Introduction

Most of us who are in the water and wastewater treatment industry are acquainted to some extent, with the iron salts and with the most effective of them all, Ferric Chloride.
Together with Ozone it is the best solution available today in terms of efficiency, quality of product water, and economical reasons for you on the customer side. The chemistry of the Ferric Chloride is a classic one. Being around for a 100 Years. Mostly in use in Europe for the treatment of potable water. This chapter are written for the treatment of potable water.
Ferric Chloride & Ozone are both classic treatment option which after a long time of silence are back on the market with growing acceptance by water treatment engineers, as well the public sector. Over the past decade there was a strong believe that soon the time will be over for the metal salts used in water treatment. Super biological systems are developed, sludge cookers, water irridators, and ultra fine filtration devices are introduced. And all of them believed that soon the time will be over for the chemicals and metal salts in the water. However ones the dust settled, what we saw in the debris where a lot of half filled promises, a lot of capital invested and wasted, in elaborate and often ill conceived systems.
The outcome of this process are quiet positive, though and hat we have now is a more knowledgeable industry, both buyer and seller, and a renewed interest and acceptance in the classical methods of treating water, including the use of iron salts. The benefit here in Thailand is that you are able to take advantage of the experiences made in the USA and not have to make any mistakes again.
Throughout the world there is a better understanding of the aspects of treating water with iron salts and Ozone. However, I still find that there remain some misconcepts how the Iron salt and Ozone function in water treatment applications.

Physical Characteristics of Ferric Chloride

Ferric Chloride is normally sold as a simple solution of the salt dissolved in water and is normally delivered to the customer in solution strengths of 12-16% iron. The solution of ferric Chloride usually has an pH at or less than 2 or 3 with some amount of free acidity left to preserve the material. Although Ferric Chloride is considered to be a hazardous material, it is on the low end of the toxicity scale and is considered more for it’s corrosivity to metals than from its danger to the environment or the human body. As iron is a basic part of the human physiology, ferric Chloride is safer to work with and less difficult to handle than some of the other chemicals we come in contact with, such as caustic Soda, Chlorine, or Hydroflurosilisic acid.

Ferric Chloride Chemistry

The chemistry that are of most interest is based on its reaction in and with water. We see that once the ferric Chloride is added into a water stream it begins to undergo a series of fairly dramatic changes.
The first thing to happen is that ferric ion reacts with whatever available anions are present, such as sulfides, phosphates and alkalinity. All these reactions are important to water industry but it is the reaction with alkalinity that really promotes ferric Chloride to a position of being more than a mere reactant. ( Chlorine is only a reactant. It is a misnomer to promote Chlorine as Oxidant). In its simplest description, ferric ion reacts with hydroxide anions to produce the gelantinous Ferric Hydroxide precipitate. After it is formed, the ferric hydroxide, being very highly charged, reacts with the colloidal material in the water. Now it is the nature of colloidal particles to remain in suspension indefinitely, as long as the charged layer that surrounds the individual particles is left intact. What this highly charged Ferric Hydroxide particle does is to neutralize this charged layer of the colloid and free up the particle inside. This process of charge neutralization is known as coagulation.
After the colloidal particles are neutralized, we then see the dual nature of the ferric Hydroxide molecule coming into play. Because what happens next is the agglomeration of the ferric Hydroxide into flocs. This occurs through hydrogen bonding and other types of weak bonding mechanisms. As these Ferric Hydroxide floc particles grow, they begin to adsorb, absorb and sweep out of the stream, not only the recently coagulated particles, but also the other suspended material in the water. This process is referred to as flocculation.
This ability to coagulate and flocculate is part of what makes the ferric ion a valuable material to the water treatment industry. One of the important features of Ferric Hydroxide is the speed at which it forms a floc. This is in large part due to the high negativ charge attraction the ferric floc has for colloidal and suspended particles.
The other feature has to do with the atomic weight of Iron which is 55.8 and is relatively high for the basic unit of a metal salt. The high molecular weight of the iron is transferred over into the ferric hydroxide molecule which makes it significantly heavier than flocs of other coagulants such as Alum.
Combining the high charge attraction, which tends to create a more tightly joined particle, and the higher weight of the ferric hydroxide molecule we can understand how the ferric ion can produce a more reactive, a faster settling and, as it begins to compact in the sludge, a denser floc particle.

