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18 March 2021

Desalination: DAF for sea water treatment

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1. Introduction: SWRO plant issues due to algal blooms

Distribution of water in the Earth is not even. The 97% of the water in our planet is in the oceans and only the 3% is fresh water on the surface. Of this fresh water, 69% belongs to ice water in glaciers, 30% is underground and less than 1% is found in lakes, swamps and rivers and available for human use.

Graph showing the distribution of water on earth: water from oceans, freshwater, groundwater, icecaps and glaciers, lakes, rivers, etc.
Figure 1. Water distribution in Earth. Credit: Timothy Bralower.

Due to the fast and uncontrollable growth of population and the associated resources consumption, there is an urgent need for obtaining water from other sources different than the freshwater directly available. The use of sea water for drinking and process water production through desalination is nowadays one of the most applied ways to obtain drinking and process water. Challenges and most applied technologies are described in this dossier.

All life in the sea depends on the marine algae and other microorganisms that convert CO2 into the biomass that forms the base of the marine food web. These algae species have a set of environmental conditions that favor growth and proliferation, thus there is a continuous succession of species through time in a given area: these are called “blooms”: proliferation of a single species that rapidly dominates the water column. Harmful algae blooms (HABs) are critical to many aspects of marine ecology as well as to human society’s utilization of marine resources, as it is the treatment of sea water.

Every coastal country in the world can be affected by HABs. Algae and the by-products released by them known as algal organic matter affect directly and indirectly the membranes filtration. The high algal organic matter (AOM) concentration in the raw water during algal blooms cause fouling issues in both the pre-treatment and reverse osmosis (RO) systems of a sea water reverse osmosis (SWRO) desalination plant. RO membranes are primarily designed to remove dissolved constituents in the water, specifically inorganic ions. Membrane systems are vulnerable to fouling and clogging due to deposition of particulates and/or growth of bacteria and other microorganisms causing a biofilm on the membrane surface, resulting in an increase in the hydraulic resistance, serious increase of the transmembrane pressure and significant flux decrease, thus lowering the membrane life spam, filtration cycles duration and performance. The algae by-products and toxins can also penetrate into the water supply pipe network, thereby affecting the water quality in the network generating huge technical and health problems.

It is not easy to clean membranes fouled by natural water, then to avoid frequent chemical cleaning, RO systems are generally preceded by a pre-treatment process to minimize the particulate, organic and biofouling potential of the feed water. Conventional techniques comprise steps of coagulation-flocculation, flotation and various types filters. The coagulation-flocculation step is applied to improve the hydraulic performance of the flotation technique and/or filter. Advanced techniques include also membranes of MF or/and UF. Dissolved air flotation (DAF) is employed before the filtration via granular media or MF/UF in SWRO systems making the pre-treatment reliable and robust.

The two potential impacts of HABs on SWRO are identified as:

  1. Significant treatment challenge to ensure the SWRO system is effectively removing algal toxins from seawater.
  2. Operational difficulties due to increased total suspended solids and organic loading from algal biomass in the raw water.

1.1. World distribution of algal blooms

HABs are distributed along the earth coastline and have severely affected desalination plants worldwide.

Typical average annual distribution of algae based on chlorophyll-a concentration in large water bodies on earth.
Figure 2. Typical average annual distribution of algae in large water bodies on earth. Ref: Villacorte et al. 2015.
Chart of a massive red tide algal bloom in the Gulf of Oman spreading to The Gulf on 22nd November 2008.
Figure 3. A massive red tide algal bloom in the Gulf of Oman spreding to The Gulf on 22nd November 2008. In red the bloom of Cochlodinium polykrikoides and in blue regions dominated by suspended sediments or shallow waters. Yellow points indicate locations of major SWRO plants. Ref: Villacorte et al. 2015.

2. Configuration of the pre-treatment in sea water treatment plants

Membrane technology emerged as an important mean of water treatment in the 1990s and is likely the most promising choice to remove algae, bacteria, viruses, and other microorganisms, however they are severely affected with fouling problems. An appropriate pre-treatment is absolutely needed to make membrane technologies efficient.

2.1 Conventional pre-treatment configuration

The conventional pre-treatment process is categorized into two ways as physical and chemical pre-treatment. The physical process does the basic mechanical screening of particulate matter through the screens and filters and the chemical process involves the addition of scale inhibitors, oxidants, pH adjustment chemicals, coagulants, flocculants and disinfectants. The key contaminants to be retained in the pre-treatment step are the particulate and suspended matter, microbial contaminants, algae and AOMs and dissolved organic matter. Conventional treatment can be adapted to the current requirements by adding, removing or modifying any of the steps described.

Image of a conventional wastewater treatment, in two paths physical and chemical pre-treatment. From water disinfection to filtered water.
Figure 4. Example of a conventional pre-treatment system. Ref: adaptation from Kavitha et al. 2019.

