Sewage Treatment

 

Sewage and Industrial Effluent Treatment

Sewage treatment (municipal wastewater treatment) and industrial effluent treatment are processes designed to remove physical, chemical, and biological contaminants from wastewater before its discharge into natural water bodies or reuse. The primary objective of wastewater treatment is to protect public health, prevent environmental pollution, and comply with regulatory discharge standards. These treatment systems generally involve physical (primary), biological (secondary), and chemical or advanced (tertiary) treatment processes.

Overall Treatment Flow

Influent → Screening → Grit Removal → Skimming → Primary Sedimentation → Biological Treatment (Trickling Filter / Activated Sludge / Oxidation Ditch) → Secondary Sedimentation → Tertiary Treatment → Treated Effluent

1. Physical Treatment (Preliminary and Primary Treatment)

Physical treatment involves the removal of large, floating, settleable, and suspended solids using mechanical and gravitational methods. It mainly reduces the load on subsequent biological treatment processes.

1.1 Preliminary Treatment

Preliminary treatment is the first step in wastewater treatment and aims to remove gross solids, grit, and floating materials that may damage equipment or hinder treatment efficiency.

a. Screening

Screening is used to remove large floating and suspended materials such as rags, plastics, paper, cloth, sticks, cans, and dead animals. Wastewater is passed through bar screens made of vertical or inclined steel bars spaced about 5 cm apart. Coarse and fine screens are used depending on the treatment requirement that trap large objects. The collected screenings are disposed of by incineration, composting, or landfilling.

b. Grit Removal

Grit consists of heavy, non-biodegradable inorganic particles such as sand, gravel, silt.  Grit removal is carried out in grit chambers by controlling the flow velocity so that grit settles while organic matter remains in suspension. Common units include horizontal flow grit chambers, aerated grit chambers, and vortex grit chambers. Grit removal prevents abrasion of pumps and accumulation of sediments in tanks.

c. Skimming

Skimming is the process of removing floating fats, oils, grease, waxes, and soap scum from wastewater. In skimming tanks, air is bubbled from the bottom, causing oily materials to float to the surface, where they are mechanically skimmed off. Removal of grease prevents interference with oxygen transfer during biological treatment.

1.2 Primary Treatment

Primary treatment focuses on removing remaining suspended solids and reducing the organic load of sewage.

Sedimentation (Primary Clarification)

Sedimentation tanks allow wastewater to flow slowly (1–2 feet per minute), enabling suspended solids to settle at the bottom as primary sludge, while lighter materials float as scum. Sludge scrapers are used to remove settled solids periodically. Primary sedimentation removes approximately 50–70% of suspended solids and 25–40% of BOD.

Note- Sometimes there is the provision of Mechanical and chemical Flocculation

Mechanical Flocculation- Mechanical flocculation improves the removal of fine suspended and colloidal particles. Wastewater is gently stirred using rotating paddles at controlled speeds (~0.43 m/s), causing small particles to aggregate into larger flocs that settle more easily.

Chemical Coagulation and Flocculation- Chemical treatment involves the addition of coagulants such as alum, ferric chloride, or lime. Alum forms aluminum hydroxide [Al(OH)₃] precipitates that trap suspended and colloidal particles, forming larger flocs which settle at the bottom. This method is effective when wastewater contains high colloidal content.

2. Biological Treatment (Secondary Treatment)

Secondary treatment removes dissolved and colloidal organic matter using microorganisms. Microbial metabolism converts organic pollutants into stable inorganic compounds, significantly reducing biochemical oxygen demand (BOD).

MajorBiological Changes

• Organic carbon → CO₂ + H₂O
• Organic nitrogen → NH₃ → NO₃⁻ (nitrification)
• Colloidal matter → biologically coagulated and settled

2.1 Aerobic Treatment (With Oxygen)

Aerobic microorganisms degrade organic matter in the presence of oxygen.

a. Trickling Filter

A trickling filter is an attached-growth system consisting of a filter bed (gravel, stones, or plastic media), rotating spray arms, and a collection system. The sewage effluent or wastewater is sprayed over the bed by using an overhead sprayer, which is rotating at a constant speed; the spraying saturates the effluent with O2. There is a development of a gelatinous microbial layer called the zoogleal layer (bacteria, algae, protozoa, fungi). The fed surface becomes covered with aerobic microbial population comprising bacterial species, including Sphaerotilus natans, Beggiatoa, Flavobacterium, Achromobacter, Zooglea and Pseudomonas, microalgae, microfungi and protozoa. As wastewater trickles through or percolates over the media, organic matter is oxidized. The aerobic microflora decomposes the organic materials into small soluble molecules. Later, the treated effluent collected at the bottom of the tank is passed through the sedimentation tank. Natural airflow provides oxygen. Treated effluent is usually passed to a secondary clarifier.

b. Activated Sludge Process

This is the most widely used aerobic biological treatment method. Wastewater is mixed with activated sludge (microbial biomass) in an aeration tank, where air or oxygen is supplied continuously. Microorganisms consume organic matter, forming flocs. The mixture is then sent to a secondary clarifier where sludge settles. A portion of sludge is recycled (return activated sludge), while excess sludge is removed. This process removes 85–95% of BOD and suspended solids.

Activated sludge system consists of an aeration tank, a settling tank and a sludge return system. At first, sewage from the primary treatment plant is mixed with sludge drawn from the previous batch, which is known as activated sludge or return sludge. The activated sludge contains a large number of microorganisms and serves as an inoculum of microorganisms. After mixing the activated sludge, sewage is placed in an aeration tank. In an aeration tank, Sewage is continuously aerated for 6-8 hours. During this period, microorganisms oxidizes the organic compounds to form CO₂, H₂0 and NO₃, etc.

