Drinking Water, Sewage and Rivers

Advantages of the use of Hydrogen Peroxide in the Treatment of Drinking Water  

• Improvement of taste and odor of water  Reduction of chlorine load in water
• Reduction in the formation of trihalomethanes
• Reduction in production cost

 

Where to Apply Hydrogen Peroxide 

• Pre-oxidation and algae control in water catchment reservoirs (direct addition) Pre-oxidation in Water Treatment Stations (direct addition and/or combined with ferrous salt —  Fenton System)
• Disinfection in Water Treatment Stations (combined with UV or ozone)

 

In drinking water treatment, hydrogen peroxide can be employed as pre-oxidizer, by the means of simple and direct addition, combined in the sequence of the simplified or conventional treatment with the conventional coagulants, flocculants and neutralizers. Also, it can be employed activated with Fe2+ ions (Fenton System), ultraviolet radiation or with ozone, in the so called advanced oxidation processes.
 
The oxidant and non-contaminant capacity of hydrogen peroxide has acknowledged usefulness in the pre-oxidation for algae control; precipitation of iron and manganese; and for oxidation of many kinds of dissolved substances, both organic and inorganic.

Hydrogen peroxide doesn’t contain heavy metals in its composition.

 

Benefits Around the World

The use of special and stabilized solutions of hydrogen peroxide in the treatment of drinking water in large scale is relatively recent in the whole world – since the year 2000. Countries like Canada, USA, France and Holland apply them for removal micro-contaminants and for disinfection.
 
In Brazil, more than 20 million people are benefited daily by consuming drinking water from stations the apply hydrogen peroxide in pre-oxidation, algae control and removal of surfactants with the Fenton System.

 

Relevant Characteristics of Hydrogen Peroxide

Powerful oxidant, non-contaminant, not persistent in the environment.

Hydrogen Peroxide is considered a clean chemical product for not leaving persistent elements and residues in the environment after its use.

This characteristics comes from the fact that its continually subjected to the spontaneous reaction of self-decomposition: 

H₂O₂ → H₂O + ½ O₂

Releasing for the environment only water and oxygen as final residues.
 
Compared with other oxidants for treatment of drinking water, it is verified that hydrogen peroxide is free of residual contaminants, besides promoting reduction of trihalomethanes levels and not oxidizing bromide to bromate.

Organic matter can be removed by simple and/ or advanced oxidation, depending on the complexity of the contaminant, generating carbon dioxide and water, as in the following reaction:

C_xH_yO_z+ w (H₂O₂ ou HO^.) → a CO₂ + b H₂O 

Effluents containing arsenic can be detoxified using hydrogen peroxide and iron sulphate.

The reactions occur as following:  

HAsO₂ + H₂O₂ → H₃AsO₄ 

2 Fe²⁺ + H₂O₂ + 2 H⁺ → 2 Fe³⁺ + 2 H₂O

 

The need of an oxidization step arises from the fact that As V compounds are much more insoluble than those of As III.
 
Arsenic can be efficiently removed from aqueous solutions by precipitation of ferric arsenate slimes in open agitated tanks. 

Fe³⁺ + H₃AsO₄ → FeAsO₄ (s) + 3 H⁺

 

Or, if there isn't enough Fe3+:

3 Fe²⁺ + 2 H₃AsO₄ → Fe₃(AsO₄)₂ (s) + 6 H⁺

 

Alternatively or additionally, the additions of Ca2+ ions (as in the addition slaked lime) to the wasterwater being treated will propitiate the occurrence of the reaction of formation of Calcium arsenate, also contributing to the removal of this metal:

3 Ca²⁺ + 2 H₃AsO₄ → Ca₃(AsO₄)₂ (s) + 6 H⁺ 

Reduction of Bromate:

In the water treatment for human consumption, the hydrogen peroxide, besides not oxidizing bromide to the toxic form bromate, is also capable of reversing through reduction the bromate that might eventually have been formed by other pre-oxidizers back to the non-toxic form of bromide, according to the following reaction:

BrO₃^- + 3 H₂O₂ → Br^- + 3 H₂O + 3 O₂ 

Hypochlorites and other Chlorinated Products

Wastewaters or purges of treated waters with chlorine or hypochlorite need to be treated before dumping, under the risk of  exceeding the low limit of active chlorine in waters of rivers of class 1 and 2 (0,01 mg/L).
 
