MIXED OXIDANT GASES GENERATED ON-SITE. José T. Masís Pres. Equipment & Systems Engineering, Inc. Miami, USA ABSTRACT

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1 Original: English MIXED OXIDANT GASES GENERATED ON-SITE José T. Masís Pres. Equipment & Systems Engineering, Inc. Miami, USA ABSTRACT Elementary chlorine has traditionally been the most common and economical chemical used in disinfecting drinking water, wastewater or industrial wastes. Chlorine gas, industrial sodium hypochlorite and calcium hypochlorite have been the most effective chemicals for the oxidation of organic water elements. It is widely used because of its easy acquisition and low cost. The increasing importance given to environmental protection, has resulted in the closing down of many industries, such as chlorinesoda factors, because they contaminate water bodies and the atmosphere. In many countries, trucks carrying chlorine gas are not allowed to circulate through cities and tunnels, or on certain bridges and roads. Chlorine plants must be built far away from schools, airports, hospitals and populated areas. However, the chlorine demand continues to increase for several reasons: the higher contamination of aquifers (leading to the need for more disinfectants); population growth; and a high increase in chlorine demand as a raw material for petrochemicals. On-site generation of disinfectants using salt and electricity is a very important alternative for water disinfection. Chlorine gas, which has traditionally been the most common and economic element used in water treatment, is in a historical process of adjustment to our society. Chlorine is continually being attacked by environmentalist movements due to its high handling and application hazards. Many chlorine factories in Latin America have closed and others cannot increase their production to cope with the growing demand. Controls and safety requirements in gas plants are intensified, increasing the prices of the disinfectant applied. Since the Treaty of Rio de Janeiro, ECO 92, almost every country has set up a new ministry for environmental protection, dealing with the standardization and control of this type of industries. In summary, chlorine gas as a disinfectant of general use in water has an uncertain future because of its supply logistics. Although some factories have limited and sometimes reduced their production for environmental reasons, the chlorine demand increases disproportionately. This is caused by the growing population that requires higher volumes of drinking water; and

2 by a higher level of pollution of aquifers used to provide water to cities and rural areas; and also by many factories producing PVC, glass, paper, etc. which use and demand high quantities of chlorine as an industrial input. Chlorine is one of the most essential chemicals in our society and at the same time, one of the most dangerous to handle, transport and store. Many countries prohibit the traffic of tank trucks through cities, some tunnels and bridges. In California (USA), it is almost impossible to build new chlorine plants in or near the cities. Marine transportation rates increase daily because of the high risk associated with chlorine, and the acquisition of chlorine gives headaches and ulcers to the sanitary engineers responsible for water treatment because of the logistics and hazards involved in its handling. Gas feeding requires special equipment and its correct operation has to be continuously verified due to its high power of oxidation and because this equipment is subjected to high working pressures. Storage cylinders have to be constantly inspected and periodically replaced to prevent leaks and accidents that are highly dangerous. There are other chemicals that offer chlorine for disinfection such as sodium hypochlorite and calcium hypochlorite. Both these chemicals are widely used in specific treatment situations when the use of chlorine gas is hindered by its supply logistics or transportation to the site where it is to be used or also by its feeding or low water flows that are not appropriate for this system of disinfection. The most common method for manufacturing industrial sodium hypochlorite is by bubbling chlorine gas through sodium hydroxide solutions. Concentrations in the order of 12% to 14% are obtained. The handling of this concentrated hypochlorite is also highly dangerous and its concentration is very unstable because as it is very avid to oxidation, its concentration is lost rapidly in relation to time. Other factors that lead it to lose its concentration rapidly are temperature, light and the material of which the storage tank is made. It is not uncommon to acquire a hypochlorite tank with 14% concentration which, by the time it has arrived at its destination for application, has lost 1% and in another week it has dropped to levels of 10-12% concentration. If chlorine is passed through calcium, calcium hypochlorite is obtained in typical concentrations of 60-65%. This chemical is also unstable in relation to storage time and temperature, although to a lesser extent than sodium hypochlorite. This is the most expensive form of chlorine compounds compared to other forms of chemicals with chlorine available for oxidation. Its handling also requires extreme care because it is highly hazardous. It is simple to feed. The hypochlorite is diluted proportionally with water and fed to the water body by gravity; or by removal of chlorine, whereby the water flow runs through controlled-flow feeders. Usually these feeding methods are rigid and cannot be adapted to variable flows, so under- or over-chlorination can easily occur, resulting in an excessive taste of chlorine in the treated water.

