Working of Raw Water Treatment Systems

There is a requirement for some type of raw water treatment system to ensure an efficient process as well as a quality product, for any industrial company that uses a raw water source for its facility. The most effective raw water treatment system will help the facility to avoid the following:

1.avoid costly plant downtime,

2.avoid expensive maintenance fees, and

3.not being able to sell its products in the market

What is a raw water treatment system?

How does the raw water treatment system function?

The answer to this query primarily depends on the quality of the raw water source in relation to the quality of water needed for the factory.

What is a raw water treatment system?

A raw water treatment system consists of multiple individual technologies which address the facility’s particular raw water treatment needs.

Treating raw water is mostly not a static process. A raw water treatment system that is designed, engineered and built to accommodate fluctuations in treatment needs will facilitate in avoiding costly replacements or upgrades in the future.

An efficient cum well-designed raw water treatment system must have the ability to handle the following:

1. seasonal variations in turbidity as well as flow

2.·variations in water chemistry needs as well as requisite chemical volumes adjustments

3. changes in water quality requirements (the quality of feed water needed for a fresh new boiler) 

What are the elements of a basic raw water treatment system?

The list of components of a raw water treatment system will depend on the quality of water being drawn from the source in relation to the quality of water required.

Generally, a basic raw water treatment system shall include the following:

1.  chemical feed to trigger the flocculation/coagulation of any suspended solid matter

2.  clarifier to settle out the bigger solid matter

3.  filtration to remove the tiny particles

4.  control panel (based on the level of automated operation required)

Based on the requirements of the plant as well as process, these standard components are normally sufficient. If the plant requires a customized or sophisticated  system, then one needs to add on some features/technologies.

What does a raw water treatment system facilitate to remove?

A raw water treatment system consists of the technologies necessary to remove the following:

• Suspended/colloidal solids: These cause

1. Unpleasant odors in food/beverage product

2. Foul process equipment

3. Energy losses for the plant

4. Silica/colloidal silica: These cause

5. Foul/scale boiler equipment

6. Reduce the efficiency of plant equipment

7. Product contamination

 Iron: These cause

1. coats fixtures,

2. fouls industrial processes,

3. foul tastes/odors in products.

• Bacteria: These cause

1.sickness and severe digestive issues/health problems

2.coating in cooling tower components.

• Hardness: These cause

1.Coating in equipment fixtures

2.Plugs in pipes

3.Scales in equipment

4.Buildup of sludge

How does a raw water treatment system function?

A raw water treatment facility process shall consist of the following steps:

Raw water intake

Raw water is defined as the untreated water found naturally in the surrounding environment. This water can come from several sources, including groundwater, rivers, oceans or lakes, An industrial plant takes in the water from the surface water source.

During this process water is pulled in through pipes or by gravitational force through a mesh-screen/grate. This mesh-screen/gate help to eliminate the larger objects, such as leaves, twigs and fish. The water is thereafter pumped into the main facility where treatment starts.

Coagulation

Subsequent to removal of all the large objects from the raw water source, different chemicals are added to a reaction tank to remove the bulk suspended solids as well as other various contaminants. 

This process begins with an assortment of mixing reactors either one or two reactors. These reactors add particular chemicals to remove all the finer particles in the water. This happens as a result of the combination of these particles into heavier particles that settle out.

The aluminum-based coagulates such as alum and polyaluminum chloride are the most widely and commonly used coagulates. Coagulation of the particles  can also be done by a slight pH adjustment also.

Flocculation

After the completion of coagulation, the water moves into a flocculation chamber. In this chamber, the coagulated particles are slowly stirred along with long-chain polymers. These long-chain polymers are charged molecules that grab all the colloidal as well as coagulated particles and pull these together. This creates visible particles that settle down and look like snowflakes.

Sedimentation

The gravity settler forms the sedimentation part of the raw water treatment process.  This is normally a large circular device where flocculated material as well as water flow into the chamber and circulate from the center out. In a very slow settling process, the water rises to the top and overflows at the perimeter of the clarifier.

This allows the solids to settle down to the bottom of the clarifier forming a sludge blanket.  

The solids are then brought to the center of the clarifier into a cylindrical tube. In this process, a slow mixing takes place, resulting in the sludge to be pumped out of the bottom into a sludge-handling/ dewatering operation. The dewatering process removes all the water out of the sludge with filter/belt presses, resulting in a solid cake.

The sludge water is put onto the press where the water runs between two belts that squeeze the water out. Subsequently, the sludge is put into a big hopper that goes to either a landfill or to a sludge reuse facility. The water from this process is normally reused as well as added to the front end of the clarifier.

Filtration

This step involves the running of the water overflow into gravity sand filters. These filters are big areas where they put 2-4 feet of sand, which is a finely crushed silica sand with jagged edges. The sand is normally installed in the filter at a depth of 2-4 feet, where it packs tightly. The feed water is then passed through the sand that helps in trapping the particles.

For smaller industrial systems, a packed-bed pressure multimedia filter can be used instead of gravity sand filtration. In some cases, based on the water source and the quantum of iron, one can also use a green sand filter in place of the sand filter. Mostly, the polishing step for conventional raw water treatment is sand filtration. 

Ultrafiltration (UF) can also be used after the clarifiers in place of the gravity sand filter in some cases. UF can replace the entire clarification process in total. Membranes have become the newest technology for treatment. 

In this technology, water is pumped directly from the raw water source through the UF after the chlorination process. This eliminates the need for the entire clarifier, filtration train.

