Difference Between primary and secondary sewage treatment

What is primary sewage treatment?

• Primary sewage treatment is a crucial initial step in wastewater management aimed at removing solid contaminants from wastewater. In this phase, wastewater is directed into primary sedimentation tanks or primary clarifiers, where the treatment process begins. These tanks are designed to allow the wastewater to flow slowly, facilitating the separation of solids from the liquid.
• The process relies on gravity to cause the heavier solids, including sand and silt, to settle at the bottom of the tank. Meanwhile, lighter materials such as oil and grease rise to the surface, forming a layer of scum. This sedimentation typically requires a hydraulic retention time (HRT) of about 1.5 to 2.5 hours.
• The settled sludge at the bottom is collected by mechanically driven scrapers, which push it toward a central hopper. From the hopper, the sludge is then pumped to sludge treatment facilities for further processing. The floating scum is skimmed off the surface and also directed for disposal.
• Once the solid materials are removed, the remaining liquid is either discharged directly into the environment or forwarded to secondary treatment processes for further purification. This initial stage of treatment, also known as the ‘primary phase’ or ‘septic process,’ primarily reduces the load of suspended solids and organic matter in the wastewater, preparing it for more advanced treatment stages.

What is secondary sewage treatment?

• Secondary sewage treatment is a critical phase in wastewater management that focuses on removing biodegradable organic matter that remains after primary treatment. This stage primarily involves biological processes to further purify the wastewater.
• During secondary treatment, the main goal is to reduce the biochemical oxygen demand (BOD) by targeting dissolved and suspended organic matter. This process is typically performed by microorganisms in either aerobic or anaerobic environments, depending on the treatment technology used. In aerobic systems, bacteria and protozoa consume biodegradable organic pollutants, such as sugars, fats, and other organic compounds. These microorganisms reproduce and form biological solids as they process the contaminants.
• There are two principal types of secondary treatment systems: fixed-film and suspended-growth systems. Fixed-film systems, such as trickling filters, bio-towers, and rotating biological contactors, involve microorganisms growing on a stationary media. The sewage flows over this media, allowing the microorganisms to degrade the organic matter. Innovations in this category include moving bed biofilm reactors (MBBR) and Integrated Fixed-Film Activated Sludge (IFAS) processes.
• Suspended-growth systems, such as activated sludge processes, use a suspended biomass of microorganisms that absorb and adsorb organic material from the wastewater. This type of system often includes a secondary clarifier to separate the biological floc from the treated water. Constructed wetlands and upflow anaerobic sludge blanket reactors are also used in secondary treatment, offering different advantages depending on the specific treatment needs.
• Therefore, secondary treatment significantly enhances wastewater quality by further reducing organic contaminants, preparing the effluent for safe disposal or potential reuse. It effectively builds on the initial primary treatment to achieve a higher standard of water purity through biological means.

Difference between primary and secondary sewage treatment

Primary Treatment

1. Nature of Process:
   • Primary treatment is a physical process that involves the removal of solid particles, both organic and inorganic, from sewage. This stage focuses on the separation of larger solid wastes through filtration and sedimentation.
   • It primarily uses filters and grit chambers to segregate solid particles from the liquid waste.
2. Method of Solid Removal:
   • Larger solid wastes are removed by physical methods, allowing heavier materials to settle at the bottom of tanks and lighter materials to float to the surface. These solids are then removed from the sewage.
3. Biochemical Oxygen Demand (BOD):
   • There is no significant reduction in BOD during primary treatment. The process does not address the dissolved organic matter that contributes to the BOD.
4. Production of Byproducts:
   • No biogases, such as methane, are produced during primary treatment. The process does not involve microbial digestion.
5. Time Required:
   • Primary treatment is relatively quick, as it mainly involves physical separation processes that take a shorter period to complete.

Secondary Treatment

1. Nature of Process:
   • Secondary treatment involves microbiological processes to degrade organic matter that was not removed during primary treatment. This stage uses biological treatment methods to further purify the sewage.
   • It utilizes aeration tanks and anaerobic sludge digesters to facilitate the biological treatment of sewage.
2. Method of Organic Matter Removal:
   • Microorganisms, including bacteria and protozoa, play a crucial role in breaking down dissolved and suspended organic matter. This process reduces the BOD by converting it into biological flocs.
3. Biochemical Oxygen Demand (BOD):
   • BOD is significantly reduced during secondary treatment. The biological activity of microbes effectively decreases the concentration of organic pollutants.
4. Production of Byproducts:
   • During the microbial digestion of organic matter in the sludge, biogases such as methane are produced. These gases can be harnessed for energy or further processed.
5. Time Required:
   • Secondary treatment takes a longer time compared to primary treatment, as it relies on the biological degradation of organic matter, which is a slower process involving microbial activity.

Difference Chart between primary and secondary sewage treatment