Disinfection, screening and pH adjustment:

Disinfection by chlorination is commonly applied to prevent biological growth responsible for the fouling of filters and membranes. It is necessary to have a dechlorination process to avoid the presence of residual chlorine in the stream. The screening process consists of a simple mesh kept at an inclination to the stream of sea water. The plants and debris in the sea water are stopped at the grids which are removed by a mechanical rake attached with the screens.

The pH of sea water is always on the alkaline side due to the presence of salts which cause extensive scaling of the membranes involved in the treatment process. The performance of RO membranes is better at lower pH values and also lesser reactions on the membrane surface. The pH of the sea water in maintained at 5.5 – 6.0 by the addition of sulfuric acid to optimize floc formation. Adjusting pH is beneficial for both algae and TOC removal. At pH lower than 5.5 cells begin to lyse releasing intracellular substances that may not be fully removed by flotation/sedimentation units like DAF and spoil the pre-treatment functionality.

Coagulation and flocculation:

The primary purpose of coagulation and flocculation process is the removal of turbidity and solids. A coagulant is added to the water to neutralize the negatively charged particles. The optimum type and amount of coagulant must be set applying a jar-test to the water. Most commonl0y used coagulants are alum and ferric salts like ferric sulfate and ferric chloride. The coagulation and flocculation step removes colloidal impurities, suspended particles, algae, AOMs and some bacteria from the sea water.

The algal cells must be completely destabilized by charge neutralization to allow the treatment to reach its maximal efficiency, this is done with the proper dosage of coagulant and flocculant along with pH adjustment (at a given coagulant dosage there is generally an optimal pH at which algae removal is at its maximum rate).

Image of the model of coagulant application in combination with different primary and secondary processes.
Figure 5. Model of coagulant application in combination with different primary and secondary processes. Ref: Villacorte et al. 2015.

Flotation:

In the flotation step particulate impurities algal cells, oil and grease which cannot be removed by sedimentation are floated and eliminated. Flocs produced in the coagulation-flocculation step are also eliminated. Dissolved air flotation has been found as the most effective technology for this step and is fully described in paragraph 3.

The DAF technology developed by SIGMA combines dissolved air flotation and sedimentation principles with optimal design of the equipment.

Filtration:

This process removes suspended particles that were not eliminated during flotation. The most common types of filters are gravity filters (sand, sand plus anthracite) and pressed filters. Gravity filters are used for both large and medium size capacity filtration plants and offer a better economic performance for desalination plants. With proper coagulation-flocculation and pH adjustment chemistry, the gravity media filtration system removes clay, colloidal silica, precipitated metal hydroxides, humic and fulvic acids, algae and bacteria.

Antiscalant addition:

The precipitation of salts and minerals of sea water on the membrane surface caused by super saturation can reduce performance and decrease water recovery. The sulfuric acid added to adjust the pH can also control the scaling produced by calcium carbonate. Recently also certain polymers are used as anti-scaling agents.

Dechlorination:

When chlorine is used as disinfectant, the residual chlorine present in the feed water to RO system has the potential to cause damage to the membrane. The most common agent used for dechlorination process is sodium bisulfite or activated carbon.

2.2 Advanced pre-treatment configuration

Membranes of MF and/or UF are sometimes applied as a standalone pre-treatment process, known as advanced pre-treatment, but it has been demonstrated that this is not an effective configuration given the fouling problem affecting all kind of membranes. Then, advanced pre-treatment can be part of the conventional pre-treatment instead of an independent pre-treatment itself. It has been demonstrated that a combination of conventional pre-treatment and advanced pre-treatment can provide a cost-effective solution for the demand of fresh water by sea water desalination processes.

MF and UF processes are the most widely used advanced pre-treatment for RO membrane filtration in desalination.

Chart of the Pilot pre-treatment line of Degrémont in the Persian Gulf with the alternatives of conventional plus advanced pre-treatment units.
Figure 6. Pilot pre-treatment line of Degrémont in the Persian Gulf with the alternatives of conventional plus advanced pre-treatment units. Ref: adapted from Peleka and Matis 2008.

A full-industrial scale pilot in Barcelona made the comparison of a dual-media filter DMF and UF as part of the pre-treatment after the DAF unit.

Chart of a comparison of a dual-media filter DMF and UF as part of the pre-treatment after the DAF unit shows the following results.
Figure 7. Simplified representation of the conventional / advanced pre-treatment configurations. Ref: Guastalli et al. 2013.

UF shows excellent and high stability in the good quality of its permeate. Installing and UF membrane after DAF allows rejecting almost 100% of algae contamination and a large fraction of the bacterial content of the water. This is a clear example of how combining conventional and advanced pre-treatment technologies lead to the best performances for sea water treatment.

