RAS Chlorination

Chlorinate to Control Filamentous

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 (aeration tank, basin, biotreater, etc.) shift unfavorably, resulting in the excessive growth of filamentous bacteria. At first, as the filamentous population begins to grow, you are likely to see a reduction in the turbidity leaving the secondary clarifier because the filaments do an excellent job of trapping solids. But as the filamentous population continues to increase, you will 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 contribute to the excessive growth of filamentous bacteria include, but are not limited to, the following:

  • Nutrient deficiency (insufficient ammonia and/or phosphate)
  • Low dissolved oxygen (DO) concentration in the aeration tank
  • Low food-to-mass (F:M) ratio
  • Low pH
  • High concentration of sulfide compounds entering the bioreactor
  • Septic wastewater
Filamentous bacteria extending from floc

A well-known and proven method for reducing the filamentous bacteria population is chlorination and several approaches are available. The most common (and recommended) approach 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 pump to maximize mixing and contact between the chlorine and the filaments in the activated sludge as illustrated in Figure 1.

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

Rule of thumb

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 three days or as long as a week or more. You will only know to stop chlorination based on a microscopic analysis of the mixed liquor suspended solids (MLSS). Chlorination is started because of an excess of filaments extending from and between floc particles as shown in Figure 2. Chlorination is stopped when you are satisfied that the quantity of filaments have been sufficiently reduced (Figure 2).

 Figure 2: Filamentous bacteria comparison

Excessive growth of filamentous bacteria

Very few filaments present

nof="te"Abundant filaments

nof="te"Few filaments

If you do not have a microscope you can use the sludge volume index as a guide to judge when to start and stop chlorination. Ideally, you want the SVI to be ≤150 mL/g. The SVI is calculated as shown in Equation 1. (For more detailed information on the SVI please click here.)

Equation 1: Sludge volume index formula

It may be that you are unable to feed bleach to the return sludge pump suction or, perhaps, anywhere along the outlet (pump discharge) or return sludge line. In this case you have no choice but to feed bleach directly into the aeration tank itself as illustrated in Figure 3. 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 of sodium hypochlorite to an aeration tank with a 2,000,000 gallon 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.

Figure 3: Bleach being fed directly into the aeration tank

Poor mix of bleach and MLSS

Caution and understanding are required because chlorination of the RAS is like chemotherapy. For the microorganisms 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. And within just a few days of stopping chlorination the microorganism population will make a rapid return to full health.

A dose of 2–10 lb of Cl2 per 1,000 lb of MLVSS per day will 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 per day. I usually start at a rate of 5 lb of Cl2 per 1,000 lb 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 per day, or even higher (20 lb of Cl2 per 1,000 lb of MLVSS per day).

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

  • MLSS = 3,240 mg/L
  • 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 80% of the MLSS concentration.)
  • Aeration volume = 2,300,000 gallons
  • Initial chlorine dosage rate = 4 (lb Cl2/1,000 lb MLVSS)/day
  • 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 of mixed liquor volatile suspended solids in the aeration tank

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

The feed rate for chlorine addition, in pounds of chlorine per day, is calculated as shown in Equation 3. Note that Equation 3 calculates the pounds of chlorine as 100% chlorine (100% basis), not as the pounds of sodium hypochlorite to be added, which is calculated below.

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

Calculation of chlorine pounds

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 (12% solution) required is calculated in Equation 4.

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

Calculation of sodium hypochlorite pounds

The specific gravity of a 12% sodium hypochlorite solution is approximately 1.2 as shown in Table 1.

Table 1: Specific gravity for different sodium hypochlorite solutions

From Table 1 the weight (density) of a gallon of NaOCl is shown to be 10.0 pounds. 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.

Equation 5: Quantity of sodium hypochlorite to add in gallons

Calculation of sodium hypochlorite gallons

Tracking Filamentous

Personally, I prefer to track biological system 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 kit, something that is relatively compact and rugged. Too often I don’t have access to a good phase contrast microscope. 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 using Table 2. By regular I would suggest conducting a microscopic analysis of the mixed liquor suspended solids at least once a week.

Table 2: Filamentous scoring table

Filamentous bacteria scoring table