Secondary treatment of sewage
The second stage of wastewater/sewage treatment is secondary wastewater treatment. Primary treatment removes suspended solids, colloidal particles, oil, and grease.
Secondary treatment involves biological treatment of the wastewater to eliminate any organic stuff present. Indigenous and aquatic microorganisms such as bacteria and protozoa accomplish this treatment, by consuming biodegradable soluble pollutants, such as sugar, fat, detergent, and food waste. These processes are temperature sensitive, and as the temperature rises, so does the pace of biological reactions.
The trickling filter, the activated sludge process, and the oxidation pond are the three basic biological treatment procedures, apart from the more advanced ones now-a-days.
Which treatment technologies are used in secondary treatment of sewage?
The various secondary sewage treatment technologies are as follows:
· Trickling filter
A trickling filter is a tank with a thick layer of stones. Sediment is constantly sprayed over the top of the stones and trickles to the bottom, where it is collected for further treatment.
Bacteria congregate and proliferate on the stones as the wastewater trickles down. The constant flow of sewage over these growths, allows the microorganisms to absorb the dissolved organics, decreasing the sewage's biochemical oxygen demand (BOD). The oxygen required for metabolic processes is provided by the air moving upwards, through the crevices between the stones.
· Secondary clarifiers
Following the trickling filters are settling tanks known as secondary clarifiers. Microbes that are swept off the rocks by the flow of wastewater are removed by these clarifiers.
To improve treatment efficiency, two or more trickling filters can be linked in series, and sewage can be recirculated.
· Activated Sludge Process
Step 1:An aeration tank is followed by a secondary clarifier in the activated sludge treatment system. Settled sewage is put into the aeration tank, along with fresh sludge recirculated from the secondary clarifier.
Step 2:Compressed air is then introduced into the mixture via porous diffusers at the tank's bottom. The diffused air supplies oxygen and a quick mixing action as it bubbles to the surface. The churning movement of mechanical propeller-like mixers situated at the tank's surface can also add air.
Step 3:Microorganisms flourish in such oxygenated environments, generating an active, healthy suspension of biological solids—mostly bacteria—known as activated sludge. The aeration tank can hold you for up to six hours. This allows the bacteria to consume dissolved organics from the sewage, lowering the BOD.
Step 4:The combination is then pumped from the aeration tank into the secondary clarifier, where the activated sludge settles out naturally. Clear water is scraped off the clarifier's surface, treated, and discharged as secondary effluent. The sludge is pumped out of the tank through a hopper at the bottom.
Step 5:Approximately 30% of the sludge is recirculated back into the aeration tank and mixed with the primary effluent. This recirculation is a critical component of the activated sludge process.
Step 6:The recycled microorganisms are highly adapted to the sewage environment, and metabolize the organic components in the main effluent with ease. The remaining 70% of secondary sludge must be processed and disposed of in a proper manner.
· Oxidation ponds
Oxidation ponds, also known as lagoons or stabilization ponds are huge, shallow ponds that use sunshine, bacteria, and algae to purify wastewater.
Algae develop by absorbing solar energy as well as carbon dioxide and inorganic compounds, generated by bacteria in water. During the photosynthesis process, algae releases oxygen that aerobic bacteria require.
Mechanical aerators are occasionally built to provide even more oxygen, lowering the area of the pond required.Dredging is required to eliminate sludge deposits in the pond.
Filtration or a mixture of chemical treatment and settling can eliminate any algae, which remains in the pond wastewater.
· Rotating Biological Contactors
A series of huge plastic disks installed on a horizontal shaft are partially buried in primary effluent, in this treatment system. The disks are exposed to air and wastewater alternately while the shaft spins, allowing a layer of bacteria to grow on the disks and metabolize the organics in the wastewater.
How can we assist?
Check out our sewage treatment services on our website. If you require additional information on wastewater or sewage treatment, our experts will gladly explain how we treat and purify sewage to ensure its safety.
Contact Netsol Water today to obtain answers to all your secondary wastewater treatment inquiries. You can approach us via phone at +91 9650608473 or by email at email@example.com.