2.3. Conclusions for pre-treatment configurations

The most important advantage of the conventional pre-treatment process including coagulation-flocculation and DAF technology is that it is a very well-known process with familiar technology while it has the door open to modifications and optimization through advanced and state-of-the art technologies for each of its steps. The system eliminates both organic matter and suspended material from the influent sea water.

Conventional pre-treatment can be adapted by changing or adding steps like filtration or UF/MF membranes.

Table 1 gathers the efficiency of the most common pre-treatment processes based on literature.

Table 1. Reported treatment efficiencies of various treatment processes based on selected algal bloom indicators. Ref: Villacorte et al. 2015.

Information table with the Reported treatment efficiencies of various treatment processes based on selected algal bloom indicators.

The research in the field of conventional pre-treatment in the recent times has focused on making the process more competent with the following reforms:

  • Alterations in seawater intake points.
  • Introducing DAF technology for the flotation step.
  • Dynamic filter or membrane backwashing/cleaning processes and modified chemical dosages to the system.

Nowadays, SWRO plants for seawater treatment are widely applied around the world as shown in the following figures:

Global distribution of major RO plants and selected locations of coastal areas where Harmful algae blooms have been reported to impact SWRO operations.
Figure 8. Global distribution of major RO plants (red dots) and selected locations of coastal areas (blue outlines) where HABs have been reported to impact SWRO operations. Ref: Villacorte et al. 2015.
Chart of the water production capacity of medium to extra-large-SWRO plants installed between 1975 and 2015 years along the world.
Figure 9. water production capacity of medium to extra-large SWRO plants installed between 1975 and 2015. Ref: Villacorte et al. 2015.

3. DAF technology as a key part of the pre-treatment in desalination plants

One of the most applied and well demonstrating effective functioning technology to be a pre-treatment for reverse osmosis applied in sea water treatment is dissolved air flotation (DAF) followed by filtration and preceded by screening and coagulation-flocculation processes. DAF constitutes a high rate, effective and familiar separation process for oil, grease and suspended solids like the algae and their AOM. The process chain of coagulation-flocculation and pH adjustment followed by DAF followed by filtration (could be gravity media or UF/MF depending on the plant requirements) is a rather common concept and it has shown along the years to be the most efficient, cost-effective and robust pre-treatment for sea water treatment using reverse osmosis.

The importance of DAF process to the economy of the whole industrial world is enormous. Without this process, may metals and inorganic raw materials would be exceedingly scarce and costly, because the high-grade ores that could be processed by simple physical and mechanical methods have long since been used up. DAF initially originated from the field of mineral processing.

Particulate solids besides minerals have been extracted from water by using DAF, that is based on the idea of applying rising gas bubbles to particles transfer the solids from the body of water to the surface. DAF technique is applied to particles whose density is lower than the liquid they are in. During DAF treatment, compressed air is introduced into a recycle stream, is dissolved, and subsequently generates 30 - 50 µm bubbles when release through a dispersion header in a DAF tank. Coagulated particles, such as algae, attach to the bubbles and float to the top of the water column where they are mechanically of hydraulically removed.

Image of the parts of Dissolved Air Flotation equipment, designed and manufactured by SIGMDAF Clarifiers, specialists in industrial wastewater treatment.

Rendering of the DAF Clarifiers equipment for the wastewater treatment, designed and built by SIGMADAF Clarifiers.
Figure 10. DAF clarifier developed and built by SIGMA DAF.

The DAF technology is currently effectively applied in other application areas like drinking water, tertiary treatment of wastewater, sludge thickening, filter backwash waste recovery and sea water pre-treatment for desalination. The trend in the development of DAF technology for sea water treatment and drinking water production is to move to very thick micro-bubbles beds with high flow rates. The process reduces space requirements, saves operation and maintenance costs and improves purification results.

Image of the typical dissolved air flotation module from SIGMADAF: pH adjustment and coagulation-flocculation tanks plus DAF chamber.
Figure 11. SIGMA DAF dissolved air flotation module: pH adjustment and coagulation-flocculation tanks plus DAF chamber.

The DAF technology accompanied by proper coagulation-flocculation and pH adjustment chemistry has been proved to be effective for removing grease, oils, suspended solids, turbidity, color, some bacteria, algae, iron, manganese, phosphorus and total organic carbon. Precipitate flotation of metal ions from aqueous solution is realized by the addition of the most appropriate coagulant, resulting in metal immobilization as a precipitate, then followed by their flotation.

Systems SIGMA DAF advantages include:

  • Better quality of the treated water.
  • Rapid start-up.
  • High-rate operation.
  • Thicker sludge (less sludge production).
  • Small footprint.
  • Easy to operate.
  • Ability to protect the sea water treatment plants against damage caused by HABs.

