TYPICAL UNIT OPERATIONS AND PROCESSES USED FOR THE TREATMENT OF SURFACE WATER

Screening and Coagulation/Flocculation

PRELIMINARY SCREENING

Screens can be:

1. Coarse screen – ex. Bar screen
2. Fine screen – ex. Microstrainer

MICROSTRAINER

A variation of the fine screen is the microstrainer . This consists of a rotating drum with a stainless steel micromesh fabric. The mesh size can range from 15 μm to 64 μm so that very fine suspended matter such as algae and plankton can be trapped. The trapped solids are dislodged from the fabric by high-pressure water jets using clean water, and carried away for disposal.

STORAGE RESERVOIR/PRELIMINARY SETTLING TANK (Optional)

Storage of the screened water in a preliminary settling tank or reservoir smoothes out fluctuations in the water quality and helps to reduce the suspended solids content. It also reduces the number of pathogenic bacteria present, and the oxidation which can occur will allow the degradation of organic matter and the precipitation of soluble iron and manganese as oxides and hydroxides. It is generally recommended that storage should be for at least seven days in the case of river-derived supplies. The storage of water is particularly valuable when abstraction is not possible, e.g. during droughts, or when the water source is badly contaminated or in flood condition.

AERATION (Optional)
After preliminary settlement, it may be necessary to aerate the water in the case of poor quality water with a low dissolved oxygen content. There are several ways in which this can be done but the simplest is to allow the water to fall over a series of steps so that it is able to entrain oxygen from the air. This is known as cascade aeration. In addition to increasing the oxygen content, aeration also helps to liberate soluble gases, such as carbon dioxide and hydrogen sulfide, and volatile organic compounds which could give an undesirable taste to the water.
Aeration can reduce the corrosiveness of raw waters which are acidic due to their carbonic acid content. When the water is aerated, some of the dissolved carbon dioxide is displaced by the oxygen dissolving in the water. This causes some of the carbonic acid that has been formed in the water by the carbon dioxide to be converted back to carbon dioxide and water in order to maintain chemical equilibrium.

Aeration is also used to remove iron and manganese from solution. Iron and manganese can cause peculiar tastes and can stain clothing. Iron is soluble in water only in the absence of dissolved oxygen and at pH values below 6.5, when it is in the ferrous (Fe2+) state. Aeration converts soluble iron into its insoluble hydroxide [Fe(OH)3] which can then be removed by filtration. Manganese can be removed in the same way.
After aeration, the water may be passed through a further fine screen before entering the treatment works proper.

Cascade Aeration

COAGULATION AND FLOCCULATION

After the source water has been screened and has passed through the optional steps of pre-chlorination and aeration, it is ready for coagulation and flocculation.  

The primary purpose of the coagulation/flocculation process is the removal of turbidity from the water. Turbidity is a cloudy appearance of water caused by small particles suspended therein. Water with little or no turbidity will be clear.
The primary purpose of the coagulation/flocculation process is the removal of turbidity from the water. Turbidity is a cloudy appearance of water caused by small particles suspended therein. Water with little or no turbidity will be clear.
The process removes many bacteria which are suspended in the water and can be used to remove color from the water.
In the flash mixer, coagulant chemicals are added to the water and the water is mixed quickly and violently. The purpose of this step is to evenly distribute the chemicals through the water. Flash mixing typically lasts a minute or less. If the water is mixed for less than thirty seconds, then the chemicals will not be properly mixed into the water. However, if the water is mixed for more than sixty seconds, then the mixer blades will shear the newly forming floc back into small particles.
After flash mixing, coagulation occurs. During coagulation, the coagulant chemicals neutralize the electrical charges of the fine particles in the water, allowing the particles to come closer together and form large clumps.

The final step is flocculation.  During flocculation, a process of gentle mixing brings the fine particles formed by coagulation into contact with each other.   Flocculation typically lasts for about thirty to forty-five minutes.  The flocculation basin often has a number of compartments with decreasing mixing speeds as the water advances through the basin.  This compartmentalized chamber allows increasingly large floc to form without being broken apart by the mixing blades. 

FLOC
The end product of a well-regulated coagulation/flocculation process is water in which the majority of the turbidity has been collected into floc, clumps of bacteria and particulate impurities that have come together and formed a cluster. The floc will then settle out in the sedimentation basin, with remaining floc being removed in the filter.

COAGULANT CHEMICALS
Types of Coagulants

1. Primary coagulants
   Primary coagulants neutralize the electrical charges of particles in the water which causes the particles to clump together.

2. Coagulant aids
   Coagulant aids add density to slow-settling flocs and add toughness to the flocs so that they will not break up during the mixing and settling processes.

Alum

There are a variety of primary coagulants which can be used in a water treatment plant. One of the earliest, and still the most extensively used, is aluminum sulfate, also known as alum. Alum can be bought in liquid form with a concentration of 8.3%, or in dry form with a concentration of 17%. When alum is added to water, it reacts with the water and results in positively charged ions.

Coagulant Aids

Nearly all coagulant aids are very expensive, so care must be taken to use the proper amount of these chemicals. In many cases, coagulant aids are not required during the normal operation of the treatment plant, but are used during emergency treatment of water which has not been adequately treated in the flocculation and sedimentation basin.

Common Coagulant Aids:

1. Lime is a coagulant aid used to increase the alkalinity of the water. The increase in alkalinity results in an increase in ions (electrically charged particles) in the water, some of which are positively charged. These positively charged particles attract the colloidal particles in the water, forming floc.

2. Bentonite is a type of clay used as a weighting agent in water high in color and low in turbidity and mineral content. This type of water usually would not form floc large enough to settle out of the water. The bentonite joins with the small floc, making the floc heavier and thus making it settle more quickly.

Factors Influencing Coagulation

1. pH
   The effectiveness of a coagulant is generally pH dependent. Water with a color will coagulate better at low pH (4.4 to 6) with alum. The pH range for operating regions of alum is 5.5 to 6.5, and iron salts is 4 to 9

2. Alkalinity

   Alkalinity is needed to provide anions, such as (OH-) for forming insoluble compounds to precipitate them out. It could be naturally present in the water or needed to be added as hydroxides, carbonates, or bicarbonates.
   Temperature

   The higher the temperature, the faster the reaction, and the more effective is the coagulation. Low temperature will slow down the reaction rate, which can be helped by an extended detention time. Mostly, it is naturally provided due to lower water demand during rainy days (cool weather).

3. Time
   Proper mixing and detention times are very important to coagulation. The higher velocity causes the breaking of floc particles, and lower velocity will let them settle in the flocculation basins. Velocity around 1 ft/sec in the flocculation basins should be maintained.

4. Zeta potential

   Zeta potential is the charge at the boundary of the colloidal turbidity particle and the surrounding water. The higher the charge the more is the repulsion between the turbidity particles, less the coagulation, and vice versa. Higher zeta potential requires the higher coagulant dose. An effective coagulation is aimed at reducing zeta potential charge to almost 0.

5.JAR TEST
Jar testing is widely used for screening the type of coagulant and the proper coagulant dosage. A jar test apparatus consists a stirring machine with six paddles capable of variable speeds from 0 to 100 revolutions per minute (rpm). In this test, batch additions of various types of different dosages of coagulants are added to the water sample. A rapid mixing stage is combined with the addition of the coagulant. This stage is followed by a slow mixing stage to enhance floc formation. The samples are then allowed to settle under undisturbed conditions, and the turbidity of the supernatant is measured as a function of coagulant dose in order to determine the proper coagulant dosage.

Source: Introduction to Wastewater Treatment Lesson 7