Operation of activated sludge system

During the operation of the wastewater treatment system , we often encounter problems such as floating sludge, foaming sludge and scum… below I would like to summarize a few common cases for us to discuss. experience.

Before starting to operate the entire system, it is necessary to perform the following operations:

engineering action, starting biological systems.

* Start-up technique:

Check the power supply system for the whole system. Check the chemicals to be supplied and the water level in the tanks. Check whether the open pile valve is suitable.

Technical check of the entire system (operating pumps, aerators, valves, programs…).

* Start the biological system:

Normally, to start a biological system, it is necessary to have biomass available in the treatment systems. Biomass can grow spontaneously through the continuous supply of wastewater to the reactor. To save time, inoculate the reactor with biomass taken from an operating wastewater treatment plant or specialized microbial biomass.

The biomass is normally cultured from active activated sludge treatment systems, or the biomass can be obtained from other sources. That will require more time. The biomass content after inoculation was in the range of 2g/l.

Start with a very low biomass load that does not exceed the design value (0.15kg BOD/kg.day). If the water quality after treatment is good (BOD, COD, and Nitrogen), increase the load. Increasing the load should ensure the appropriate biomass content.

Parameters to consider:

– COD; BOD; MLSS; MLVSS; N (N-NH3; N-NO2; N-NO3; N kiejdahl), P (ortho P, Poly P)

– Volume of biomass: volume of sludge settled after 30 minutes (V Experiment = 1 liter)

– Biomass volume index: SVI (ml/g) = biomass settling volume/ biomass content.

■ Organic load:

With COD: OLR = COD (kg/m3) x Q (m3/day)/ V tank (m3)

With BOD: OLR = BOD (kg/m3) x Q (m3/day)/ V tank (m3)

■ Loading biomass:

F/M = {COD (kg/m3) x Q (m3/day)}/ {V tank (m3)x MLSS (kg/m3)}

■ Surface load: is the amount of water flowing into the settling tank in one hour per square meter of settling surface bề

Vs (m3/m2.h) = Flow (m3/h)/settling surface area (m2)

■ Average retention time of biomass: is the age of biomass

MCRT (day) = MLSS (kg/m3) x total volume (m3)/biomass removed daily (kg/day)

* During operation, pay attention to:

Mastering technology

Monitor and analyze periodically, observe the fluctuations of wastewater, abnormal factors, record and keep accurate information, easy to trace, enough documents to look up

 Parameters checked during operation

Flow: determines the load capacity of the system and the surface load of the settling tank. It is necessary to ensure a stable flow before entering the biological work.

F/M : suitable about 0.2 – 0.6. Limiting low pH, floating sludge, poor settling. If F/M is low: it is caused by bacteria with special structure – fungus, high F/M: low DO, overload, black sludge, poor settling, fishy smell, low treatment efficiency.

pH: Suitable is 6.5 – 8.5. High pH due to good conversion of N to N-NH3, high buffering capacity. Low pH: Nitrification process, low HCO3- content. Need to increase alkalinity chemicals. The way to overcome this pH fluctuation is to provide enough nutrients, organic content, limit the process of intracellular decomposition, use chemicals to increase alkalinity.

BOD/COD > 0.5 => Suitable for biodegradation

Regularly check BOD and COD to avoid underload or overload

Nutrients: N, P ensure the ratio BOD:N:P = 100:5:1, if lacking, must be supplemented from outside sources. Domestic wastewater, it is not necessary to add N, P

Toxic substances: heavy metals, apricot oil, high content of Cl, sulfate, N-NH3 …

 Organic Load Control

   High organic load: low DO; light brown mud, poor settling, foaming

Low organic load: high DO, fast settling sludge, good compaction, porous, brown sludge. A layer of fat and scum appears on the surface

   Surface load: high will affect the settling process. Biomass drifts out

   Suitable surface load: 0.3 – 1 m3/m2/h

Poorly settled sludge:

Floating on the surface: The process of denitrification, producing N2, lack of nutrients appear filamentous bacteria, or nutrients, dead sludge floats on the surface.

   Biomass grows scattered: due to high or low organic load, excess oxygen, toxicity oxy

   Coagulated biomass: Lack of oxygen, lack of nutrients, biodegradable organic matter

Dissolved oxygen:

Depends on organic load and biomass content. Suitable DO: 1-2 mgO2/l. Lack of oxygen will reduce treatment efficiency, appear rod-shaped bacteria, fungi, reduce settling capacity and inhibit nitrification process.