 

Rendering of DAF model FPAC-CW, full unit for desalination pre-treatment, design by SiGMADAF Clarifiers.

Image of the DAF FPHF® system with pH adjustment and coagulation-flocculation processes is applicable for sea water pre-treatment, flowrates applicable from 100 to 1000 m3/h.
Figure 12. SIGMA DAF models for pre-treatment of sea water treatment: (a) Sigma DAF FPAC-CW® full unit specially designed for desalination pre-treatment including pH adjustment, coagulation and flocculation chambers plus DAF chamber, flowrates applicable from 1000 to 4000 m3/h; (b) Sigma DAF FPHF® system that installed along with pH adjustment and coagulation-flocculation processes is applicable for sea water pre-treatment, flowrates applicable from 100 to 1000 m3/h.

Removal efficiencies for algae with coagulation-flocculation with pH adjustment and peroxidation, followed by DAF followed by gravity filtration range between 99 - 99.9% when chemicals dosing are optimized even during HABs events.

Table 2. Comparative algae removal efficiencies using various treatment lines n optimized conditions. Ref: Mouchet and Bonnelye 1998.

Comparative table of algae removal efficiencies using various treatment lines on optimized conditions, e.g., DAF-filtration.

Table 3. Sigma DAF model FPAC-CW® removal efficiencies. The unit includes pH adjustment, coagulation-flocculation system plus DAF system, shown in Figure 12 (a). The water quality used for this estimation is: Type of influent: sea water plus river water; flowrate = 10500 m3/h divided in three lines of operation of 3500 m3/h average each; total dissolved solids <40000 mg/L; pH = 8.2-8.3, temperature = 17-39ºC; algae: 5 Mcells/L; total suspended solids < 200 mg/L; turbidity = 5-30 NTU; oil and grease < 3 mg/L.
ParameterRemoval efficiency
Total suspended solids95%
Oil and grease98%
Algae98%
Floated sludge concentration> 2%

 

Table 4. Sigma DAF model FPHF® removal efficiencies, shown in Figure 12 (b). The installed process also includes pH adjustment, coagulation-flocculation system plus DAF system. The water quality used for this estimation is: Type of influent: sea water; flowrate = 832 m3/h divided in two lines of operation of 416 m3/h average each; total suspended solids <50 mg/L; pH = 8.0, temperature = <34.6ºC; algae <100000 cell/mL; oil and grease < 3 mg/L.
ParameterValue in effluent
Total suspended solids< 10 ppm
Turbidity< 5 NTU
Total organic carbon< 7 ppm
Algae removal> 98%
Oil and grease removal> 90%

 

Because of the buoyant nature of algae, DAF is increasingly applied as a pre-treatment process for seawater desalination using RO. High algae removal rates achieved by DAF protect downstream membrane process from fouling and shutting down during HABs events.

RO membranes, extensively used in sea water treatment plants, are very sensitive to fouling by colloids, inorganic scale, biofilm development, etc. With an open seawater intake of the plant, the membranes are sensitive to other types of pollution: algae, precipitated metal, organic matter hydrocarbons, particles, turbidity, etc. The pre-treatment should be designed to face the worst water quality, providing a constant and good feed water quality for the RO units, even when HABs are occurring. Pre-treatment combinations for RO units reduces also RO membrane cleaning costs.

The severe HAB event in 2008 – 2009 in the Gulf of Oman that led to the shutdown of several desalination plants in the region redirected the attention of the desalination industry to DAF as part of SWRO pre-treatment schemes. A pilot plant fitted with a DAF was installed in the Al-Dur plant in reporting more than 99% removal of algae cells during pilot testing combined with coagulation-flocculation prior to GMF. The Al-Shuwaik desalination plant in Kuwait equipped with DAF and UF as pre-treatment consistently proved good quality feed water for deteriorated conditions during HABs events. DAF is now being regularly incorporated in new SWRO plants in the Persian Gulf upstream of GMF or MF/UF. Expansion of the Fujairah plant incorporates DAF as an essential part of the pre-treatment scheme.

The best combination of units that constitute the pre-treatment and the chemicals for coagulation-flocculation and pH adjustment steps, must be designed on basis of analysis of the water, applying a jar-test analysis to water samples of the whole range of water quality: from the less concentrated sea water to the worst case scenario given by HABs episodes. AGUASIGMA offers to do these analyses and design the coagulation-flocculation and pH adjustment step plus DAF design of the pre-treatment chain. It is also a must to have a continuous monitoring of algae blooms in each ecosystem.

4. Glossary

AOM: algal organic matter

DAF: dissolved air flotation

DMF: dual media filter

GMF: granular media filters

HAB: harmful algal bloom

MF: microfiltration

RO: reverse osmosis

SWRO: sea water reverse osmosis

TOC: total organic carbon

UF: ultrafiltration

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