Biological Phase & Operational Status Judgment – Activated Sludge Microscopic Examination (Part IV)

I. Understanding Activated Sludge Floc Structure

The size, shape, compactness of activated sludge flocs, as well as the ratio and growth status of zoogloeal bacteria and filamentous bacteria constituting flocs, can well reflect wastewater treatment performance. Microscopic observation of floc microstructures enables preliminary judgment of sludge health status.

Note: Floc particle size is greatly affected by shear force generated by aeration and agitation; values vary under different operating conditions.

1. Newly-formed Zoogloeal Flocs

Observation of newly-formed zoogloeal flocs indicates abundant organic matter and vigorous bacterial proliferation. This is a typical feature in the initial sludge cultivation stage or under high organic loading, with clearly distinguishable bacterial morphology.

2. Healthy Flocs

Healthy flocs possess the following characteristics:

• Good compactability and dense structure
• Dark brown color
• No pinpoint-sized micro-flocs observed between flocs
• Clear, closed edges

Such flocs feature excellent adsorption and settling performance.

3. Disintegrated Flocs

Disintegrated flocs are fundamentally different from newly-formed zoogloeal flocs. Although individual bacterial shapes are distinguishable in newly-formed zoogloeal flocs, bacteria inside poorly compacted, disintegrated flocs are ruptured. The floc structure becomes loose, and settling performance deteriorates significantly.

II. Biological Phase Judgment for Nine Typical Operating Conditions

Sludge under different conditions shows obvious differences in sedimentation tests.

Condition 1: Normal Operation

Large floc particles with clear edges, dense and closed structures. Flocs are supported by zoogloeal bacteria as the skeleton, with a small quantity of filamentous bacteria interspersed (far less abundant than zoogloeal bacteria). No free bacteria are detected. Microfauna are dominated by sessile ciliates (Vorticella, Opercularia, Epistylis, etc.); Aspidisca can be seen crawling on flocs, and rotifers grow actively. Countermeasure: Strengthen routine management and maintain current operating parameters.

Condition 2: Low Sludge Loading

Loose floc structure and reduced floc particle size. Massive increase in free-swimming ciliates (Colpidium, Paramecium, Bodo, Monas) and sarcodines (Amoeba). Cause: Bacteria consume extracellular polysaccharide matrix of zoogloea for self-maintenance, leading to loose floc structure. Countermeasure: Reduce sludge return flow, supplement nutrients, shorten sludge retention time (SRT) to raise sludge loading.

Condition 3: High Sludge Loading

Similarly loose and smaller flocs, accompanied by large quantities of free bacteria. Cause: Abundant nutrients stimulate massive growth of free bacteria; flocculated zoogloeal bacteria tend to deflocculate into single free cells. Countermeasure: Reduce influent flow and sludge wasting to lower sludge loading.

Condition 4: Insufficient Dissolved Oxygen (DO)

Appearance of sulfur bacteria, spirochetes and Metopus. Countermeasure: Boost aeration in the aeration tank to elevate dissolved oxygen concentration.

Condition 5: Excess Dissolved Oxygen

When DO exceeds 5 mg/L, abundant sarcodines and rotifers emerge. Countermeasure: Reduce aeration volume.

Condition 6: Low Loading with Nitrification Ongoing

Euplotes, Spirostomum, Rotaria, Arcella become dominant, indicating active nitrification. Countermeasure: Appropriately increase BOD loading.

Condition 7: Sludge Recovery

Appearance of Litonotus, Loxophyllum, Trachelius signals activated sludge is recovering from deteriorated status toward normal. Tip: Positive signal; continue existing adjustment strategy.

Condition 8: Sludge Aging

Massive occurrence of rotifers and nematodes is associated with activated sludge aging, usually observed in the early aging phase.

Condition 9: Early Warning of Sludge Bulking

Rising populations of Epistylis, Aspidisca, Amphileptus, Vorticella indicate improved effluent quality. However, when sludge structure deteriorates and loosens, free-swimming ciliates multiply drastically, causing turbid effluent.

III. Quick Reference Table: Biological Phase vs Operating Parameters

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