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The phosphate and nitrogen challenge
Removing phosphorus and nitrogen cause challenges at many wastewater treatment plants. Regulatory discharge limits require phosphorus removal processes that convert soluble phosphates into insoluble solids. Phosphorus removal from wastewater can be achieved either through chemical removal, advanced biological treatment or a combination of both.
The chemical removal of phosphorus involves the addition of iron or aluminium salts to achieve phosphorus precipitation. These chemicals are costly, increase the volume of inorganic waste, create chemical waste and cause operational side effects, such as alkalinity consumption and corrosion. In addition, costly polymers are used during the sludge treatment process to enhance the dewatering process.
Furthermore, the concentration of phosphorus causes uncontrolled formation of struvite, which causes scaling in pipes and valves, thereby reducing the capacity and requiring high maintenance costs. Figure: Struvite scaling.
The solution: Magnesium hydroxide
When using or in the sludge treatment process (see figure 2), a significant reduction of operational costs can be achieved. Flocculation agents as FeCl3 and AlCl3 are no longer needed and no additional aeration energy or NaOH is required to increase the pH before the dewatering step.
Figure: Magnesium dosing points in case of phosphate removal & recovery.
Because of the formation of struvite from magnesium ions with free phosphates the dewatering characteristics of the sludge are changed.
The sludge is more easily compressed what results in a significant reduction of dewatering polymer usage up to 25%.
When utilizing or , the following is achieved:
• Reduced chemical usage (iron or aluminium salts) up to 100%
• Reduced sludge water content by 10-20%
• Reduced polymer usage, up to 25%
• Reduced energy costs (aeration)
• Reduced maintenance costs, due to controlled struvite production
• Opportunity to commercialise struvite
Commercialisation of struvite
Several commercial processes are now available which solve the main challenge to recover struvite and meeting the phosphorus discharge limits.
It is now possible to recover phosphorus and nitrogen to consistently produce struvite that can be used as a fertiliser with or without further treatment. To implement this, magnesium needs to be dosed into the sludge just before the dewatering step or into the process water extracted out of the dewatering process.
About struvite Struvite (magnesium ammonium phosphate) is produced at pH > 8. Dosing of magnesium results in the formation of struvite through the following equation: Mg2+ + PO43- + NH4+ + 6H2O -> MgNH4PO4·6H2O. Struvite is sparingly soluble in neutral and alkaline conditions, but readily soluble in acid.
is used for pH neutralisation. It offers more alkalinity per kilogram compared to sodium hydroxide or calcium hydroxide and it has a natural buffering effect at pH 9 – 10. An accidental overdose will not increase the pH above this value unlike lime or caustic soda where the pH will instantaneously rise to 12 – 14. Magnesium hydroxide is safe to handle and it provides a long lasting alkalinity due to its high buffering capacity.
is able to remove metals from wastewater by precipitation. Magnesium hydroxide increases the pH to a level where metals precipitate as hydroxides. It can remove most of the metals with the exception of nickel and cadmium.
Odour control and corrosion protection
is a safe and cost effective solution to eliminate hydrogen sulfide odour and to avoid corrosion, especially in sanitary sewer systems. Hydrogen sulfide is the principal source of odour and corrosion and the generation is a function of dissolved sulfide concentration, water temperature, turbulence and pH. At pH 7 roughly 50% of the dissolved sulfide can exist as hydrogen sulfide and this results in a bad smell and corrosion of the sewage system infrastructure. Magnesium hydroxide raises the wastewater pH to 8.0-8.5 resulting in:
• Reduced sulfide production
• Elimination of hydrogen sulfide gas production
• Reduced corrosion rate of sewer pipes
• Reduced FOG (Fats, Oils and Grease)