13.21
Biological Treatment of Effluent and Waste Water
Biological wastewater treatment relies on the metabolic activity of microorganisms to remove pollutants from sewage. In modern treatment systems, this…
Biological wastewater treatment uses microorganisms to degrade pollutants in the sewage.
Primary treatment uses screening to remove solids and sedimentation to settle suspended particles.
Secondary treatment uses microbial oxidation under aerobic conditions to break down organic waste in sewage. This can be done using trickling filters or the activated sludge process.
In trickling filters, wastewater can be sprayed onto crushed stone or synthetic media coated with biofilms of aerobic bacteria.
Dissolved organic matter diffuses into the biofilms, where aerobic microbes oxidize it into simpler metabolites.
In the activated sludge process, bacteria such as Zoogloea ramigera form flocs with other microbes and oxidize organic carbon, which greatly reduces biochemical oxygen demand.
The sludge then flows to a settling tank, where part of it is recycled, and the excess is removed for further treatment.
Tertiary treatment uses UV disinfection to refine clarified effluent for discharge or reuse.
Finally, anaerobic digestion breaks down the sludge under anaerobic conditions, producing methane-rich biogas and reducing sludge volume.
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Q1: What is the purpose of primary treatment in wastewater systems?
Primary treatment removes large and settleable solids from wastewater through physical processes. Screening captures coarse debris, while sedimentation allows suspended particles to settle out of the sewage. This initial step reduces the load on downstream biological processes by preparing wastewater for secondary treatment.
Q2: How do trickling filters remove organic matter from wastewater?
Trickling filters spray wastewater over crushed stone or synthetic media coated with biofilms of aerobic bacteria. Dissolved organic compounds diffuse into the biofilm, where aerobic microbes oxidize them into simpler metabolites. This secondary treatment method effectively reduces organic pollutants under aerobic conditions.
Q3: What role does Zoogloea ramigera play in the activated sludge process?
Zoogloea ramigera bacteria aggregate with other microorganisms to form flocs in the activated sludge process. These microbial flocs oxidize organic carbon in sewage, greatly reducing biochemical oxygen demand. The flocs then settle in a tank, where excess sludge is removed for further treatment while some is recycled.
Q4: How does tertiary treatment prepare effluent for discharge or reuse?
Tertiary treatment provides additional refinement of clarified effluent through ultraviolet disinfection, which reduces microbial contamination before the water is discharged or reused. This final stage ensures the treated water meets safety standards for environmental release or recycling applications.
Q5: What happens to sludge during anaerobic digestion?
During anaerobic digestion, microorganisms break down organic matter in sludge under anaerobic conditions without oxygen. This process decreases sludge volume significantly and produces methane-rich biogas, which can be recovered as an energy source for facility operations or external use.
Q6: Why is sludge recycled in the activated sludge process?
Sludge is recycled in the activated sludge process to maintain microbial activity and microbial populations in the system. A portion of settled sludge is returned to the aeration tank, ensuring sufficient bacteria remain to continue oxidizing organic carbon and sustaining treatment efficiency.
Q7: What is the difference between secondary and tertiary treatment objectives?
Secondary treatment targets dissolved and fine particulate organic matter through microbial oxidation under aerobic conditions, reducing biochemical oxygen demand. Tertiary treatment refines the clarified effluent using ultraviolet disinfection to eliminate microbial contamination, preparing water for safe discharge or reuse.