R-Squared Energy Blog

Pure Energy

German Robot Pigs

“What I need for this project”, I explained to my young son, “is an army of German robot pigs.” His eyes went wide. “And I know just where to get them…”

I have a project on my desk right now to look at avenues of disposal for sewage sludge in a specific location. There are lots of things you can do with sewage sludge, but it is obviously dependent on a number of factors. There are places where farmers spread it on their fields, there are places where it is incinerated, and there are places that it is land-filled. And there are other options beyond that.

In order to utilize sludge for energy, the water content is obviously critical. Too much water, and any energy you create from the sludge is lost due to the need to dry the sludge. So what I really needed in this specific case was a method of drying that could help generate net energy from the project.

Obviously you could just spread the sludge out in a sunny location, but there are many problems with that. One good rainstorm can ruin a week’s worth of drying and create a brown river to the nearest waterway. Further, as the outside layers dry, the drying slows down unless the sludge is constantly mixed.

I thought that this particular situation could benefit from a solar dryer. I am familiar with solar kilns and solar ovens, so why not a solar sludge dryer? I envisioned a system like a greenhouse that allows the sludge to dry, but also ventilates and removes the moisture. Several of the functions would need to be automated if this has any hope of being economical.

So I thought “Hmm, I wonder if anyone has invented such a system.” Since about 98% of anything that I ever think of has already been invented, I Googled it. Turns out that there are lots of solar sludge dryers to choose from, but one in particular caught my eye. Enter the German robot pig:

German robot pig the star of sludge drying plants

A group of young, up-and-coming German scientists in the late 1990s founded a company that today is the world leader in solar sludge drying. The star of the environmentally friendly drying process is an unusual pig that works all day without food or complaint.

What would a machine look like if it lived in the mud? Well, probably like a pig. Some 12 years ago, Tilo Conrad, together with two of his fellow students from the University of Hohenheim, built the first electrical pig, pioneering a device that is now being used as a solution to waste disposal problems throughout the world.

The stainless steel pigs, which in a sense resemble big beetles, are an important part of a larger solar drying process was patented by Thermo-System GmbH, a company Conrad founded in 1997 in Filderstadt.

Today, nearly 200 mechanical pigs do what normal pigs do: wallow in and shuffle through the mud — only these pigs do it to reduce sewer sludge disposal costs and protect the environment. Conventional drying processes use non-renewable energy, but Thermo-System’s process harvests solar power to dry the mud. It spreads the wet sewage sludge into sheds that are similar to greenhouses and puts the pigs to work.

In the sheds, the sludge absorbs heat from the solar rays and an innovative ventilation system keeps the air inside the shed warm and dry. The electrical pig, which is a fully automated robot complete with stainless steel mixing tools, tills and aerates the microbiologically active sludge, thereby accelerating the drying process. The whole system is fully automatic, uses very little energy and can be easily maintained.

Alas, another invention that someone thought of 10 years before I did. But this system really has many of the characteristics that I am looking for. Whether the economics pan out is still entirely unclear. The company does have an impressive project portfolio, though, having already built over 100 sludge-drying plants worldwide.

But one doesn’t get a chance to work with an army of German robot pigs every day, so I will continue to investigate and maybe this works out. Besides, that sounds so much more interesting than “I am working on sewage sludge.”


December 20, 2009 - Posted by | solar drying, solar thermal


  1. Robert,Thanks for this post. I figured the company you spoke of was also involved in solar kilns for drying lumber/firewood, etc."With solar-assisted drying, firewood is dried to a consistent, premium quality in one to two weeks. It is thus possible to react flexibly and at short notice to market developments and customer wishes."Normal air dying of firewood might take a year, A two week turn-around time-frame is amazing.I have often thought that a company like Poet might also use solar drying for their corncobs.John

    Comment by Anonymous | December 20, 2009

  2. This is yet another reason why, in this time and place, we need not fear "energy shortages." We have a plethora of options, and man's ability to adapt in incredible.Happily, many of our options are also cleaner options.Man's stupendous productivity always wins.

    Comment by Benny "Boom, No Doom" Cole | December 20, 2009

  3. Yeah Benny? Did it win on Easter Island when the natives produced themselves to death by cutting every last tree and sealing their own fate in the process? I'm sure someone stood there and said technology will save the day when all the trees are gone. Pride comes before the fall…

    Comment by Anonymous | December 20, 2009

  4. “Obviously you could just spread the sludge out in a sunny location ..”Biosolids is another term for sewage sludge. The first problem is it smells terrible.Of course biosolids is an excellent fertilizer. Using it for energy is a waste. It really does not matter how wasteful you are when you are giving up on good soluttions.

    Comment by Kit P | December 20, 2009

  5. Anon:The world today is connected by Internet. Unlike the Easter Island days, good ideas are transmitted globally and instantly. And unlike Easter Island, we have the price mechanism. The wonder of the price mechanism is that it both reduces energy demand and creates more energy production, when needed.To wit, right now we have gluts in all fossil fuels, especially natural gas, but including crude oil. We are almost upon 2010–yet another year in which some terrible global calamity is assured due to oil shortages. Instead we have gluts, that an oil cartel –OPEC– is attempting to manipulate out of existence. In fact, I worry about USA oil shortages–due to thug oil states. There is no assurance that oil thug states will continue to supply oil, and they control the vast bulk of the world's oil supply. That is why I advocate a migration of our auto/truck fleets to CNG and PHEV, using the price mechanism to push things along, As in $4 a gallon gasoline taxes, phased in at a rate of $1 per year (25 cents per quarter). If OPEC overplays its hand, this will happen anyway. Easter Island has lessons, but let's not get doomy about them. It was an island ecosystem, and evidently they had a religious belief in rolling big stone heads around on logs (and I assume burning the logs and building houses out of them). Yes, not a good game plan.And as to my larger statement, "Man's stupendous productivity always wins"–well here we are, communicating by Internet, while the shortcomings of Easter Island were relegated to Easter Island. We have already triumphed over a wood-based fuel system. We have nukes, wind, solar, coal, natural gas, geothermal, hydro power and even oil-fired power plants. I guess we will have biomass power pants soon. I count nine new energy sources from the wood days. Like the wood-as-fuel days, I think we will easily surmount the Oil Age–indeed, I think it is possible we will migrate away from oil long before "we "run out" of oil. The PHEV is already close to commercialization.Imagine spending 2 cents a mile. Imagine an e-bike. If costs comes down on PHEVs–and they are looking at mass production in China–then who will want a simple ICE?The e-cars are said to accelerate like crazy, easily blow the doors off of an ICE. With an onboard generator, you have hundreds of miles in range.What a world. Wish I could live another 100 years.

