DELTA. 14 15-04-2010
06
science
opinion please
Energy down the drain Sewage water is regarded as waste, but according to professor Jules van Lier it should be viewed as a valuable source of energy and fertilizers. xTomas van Dijk Western countries are proud of their sewer systems, and for good reason: infectious waters are effectively removed from residential areas and the effluent water from treatment installations is of reasonably good quality. But recently appointed professor, Jules van Lier (wastewater treatment and environmental engineering), believes that the way we deal with our water and waste streams is questionable. By flushing the toilet and mixing that water with all other household waters, prof. Van Lier says: “We dilute our sludge with a factor of hundred.” Subsequently, we aerate the sewage water in treatment installations, in order for aerobic (oxygen consuming) bacteria to break down the organic compounds. But this aeration – pumping air inside the basins - costs lots of energy. Instead, sewage treatment could produce energy. “For this we have to concentrate the sludge and use an anaerobic waste treatment technique that produces methane”, Van Lier says. “And what’s more, by doing so we can also recuperate nitrates and phosphates that we can use as fertilizers.” The professor explains that in the
agro food business, waste streams are already widely being treated through fermentation by methane bacteria in so called biogas plants. “These methane producing bacteria thrive well in waste streams because the temperature is relatively high”, he says. “The optimum temperature for these microorganisms is 38°C. Sewage water is generally much cooler than that. Heating the low-concentrated, large quantities of sewage is out of the question, however, because of the
Experiments with separate collections for faeces and urine are currently being conducted in the Frisian town of Sneek energy requirement. This is why the waste must be concentrated prior to anaerobic treatment.” One way of doing this is through the use of vacuum toilets. Experiments with separate collections for faeces and urine are currently being conducted in the Frisian town of Sneek. Van Lier, and a PhD student from Wageningen University, are working on a technique to treat the highly concentrated waste streams deriving from these kind of toilets. They are using a high pressure fermentation tank, and their goal is not simply to produce methane, but also to produce high quality green gas, which is a gas consisting of almost pure methane. Because of this high pressure, the carbon dioxide, which is normally part of the biogas, largely remains dissolved in the sludge.
“We found that these methane bacteria can survive a pressure of ninety atmosphere”, Van Lier says, “and that’s more than enough. We already obtain high quality gas with ten atmosphere.” The trick now is to make this fermentation process suitable for the black waters that are collected via vacuum sewers. Instead of batch digestion processes, a continuous flow process that can be operated in a decentralized mode is needed. With the support of a TU Delft PhD student, the professor will now try to build a user friendly, continuous flow fermentation tank. Because most sewer systems in the Netherlands date from just after the Second World War, large sections of these aging systems must be replaced in the coming years. According to Van Lier, now is the time for a paradigm shift. Decentralised sanitation, coupled with resource recovery, might be a better solution, he believes. The advantages of this technique are even more obvious for developing countries. Since such countries cannot afford to build sewer systems like we have in the west, why waste the energy that’s locked inside the waste?
Faeces contain energy that can be recuperated. Our sewer systems waste this energy. (Photo: Tomas van Dijk)
Energy devouring searches Last week’s opinion page of New Scientist magazine featured an article by an American physicist, describing the huge energy costs of a simple Google search query. It hardly takes a second to type in a word in Google and hit the ‘search’ button. And while this may seem a trivial act for most people, it most assuredly is not. During that second you consume a huge amount of energy: the equivalent of a 100-watt light bulb burning for one hour, or so James Clarage, a physicist at the University of St Thomas in Houston, has calculated. Clarage reasons that Google’s data centres contain nearly a million servers, each using approximately one kilowatt of electricity (mostly for cooling). So every hour Google’s engine burns through one million kilowatt-hours. Since Google serves up approximately ten million search results per hour, this means that one search has the same energy cost as turning on a 100-watt light bulb for an hour. Following the article on the website is a long list of comments from readers, mostly mocking the statistics and calculations. Clarage’s figures for the number of searches that Google performs were based on information deriving from a consultancy company called Comscore, which specializes in IT. Some critical readers remarked that Google processes almost ten times more searches (their figures are backed up by yet another consultancy firm) and that Google’s servers each use four times less electricity than Clarage states. According to TU Delft professor, Henk Sips, of the parallel and distributed systems section (Electrical Engineering, Mathematics and Computer Science), it is very difficult to calculate exactly how much energy a Google search costs. The 100-watt light bulb comparison does not seem far fetched to him, however. “But”, he adds, “it could easily be wrong by a factor of two. These kinds of calculations are always made on the back of a napkin, because nobody is given access to data about how Google actually functions.” Prof. Sip says that a certain degree of nuance is required. He points out that the hard disks on Google’s servers are also used for its Gmail email service, meaning the 100-watt theory cannot only be accounted for by net searches. “And”, the professor continues, “an increase in the number of searches won’t lead to a linear increase in energy consumption. The servers are continuously indexing websites around the world. They are doing the preparative work, so to say, so that when you type in a query, Google can immediately give you the results.” While they are busy indexing, the servers heat up and then must be cooled. But like Clarage, Prof. Sips also believes that Google’s energy consumption will rise. But whereas the former warns about what will happen when six billion people on Earth have access to internet, Sips is primarily concerned about the effects of video streaming: “Youtube is an energy devourer. In our field of work, we are left wondering how long Google can continue with this service. According to most analysts, the company is losing money with Youtube, but then again, all that comes through the grapevine.” Another influential factor in the future energy consumption of Google queries is the use of optical fibre in internet networks, which is rising. Sending information through optical fibre costs a lot less energy than through copper cables. (TvD)