Sludge thickening by dissolved air flotation (DAF)

• 1. Introduction to sludge thickening
• 2. Thickening of wastewater sludge
• 3. Sludge thickening by sedimentation
• 4. Sludge thickening using Dissolved Air Flotation (DAF)
• 5. Comparison of thickening processes
• 6. Summary

1. Introduction to sludge thickening

As a consequence of the treatment of wastewater generated in industrial processes, a concentration of waste is produced in the form of sludge, also called sludge, which can be more or less hydrated.The waste is produced in the form of sludge, also known as sludge, which can be more or less hydrated.

Wastewater purification processes are mainly classified into physical, chemical and biological.

• In the case of physicochemical treatments, the sludge obtained is fresh sludge in the form of suspended solids in water (TSS). The sludge generated in physicochemical processes (primary sludge) is usually made up of denser materials than the sludge from biological treatments (secondary sludge).
• Biological and chemical treatments involve oxidation and reactions that transform them. Biological sludge is composed of organic and mineral matter, as well as the remains of the bacteria that feed on it (active sludge). The density of these sludges is lower than primary sludges and they are more difficult to dewater.

Effluent treatment plants have a wastewater treatment line and another line for sludge treatment. The objective is to obtain water that meets the parameters required by the applicable standards and to reduce the volume of sludge until it can be sent to a landfill or used in composting or for the production of by-products.

As we have seen, one of the main objectives is to reduce the sludge volume as much as possible. Most landfills require a minimum DM concentration of 30%, which can only be obtained by mechanical means (filter presses, belt filters, centrifuges, etc.) or by thermal drying systems.

An intermediate process that reduces the size of expensive sludge drying facilities is thickening, or dewatering.

2. Wastewater sludge thickening

Sludge thickening is a process that reduces the volume of sludge prior to any other dewatering treatment. The two main sources of sludge production are primary and secondary treatment. The settled solids removed from the bottom of the primary and secondary settling tanks are an aqueous mixture containing the separated suspended solids. We call this mixture sludge or slurry.

The sludge produced in the primary and secondary treatment of the water line of a WWTP has the following characteristics:

• They have a high water content (95-99%), so they occupy a large volume and are difficult to handle.
• They have a large amount of organic matter, so they easily decompose (putrefaction), producing bad odors.
• They contain a large number of disease-causing pathogenic organisms.

All this means that they must be processed with great care. Their treatment consists of three phases, designed to reduce the above-mentioned problems as much as possible:

• Reduction of the water present in the sludge to reduce its volume.                                         
• Stabilization of organic matter to avoid fermentation and putrefaction problems.
• Achieve a suitable texture to make them manageable and transportable.

The quantity and properties of the sludge to be treated in a wastewater treatment plant will depend on the characteristics of the wastewater from which it comes, as well as the treatment to which it is subjected.

The origin and main characteristics of the sludge are as follows:

a) Primary sludge

They originate from primary decantation. These sludges have not undergone biological treatment, nor have they decomposed, so they are highly unstable and putrescible. After a certain time they produce a bad odor. About 60% of these sludges are considered sedimentable.

Their color is usually gray, with high contents of biodegradable solids and other types of debris. They easily release their constituent water and thicken well. Their moisture content varies between 95-99%.

b) Biological sludge

They are commonly called excess sludge and come from the activated sludge process. They are usually dark brown in color, and their organic matter content is partially decomposed.

They have a damp earthy odor, not unpleasant, but, in their subsequent decomposition, they become septic and produce bad odors. Their moisture content varies between 98-99.5% and they are difficult to concentrate.

They can be thickened directly, or sent to primary settling, where they are decanted together with the primary sludge, giving rise to mixed sludge.

c) Aerobically digested sludge

They are dark brown sludge. They have a flocculated appearance and their odor is not particularly strong.

d) Chemical precipitation sludge

Their rate of decomposition is somewhat slower than that of the primaries. They are dark in color, reddish if they have a lot of iron. The odor can be unpleasant, although not as unpleasant as that of primary sludge.

3. Thickening of sewage sludge by sedimentation

Sludge thickening by sedimentation, or mechanical thickening, is a process that takes place in circular plant equipment containing a slowly rotating bridge fitted with bottom scrapers. The bridge includes a set of vertical picks that facilitate the removal of volatile compounds by opening grooves in the sludge blanket, thus facilitating thickening.

Bottom scrapers sweep the thickened sludge to an outlet, from where it is periodically purged.

A torque limiter is usually provided on the drive motor to prevent the rotating bridge from jamming and failure.

The total height of this equipment does not usually exceed 3.5 meters, and the slope of the bottom walls is of the order of 15%.

The sizing of a mechanical thickener is based on the sludge sedimentation curve, according to Kynch's theory, according to which the rate of fall of a type of particle depends only on the local concentration in that particle.  