3 In different countries, the water industry s interest in developing systems that will offer a reliable alternative in the acquisition and application of disinfectants has been promoted for the following reasons: The growing difficulty of working with chlorine gas; Distribution logistics of chlorine gas in populated areas; The closing down of production plants and/or prohibition to increase production; Increase in chlorine demand due to higher consumption because of population growth; Higher contamination of aquifers, leading to need for more disinfectants; Greater demand for chlorine in petrochemicals, papers, PVC, glass, etc., manufacturing; Difficulty of supplying chlorine to rural areas, which are considered a priority in water treatment since it has been determined that they are the focus of waterborne disease epidemics; Difficulty in dosage of chlorine for small aqueducts; Proliferation of environmentalist movements against this type of potentially polluting chemicals; Creation of governmental environmental protection entities in the wake of ECO 92, which impose strict handling controls and regulations; Oscillating prices of chlorine and its compounds make for little stability and are subject to monopolization and speculations regarding availability. In addition, marine and overland transportation prices are continually fluctuating. Many non-producing countries depend on other countries for this basic commodity, essential for public health. ALTERNATIVE SOLUTIONS FOR DISINFECTION 1. Mixed oxidant gases generated on-site (MOGOD) 1.1 Basic technology: This system is based on the generation of chlorine and other oxidant gases in an electrolytic cell with two chambers separated by a special semi-permeable membrane. The anodic chamber is filled with brine (salt and water) and the cathodic chamber is filled with a sodium hydroxide solution. The anode, usually made of titanium, is separated from the cathode, that can be made of titanium, Hastelloy-C or stainless steel, by a selective membrane. When energizing the electrolytic cell by passing direct current (DC) from the anode to the cathode through the electrolytic environment (brine), sodium ions cross the membrane that selectively traps the chlorine ions that remain in the anodic chamber. These chlorine ions plus a few others form oxidant gases used in water disinfection.

4 The process is not really new in the chemical industry since this industry had previously been obtaining such gas mixtures by using electrolysis. However, these results were considered undesirable because the objective was the acquisition of pure oxidants. The water treatment industry, on the other hand, observed that the gases generated had a disinfection potential even greater than that of chlorine gas. Gases generated in the anodic chamber include chlorine, hydrogen peroxide, hypochlorous acid, ozone, hydroxyl radicals, traces of chlorine dioxide and oxygen compounds of very short life, etc. The broad range of gases generated and the proportion among them apparently depend on the cell design, the material of which the anode is made (type of coating), the concentration of sodium chloride in the electrolyte, the density of the current applied during electrolysis, and even the temperature of the process. The feeding of gases created in the anodic chamber is effected through gas extraction by vacuum or Venturi caused by water flow from pumping or by compression of the gases and their discharge into the water flow. The oxidant gas mixture is highly effective as a means of eliminating the microorganisms in water, including some that are among the most resistant to disinfection. This oxidation is performed under a broad specter of ph and temperature conditions providing residual chlorine. Tests show that the effectiveness of this disinfection process is equal to, or higher than, that of chlorine gas. Apparently, the contact time of these oxidant gases is less than the contact time required by chlorine gas to obtain the same disinfection. Its use in certain areas is somewhat limited by the strict requirement of very good quality salt with low carbonate content and calcium phosphates. Dirty salts usually tend to obstruct the membrane and specialized labor is required to clean it. This membrane is usually cleaned once a month, depending on the quality of the salt used. The membrane has to be changed frequently (approximately every year) while the anodes may last two to three years, if they have a suitable liner and the proper current density through them is used (not increasing density to obtain greater production). The duration of the electrodes depends on the material of which they are made; they usually have a long service life. This system is offered in a broad range of production capacities. It generates gases on-site, which have to be fed at the time of generation as they cannot be stored for subsequent use. This condition can be very relevant in countries where the electric current frequently fails, since when there is no electric current there is no disinfection. The system does not produce disinfectant batches that can be used afterwards, a factor which limits its use in rural areas. Salt and a small quantity of caustic soda have to be stored to be added in the cathodic chamber to start up the system. This procedure has to be followed whenever the system initiates its operation.