    Comment by Benny "Boom, No Doom" Cole | December 21, 2009

  6. Some might say it smells like Sh*t8) You do get use to it and it doesn't stink the whole season. It would be awful if the Nimby philosophy extended to Farms. The Pig reminds me of a cross between a Roomba and a RotoTiller.When I saw this post earlier today I thought I wouldn't hear any Troll like comments. There is your proof nothing is Troll Proof.

    Comment by takchess | December 21, 2009

  7. I'm surprised Rufus doesn't have one of these already.

    Comment by Wendell Mercantile | December 21, 2009

  8. Pigs is fer eatin.

    Comment by rufus | December 21, 2009

  9. Why not just run it through an anaerobic digester?More, and more Dairies are doing that.

    Comment by rufus | December 21, 2009

  10. Perhaps he's waiting for the E85 version.

    Comment by PeteS | December 21, 2009

  11. "Why not just run it through an anaerobic digester?"That is certainly what many municipalities have been doing for decades to reduce the volume of sludge — some making use of by-product methane.One issue with using digested sludge as a soil amendment has been the potential presence of heavy metals (depending on the source of the sludge). Could that issue have air quality implications if the intent is to burn the dried sludge?

    Comment by Kinuachdrach | December 21, 2009

  12. "Of course biosolids is an excellent fertilizer. Using it for energy is a waste. It really does not matter how wasteful you are when you are giving up on good soluttions."I think RR already mentioned that different options are apt for different locations. Not all sludge production is close to sufficient agricultural land, or sufficiently non-toxic even after treatment. The primary purpose of incineration is not energy generation, but sterilisation and reduction of bulk for disposal. Energy recovery just makes the process more efficient.

    Comment by PeteS | December 21, 2009

  13. Robert said: "Since about 98% of anything that I ever think of has already been invented"This happens to me all the time. I start thinking, "I wonder if this or that would be a good idea", then find out someone is wayyy ahead of me somewhere on the internet.Makes sense, really. I mean, its not like I work in cutting edge R&D so it would make sense that others working in those fields would have already thought of it, if its a logical idea.Andy

    Comment by Andytk | December 21, 2009

  14. “I think RR already mentioned that different options..”Energy recovery is the worst option. Using biosolids for energy is just another example of lazy big city managers making bad environmental choices and calling it green to be politically correct.“Energy recovery just makes the process more efficient.”Very true but if you extend the boundaries of the energy balance to fertilizer nutrients (N. P, K) production then burning biosolids is not more efficient.

    Comment by Kit P | December 21, 2009

  15. Energy recovery is the worst option.The world is too complex to make such a blanket statement, Kit. The options are entirely dependent upon specific conditions that are in place. Your comment really means “There are better things that could be done with it.” Well, I could say the same thing about corn ethanol. We are using it because of a system of regulations, incentives, and mandates that we have in place. The options for sludge will be dependent on the same sort of issues.Using biosolids for energy is just another example of lazy big city managers making bad environmental choices and calling it green to be politically correct.There are so many counters-examples to that. As noted, there are sludge plants with high levels of things you don’t want to spread on the fields. By incinerating, you can recover or destroy those things.RR

    Comment by Robert Rapier | December 21, 2009

  16. RR Do you know what a WWTP is? Do you know what biosolids are? Have you been to a WWTP?What do you think are the penalties for dumping hazardous waste down the sewer? What do you think are the penalties disposing of hazardous waste on fields? What do you think will be the outrage if you suggesting putting a hazardous waste incinerator in a city?So yes, RR there are all kinds of sludges that are not called biosolids. If you have a sludge from a pulp mill and it has a high concentration of dioxin, I would not have an objection to recovering the energy.There is a worse option than energy recovery which requires becoming knowledgeable of RCRA and CERCLA regulations. One innovative PNW ended up with ash from their 'renewable energy' facility being disposed of as hazardous waste. Ouch! Very expensive.

    Comment by Kit P | December 21, 2009

  17. Do you know what a WWTP is? Do you know what biosolids are? Have you been to a WWTP?LOL! Do you think you can straw man me to death? I will bet you that I have spent more time around WWT than you have. In fact, my thesis is on sewage sludge and municipal solid waste conversion, and it wasn't a desk exercise. RR

    Comment by Robert Rapier | December 21, 2009

  18. So yes RR you know the difference and you misspoke before. Please go back and read what you wrote for technical accuracy. I would interested in what you know about MSW (or other paper/plant waste) being added to a dairy farm AD to increase methane and organic fertilizer production. WWTP operators are trying to get rid of nitrogen with as little energy as possible. I maintain the goal should be to preserve the nutrients to grow plants.

    Comment by Kit P | December 22, 2009

Sorry, the comment form is closed at this time.

%d bloggers like this: