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The activated sludge settling ratio (SV30%) is one of the most important control parameters in the daily management of wastewater treatment plants. By observing SV30%, operators can quickly evaluate the settling characteristics of activated sludge and gain valuable insights into the operational condition of the biological treatment system. Drawing on over 20 years of practical experience, this article provides an in-depth analysis of the key observation points of SV30%, helping readers comprehensively understand this critical indicator.
The operation and management of the activated sludge process involve multiple control parameters, each serving a unique function. However, SV30% stands out for its ease of measurement and rich informational value, making it almost capable of substituting for other parameters in system performance assessment.
Food-to-Microorganism Ratio (F/M): Requires testing of influent substrate concentration and activated sludge concentration; complex to calculate.
Dissolved Oxygen (DO): Requires online monitoring; primarily used to assess the degree of organic matter oxidation.
Return Sludge Ratio: Commonly used to adjust hydraulic retention time and shock resistance.
Sludge Age (t): Indicates the degree of sludge aging based on calculated values.
Biological Population (Microscopic Observation): Reflects sludge activity and possible toxicity.
Mixed Liquor Suspended Solids (MLSS): Indicates the sludge concentration; requires laboratory analysis.
Sludge Volume Index (SVI): Used to evaluate sludge bulking through calculation.
1.Simple Measurement: Requires only a graduated cylinder and mixed liquor; easy to perform on-site.
2.Represents the Secondary Clarifier Settling Process: Effectively simulates the actual settling conditions in the secondary sedimentation tank.
3.Early Problem Detection: Variations in the settling process provide early warning of potential system abnormalities.
| Item | Description | Causes |
| Supernatant surface — Oily substance | A hazy, oil-like film covering the liquid surface, usually thin and easily overlooked. (Mixed liquor viscosity increases overall.) | 1. Influent contains mineral oil or emulsified oil 2. Influent contains detergents or defoamers 3. Insufficient influent flow causing relative over-concentration and excessive break-up of flocs 4. Aging and disintegration of activated sludge. |
| Supernatant surface — Scum | Brown or black flocculent masses floating on the surface. | 1. Over-aeration 2. Aging of activated sludge 3. Caused by surface oil film 4. Sludge poisoning/toxicity 5. Filamentous bulking 6. Oxygen deficiency in activated sludge. |
| Supernatant surface — Bubbles | ※ Rows of relatively large bubbles between the liquid surface and the cylinder wall. ※ Smaller bubbles attached to scum on the surface. |
1. Over-aeration 2. Aging of activated sludge 3. Caused by surface oil film 4. Denitrification 5. Filamentous bulking. |
| Supernatant surface — Odor | Odor emitted (during initial settling) | 1. Strong earthy (musty) odor indicates high biological activity 2. Strong acidic or alkaline smell indicates abnormal mixed liquor pH 3. Strong foul odor suggests oxygen deficiency 4. Other unusual odors may indicate inflow of specific industrial wastewater. |
| Item | Description | Causes |
| Overall Settling Characteristics | The stage from free settling to flocculent settling; overall settling is characterized by a clear sludge-water interface and integrated sedimentation. | 1. Low activated sludge activity (the older, the worse); 2. High loading rate (higher = poorer); 3. Insufficient aeration (excessive adsorption); 4. Sludge toxicity (toxic sludge flocs become dispersed); 5. Filamentous bulking (forms mass but without settling). |
| Settling Speed | Initial flocculation speed and the rate of free and flocculent settling; speed at which the sludge-water interface forms. | 1. Activated sludge activity (higher = faster); 2. Sludge aging (older = slower); 3. Sludge toxicity (toxic = slower); 4. Sludge viscosity (higher = slower); 5. Filamentous bulking (overall slower); 6. Sludge concentration (early flocculent settling); 7. Suspended solids content (higher = slower); 8. Water temperature and viscosity. |
| Interfloc Water | Characteristics of water between flocs after aggregation (turbidity, fine suspended solids). | 1. Over-aeration (produces finer floc particles); 2. Aging of activated sludge (floc disintegration); 3. High organic loading (floating sludge); 4. Filamentous bulking (causes high turbidity). |
