Algae, organic matter and metal removal in drinking water treatment plant

Scenario

EFACEC was contracted to design-supply and build the complete process equipment for the Póvoa Water Treatment Plant (ETAP Póvoa) and entrusted SIGMADAF with the supply of the DAF units. The PÓVOA DWTP owned by AGUAS DO VALE DO TEJO has a capacity of 740 m3/h (54,000 PE) and has a water treatment line with pre-ozonization, to oxidize organic matter, algae and metals (Fe and Mn), lime dosing, coagulation/flocculation, dissolved air flotation (DAF), post-ozonization, activated carbon contact, sand bed filtration and final disinfection with chlorine.

SIGMADAF supplied EFACEC with the DAF system as part of the pretreatment, to remove algae, organic compounds and metals precipitated after oxidation. The technology used is with the lamellar package with counter-current downflow and cross-flow, to optimize and ensure uniform distribution and laminar water flow through the flotation tank with lateral water outlets. Before the DAF there is a physical-chemical treatment with dosing and mixing of chemicals (coagulant and polymer), this will help the formation of larger flocs that can be better separated by flotation in the DAF tank.

SIGMADAF FPHF Equipment Features 

The FPHF is a compact unit especially effective for high flow rates and high suspended solids content, using inclined plate separation (IPS) technology. The FPHF uses a combination of cross-flow and countercurrent flow, the heavier particles are separated by sedimentation with relatively high efficiency in the lamellae. 

High flow equipment requires a better flow distribution in the flotation tank. The inlet surface of the lamella plates is equally divided in proportion to the laminar flow within the system. In practice, it offers a water volume treatment of over 100%, compared to conventional plate separators, which mostly have a surface load of 3 m/h. 

In this project the FPHF equipment is required to remove algae and TSS suspended solids. The suspended matter is aggregated into flocs in the flocculation tank and then separated with the help of air bubbles, which enhance the flotation separation process. The use of microbubble flotation technology (30 to 50 micron range) increases the performance of the flocculator, 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 in small bubbles in quantity and well distributed in the filled tank volume.

Unique Characteristics of the System 

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

• Efficient laminar distribution of water flow, enhanced by the lamella pack and lateral piped water outlets along the length of the tank, with adjustable outlet weirs for flow and water level control in the tank. 
• Floated sludge collector with continuous longitudinal movement, which rotates against the water flow, helping to minimize the path of the floated sludge and eliminating the entrainment of solids with the clarified water. The floated sludge is drawn into a collection hopper and is extracted at 3 to 4 times the concentration of a conventional system, minimizing dewatering or drying costs. 
• Solids settling at the bottom of the tank are collected in the central extraction trough with a shaftless screw conveyor. The accumulation and concentration of these solids are controlled by the running time of the extraction screw and the pneumatic blowdown valve. The discharge cycle assists in cleaning the bottom of the tank and the particles adhering to the walls are peeled off and will follow their downward trajectory into the collection channel. 

Performance