Magnesium Hydroxide liquid dosing for odour control

Mission Beach is a mixed residential and tourist area and is home to the iconic Cassowary. A sewerage scheme was commissioned by Cassowary Coast Regional Council in 2006 to collect the wastewater from three constituent communities and pumped by two major stations, at Wongaling and North Mission Beach, to the Main Transfer Pump Station (MTPS). The MTPS then pumps 23km to the Tully Sewerage Treatment Plant (STP).

This sewer main suffered significant corrosion due to “rotten egg gas” (H₂S) release in the drained sections. The Tully plant is designed for phosphate removal with liquid alum dosing for phosphate trimming and powered lime dosing for alkalinity and pH correction.

A trial of Calix’s ACTI-Mag Magnesium Hydroxide Liquid (MHL) dosing commenced in December 2017 with the primary objective to control septicity, odour and corrosion issues. Calix proposed additional objectives, i.e. the control of Fat, Oil & Grease (FOG) build-up, elimination of lime dosing at the Tully plant, and partial elimination of alum dosing for phosphate removal.

The trial eliminated odours by maintaining pH between 8.2 and 8.5. Pump station cleaning has been substantially reduced due to decreased FOG accumulation. The lime dosing system was prone to blockage and breakdowns, however dosing of ACTI-Mag has allowed suspension of lime dosing. Also, the dose rate has been reduced by more than 75% whilst still meeting phosphate effluent discharge limits.

That’s out of a total of 3,500 blockages in QUU sewer pipes that together cost $1.2 million to clear. Fatbergs also caused almost 250 sewage overflows.

But fats, oils and grease also play an important role in wastewater treatment plants, particularly those that capture biogas to generate electrical and thermal energy.

ACTI-Mag is a concentrated and stabilised suspension of magnesium hydroxide also known as milk of magnesia. An inorganic compound with the chemical formula Mg(OH)2.

Highly fluid, ACTI-Mag has proven to flow easily in most dosing systems. With a higher neutralising value per dry kilo when compared with caustic and lime and being significantly safer than other traditional alkalis, ACTI-Mag is a very cost competitive option for hydrogen sulphide gas (H₂S) control in sewers and industrial waste treatment as well as for several other applications including phosphorus and nitrogen removal from wastewater.

Calix’s ACTI-Mag has several advantages over other generic MHLs.

• Neutralises waste streams 30% faster than generic brands.
• The pH peaks at 10.5 and buffers in the solution between 8.5 and 9.
• The MHL dissolves slowly over time with particles remaining in suspension for as long as needed. (low “GI” alkali)

Magnesium Ammonium Phosphate (MAP), commonly known as Struvite, is a crystalline precipitate of Magnesium, Ammonia, and Phosphate.

Mg²+ + NH4+ + PO4³- + H2O = Mg.NH4.PO4.6H2O (s)

Struvite forms in pipe bends due to pressure differential in the flow. This will lead to a solid precipitate creating blockages and restricting flow.

The advantage of dosing ACTI-Mag is that it precipitates ammonia and phosphate directly in the sludge and stops struvite forming on key infrastructure.