The treatment of water and wastewater relies on a sequence of distinct, engineered steps called unit operations (physical forces) and unit processes (chemical and biological reactions). While drinking water treatment focuses on removing low-level contaminants (like turbidity and pathogens) to protect public health, wastewater treatment focuses on removing massive quantities of organic matter and nutrients to protect the environment.

Surface water (from rivers or lakes) typically requires more extensive treatment than groundwater because it is exposed to the atmosphere and surface runoff, resulting in higher turbidity, organic matter, and microbial contamination. The standard sequence for a conventional surface water treatment plant is as follows:

1. Coagulation and Flocculation: Raw water contains tiny, negatively charged particles (colloids) that repel each other and will not settle. A chemical coagulant, such as alum $Al_2(SO_4)_3$, is rapidly mixed in to neutralize these charges. Slow mixing then causes the neutralized particles to collide and stick together, forming larger, heavier flocs.

2. Sedimentation (Clarification): The water flows slowly through a large basin. Gravity causes the heavy flocs to settle to the bottom as sludge, which is periodically removed. The clarified water exits over weirs at the top.

3. Filtration: The clarified water passes through beds of porous media, typically sand and anthracite coal. This traps remaining fine particles and some pathogens that did not settle in the clarifier. The filters must be periodically backwashed to clean them.

4. Disinfection: The final step ensures pathogenic organisms are destroyed. Chlorine is the most common disinfectant because it leaves a residual in the pipes to prevent recontamination. Alternatives include ozone $O_3$ and ultraviolet light, though they provide no residual protection.

Colloidal particles in natural waters (like clays, silts, and organic matter) generally carry a negative surface charge. This charge creates electrostatic repulsion, preventing the particles from aggregating. Coagulation is the chemical process of overcoming this repulsion.

• Double Layer Compression: Adding high concentrations of electrolytes (salts) shrinks the electrical layer surrounding the colloid, allowing particles to get close enough for attractive van der Waals forces to take over.
• Charge Neutralization: Adding highly positively charged ions, such as $Al^{3+}$ from alum or $Fe^{3+}$ from ferric chloride, directly neutralizes the negative surface charge of the colloids.
• Sweep Coagulation: Adding excess coagulant forms massive, voluminous hydroxide precipitates, such as $Al(OH)_3$. As these heavy precipitates settle, they physically sweep up and entrap smaller colloidal particles.
• Interparticle Bridging: Long-chain synthetic polymers (polyelectrolytes) attach to multiple colloids simultaneously, physically binding them into a large matrix.

The design of a sedimentation basin (clarifier) is fundamentally based on how fast particles fall through water. This is governed by Stokes' Law, which balances the gravitational force pulling the particle down against the buoyant force and fluid drag pushing it up.

As water passes through a sand filter, the trapped particles cause the pores to clog, increasing the resistance to flow. This resistance is measured as Head Loss.

The effectiveness of a chemical disinfectant (like chlorine) depends on its concentration and how long the water is exposed to it.

Wastewater (sewage) from homes, commercial buildings, and industries contains exceptionally high levels of organic matter (measured as BOD/COD), excess nutrients (such as nitrogen and phosphorus), heavy metals, and pathogenic microorganisms. Untreated discharge can completely deplete oxygen in receiving waters (leading to fish kills) or spread waterborne diseases (like cholera or typhoid). The standard sequence of treatment involves physical, biological, and advanced unit processes to safely clean the water prior to reuse or environmental discharge.

1. Primary Treatment: This physical process involves screening to remove large debris and primary clarifiers where heavier organic solids settle to the bottom as raw sludge while lighter materials such as grease float to the top.

2. Secondary Treatment: This biological process commonly uses activated sludge. Microorganisms are actively mixed with wastewater and supplied with air to consume dissolved organic matter. A secondary clarifier then separates the dense microbial mass from the clear, treated water.

3. Tertiary Treatment: This advanced process is used when the receiving water is highly sensitive. It focuses on removing nutrients such as nitrogen and phosphorus to prevent eutrophication, followed by final filtration and disinfection.

Alternative: Trickling Filters: Trickling filters are attached-growth processes where wastewater is sprayed over a bed of highly permeable media, such as rocks or plastic. Microorganisms form a biological film on the media, absorbing and digesting organic matter as the water trickles down.

Wastewater treatment generates massive volumes of semi-solid waste (sludge) that requires stabilization and volume reduction before disposal.

• Nutrient Removal: Biological or chemical processes remove nitrogen through nitrification/denitrification and phosphorus through chemical precipitation or enhanced biological phosphorus removal (EBPR).
• Filtration: Sand filters or multimedia filters remove residual suspended solids.
• Carbon Adsorption: Activated carbon removes recalcitrant organic compounds that cause taste, odor, or toxicity.

• Microfiltration (MF) and Ultrafiltration (UF): Remove suspended solids, bacteria, and large macromolecules. These processes are often used as pretreatment for reverse osmosis.
• Nanofiltration (NF): Removes multivalent ions, such as hardness-causing calcium and magnesium, and smaller organic molecules.
• Reverse Osmosis (RO): Removes monovalent ions, such as sodium and chloride, and almost all other dissolved impurities. It requires high pressure to overcome osmotic pressure and is used extensively in desalination.