BOD after treatment is high due to: Overload, Lack of oxygen, pH change, poisoning, poor mixing

N after treatment is still high due to: Unstable technology, Presence of non-degradable N compounds, High sludge biomass in the tank, Poisoning, bacterial death

High N-NH3 due to: inappropriate pH (<6.5 or > 8.5), Low sludge age < 10 days, low DO < 2 mgO2/l, High N load, Toxic presence, Unstable operation steady, low temperature

N-NO3; High N-NO2 due to: Inappropriate pH (<6.5 or > 8.5), High N Load, Toxic presence, Unstable operation, low temperature, Excess oxygen (anaerobic tank), Lack of organic matter.

P: required ortho phosphate : 1-2 mg/l, Lack must be supplemented.

Operational Observation

– The color change indicates the operation of the processing system

– Dissolved, fine suspended solids also cause color

– The color of the original wastewater itself

Sensory: Odor, color, foam. A well-functioning system usually does not cause odors. During aeration -> small, white foam; if there is too much white foam it is due to: Biomass is in acclimatization or recovery phase, Overload, Lack of oxygen, lack of nutrients, Variable temperature, High content of surfactants, Presence of toxic substances .

 Stop working

There are many different reasons for deciding to stop the operation of a wastewater treatment plant. Result:

– Populations of organisms are starving, lacking food, and decomposing intracellularly

– Dead biomass drifts out, increasing the amount of suspended solids in clean water.

Oxygen is still required to avoid anaerobic conditions and odor problems, but must be kept to a minimum.

– If possible, try to store as much wastewater as possible in the conditioning tank or storage tank.

Reduce the amount of wastewater entering to 20-30% of normal level.

– Reduce oxygen supply to the lowest possible level (DO about 1-2mg/l).

– Maintain normal operation for as long as possible.

Maintain nutritional supplements if possible.

– If necessary, carbon sources must be added from the outside (such as acetate, methanole…) to prevent the biomass from rotting and remove as much as possible.

Operation of activated sludge system:

Prepare the required amount of activated sludge and start biological works (aerotank, oxidation ditch) in the following order:

– First, let a part of wastewater with BODtp concentration of about 200¸250mg/l flow through the work. If industrial wastewater has a high concentration, dilute it with production water or river water. The sludge settled in the second settling tank is continuously circulated to the aerotank.

– Activated sludge will increase over time. As the sludge increases with the presence of nitrate and nitrite, gradually increase the amount of water to be treated or decrease the dilution.

Can use sludge from any aerotank or activated sludge exposed at 60oC,

or biofilm drifting from biofilters or pond sludge. Activated sludge can be obtained from river or lake mud that is not contaminated with grease or mineral oil. Before being added to the aerotank, river or pond mud must be preliminarily removed from heavy mineral impurities (gravel and sand). For this purpose, the sludge is mixed with water, and after a short settling time (3¸6 minutes) is poured into the aerotank. There the sludge is aerated, no waste water is required. After the sludge preparation is completed, put the wastewater into the aerotank tank, initially in a small amount, then according to the degree of sludge accumulation, gradually increasing until the design flow is reached.

In the activated sludge works well, in addition to the cotton concentrates, micro-organisms also encounter a small amount of cordyceps (hairworms), spirulina, and worms.

When stable working conditions are disrupted, in the slurry grow phylum bacteria (sphacrotilus, cladothrix) phylum (zooglea ramigeras, aquatic fungi, etc.). These plant forms make the sludge float, which is difficult to settle in the second settling tank and is washed away with the water in a significant amount.

The cause of sludge floating is an overloaded aerotank, a large amount of carbon in the wastewater, insufficient oxygen supply, and low pH of the aerotank water. In order to control the sludge floating, it is necessary to reduce the load of the aerotank tank. Even temporarily stopping the wastewater inlet, or increasing the amount of dissolved oxygen in the aerotank, raising the influent pH to 8.5,9.5 for a period of time.

If high-concentration wastewater is discharged in abnormal batches, the plant leader must be asked to correct the technological principle or change the discharge mode by installing a regulator or a reserve tank.

When operating multiple clarifiers 2, it is necessary to evenly distribute the flow of wastewater and activated sludge between them as well as separate the activated sludge from the clarifiers.

The complete separation of activated sludge can be carried out continuously and does not form a layer of sludge in the settling tank. Improper sludge separation will contaminate and reduce the quality of treated water, in addition to floating the settled sludge.

The cause of sludge withdrawal from sump 2 could be due to a higher sludge concentration than the limit for a given load.

Sometimes it is difficult to ensure the separation of sludge from the vertical settling tank. Therefore, in these settling tanks it is necessary to systematically scrape the sludge from the bottom of the hopper (or several times a day), this difficulty can also be solved by increasing the volume of circulating sludge.

If in the treatment station there are several second settling tanks and there is no measuring device for circulating sludge, separated from each hopper, then on each settling tank, a tool to check the depth of sludge is required, these tools can be:

– The surge pump device (communication pipe) is set at the respective levels to control the highest and lowest sludge levels in the tank.

