Primary wastewater treatment with DAF clarifiers

• 1. Primary clarification in wastewater treatment plants
• 2. Primary treatment options in water purification
• 3. DAF clarifiers for primary wastewater treatment
• 4. Comparison between primary sedimentation and dissolved air flotation
• 5. SIDMADAF DAF systems in primary treatments
• 6. Summary

1. Primary clarification in wastewater treatment plants

Both surface water (rivers, reservoirs and lakes) and wastewater, both from urban and industrial sources, require appropriate treatment for their use, i.e., discharge for recovery. An important part of this treatment is the initial treatment, also called primary treatment, which consists of the separation of solids and larger contaminants.

Depending on the origin of the water to be treated, the stages involved in this process may vary. The most common treatment consists of a first stage of coarse separation, followed by a second stage of fines separation, which is usually carried out by means of mechanical systems such as screens and sieves. The final filtration span is usually not less than 3 mm, so it is necessary to install a complementary treatment to remove the rest of the suspended solids present in the water.

After these treatments to remove coarse solids, clarification systems are usually installed, which, depending on the application, may use chemical products for pH adjustment, sterilization, and addition of coagulant and flocculant reagents. In these cases we speak of physicochemical treatments.

Correct primary treatment is essential for the subsequent purification stages to function correctly.

2. Primary treatment options in water purification

In water treatment plants, different types of primary treatments are used to reduce the content of suspended solids to acceptable values for secondary and tertiary processes.

In the case of urban wastewater treatment plants, primary settling tanks are installed, which are sedimentation tanks, since in them the solid particles are separated by density and ascensional velocity, without the addition of reagents. These tanks are circular in shape and have a system for collecting floating materials with a surface sweeper, in addition to bottom scrapers that separate the decanted solids; in these cases, the separation of SS does not usually exceed 60 - 65%, and the reduction of COD rarely exceeds 40%.

The sizing of this equipment is based on an ascensional velocity of 0.5 - 2 ^m3/m2/h, and a retention time of approximately 1.5 - 3 h. The resulting sludge consists of floating and settled sludge, which is susceptible to rapid decomposition. The resulting sludge is composed of floating and settled sludge, which is susceptible to rapid decomposition. This sludge is usually mixed with activated sludge from secondary treatment and then sent to an aerobic or anaerobic digester, the selection of which depends on the quantity and type of sludge produced.

Primary decanter scheme 

In drinking water plants from surface water (rivers, lakes, reservoirs, etc.), clarification is also carried out after mechanical separation of solids, but in this case, a physicochemical treatment is performed.

During water collection, a sterilizing reagent (NaOCl, _Cl2, etc.) is usually dosed in order to eliminate the microorganisms present in the natural surface water. The colloidal materials, many of which are organic, are then coagulated by dosing reagents such as _FeCl3 or _Al2(_SO4). The corresponding pH adjustment is carried out by means of acid or alkali to optimize efficiency.

Colloid coagulation is performed in a high-speed agitated chamber. A polyelectrolyte-type flocculant reagent is then added, which is a polymer that will bind the clots formed to create clouds of fluffy solids (flocs). The flocculation chamber is larger than the coagulation chamber and the mixers used are of slow speed, ideal for not breaking the floccules formed.

The formed flocs will tend to sediment (settling) or float, depending on their density and nature.

For sedimentation, there is also the option of using static or lamellar decanters.

Lamellar decanters are designed with much higher ascension velocities than conventional decanters (up to 5 times higher), since they are sized based on the surface projected by the lamellae and their retention times do not usually exceed 1 hour. This has the following advantages:

• Reduction of occupied space
• Construction of the equipment based on the results of the Jar test trials.

Flotation systems are also available for certain cases:

• When oils and greases, surfactants and other light contaminants are present.
• When the flocs have a tendency to float, as for example in seawater desalination plants, where it is common to find algae, marine biological material and even hydrocarbon spills.
• When there are secondary treatments, such as reverse osmosis or ultrafiltration, which require high quality inlet water (low turbidity, low organic matter load, free of oil and grease, and well sterilized), to avoid clogging, scaling and biofilm, which reduce their operating capacity to the point of making them unviable.

Flotation clarification can reduce the space occupied and improve the efficiency of suspended solids separation, which can exceed 90%, while forming a supernatant sludge blanket with a higher concentration (3- 4%) than in primary settling tanks, where a concentration of 1% is not usually exceeded.

It must be taken into account that the flotation phenomenon is associated with the supply of air, which represents an additional energy consumption.

With regard to industrial wastewater treatment plants (WWTP), there is as much variety of discharges as there are industries and, therefore, the most appropriate pretreatment must be studied in each case.

Industrial wastewater is usually collected in a homogenization tank in order to obtain a discharge as continuous and constant as possible, since the discharges from the same factory can be very varied. There are occasions when it is convenient to separate them in order to treat them independently.

The first stages of pretreatment of these discharges are aimed at eliminating the coarser elements and larger solids. This process is carried out before the wastewater enters the homogenization basin to avoid mechanical problems in the rest of the installation. Due to the chemical variations that occur in industrial processes, it is also common to adjust the pH to make the coagulation-flocculation process more efficient.

