DAF flotation clarification treatment for seawater desalination plants

Characteristics of the water to be treated 

Description of supply

CORODEX industries, a water treatment and purification company, purchased from SIGMADAF a DAF primary treatment flotation clarification system for the Surf Ranch seawater desalination plant in Abu Dhabi (UAE). 

The scope of supply includes: Two SIGMADAF FAPC100 floats arranged in parallel, composed, each one, of rectangular plant tank built in PP containing a lamella pack of the same material, supported on support structures in AISI 304. Float sweeper system, recirculation pumping set with pressurized air injection manifold, sedimented sludge extraction system and pneumatic control panel are included. Engineering documentation and installation manuals are also provided. 

The expected performance of the equipment supplied reaches the following yields, provided that it is operated correctly with the physical-chemical system of coagulant and flocculant addition available to the customer:

No. of units: 2 in parallel

Unit characteristics

• Equipment: SIGMADAF FPBC100
• Materials: PP /AISI 304                        
• Dimensions: 5,000 x 2,400 x 4,000 mm (L x W x H)
• Filling: Honeycomb H80 PP lamella packs
• Design flow rate: 104.2 ^m3/h
• Free area: 6.5^m2
• Effective area: 32.8^m2
• Hydraulic load: 2.45 ^m3/m2/h
• Mass load: 2.9 ^kg/m2/h
• HRT: 7 minutes
• Air consumption: 45 Nl/min.
• Power consumption: 12.6 kW
• Surface sweeping system with 0.55 kW motor
• Sedimented solids extractor with 0.37 kW motor
• Pressurization and recirculation pumping with 11 kW engine

Membrane systems used for seawater desalination are vulnerable to fouling and clogging, which reduces their lifetime and performance. In addition, algae by-products and the toxins they contain can also enter the water supply network, thus affecting the quality, generating technical and health problems. To avoid these situations, seawater must be subjected to pretreatment prior to reverse osmosis. The most efficient one consists of a combination of a coagulation-flocculation process followed by clarification by means of a DAF system, as has been demonstrated in numerous examples.

The DAF FPBC: high performance for seawater clarification

The operation of a DAF flotation unit is based on the supply of dissolved air that is inoculated in the recirculation of part of the treated water, to which pressurized air is injected and then depressurized in the liquid, thus creating a stream of micro air bubbles (30 - 50 µm) that adhere to the particles, causing them to rise as they lose apparent density. On the surface, a foam blanket is formed which is swept by a system of scrapers. During this process a sludge is produced with a dry solids content that is 3 to 4 times higher than that created by other conventional processes.

In this case and due to the characteristics of the water to be treated, a SIGMADAF FBPC type equipment built in anticorrosive materials suitable for seawater has been proposed. Its technology is based on the countercurrent flow principle, through a package of inclined plates (lamellae), which significantly reduces the distance that the solids must travel to separate effectively. 

It is an ideal equipment for physical-chemical processes and waters with medium-high pollutant load, easy to handle and low maintenance. It is built for a wide range of flow rates (between 5 and 175 ^m3/h).

The DAF allows the separation of suspended solids, oils and fats, as well as organic matter typically contained in seawater such as algae, colloidal organic matter and hydrocarbon residues.