Pre-treatment of water in a desalination plant for combined cycle power plant

Scenario

TRSA was contracted by the Client (SHIPCO), to execute together with GE an 1800 MW CCGT Power Generation Plant. The end Customer is Sharjah Electricity and Water Authority (SEWA), while the Sharjah Hamriyah Independent Power Co. (SHIPCO) is an IPP of Sumitomo, GE, Shikoku Electric Power Co. and Sharjah Asset Management (SAM). 

Due to the lack of fresh water, the project includes a 7 ML/d capacity SWRO Desalination plant, with pretreatment including coarse and fine screening, coagulation-flocculation, DAF flotation and DMF mixed bed filters, cartridge filters and RO membranes.

SIGMADAF provided the desalination plant pretreatment DAF, complete with coagulation-flocculation tanks and two DAF units for the removal of TSS and eventually algae in seawater encountered during bad weather conditions or algal bloom episodes. The DAFs prevent clogging of the DMF filters and the collapse of the pretreatment and shutdown of the RO plant, with consequent loss of feedwater supply to the power generation plant. 

SIGMADAF's order included the design and supply of the DAF package, including coagulation-flocculation, piping, valves, access ladders and platform.

The technology for the DAF incorporates the LAMELAR PACK with countercurrent flow and cross-flow, which optimizes water distribution in the tank and facilitates uniform water outflow through pipes along the length of the tank.

The physical-chemical treatment prior to DAF is carried out in 1 coagulation tank of 35 ^m3 (Ø3.5 x L 6.5 m) and 4 flocculation tanks (2 per DAF unit) of the same unit dimensions, designed for retention times of 2.5 and 20 min respectively. The tanks are built in GRP and equipped with vertical multi-blade agitators in superduplex material (SAF2507). 

SIGMADAF FPHF Equipment Characteristics 

The FPHF is a compact unit especially effective for high flow rates and high suspended solids content, which incorporates inclined lamellae to improve laminar flow and sedimentation of larger or heavier solids. The FPHF uses a combination of counterflow (downflow) and crossflow, which improves solids separation performance. 

High treatment flows require a better distribution of the water flow in the flotation tank. The inlet surface of the lamella packs makes the water flow laminar and distributes the water in the flotation tank. 

equally over the entire surface of the tank. In practice, this equipment achieves a volumetric treatment of over 100% compared to conventional plate separators, which mostly have a hydraulic load of 3 m/h. 

In this project, the FPHF equipment is expected to remove a high level of TSS and algae under HAB episodes. The suspended matter is aggregated into flocs in the flocculation tank, but as they do not possess sufficient buoyancy, air bubbles enhance the separation process. The use of microbubble flotation technology (30 to 50 micron range) increases the efficiency of the float, as smaller bubbles easily attach to particles of equal or larger size, which increases the overall effectiveness of the system. The SIGMADAF air aerator generates the required amount of air to be injected into small bubbles in quantity and well distributed throughout the entire tank volume. 

Singular Characteristics 

The FPHF is a high-performance piece of equipment with a number of unique features. 

• Efficient distribution of water flow in a laminar regime, enhanced by the lamella pack and piped side outlets along the length of the tank, with adjustable outlet weirs to control the flow rate and water level in the tank. 
• Floated sludge collector with a single-motion chain scraper, which advances against the water flow, helping to reduce the sludge evacuation distance at the surface and eliminating solids entrainment with the clarified water. The floated sludge is extracted to a collection hopper, with a solids concentration 3 to 4 times higher than a conventional system. This minimizes the cost of sludge dewatering or drying.

Water characteristics

Performance