Wastewater Secondary Treatment Process

A comprehensive set of vocabulary flashcards covering key terms, processes, and regulatory concepts from the provided wastewater engineering and environmental science lecture notes.

Fixed-film reactor

A biological treatment unit where microorganisms attach to a medium and degrade organics as wastewater passes over the surface.

Activated sludge process

Suspended-growth aerobic treatment that mixes wastewater with recycled biomass in an aeration tank followed by secondary clarification.

Biochemical Oxygen Demand (BOD)

The amount of dissolved oxygen required by aerobic organisms to break down organic matter in water over a specified time, usually 5 days at 20 °C.

Chemical Oxygen Demand (COD)

A rapid measure of the oxygen equivalent of organic matter in water, determined chemically rather than biologically.

Mixed Liquor Suspended Solids (MLSS)

Concentration of all suspended solids in the mixed liquor of an aeration tank, expressed in mg L⁻¹.

Mixed Liquor Volatile Suspended Solids (MLVSS)

The organic (volatile) fraction of MLSS that represents the active biomass in an aeration tank.

Food-to-Microorganism ratio (F/M)

The mass of substrate (BOD or COD) applied per mass of MLVSS in the aeration tank each day; key process-control parameter.

Return Activated Sludge (RAS)

Settled biomass pumped from the secondary clarifier back to the aeration tank to maintain mixed-liquor concentration.

Waste Activated Sludge (WAS)

Portion of surplus biomass intentionally removed from the system to control solids inventory and sludge age.

Primary settling tank

Clarifier that removes settleable solids and floating material before biological treatment.

Weir overflow rate

Flow per linear foot of weir in a clarifier; typical value for primary settling tanks is ~1,500 gpd ft⁻¹.

Flocculation

The gathering and bridging of small particles into larger, settleable flocs by biological or chemical action.

Trickling filter

An attached-growth aerobic process where wastewater trickles over a bed of media coated with biofilm.

Rotating Biological Contactor (RBC)

A series of discs partially submerged in wastewater that rotate to alternately contact air and water, supporting attached biofilm growth.

Aerobic lagoon

Shallow oxidation pond with mechanical or diffused aeration where aerobes stabilize organics.

Anaerobic lagoon

Deep pond that stabilizes high-strength wastes under oxygen-free conditions, often producing odorous gases.

Denitrification

Anoxic biological conversion of nitrate to nitrogen gas, reducing total nitrogen in wastewater.

Nitrification

Two-step aerobic oxidation of ammonia to nitrite and then nitrate by autotrophic bacteria.

Disinfection

Process that reduces pathogenic microorganisms in effluent, commonly by chlorination or UV irradiation.

Pathogen

Any disease-causing organism present in water, such as bacteria, viruses, or protozoa.

Sludge Volume Index (SVI)

mL occupied by 1 g of MLSS after 30 min settling; indicator of sludge settleability.

Biosolids age (Sludge age)

Average number of days solids remain in the aeration tank; equals solids inventory divided by daily solids input.

Mean Cell Residence Time (MCRT)

Average time biomass stays in the entire activated sludge system, including aeration and clarifiers.

Complete-mix activated sludge

Configuration in which influent, biomass, and oxygen are uniformly mixed, yielding uniform conditions throughout the aeration tank.

Plug-flow activated sludge

Aeration tank where wastewater moves like a “plug,” with substrate concentration highest at the inlet and lowest at the outlet.

Contact stabilization

Two-tank modification where raw wastewater briefly contacts biomass, then solids are stabilized in a separate aerated basin.

Step-feed aeration

Modification where influent is introduced at multiple points along the aeration tank to equalize oxygen uptake and reduce clarifier loading.

Extended aeration

Low F/M, long retention activated sludge (≥18 h) that produces highly treated effluent and minimal excess sludge.

Oxidation ditch

Continuous-loop, extended-aeration system where mechanical rotors circulate and aerate mixed liquor.

High-purity oxygen system

Activated sludge operated under covered tanks with pure O₂ gas, allowing high loading and reduced tank volume.

High-rate aeration

Activated sludge operated at low MLSS and high volumetric loadings to achieve short detention and high F/M.

Equalization tank

Basin that evens out flow and concentration variations before biological treatment.

Aerator – Diffused air

Device using submerged porous diffusers to release fine bubbles and supply oxygen in aeration tanks.

