Water and Industrial Wastewaters

The discarding of wastewaters accordingly with the environmental legislation requires the control of the COD and BDO. This can be achieved through operations of biological treatment, that lead to the reduction of BDO. 
In the cases where the effluent contains excessive levels of non-biodegradable substances, the biological treatment is not enough, being necessary the addition of previous or subsequent treatment with more potent oxidant systems. 
 
In these cases the hydrogen peroxide can be employed in addition with an advanced oxidation process to oxidize the recalcitrant compounds, leading to the reduction of the COD to the limits defined by the environmental laws.
 
Note: The residual hydrogen peroxide (non-reacted residual) must be eliminated of the treated wastewaters in which there is COD control, because it decomposes in oxygen and water, generating a higher value than that of the real COD.

Deionized water is largely used in the chemical industry and pharmaceutical production.  

This chemically pure water is, however, easily contaminated by bacteria of low nutrition requirements, such as: non–fermentative gram-negative bacteria, P. aeruginosa, among others. These populations can easily contaminate liquid pharmaceutical formulations (such as syrups, liquid solutions and emulsions) as well as to increase the microbial load in equipment and flasks, also reducing the thrust level of the water used for injections. 

So, to meet the Good Manufacturing Practices (GMPs), according to the FDA standards, requires constant monitoring and periodical disinfection of the deionized systems. 

Many kinds of solutions have been proposed to solve the contamination matters. The one of highest investment is the use of a complete stainless steel system (tanks, filters, lines and pumps), sterilized via flowing steam. Sodium hypochlorite, formalin, quaternary ammonium compounds are largely employed, even tough this last one can produce resistant strains. With all those disinfectants, the rinse of the residues os sterilizing solutions becomes a great problem. Also, the sodium hypochlorite causes corrosion to the stainless steel. 

The solution is the use of hydrogen peroxide. Its capacity of disinfection is extremely high compared to other disinfectants, meeting the specification needs. It requires a simple procedure with short total time of machine stop, without generating any toxic residue after rinsing. The frequency of disinfection is also reduced, as well as the amount of deionized water used for rinsing. No corrosion occurs.

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⁺

Alternatevelly 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⁺

Wastewaters containing cyanides are generated mainly by hydrometallurgical operations of gold and silver extraction, galvanic industries, production of nitrated compounds, steel mills and oil refineries.

The current treatment options of include the use of an oxidizing agent that converts cyanide into the much less toxic cyanate. This one, in turn, hydrolyzes spontaneously, forming as final products of this operation ions of carbonate/ bicarbonate and ammonia/ ammonium.

The hydrogen peroxide is indicated for treatment of clarified wastewaters, which already present less than 10 mg/l of dissolved copper. The Cu2+ ion acts as a potent reaction catalyst. In case it’s insufficient, a CuSO4 solution is added to the treatment – The added Cu2+ precipitates itself at the end of the reaction as Cu(OH)2.

In case of wastewaters in pulp, the best oxidant indication  is Caro’s Acid (H2SO5), that, for having a very quick action, eliminating the need of catalyst addition.

Process Chemistry 

With hydrogen peroxide, the action of cyanides occurs according to the following reaction:

CN^- + H₂O₂ → CNO^- + H₂O

When using Caro's acid, sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) are mixed together to generate H2SO5: 

H₂SO₄ + H₂O₂ → H₂SO₅ + H₂O

The action of Caro’s acid over free cyanide produces the cyanate ion. It hydrolyses spontaneously to carbonate and ammonium ions. The reaction must be made in alkaline medium to avoid generation of hydrogen cyanide (HCN), usually by adding slaked lime or caustic soda:

CN^- + H₂SO₅ + 2 OH^- → CNO^- + 2 H₂O + SO₄^2-

CNO^- + 2 H₂O → NH4⁺ +CO₃^2-

In analogous reactions to the free cyanide, metallic cyano-complexes, moderably stable, such as copper, zinc and nickel, are oxidized, generating, besides carbonate and ammonium, precipitated hydroxides. This way, it is also achieved a removal of heavy metals from the effluent:

M(CN)_n^(2-n) + n H₂O₂ → n CNO^- + M²⁺ + n H₂O

n CNO^- + M²⁺ + (2n+2) H₂O → n CO₃^2- + n NH4⁺+ M(OH)₂(s) + 2 H⁺

where M = Cu, Zn, Ni, etc.

In the case of iron-cyanide complexes, the removal is not by oxidation, but by precipitating insoluble complexes with  ions of transition metals: 

Fe(CN)₆^4- + 2 M²⁺ → M₂Fe(CN)₆(s)

where M = Fe, Cu, Zn, Ni, etc.