_Zonat1 - t2_corresponds to vertical hydraulic decanters with a fixed sludge bed. From point t2 there_is a progressive decrease in the rate of fall of the solid particles, up to point t3_where the solids touch and exert a compression on the lower layers. This zone _t2 - t3_is used for mechanical sludge thickeners.

In a mechanical sludge thickener, the specific load is between 25 and 70 kg ^DM/m2. d. Throughout the operation, a wide layer of sludge is kept on the bottom to facilitate better concentration. An operational variable of this equipment is the sludge volume ratio (SVR), which is the volume of the thickener sludge layer divided by the volume of sludge thickened daily. RVF values range from 0.5 to 2 days, with the lowest values in warm weather.

4. Sludge Thickening using Dissolved Air Flotation (DAF)

Dissolved air flotation (DAF) is a technique used for solid particle, liquid and sludge separation/thickening. is a technique used for the separation of solid particles, liquids and sludge separation/thickening.

In this process, the sludge rises at a much higher velocity than gravity sedimentation. This sludge, already thickened, is collected on the surface of the float by means of a scraper system. DAF equipment achieves very high performances in urban and industrial treatment plants.

Compared to other sludge thickening systems, dissolved air flotation (DAF) has several advantages:

• First, the DAF process is faster than gravity sedimentation, which allows for higher throughput in wastewater treatment plant applications . 
• Secondly, DAF flotation is more effective in removing fine particles and colloids than gravity settling. 
• In addition, DAF equipment can handle higher solids loads, resulting in higher efficiency in the separation of solids and liquids. 

DAFs are also used as physicochemical separation systems for biological sludge, as they occupy approximately 5 times less space than a static settler for the same flow rate treated. In addition, they can be used to thicken already separated sludge, and generally allow increasing the sludge concentration up to 8-10% of dry matter, depending on its origin.

DAF Sludge Thickener Scheme

Dissolved air flotation (DAF) is a process used for the separation of solid particles from liquids containing them. In this process, the solid particles are attached to very fine air bubbles, which gives them a very fast rising velocity. The air bubbles are generated by dissolving air in water at high pressure and then depressurizing and releasing it in a flotation tank.

SIGMADAF has installed DAF equipment for the separation of TSS in primary and secondary clarification, as well as in sludge thickening.

Pre-coagulation / flocculation favors the efficiency of the DAFs, and increased sludge dewatering.

SIGMADAF offers a wide range of DAF floats for these applications:

• SIGMADAF FPAC Compact: From 3 - 200 m3/h built in metal tank.
• SIGMADAF FPAC-CWFrom 1000 - 4000 m3/h in concrete tanks.
• SIGMADAF FPHF Compact: From 100 - 1000 m3/h Compact equipment in metal or plastic.

All these equipments resist the action of high salinity in the effluents.

Dissolved Air Float Design

If we consider a DAF flotation system versus the use in primary and secondary settling tanks, we observe that flotation equipment has important advantages over conventional sedimentation technologies, since its ascending velocity is approximately 5 times higher than the gravity clarifier settling velocity, which saves time and space.      

The surface area of the equipment is decided based on parameters such as solids loading and surface loading (also known as ascent velocity). The solids loading is expressed in kgSS/(m²-d) and the surface loading in m³/(m²-h) . Typical values for these parameters are: 50-100 kgSS/(m²-d) and 3-5 m³/(m²-h).

The dissolved air float can be designed with or without recirculation, depending on the specific needs of the treatment plant.

Float Sludge Thickening

A static system based on inclined plates anda scraper is used to dewater the floated sludge. scraper scraper is used. This system achieves a dry solids content 3 to 4 times higher than any conventional system. 

Pressurized air flotation is normally used for activated sludge thickening and cellulosic fiber recovery, but it is also applicable for thicker sludge, such as the resulting mixture of primary sludge and activated sludge.

Although flocculant is usually dosed to aid the process, thickening can also be done without the addition of flocculant, but with lower DM concentrations.

The sizing of sludge thickening floats is calculated based on the following parameters:

Parameter	Design values
Specific load	5 - 10 Kg of DM/m2.h
Speed	1 - 3 m3/m2.h
Floating activated sludge concentration	30 - 50 g/l
Primary sludge + activated floating activated sludge	50 - 70 g/l
Volumetric mass of floating mantle	0.75 - 0.9 kg/l

In summary, a DAF system is an effective tool for concentrating sludge in wastewater treatment plants, reducing its volume and facilitating its subsequent treatment.