Disinfection

The next step after the water flows through the gravity sand filter, is disinfection/chlorination to eliminate the bacteria in the water. In some cases, this particular step is performed upstream prior to filtration which will keep the filters disinfected and clean.

In such a case where this step is prior to filtration, larger quantities of disinfectant will be required. In this manner, the filters are disinfected as well as kept free from bacteria as well as the filtered water.

If chlorine is added upfront, then bacteria gets killed and will result in less fouling. If there is bacteria in the bed, one need to grow slime and also need to backwash the filters very often. Hence it depends upon how the system operates, whether the system is set up to chlorinate upstream before filtration or to chlorinate downstream subsequent to filtration.

Distribution

In the case of the raw water treatment being used in an industrial process, the raw water is normally pumped into a holding tank. The water can then be used based on the demands of the facility. For municipal use, the treated water is normally pumped into a distribution system of water towers. Water is also pumped into different collections as well as distribution devices in a loop throughout the city.

Other steps for the raw water treatment process

In some cases, depending upon the plant’s process and quality requirements, some raw water treatment systems might include:

Lime softening

In case of high hardness or sulfates, or other constituents in the water, one need to precipitate or take out, a lime/lime soda process must be used. This increases the pH, resulting in hardness in the water to precipitate out. Cold, hot or warm lime processes will yield a different efficiency in each case. Normally, hotter water eliminates more hardness.

Ion exchange softening

In some industrial as well as municipal applications, if there’s high hardness, there can be post treatment required for the elimination of the hardness. A softening resin can be used instead of using lime. This is a strong acidification exchange process. In this process, resin is charged with a sodium ion. 

As the hardness comes through, the resin has a higher affinity for calcium, magnesium, and iron. Hence, the resin will grab that molecule and release the sodium molecule into the water.

How to choose a Facility’s Raw Water Treatment System

How does one choose the best raw water treatment system for the plant/factory?

The main factors to be considered whilst choosing a raw water treatment system.

There are three main factors that can help one to choose the ideal raw water treatment system for the plant:

 • Quality.What is the quality of the raw water source ?

What are the purity requirements for the treated water?

• Raw water testing/treatability study results.

What are the variations of the feed water chemistry over a period of time ? How does this affect the plant process?

Will the proposed treatment options assist in solving the issues as well as meet local discharge regulations for the secondary wastes generated by the plant ?

• Plant lifespan. How long is needed to run the system?

How many years?

This needs to be worked out with the system engineering company to analyze these key points. This will give a clear path in the right direction while choosing the best system for one’s plant.

The following gives an in depth analysis into each of the factors thereby simplifying the decision making process:

The quality of the raw water source in relation to the quality requirements post treatment

This is one of the critical factors that will determine how to choose the raw water treatment system. The equipment that forms the actual makeup of the system is primarily determined by the quality of the raw water source in relation to the quality of water needed post treatment.

What is the quality of the water source?

Water source is the foremost thing to be understood whilst choosing the best raw water treatment system for the plant..In every case, one should carefully consider looking at the source water and whether it’s best to use the local municipal-fed, groundwater or to use the surface water. Both of these are affected by different types of contaminants.

In certain cases, it’s preferrable to treat one’s own raw water from ground or surface sources. Alternatively one can buy raw water from a secondary source, such as a municipality. However, the water quality needs to be evaluated.  

One needs to weigh the options - a poor quality water from the municipal water source which needs to be treated further to make it useful in the facility; or, access to one’s own better quality surface or ground water. The contaminants contained in the water source in relation to the water quality needs will affect the technology present in the makeup of the system.

What is the quality of water needed?

The second important factor to be understood whilst choosing the best raw water treatment system for the plant is the quality of water needed for the plant. Is the water required to be pure for drinking?

Is the water required to be Ultrapure for microelectronics production? Is the water not required to be so pure for domestic use such as flushing a toilet/equipment wash down?

The water quality shall depend on the industry. For instance, many industrial facilities in industries such as petrochemical, power, refineries and chemical require large volumes of water for boiler makeup.

Due to this fact, selection of the raw water treatment technologies should ensure that the system will appropriately prepare the water for polishing treatment as eliminating colloidal contaminants from the water, which will foul membrane systems as well as plug deionizers. Specifically, this can relate to The colloidal silica  will affect the downstream treatment technologies as well as their ability to run efficiently.

Cold as well as warm lime softening and silica reduction campaigns such as aluminum-based coagulants or membrane processes can be utilized to remove colloidal materials. These can also be used to polish with carbon and ion exchange for the required boiler feed.

So, when choosing the optimal raw water treatment system,specific process requirements will largely affect the needs.

Firstly assessment as to which water source is best and evaluation of the quality of the chosen water source needs to be done. This will help in considering the technologies needed to treat the raw water source for the facility to get the water quality required for the process.

The outcome of a thorough treatability study

A raw water treatability study is a study (test) that will determine whether the raw water can be treated for the process as well as how it needs to get treated.

By a proper study, the problems in the feed water stream can be clearly identified. This will ensure that the appropriate treatment solutions are considered as well as implemented in the raw water treatment system.

This step is the most critical and important step whilst choosing the best raw water treatment system for the plant. Firstly one need to have a roadmap of two/three technology platforms that meet the base as well as operating cost. Then  run an efficient treatability test to validate the assumptions  made about possible contaminations as well as solutions to remove them.

It is better to run pilot testing in the field to validate the treatment-technology assumptions and also to optimize design. During this phase, other problems can arise and be traced prior to choosing the components of the system. This can help save a lot of plant downtime as well as costly system equipment change-outs in the future.