Chlorinate to-control-filamentous-bacteria

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Chlorinate to Control Filamentous Bacteria

Richard Fuller

If you spend enough time around an activated

sludge system you will, at some point, find

yourself in a situation where conditions in the

bioreactor shift unfavorably, resulting in the

excessive growth of filamentous bacteria. At

first, as the filamentous population begins to

grow, you might see a reduction in the turbidity

leaving the secondary clarifier because the

filamentous do an excellent job, at least initially,

trapping suspended solids. But as the

filamentous population continues to increase,

you will begin to see deteriorating conditions

that include an expansion in the clarifier sludge

blanket, an increase in the sludge volume

index(SVI),an increase in clarifier effluent

turbidity, and an increase in solids loss

(suspended solids) from the clarifier.

Factors that can contribute to the excessive growth of filamentous bacteria include, but are not

limited to, the following:

Nutrient deficiency (insufficient ammonia nitrogen and/or phosphate)

Low dissolved oxygen (DO) concentration in the bioreactor

Low food-to-mass (F:M) ratio

Low pH

High concentration of sulfide compounds entering the bioreactor

Septic wastewater

An established, proven method for reducing the filamentous bacteria population is

chlorination.The most common approach, and the approach I recommend, involves the

chlorination of the return activated sludge (RAS).With this controlled approach, industrial

strength bleach (≥10% sodium hypochlorite) is fed to the suction of the return sludge pumps to

maximize mixing and contact between the chlorine and the filaments in the activated sludge as

portrayed below in Figure 1.

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Figure 1: Dispersion of bleach into MLSS (RAS) as a function of mixing

Bleach

Bleach and MLSS

Well Mixed

To AerationRAS Pipeline

Effective chlorination of the return sludge requires, as a rule-of-thumb, contact between the

return sludge and bleach ≥3 times per day.The period of chlorination may be as short as a few

days or as long as a week or more. You will know to stop chlorination based on a microscopic

analysis of the mixed liquor suspended solids (MLSS) if a microscope is available. Lacking a

microscope, you will have to rely on a decrease in the sludge volume index, discussed in more

detail shortly.

Following a microscopic analysis, you would start chlorination because of an excess of filaments

extending from and between floc particles as shown in Figure 2A. Chlorination would be

stopped when you are satisfied that the quantity of filaments have been sufficiently reduced

(Figure 2B). You are not trying to get rid of all the filamentous bacteria because a small

population helps increase the integrity and strength of the floc structure which improves solids

settling in the secondary clarifier.

Figure 2: Filamentous bacteria comparison

A. Excessive growth of filamentous bacteria B. Very few filaments are present

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If you do not have a phase contrast microscope you can use a combination of the sludge settling

rate and the sludge volume index as a guide to judge when to start and stop chlorination. By

“sludge settling rate” I’m referring to the simple practice of recording the settled sludge volume

in a 1 liter jar every two minutes for 30 minutes and then graphing these values to observe the

settling curve. All you are doing is monitoring the settling of the MLSS as you wait to get your

settled sludge volume at the end of 30 minutes to calculate the sludge volume index. Settling

curves for an industrial wastewater system spanning a four month period are shown in Figure 3.

Figure 3: MLSS Settling Curves

0

100

200

300

400

500

600

700

800

900

1,000

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Sett

led

Slu

dge

Vo

lum

e, m

Ls

Time, minutes

MLSS Settling Rate

April May June July

The associated MLSS concentrations and SVI values for the data graphed in Figure 3 are

summarized in Table 1. Note the low SVI of just 46 mL/g which occurred in April. In this case

the MLSS was actually settling too fast, producing a water with slightly higher turbidity because

the rapidly settling MLSS leaves many dispersed fineparticles that remain suspended in the water

which ultimately get carried over in the clarifier effluent.

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Table 1: MLSS Concentration and SVI

April May June July

MLSS concentration, mg/L 3,230 3,450 3,116 4,122

SVI, mL/g 46 72 87 164

There are a few more comments to make about the graph in Figure 3 showing the MLSS settling

curves. The water temperature in the bioreactor had been steadily increasing from spring into

summer, lowering the DO saturation point. By July, due to high water temperature and very low

DO conditions, combined with a phosphate nutrient deficiency, the SVI had increased to 164

mL/g and the MLSS was settling poorly due to a significant increase of filamentous bacteria.

During this time it was easy to look down into the clarifier and see the “fluffed” or suspended

sludge blanket rising to near the surface in different parts of the clarifiers and a large quantity of

solids were being lost in the effluent. Under these conditions the return activated sludge

concentration also decreased which resulted in returning fewer solids to the bioreactors.

