Activated Sludge: Basic Concepts

Bacteria that require free dissolved oxygen to survive are called:

Facultative bacteria are best described as:

The activated-sludge process relies on:

Mixed liquor refers to:

The main purpose of returning activated sludge (RAS) is to:

Wasting activated sludge (WAS) is performed to:

In a conventional activated-sludge system, aeration detention time is typically about:

Activated sludge forms floc primarily through:

The 'solids inventory' of an activated-sludge system is best described as:

Which is NOT a goal of the secondary clarifier in activated sludge?

An 'anoxic' zone in activated sludge has:

Which organism group is largely responsible for nitrification in activated sludge?

Sludge that never settles well and stays suspended in the clarifier is described as:

The biological growth phase associated with extended aeration (very low F/M, long SRT) is:

The food-to-microorganism ratio (F/M) is most commonly expressed as:

To calculate F/M you need which two analyses?

Typical F/M for a conventional activated-sludge plant is approximately:

Compared with conventional activated sludge, high-rate (high F/M) systems tend to:

An extended-aeration plant typically operates at which F/M and SRT?

A rising F/M (e.g., from a load increase) without raising MLVSS usually causes:

If F/M is very low for a long time, the sludge tends to:

Which units must match before computing F/M?

An operator raising the MLVSS (e.g., by reducing wasting) is trying to:

F/M is most useful for comparing plants or operations when:

Which best describes F/M controlling strategy during a load spike?

A CBOD-based F/M differs from a total-BOD F/M because:

Mean Cell Residence Time (MCRT), also called SRT, is defined as:

Typical SRT for a conventional activated-sludge plant is about:

An extended-aeration plant typically targets an SRT of about:

In cold weather, the SRT needed to maintain nitrification:

To lengthen SRT, an operator should:

Shortening SRT (more wasting) generally:

Which solids are normally counted in the numerator of SRT?

Which daily losses are subtracted in the SRT denominator?

If SRT is far longer than needed, the risk is mainly:

Gould Sludge Age (GSA) differs from MCRT in that GSA is based on:

An SRT shorter than the nitrifier doubling time will cause:

MLSS measures:

MLVSS is used as a proxy for:

Typical MLVSS as a fraction of MLSS is about:

Typical MLSS in a conventional activated-sludge plant is about:

MLSS in an extended-aeration or oxidation-ditch plant is typically:

Total pounds of solids in an aeration tank is calculated as:

An operator wants to raise MLSS. The usual action is to:

A sudden large drop in MLSS is most likely caused by:

Polymer is sometimes added to secondary clarifiers to:

Carrying too-high MLSS in a fixed basin risks:

The Sludge Volume Index (SVI) is calculated as:

The settled volume used for SVI is read after how many minutes?

An SVI below about 100 generally indicates:

An SVI above about 150 suggests:

A very low SVI (e.g., <70) combined with turbid effluent may indicate:

Settled volume in the cylinder is reported in:

SVI is most useful when compared:

A dilute settleometer (diluted SVI) helps:

When the sludge blanket in the secondary clarifier is rising while SVI is normal, suspect:

An operator should generally respond to a rising SVI by first checking:

Chlorinating the RAS to control filaments works because:

If chlorination of the RAS reduces bulking but also stops nitrification, the most likely cause is:

RAS flow is most commonly expressed as:

Typical RAS return rate for a conventional activated-sludge plant is about:

If RAS flow is too low for the conditions, the clarifier sludge blanket will:

RAS solids concentration is highest when the sludge:

Wasting (WAS) is normally taken from:

Wasting from the RAS line versus directly from the aeration tank mainly differs in:

If you stop wasting for a week, the MLSS will:

A sudden, sharp decrease in wasting rate will, over days, cause:

Target sludge blanket depth in a secondary clarifier is commonly about:

Why is constant RAS flow usually preferred over constant MLSS-only control during wet weather?

