Microbiology of Activated Sludge...

Activated sludge can be defined as "a mixture of microorganisms which contact and digest bio-degradable materials (food) from wastewater."

Activated sludge is microorganisms. The Activated sludge process is a biological process. To properly control the activated sludge process, you must properly control the growth of microorganism.

This involves controlling the items which may affect those microorganisms.

Bacteria... Make up about 95% of the activated sludge biomass. These single celled organisms grow in the wastewater by consuming (eating) bio-degradable materials

such as proteins, carbohydrates, fats and many other compounds.

Protozoa & Rotifers...

The presence of particular types of protozoans is related to effluent quality and plant performance. Protozoan play secondary but important role in purification of aerobic wastewater. The protozoans in the activated sludge treatment process fall into four major classes: amoebae, flagellates, and ciliates (free-swimming, crawling, and stalked).

Amoebae : Amoebae are the most primitive, single-celled protozoans. They move by false feet. They are frequently present in raw influent, and their presence is short in the aeration basin. Amoebae can only multiply when there is an abundance of nutrients in the aeration tank. They move very slowly and it is difficult for them to compete for food the there is a limited amount available. They are only dominant in the aeration basin for a short time. They feed on small organic particulates. When amoeba are present in large numbers in the aeration basin this usually indicates that there has been some sort of shock loading to the plant (there must be a lot of food available). Their presence may also indicate that there is a low D.O. environment in the aeration basin, because they can tolerate very low amounts of D.O.

Flagellates : Most flagellates absorb dissolved nutrients. Soon after amoebae begins to disappear and while there is still high concentrations of soluble food. Flagellates and bacteria both feed on organic nutrients in the sewage so as the nutrient level declines they have difficulty out competing the bacteria for soluble food so, their numbers begin to decrease. If large amounts of flagellates are present in the later stages of the activated sludge development this usually indicates that the wastewater still contains a large amount of soluble organic nutrients.

Ciliates : Ciliates feed on bacteria not on dissolved organics. While bacteria and flagellates compete for dissolved nutrients, ciliates compete with other ciliates and rotifers for bacteria. The presence of ciliates indicate a good sludge, because they dominate after the floc has been formed and after most of the organic nutrients have been removed.

Free-swimming ciliates - These ciliates appear as flagellates begin to disappear. As the bacterial population increases, a lot of dispersed bacteria is available for feeding and as a lightly dispersed floc appears, free-swimming ciliates begin to dominate and feed on the increased numbers of bacteria.

Crawling ciliates - As floc particles enlarge and stabilize, crawling ciliates graze on floc particles. Crawling ciliates out compete free-swimming ciliates for food because they can find food within the floc.

Stalked ciliates - Stalked ciliates appear in the mature sludge. Within the mature sludge the crawling and stalked ciliates compete for dominance.

Factors Influencing Protozoa...

Temperature : Most protozoans can survive and reproduce in a temperature range at which activated sludge processes are carried out. They grow best in ambient temperatures (15 - 25 oC).

pH : Protozoans are more sensitive to pH than floc-forming bacteria. They have an optimum pH range of 7.2-7.4 and a tolerance range of 6.0 - 8.0.

Dissolved Oxygen : Like bacteria, protozoan must have oxygen to survive. Thus lack of DO will severely limit both the kind and number of protozoans.

Nutrition : Most municipal wastewater treatment plants, however dilute, contains sufficient nutrients to support most of the protozoan associated with wastewater.

Rotifers...

Rotifers are rarely found in large numbers in wastewater treatment processes. The principal role of rotifers is the removal of bacteria and the development of floc. Rotifers contribute to the removal of effluent turbidity by removing non-flocculated bacteria. Mucous secreted by rotifers at either the mouth opening or the foot aids in floc formation. Rotifers require a longer time to become established in the treatment process. Rotifers indicate increasing stabilization of organic wastes.

Indicator Microorganisms... The indicator microorganisms that feed on bacteria can be observed by using inexpensive microscopes at lower powers of magnification. Some forms may need to be stained for viewing. Process status can then be interpreted from these observations. For operational purposes, the relative types of microorganisms present in an activated sludge sample, the predominance of each of the various types and the mobility of the microorganisms are the primary concerns. The microorganisms that appear most frequently and the activity of these microorganisms enables the operator to interpret occurrences in the process. The more common microorganisms encountered and their relative numbers are shown below.

"Relative Predominance of Microorganisms"...

One important operational philosophy that merits a constant reminder is that ; (1) any specific microorganism predominates in a given environment because it thrives in the existing conditions, (2) normally the operator has created the conditions favorable to the microorganism and (3) to decrease the microorganism predominance, the operator need only change its environment (that is control the process) to provide less favorable conditions. If the environment is changed drastically, a different microorganism will quickly predominate (unless all microorganisms are destroyed). A normal transition results in the gradual disappearance of the former, during routine observation, with the latter gradually taking over.

Introduction to Filament Identification...

Filament Identification... In order to identify many of the following filament characteristics, the mixed liquor must be examined under 100X using immersion oil. It is difficult to see many of these characteristics under lower magnifications.

Filament Shape and Length...

Filaments may be long, short, smoothly curved, coiled, irregularly bent, straight, or bundled.

Individual Cell Shape...

