WATER TEATMENT AND DESALINATION Chapter 6 WATER TEATMENT AND DESALINATION

1- Water Treatment Processes Dr Salam Al-Dawery

Water treatment It is common to treat raw water to produce safe drinking water for the protection of public health, as most raw water quality does not meet safe drinking water standards. The most cited reasons for treatment water are: • To remove disease-causing pathogens; • To remove potentially toxic natural or synthetic substances; • To remove dissolved and gaseous radioactivity; • To improve organoleptic quality of water to prevent consumer rejecting water due to its physical appearance, taste or odour; • To prevent bacterial after-growth in the distribution system; • To prevent deposition and silting up of pipes; • To prevent corrosion and dissolution of pipes and fittings; and • To comply with local, national and international law on water quality.

Disinfection Any process to destroy or prevent the growth of microbes Intended to inactivate (destroy the infectivity of) the microbes by physical, chemical or biological processes. Use one of the following: Free chlorine: HOCl (hypochlorous) acid and OCl- (hypochlorite ion) strong oxidant; relatively stable in water (provides a disinfectant residual) ozone, O3: strong oxidant; provides no residual (too volatile, reactive) Chlorine dioxide, ClO2,: strong oxidant; unstable (dissolved gas) UV radiation low pressure mercury lamp: low intensity; monochromatic at 254 nm medium pressure mercury lamp: higher intensity; polychromatic 220-280 nm) Boiling: efficient kill; no residual protection; fuel/environmental costs

Summary of Mainline Water Treatment Processes Storage Disinfection Physical: UV radiation, heat, membrane filters Chemical: Chlorine, ozone, chlorine dioxide, iodine, other antimicrobial chemicals Filtration Rapid granular media Slow sand and other biological filters Membrane filters: micro-, ultra-, nano- and reverse osmosis Other physical-chemical removal processes Chemical coagulation, precipitation and complexation Adsorption: e.g., activated carbon, bone char, etc, Ion exchange: synthetic ion exchange resins, zeolites, etc.

Typical Surface Water Treatment Plant Alum Some of the main treatment processes can be found in Table in following slide. Typical water treatment processes usually comprises of pre-treatment, main treatment and disinfection.

Screening

Chemical Coagulation-Flocculation1/2 Removes suspended particulate and colloidal substances from water, including microorganisms. Coagulation: colloidal destabilization Typically, add alum (aluminum sulfate) or ferric chloride or sulfate to the water with rapid mixing and controlled pH conditions Insoluble aluminum or ferric hydroxide and aluminum or iron hydroxide complexes form These complexes entrap and adsorb suspended particulate and colloidal material.

Chemical Coagulation-Flocculation1/2 Slow mixing that provides for a period of time to promote the aggregation and growth of the insoluble particles (flocs). The particles collide, stick together and grow larger The resulting large floc particles are subsequently removed by gravity sedimentation (or direct filtration) Smaller floc particles are too small to settle and are removed by filtration

Granular Media Filtration Used to remove suspended particles (turbidity) including microbes. Two types of granular media filters: Slow sand filters: uniform bed of sand; low flow rate Rapid sand filters: 1, 2 or 3 layers of sand/other media Diatomaceous earth filters siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder used as filtration aid fossilized skeletons of diatoms (crystalline silicate); powdery deposit;

Roughing Filter gravel: rock that is of a specific particle size range Widely Used inexpensive low maintenance local materials Remove large solids Remove microbes 90% turbidity reduction

Membrane Filters More recent development and use in drinking water Microfilters: several tenths of μM to μM diameter pore size nano- & ultra-filters: retention by molecular weight cutoff Reverse osmosis filters: pore size small enough to remove dissolved salts; used to desalinate (desalt) water as well as particle removal High >99.99% removal of cellular microbes Virus removals high >9.99% in ultra-, nano- filter.

Adsorbers and Filter-Adsorbers Granular activated carbon adsorption remove dissolved organics poor retention of pathogens, esp. viruses biologically active; develops a biofilm can shed microbes into water Filter-adsorbers Sand plus granular activated carbon reduces particles and organics biologically active microbial retention is possible

Water Softening and Microbe Reductions ”Hard" Water: contains excessive amounts of calcium and magnesium ions iron and manganese can also contribute to hardness. Hardness ions are removed by adding lime (CaO) and sometimes soda ash (Na2CO3) to precipitate them as carbonates, hydroxides and oxides. This process, called softening, is basically a type of coagulation‑flocculation process. Microbe reductions similar to alum and iron coagulation when pH is <10 Microbe reductions >99.99% possible when pH is >11 microbial inactivation + physical removal

Rapid Gravity Filtration

Pressure Filters

Membrane Micro-filtration

Activated Carbon Adsorption

Ultra- violet (UV) Disinfection

2- Desalination Industry

Introduction to Desalination 97% of the earth’s water is seawater salt content > 30,000 mg/liter Domestic water supply requires dissolved solids content<1000 mg/l Drinking water < 500 mg/l Desalination refers to the wide range of technical processes designed to remove salts from water.