5. Comparison of sludge thickening using DAF technology and mechanical settling technology.

Dissolved Air Flotation (DAF) and Gravity Thickener are two technologies used for sludge thickening in wastewater treatment plant sludge lines. Below, we compare their advantages and disadvantages:

Dissolved Air Flotation Thickener (DAF)

Advantages:

• Thickening Efficiency: The DAF achieves a high concentration of suspended solids in the water by forming micro air bubbles. This significantly reduces the sludge volume. The concentration obtainable in thickened sludge is in the order of 4 - 6%, with mixtures of primary sludge and activated sludge in excess (even ≥ 8%). The use of polyelectrolytes as sludge flocculants allows to improve solids separation from 85% to 98-99%.
• Rapid Separation: Flotation allows for rapid separation of solids, which is beneficial in situations with high sludge loading.                                                                       
• Less Space Required: The DAF takes up less space than gravity thickeners.

Disadvantages:

• Initial Costs: Microbubble air injection technology and recirculation systems can increase initial costs.
• Energy Consumption: Requires more electrical energy to generate microbubbles and recirculate the sludge.

Gravity Thickener:

Advantages:

1. Low Initial Costs: Gravity thickeners are more economical in terms of initial investment.
2. Lower Energy Consumption: No aeration systems are required, which reduces operating costs.
3. Robustness: They are simple and robust equipment.

Disadvantages:

1. Thickening rate: The sludge velocity is significantly lower than in the DAF.
2. Greater Space Required: They occupy more space in the plant.
3. Lower Efficiency: The solids concentration is not as high as in the DAF.

Cost comparison

The DAF process is an effective option for sludge concentration in wastewater treatment plants. However, it is important to consider the costs compared to other processes. Here are some considerations:

Initial Investment Costs

1. The DAF requires an initial investment for the purchase and installation of the equipment. Compared to other processes, such as conventional decanters, it can be more expensive due to the compression system, air decompression and the need for recirculation systems.
2. Other processes, such as decanters, may have lower initial costs for mechanical equipment, but the civil work required for larger equipment must also be evaluated.          

Operating Costs

1. DAF operating costs include electrical power for microbubble generation by air compression and dissolution and sludge recirculation. These costs can vary depending on the size of the plant and the efficiency of the equipment.
2. In comparison, conventional decanters have lower operating costs because they do not require aeration systems.

Efficiency and Space Saving

1. The DAF offers a higher sludge rising velocity, which saves space in the plant. This can be beneficial in areas with space limitations.

Maintenance and Repair Costs

1. DAF maintenance and repairs can be more costly due to the complexity of the aeration and recirculation systems.
2. Mechanical thickeners and other simpler processes may have lower maintenance costs.

Sludge drying Despite its effectiveness, DAF is not suitable for achieving extremely high solids concentrations. However, it can improve alternative processes, thereby reducing the size and cost of downstream dewatering systems.

To obtain more concentrated sludge, other systems are used, such as filter presses or centrifuges.

Type of sludge	% DM Unthickened	% DM Thickened	Solid load (kg/m2.h)
Separated sludge
Primary mud	2,5 - 5,5	8 - 10	4 - 6
Trickling filter	4 - 7	7 - 9	1,6 - 2
Modified aeration	2 - 4	4,3 - 7,9	1,4 - 3,7
Activated	0,5 - 1,2	2,5 - 3,3	0.8 - 1,6
Combined sludge
Primary and trickling filter	3 - 6	7 - 9	2,4 - 4
Primary and modified aeration	3 - 4	8,3 - 11,6	2,4 - 4
Primary and activated	2,6 - 4,8	4,6 - 9	1,6 - 3,2

Thickened and unthickened sludge concentrations and solids loadings for mechanical thickeners (Metcalf - Eddy. Ed. Labor).

In activated sludge plants using digestion, excess activated sludge, or combined raw sludge prior to digestion, should be thickened to reduce construction and operating costs.

Considerations to be taken into account when designing a thickener are:

• Provide it with sufficient capacity to meet peak demands.
• Avoid septicity and the inherent odor problems that occur during this process.

Type of sludge	Load in kg/m2. h
Activated (mixed liquid)	1 - 3
Activated (sedimented)	2 - 4
50% primary + 50% activated	4 - 8
Primary only	Up to 10

Calculation load in DAFs from different sludge types

It is preferable to use plant effluent, rather than flotation effluent, for pressurized recirculation, due to the possibility of fouling the equipment involved.

6. Conclusion

The sludge produced in wastewater treatment plants must be concentrated and treated to such a degree of loading and inertization that it can be sent to authorized landfills. Gravity settling and thickening systems (mechanical thickening) or clarification and thickening systems using dissolved air flotation (DAF) equipment are usually used.

The DAF process is more efficient in terms of solids concentration and separation, but can be more costly in terms of investment and energy. The gravity thickener is more economical and robust, but requires more space and larger civil works. The choice depends on the specific needs of each plant and its available budget.

Consultations and bibliography

• Wastewater Engineering/Sludge Line - Wikibooks (wikibooks.org)
• Technical Water Manual (Degrémont)
• Wastewater treatment and purification - Metcalf-Eddy (Edit. Labor S.A.)