– Photocells.

If a measuring instrument is not available, the sludge level is determined by sampling at different depths. The appearance of air bubbles and activated sludge clusters on the surface of the sedimentation tank is due to the retention time of the sludge in the settling tank for too long, to control this phenomenon, the volume of sludge removed should be increased.

The presence of grease, petroleum products, and fat in the aerotank causes the sludge to float and attracts the sludge from the clarifier 2. In this case, it is necessary to increase the efficiency of the pre-oil separator and if possible. stop receiving wastewater containing grease, petroleum products.

 System operating parameters:

To operate the activated sludge system, it is necessary to pay attention to the following factors:

* Alkalinity: Alkalinity control in the aeration tank is necessary to control the whole process. Insufficient alkalinity reduces microbial activity and can also affect pH.

* DO: The operation of the activated sludge tank is an aerobic process so it requires the amount of DO to be present at all times. This amount of DO depends on the incoming BOD, the nature of the activated sludge and the treatment requirements.

* pH: The pH in an aerobic system is usually in the range of 6.5 – 9.

* MLSS, MLVSS, and MLTSS:

* Activated sludge concentration and recirculation rate: The operator must maintain active sludge circulation in the system. If this rate is too low, the aeration tank may be hydraulically overloaded, reducing aeration time. The circulating concentration is also important because it can be used to determine the rate of circulation required to hold the required MLSS.

* Waste activated sludge flow rate: Because activated sludge contains live microorganisms that grow, the amount of activated sludge can continue to increase. If the activated sludge remains in the system for too long, the efficiency of the process will decrease. If too much activated sludge is removed from the system, the solids will not settle fast enough to be removed in the secondary clarifier.

* Temperature: Temperature directly affects the activity of microorganisms.

* mud cover depth: If the solids are not removed from the system from the filter at the same rate they were introduced, the coating depth will increase. Sludge cover depth can be affected by many conditions: temperature, toxicity in wastewater, etc.

Control system operation;

* Cycle speed

– The circulation rate is too high, resulting in: Aeration and deposition in hydraulically overloaded tanks; reduced aeration and deposition time, etc.

– The circulation rate is too low, resulting in: rotten circulation, trapped solids in the clarifiers, reduced MLSS in the aeration tank, etc.

* Wastewater speed:

– The wastewater rate is too high, resulting: decrease in MLSS, decrease in sludge density, increase in SVI, decrease in MCRT, increase in F/M ratio.

– Wastewater rate is too low, resulting in: increase in MLSS, increase in sludge density, decrease in SVI, increase in MCRT, decrease in F/M ratio.

* Ventilation speed:

– The aeration rate is too high, resulting in: wasted energy, increased operating costs, floating solids, breakdown of activated sludge.

– The aeration rate is too low, resulting in: rotten aeration tank, poor efficiency, loss of nitrification.

  Possible problems during operation and how to fix them:

* Symptom 1: The mulch layer is drained out along the waste stream, there is no more sediment.

– Due to overload of organic matter. Fix: reduce organic load.

Due to low pH. Fix: add alkalinity.

Due to the growth of filamentous fungi. Remedy: add nutrients, add chlorine or peroxide to recirculate.

– Due to nutritional deficiencies. Fix: add nutrition.

– Due to toxicity. Fix: define source, add preprocessor.

Due to excessive ventilation. Correction: hypoventilation during periods of low flow.

* Symptom 2: a large amount of small solid particles leave the clarifier.

– Cause: old mud. Remedy: reduce sludge age, increase waste flow rate.

– Cause: excessive chaos. Fix: reduce turbulence (control blowout when flow is low).

* Symptom 3: A large amount of small, translucent molecules leave the clarifier.

– Due to the growth rate of the sludge. Fix: increase mud age.

– Due to the new, weak activated sludge. Fix: reduce wastewater.

* Symptom 4: The sludge settles well, but floats to the surface in a short time.

Due to denitrification. Remedy: increase circulation rate, adjust sludge age to limit denitrification.

– Due to excessive ventilation. Correction: decreased ventilation.

* Symptom 5: microorganisms in activated sludge die in a short time.

Due to the influent containing toxic substances. Remedy: remove activated sludge (if possible). Cycle all the solids present. Stop supplying wastewater. Increase circulation speed. Additional preprocessor programs.

* Symptom 6: The surface of the aeration tank is covered with thick, greasy foam.

– Because the mud is too old. Fix: reduce mud age. Increase the amount of wastewater, use foam control pumps.

Due to too much oil and fat in the system. Remedy: enhanced fat removal. Use foam control pumps. Additional preprocessor programs.