Filamentous bacteria are made up of a chain of cells. The shape of the individual cells is a characteristic that can help us to identify the different filamentous bacterial types. Cell shape may be round, square, rectangular, oval, or discoid.

Cell Septa...

The cell septa is the "line" which separates each individual cell which makes up the bacterial filament. The septa are clearly seen in some filaments an is very difficult to see in others. Some septa are "indented" and some are not. Indentations and the ability to clearly see the cell septa are other characteristics which can help us to identify the different filamentous bacteria.

Motility...

Motility is the ability of an organism to produce motion or to move. Beggiatoa spp is only one filamentous bacterium found in activated sludge that is motile.

Intercellular Granules...

Some filaments store by-products as intercellular granules (mostly sulfur granules). Sulfur granules can be seen very clearly under phase contrast and are found usually in septic wastes. Sulfur granules are commonly found in Beggiatoa, Thiothrix and Type 021N.

Branching...

Branching may be "true" or "false". If a filament has true branching the intercellular fluids will flow freely throughout all the branches of the filament. Intercellular fluids cannot flow through false branches. In false branching the filament are simply attached to each other simulating a branch. There are only two filaments which exhibit branching; one has true branching and the other false. Nocardia spp has true branching and Sphaerotilus natans exhibits false branching.

Sheath...

The cells of some filamentous organisms are contained in a tight fitting sheath. The easiest way to detect a sheath is to look for "missing spaces" between the cells. Some filaments which have a sheath are Haliscomenobactor hydrosis, Sphaerotilus natans, Type 1701, Type 0041, and Type 0675.

Attached Growth...

Some filaments have bacterial cells attached along the side, perpendicular to the filament. There are three filaments on which this commonly occurs. Type 0041, Type 0675, and Type 1701.

Troubleshooting Tests...

The operator must determine the probable cause and select one or more of the corrective measures to restore the process to full efficiency with the least adverse effect on the final effluent quality and at the lowest cost. To do this, the operator needs a thorough knowledge of the plant's activated sludge process and how it fits into the overall plant operation.

Mixed Liquor Settleability Test :

This section refers frequently to the mixed liquor settleability test and a settleometer, a specially marked wide - mouth beaker, for observing how sludge settles. Some operators use a 1 - L graduated cylinder instead of the settleometer for this test, but the sidewall effects of the narrow cylinder interfere with the settling. This test, which shows the mixed liquor settling characteristics under controlled conditions, is one of the most useful tools for understanding what is happening to the process. When performing the test, observe the floc in the settleometer to determine if it is granular with well - defined edges and interspaces.

Troubleshooting by Settling Test...

"Settleability Test for Bulking Sludge"...

Perform microscopic examination... 1. Very few or no filaments, dispersed floc, supernatant turbid or cloudy

o Check for toxics and low specific oxygen uptake rate Enforce sewer use ordinance

o Check for high F:M bulking and MLVSS, SRT, F:M and influent BOD changes Switch to contact stabilization, if possible Decrease wasting, put another aeration tank on-line if necessary Decrease return rate Use settling aid, if possible

2. Very few or no filaments, dispersed floc, supernatant clear but clarifier effluent cloudy o Check for high F:M bulking and MLVSS, SRT, F:M and influent BOD changes

Switch to contact stabilization, if possible Decrease wasting, put another aeration tank on-line if necessary Decrease return rate Use settling aid, if possible

3. Very few or no filaments, dispersed floc, supernatant clear o Check for high MLSS, low F:M and old, thick sludge causing hindered settling (not bulking)

Conduct dilution test, if dilution improves settleability, increase wasting to raise F:M

4. Moderate to large number of filaments, (try to ID filaments involved and determine if they are bacteria or fungi), supernatant very clear

o Chlorinate RAS at 1 - 10 kg/1,000 kg MLVSS.day, starting low, check frequency of exposure Monitor settleability, monitor effluent turbidity, and observe filaments with microscope,

correct underlying cause of bulkingo Check available N, P, and Fe for nutrient deficiency

If BOD:N > 100:3, try adding N If BOD:P > 100:1, try adding P If BOD:Fe > 100:0.5, try adding Fe

o Check for too low DO for F:M or specific oxygen uptake rate If DO too low, increase aeration If DO uneven; (a) Clean diffusers (b) Increase aerator speed or raise weir (c) Change to

step feed, complete-mix or tapered aeration, if possible Decrease F:M, if possible Add a settling aid Use upstream attached growth process, if possible

o Check pH for wide fluctuations or low value Chlorinate RAS (see above) If influent pH < 6.5, identify source and correct or try raising pH If widely varying, control at source If process is nitrifying unnecessarily, increase wasting, if nitrification is required, raise and

maintain constant pHo Check influent wastewater and in-plant side-streams for filaments

Chlorinate influent at 5 - 10 mg/L Pre-aerate upstream, if possible Optimize performance of or upgrade in-plant processes

o Check for insufficient soluble BOD gradient causing low F:M bulking Consider using a selector or switching to plug-flow Increase F:M Decrease aeration tankage on-line

o Check for high F:M bulking Switch to contact stabilization, if possible Decrease wasting, put another aeration tank on-line, if necessary

o Check for septic wastewater with sulphides Add an oxidizing agent

Pre-aerate upstream, if possible