With degradable substrates at low concentrations, the filaments tend to grow. This explains why complete mix systems with low mixed liquor substrate concentrations favor filamentous growth.
One of the more common causes of filamentous bulking in industrial wastewaters is inadequate nitrogen or phosphorus. There are numerous examples, particularly in the pulp and paper industry, in which severe filamentous bulking resulted from inadequate nitrogen. Restoration of adequate nitrogen restored a flocculated sludge within three sludge ages. Studies on a Wisconsin pulp mill indicated a minimum concentration of NH3-N in the effluent of 1.5 mg/L to favor zoogleal growth. Other studies indicate that higher ammonia concentrations may be required in some cases. Minimum soluble phosphorus concentrations in the effluent of 0.5 mg/L have been reported as required for optimal zoogleal growth.
Therefore deficiency in substrates, such as the macro- or micronutrient concentration, residual soluble BOD, and/or dissolved oxygen concentration in the biological floc, can promote filamentous growth and sludge bulking.
Finally, the bacterial populations require nutrients in addition to carbonaceous food, as indicated by the empirical chemical formula for bacterial protoplasm, C60H87O23N12P. As a rule of thumb, the system requires approximately 5 parts of nitrogen as N and 1 part of phosphorus as P for every 100 parts of BOD in the wastewater. Many wastewaters, particularly municipal sewage, contain the required nitrogen, but industrial wastewater may require supplemental nutrients usually added as ammonium salts and phosphoric acid.
The phenomenon called sludge bulking is thought to be caused by inadequate nutrient levels, although this is probably an oversimplification of a difficult problem. In sludge bulking, the sludge settles slowly because of the development of filamentous organisms in the biomass. The projecting filaments make the biological floc similar to thistledown, so that it moves with the water currents and resists settling. A bulking sludge produces a high sludge volume index (SVI) and is readily identified in microscopic examination. Other factors that are believed to promote bulking conditions may include low pH, which favors the growth of mold and yeasts; the presence of high concentrations of carbohydrates; and low levels of dissolved oxygen.
Effluent Suspended Solids Control
Carryover of suspended solids in the secondary clarifier effluent can have several causes:
- Floc shear due to high aeration basin power levels
- Poor clarifier hydraulics
- High wastewater TDS concentration
- Low or high mixed liquor temperature
- Rapid change in mixed liquor temperature
- Low mixed liquor surface tension