Sludge blanket level is most commonly measured by:

Filamentous bulking is most directly caused by:

Which filament is classically associated with LOW dissolved oxygen bulking?

Which filament is classically associated with LOW F/M (old sludge) bulking?

Which filaments are associated with septicity / sulfide (high H₂S) in the influent?

Which filaments are linked to nutrient deficiency (low N or P)?

Which filament is famous for grease/foam and low-F/M conditions?

Short-term control of a severe filamentous bloom often uses:

Long-term control of low-DO bulking is best achieved by:

Selectors control filaments by:

Which selector type uses nitrate (anoxic) to favor floc-formers?

Filamentous organisms examined microscopically are best identified using:

A few filaments in healthy activated sludge are:

Stable, viscous brown foam on the aeration tank is commonly associated with:

Foaming is often linked to which influent characteristic?

Nocardia foam is notably resistant to control because the organisms:

Common foam-control approaches include all EXCEPT:

White, sudsy, fluffy foam in a young system often indicates:

A thick, greasy dark-tan foam clinging to the clarifier and aeration tank, combined with high SVI, most likely indicates:

Physical removal of foam (skimming, spray) is most useful as:

Which condition makes foam problems worse in cold months?

Rising sludge in a secondary clarifier is most often caused by:

The quick fix for rising sludge is usually to:

Pin floc is best described as:

Pin floc is most commonly associated with:

Corrective action for pin floc usually includes:

Straggler floc (light strings of floc) settling slowly but compacting usually indicates:

Diffuse, non-settling turbidity with normal-to-low SVI, but no filaments, suggests:

Rising sludge differs from bulking because rising sludge solids originally settled but:

Typical target DO in an activated-sludge aeration tank is about:

Low DO over time most commonly promotes:

Dissolved oxygen is measured most reliably with:

Excess DO (well above need) mainly wastes:

Oxygen demand in the aeration tank is driven mainly by:

On-line DO probes should be:

During a storm-load spike, oxygen demand typically:

An operator noticing DO sag at the head of the aeration tank during peak load should:

A selector is placed at the head of the aeration tank to:

An anoxic selector most directly also provides:

Contact stabilization splits the process into a:

Step-feed (step aeration) means:

An oxidation ditch is best described as:

A Sequencing Batch Reactor (SBR) cycle includes, in order:

Pure oxygen activated sludge is best suited for:

An MBR (membrane bioreactor) eliminates which conventional unit?

Compared to conventional, extended aeration typically features:

A selector failing to control filaments usually means:

Scenario: high SVI, turbid effluent, low DO. The likely cause and first action?

Scenario: settled sludge floats to the surface after sitting; effluent nitrate present. The cause is:

Scenario: very low SVI but effluent turbid, lots of small particles. Most likely:

Scenario: MLSS dropping fast after a thunderstorm; influent much higher flow. Cause and action?

Scenario: ammonia breakthrough in winter with normal MLSS. Most likely cause?

Scenario: persistent greasy foam on aeration and clarifier, high SVI, long SRT. Cause?

Scenario: white fluffy sudsy foam in a brand-new plant startup. The best interpretation is:

Scenario: aeration tank DO very low at peak, effluent ammonia rising. Immediate priority?

Scenario: SVI climbing steadily while DO and F/M look fine; microscopy shows many Type 0041/0675. Best control?

Scenario: belt press can't dewater WAS well and SVI is very high. The dewatering trouble is likely linked to:

Stalked ciliates (e.g., Vorticella) and rotifers generally indicate:

Large numbers of free-swimming ciliates and flagellates often suggest:

A bloom of spirochetes often signals:

When examining wet mounts, 'inter-floc water' clarity is best interpreted as:

A healthy floc examined at 100× typically looks:

Which is the most immediate benefit of routine microscopic exam?

Gram staining of filaments helps because:

If rotifers and stalked ciliates suddenly disappear and flagellates dominate, the operator should suspect: