Water supply and sanitation.

The purpose of this study is to understood how activated sludge works and how it is handled. (And gain some studying points.) The main sources in this have been What is activated sludge, Activated sludge microbiology problems and their control and The Microbiology of Activated Sludge.

Activated sludge is mass of microorganisms, which digest bio-degradable material from wastewater. About 95% of the activated sludge biomass is bacteria. Sludge process is a biological process and the process control is controlling the growth of microorganisms. This involves controlling the items, which can affect those microorganisms.

Almost all biochemical reactions need some kind of enzymes. Microbes produce them and use them to break down nutrients or rebuild nutrients into the new compounds. Enzymes do only certain things and they work right only if environmental conditions are right. And that means that the microorganisms don’t survive.

If there is very much nutrition available, bacteria use most of it to growth and reproduction. If there is only little nutrition, bacteria try to save energy. "Resting" bacteria don’t move and there is much less reproduction and growth. If there is not much nutrition, the waste products build a thick slime outside the cell wall and that makes cells stick together.

The growth characteristics of bacteria are better understood by studying the growth curve.
1: Lag-phase: Bacteria arrives their new surroundings. They digest food and develop enzymes and other things required for growth.
2: Accelerated Growth-phase: There is no lack of food, so bacteria grow as fast as they can. Cells are not stick together.
3: Declining Growth-phase: Lots of food has been eaten, and there is large mass of bacteria. There is lots of bacteria competing. Bacteria do not have enough remaining food to keep the growth rate at a highest speed.
4: Stationary-phase: Now here is as many bacteria that it is possible, but there is not much food is left. Reproduction happens, but cells are also dying. The amount of bacteria remain constant. The bacteria have now lost their flagella, and they are covered by sticky substance. They are sticking together and so generating floc. If floc get big enough, and if aeration and mixing are stopped, the floc could settle to the bottom.
5: Death-phase: Cells die fast and there is only little if any growth. Total number of living bacteria reduce. The remaining bacteria are just trying to keep alive.

F/M (Food to Microorganism ratio) is extremely impordant indicator in diagnosing activated sludges states. It is counted from the amount of matter, which is eatable to microbes, is measured by counting aeration basins BOD (biochemical oxygen demand) and / or COD (chemical oxygen demand). Estimating how much there is microorganisms in the mixed liquor is done measuring the amount of VSS (volatile suspended solids) in the activated sludge.
These tell, which is F/M ratio (food / microorganism). The F/M ratio tells about the cells growth and conditions. Mostly it tell how much food there is for the bacteria, protozoa and other micro organisms. If the F/M ratio is high, the micro organisms grow rapidly. If the F/M ratio is low, the bacteria grow slowly.

Many microorganisms need oxygen to live. Growing organisms metabolize the food, so they use oxygen fast. The rate, which the oxygen is used is a OUR (Oxygen Uptake Rate). Normally high OUR is in correlation with high F/M ratios and younger sludges. Lower OUR is typical with lower F/M and older sludges. Usually bacteria develop small clumps and other small formations. If they are in food rich enviroment, they are very active and motile. That means that if mixing occurs, the small clumps are broken up and the bacteria are dispersed, and they will not flocculate or settle. As the sludge is allowed to age, the bacteria move less and create more slime. And clumps can stick together and grow bigger. Eventually floc get large enough and compact and begin to settle. The mixing in the aeration tank break the holdings and make floc small. This is good because it allows the cells, food, and oxygen to contact each other. Oxygen is required to metabolize food for cell maintenance and growth. Although they need oxygen, some can get along with less oxygen than others. Each bug must have enough DO (dissolved oxygen) to function properly. If the DO is less than 2 mg/L, the bugs on the outside of the floc use the DO before it can get to the center of the floc. Then the floc’s core can die ant floc break. Mixing is important, because it mix organisms, oxygen, and nutrients together and remove bacteria waste products. Too strong mixing breaks floc. Many enzymes are working right only in certain pH. pH should be between 7.0 and 7.5 for the right activated sludge microorganisms to dominate. Lower temperatures slow down many reactions. Thus, more bugs are required to do the same job during the winter than in the summer.

Microorganisms need certain nutrients for growth. The basic nutrients in normal raw sewage are carbon (C), nitrogen (N), phosphorus (P). In addition to C, N and P, smaller amounts of sodium (Na), Potassium (K), magnesium (Mg), iron (Fe) and others are needed. If there is not enough these, cells activity slows down because it cannot produce enough enzymes and because needed nutrients cannot penetrate the slime layer normally. In this kind of situation the sludge will not settle and BOD removal slows down. 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. Amoebae are very primitive, single-celled protozoans. Amoebae can only multiply when there is an much in the aeration tank. Amoebae are slow, and they often lose competition with faster organisms if there is limited amount of food available. They are dominant in the aeration basin for a short time. Amoebae tolerate very low amounts of DO. Most flagellates absorb dissolved nutrients. If there are lots of flagellates present, it usually indicates that the wastewater contains a large amount of soluble organic nutrients. Ciliates feed on bacteria and most importantly not on dissolved organics. Large amount of ciliates tells that it is a good sludge. They dominate after the floc has been formed and after most of the organic nutrients have been removed. Most protozoans live and multiply in normal activated sludge temperatures. They grow best in ambient temperatures (15-25 C). Protozoans are sensitive to pH. They have an optimum pH range of 7.2-7.4 and a tolerance range of 6.0-8.0. Like bacteria, protozoan must have oxygen to survive. Lack of DO will severely limit both the kind and number of protozoans. Most municipal wastewater treatment plants, even dilute ones, contains enough nutrients to support most of the protozoan associated with wastewater. Rotifers are rare in wastewater treatment processes. Rotifers role is the removal of bacteria and the development of floc. Rotifers remove non-flocculated bacteria. They also secrete mucous, which aids in floc formation. Rotifers need a longer time to become established in the treatment process and they indicate stabilization of organic wastes.

Filamentous bacteria generate 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. Filaments may be long, short, smoothly curved, coiled, irregularly bent, straight, or bundled. The cell septa is the "line" which separates each individual cell which makes up the bacterial filament. In some filaments, the septa are easy to see and in others it is hard to see. Also some cells are indentations ant others aren’t. These differences help to identify the different filamentous bacteria. Other differences are organisms ability to move, Some filaments store by-products as intercellular granules. Filaments can also generate branches. If a filament has true branching the intercellular fluids flow freely. Intercellular fluids don’t flow through false branches. In false branching filament are just attached to each other simulating a branch. Some filamentous organisms have a tight fitting sheath. 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.

Handling.

Floc forming species may grow more fast than nonflocformin if the growth rate is too fast. That condition is rare in domestic waste but is common in industrial waste treatment. Dispersed growth problem happen after toxic or hydraulic washout; Then there is only a bit of biomass and lots of nutrition for them. Free floating filaments can also cause dispersed growth. Starwation conditions generate weak flocs. Pin flocs consist only bacteria and they are small. Constat toxicity can also cause that condition. Poisonous matter can be severe problem in activated sludge and actually it is quite common. Toxic shocks are caused by many different ways, like dumping diesel or organic acids or sulfides. Toxic wastes generally do not harm filaments directly. It rather upset the conditions, which allow filaments to multiply.

Nitrification can create problems in activated sludge operation. Nitrification cause dispersed growth and filamentous bulking. Nitrite has a significant chlorine demand. Extensive nitrification lowers pH. That is large problem in some plants if wastevater’s alkalinity is low. Many plants cause dispersed growth and filamentous bulking every spring when warmer temperatures induce nitrification. Nitrification can be controlled by lowing dissolved oxygen concentration, but that can cause filamentous bulking. Denitrification happens when bacteria don’t have enough oxygen. Then they respire using nitrate and that generate nitrogen. Nitrogen gas did not dissolve in water well, and it create bubbles, which cause sludge blanket flotation in clarifier. Denitrification problems happens usually during the warmer times. Denitrification control can be done two ways:
1: Sludge is reduced or aeration is reduced.
2: Removing nitrification removing the sludge faster from final clarifier or increasing dissolved oxygen concentration in the final clarifier.

Nitrification and denitrification are common problems in industrial waste, where is ammonia. Many industries have experienced floating sludge in aeration basin, which leads to situation where is lot of sludge inventory in foam.

Nitrogen and phosphorus can limit micro organisms growth if its amount in wastewaters are too low. That is usual problem with industrial waste and rare in domestic wastes. Other nutrients might be limiting too, but those are rare. Extracellular polysaccharides are responsible for floc formation and these are produced by activated sludge bacteria. If it is overproduced it can generate nutrient deficiency, and that builds up sludge which is poorly degraded and it is not settled well.

Fingered zoogloea can multiply in activated sludge in such a force that sludge did not settle well. The responsible organism is Zoogloea ramigera, which is common floc-former. Zooglocea happen when there is high food to microorganism ratio and when there is large amounts of specific organic acids and alcohols and low oxygen conditions or septicity.