– Due to the bacteria that cling to foam. Fix: remove these bacteria.

* Symptom 7: appearance of large bubbles on the surface of the aeration tank.

– Due to the young activated sludge, the amount of sludge is small. Remedy: increase sludge age, reduce wastewater supply, use foam control pumps.

– Due to detergents. Remedy: limit surfactants, use foam control pumps.

Common problems when operating the activated sludge process

No. Problem Cause Consequence

1 Dispersed growth Microorganisms that do not form flocs but disperse as isolated individuals or small clusters with a diameter of 10mm-20mm. The efficiency of the second settling tank is low, the water coming out of the tank is cloudy. The amount of circulating sludge is small.

2 Pinpoint floc The floc is usually small compressed sphere, 50-100mm in diameter, caused by the division of large flocs, lack of food, microorganisms must use polysaccharides extracellular as a source of C and energy for life processes Low SVI sludge volume index, water coming out of the tank is turbid.

3 Bulking Sludge Overgrowth of filamentous bacteria in the sludge causes poor compaction and poor settling of the sludge. High SVI Difficult to maintain required sludge concentration in aeration tank Reduced sludge dewatering capacity

4 Rising sludge In the second settling tank, denitrification process takes place, producing N2 gas, N2 gas moving upwards, pulling activated sludge flocs to the surface.

Sludge retention time less than 1 hour is enough to form air bubbles Formation of activated sludge layer on water surface.5 Foaming/scum Due to the presence of bacteria Norcadia spp and Microthrix parvicella Causes bad odor

Increases SS, BOD in the outlet wastewater The scum layer will retain a layer of activated sludge, which affects the sludge retention time.6The bulking sludge is not caused by filamentous bacteriaThe sludge contains too many extracellular polymers that make the sludge porousIncrease SS, BOD in the output wastewater, dilute the amount of sludge.

Measures to control hard-to-settle sludge:

Bulk sludge control:

Factors to consider when controlling bulk sludge:

– Wastewater composition

Wastewater containing many trace elements also causes sludge formation. For industrial wastewater, the nitrogen and phosphorus content should be checked before being put into the treatment system, nutritional deficiencies in industrial wastewater with high BOD content will form sludge. pH fluctuations are also detrimental to the design. The difference in load when operating a single-batch system also causes sludge formation.

Oxygen concentration should be maintained at least 2 mg/l.

– Sludge retention time (SRT): should be checked and calculated to be within an acceptable value. In many cases, in fully agitated systems with large sludge retention times and low F/M ratios, filamentous bacteria are often present. In such systems, these bacteria often compete for food. Numerous laboratory studies along with complete models have shown reactor configurations that support flocculation rather than the growth of filamentous bacteria. These tanks are called selectors.

– Nutrient concentration

Although the nutritional dose required to prevent overgrowth of filamentous bacteria is unique to each system, when it is not possible to determine the exact number, Richard should maintain:

  + Inorganic N concentration (NH4 _N, NO2 _ N, NO3 _N ) is greater than 1mg/l.

 + Phosphorus orthophosphate (PO43-_P) concentration greater than 0.2mg/l.

– pH is an important parameter of activated sludge but is often ignored because its value often changes only at 2 levels. Microorganisms are only affected when the pH is less than 6.0-6.5 and above 8.5. To avoid overgrowth of fungi due to low pH, it is necessary to equip pH adjustment devices, at least a lime dosing device.

Temperature affects the following two parameters:

 An increase in temperature decreases the solubility of oxygen in water

 Increasing temperature increases the rate of metabolism. This process requires dissolved oxygen, so the DO consumption rate also increases.

Therefore, temperature affects mainly the growth of low-DO filamentous bacteria such as Sphaerotilus natans and species 1701. Richard ran a model investigating the effect of temperature on competitive growth between species. 1701 and a cotton-producing microorganism. From that, it was concluded that at a temperature greater than or equal to 28oC, species 1701 thrived more than the cotton species under study. Thus, we can draw the conclusion that as temperature increases, DO concentration needs to be increased to prevent bulk sludge process caused by filamentous bacteria growing in low DO water.

Foam control

Systematic de-foaming to remove and destroy floating sludge in the aeration tank. The de-foaming agent does not recirculate the clarifier, as this will lead to the proliferation of scum-causing bacteria. In addition, this substance should not be recirculated to the aeroten bath.

Use chlorine spray to spray the surface of Nocardia foam.

Controlling floating sludge:

Floating sludge can be controlled by the following measures

Increase the amount of circulating sludge discharged from the second settling tank, in order to reduce the sludge retention time in the settling tank.

Reduce sludge retention time to avoid nitrification. In warm climates, it is difficult to operate at short sludge retention times to avoid nitrification.