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.

"Denitrification"...

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.

Foaming - 1...

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 ;

• What allows Nocardia to grow in activated sludge ?
• What conditions cause foaming ?

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.

More Info About Foaming...

Foaming is the formation of a thick stable scum on aeration vessels and sometimes sedimentation tanks. There are other types of foam experienced due to non-biodegradeable detergents and foaming at plant startup, but these foams are not stable. Deleterious effects of foaming include extra maintenance, reduction in oxygen transfer, poor quality effluent, foam in anaerobic digesters and the spread of pathogens. Foam formation involves a flotation process and requires three components for foam formation: (1) air bubbles, (2) surfactants and (3) hydrophobic particles. Foam is stabilised by surfactants and hydrophobic particles. Gas bubbles are supplied by aerators, both surface and diffusers. Bubble sizes vary - smaller bubbles result in "better", or more stable foam. Surfactants are present in raw sewage and industrial discharge, but are also produced by organisms (foam formers and others). Surfactants can be cell associated or extracellular. Pilot plant studies have shown an increase in growth of foam-formers when surfactant was added. The hydrophobic particles are the microorganisms causing foam.

There is a need to identify foam-formers more precisely, ( 1 ) to assist with control, as different organisms have different growth rates which may determine the success of reduction of sludge age as a control measure, and ( 2 ) since some are pathogens. Although G.amarae, S.piniformis and "M.parvicella" are not pathogens, N.asteroides, N.farcinica and R.equi are potential pathogens.

The following microorganisms have been found in various surveys using microscopic methods to identify the organisms causing foam.

"Microthrix parvicella"	M.P. : Long coiled unbranched Gram positive filaments. A major cause of foaming in Europe, Australia and South Africa, but less common in published reports from USA. More prevalent in cooler climates.
"NALO" or "GALO"	Nocardia : Shorter branched filaments with branches at approximately right angles (so called "Nocardia" or Nocardia amarae-like organisms (NALO), but since N.amarae has recently been reclassified as Gordona amarae these are now sometimes called Gordona amarae like organisms or GALO). All are members of the mycolata (mycolic acid producing organisms) and may belong to genera such as Nocardia, Gordona, Rhodococcus, Tsukamurella, Dietzia and Mycobacterium.
"PTLO"	Nocardia pinensis and Skermania piniformis : The only "Nocardia" with morphology sufficiently different to allow identification in its own right. Originally called PTLO (Pine Tree-Like Organism) because of its tree-like branching morphology. Now classified as Skermania piniformis. Member of the mycolata.
"Actinomycetes"	Gram positive branched rods without the distinctive branching patterns of NALO and Skermania piniformis. This description includes coryneform bacteria. Probably members of the mycolata.
"Gram positive cocci"	Found in non-filamentous foams in Australia. May be members of the mycolata, including coccal stage of Rhodococcus and Nocardia farcinica.
"Eikelboom Type 0675"	Type 0675 was significant in recent French studies. Some studies don't differentiate between Types 0041 and 0675.
"Eikelboom Type 0092"	More common in nutrient removal plants. Its role as a foam-former is disputed.
"Nostocoida limicola"	Recent surveys suggest this is becoming more common as a foam former.
"Eikelboom Types 0803, 0413, 1851, 021N, 0914, 0581, 1863, 1701, Haliscomenobacter hydrossis, Sphaerotilus sp, Acinetobacter sp and Cyanobacteria"	Occasionally reported as the dominant organism in foam, but their incidence is low.

Taxonomy of mycolata (mycolic acid producing nocardioforms) has recently been clarified using 16SrRNA sequences and include Mycobacterium, Gordona, Tsukamurella, Skermania, Dietzia, Nocardia, Rhodococcus, Corynebacterium. These genera can be clearly delineated using chemotaxonomic characters such as mycolic acid composition and predominant menaquinones.

