Rising Sludge (Denitrification)...

Occasionally sludge that has good settling characteristics will be observed to rise or float to the surface after a relatively short settling period. The cause of this phenomenon is denitrification, in which the nitrites and nitrates in the wastewater are converted to nitrogen gas. As nitrogen gas is formed in the sludge layer, much of it is trapped in the sludge mass. If enough gas is formed, the sludge mass becomes buoyant and rises or floats to the surface. Rising sludge can be differentiated from bulking sludge by noting the presence of small gas bubbles attached to the floating solids.


Rising sludge problems may be overcome by ; (1) increasing the return activated sludge withdrawal rate from the clarifier to reduce the detention time of the sludge in the clarifier, (2) decreasing the rate of flow of aeration tank mixed liquor into the offending clarifier if the sludge depth cannot be reduced by increasing the return activated sludge withdrawal rate, (3) where possible, increasing the speed of the sludge-collecting mechanism in the settling tanks and (4) decreasing the mean cell - residence time by increasing the sludge wasting rate.


Three filamentous organisms can cause activated sludge foaming ; (1) Nocardia sp. (most common), (2) Microthrix (less common), and (3) Type 1863 (rare). These filamentous organisms can produce a stable, viscous, brown foam on the aeration tank surface that can carry over onto the clarifier surface and may even escape in the effluent, violating permit limits. The foaming can range from being a nuisance to a serious problem. In cold weather, this foam may freeze solid and have to be manually removed with pick and shovel. In warm weather it often becomes odorous.

The analysis of Nocardia foaming involves two questions ; Nocardia growth is associated with warmer temperatures, grease, oil, and fat present in treated wastes, and longer SRTs (generally greater than 9 days), although it has been frequently encountered at SRTs of 2 days. Foam, however, usually has a longer SRT than the underlying MLSS, and sludge age calculations may be incorrect. Plants prone to Nocardia foaming often ; (1) receive oil and grease wastes (for example, restaurants without grease traps), (2) have poor or no primary scum removal and (3) recycle scum rather than remove it from the plant.

The best way to deal with Nocardia foaming is to prevent the conditions from developing that encourage Nocardia growth. Once established, Nocardia foaming can be extremely difficult to eliminate because ; (1) the foam is difficult to knock down with water sprays, (2) the foam generally does not respond to chemical antifoamants, (3) chlorinating return sludge, although often helpful, does not eliminate Nocardia since most of it is in the floc particle and not exposed to chlorine and (4) increased wasting has its limitations since ; (4-1) foam is not wasted with the sludge, (4-2) even if foam and scum are removed from the process, they can cause problems in downstream units like digesters and also can be recycled back with decant or supernatant to the activated sludge process and (4-3) reducing the SRT to less than 9 days may be inadequate.

Filamentous organism Factors promoting rapid growth
Haliscomenobacter hydrosis, Sphaerotilus natans, type 1701 Low DO
Haliscomenobactor hydrosis, Microthrix parvicella, Nocardia spp., type 021N, type 0041, type 0092, type 0581, type 0675, type 0803 and type 0961 Low F / M
Sphaerotilus natans, Thiothrix spp. fungi, type 0675 and type 021N Low Nutrients (nitrogen or phosphorus)
Nocardia spp fungi Low pH
Type 0041, type 0092, and Microthrix parvicella Low organic load
Beggiatoa spp, Thiothrix spp and type 021N Septic wastewater / Sulfides

Foam producing filaments Factors influencing their growth
Microthrix parvicella Low F : M and high wastewater grease and fat, colder temperatures
Nocardia spp Longer MCRT, excess grease, oils and fats and warmer temperatures
Type 1863 Low DO, excess grease and fat, and low pH

To determine if aeration tank or clarifier foam is due to the growth of foam - producing filamentous organisms, a sample of fresh foam should be spread thinly and evenly over a clean microscope slide, and the slide stained by Gram staining. Microthrix parvicella and Nocardia spp each stain Gram positive (staining purple or dark blue). Microthrix is a long thin filament while Nocardia is a short branched filament. So, if you have foam and slide of mixed liquor stains Gram positive, you can easily determine which filament is responsible. Type 1863 stains Gram negative (staining pink). Type 1863 is a long filament which looks like a dashed line. If the foam is not due to foam-producing filamentous organisms, it may be due to the presence of a nutrient deficiency. To determine if a nutrient deficiency is the cause of foam production, a representative sample of mixed liquor should be treated with India ink and examined under phase contrast microscopy for the presence of nutrient deficient floc particles.

