Surface pH/Corrosion

H2S gas generated in the sewer system is converted by Thiobacillus bacteria residing on concrete and metal surfaces above the waterline to corrosive sulfuric acid. This acid attacks exposed concrete and metal surfaces.

A simple method to measure relative strength of the sulfuric acid and rate of decay of the infrastructure is surface pH testing, conducted with contact pH paper on the wetted surfaces inside wetwells, manholes and sewer lines.

The measured surface pH can then be used to correlate corrosion rate and subsequently, remaining years of useful life of a concrete structure.

The use of THIOGUARD® by direct addition has been demonstrated to elevate surface pH from a highly corrosive pH = 1-2 up to a more desirable range of pH = 5-7.

Under some circumstances, THIOGUARD® CROWN SPRAY can be applied to surfaces to provide instant neutralization and a protective, sacrificial barrier on infrastructure surfaces.

When THIOGUARD® is injected into a wastewater stream, it enters in 3 distinct phases as shown below. These phases are a result of magnesium hydroxide's unique solubility and reactivity properties and the way it reacts to other qualities of the wastewater (i.e. pH, CO2 concentration, free acid H+, biological activity, etc.).


THIOGUARD® - 3 Distinct Phases make it unique among its alkaline peers, and highly suited to biological treatment systems

Phase I (Mg(OH)2):
Technical grade magnesium hydroxide as a particle has a solubility of 9 mg/L. While the solubility is considered relatively low, the particle, having a surface area of nearly 1 acre per gallon, is reactive and/or absorptive to acids, H2S, CO2, and some organics. The particle has a positive surface potential and is capable of improving flocculation and settling. In the collection system, it slowly dissolves as it reacts with H2S, CO2, acids, FOG, etc… At the plant, it usually enters the biosolids stream through Primary settling. That which passes through to Secondary processing is typically fully consumed by biogenic acids and CO2 produced during secondary treatment.

Phase II (MgOH+ + OH-):
This ionic phase is transitional from the particle phase to soluble phase and is indicative of technical grade magnesium hydroxide's unique abilities to "buffer" both acids and bases. This species also explains why pH's in a magnesium hydroxide supplemented stream do not accurately reflect the total amount of hydroxide present (neutralizing capability), since only the first ionized OH- contributes to the pH values. This is why pH, OH- and total alkalinity are critical measurements collectively rather than individually.

Phase III (Mg+2 + 2OH-):
This ionic phase is the result of complete dissolution of the Mg(OH)2 molecule. Complete dissolution occurs as a result of free proton (H+) acid neutralization or the formation of bicarbonate (HCO3-) from the reaction of OH- with CO2. Once dissociated, the divalent magnesium cation (Mg+2) aids wastewater treatment by 1) being utilized as a bridging particle for improved flocculation, settling and clarification in both Primaries and Secondaries and improved dewatering and densification in bio-solids processing 2) facilitating the transport and stabilization of P during ADP/ATP conversion and ATP hydrolysis, and 3) supplementing biological nutrients.

Residual OH- is reflected in pH readings. Since the reactive pH is 9, it "buffers" strongly in the pH range between pH 8 and 9, and most preferably near 8.4. As pH rises, magnesium shifts back towards Phases I and II. This is how magnesium hydroxide is extracted from brines and seawater for commercial production.

 

 

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