Nowadays we find it quite normal that our waste water disappears through the sewer, but less than a century ago the "bucket poo" in Amsterdam was collected by horse carts and used on the farmland. We have to reinvent the reuse of our waste water. As of this week, the Amsterdam water company Waternet, commissioned by the Amstel, Gooi en Vecht Water Board, has been extracting large-scale phosphate from the city's sewage water.
With the new phosphate plant, Waternet will produce 1000 tons of struvite per year. “Struvite consists of pure crystals of magnesium ammonium phosphate (MgNH4 PO4 .6H2 O) and already looks like a sugar crystal,” says Jacqueline de Danschieter, waste water process technologist and resource recycling coordinator at Waternet. ”When it comes out of the reactor, it is still a bit moist, but clean.”
Dilute with drinking water
Everything that now disappears into our toilet bowl, we dilute so strongly with (drinking) water that it is no easy task for the sewage treatment companies (WWTPs) to remove valuable substances from it again. However, there is a lot of usable phosphate and nitrogen in domestic waste water, which in turn can serve as fertilizer. Particularly due to the worldwide scarcity of phosphorus, which is expected in the middle of this century, the reuse of phosphate is becoming increasingly interesting. Phosphate – an important raw material in fertilizer – now mainly comes from phosphate ore, which is mined in countries such as Morocco and China.
Clogged pipes
With the new recovery installation, approximately 25 percent of all phosphate that enters the WWTP via the sewage system will be recovered. But the future worldwide phosphate scarcity is not the direct reason for the construction of the phosphate factory. Also, the sale of the struvite will not be very profitable for Waternet. The main reason for the construction of the phosphate factory has to do with the problems in the 'sludge line' of the treatment plant, in which struvite naturally precipitates at high temperatures and clogs the pipes and pumps.
Sludge dewatering
The dewatering process of the sludge also goes much better if the sludge contains less phosphate. De Danschieter:"It is still a mystery exactly how that works chemically, but the fact is that we can centrifuge the sludge much better if it contains less phosphate." By reducing the dewatered sludge, Waternet saves on costs, because the dewatered sludge has to be disposed of as waste to an incineration plant.
'Green' ore
Incineration of sewage sludge has been mandatory throughout the Netherlands for several decades. Before that time, the sludge was used as fertilizer in the agricultural sector, but because it contained too many contaminants, such as heavy metals, that practice was stopped. The ash – the combustion product of sludge – is also a source of phosphate. At Thermphos, now bankrupt in Zeeland, the largest European phosphorus producer until last year, the sludge ash served as a raw material for the production of phosphorus, as a kind of 'green' ore.
Fooling
“The bacteria are fooled in the WWTP,” explains De Danschieter. “First we give them food, but no oxygen. If we do administer oxygen, they will absorb phosphate like mad. In the next step – fermentation with energy recovery in the form of biogas – the bacteria die and the phosphate is largely released from the broken bacterial cells. It is during the process step between fermentation and centrifugation of the water-rich sludge that we precipitate struvite by adding magnesium chloride.”
German technique
Waternet processes the sewage sludge produced by two million Dutch people. This partly comes from the WWTP, which processes the sewage water from Amsterdam and the surrounding area, and from the other WWTPs of the Amstel, Gooi and Vecht Water Board in the area. To increase the concentrated supply of phosphate, Waternet will collect urine at pop festivals.
The new phosphate factory is the largest in the Netherlands and will process 2000 cubic meters of sewage sludge every day. The process – Airprex – is a German invention, developed by the Berliner Wasserbetriebe, and has been specially adapted for the Waternet factory. There is an installation of comparable size in Berlin.
Sewage old-fashioned?
With modern eyes we see the major drawbacks of the discharge of faeces and urine through the sewer, which is diluted with large amounts of clean drinking water flushed. But at the end of the 19th century, the sewage system was a necessary 'luxury' to prevent the discharge of faeces to open water, such as canals, from leading to unsanitary conditions.
Cholera
At the end of the 19th century, the number of inhabitants in the major cities doubled. The last cholera epidemic occurred in Amsterdam in 1866, with most victims in the poor neighbourhoods. In some old Amsterdam city districts it took until 1934 before a sewer connection was constructed and the canal houses even discharged into the canals until forty years ago. Well into the 20th century, a 'poo cart' - popularly called the "Boldootwagen", named after the soap factory - came to collect the buckets in some neighborhoods. Until cheap fertilizers became available around 1900, faecal matter was considered a valuable fertilizer and used in agriculture.
Struvite for sale
As normal it used to be to reuse organic waste, it is so innovative today. Legislation also needs to be amended accordingly. In Germany it was already arranged, but from 1 January 2014 it will also be allowed to sell struvite in the Netherlands. Previously, this was not allowed because recycled phosphate was classified as 'waste' and could not be regarded as fertilizer.
De Danschiete:“It would be great if the struvite could be sold as ready-to-use fertilizer, because then we close the cycle. We also try to sell it to fertilizer manufacturers, who mix it with other fertilizers to make it suitable for agriculture."