Struvite that is found in wastewater treatment plants (WWTP) is a phosphate mineral crystal whose elemental composition of magnesium ammonia and phosphate forms in water and is chemically written as NH4MgPO4•6H2O. Charles Jennings in his article on Struvite Inhibitors
also noted that the Struvite crystal is referred to as MAP in the wastewater industry, which is short for magnesium (Mg2) ammonium (NH4) phosphate (PO4-3) compound.
Struvite crystals will be found in colors of white to yellow to brown, depending upon the MAP content in the water. Jennings also notes that because struvite crystals are not produced with a pure stoichiometry, i.e., it is different depending on local water conditions (see below) and local process applications at treatment plants, that each struvite formation needs its own treatment regime for elimination.
Struvite Formation - How do Struvite Crystals form?
The chemical formula for the formation of Struvite is Mg2 + NH4+ PO4-3 + 6H2O → NH4MgPO4•6H2O (crystal form), which says that struvite crystals are created when magnesium, ammonium and phosphate combine in water or an aqueous solution in a mole to mole to mole ratio of 1:1:1:. The wastewater’s temperature and pH are also contributing factors to the formation of Struvite. The higher the pH the more likely the elemental mole to mole to mole balance will be affected and the formation of Struvite crystals will occur.
In wastewater treatment plants and concentrated animal feeding operations (CAFO) Struvite forms in those anaerobic digesters. The reason the formation occurs in WWTPs and CAFOs is because magnesium is a major ingredient and contributor to hard water, ammonia is a byproduct of urea and urine and phosphate is a major element in the organic matter processed at the plant. Depending upon the feed stock elements at the treatment plant, wastewater managers will have a major or minor problem with Struvite crystal formation.
Wastewater treatment plants find Struvite crystal formations in many of the following areas:
- Anaerobic digesters
- Aeration assembly
- Rollers of the Belt Press
- Rotary drum filters
- Screw presses
- Belt presses
- Places of high kinetic energy (high turbulence)
- Pipe connections - elbows
- Internal pump components
- Overflow box of the anaerobic digesters
- Sludge transfer lines
It should also be noted that rough surface areas are also excellent nucleic sites for Struvite crystals. When Struvite is not 100% removed from external and internal surfaces, the remaining rough edges are where struvite and vivianite will form again.
Struvite Control and Struvite Prevention
There are a number of wastewater treatment plant managers that are now using specialty chemicals like JenFitch’s JS 9325 and JS 9310 to prevent the formation of Struvite or control the growth rate of Struvite. The key to controlling Struvite is having the ability to deal with the magnesium level, carbon dioxide release into the wastewater from degassing and manipulate its pH. If you control the magnesium, you are able to control the precipitation of the Struvite. By using a liquid product like JS 9310 to maintain a low pH of the water and effectively deal with the CO2, one is able to prevent the formation of Struvite and therefore, you control its formation. When it does form, JS 9325 has the ability to break through the struvite water bond and dissolve the struvite crystal in an environmentally friendly way.
Another method for the prevention and control of Struvite is the use of an iron salt or an aluminum salt to maintain a low pH and counteract the degassing of the CO2. Furthermore, since phosphorus and ammonia are highly soughtelements for fertilizer one is able to use Ferric Iron (Fe3) or aluminum (Al3) to control the phosphorus in the wastewater by precipitating the phosphorus and removing it from the water as a solid. The same is true for the control and precipitation of ammonia.
In the report by Enrique Vadiveloo, P.E.
and colleagues concerning the Miami-Dade Water and Sewer Department's struvite deposits in the solids handling process equipment at the Central District Wastewater Treatment Plant and the South District Wastewater Treatment Plant, they found no significant difference in dosage or effectiveness in using ferric chloride and ferric sulfate in controlling Struvite formation. Their suggested choice was to use ferric sulfate because it is cheaper than ferric chloride, easier to handle and less corrosive to use. However, struvite can still be a problem and JS 9325 is recommended for removal.