Biology of Wastewater Treatment and Sewer Systems Quiz

Biological Wastewater Treatment Overview

• Biological treatment uses microorganisms to convert organics in wastewater into biomass.
• Wastewater requires a variety of organisms for complete treatment.
• Microorganisms involved in wastewater treatment are similar to those in natural systems but are carefully controlled in engineered reactors.
• Factors affecting biomass production and food utilization include temperature, pH, toxins, salt concentration, oxidants, and nutrient deficiency.
• Microorganisms need air to maintain growth and can adjust to a wide range of environmental factors.
• Design of biological systems requires understanding of biological principles, kinetics of metabolism, and mass balance.
• Two types of biological growth used in wastewater treatment are suspended cultures and attached growth.
• Suspended cultures include activated sludge, ponds, and lagoons.
• The activated sludge process is a suspended-culture process with various common variations.
• Design and operational parameters of the activated sludge process include F/M ratio, MLSS, MCRT/SRT, and HRT.
• The F/M ratio affects the aeration period and treatment efficiency in the activated sludge process.
• MLSS, MCRT/SRT, and HRT are important parameters for the suspended-culture process.

Wastewater Treatment and Sewer Systems

• Types of sewer systems include sanitary sewer system, storm sewer system, and combined water system
• Contaminants of concern in wastewater treatment include suspended solids, biodegradable organics, pathogens, nutrients, priority pollutants, refractory organics, heavy metals, and dissolved inorganics
• Wastewater treatment objectives involve the removal of suspended and floatable material, treatment of biodegradable organics, elimination of pathogenic organisms, removal of toxic compounds, and removal of nutrients
• Division of wastewater treatment systems includes preliminary wastewater treatment, primary wastewater treatment, secondary wastewater treatment, tertiary wastewater treatment, and solids treatment and disposal
• Preliminary wastewater treatment involves the removal of large solids to prevent damage to the remainder of the unit operations through screening, comminution, grit removal, and flotation
• Primary wastewater treatment aims to remove a portion of suspended solids and organic matter by settling or sedimentation, removing about 60% of the solids and about 30% of BOD
• Secondary wastewater treatment is directed principally for the removal of biodegradable organics and suspended solids, including biological treatment by activated sludge, fixed-film reactors, lagoons, and pond systems
• Tertiary wastewater treatment involves the polishing of secondary effluent, primarily focusing on the removal of nutrients, toxic compounds, increased amounts of organic material, suspended solids, and removal of ions and salts through ion exchange and reverse osmosis processes
• Solids treatment and disposal includes the collection, stabilization, and subsequent disposal of the solids removed by other processes
• Classification of wastewater treatment methods includes physical unit operations, chemical unit processes, and biological unit processes
• Preliminary treatment methods include screening, which is the first operation performed on incoming wastewater for the purpose of removing materials that might damage equipment or hinder further treatment
• Coarse screens are used to remove coarse solids from wastewater, consisting of vertical bars spaced 1 or more centimeters apart and inclined away from the incoming flow, with solids retained by the bars usually removed by manual raking in small plants while mechanical cleaner units are used in larger plants

Wastewater Treatment Techniques and Challenges

• Dissolved oxygen (DO) concentration is crucial for aerobic processes, with an optimum value of 1.5 – 2.5 mg/L.
• High aeration levels (DO 4) are beneficial, but over-aeration (DO 5-6) can lead to undesirable microorganisms and filamentous bulking.
• Filamentous bulking in secondary clarifiers hinders settling and can impact the performance of activated sludge systems.
• Causes of poor settling due to filamentous bulking include wrong DO levels, insufficient nutrients, and fluctuating organic waste loading.
• Operational and wastewater characteristics causes of non-filamentous organisms include over-aeration, improper loading, and toxic substances.
• Remedies for filamentous bulking include adjusting F/M ratio, DO levels, dosing with H2O2, and pre-treating wastewater to remove toxins.
• Aeration techniques include air diffusers (fine and coarse bubble) and mechanical aerators used in plug flow and completely mixed reactors, respectively.
• Wastewater ponds and lagoons provide natural purification processes through aerobic and anaerobic zones, with oxygen for lagoons being provided by artificial aeration.
• Attached growth systems like trickling filters, biotowers, and rotating biological contactors facilitate wastewater treatment through attached biomass.
• Trickling filters use stationary medium and intermittent wastewater doses to achieve treatment efficiency, requiring a primary clarifier to avoid media clogging.
• Biotowers use modular synthetic media to enable a vertical arrangement for wastewater treatment, addressing issues such as odor and vector problems.
• Trickling filters and biotowers have distinct features and limitations, such as odor problems, vector issues, and the need for pre-treating wastewater.