Reverse Osmosis, Desalination, and Water Treatment Explained
Reverse Osmosis Membranes
A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not the majority of dissolved salts, organics, bacteria, and pyrogens. However, you need to ‘push’ the water through the reverse osmosis membrane by applying pressure that is greater than the naturally occurring osmotic pressure in order to desalinate (demineralize or deionize) water in the process, allowing pure water through while holding back a majority of contaminants. When pressure is applied to the concentrated solution, the water molecules are forced through the semi-permeable membrane, and the contaminants are not allowed through.
Desalination: Removing Salts and Minerals
Desalination is the process of removing dissolved salts and minerals from seawater or brackish water. Desalination produces drinking water and concentrate.
Seawater vs. Brackish Water
Seawater
- Advantages: Unlimited resources due to its abundance, constant salinity, known physical and chemical characteristics.
- Disadvantages: High salinity, corrosive, frequent and abundant biological fouling, restricted use, and high cost of desalination.
Brackish Water
- Advantages: Less salinity, less biological fouling, not restricted use, and lower cost of desalination.
- Disadvantages: Limited resources, salinity increases with time, problematic elements: heavy metals, silica, etc.
Water Quality Parameters
COD (Chemical Oxygen Demand): The total measurement of the chemicals in the water that can be oxidized.
BOD (Biochemical Oxygen Demand): Supposed to measure the amount of organic carbons that bacteria can oxidize.
TOC (Total Organic Carbon): The measurement of organic carbons.
Environmental Management Systems
EMAS (Eco-Management and Audit Scheme): Identifies and evaluates environmental aspects (direct and indirect). Obligatory to demonstrate it. There is a compliance audit. Requires annual improvement. Applies in Europe. Under legal bases. Is obligatory.
ISO (International Organization for Standardization): Requires only a procedure able to identify environmental aspects. Only commitment to comply with applicable legal requirements. There is no compliance audit. Requires periodic improvement without a defined frequency. Applies internationally. Under no-legal bases. Is recommended but not required.
Environmental Impact Assessments
EIA (Environmental Impact Assessment): An administrative process involving a promoter and previous consultations with institutions.
ESIA (Environmental Impact Study): A technical study made by an expert or a technician. It is one of the stages of EIA.
Osmosis and the Water Cycle
Osmosis: Is a naturally occurring phenomenon and one of the most important processes in nature. It is a process where a weaker saline solution will tend to migrate to a stronger saline solution. Examples of osmosis are when plant roots absorb water from the soil and our kidneys absorb water from our blood. The solution that is less concentrated will have a natural tendency to migrate to a solution with a higher concentration.
Water Cycle:
Water Intake (Sources of Water Supply)
- Rainwater
- Surface water
- Groundwater
- Meltwater, iceberg, or icecaps
- Reuse water
Water Treatment
Transforms raw surface and groundwater into safe drinking water. Water treatment involves two types of processes: physical removal of solids and chemical disinfection.
Techniques
- Coagulation and flocculation
- Sedimentation
- Filtration
- Disinfection