The following mycolata have been isolated in pure culture from foam (current taxonomic name) showing the diversity of mycolata in activated sludge: Nocardia asteroides, Nocardia farcinica, Nocardia otitidiscaviarum, Nocardia spp., Rhodococcus coprophilus, Rhodococcus equi, Rhodococcus erythropolis, Rhodococcus globerulus, Rhodococcus rhodochrous, Rhodococcus ruber, Rhodococcus rubra, Rhodococcus spp., Mycobacterium spp., Dietzia maris, Gordona amarae, Gordona spp., Tsukamurella paurometabola, "Tsukamurella spumae", Tsukamurella spp., Skermania piniformis. Most of these show right-angled branching at some stage in their life cycle and therefore fit the category of "Nocardia" or NALO foams. We expect that many more new species and genera of mycolata will be found in activated sludge.

Taxonomy of "M. parvicella" - member of high G+C group (actinomycetes) closest to Acidimicrobium ferroxidans. Organisms related to "M. parvicella" are found in diverse environments - peat bog in Germany, geothermally heated soil in NZ, soil in Australia, Japan and Finland, marine environments. "M. parvicella" still has Candidatus status, as it has not yet been fully characterised due to poor growth on conventional media.

More Info About "Microthrix Parvicella"...

Microthrix parvicella, a gram positive, unbranched filament (Fig 1.), can confidently be said to be the most troublesome filamentous bacterium in activated sludge. Surveys carried out in different countries have all shown that this organism is the one which dominates bulking sludges and foams, in plants with widely different process configurations and operating conditions.


Fig. 1. Microthrix parvicella, Gram stain x 1,000.

This organism was isolated by Slijkhuis in the 1980's who showed that it had peculiar metabolic requirements, preferring certain fatty acids to sugars and amino acids as nutrients. Repeated attempts by other groups to isolate M. parvicella using Slijkhuis' medium failed, indirect evidence that there may be significant physiological differences among strains of this organism. However the group at Bendigo were ultimately successful in growing this filament in pure culture following micromanipulation of sludges taken from Australian wastewater treatment plants (Fig. 2). In collaboration with Dr Linda Blackall (University of Queensland), the group showed by 16S rDNA sequence analysis that M. parvicella is in the Actinomycetes group, albeit a very unusual one with no close relatives.


Fig 2. Scanning electron micrograph of Microthrix parvicella in pure culture.

Dr. Valter Tandoi's group at CNR in Rome were also successful in culturing this organism, and again with Dr Blackall's assistance showed that the Italian isolate was almost identical to the strain grown by the Bendigo group by 16S rDNA sequence analysis. Gene probes designed using one of the Bendigo isolates (DAN 13) were tested against activated sludges from several countries and the filaments identified as "M. parvicella" present all showed a positive response to the Australian probe. This suggests that the "M. parvicella" morphotype is a single organism, but does not rule out the strong possibility that physiological variation may exist.

It grows very slowly in pure culture but seems to accumulate polyphosphate which may provide it with some ability to survive in activated sludge systems. It also seems to produce resting structures visible in scanning electron micrographs (Fig 3.) which may be a reaction to stress in pure culture. This suggests that the "M. parvicella" morphotype is a single organism, but does not rule out the strong possibility that physiological variation may exist.


Fig 3. Scanning electron micrograph of Microthrix parvicella in pure culture with resting structures visible.

Cell Surface Hydrophobicity ( CSH )...

Foam-formers are thought to have hydrophobic cell walls to help them in the "flotation" process.

Mixed Liquor Biomass Studies : Mixed Liquor biomass is more hydrophobic in foaming plants than in non-foaming plants. The onset of foaming incidents is often correlated with increase in cell surface hydrophobicity.

Pure Culture Studies : Foam isolates are hydrophobic. CSH showed variation with culture age, C:N ratio and temperature.