Chlorination was started, a regular practice at this particular industrial wastewater plant, using a

tapered dosing program. For two days the bleach dose into the suction of the return sludge

pumps was at 10 lb of Cl2 per 1,000 lb of MLVSS (10 kg of Cl2 per 1,000 kg of MLVSS). The

dose was lowered to 7 lb/kg of Cl2 per 1,000 lb/kg of MLVSS for two more days. Finally, three

days of dosing at 5 lb/kg of Cl2 per 1,000 lb/kg of MLVSS lowered the SVI to a value of 97

mL/g.

Ideally, you want the SVI to be ≤150 mL/g with the optimal SVI range usually stated as being

50–150 mL/g. This is the range you will find in most textbooks. The range is a guide, and a good

one at that, but you may find your experience at your plant differs from this range yet you are

still able to produce good water. I have seen plants consistently carry SVI’s at 175 to 200 mL/g

and never have a problem with solids loss. So use the guide but trust your own experience. The

SVI is calculated as shown in Equation 1.

Equation 1: Sludge volume index formula

3, 10

,

mL mgSettled volumeof sludge

L g mLSVI

mg gMLSS

L

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It may be that you are unable to feed bleach to the return sludge pump suction or, perhaps,

anywhere along the outlet (pump discharge) of the return sludge line. In this case you have no

choice but to feed bleach directly into the aeration tank (bioreactor) itself. With this somewhat

uncontrolled approach you are slugging (shock-loading) the aeration tank with a large quantity of

bleach in a very short period of time (minutes). For example, you might add 1,000 gallons (3,785

liters) of sodium hypochlorite to an aeration tank with a 2,000,000 gallon (7,571 m3) volume.

Though you may feel uncomfortable with this approach I can tell you that it does work and it

works quickly. And if you don’t have sodium hypochlorite you can substitute calcium

hypochlorite tablets or powder from pails, dumping the product directly into the inlet of the

aeration tank. The heavy hit of dumping a large volume of bleach directly into the inlet of a

bioreactor is somewhat offset by the poor mixing and contact that takes place with this approach,

as illustrated in Figure 4. Though you are slugging the system the impact is dramatic during first

contact but gets diluted very quickly. So though you may negatively impact “some” of your good

bugs you will spare many of them due to the lack of mixing in the rest of the tank.

Figure 4: Bleach being fed directly into the aeration tank

Bleach

MLSS

Bleach and MLSS

Poorly MixedAeration Tank

Caution and understanding are required because, with no intention to be overly dramatic,

chlorination of the RAS is a bit like chemotherapy. For the microorganisms in the bioreactor

chlorination is, in general, a relatively aggressive and harsh process. This will be evident by the

increase in turbidity you will see in the clarifier effluent during the period of chlorination and for

a short time after. But while chlorination tends to weaken the entire population of

microorganisms it is particularly effective in killing the filamentous bacteria. The filamentous

bacteria are more vulnerable because their filament strands extend out from the floc mass into

the bulk solution increasing their exposure to the chlorine. The good news is that within just a

few days of stopping chlorination the microorganism population will make a rapid return to full

health.

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A dose of 2–10 lb of Cl2 per 1,000 lb of MLVSS(2–10 kg of Cl2 per 1,000 kg of MLVSS)per

daywill effectively control filamentous microorganisms. If you don’t know where to start you

can begin dosing in the range of 3 to 5 lb of Cl2 per 1,000 lb of MLVSS(3–5 kg of Cl2 per 1,000

kg of MLVSS) per day. I usually start at a rate of 5 lb/kg of Cl2 per 1,000 lb/kg of MLVSS per

day. But it is not unusual to have to dose at a higher rate, in the range of 8 to 10 lb of Cl2 per

1,000 lb of MLVSS(8–10 kg of Cl2 per 1,000 kg of MLVSS) per day, or even higher (20 lb/kg of

Cl2 per 1,000 lb/kg of MLVSS per day). So be prepared to adjust your dose upward.

Chlorination works as well as it does because the filamentous microorganisms are more readily

affected by the addition of oxidizing agents (chlorine, hydrogen peroxide) due to the filaments

having a greater surface area to volume ratio than bacterial cells. The use of hydrogen peroxide

in dosages of 100 to 500 mg/L or more can also be used to control (reduce) filamentous bacteria.

A detailed example of calculating the required quantity of chlorine is provided below.

Calculation of Chlorine Feed Rate

A. Data Required

1. MLSS = 3,240 mg/L

2. MLVSS = 2,592 mg/L (If the mixed liquor volatile suspended solids concentration is not

known, as is often the case, assume the MLVSS to be 75–80% of the MLSS

concentration.)

3. Aeration volume = 2,300,000 gallons (8,706 m3)

4. Initial chlorine dosage rate= 4 (lb/kg Cl2/1,000 lb/kg MLVSS)/day

5. Sodium hypochlorite solution strength = 12% (This is an assumed solution strength. Keep

in mind that sodium hypochlorite degrades over time and the rate of degradation

increases with increasing ambient temperature.)

