Brunswick, the ?Port of Georgia?, is located in southeastern Georgia along the coast of the Atlantic Ocean. Over the last 20 years the city has experienced a mix of industrial, commercial, and residential growth. With growth came H2S gas problems, primarily due to additional force mains with long detention times.
Lift Station 6, located on the grounds of Brunswick College just across the street from a new car dealer, had suffered from H2S gas odor problems since it was constructed. Severe H2S gas-related corrosion also was destroying the concrete gravity lines, manholes, and wet wells.
Due to the dynamics of this particular collection system, treating the H2S problem had proven difficult. H2S gas headspace concentrations exceeded 550 ppm with a 24-hour average of 281 ppm. The city previously had tried calcium nitrate, sodium hydroxide, chlorine gas, potassium permanganate, and two different varieties of sulfide reducing bacteria in an effort to reduce the H2S gas to acceptable levels.
In September 1997, the city contracted with Premier Technologies to continuously feed an alkaline slurry into the wastewater system at two locations with the objective of solving H2S gas odor and corrosion problems at lift stations down stream, particularly the master repump station on the grounds of Brunswick College.
The company?s Thioguard product, Mg(OH)2, is designed to prevent the formation of hydrogen sulfide gas by raising the pH of the wastewater, causing the H2S to dissociate into HS and H+ ions.
How the Additive Works
There are three principal mechanisms by which Thioguard reduces wastewater sulfide emissions. Because Thioguard is an alkaline material, it raises the wastewater pH via the controlled slow release of hydroxyl (OH-) ion. H2S gas in the headspace of sewer pipe and other sewer structures will be greatest below a pH of 5. Typically, raising the pH to about 8.5, which on average takes about 100 gallons of Thioguard for each million gallons treated, is sufficient to achieve a greater than 95 percent reduction in hydrogen sulfide gas emission.
How the Additive Works
By raising the wastewater pH the speciation of dissolved sulfides also is altered. At a pH of 7.0, approximately 50 percent of the sulfides are present as dissolved hydrogen sulfide gas and the other 50 percent present as hydrosulfide, HS- (or bisulfide) ion. As the pH is raised the equilibrium shifts and more bisulfide ion is generated. The bisulfide ion, unlike hydrogen sulfide, cannot escape from solution into the atmosphere, thus sulfides are prevented from escaping from solution.
How the Additive Works
The second mechanism involves adsorption of the sulfide from solution onto the surface of the magnesium hydroxide where complex polysulfides form. It also is believed that a complex is formed between Mg2+ ions and bisulfide ions in solution. Both of these complexes are stable and do not readily break down to release hydrogen sulfide if the pH of the waste stream decreases.
How the Additive Works
Raising the pH of the waste stream also impacts on microbial activity in the slime layer present on the bottom of collection system pipes. It is the activity of sulfate reducing bacteria (SRB) present in the slime layer that is responsible for generation of sulfides. Like any bacterial species, SRB have an optimal pH range in which they attain maximum activity. Thioguard produces a pH environment that is non-optimal for SRB, thus decreasing their sulfide generating ability. Consequently, the continuous addition of Thioguard to wastewater results in fewer sulfides actually being formed by SRB.
Results
Within just a few days, average H2S gas concentrations at Lift Station 6 had been reduced by 93 percent. A follow-up study performed after six months revealed a 97 percent reduction in H2S gas.