Role of Mycolic Acids : Mycolic acids (which are very hydrophobic)are a major component of the cell walls of mycolate. The mycolic acid composition of a foam isolate of Rhodococcus rhodochrous varies with culture age growth temperature carbon source, but the composition of mycolic acids had little influence on their CSH or foaming ability. Using cell-water contact angles as a measure of CSH, studies of Corynebacterium, Rhodococcus, Gordona and Mycobacterium revealed a tendency for CSH to increase with mycolic acid size However, an exception was Mycobacterium. Conclusion: that maybe other components "neutralise" hydrophobicity of the mycolic acids. Thus foam-formers are hydrophobic, but the relationship between CSH and mycolic acids is complex.

Hydrophobic Substrates : There are many suggestions that foaming occurs when waste contains oils and fats, although some evidence is anecdotal. Using pure cultures, vegetable oils, long chain fatty acid esters and paraffin were readily used by all mycolata tested, but with varying growth rates. There was a varied response to kerosene and hexadecane and a weak response to xylene. It is interesting to note that Skermania piniformis grows better on hydrophobic substrates like olive oil and Tween 80 than on glucose. Metabolism of long chain fatty acids by "Microthrix parvicella" is also thought to be important. The hydrophobic nature of foam-formers allows them to attach to hydrophobic substrates. This may give them a means of competing with faster growing organisms present in the aqueous phase of activated sludge.

Control of Foam : Four Main Approaches...

Washout of Foam-Forming Organisms by Manipulation of Sludge Age or MCRT...

Reduction of MCRT should washout organisms but is not always effective. This is probably because different organisms have different growth rates - which may also be affected by growth temperature (different mycolata have quite different temperature ranges). Therefore, slowly growing Microthrix parvicella or Skermania piniformis, both of which can be easily detected by microscopic examination, are likely to be washed out by reducing sludge age, but the right-angled branching NALOs have a wide range or growth rates, and this strategy may not work with some of them.

Selectors...

Anoxic selector favours growth of floc formers at the expense of "Nocardia", but is not useful for Microthrix foams because M.parvicella grows well anoxically.

Physical Methods...

Use of water sprays to break down foam, or encouragement of flotation so that foam formers rise to surface more rapidly (for removal). Some plants physically remove scum as it accumulates.

Recycling trapped foam simply reseeds the mixed liquor with the foaming organisms, and hence it recycles the problem. Disposal of foams into an anaerobic digester may cause foaming problems there.

Use of Additives to Help Reduce Foam...

Chlorine : Kills filaments protruding from flocs but doesn't kill flocs unless too much chlorine applied. Useful, but must be performed with caution.
Antifoam : Doesn't always work and is expensive.

Commercial mixtures of microbes, sometimes supplemented with enzymes are expensive and often do not work, or need regular addition or supplementation for continued beneficial effect.

Foaming - 2...

The Role of Bacterial Cell Surface Hydrophobicity in Biological Foams...

Biological foams have been a problem in activated sludge plants world wide for at least three decades. Foam formation results in increased cost due to extra maintenance required for cleaning clarifiers, and solids carry over from clarifier can significantly add to increased suspended solids and total phosphorus levels in the final effluent. The increased risk of the spread of pathogens is also of major concern. Attempts to consistently and reliably control these foams continue to elude plant designers, operators and researchers. Considerable capital costs has been expended on facilities such as aerobic/anoxic selectors which have not proven successful in eliminating foams. Recent efforts by several groups have led to a better understanding of the bacteria responsible for foam formation, their growth characteristics (substrates utilised and growth rates) and mechanisms for foam formation. Factors such is the types of substrates available and the physicochemical properties of the mixed liquor have been shown to be key factors affecting foam formation.

Proposed Mechanisms for Foam Formation...

Several mechanisms for foam formation in the activated sludge process have been proposed but none have been proven. The theory of selective flotation has been alluded to by several groups and is proposed as the mechanism responsible for the flotation of microorganisms in activated sludge resulting in foam formation described below.

The Theory of Froth Flotation...

Foam formation is initiated when gas bubbles form within a liquid. The presence of a surfactant whether added or produced by the bacteria (biosurfactant) reduces the surface tension of the liquid walls and allows the bubbles to remain elastic. Stabilisation of the foam is possible if liquid drainage from the bubble lamellae is reduced, and this is achieved by the presence of solid particles within the foam which bridge each bubble and minimise the distance between them, entrapping the liquid that constitutes the bubble wall.