Interruption of Floc Formation...

When floc particles first develop in the activated sludge process, that is, at a relatively young sludge age, the particles are small and spherical. Because filamentous organism do not develop or elongate at relatively young sludge ages, the floc - forming bacteria can only "stick" or flocculate to each other in order to withstand shearing action. Bacterial flocculation and the absence of filamentous organisms result in spherical floc particles. As the sludge age increases and the short filamentous organism within the floc particles began to elongate, the floc forming bacteria now flocculate along the lengths of the filamentous organisms.

These organism provide increased resistance to shearing action and permit a significant increase in the number of floc - forming bacteria in the floc particles. The presence of long filamentous organisms results in a change in the size and shape of floc particles. The floc particles increase in size to medium and large and change from spherical to irregular. Factors interrupting floc formation can be listed as follows ;

Dispersed Growth...

Dispersed growth is a population of bacteria that is suspended in the liquid portion of the mixed liquor. These bacteria are still growing rapidly and have not begin to flocculate. Most dispersed growth is bacterial. Only a little dispersed growth should be present in a properly operating activated sludge process. Ciliated protozoa play an important role in the removal of dispersed growth. Dispersed growth is also removed from the bulk medium by its adsorption to the surface of floc particles. A significant amount of dispersed growth is present at the start-up of an activated sludge process. A lot of food is available, and the bacteria are very active and are multiplying rapidly. The presence of significant or excessive dispersed growth within the mixed liquor can also be due to the interruption of proper floc formation.

Slime Bulking...

Often in industrial and municipal activated sludge processes a nutrient deficiency may occur. The nutrients that are usually deficient in these processes are either nitrogen or phosphorus. This deficiency results in the production of nutrient deficient floc particles, loss of settleability, and, possibly a billowy white or greasy gray foam on the surface of the aeration tank. During a nutrient deficiency, the bacteria within the floc particles remove soluble BOD from the wastewater. However, when nitrogen or phosphorus is deficient, the soluble BOD is not degraded but it is stored within the floc particles as an exocellular polymer-like material. This slimy material interferes with settling and may cause foam upon aeration.

Operational Considerations...

The solution usually involves addition of the limiting nutrient, such as ammonia to provide nitrogen, or phosphoric acid to provide phosphorus. There is usually enough nutrient if the ammonia plus nitrate in filtered (0.45 um) effluent is greater than 1 mg / L and the soluble orthophosphate is greater than 0.5 mg / L. However, in cases where easily degradable, soluble BOD is available, higher N and P concentrations may be necessary.


Toxicity assessment is one of the most valuable applications of microscopic observation of microorganisms in activate sludge. The higher life forms, particularly the ciliates and the rotifers, are generally the first to be impacted by toxic materials and may serve in essence as an in-plant biomonitoring test for toxicants or other adverse stresses. The first noticeable sign of toxicity or stress is usually the slowing or stopping of cilia movement for these organisms and small flagellates and ciliates begin to predominate. This is an indication of the break up of the floc and an over abundance of free bacteria used by these organism as a food source.

Indications of toxicity upset include ; What can you expect to see under the microscope if toxic conditions exist? Toxic wastes generally do not favor filaments directly (except in the case of H2S), rather upset conditions allow filaments to grow. Microscopic examination of activated sludge can diagnose toxicity, however, this is usually "after the fact". A better method of toxicity detection is the use of oxygen uptake rate testing to detect toxicity early and to find the source.


There are many odors involved with the collection and treatment of domestic wastewater. Some of these odors are tolerable, while others are not. Combining industrial and commercial wastewater may influence the strength and types of odors many of us encounter. Probably, the most common odor involved with wastewater treatment is Hydrogen Sulfide. It is that distinctive, "rotten egg," type odor that is both toxic and corrosive, and argued by some, the most offensive. It is slightly heavier than air, and can cause headaches, nausea, and eye irritation in very small concentrations. Hydrogen Sulfide has been identified as the cause of death for a number of collection system and treatment plant personnel. It is very toxic at low level concentrations such as 300 ppm by volume in air. Such concentrations can be produced in confined spaces with turbulence from wastewater containing as little as 2 mg/L of dissolved sulfide.