Consumers Desalination has developed rapidly, is an important source in parts of : Middle east Arabian Gulf North Africa and some islands

Simplified Box Diagram for Desalination Feedwater (seawater or brackish water)

Desalination Processes Thermal processes: Multi Stage Flash Multi effect distillation Vapor compression distillation - Thermal vapor compression (TVC) - Mechanical vapor compression (MVC) Membrane Processes: Reverse osmosis (RO) Electrodialysis Combined Processes Minor processes: freezing, solar humidification

1-Multi Stage Flash Technology1/4 Well proven track record Large capacity units Low operating & maintenance cost High quality product water Used where adequate steam and power is available DESALINATION Page 6 Dr Salam Al-Dawery

1-Multi Stage Flash Technology2/4

1-Multi Stage Flash Technology3/4 Seawater feed passes through tubes in each evaporation stage where it is progressively heated. Final seawater heating occurs in the brine heater by the heat source. Subsequently, the heated brine flows through nozzles into the first stage, which is maintained at a pressure slightly lower than the saturation pressure of the incoming stream. As a result, a small fraction of the brine flashes forming pure steam. The heat to flash the vapour comes from cooling of the remaining brine flow, which lowers the brine temperature.

1-Multi Stage Flash Technology4/4 Subsequently, the produced vapour passes through a mesh demister in the upper chamber of the evaporation stage where it condenses on the outside of the condensing brine tubes and is collected in a distillate tray. The heat transferred by the condensation warms the incoming seawater feed as it passes through that stage. The remaining brine passes successively through all the stages at progressively lower pressures, where the process is repeated. The hot distillate flows as well from stage to stage and cools itself by flashing a portion into steam which is re-condensed on the outside of the tube bundles.

Boiling Temperature of Water at Different Pressures Pressure (bar) Temperature (deg. C) 2 120 1 100 0.25 65 0.1 45

2-Multi Effect Distillation Technology1/3 Well proven track record Mid-size units Low Operating&Maintaince cost High quality product water Used in IWPPs where adequate steam is available but may be some constraints on power DESALINATION Page 8 Dr Salam Al-Dawery

2-Multi Effect Distillation Technology2/3

2-Multi Effect Distillation Technology3/3 In each effect, heat is transferred from the condensing water vapour on one side of the tube bundles to the evaporating brine on the other side of the tubes. This process is repeated successively in each of the effects at progressively lower pressure and temperature, driven by the water vapour from the preceding effect.

2-Multi Effect Distillation Technology3/3 In the last effect at the lowest pressure and temperature, the water vapour condenses in the heat rejection heat exchanger, which is cooled by incoming seawater. The condensed distillate is collected from each effect. Some of the heat in the distillate may be recovered by flash evaporation to a lower pressure. As a heat source, low pressure saturated steam is used, which is supplied by steam boilers or dual-purpose plants (co-generation of electricity and steam).

3-Vapor compression1/2 diesel engines in 1940 VC plants were first used in naval vessels powered by diesel engines in 1940 Vapor compression is generally integrated with MED plant They are used for small or medium scale applications Two types of vapor compression Thermal vapor compression Mechanical vapor compression Developments in the technology are: Unit size increased by 5 times Energy consumption is reduced; it is 8-14 kWh/m3

3-Vapor compression2/2

Evaporation plant with mechanical vapour compression

Reverse Osmosis Process (RO) Reverse osmosis is a membrane separation process in which pure water is “forced” out of a concentrated saline solution by flowing through a membrane at a high static transmembrane pressure difference. This pressure difference must be higher than the osmotic pressure between the saline solution and the pure water. The saline feed is pumped into a closed vessel where it is pressurised against the membrane. As a portion of the water passes through the membrane, the salt content in the remaining brine increases. At the same time, a portion of this brine is discharged without passing through the membrane.

Components of a Reverse Osmosis Plant

Reverse Osmosis Process  Dr Salam Al-Dawery

Two Types of Reverse Osmosis Membrane

Reverse Osmosis Low capacity units Higher O&M Cost Preferred option for stand alone water plants Easy O&M Lower installation cost

Electrodialysis1/3 What is electrodialysis (ED)? General Principles Electrodialysis is a membrane process in which ions are transported through ion permeable membranes from one solution to another under the influence of an electrical potential gradient. ED is generally applied for brackish water desalination General Principles Salts dissolved in water forms ions, being positively (cationic) or negatively (anionic) charged. These ions are attracted to electrodes with an opposite electric charge. Membranes can be constructed to permit selective passage of either anions or cations. Dr Salam Al-Dawery

Electrodialysis2/3

Electrodialysis3/3

Combined Process: Solar Distillation1/2

Combined Process: Solar Distillation2/2