ABSTRACT
Solving the Problem of Hydrogen Sulfide in Sewage Systems
By
Engr. John L. Relayson
(MS ENVI. ENG’G)
Hydrogen sulfide (H2S) is one of the gases found in untreated wastewater and is derived from the decomposition of the organic matter present in the wastewater containing sulfur or from the reduction of the mineral sulfites and sulfates. This gas is a result of septic condition during the collection and treatment of wastewater and has long been recognized by civil/sanitary engineers as a major problem in municipal wastewater systems. This colorless gas, known for its rotten egg odor, forms at virtually every point in a system from interceptors, force mains, and lift stations, to holding tanks, dewatering presses and drying beds. The unpleasant odor is caused by lack of oxygen. Since there is no constant supply of oxygen in the sewers, the oxygen dissolved in the water is consumed in a relatively short time, by both bacteria (Thiobacillus) and chemical reactions in the effluent. As long as oxygen is dissolved in the wastewater, no hydrogen sulfide will form. If, however, the oxygen present in the water is used up, the aerobic bacteria Thiobacillus begin to reduce the oxygen-rich compounds in the influent. In this way, first nitrates (NO3) and then sulfates (S04) are consumed. Gaseous nitrogen is produced when the nitrates are reduced. The formation of hydrogen sulfide occurs when the sulfates are reduced.
The unpleasant odor caused by the hydrogen sulfide is noticeable at concentrations of less than 0.1 ppm (parts per million) in air. Because of its low solubility and high volatility, hydrogen sulfide can be very easily released at collection points, pumping stations and other areas of the sewage system with high turbulence. If, however, hydrogen sulfide is not removed, it will have toxic effects on all organisms within the purification plant. Gaseous hydrogen sulfide and atmospheric oxygen from the free surface of a partly-filled sewage channel will be oxidized by Thiobacillus on the walls of the pipe to sulfuric acid. In the wastewater, itself, this acid is diluted, so that is has little influence on the pH (acidity) value of the effluent. However, there is a completely different situation on the walls above the water surface. Considerably higher levels of acid can build up there, which can lead to serious corrosion and premature destruction of concrete pipes. Foul sewage therefore has a high risk factor. In the past, reaction of local communities to the unpleasant odor of H2S was often viewed as the primary problem. But H2S also poses a serious problem for the structural integrity of the collection system and the STP (sewage treatment plant) as a whole. Millions of pesos are lost each year to corrosion caused by sulfuric acid formed from the interaction of the H2S with moisture. Of equal importance are the safety hazards associated with H2S. Hydrogen sulfide gas is a leading cause of death among workers in sanitary sewer systems. The methods for solving the problem can be divided into two groups: (1) elimination of the existing hydrogen sulfide, and (2) prevention of the formation of hydrogen sulfide.
Elimination of Hydrogen Sulfide
Oxidation rate of H2S with molecular oxygen is very low at ambient temperature and in the absence of catalysts. The only remaining possibility is to use a powerful oxidizing agent such as hydrogen peroxide (H2O2). As such substances are not specific, an excess over the stoichiometric requirement must generally be used. This has a negative effect on the cost-benefit ratio of these measures.
Preventing H2S Formation
The second possibility is to prevent the formation of H2S from the outset. This can be done in one of two ways:
1. Addition of Nitrates. As the added nitrates are more rapidly reduced by aerobic bacteria Thiobacillus than the sulfates already present in the effluent, the formation of H2S will be prevented as long as dissolved nitrates remain in the water. It is very difficult, however, to add nitrates accurately to fluctuating flowrates and compositions of effluent in a way which avoids harmful over-dosing and the resulting additional load to the effluent.
2. Maintaining a certain concentration of Dissolved Oxygen (DO) in the effluent. In order to maintain a concentration of oxygen in the wastewater, it must be constantly treated with oxygen. This can be achieved by dissolving air or pure oxygen. As air is used for maintaining the oxygen concentration in the effluent, it must be taken into account that air contains 70-80% nitrogen. Only a minor part of the gaseous nitrogen in the air is dissolved in the water because the nitrogen saturation level has often already been reached. This frequently causes the undissolved nitrogen to strip other toxic or odorous substances contained in the effluent. As a major part of the gas used is not dissolved, this usually leads to a drop in the flow performance of the pressurized sewer pipe due to the formation of gas pockets.
By using pure oxygen, the dissolving of gas can be controlled according to need. Equipment can be designed in such a way that an almost complete dissolution of gas can be achieved.
The unpleasant odor caused by the hydrogen sulfide is noticeable at concentrations of less than 0.1 ppm (parts per million) in air. Because of its low solubility and high volatility, hydrogen sulfide can be very easily released at collection points, pumping stations and other areas of the sewage system with high turbulence. If, however, hydrogen sulfide is not removed, it will have toxic effects on all organisms within the purification plant. Gaseous hydrogen sulfide and atmospheric oxygen from the free surface of a partly-filled sewage channel will be oxidized by Thiobacillus on the walls of the pipe to sulfuric acid. In the wastewater, itself, this acid is diluted, so that is has little influence on the pH (acidity) value of the effluent. However, there is a completely different situation on the walls above the water surface. Considerably higher levels of acid can build up there, which can lead to serious corrosion and premature destruction of concrete pipes. Foul sewage therefore has a high risk factor. In the past, reaction of local communities to the unpleasant odor of H2S was often viewed as the primary problem. But H2S also poses a serious problem for the structural integrity of the collection system and the STP (sewage treatment plant) as a whole. Millions of pesos are lost each year to corrosion caused by sulfuric acid formed from the interaction of the H2S with moisture. Of equal importance are the safety hazards associated with H2S. Hydrogen sulfide gas is a leading cause of death among workers in sanitary sewer systems. The methods for solving the problem can be divided into two groups: (1) elimination of the existing hydrogen sulfide, and (2) prevention of the formation of hydrogen sulfide.
Elimination of Hydrogen Sulfide
Oxidation rate of H2S with molecular oxygen is very low at ambient temperature and in the absence of catalysts. The only remaining possibility is to use a powerful oxidizing agent such as hydrogen peroxide (H2O2). As such substances are not specific, an excess over the stoichiometric requirement must generally be used. This has a negative effect on the cost-benefit ratio of these measures.
Preventing H2S Formation
The second possibility is to prevent the formation of H2S from the outset. This can be done in one of two ways:
1. Addition of Nitrates. As the added nitrates are more rapidly reduced by aerobic bacteria Thiobacillus than the sulfates already present in the effluent, the formation of H2S will be prevented as long as dissolved nitrates remain in the water. It is very difficult, however, to add nitrates accurately to fluctuating flowrates and compositions of effluent in a way which avoids harmful over-dosing and the resulting additional load to the effluent.
2. Maintaining a certain concentration of Dissolved Oxygen (DO) in the effluent. In order to maintain a concentration of oxygen in the wastewater, it must be constantly treated with oxygen. This can be achieved by dissolving air or pure oxygen. As air is used for maintaining the oxygen concentration in the effluent, it must be taken into account that air contains 70-80% nitrogen. Only a minor part of the gaseous nitrogen in the air is dissolved in the water because the nitrogen saturation level has often already been reached. This frequently causes the undissolved nitrogen to strip other toxic or odorous substances contained in the effluent. As a major part of the gas used is not dissolved, this usually leads to a drop in the flow performance of the pressurized sewer pipe due to the formation of gas pockets.
By using pure oxygen, the dissolving of gas can be controlled according to need. Equipment can be designed in such a way that an almost complete dissolution of gas can be achieved.
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