Particles selectively removed by flotation must fulfil certain criteria. They must be less than 300 mm and have a hydrophobic surface. The ability of a solid particle to float in the froth flotation process can be controlled by manipulating the surface hydrophobicity of the solid particle or by adding surface active agents (frothers) and work is continuing in this area with relation to activated sludge foaming. By understanding the nature of the hydrophobic effect occurring between the foam foaming bacteria and the surrounding environment sensible control strategies will be developed. The relative contribution or hydrophobic and other interactions between the cells and the substrate or bubbles depends not only on the cell surface properties but also on the substratum characteristics which can vary diurnally within the activated sludge process. When these interactions are fully understand we will most certainly control foaming in activated sludge processes.

Surfactants...

The presence of surfactants in activated sludge is imminent as they are present in the influent and may be produced by bacteria within the sludge.

Gas Bubbles...

Gas bubbles are an inherent component within the aeration tanks and cannot be removed. The gas bubbles are formed by the aeration system whether mechanical surface aerators or a submerged diffused air system.

Hydrophobic Particles...

Most foam isolated from activated sludge has a hydrophobic cell surface and can produce stable foams when grown in pure culture. This hydrophobicity has been shown to vary under different culture conditions and with culture age. Foam isolates belonging to the Mycolata group of bacteria (Nocardia, Rhododoccus, Tsukamurella and Mycobacteria)have hydrophobic surfaces due to the relatively high component of mycolic acids (long chain hydrocarbons) on their cell surface. The mycolic acids vary in carbon chain length and degree of unsaturation which is thought to reflect the changes in cell surface hydrophobicity. The hydrophobic cell surface of foam forming bacteria provides them with a competitive advantage in the presence of hydrophobic and/or coIloidal substrates allowing them to proliferate in the presence of such substrates. Their cell surface chemistry also allows them to participate in the flotation process and form a stable foam.

History and Development of Foaming Control...

The ability of some microorganisms to float and create foams is a well-known fact in general microbiology. The operational troubles resulting from this ability in activated sludge plants were described for the first time at the wastewater treatment plant Jones Island East Plant in Milwaukee in February 1969. The phenomenon was called "Milwaukee mystery". The phenomenon of foaming has spread very quickly throughout the world. The biological foaming is mostly reported in nutrient removal plants with different cultivation conditions (i.e., anaerobic, anoxic, oxic) but the conventional oxic activated sludge systems are not resistant to foaming, either.





Surveys of filamentous microorganisms in biological foams done in South Africa, USA, Australia and Europe showed that the problem of foaming is connected only with certain types of filamentous microorganisms. At present the following filaments are considered to be the main foam-formers :

Microthrix parvicella : Probably the most widespread filamentous microorganisms in activated sludge plants in the world.



Nocardia amarae : Like organisms (now called Gordona amarae like organisms) and other actinomycetes like Skermania piniformis.

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 ;

• Young sludge age (< 3 days)
• Toxicity (heavy metals etc.)
• Slug discharge
• Lack of active and abundant ciliated protozoan population
• Excessive shearing
• Excessive surfactant

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...

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 ;

• Loss of the higher life forms in the activated sludge (these are the most toxic sensitive microbial components).
• A dispersed activated sludge biomass with poor floc formation and pin floc.
• Unusually low oxygen use, caused by poor biomass growth.
• Poor BOD removal.

What can you expect to see under the microscope if toxic conditions exist?

• There will be a sudden increase in flagellates. This is sometimes called a flagellate "bloom".
• The of protozoa and higher life forms will begin to die off.
• Break-up of floc, sometimes accompanied by foaming.
• Loss of BOD removal.
• Filamentous bulking upon process recovery. Filamentous bacteria are very often the first to recover after a toxic upset.

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.

Odor...

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.