Struvite Reactor Tank
The Struvite Reactor Tank converts urine into fertilizer.
The Eawag low-cost struvite reactor tank is a tool to produce struvite powder from urine. Struvite contains nutrients such as phosphorus necessary for plant growth. The goal of this product is to improve crop production in low-income communities and improve resource reuse. Phosphorus is becoming scarce and it is hence increasingly important to find new phosphorus sources. Urine is available wherever there are humans and by constructing a struvite reactor tank and selling/using struvite on farmland, individuals can improve their economic situation while also potentially simplifying waste management since it is easier to handle and make use of urine and faeces separately than when mixed.
Construction manual in the Nepal context estimated 7000 Nepal rupee ($68 USD) for a 50 L tank and 43000 Nepal rupee ($418 USD) for a 500 L tank.Converted to USD on 1 July 2017
Fertilizers sold in stores, applying urine directly or after mixing with water.
Communities and farmers in areas where farming is common and there hence is a market for fertilizer and a stronger incitament to recover nutrients. The struvite reactor tank is adapted to be simple to use so that anyone with some building skills and information on how the process works should be able to construct and use the tank.
This product is not for sale; open source instructions
Unknown – open source
Continuous, Recharging only (V, time required, battery life), Other
The tank is made out of metal such as galvanized sheet metal or stainless steel. The stirring mechanism is also made of metal. The bag is a nylon filter bag that lets liquid through but keeps the struvite inside the bag. There is a plastic switch between the tank and the bag so that flow can be regulated. The dimensions of the tank can be adjusted according to the users wishes. For more info about the design see product schematics.
Source-separated urine and magnesium is added to the tank and mixed manually for 10 min with a crank connected to stirring blades in the tank. A valve connecting the tank to a nylon filtering bag is then opened. The mixture enters the filtering bag so that struvite stays in the bag while liquid runs through. The bag is then removed from the tank and dried for 1-2 days. The struvite in the bag can then be applied as fertilizer.
Open source- user applies this technique and maintenance.
If the product can be built on site spare parts are also available. The components and exact materials of the product can vary while still maintaining the same function.
The product is mainly constructed by recyclable materials such as metal and plastic. The nylon bag is the most challenging to recycle correctly, especially in the global south. (The lifetime of the tank is long but some components might be needed to be replaced continously such as the nylon bag. )
The amount of struvite depends on the size of the tank and the efficiency of the process. When filled and magnesium has been added it should be enough to stir for 10 min and then air dry for about 1-2 days. The goal of the product is to increase crop production in a sustainable way and also potentially give another source of income for people selling the struvite. The tank is stated by Eawag as low-cost and easy-to-use. Even though urine can be applied directly to crops and in mix in water, the fertilizer becomes easier to transport in the shape of struvite. It is also more socially accepted by people to apply the struvite powder than urine directly.
The tank is driven manually so there is no concern regarding electricity. Contact with the urine is one risk when filling the tank but this is not a big security risk and can be avoided.
Urine diverting toilet and storage tank. It is possible to make stirring automated and also add a rotating biological contractor (RBC) to reduce nitrogen content in the effluent. A possible schematics is found in this operation manual
Etter, B., Tilley, E., Khadaka, R. & Udert, K. Low-cost struvite production using source.seperated urine in Nepal. Water Research.
Tilley, E., Gantenbein, B., Khadka, R., Zurbrügg, C. & Udert, K. Social and economic feasibility of struvite recovery from urine at the community level in Nepal. International Conference on Nutrient Recovery from Wastewater Streams.
Application of urine as fertilizer is legal but if the struvite product is going to be patented and sold on the market as fertilizer a permission might be needed. An example of this is Aurin.
The reactor has been developed and tested in regards to percentage of nutrient recovery, production cost and demands of operation. There is also an academic article written about the social and economical feasibility using the product in communities in Nepal.
Struvite is easier to transport than liquid and applying a powder on the fields rather than urine directly is more socially accepted.
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