Aerator – Surface

Mechanical unit that splashes water into air, transferring oxygen at the surface of tanks or ditches.

Jet aerator

System injecting water and air through nozzles to mix and oxygenate wastewater.

Centrifugal pump

Rotodynamic pump that imparts energy to liquid via a rotating impeller, relying on inertia and centrifugal force.

Global warming

Long-term rise in Earth’s average surface temperature due to greenhouse gas accumulation.

Chlorofluorocarbons (CFCs)

Stable compounds once used as refrigerants and propellants that deplete stratospheric ozone.

Materials Recovery Facility (MRF)

Site that houses equipment for sorting, composting, and recycling components of solid waste.

Coagulation

Chemical destabilization of colloidal particles in water by charge neutralization, often using alum or iron salts.

Sedimentation

Gravity settling of suspended particles after coagulation/flocculation or in primary clarifiers.

Cesspool

Pit lined with stone or brick that allows sewage effluent to seep into surrounding soil; obsolete and unsanitary.

Septic tank

Underground watertight chamber where sewage undergoes sedimentation and anaerobic digestion before soil absorption.

Privy

Simple pit or vault toilet where excreta drop directly into the ground or a sealed vault below the seat.

Copper sulfate

Chemical applied to surface reservoirs at ≈1 mg L⁻¹ to control algal growth, often used with lime.

Trihalomethanes (THMs)

Chlorinated organic by-products formed when chlorine reacts with natural organics; potential carcinogens.

Surge tank

Stand-pipe or reservoir installed on a pipeline to protect against pressure transients (water hammer).

Coriolis effect

Apparent deflection of moving fluids due to Earth’s rotation, influencing large-scale atmospheric and oceanic flows.

Pitot tube

Instrument that measures fluid flow velocity by converting kinetic energy into potential (pressure) energy.

Bernoulli’s equation (assumptions)

Energy relationship for steady, incompressible, frictionless flow along a streamline in a homogeneous fluid.

Nitrate contamination

Elevated NO₃⁻ levels in groundwater, often linked to agricultural fertilizer application.

Lead in drinking water

Metal that leaches primarily from household plumbing and service lines when water is corrosive.

Acidity (water)

Capacity of water to neutralize bases, commonly caused by dissolved CO₂, mineral acids, or metal salts.

Coagulation vs. flocculation

Coagulation destabilizes particles; flocculation gently mixes to form larger, settleable flocs.

Masking (acoustics)

Interference with speech intelligibility caused by background noise relative to listener distance and frequency.

Pathology

Medical science studying disease nature and the structural/functional changes it produces.

Waterborne hepatitis

Viral infection (commonly Hepatitis A or E) transmitted via contaminated water.

Inertial separator

Dust collector that removes particulates by abrupt changes in gas direction causing heavier particles to depart from the airstream.

Baghouse

Fabric filter system that captures fine particulate matter from flue gases.

Electrostatic precipitator

Device that removes particles by charging them electrically and collecting them on oppositely charged plates.

Catalytic reactor (for NOx)

Post-combustion control unit that converts nitrogen oxides to N₂ using a catalyst and reducing agent (e.g., ammonia).

Gram-positive Clostridia

Spore-forming bacteria capable of causing food poisoning and anaerobic infections.

Materials Recovery Act – RA 9003

Philippine law establishing an ecological solid waste management program, including MRFs and waste reduction.

Philippine Mining Act – RA 7942

1995 legislation governing mineral resources exploration, development, and utilization in the Philippines.

Global permissible noise (3 h)

Occupational exposure limit of ~100 dBA for a three-hour duration per day.

Sludge blanket

Layer of settled solids accumulated at the bottom of a clarifier, through which incoming flow must pass.

Anaerobic digestion

Biological conversion of organic solids to biogas (CH₄ + CO₂) in the absence of oxygen, stabilizing sludge.

Aerobic digestion

Stabilization of excess activated sludge under aerated conditions, reducing volatile solids.

Upflow Anaerobic Sludge Blanket (UASB)

High-rate anaerobic reactor where influent flows upward through a dense granular sludge bed.

Ozone layer depletion

Thinning of stratospheric ozone caused primarily by CFCs and related halogenated compounds.

Threshold shift (hearing)

Permanent or temporary change in hearing sensitivity due to noise exposure.