should be restricted to 4.5 lbs Cl₂/1,000 lbs MLSS-d since the nitrifiers are more sensitive and tend to concentrate on the outer floc layers where the dissolved oxygen levels are higher.

When treating low-strength industrial wastewaters that require a short aeration basin detention time (<6 to 8 hrs), chlorine should be applied to the return sludge. For high-strength industrial wastewaters, however, with long hydraulic retention times, the chlorine must be directly applied to the aeration basin. In some cases, hydrogen peroxide has successfully been applied for sludge bulking control. It is probable, however, that this relates to low dissolved oxygen filaments, which are suppressed by the oxygen released by the hydrogen peroxide.

The major suggested causes of filamentous bulking include: low aeration basin dissolved oxygen (DO) concentration for the applied organic loading, low F/M in completely-mixed, continuously-fed systems, septic wastes, nutrient deficiency (principally nitrogen and/or phosphorus), and low pH (pH < 6.5).

The calculation of SVI is shown in Equation 1:

Equation 1: Sludge Volume Index (SVI) Calculation

Sludge Quality Considerations

One of the factors essential to the performance of the activated sludge process is effective flocculation of the sludge, with subsequent rapid settling and compaction. McKinney related flocculation to the food/microorganism ratio and showed that certain organism normally present in activated sludge deflocculate rapidly under starvation conditions. More recently, it has been shown that flocculation results from the production of a sticky polysaccharide slime layer to which organisms adhere. Flagellates are also entrapped in this slimy material. Filamentous organisms are present in most activated sludges (exceptions are found in the chemical and petrochemical industry).

Palm, Jenkins, and Parker have identified three generic types of activated sludge, as shown in Figure 1. Nonbulking sludge will result from plug flow or selector plant configuration or from complex organic wastewaters. Bulking sludges result from degradable wastewaters treated in a complete mix process or from oxygen or nutrient deficiencies. Pin floc usually results from low F/M (long sludge age) operation.

A number of filamentous organisms have been identified in activated sludge treating municipal and industrial wastewaters. Depending on process operating conditions, one or more of these organisms may dominate in the process. Proper process design and operation should not permit the filaments to overgrow the floc formers.

Filamentous growth is affected by:

• Wastewater composition. Wastewaters containing glucose-like saccharides (glucose, saccharin, lactose, maltose, etc.) promote filamentous growth while laundry, textile, and complex wastewaters inhibit filamentous growth in a completely mixed system. In general, the more readily degradable the substrate, the more prone the system is to filamentous bulking.
• Dissolved oxygen concentration. Oxygen must diffuse into the floc in order to be available to the organisms within the floc. The depth of oxygen penetration within the floc depends on the bulk concentration of oxygen in the surrounding liquid and on the oxygen-utilization rate of the floc. The oxygen-utilization rate is proportional to the organic loading (F/M). Hence, as the organic loading increases, the dissolved oxygen necessary to maintain a fully aerobic floc also increases. The thin filaments (1 to 4 µm) can readily obtain oxygen at concentrations less than 0.1 mg/L.

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Figure 1: Activated Sludge Characteristics

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.

Trace Nutrients

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

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