Causes of green foam and scum
Causes of green foam in biochemical pools
Algae reproduction
(Main source of green):
Under light conditions, photosynthetic microorganisms such as green algae and blue algae reproduce in large numbers in open biochemical pools, secreting sticky extracellular polymers (EPS) to wrap bubbles and form green foam.
Filamentous bacterial expansion
Nocardia, Microthrix and other filamentous bacteria grow excessively, and their hydrophobic hyphae adsorb bubbles to form stable foam, which may be gray-green due to the metabolic products of the bacteria.
High-load shock
The concentration of organic matter (COD/BOD) in the influent increases suddenly, resulting in abnormal microbial metabolism and excessive secretion of EPS. The foam may carry sludge particles and appear green.
Abnormal dissolved oxygen (DO)
A low DO (<1 mg/L) triggers anaerobic fermentation and gas production, or a high DO (>4 mg/L) causes sludge dispersion, both of which may form foam.
Causes of green scum in secondary sedimentation tank
Algae migration
Algae in the biochemical tank enter the secondary sedimentation tank with the mixed liquid, and continue to reproduce under surface light conditions to form green scum.
Denitrification
When the secondary sedimentation tank is partially hypoxic, nitrate is converted into nitrogen by denitrifying bacteria, and bubbles attach to the sludge and float to form scum. If the sludge carries algae, it will be green.
Poor sludge settling
Sludge swelling (SVI >150 mL/g) or aging leads to slow settling, and the floating sludge combines with bubbles to form scum, and the color is affected by algae or filamentous bacteria.
Comprehensive treatment measures
Source control
Pretreatment enhancement
Intercept grease and surfactants (such as adding flotation units); industrial wastewater needs to detect and remove toxic substances (such as heavy metals and phenols).
Balance water quality
Set up a regulating tank to buffer shock loads and avoid drastic fluctuations in COD/BOD.
Biochemical pool regulation
Inhibit algae and filamentous bacteria
Add a cover to block light or shorten the hydraulic retention time (HRT <12 h) to inhibit algae growth;
Add sodium hypochlorite (10-20 mg/L) or hydrogen peroxide (50-100 mg/L) to selectively kill filamentous bacteria;
Control DO at 2-4 mg/L to avoid hypoxia or over-aeration.
Optimize sludge state
Adjust sludge age (SRT 5-10 days) to prevent sludge aging;
Supplement carbon sources (such as sodium acetate) or nitrogen and phosphorus (urea, phosphate) to maintain C:N:P=100:5:1.
Secondary sedimentation tank management
Inhibit denitrification scum
Reduce the sludge return ratio (30-50%) to reduce the entry of nitrate into the secondary sedimentation tank;
Add pre-aeration (DO 0.5-1 mg/L) in the return sludge channel to destroy the denitrification conditions.
Improve sedimentation effect
Add PAC (50-100 mg/L) or PAM (0.5-1 mg/L) to enhance flocculation;
Control the surface load ≤0.8 m3/(m2·h), and add baffles to eliminate short-circuit.
Physical and chemical emergency treatment
Foam and scum removal
Spray water mist or silicone defoamer (such as polydimethylsiloxane) for rapid defoaming;
Install mechanical skimmers or manually salvage scum to avoid accumulation and corruption.
Algae control
Add copper sulfate (0.1-0.5 mg/L) for short-term emergency, and shading or ultraviolet sterilization is required to inhibit algae in the long term.
Process upgrade and maintenance
Process modification
Change the activated sludge process to MBBR or MBR process to reduce foam generation;
Add inclined plate fillers to the secondary sedimentation tank to improve sedimentation efficiency.
Monitoring and maintenance
Microscopically check the abundance of filamentous bacteria daily, and monitor parameters such as SVI, DO, and pH;
Regularly clean the algae biofilm on the pool wall to prevent secondary pollution.