DAF flotation clarification treatment for sea water

  • Year: 2023
  • Project location: Cape Town (South Africa)
  • Objectives: Primary seawater treatment
  • Equipment installed: 2 SIGMADAF FPBC100 units
  • Capacity: 8,640 m3/d (360 m3/h)

Characteristics of the seawater to be treated

ParametersUnitValue
pH-8,1
TemperatureºC10 - 22
TDSmg/l39.000
TSSmg/l90
TurbidityNTU50
CODmg/l4
TOCmg/l6
AlgaeCell/ml100.000
Chlorophyll-20
Oils and fatsmg/l10

Description of supply

PROXA, a water treatment and purification company, purchased from SIGMADAF a DAF primary flotation clarification system for the Cape Town seawater desalination plant in South Africa.

The supply comprises two SIGMADAF FPBC100 floats arranged in parallel, each consisting of a rectangular tank built in PP containing a pack of lamellae of the same material, with the following unitary design characteristics:

  • Equipment: SIGMADAF FPBC100
  • Materials: PP/AISI 316
  • Dimensions: 6,400 x 2,400 x 3,275 mm (L x W x H)
  • Filling: PP lamella pack, Honeycomb H80
  • Design flow rate: 180 m3/h
  • Free surface: 10m2
  • Effective area: 82m2
  • Hydraulic load: 2.5 m3/m2/h
  • Mass load: 1.62 kg/m2/h
  • HRT: 7 minutes
  • Air consumption: 45 Nl/min. (6.5 Bar)
  • Power consumption: 16.6 kW
  • Surface sweeping system with 0.55 kW motor
  • Extractor of sedimented solids with 0.37 kW engine
  • Pressurization and recirculation pumping with 11 kW engine
  • PP/PVC air manifold
  • 2.2 kW air compressor

Included are power and control panels with PLC, electro-pneumatic cabling, PP/PVC piping, instruments and accessories for the compact equipment supplied, as well as engineering, erection supervision and start-up. The supplied equipment will allow to obtain the following values in the treated water:

ParametersUnitValue
TSSmg/l≤ 10
TurbidityNTU≤ 5
Algae removal%≥98
Reduction of fat and fatty acids%≥90

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 its quality, generating technical and health problems. To avoid these situations, seawater must be subjected to pretreatment before accessing the UF or RO membranes. 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 air micro-bubbles (30-50 µm) that adhere to the particles, causing them to rise as they lose apparent density. A foam blanket is formed on the surface, which is swept away 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 FPBC type equipment built in anticorrosive materials suitable for seawater has been proposed. Its technology is based on the principle of countercurrent flow, through a package of plates and inclined (lamellae), which significantly reduces the distance that solids must travel to separate effectively.

It is an ideal equipment for physicochemical processes and waters with medium-high pollutant load, easy to handle and low maintenance. It is built for a wide range of flow rates.

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.

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