Scum floating on the surface falls into two core categories: floating of activated sludge itself (sludge bulking, denitrification flotation) and scum formed by inert impurities plus dead bacterial flocs (sludge aging, abnormal organic loading, flocculation with grease). Chemical wastewater is far more prone to this issue than domestic sewage due to its complex water quality, including toxic substances, high salinity, fluctuating COD and refractory organic matter.
I. Four Most Common Root Causes
1. Sludge Flotation Caused by Denitrification
(Off-white fine scum, easily broken up when stirred with tiny air bubbles entrapped)
Return sludge from secondary sedimentation tank carries high nitrate nitrogen to the rear section of aerobic tank or anoxic zone. With low dissolved oxygen (DO), denitrification occurs inside sludge flocs to generate nitrogen gas, which entraps sludge and causes flotation.
This problem frequently occurs in chemical wastewater with high total nitrogen, over-nitrification, and insufficient DO at the tank outlet.
Characteristics: Loose floating sludge; tiny air bubbles visible when crushed; part of the sludge resettles after static placement.
2. Filamentous Sludge Bulking
(Brown flocculent floating sludge with poor settleability, SVI＞150)
High-incidence inducing factors for chemical wastewater:
Sharp fluctuation of influent COD, long-term excessively high or low organic loading
Imbalanced trace nutrients (deficient nitrogen and phosphorus, a common issue in chemical wastewater)
Persistently low DO during aeration
Inhibitory substances in influent (organic solvents, phenols, heavy metals, biocides) suppress zoogloea, allowing filamentous bacteria to gain competitive dominance
Characteristics: Extremely slow settling in SV30 test with turbid supernatant; flocculent sludge drifting all over the tank.
3. Scum from Aged & Disintegrated Sludge
(Dark brown fine sludge foam with sludge broken into powder particles)
Excessive aeration and long-term overhigh DO trigger intensive endogenous respiration, leading to sludge aging and disintegration.
Infrequent sludge wasting results in excessively high MLSS and prolonged sludge retention time (SRT), causing dead sludge to float.
Long-term micro-toxic inhibition from chemical wastewater kills massive active microorganisms, and fragmented zoogloea floats and accumulates on the surface.
Characteristics: Fine dark floating scum; clear supernatant after settlement yet fragmented sludge residue.
4. Clotted Floating Sludge from Grease & Surfactants Carried in Influent
Common pollutants in chemical wastewater include cutting fluid, emulsified oil, solvents, surfactants, and flocculated alum flocs incompletely removed by pre-treatment.
Grease wraps sludge flocs to reduce density and induce flotation.
Surfactants generate abundant foam that entraps sludge and forms thick surface scum layers.
Unremoved suspended solids and colloids from inadequate pre-sedimentation directly flow into the aerobic tank.
Characteristics: Viscous floating sludge with oily luster, clumped when skimmed and hard to break apart.
II. Rapid Inspection & Diagnosis Procedures
Test SV30 and SVI: High SVI indicates filamentous bulking; normal SVI with abundant fine residue points to sludge aging or disintegration.
Monitor tank DO: Maintain 2~3 mg/L DO at the front section and 1~2 mg/L at the outlet; low outlet DO highly suggests denitrification flotation.
Microscopic examination: Massively multiplied filamentous bacteria = sludge bulking; fragmented zoogloea and excessive rotifers = sludge aging.
Knead floating scum to judge types: Bubbles inside → denitrification; oily and sticky texture → grease/surfactant contamination; dry powdery residue → dead aged sludge.
Laboratory analysis of influent indexes: COD, total nitrogen, total phosphorus, salinity, and characteristic toxic pollutants.
III. Targeted Treatment Countermeasures
1. For Denitrification Flotation
Mitigate nitrification intensity and reduce nitrified liquid reflux ratio.
Appropriately raise DO at the aerobic tank outlet to eliminate anaerobic micro-environments inside sludge.
Strengthen denitrification in the upstream anoxic tank to cut down nitrate nitrogen carried into the aerobic section.
2. For Filamentous Sludge Bulking
Trace and cut off the source of influent toxic wastewater to avoid shock loads from high-concentration inhibitory wastewater.
Supplement nitrogen and phosphorus nutrients to maintain the BOD:N:P ratio of approximately 100:5:1.
Adjust aeration volume to stabilize DO and avoid long-term low oxygen conditions.
Mild bulking: Dosing small amounts of PAC and PAM to flocculate and skim surface scum. Severe bulking: Short-term low-dose sodium hypochlorite to inhibit overgrown filamentous bacteria (strictly control dosage to prevent zoogloea poisoning).
3. For Aged & Disintegrated Sludge
Increase sludge wasting volume to lower MLSS and shorten sludge retention time.
Reduce aeration air volume to avoid over-aeration.
Alleviate intermittent shock loads from toxic wastewater and stabilize influent organic loading.
4. For Floating Sludge Caused by Grease & Surfactants
Install oil separation and dissolved air flotation (DAF) units in pre-treatment to remove emulsified oil and surfactants in advance.
Foam scum induced by surfactants: Install mechanical defoaming devices (preferred over chemical defoamers, which may cause secondary microbial inhibition).
Manually skim clotted surface scum to prevent anaerobic putrefaction of sludge and further deterioration of effluent quality.
专业术语注释(便于设备海外资料使用)
good oxygen tank = aerobic tank 好氧池
activated sludge 活性污泥
denitrification 反硝化作用
sludge bulking 污泥膨胀
filamentous bacteria 丝状菌
zoogloea 菌胶团
sludge aging 污泥老化
MLSS (Mixed Liquor Suspended Solids) 混合液悬浮固体
SRT (Sludge Retention Time) 泥龄
SV30 / SVI (Sludge Volume Index) 30 分钟沉降比 / 污泥体积指数
dissolved oxygen (DO) 溶解氧
PAC (Polyaluminum Chloride) 聚合氯化铝
PAM (Polyacrylamide) 聚丙烯酰胺
DAF (Dissolved Air Flotation) 气浮