| Floc Appearance | Floc size after aggregation, floc movement direction (upward/downward), and color. | 1. Over-aeration (loose flocs); 2. Aging of activated sludge (coarse, dark-colored flocs); 3. High loading (fine floc structure); 4. Filamentous bulking (dense flocs). |
| Item (English) | Description (English) | Causes (English) |
| Clarity | Overall chroma (color) and turbidity. | 1. Load level (the higher, the worse); 2. Degree of aeration (excessive aeration leads to poor clarity); 3. Whether the sludge is poisoned (poisoned sludge results in poor clarity); 4. Filamentous bulking (clear). |
| Particles | Quantity of suspended particles. | 1. Degree of sludge aging (the more aged, the more particles); 2. Whether the sludge is poisoned (turbidity accompanied by fine, scattered particles); 3. Activated sludge load (the higher, the more turbid); 4. Inert substance content (the higher, the more turbid). |
| Interstitial Water | Clarity of water between scattered particles. | 1. Excessive aeration (small particles still visible in the water between large particles); 2. Aging of activated sludge (clear interstitial water); 3. Excessive load (turbid interstitial water); 4. Sludge poisoning (turbid interstitial water). |
| Wall Adherence | Activated sludge floc particles adhering to the wall of the measuring cylinder. | 1. Aging of activated sludge; 2. Excessive aeration. |
| Item | Description | Causes |
| Compactability | The final density of the settled sludge. | 1. Amount of inert substances (the more, the denser); 2. Load level (the lower, the denser); 3. Degree of aeration (excessive aeration leads to poor compactability); 4. Whether the sludge is poisoned (fine, fragmented, and dense); 5. Filamentous bulking (varies with the degree of bulking). |
| Color & Luster | 1. Depth of color; 2. Gloss of the color; 3. Brightness of the color. | 1. Activity of activated sludge (the higher, the lighter the color); 2. Degree of sludge aging (the more aged, the darker and duller the color); 3. Sludge poisoning (dull color); 4. Activated sludge load (the higher the load, the lighter the color); 5. Filamentous bulking (white liquid surface); 6. Sludge concentration (the higher the concentration, the darker the color); 7. Sludge denitrification (bright and vibrant color). |
| Scum Formation | After sedimentation, the flocculability of the sludge is further enhanced, the non-compressible surface layer increases its adsorbability. | 1. Normal activated sludge shows moderate scum formation; 2. Becomes pronounced when activated sludge is excessively aged (even when system performance issues are not yet obvious); 3. Not present when sludge is poisoned or under high load conditions. |
| Bubbles | Bubbles trapped within the settled flocs. | 1. Excessive aeration (small bubbles visible immediately after settling); 2. Filamentous bulking; 3. Increased viscosity due to aging of activated sludge; 4. Denitrification of activated sludge (released after stirring); 5. Expansion of fine bubbles due to high temperature after sampling. |
| Indicator / Sludge Condition | Normal Activated Sludge | High Sludge Load | High Sludge Concentration | Sludge Aging | Filamentous Bulking | Sludge Poisoning | High Inert Content | Sludge Denitrification | Over-aeration |
| Oil-like substance on liquid surface | × | × | ∘ | ∘ | × | ∘ | × | ∘ | ∘ |
| Scum on liquid surface | × | × | ∘ | ∘ | □ | ∘ | ∘ | ∘ | ∘ |
| Bubbles on liquid surface | × | × | □ | ∘ | □ | ∘ | × | ∘ | ∘ |
| Earthy smell of activated sludge | ∘ | □ | ∘ | □ | □ | × | □ | □ | □ |
| Good settling characteristics | ∘ | × | ∘ | × | × | × | □ | × | × |
| Fast settling velocity | ∘ | × | □ | ∘ | × | □ | ∘ | □ | × |
| Clear interstitial water | ∘ | × | × | × | × | × | × | × | × |
| Good flocculation state | ∘ | × | ∘ | × | × | × | × | × | × |
| High supernatant clarity | ∘ | × | × | × | × | × | × | × | × |
| Many particles in supernatant | × | ∘ | × | ∘ | ∘ | ∘ | ∘ | × | ∘ |
| Wall adherence phenomenon | × | × | □ | ∘ | × | × | × | × | ∘ |
| Good sediment compactability | ∘ | × | ∘ | ∘ | × | × | × | × | ∘ |
| Deep and bright color | ∘ | × | □ | × | ∘ | × | × | ∘ | ∘ |
| Good scum formation | × | × | ∘ | ∘ | × | × | × | ∘ | ∘ |
| Easily traps bubbles | × | × | ∘ | □ | ∘ | × | × | × | ∘ |
∘: Present
×: Not Present
□: Partially Present/ Variable
In summary, SV30 is an indispensable tool in the operation and management of the activated sludge process. By mastering the key observation points, operators can quickly assess the system's status, optimize operational parameters, and ensure the efficient and stable performance of the wastewater treatment system. It is hoped that this article provides valuable guidance and supports the smooth execution of wastewater treatment operations.