5 The hardness deposited in the cathodes is cleaned with a hydrochloric acid solution at 5%. The cleaning of the membrane and anodes is performed mechanically with a smooth brush and clean water. The maintenance requires a certain degree of technical preparation on the part of the worker. It should be noted that the system generates highly toxic and corrosive chemical gases which have to be handled very carefully. Graphs 1 and 2 show the oxidant gas generation system (MIOX) and the MOGOD system. The basic principle of the two systems is the same although there are important variations in their construction. 2. On-site generation of sodium hypochlorite On-site generation of sodium hypochlorite is nothing new. It is a simple process that has become economic and reliable over the past years, thanks to the development of special anodes of low electric consumption. Generally speaking, the electricity consumption of a modern cell is 2.5 kwh and 3.5 pounds of salt is needed per each pound of chlorine equivalents produced. The first plant of sodium hypochlorite generators was built in Brewster, New York, in 1893 and was known as the WOOL Process. It was used in the treatment of industrial wastewater. In 1930 electrolytic generators were used by YMCA in the disinfection of swimming pool water but because of the high electricity consumption of electrodes, chlorine equivalents proved much more expensive than chlorine gas and, consequently, were discontinued. During the First World War in , a solution obtained by this electrolytic method was used as an antiseptic in hospitals to treat open wounds. This solution was called Carrel Dankin Solution. Later the first electrolytic cell was developed by Van Peursem but the low cost of chlorine gas limited the development and progress of this technology. 2.1 Basic technology The technology of on-site sodium hypochlorite generators is very simple and reliable. It is based on the principle of brine electrolysis or the passing of electricity between the anode and the cathode through salt water which makes H 2 O and ClNa react and form the ClONa, releasing hydrogen during its reaction in the cathodic compartment. This technology is almost the same for elementary chlorine.

6 3. Cell reactions Chlorine is generated in the anode, while the cathode produces hydrogen as follows: (1) 2 Cl > Cl2 + 2 e - (2) 2 H 2 O + 2 e > H OH - While still in the cell, chlorine reacts immediately and forms hypochlorous acid according to this reaction: (3) Cl2 + H 2 O > HOCl + H+ + Cl- Considering that we began with salt (NaCL), all reactions lead to the following: (4) 2 NaCl + 3 H 2 O > 2 NaOH + H 2 + HOCl + HCl In the same cell, the hypochlorous acid is separated and forms the hypochlorous ion which is considered as Free Available Chlorine or FAC according to the following reaction: (5) HOCl < > OCl - + H + If HOCL and OCL- concentrations are the same, the total reaction in the cell is as follows: (6) 2 NaCl + 3 H 2 O > NaOCl + HOCl + NaOH + 2H 2 It should be noted that when elementary chlorine gas is used for disinfection, one of the two atoms of the chlorine molecule (Cl 2 ) forms hydrochloric acid (ClH), which is wasted since it is not a reactive oxidant; and only one atom of the Cl 2 forms the hypochlorous acid (ClOH) which is the disinfecting agent. When 100 pounds of Cl 2 are added, only 50 pounds are used, the other 50 are wasted. It acidifies the medium and reduces ph, which is not desirable, sometimes requiring neutralization with caustic soda. Today s technology uses anodes with a basic structure of titanium, which is almost indestructible to oxidation. These anodes are coated with oxides of precious metals such as iridium, rhodium, platinum. With this coating, electric conductivity is greater and more uniform through all the electrodes, increasing production efficiency and reducing the consumption of electricity. Electricity is the most expensive raw material. The cathode can be made of titanium or of special alloys such as Hastelloy-C or high grade stainless steel. Both plates form the core of the system or the electrolytic cell.

7 These systems generate sodium hypochlorite at a concentration of 0.6% to 1.0% (6,000 ppm 10,000 ppm). Such concentrations are environment-friendly and they are non-hazardous for the operator, although they have a high disinfection power. Unlike chlorine gas, sodium hypochlorite solutions at low concentrations are accepted by environmentalists for widespread use as a disinfectant that can gradually replace elementary chlorine gas. The solution is very stable: its oxidation avidity is considerably reduced because of its low relative concentration compared to commercial hypochlorite that is typically offered at 12%. Graph 3 relates different concentrations of sodium hypochlorite to time and temperature. Generators are of simple construction, durable and very easy to maintain and operate. They consist of an electrolytic cell and a power source or solar photovoltaic panels. The anodes last at least three years providing that the density of the current applied does not exceed the limits established for the coating stability, and providing that the quality of power source which supplies the direct current is also good. In the course of time, the coating is lost and the consumption of electricity gradually increases until the process becomes uneconomic and the electrode has to be changed. Modern systems have greatly developed the technology. Some examples are the power sources of the switching type, that besides being small and light compared to conventional linear designs, have all sorts of protections for tropical working environments. They are highly resistant to poor handling by unskilled workers, they are protected against current fluctuations common in third-world countries and against short circuits caused by dirty electrolytes (leaves, insects), etc. These modern sources have mechanical/ electrical timers that accumulate electrolysis time when the current fails and resume electrolysis when the current returns accumulating the real time of the electrolytic process. These systems operate efficiently with solar panels, due to their relative low electricity consumption. The only maintenance that the system requires is the periodic immersion of the cell in white vinegar to clean carbonate and phosphate hardness settled in the cathode. It is very important to note that these systems work in batches which generate disinfectant that can be stored and used even when there is no electric power. This is very significant in rural areas where electricity frequently fails. The efficiency and effectiveness of water disinfection using sodium hypochlorite solutions have been widely verified. This system produces stable residuals. Graph 4 shows a typical plant of on-site simultaneous generation and feeding of sodium hypochlorite. It uses three polyethylene tanks connected among themselves with a manifold and a by-pass valve in between. In one of them, the cell is introduced into the previously prepared brine and the timer is regulated for a given time. Brine is prepared in a second tank. The generated hypochlorite is fed the next day, while the