So, activated sludge is a process for the treatment of domestic and industrial sewage using air and biological flocs composed of bacteria and protozoa that substantially reduce organic materials. This process starts when air is introduced into a sewage that is held in a large aeration tank, combined with the following aerobic microbial decomposers to develop biological floc that decomposes organic matter into simple soluble molecules, amino acids, ammonia, phosphorus, nitrates, CO₂, H₂O, etc.

·        Bacteria: Escherichia, Enterobacter, Achromobacter, Flavobacterium, Pseudomonas, Zooglea, Micrococcus, Sphaerotilus, Beggiatoa, Thiothrix , etc.

·        Protozoa: Amoaebe, Spriotrich and Verticillium. 

·        Filamentous fungi: Geotrichum, Cephalosporium, Penicillum and Cladodsporium.

After oxidation, sewage is passed to the settling tank and left undisturbed for 2-3 hours. Sludge settles to the bottom. This sludge is called activated sludge, which is fully oxidized and is very offensive. This activated sludge can be used as inoculum for the next batch of sewage. Most of the sludge is removed and some is returned to the aeration tank for the next round of treatment.

By the sludge digestion process, the BOD of sewage is reduced by 5-15%.

c. Oxidation Pond (Lagoon / Stabilization Pond)

Oxidation ponds, also known as lagoons, stabilization ponds, or reduced ponds, are large, shallow, open basins used for biological treatment of wastewater. This method is primarily aerobic in nature and is suitable for areas with warm climates and sufficient land availability. In this system, sewage from the primary treatment plant is retained in the pond for a long detention period, typically 10 to 40 days. During this time, microorganisms oxidize the organic matter present in the sewage.

A symbiotic relationship exists between algae and bacteria in oxidation ponds. Algae release oxygen during photosynthesis, which is utilized by aerobic bacteria to oxidize organic compounds. In turn, the carbon dioxide and nutrients released during bacterial oxidation are used by algae for photosynthesis. Additional oxygen is also supplied from the atmosphere since the pond is an open system. The oxidation pond generally remains aerobic during daytime and the early hours of night, allowing aerobic decomposition of organic matter. During late night hours, when photosynthesis ceases, oxygen levels may drop, leading to partial anaerobic conditions and anaerobic decomposition. Oxidation ponds are cost-effective, require minimal mechanical equipment, and provide satisfactory removal of BOD and pathogens.

 

2.2 Anaerobic Treatment (Without Oxygen)

Anaerobic treatment is used for high-strength wastewater and sludge stabilization.

a. Anaerobic Digestion

In sealed, oxygen-free digesters, anaerobic bacteria break down organic matter into biogas containing 60–70% methane (CH₄) and carbon dioxide. Anaerobic digestion reduces sludge volume, stabilizes organic matter, and produces renewable energy.

Anaerobic Treatment of Wastewater

Anaerobic treatment is a biological wastewater treatment process carried out in the absence of oxygen. It is mainly used for high-strength wastewater and for stabilization of sludge produced during primary and secondary treatment. This method is especially suitable for wastewater with high organic load, such as that from distilleries, food processing industries, and municipal sludge. Anaerobic treatment reduces organic matter, produces less sludge compared to aerobic processes, and generates useful energy in the form of biogas.

Septic Tank

A septic tank is a simple anaerobic treatment system commonly used for domestic sewage in areas without centralized sewer systems. It is a closed underground tank where sewage is retained for several hours to days. During this detention period, heavier solids settle at the bottom forming sludge, while lighter materials such as oils and grease float on the surface forming scum. Anaerobic bacteria present in the tank partially digest the organic matter, converting it into gases and stabilized sludge. The clarified effluent flows out of the tank for further treatment or disposal through soak pits or drainage fields. Septic tanks are low-cost systems but provide only partial treatment.

Anaerobic Sludge Digestion

Anaerobic sludge digestion is used for stabilizing sludge generated from primary and secondary treatment units. The process takes place in sealed digesters under controlled temperature conditions (mesophilic or thermophilic). Anaerobic microorganisms decompose complex organic matter through stages such as hydrolysis, acidogenesis, acetogenesis, and methanogenesis. This process significantly reduces sludge volume, destroys pathogens, and produces biogas. Digested sludge is more stable, less odorous, and easier to dewater and dispose of safely.

Biogas Production

Biogas is a valuable by-product of anaerobic treatment and sludge digestion. It mainly consists of methane (60–70%), carbon dioxide (30–40%), and trace amounts of hydrogen sulfide and other gases. Methane-rich biogas can be used as a renewable energy source for cooking, heating, or electricity generation in wastewater treatment plants. Biogas production not only recovers energy but also improves the overall sustainability of wastewater treatment systems.

Advantages of Anaerobic Treatment

·        Suitable for high-strength wastewater

·        Low energy requirement (no aeration needed)

·        Produces renewable energy (biogas)

·        Low sludge production

 

3. Chemical and Advanced Treatment (Tertiary Treatment)

Tertiary treatment is employed when high-quality effluent is required, particularly for reuse or discharge into sensitive environments.

Major Processes

• Coagulation and Flocculation: Removal of fine particles and color
• Filtration: Sand filters, membrane filtration
• Disinfection: Chlorination, UV radiation, ozonation
• Nutrient Removal:
• Nitrogen: Nitrification–denitrification
• Phosphorus: Chemical precipitation using alum or lime

Sludge Treatment and Disposal

Sludge generated during primary and secondary treatment is thickened, digested (aerobically or anaerobically), dewatered, and safely disposed of or reused as fertilizer if free from pathogens and toxic substances.