Water Treatment: Processes and Technologies

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 (coastal areas)
• High cost of desalination

Brackish Water

Advantages:

• Less salinity
• Less biological fouling
• Not restricted use
• Lower cost of desalination

Disadvantages:

• Limited resources
• Salinity increases with time
• Problematic elements: heavy metals, silica, etc.

• Equipment size may be decreased
• Do not involve phase changes or chemical additives
• Low energy consumption and typically a-thermal
• Greater efficiency for raw materials use and potential for recycling of by-products
• Simple in concept and operation
• Modular and easy to scale up

The main objective of wastewater treatment is to remove or reduce the concentration of different chemical substances found in waters due to their use, which can be hazardous to the environment and health. The treatments to be applied will depend on the use foreseen for treated waters.

The purpose of preliminary treatment is to protect the operation of the wastewater treatment plant. This is achieved by removing from the wastewater any constituents which can clog or damage pumps, or interfere with subsequent treatment processes. They are designed to:

1. Remove or reduce in size the large, entrained, suspended, or floating solids.
2. Remove heavy inorganic solids such as sand and gravel, as well as metal or glass.
3. Remove excessive amounts of oils or greases.

Preliminary treatment processes consist of the following:

• Screening
• Grit chamber
• Flotation units
• Skimming tanks

Primary Treatment: A physical process.

• Wastewater flow is slowed down, and suspended solids settle to the bottom by gravity.
• The settled material is called sludge or biosolids.
• Sludge from the primary sedimentation tanks is pumped to the sludge thickener. More settling occurs to concentrate the sludge before disposal.
• Primary treatment reduces the suspended solids (approximately 50 to 70%) and the B.O.D. of the wastewater (approximately 25 to 50%).
• From the primary treatment tanks, water is pumped to the trickling filter for secondary treatment.

The objective of secondary treatment is the further treatment of the effluent from primary treatment to remove the residual organics and suspended solids.

• Secondary treatment follows primary treatment and involves the removal of biodegradable dissolved and colloidal organic matter using aerobic biological treatment processes.
• Utilizes bacteria and algae to metabolize organic matter in the wastewater.
• The biological solids removed during secondary sedimentation, called secondary or biological sludge, are normally combined with primary sludge for sludge processing.
• Common high-rate processes include:
  • Trickling filters or bio-filters
  • Rotating biological contactors (RBC)

Tertiary Treatment: Its objective is to improve the quality of waters coming from the secondary treatment. It is mainly used when there are pollutants non-affected by the biological treatment, such as heavy metals or some drugs. At least, disinfection is applied to remove microorganisms before disposal.

Tertiary treatment includes:

• Chlorination and disinfection
• Reverse osmosis
• Ion exchange