8 electrode is introduced into the second tank. The brine in the third tank is prepared for the following day and so on. The feeding of these solutions both in water bodies and reservoirs, standpipes or pressurized lines is facilitated by their relatively low corrosive power compared to chlorine gas and the concentrated hypochlorite. Therefore, material available in unspecialized hardware stores can be used for its feeding. Feeding by gravity through PVC valves is common. The feeding of chemicals through variable impulse chemical pumps is also very common in pressurized lines or reservoirs, as well as being economic and reliable. The systems are manufactured in sizes ranging from 5 to 100 grams/hour of equivalent chlorine in batches. Automatic systems that generate 20 to 150 pounds/day are also offered. These systems are suitable for plants in towns with a population of inhabitants. Graph 5 shows typical cells of on-site sodium hypochlorite generation. Production costs Production costs are an important factor. The following table is used to analyze these costs, comparing the costs of on-site sodium hypochlorite generation with the cost of chlorine gas or concentrated hypochlorite, using national parameters. Usually the onsite production cost is up to 50% of the cost of hypochlorite of industrial concentration.

9 Comparative cost of on-site generation vs. commercial sodium hypochlorite 3.5 pounds of salt + 15 gallons of water kilowatts hour of electricity (AC) = 1 pound of equivalent chlorine I. On-site generation - Operation cost per pound of equivalent chlorine: A = Cost of salt, $/lb. B = Cost of electricity, $/kwh C = Cost of water, $/gallon (A X 3.5) + (B X 2.5) + (C X 15.0) = D = $/lb of equivalent chlorine II. Operation cost per pound of chlorine 1. Chlorine 2. Commercial sodium hypochlorite E = $/pound of chlorine gas F = Cost per gallon G = % concentration G x = H (lbs of chlorine/gallon) F/H = I = ($ per pound of equivalent chlorine) III. Cost savings by on-site generation Compare D with E or with I and multiply it by the number of pounds used per day. Tritation procedure to determine the content of available chlorine in sodium hypochlorite solutions 1. Add 10 ml potassium iodide (KI) at 10% in a 250-ml test tube. (The exact quantity of potassium iodide is not very important, since it is merely a catalytic). 2. Add approximately 20 ml of distilled and de-ionized water to the test tube. 3. Stir it until potassium iodide is completely dissolved. 4. Add 6 drops of sulfuric acid to the mixture. (The quantity of sulfuric acid is not important, since it is merely an activator). 5. Add exactly 3 ml of sodium hypochlorite solution to the test tube. 6. Tritate the mixture with sodium thiosulfate until it turns a light yellow color. 7. Add 4 drops of starch to the test tube. The solution will get a dark color. (The quantity of starch is not very important since this is only an indicator.) 8. Tritate the solution with sodium thiosulfate until its dark color becomes completely light.

10 Estimate of the available chlorine concentration from the tritation results: Concentration in mg/liter = (ml of thiosulfate used) x (% of concentration of thiosulfate used) x (1,000) x (35.5) (ml of sodium hypochlorite of the sample.) Figure 1. System of on-site generated oxidant gases (MIOX)

11 Figure 2. System of mixed oxidant gases generated on-site (MOGOD)

12 Figure 3

13 Figure 4. Scheme of continuous sodium hypochlorite generation Figure 5. Sodium hypochlorite generation system (AQUACHLOR)