry about spikes in energy costs on their budget.  But ultimately, it’s what isn’t in the budget sheets that matters: the plant, by being anet energy producer, is a net asset in the fight against climate change, because of all the carbon that is no longer emitted as a result of its operation.  This is what we can no longer afford not to do.

But, true to form, Garleff and his colleagues are not resting on their laurels.  They are now looking at ways to remove phosphorus from the incinerator ash.  Phosphorus is an essential fertilizer for agriculture, but there is only a finite amount of it that can be dug up in mines.  Once that runs out, we may be in deep trouble.  Unless, like the good folks at the Hamburg wastewater treatment plant, we find ways to turn our waste into a resource.  But the solutions exist; under this grey, leaden November sky, I couldn’t help but be totally optimistic.

Read more about the Hamburg wastewater treatment plant here, here, here and here.

By the numbers:

  • 160 million cubic meters of sewage, from 3 million people, treated per year
  • Among the sewage contaminants that must be treated every day are 380 tonnes of COD, 30 tonnes of nitrogen, and 4 tonnes of phosphorus.  Removal efficiencies are 94% COD, 99% BOD, 81% N, and 93% P.  Cod and BOD are removed through conventional aeration process, nitrogen by nitrification/denitrification, and phosphorus by chemical precipitation.
  • From this, 1.5 million m3 of digested sludge is produced, down to 100,000 m3 dried sludge, which after incinerator represents 20,000 tonnes of ash, inclusing 3000 tonnes of gypsum and 300 tonnes of heavy metals.
  • The sludge digesters run at 36C and produce 36 million m3 biogas per year.
  • The replacement of the surface aerators with fine bubble diffusers, 5MW total, saves 18,000 MWhr/yr, and prevents the emission of 9500 tonnes of CO2 per year. Three wind turbines contribute 24000 MWhr/yr.