Potable Water Treatment

Many people think that the use of iron in treating water is new but in fact it’s been around for at least a 100 years. It was the first coagulant in the City of Louisville water plant, when it was build over 100 Years ago. Ferric coagulation has been considered a standard method for potable water treatment in Europe for decades, however, in Thailand it’s use has recently developed new momentum. This is due to abundant supplies of ferric Chloride by the production plant in the Wellgrow Industrial Estate. With completion of the new manufacturing facility the ability exists to provide a continuous, reliable source of Ferric Chloride to water treatment plants in Thailand.
Earlier, I mentioned the ability of Ferric Chloride to coagulate solloidal particles and then flocculate those particles so that they settle out of the water stream. This is precisely what occurs when Ferric is used to treat Drinking water and there are a number of benefits of using Ferric Chloride to do this Job. As you will remember when ferric ion is added to the water it is converted into Ferric Hydroxide through a reaction with the available or added alkalinity. Unique characteristics of the ferric Hydroxide floc make it an excellent choice for the treatment of potable water.
First, the Ferric ion has a significant higher charge potential than other coagulant ions. When the ferric ion is converted into ferric hydroxide it carries over some of this characteristic. When the ferric hydroxide reacts with colloidal material the reaction is so strong that it is uncommon to find a disproportionate molar advantage in using the iron over other coagulants. In addition the charge demand of Ferric Chloride is nonspecific. Contrary to what might be found with other coagulants such as coagulant polymers, so the charge density of the colloidal particles are not an issue. When the floc is formed with ferric hydroxide it tends to be more dense and less fluffy. This then works with the second important characteristic of the ferric ion.
Because the ion molecule is comparatively heavier in molecular weight, twice that of the aluminum molecule, it forms a floc that is proportionately heavier. Taking this, and a tendency to form a denser floc particle, we see that the floc that is formed not only settles 3 to 4 times faster, it also creates a much more compact sludge.
These features lead to some obvious benefits:

• First a stronger reaction between coagulant and colloid allows for less coagulant to be used. The ferric molecule ends up being significantly more efficient than the aluminum molecule.
• Second a denser floc particle settles faster and a faster settling floc allows for smaller plants to built or more flow to be pushed through existing facilities.

All these features add up to benefits that are both operational and economic.

One of the major applications of the ferric Chloride is the removal of color from raw water. Many plants using ferric Chloride to treat the color in their water have no other treatment option other than Reverse Osmosis and it’s high capital and maintenance and operational costs.

With Ferric Chloride and Ozone this way of water treatment changes.

One of the newer applications that is still being studied has to do with the prevention of Disinfection By-Product formation. Disinfection By-Products are compounds that are created when natural organic materials are exposed to the chlorine that is used to disinfect our drinking water.
The approach we are after to remove the organic compounds and materials from the water prior to the disinfection step to prevent their reaction to form halogens, carciogenic substances and other toxic compounds.
The real solution for it are the process, called:

Enhanced coagulation

Enhanced Coagulation is the process by using Ozone and ferric Chloride in an symbiotic way of treatment.
This  are explained in this paper and will be further explained by the plant layout.

Step 1.

The incoming raw water are treated by Ozone through diffuser disk in the raw water storage tank the discs are set in a depth of 130 – 150 cm below water level, operation is continous 24 hour a day. Even if no raw water inflow this will improve the quality of the water manifold and will stabilize even by excess inflow through the memory effect by the water molecules.
Benefits:
Removal of Organic substances up to 80%
Removal of BOD, COD up to 75%
Removal of Bacteria & Virus up to 95%
Removal of Turbidity up to 95%

Step 2.

The water then are pumped to the Ferric Chloride injection Line where it will be mixed with the Amount of ferric Chloride. ( On site testing for amount will be done by the manufacturer of the Ferric Chloride) the Ferric Ion will then remove, coagulate, and flocculate the remaining unwanted substances in the Water and will be settled down in the Flocculation section of the treatment. After sedimentation the water will then pass through the Sandfilter and flow in to the Clearwater storage Tank.

Step 3.

In the clear water storage tank again Ozone will be applied to the Water by Diffuser Disks to prevent the grow of Bacteria and Viruses in the Water, and enriching the water with Oxygen for when Ozone braks down it converts back to Oxygen.

Step 4.

The most important step in the treatment.
In my opinion here pure Oxygen should be used to produce Ozone, for containing a safe residual amount of Ozone in the treated water before it passes to the houses and other facilities.
Out of cost saving options Chlorine could be used in an very low amount that it is not traceable by Humans, in this situation the treatment of Step 3 would not be needed. Therefore 1 more unit in Step 3 should be installed to lift the water quality in this stage to the highest point possible.
If using Step 4 with Oxygen the highest water quality are guaranteed. If using in this stage out of Cost savings on equipment Chlorine in small amounts the water quality will be high, but not as good as with Oxygen Ozone by himself.
The benefits and drawbacks will and should be discussed with the Customer.