P3 Flashcards
Why must the feedwater be pre-treated before going into desalination facility?
- Ensures materials inside the water do not enter the facility.
- Seawater is a mixture of algae, bacteria, salts, microparticles etc which can cause fouling and scaling.
- Corrosive constituents must be removed for thermal processes as they are prone to pipe erosion and scaling.
- Membrane fouling can be caused by solids of mineral and biological origin.
- Biological growth must be removed by using disinfectants.
What are the main scale forming compounds in sea water?
- Calcium carbonate: favoured by decreasing co2 concentration and increasing calcium or bicarbonate concentration.
- Calcium sulphate: Forms as dehydrate, anhydrous, and hemihydrate where increasing the ionic concentration of the two ions can lead to scaling.
- Silica: Solubility depends on pH and temperature which must be below 10 degC. When concentration increases so does scaling. Silica forms precipitates.
What pre-treatment can be used for calcium carbonate?
Addition of acid or using scaling control agents (homopolymer of polymaliec or polycarboxylic acid)
The choice depends on process economics and material of inner tubes.
What pre-treatment can be used for calcium sulphate?
The removal of calcium sulphate can be achieved by adding anti scaling agents to prevent precipitation.
List the different types of membrane module configurations that are commonly used in industry?
- The flat sheet
- Hollow fibre membranes
- Tubular membranes
- Spiral wound membranes
What are the major techniques that are available for the manufacture of polymeric membranes and which one is the most predominant technique?
- Sintering: when polymeric particles are pressed into the film or plate below the melting point of the material.
- Stretching: when a polymer film is stretched.
- Track-etching (uniform pores): thin films are exposed to irradiation with charged particles damaging the polymeric matrix. The polymeric material is then etched away when immersed in alkali/acid which leaves uniform pores.
- Phase inversion: dissolving a polymer in a solvent to form a homogeneous solution, casting this solution into a thin film, and then immersing the film into a non-solvent bath where the polymer precipitates to form a porous membrane structure due to solvent exchange.
Which technique is predominantly used to make polymeric spiral wound membranes?
Phase inversion
What are four methods for concentrate disposal?
- Surface water discharge (main) : Disposing of concentrate directly into natural water bodies like oceans, rivers, or lakes.
- Deep well injection (main): Injecting concentrate into deep underground wells, isolating it from surface water and ecosystems.
- Evaporation ponds: Using large, shallow ponds where concentrate is allowed to evaporate, leaving salts and other residues behind.
- Sewer discharge to WWTP (main): Discharging concentrate into the municipal sewer system for further treatment at wastewater treatment plants.
What are the advantages and challenges for the surface water discharge method for concentrate disposal?
Advantages:
Simple and cost-effective.
Direct disposal into oceans, rivers, or lakes with adequate dilution can minimize local environmental impact.
Challenges:
Potential ecological damage due to increased salinity and chemical content.
Regulatory constraints and environmental permits may be difficult to obtain.
What are the advantages and challenges of the deep well injection method for concentrate disposal?
Advantages:
Isolates concentrate from surface environments, reducing ecological risks.
Suitable for inland desalination plants.
Challenges:
High costs and technical complexity of drilling and maintaining injection wells.
Risk of groundwater contamination and induced seismicity.
What are the advantages and challenges of the evaporation pond method of concentrate disposal?
Advantages:
Simple technology with low operational costs.
Effective in arid regions where evaporation rates are high.
Challenges:
Requires large land areas, which can be costly or unavailable.
Slow process and potential for ground and air pollution from concentrate constituents.
What are the advantages and challenges of the sewer discharge to WWTP for concentrate disposal?
Advantages:
Utilizes existing infrastructure, reducing initial costs.
Provides additional treatment of concentrate through the wastewater treatment process.
Challenges:
Limited by the capacity and acceptance criteria of wastewater treatment plants.
May require pre-treatment of concentrate to avoid disrupting wastewater treatment processes or exceeding regulatory discharge limits.
Why must desalinated water be post treated prior to final distribution?
Desalinated water must be post-treated prior to final distribution to adjust its chemical composition and quality to meet health and safety standards, ensure compatibility with existing water infrastructure, and enhance its taste and stability. This process includes steps like remineralization to add essential minerals, pH adjustment to prevent corrosion in pipes, and disinfection to eliminate any remaining pathogens.
What would be some examples of post treatment processes that may be used at the end of an RO desalination facility?
- Recarbonation
- lime addition
- calcite bed filtration
- pH adjustment
- addition of corrosion inhibitors
- primary and secondary disinfection
- blending with freshwater supplies
Explain briefly three alternative methods of desalination.
Capacitive Deionization (CDI):
Description: CDI is an electrochemical water treatment technology where salt ions are removed from water by applying an electrical potential difference between two porous electrodes. Ions are attracted to and held on the surface of the electrodes, effectively desalinating the water.
Hybrid Plants:
Description: Hybrid desalination plants combine two or more desalination processes, such as reverse osmosis (RO) and multi-stage flash distillation (MSF), to optimize water production and energy use. These plants can leverage the strengths of different technologies to improve efficiency and reduce costs.
Microbial Desalination Cells (MDC):
Description: MDC is a bio-electrochemical system that uses bacteria to generate electricity while simultaneously removing salts from water. In an MDC, bacteria oxidize organic matter in wastewater, producing electrons that drive the desalination process.