Filamentous bulking and foaming are common and serious problems in activated sludge operation. A bulking sludge settles and compacts slowly. Bulking sludge may or may not lead to
a bulking problem. That depends on the treatment plant's ability to contain the sludge within
the clarifier. A certain amount of filamentous bacteria can be good in activated sludge process. Lack of them might lead to small and easily breaking flocks. Filaments build bigger and stronger flocs. They might also catch and hold some small particles from the wastewater. Only large amount of filaments cause problems. Which kind of settling happen, depends on the type of filament, which is involved:
1: Interflock bridging, filaments extend from floc surface and physicallu hold particles apart.
2: Open-floc structure, filaments grow mostly within the floc. Flocs are big and irregulary shaped.

Bulking sludge can cause environmental damage and loss of sludge inventory. In severe cases loss of sludge inventory can lead to loss of its activity. In less critical cases bulking leads to massive return sludge recycle rates. There might also have problems in waste activated sludge disposal.

Foaming is also a possible source of problems. There is lot of different types of foams. Usually three filamentous organisms can cause foaming, Nocardia and Microthrix parvicella and type 1863 (rarely). Nocardial foaming is most common, it generate a thick stable brown foam. This foam consist lots of solids. Foam is stable because Nocardia’s filaments interlock with each other.
Nocardial foam occurs in all types of plants. They are more harmful in pants where is fine bubble or jet aeration and oxygen activated sludge plants. Nocardia foam is common if in waste water is grease, oil or fat. Severe Nocardial foam cause many operational problems. It is smelly and unaestetic, it might cover walkways, which make them slippery. In cold weather theu freeze easily. Foam might increase effluent suspended solids. In covered aeration bases foam can go through the basin.

The core of problem solving is microscopic examination of the activated sludge. It reveals if problem is caused by filaments. Problems can fix two ways:
1: Short term, treating the symptoms
2: Long term, treating the cause

Short term measures sludge juggling, changes in return activated sludge rates and in feeding points. Adding polymer and coagulant in aid sludge settling, and chlorination.
1: Sludge juggling contains several methods, which are useful surviving under problems. These will not solve a chronic problem. In process must remove solids from the final clarifier faster than they are added. Therefore, the flowrate of return activated sludge must be increased in a bulking situation to prevent loss of solids. There is a limit to the increase flowrate. It increases return flow and effluent conditions worsening. Bulking can be controlled by holding sludge in the clarifier for lengthy time periods. This works, because it proably create septic and toxic conditions. Sometimes it worsen the problem, because it encourage sulfide-oxidizing filaments.
A reduction in solids loading to the clarifier can be achieved reducing the system's
sludge inventory. This can cause problems, because it can encourage filament growth. It is also possible to change mixed liquor suspended solids (MLSS) concentration without reducing system’s sludge inventory. MLSS concentration is highest in head end of aeration basin and its amounts decrease in the clarifier feed. Selecting right place, redistribution of solids can made in a day.
2: Polymer and Coagulant addition, which means different chemicals, which are used to make activated sludge settling better. Most of them are synthetic anionic large polymers, which might be used with cationic polymers. In some cases inorganic coagulants or precipitates such as lime or ferric chloride can be useful. These improve settling because they produce a voluminous precipitate that sweeps down the activated sludge. They increase sludge production. The weighting action of inert biological solids has also been used to aid sludge settling in activated
sludge modifications. It is also possible to recirculate anaerobic digester contents through the aeration basin. One method is to release fiber or clay to the wastewater system, in some cases that help sludge settling during a bulking episode.
3: Chlorine or hydrogen peroxide are useful solver of bulking problem, because they damage filaments extending from the floc surface, and leave orgaisnms within the floc largely untouched. Dosage of chlorine is adjusted so that its concentration are not lethal to the core of the floc, but kills the surface of flocs. Chlorination will make problems worse, if problem is from poor floc development or slime bulking. Chlorine must be used correct, it affects only if chlorine is used in right place and is mixed well. Most important parameters are (1) Chlorine dosage and (2) Exposure frequency. Dosage should be started low and increased until it is effective. Correct dosage corrects settling within couple days. The needed frequency depends of the relative growth rates and efficiencies of kill of filamentous and floc-forming organisms. Success is plant specific. Chlorination controls filament extension from the floc surface. That means that it reduces the symptoms of bulking, not cure the reason behind of it. Filaments will rapidly grow back after chlorination is ended. Over chlorination cause problems like milky effluent, and it kills higher life forms (protozoa). Problems can be avoided if chlorination is ended when about 70% of the filament’s cells are damaged or missing.