3. DAF clarifiers for primary wastewater treatment

In the case of wastewater from industries, where pollutants are usually less dense than water, dissolved air flotation (DAF) is the most suitable solution thanks to its high efficiency and the small space required for its installation.

DAF technology is based on the supply of pressurized air (0.15 - 0.30 kg air / kg TSS) at 5 - 7 bar pressure, in a recirculated part of the treated water (approx. 10 - 25% of the total).

A DAF unit is divided into two zones:

• The front zone, also called the reaction zone, where the suspended floccules come into contact with the air bubbles that are introduced to trap them.
• The separation-clarification zone, where the air bubbles perform their function of separating the flocs, sending them by flotation to the surface. A baffle is fixed between the two zones to separate the operating conditions.

Dissolved air flotation flotation (DAF) clarification scheme

The clarified water is extracted from the bottom of the separation tank. This process causes the aggregates of bubbles and flocs that do not reach the surface to be swept away with the clarified water.

4. Comparison between primary sedimentation and dissolved air flotation

In relation to primary decantation, dissolved air flotation has a number of advantages, which are detailed below:

1. Higher efficiency: Dissolved air flotation is more efficient than primary settling in the removal of suspended solids, fats, oils and grease. 
2. Less space required: Dissolved air flotation requires less space than primary decantation due to its higher efficiency and shorter retention time. 
3. Greater flexibility: Dissolved air flotation is more flexible than primary settling in terms of treatment capacity and variations in water quality. 

However, like any process, it also has some disadvantages. Some of the disadvantages of dissolved air flotation are:

1. Cost: Dissolved air flotation can be expensive due to the need for specialized equipment and the complexity of the process. 
2. Energy requirements: Dissolved air flotation requires a significant amount of energy to generate the air bubbles necessary for the process.
3. Need for chemicals: Very often, dissolved air flotation requires the addition of chemicals to help separate solids from the water. 

Comparative table clarification technologies in primary treatments        

Technology	V. ascens. (m3/m2/h)	T. retenc. (hr)	SS reduc. SS (%)	COD reduc. COD (%)	% reduction Ac. and fats
Decantation Primary (WWTP)	0,5 - 1	1 -2	60 - 70	25 - 40	40 - 50
Decanting lamellar	3 - 5	0.5 - 1	80 - 90	30 - 40	50 - 60
DAF Flotation	5 - 20	– 1	90 - 95	30- 50	90 - 99

5. SIDMADAF DAF systems in primary treatments

SIGMADAF offers several models of DAF systems for primary clarification.

The FPHF/FPBC models operate at a higher hydraulic loading rate, with lamellas to improve the separation process and a smaller flotation surface.                                           

Thecompact FPBC consists of a steel or plastic tank used for a smaller capacity, in the range of 10 to 250 m3/h, while the compact FPHF is a larger capacity unit in the range of 300 to 1000 m3/h with a steel tank.

The FPBC-CW with the tank in RCC can treat flow rates from 500 to 3500 m3/h.

Todos estos modelos pueden construirse en tanques de acero recubiertos con recubrimiento epoxi especial o revestimiento interno de PRF. Los tanques de plástico (PP) son empleados para capacidades más pequeñas (< 250 m3/h), o en tanques RCC si la capacidad de tratamiento supera los límites de las unidades compactas construidas en tanque de acero.

The differential technology used in the FPBC is the lamella pack that works countercurrent to the downward flow of water in the tank, through the lamellae. This ensures uniform water distribution and improves the adhesion of air bubbles to suspended solids as the water flows through the reed tube channels. The lamellae are countercurrent type with a wide plate spacing of 80 mm and an inclination of 60º. Lighter particles are separated by flotation in the first section of the lamella pack and heavier particles are separated with relatively high efficiency in the lamellae as the water flows downward. The floated solids form foams in the upper zone of the tank that are removed with the surface sweeper, while the sediments at the bottom are removed with a skimmer at the bottom of the tank, in the case of an RCC tank, or a screw conveyor in the case of a compact unit with a steel or plastic tank.

which are removed with the surface sweeper, while the sediments at the bottom are removed with a skimmer at the bottom of the tank, in the case of an RCC tank, or a screw conveyor in the case of a compact unit with a steel or plastic tank.       

In primary treatment, SIGMADAF's dissolved air flotators, which use lamellae or separator plates, achieve upward velocities of more than 30 ^m3/m2/h, which gives us an idea of the reduction of space they represent in relation to static or lamellar primary decanters.

6. Summary

The primary treatments or pretreatment of water treatment plants, both wastewater and drinking water, are of great importance in the sizing and performance of the facilities as a whole.

Dissolved air flotation DAF is the best solution for the separation of suspended solids (TSS), in most of the wastewater from industries, and for drinking water, due to its remarkable performance in reducing colloidal material, oils, detergents and other low density contaminants, which if they reach the subsequent stages of treatment can cause mechanical problems of clogging and contamination in pipes, equipment and filter media such as RO and UF membranes. SIGMADAF has the most appropriate solutions for each case.

Bibliography and consultations

Metcalf-Eddy . Editorial Labor S.A.   

Schematic diagram of a DAF clarifier unit | Download Scientific Diagram