Os agentes redutores convencionalmente utilizados na eliminação do cloro são compostos de enxofre do tipo bissulfito e tiossulfato. O dificilmente evitável excesso desses compostos no tratamento apresenta como desvantagem um aumento da DQO do efluente e deve, portanto, ser eliminado antes do descarte final do efluente. 
 
The reaction of these compounds with chlorine produces sulphates as byproducts, causing also an increase in the salinity of the wastewater.

 

Reduction with Hydrogen Peroxide 

Hydrogen peroxide is an alternative that presents the advantage of not generating undesirable byproducts, because it decomposes in water and oxygen. 
 
Hydrogen Peroxide reacts quickly with chlorine/hypochlorite, as in the following reaction:  

ClO^- + H₂O₂ → Cl^- + H₂O + O₂

As this reaction releases gaseous oxygen, the possibility of a pressure increase must be considered in closed systems. 
 
The hydrogen peroxide action over chloramines is slow at ambient temperature, and may be accelerated in the presence of an alkali at 50ºC.

R-NHCl + H₂O₂ + OH^- → R-NH₂ + Cl^- + H₂O + O₂ 

Process solutions and wastewaters are frequently contaminated with iron ions. Even tough the removal of this contaminant by precipitation is simple and well-known; it is convenient to assure that all the dissolved iron is in the 3+ oxidizing state, so that the precipitation is efficient from a pH starting on 3.5, with low base consumption.
 
The removal of iron by oxidation and precipitation with hydrogen peroxide is very fast, according to the following equation: 

Fe²⁺ + ½ H₂O₂ + 2 OH^- → Fe(OH)₃ (s) 

Process solutions and wastewaters from metallurgical processes are frequently contaminated with manganese ions.
 
The removal is eased by oxidizing the metal from the 2+ state to the 4+ state, which allows to reach, in a pH up to 9, a high level of precipitation. The same efficiency of hydroxide precipitation would only be possible with pH 10 or more.
 
The reaction occurs as following:

Mn²⁺ + H₂O₂ + 2 OH^- → MnO₂ (s) + 2 H₂O 

Wastewaters containing nitrite can be treated with hydrogen peroxide, oxidizing it to nitrate, according to the following reaction:

NO₂^- + H₂O₂ → NO₃^- + H₂O 

Waters and wastewaters containing selenium are efficiently treated with hydrogen peroxide and Calcium hydroxide (slaked lime), leading to precipitation of calcium selenate, according to the following reaction:

 

Ca²⁺ + SeO₃^2- + H₂O₂ → CaSeO₄(s) + H₂O

Alternatively or additionally, the possibility of formation of precipitates of zinc and manganese selenates should be considered if the effluents treated with H2O2 contain these metals. 

Sulfides 

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:

Odor 

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. 

 

Toxicity 

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.

 

Corrosion 

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⁺ + S^2- (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: 

S^2- + 4 H₂O₂ → SO₄^2- + 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 
• Polishing

 

Complete 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.

 

Preventive Oxidation 

When used as a preventive measure to the development of sulfides, hydrogen peroxide is primarily used as source of oxygen. 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. 

H₂O₂ → H₂O + ½ O₂

 

Auxiliary Oxidation

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.

 

Polishing 

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.

 

Disinfection 

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.

Treatment of Wastewaters Containing Sulfites 

Hydrogen peroxide can be used in sulfite oxidation, generating water and sulphate ions according to the following reaction:  

SO₃^2- + H₂O₂ → SO₄^2- + H₂O