B. Calculate the volatile solids inventory under aeration

Equation 2: Pounds (kilograms) of mixed liquor volatile suspended solids in the aeration tank

, , , 8.34

2,592 2.3 8.34

49,720

mg lbMLVSS lb MLVSS Aeration volume mil gal

L gal

mg lbmil gal

L gal

lb of MLVSS

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3 3

3

3 3

3

, , , 10

2,592 8,706 10

22,566

g kgMLVSS kg MLVSS Aeration volume m

m g

g kgm

m g

kg of MLVSS

C. Calculate the chlorine(100% basis) feed rate

The feed rate for chlorine addition, in pounds (kilograms) of chlorine per day, is calculated as

shown in Equation 3. Note that Equation 3 calculates the pounds (kilograms) of chlorine as

100% chlorine (100% basis), not as the pounds (kilograms) of sodium hypochlorite to be added,

which is calculated below.

Equation 3: Calculation for pounds (kilograms) of chlorine (100% basis)

2

2

2

4 49,720,

1,000

199

lbCl lbof MLVSSlbCl

day lbof MLVSS

lb Cl

day

2

2

2

4 22,566,

1,000

90

kg Cl kg of MLVSSkgCl

day kg of MLVSS

kg Cl

day

If you use sodium hypochlorite (NaOCl), which is what I would recommend, you need to keep in

mind that an industrial-strength NaOCl solution, bleach, typically has 10–15% available

chlorine. The quantity of bleach (using a 12% solution in our example) required is calculated in

Equation 4.

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Equation 4: Calculation for pounds of a 12% sodium hypochlorite solution

2199

, 1,65712.0%

lbCl

lb dayNaOCl

day

290

, 75012.0%

kg Cl

kg dayNaOCl

day

The specific gravity of a 12% sodium hypochlorite solution is approximately 1.2 as shown in

Table 2.

Table 2: Specific gravity for different sodium hypochlorite solutions

% Strength Density, lb/gal Density, kg/L

15% 10.43 1.25

12% 10.02 1.20

10% 9.70 1.16

8% 9.39 1.13

Sodium Hypochlorite

From Table 2the weight (density) of a gallon of NaOCl is shown to be10.0pounds. Based on this

weight per gallon, you would need to add 159 gallons of NaOCl, at a 12.0% solution strength, to

the RAS line per day, as shown in Equation 5.

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Equation 5: Quantity of sodium hypochlorite to add in gallons (liters)

1,657

, 165

10.02

lb NaOCl

gal dayNaOCl

lbday

gal

750

, 625

1.2

kg NaOCl

L dayNaOCl

kgday

L

Tracking Filamentous

Personally, I prefer to track biological system health and performance using a combination of

oxygen uptake rate (OUR) testing and sludge volume index monitoring. Part of my preference is

based on being able to easily travel with an OUR test kit, something that is relatively compact

and rugged. I usually don’t have access to a phase contrast microscope at the industrial

wastewater systems I visit. But for those of you who do have use of a high quality microscope

you might want to conduct a regular scoring assessment of the filamentous population in your

bioreactor using Table 3 as a guide. By “regular” I would suggest conducting a microscopic

analysis of the mixed liquor suspended solids at least once a week. With this frequency of

monitoring you are not likely to find yourself surprised by an increase in filamentous growth.

Table 3: Filamentous scoring table

Numerical Value Abundance Explanation

0 None No filaments observed.

1 Few Filaments are present, but only observed in an occasional floc.

2 Some Filaments are commonly observed, but they are not present in all flocs.

3 Common Filaments are observed in all flocs, but at a low density of 1 to 5 filaments per floc.

4 Very Common Filaments are observed in all flocs at a medium density of 5 to 20 per floc.

5 Abundant Filaments are observed in all flocs at a high density of >20 per floc.

6 ExcessiveFilaments are observed in all flocs and there are more filaments than there are flocs

and/or the filaments are growing in high abundance in the bulk solution (MLSS).

Subjective Scoring of Filament Abundance

Source: Jenkins, David, Michael G. Richard, and Glen T. Daigger. Manual on the Causes and

Control of Activated Sludge Bulking, Foaming, and Other Solids Separation Problems. 3rd

ed.

Boca Raton: CRC Press, 2004.

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References

Jenkins, David, Michael G. Richard, and Glen T. Daigger. “Manual on the Causes and Control

of Activated Sludge Bulking, Foaming, and Other Solids Separation Problems.” 3rd

ed. Boca

Raton: CRC Press, 2004.

Tandoi, Valter, David Jenkins, and Jiri Wanner, eds. “Activated Sludge Separation Problems:

Theory, Control Measures, Practical Experience.” London: IWA Publishing, 2006.

Richard Fuller is a Senior Technical Advisor based in Chicago for Athlon Solutions.

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