Sulfide can be produced by the biochemical reduction of inorganic and organic sulfur compounds. Sulfate and sulfur-containing matter come from some industrial wastes, human domestic waste, and sometimes from groundwater infiltration. The odor and corrosion problems are the result of the reduction of sulfate to hydrogen sulfide under anaerobic conditions and the subsequent release of this compound to the atmosphere. Sulfate-reducing bacteria are the anaerobic bacteria that utilize sulfate as an oxygen source. When the dissolved oxygen concentration is below 1.0 mg/L sulfate reducing bacteria thrive and dominate the bacterial population. At dissolved oxygen levels above 1.0 mg/L sulfate reduction is usually prevented, due to the ability of the aerobic bacteria, the "GOOD GUYS," to thrive and dominate the population. Hydrogen sulfide is only moderately soluble in water and the solubility of hydrogen sulfide in the water decreases with increasing water temperatures. This means that as the temperature increases, any dissolved hydrogen sulfide in the wastewater tends to be released more readily to the atmosphere as hydrogen sulfide gas. Hence, summertime and its warm temperatures is often accompanied by phone calls with odor complaints, more so than any other time of the year. Corrosion and deterioration of collection system infrastructure, concrete pipe/tanks, and metal equipment such as grit collectors, bar screens, gratings and walkways is usually caused by hydrogen sulfide gas. In the presence of moisture and oxygen, certain bacteria can convert the hydrogen sulfide gas into sulfuric acid, thereby causing the corrosion.

The collection system not only transports the wastewater to the treatment plant, it also provides an environment for the wastewater treatment process to begin. In the collection system, organic matter begins to breakdown, aerobic and anaerobic bacteria are present and forming the zoogleal slime layer, much the same as they would in a fixed-film treatment process. Sloughing of the slime layer occurs, the same as it would from a Trickling Filter, or Rotating Biological Contractor. These bacterial flocs demand oxygen, thereby depleting the dissolved oxygen content throughout the length of the collection system. It is therefore imperative in the design of collection systems that dissolved oxygen levels be maintained such that oxygen demands are met. Locations in the collection system where the slower velocity of the wastewater allows solids to settle out and build up in the pipe invert, or in manholes, produce conditions that promote anaerobic bacteria, (sulfate-reducing bacteria) and subsequent hydrogen sulfide production. Force mains that have long detention times are prime environments for hydrogen sulfide production, as well as lift stations during low flow periods that require infrequent pumping.

Hydrogen sulfide odor has the potential to be generated at most unit processes in the wastewater treatment plant. If hydrogen sulfide is dissolved in the wastewater due to conditions upstream in the collection system, it is usually released as hydrogen sulfide gas when it enters the headworks of the treatment plant. This is due to the turbulence that may be created by bar screens, flumes, or aerated grit chambers. Wet wells, or flow equalization tanks that are not aerated and have long detention times are also the potential source of hydrogen sulfide odors. The buildup and accumulation of rags, plastics, and other organic material on bar screens, and comminutors, can cause other odors in addition to hydrogen sulfide. A common practice at smaller treatment plants is to allow grit to accumulate, and essentially be stored, in the grit channels for long periods of time prior to disposal. This practice results in strong odor production during storage, decreased grit channel efficiency, and even greater release of hydrogen sulfide gas during cleaning. Odors generated at the primary clarifiers are usually caused by the following two reasons: (a) scum accumulation, and (b) infrequent, or incomplete sludge withdrawal. Trickling filters and rotating biological contractors can also be the source of odor production, if sufficient oxygen is available to generate aerobic biological activity. Aeration basins are not usually the source of severe odor problems, however if there are dead spots, where solids are allowed to accumulate, or if aeration is discontinued for an extended period of time, septic conditions may be experienced. Final clarifiers, like primary clarifiers, may have sludge withdrawn frequently enough to prevent septic conditions and rising sludge from escaping over the weirs. Sludge handling, storage, treatment, and disposal are probably the most significant sources of odors at a majority of treatment facilities. Sludge thickeners may produce offensive odors due to exposure of raw sludge to the atmosphere. If sludge is not adequately stabilized prior to dewatering and disposal, it can also create quite intense and persistent odors.