One of the most utilized processes to treat effluents containing chrome (VI) consists in the chemical reduction to chrome (III), followed by precipitation of its hydroxide. The reducing agents conventionally used in industry are sodium bisulfite and sodium metabisulfite. Even tough they are efficient, they have the disadvantage of aggregating to the treated wastewaters additional loads of sulfite (non-reacted residual) and sulfate (reaction product).

The hydrogen peroxide can be used instead, as a clean reducing agent of the chrome (VI) in acid medium as an alternative to sulphited compounds. All the residual hydrogen peroxide decomposes into water and oxygen, which eases the employing of treated wastewaters as reuse water in the industrial process.

The following reactions represent the global treatment process:

Cr₂O₇^2- + 3 H₂O₂ + 8 H⁺ → 2 Cr³⁺ + 7 H₂O + 3 O₂ (g) 

Cr³⁺ + 3 OH^- → Cr(OH)₃(s) 

Introduction

Many industrial wastewaters, such as those generated in oil refining, synthetic materials production and in chemical synthesis, may contain phenols with many structures. When they get to the receiving water bodies, those compounds can cause deleterious effects, especially in the drinking water treatment.

The chlorination of waters contaminated by phenols might, under certain conditions, lead to the formation of chlorinated phenols, more toxic than the originals, and that, even in small concentrations, may give unpleasant smell and taste.

For this reason and due to its toxicity, the admissible limits of phenol in the wastewaters are very low and rigorously controlled. 

Chemical/Biological Treatment 

The biological treatment of phenols is widely used, but, in the majority of the systems, even after previous adaptation, is too sensible to increases in the organic load. Besides that, those systems have a restricted capacity of phenol degradation. 

The chemical oxidation with chlorine leads, as mentioned above, to the formation os chlorates compounds, more toxic than the originals.

Oxidation with hydrogen peroxide 

The oxidation of phenols with hydrogen peroxide, catalyzed by salts of bivalent iron (Fenton reaction), occurs quickly in acid medium (pH between 3 and 4). 

Hydrogen peroxide reacts with phenols, forming more hydroxylated compounds, such as catechol, hydroquinols and resorcinol. Depending of the reaction conditions, a total decomposition of the phenol can be obtained, such as in the reaction below:

C₆H₅OH + 14 H₂O₂ → 6 CO₂ + 17 H₂O

After the oxidation it is usually necessary to increase the pH back to neutrality with sodium or calcium hydroxide. With this procedure the flocculation of ferric hydroxide occurs, together with the products originated by the oxidation, which are subsequentely separated by filtration.

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 (Mn(OH)2) precipitation would only be possible with pH 10 or more.

The reaction occurs as following:

Mn²⁺ + H₂O₂ + 2 OH^- → MnO₂ (s) + 2 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

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.

The constant search for efficiency increase and industrial cost reduction in the paper sector has led to the closing of many whitewater circuits, reducing the loss of fibers, chemical products and industrial water. Besides that, there is the social and legal necessity of reducing the volume and improvement of the quality of liquid effluents.

The water reuse obtained by the closing of the water circuits brings direct economical advantages to the paper industry. However, the presence of fibers, minerals, resins, organic loads (binders, dispersants, etc.), moderately elevated temperature and aeration cause a lot of operational and quality problems. The majority of these products is related to the increase of salinity, odors caused by generation of H2S by sulfate-reducing bacteria, microbiological corrosion and formation of bacterial slime - and cause production interruptions, breakages and paper defects. In most cases, the more closed the system, higher will be its microbial contamination (i.e. recycled paper is potentially more contaminated than virgin cellulose).

The hydrogen peroxide is recognized as a microbicide, being also compatible with the environment. In view of this and with the objective of efficiency increase and cost reduction of whitewater circuits, Peróxidos do Brasil has developed an application of hydrogen peroxide as a microbicide agent for this area.

Its effect is sufficiently powerful, reducing the formation of biological deposits (slime) and number of chemical cleanings required, being able to effectively reduce the bacterial count to the required levels. Corrosion rates caused by sulfate-reducing bacteria are also greatly improved due to increased oxygenation, since they are anaerobic.

Sulfide level reduction is also achieved, eliminating odor in the circuit and reducing need of adding other compounds for H2S control. (See how hydrogen peroxide works for sulfide reduction.

With all these effects, the frequency of paper breakage is reduced and machine speed is increased. Also,  less cellulose is lost by the dumping of whitewater during the paper machine cleaning.