The generation of offensive odors in wastewaters, domestic sewage and other installations of water treatment is mainly due to the action of reductive bacteria, which act anaerobically over sulfates present in the medium. In oil refineries and terminals, the waters of production and processes are contaminated due to the natural existence of sulfides in oil.
Problems caused by sulfides:
The odor of rotten eggs of sulfides is well known and is perceptible even in concentrations below 0.3 ppm. In elevated concentrations, the sulfidric gas inhibits the olfactory system, eliminating a factor that would serve as alarm in a danger situation.
Sulfides constitute an environmental threat for being poisonous to aquatic life in general.
The sulfidric gas is equally toxic and, in concentrations over 1000 ppm in the air, causes death in a few minutes. Eye and respiratory tract irritation, headaches and fatigue sensation are symptoms of an exposition to concentrations over 5 ppm.
Due to its low solubility and elevated volatility, the danger represented by the sulfidric gas is of the same order of the cyanide gas.
The presence of sulfidric gas in the working environment in tanneries depends mainly of the processing step that the hides go through and air circulation inside the factory.
It also corrodes piping, pumps and installations, even concrete parts.
Unfavorable action over biological treatment
In elevated concentrations, sulfides are toxic to the biological treatment, reducing the efficiency of the process and inhibiting microbial activity. They also favor the growing of filamentous bacteria in the processes of treatment by activated sludge.
To avoid disturbances of the active biomass, sulfide concentration must not be superior to 25 mg/l, and should be kept constant to avoid shocks that hamper the biological activity in the processes.
Oxidation of sulfides by hydrogen peroxide:
Our hydrogen peroxide products are easily handled and their application doesn’t involve large increases in cost. They provide partial oxidizing of sulfides to intermediate compounds that do not exhale bad smell, and that can be treated by aeration, efficiently and at low cost.
Sulfides are formed as follows:
H+ + HS- ↔ H2S (Meio neutro ou ácido)
HS- ↔ H+ + S2- (Meio básico)
In a basic medium, there are species of low corrosive power in equilibrium: S2- and HS-, with a low concentration of H2S (less than 1% of dissolved sulfur).
However, in this pH range, nor domestic sewage nor industrial effluents could be dumped over rivers. They are dumped with neutral pH, which is when S2- and HS- ions are converted into the volatile, toxic and corrosive H2S.
H2S + H2O2 → S(s) + 2 H2O
Where the biggest part of sulfide is transformed into elemental sulfur. The rest is constituted of different soluble compounds of sulfur, and according to their structure might be oxidized later.
The reaction is relatively slow in acid medium, but can be catalyzed through ions of transition metals. After addition of dissolved iron (such as iron (III) sulfate), the reaction completes in few minutes, even at room temperature.
In reactions in alkaline medium, the oxidation occurs according to the following equation:
S2- + 4 H2O2 → SO42- + 4 H2O
In that case, the reaction is considered faster than in acid medium. At ambient temperature it concludes itself in few minutes, even without catalystaddition. To avoid byproducts, the proportion of hydrogen peroxide that must be used should be above the stoichiometric.
There are mainly four practical and economically viable ways to oxidize the sulfides with our products:
- Complete Oxidation
- Preventive Oxidation
- Auxiliary Oxidation
The complete oxidation of sulfides to sulfates by hydrogen peroxide is used preferably by factories with a great wastewater with sulfides flow and small area for treatment, or by those who are still constructing their wastewater treatment.
It can also be used in case of accidental spill, momentary inoperability of the treatment station or emergency cases.
The speed of implanting the dosage system, the operational easiness and reaction effectiveness in a very short time are inherent characteristics of the hydrogen peroxide.
When used as a preventive measure to the development of sulfides, hydrogen peroxide is primarily used as source of oxygen.
H2O2 → H2O + ½ O2
The decomposition level to oxygen will depend of the presence of contaminants in the wastewater, particularly transition metals. The concentration of hydrogen peroxide to keep the aerobic conditions is lower than those necessary of molecular oxygen.
To the companies that already oxidize its wastewaters by aeration or oxygen injection, the hydrogen peroxide is a valuable partner, since the use in large enough amounts for a partial oxidation of the wastewater eliminates the bad smell immediately and accelerates the operation of the subsequent aeration, guaranteeing an increase in efficiency of the process with energy economy.
The polishing is an adequation of the wastewater to the conditions required by the environmental control organs. In case of the wastewater still having sulfide content above the allowed emission levels, it can be adequated by adding a small dosage of hydrogen peroxide.
In some countries, for the disposal of sewage in rivers and the sea, where is foreseen use for recreation of primary contact, it is mandatory by law to disinfect it to eliminate the thermo tolerant coliforms. For this application, the oxidizing action of the hydrogen peroxide works as a first step of disinfection, reducing the peracetic acid necessary for complete elimination of pathogenic microorganisms.