Long term measures include activities which cure the cause behind problem. They control influent waste septicity, nutrient additions, changes in aeration; biomass changes and changes in waste feeding.
1: Low DO means that the rate of biochemical oxygen demand removal is near at 1.0 mg/L dissolved oxygen concentration. The rate of nitrification is near maximum at 2.0 mg/L dissolved oxygen. The actual dissolved oxygen concentration within the biological flocs is less than that measured in the bulk solution around the flocs, due to oxygen use as it penetrates into the flocs. If there is much food per microbs, it is possible to need more oxygen, because it is important to keep the floc interiors aerobic. Low dissolved oxygen filaments are often eliminated by an increase in the mixed liquor suspended solids concentration.
2: Septicity mean the conditions where the wastewaters bacteria becomes anaerobic. That means that organic material is fermented to organic acids. A septic wastewater thus contains a relatively high amount of organic acids and hydrogen sulfide. Septicity can occur before the plant, in collection system or in treatment plant. Septicity cause bad smell, dark color and corrosion. Septicity occurs in places which are poorly aerated or poorly mixed. Common cause of septicity is the use of primary clarifier to sludge thickening or if waste activated sludge is returned to primary clarifier. Problem is handled using pre aeration, chemical oxidation, chemical precipitation or using other oxygen source. If the septicity is still high, then it is possible to cofigure aeration base.
3: If there is Low F/M –problems (high food / micro organism ratio) and selectors it is possible to control low F/M bulking by reducing the aeration basin mixed liquor suspended solids (MLSS) concentration, or increasing F/M by manipulating micro organism ratio. Usually MLSS concentration lowering is not suitable on plants because it can cause loss of nitrification and increase waste sludge production. Low F/M is most common problem in completely - mixed aeration basin systems, which have low aeration basin substrate (BOD) concentration. Intermittently - fed and plug - flow systems are more resistant to this. A selector is a mixing basin or channel. Using a selector means that conditions are modified so that certain floc forming bacterias can live easily, but which is not optimal in filaments growing. Selectors must be used right.
4: In case of Nutrient Deficiency it is possible that bacterias don’t get enough nitrogen and/or phosphorus, and it can’t grow enough. That is problem with industrial wastes and rare in domestic waste. Signs of nutrient deficiency are (1) certain kind of filamentous bulking and (2) a viscous activated sludge, which has much polysaccharide. Problem can be solved by adding needed nutrient in wastewater or in the aeration basin. It is important to look that nutrition adding is done right. Wrong handling can cause BOD "spikes", and these causes aeration basin to become nutrient limited for short time periods.
5: If sludge is foaming, there are three filaments cause foaming. All of them grow on grease and oil. These can become problem when wastewaters grease and oil concentration is high. Systems which lack primary clarification, are suffering more this problem, because primary clarification is good grease and oil removal mechanism. Nocardia and M. parvicella, which are common bacters behind foaming, occur at longer sludge age. Nocardia is succesfull at higher temperatures and M. parvicella in lower temperatures. Many plants reduce sludge age, which weakens nitrification and sludge handling capacity. They can be controlled: Nocardia can by a sludge age below 6-8 days and M. parvicella at a sludge age below 8-10 days at moderate temperatures. Nocardia and M.parvicella grow well in low oxygen levels and they grow better if there is lot of unsaturated fatty acids. Anaerobic bacterias modify fatty acids to unsaturated, which means that they prove Nocardia and M.parvicella a source of unsaturated fatty acids. Type 1863 is another foaming cause. It grow usually faster, it needs only 3-4 days. It needs a high amount of grease and oil and a young sludge condition. Control of Nocardia and M. parvicvella is difficult. Chemical antifoam agents are usually not effective. Many plants reduce aeration to control foaming, but process starts to suffer becouse oxygen levers are low. And low oxygen cause bulking. Physical foams handling is most common way to handle problem. Surface scum traps and forceful water sprays. If foam is removed entirely from the system and not recycled back into the plant, it may solve the problem. If foam is disposed in aerobic or anaerobic digesters, it can cause foaming there. It is also propable to control foaming by reducing the grease and oil content of the wastewater. Also many operators reduce aeration. That reduce the foam, but this causes more filament growth in the long term.

Tuomo Hämäläinen, Laurea University of Applied Sciences.