Struvite is a hydrated ammonium magnesium phosphate represented by the following formula: MgNH4PO4 x 6H2O. In nature, it occurs as a mineral with a crystalline structure. It can also be obtained synthetically as a result of the following reaction:
Mg+2 + NH4+ + PO4-3 + 6H2O = MgNH4PO4 x 6H2O
The above reaction shows that for the formation of struvite, an appropriate molar ratio of individual components is needed, which, according to research, is 1:1:1, respectively. As a result of further research, its characteristic features were revealed:
- colour – white transparent or translucent,
- fracture – uneven
- specific mass – 1700 kg / m3
- Mosh hardness -2
The most susceptible place of struvite generation is the set of supply pipelines and devices (e.g. pumps located after the fermentation chamber) within sludge process line. And the process of struvite generation in these places can be characterized as follows: Polyphosphates flowing into the fermentation chambers are hydrolysed to orthophosphates containing PO4-3 ions. The aqueous phase of the fermenting sludge, after biological removal of nitrogen and phosphorus, is enriched with orthophosphate and ammonium ions. If magnesium is present in the wastewater, which usually comes from tap water needed at sewage treatment plants, there is nothing to prevent struvite from forming at appropriate concentrations of these ions. This way, struvite is formed in an uncontrolled manner, resulting in the formation of hard deposits of struvite in pipelines, pumps, etc. Hard struvite deposits are difficult to remove and often create huge problems in sewage treatment plants. It also results in increased costs of repairs and replacement of damaged devices.
However, this problem was dealt with at sewage treatment plants by controlled formation of struvite for further use as a life-giving compound for fertilizing crops, because, as already mentioned, it contains life-giving elements such as phosphorus, nitrogen and magnesium. Research centres around the world have faced this problem. As a result a number of technologies of controlled struvite formation were developed.
As for chemistry, each technology is similar to each other. They differ slightly in the way the struvite is manufactured in different apparatuses and process parameters. Struvite technologies originate from phosphorus recovery from sludge mentioned earlier. The difference is that in the previous examples phosphorus was recovered in a form of easily digestible phosphates, e.g. calcium.
A complex process in which about 95% of phosphorus contained in sewage is recovered and the production of struvite is about 550 kg / day. First, a mixture of sewage sludge and ashes from thermal transformation is hydrolysed by sulfuric acid in the reactor. The reaction mixture is subject to a filtration process in which the organic sludge for incineration (ash is recycled to the hydrolysis) and the liquid phase in the form of acid phosphates and excess sulfuric acid are separated. Gas from fermentation chambers is passed through such solution, causing its desulphurisation. Precipitated metal sulphides are subject to filtration process from which the solution is used to produce struvite. This solution is treated with sodium carbonate, magnesium compounds and orthophosphates in an ammonia atmosphere to precipitate struvite. Precipitated struvite undergoes a filtration process and leaves the filtering devices in the form of a cake. Contaminated water after filtration is recycled to the treatment plant.
“PHOSPHOGREEN” reactor technology
one of the newer processes of sludge treatment, which ultimately leads to the formation of granulated struvite. It uses biological sludge, from which phosphorus compounds are first released in a special tank. Then such sludge is subject to thickening process, after which the concentration of phosphorus compounds increases. Primary sludge which first undergoes anaerobic digestion and then dewatering is also used in the process of obtaining struvite. The phosphorus-rich solution from dewatering process as well as the one after thickening are fed to the “PHOSPHOGREEN” reactor. The “PHOSPHOGREEN” reactor is a cylindrical fluidized bed reactor, with recirculation and water washing. The lower part of the reactor is narrowed ending with a conical bottom, the upper part is about three times wider than the lower part. A concentric tube is placed in the centre, through which air is introduced from below to lift the layers of formed struvite, while stripping away carbon dioxide. The phosphate-rich solution is introduced from the side at the bottom of the conical reactor, as well as a solution of magnesium chloride and sodium hydroxide. As a result of the reaction, struvite is formed in the form of granules, which are collected from the bottom of the conical reactor and further washed, dried and packaged as a commercial product. Sodium hydroxide serves as a pH stabilizer. The entire reactor is covered with an outer insulation layer to avoid energy loss.
The resulting struvite has the following characteristics:
- granules size 1.0 to 3.0 mm
- phosphorus content approx. 12.0%
- nitrogen content approx. 6.0%
- magnesium content approx. 10.0%
It is therefore a classic fertilizer that can be used directly or as an addition to the production of synthetic fertilizers.
Main parameter: form of struvite, metals content level