P1 Flashcards
What is meant by the term impingement?
When fish/sea life get stuck to intake screen due to high intake velocity.
What is meant by the term entrainment?
When organisms which are smaller than the screen are drawn into the intake.
What are the main considerations that need to be taken into account when designing a seawater intake system?
- Source water quality
- Construction impacts
- Operational impacts to marine life
- Capital and maintenance costs
- Regulatory permits
Why are scaling and fouling an issue?
Fouling and scaling causes reduced system efficiency.
Can block membranes and heat exchange surfaces decreasing the efficiency and increasing energy consumption.
Increased maintenance costs: frequent cleaning and replacement of scaled components leads to greater operational and maintenance costs.
Shortened equipment lifespan: Can cause irreversible damage to equipment.
Operational downtime: regular cleaning and maintenance can increase the process downtime and hence affect water production.
What are treatments used to prevent scaling in desalination processes?
- Removal of all or some of he bicarbonate alkalinity by feeding acid. (bicarbonate removed using scale control agents such a polycarboxylic acid and homopolymer of polymeric)
- Use of scale control agents depending on the composition of tube material and the process economics.
- Polyphosphates cause increased scaling in MSF and MED so should be avoided.
- Softening pre-treatment can be used to removed hard ions which cause scaling.
What causes scaling?
Precipitation of minerals, such as calcium carbonate, from solution. Scaling material includes:
CaSO4 - CaCO3 - BaSO4 - SiO2 (silica)- SrSO4 -Mg(OH)2
What are the advantages of desalination using an RO system?
- High efficiency and purity
- Scalability
- Lower energy consumption when compared to thermal methods such as MSF and MED.
- Automated and reliable.
- Can handle variety of feed water (versatile) : Seawater, Brackish, and Wastewater.
What are the practical limitations of using an RO system for desalination?
- High initial capital costs
- High energy consumption
- Prone to membrane fouling and scaling
- Brine must be disposed of which is costly and challenging
- Feedwater must be pre treated to protect the membranes.
What are the 6 major steps which must be considered when designing an RO process?
- Select the membrane type most suitable for the design.
- Select the flux rate (L/m^2h) according to expected feed water quality.
- Determine the number of membrane elements required to produce the desired plants capacity.
- Determine the number of pressure vessels required to hold the calculated number of membrane elements (assume 6 elements per vessel)
- Select the appropriate array to achieve the desired recovery, cake feed, and product flows.
- Calculated feed pressure + 15% NDP + manifold pressure losses < guideline max pressure (this condition must be met at the lowest operating temperature.
what are the advantages of the freeze desalination method?
- High production rate per unit volume at low driving force.
- Small power consumption is because no new transfer surface involved in the system.
- Absence of moving parts.
- Unit is compact and efficient.
What are the disadvantages of the freeze desalination method?
There is part retention of the refrigerant in the ice generated, making the water obtained non-potable.
What are processes which can be used for post treating desalinated water?
- Aeration : Water is sprayed or bubbled with air, allowing gases to escape and oxygen to enter, facilitating the oxidation of certain dissolved materials.
- Re-carbonation: CO2 gas is bubbled through the water, forming carbonic acid, which dissociates to bicarbonate and hydrogen ions, thus increasing alkalinity and stabilising pH.
- Lime addition: Lime added to water, precipitating calcium carbonate, which can be filtered out or left to increase water hardness.
- pH adjustment: controlled addition of acid or base to achieve a desired pH
- Corrosion inhibitors: Addition of corrosion inhibitors which form a thin protective layer on pipes and metal.
- Calcite bed filtration: Water flows through a calcite bed, where calcium carbonate dissolves into the water enhancing alkalinity and buffering pH.
- Primary and secondary disinfection: Chlorine or UV light is applied initially (primary) and chloramines or other long lasting disinfectants are added for secondary disinfection.
- Blending with freshwater supplies: Controlled mixing of desalinated water and natural fresh water sources to achieve desired quality and mineral balance.
- Removal of other ions: Ion exchange resins or specific adsorbent materials to remove targeted ions from the water.
Why is concentrate management such a problem?
Environmental impact:
Marine ecosystems affected by increased salinity and chemical contaminants.
Groundwater contamination such as soil salinization and groundwater pollution.
Technical and economic challenges:
High disposal costs from transport, treatment caused by energy consumption.
Limited disposal options due to regulatory restrictions and geographical constraints.
Social and regulatory issues:
Community concerns such as public opposition and health risks.
Regulatory compliance of stringent regulations and permitting processes.
What factors need to be considered when disposing of concentrated brine?
- Concentrate quality and quantity
- Permitting requirements
- Geography and geology
- Costs
- Environmental impact
