Removal of foaming from industrial wastewater treatment plants

In the activated sludge method, wastewater is in contact with bacteria in a tank for several hours and sufficient oxygen for creating aerobic conditions is provided. The bacteria degrades organic matter into carbon dioxide and water, and produces substantial amounts of bacteria. Afterward, a mixture of degraded wastewater, including microorganisms flows through the settling tank for a duration of time, then part of the settled sludge in the settling tank is returned to aeration tanks and treated wastewater leads to effluent of the settling tank (Shariat Panahi 2001). Because microorganism activity is a significant part of the activated sludge process, any changes in the existing conditions may affect the process and create difficulty in treating wastewater. Any disturbance in the process may have an effect on the effluent quality of the system. The effluent quality of the system sometimes will be lower than the influent. Therefore, control of the operation and maintenance of the system and solving any flaws in the sludge of wastewater treatment plants are very significant. The most common problems in the operation of an activated sludge system include sludge bulking, foaming, and sludge rising, which is caused by Nocardia bacteria. These problems are the result of changes in the microbial ecology of the reactor (Davis 2010). In about 40% of wastewater treatment plants, bulking and foaming was observed (Takdastan & Azimi 2009). The research showed that bulking is a major and considerable problem in wastewater treatment plants in South Africa, and 78% of existing activated sludge systems in the country encounter bulking and foaming problems (Ekama et al. 1985).

General statistics regarding foaming and bulking indicate that in large wastewater treatment plants, especially industrial wastewater treatment plants, growth of filamentous bacteria has caused bulking, and in small wastewater treatment plants, foaming is a major problem; however, this is a general conclusion and the main and principal reasons are not given (Ekama et al. 1985). Another problem in the activated sludge process that gradually stops biological treatment is the presence of foam on the surface of aeration and settling tanks. Creating foaming phenomena on the surface of settling or aeration tanks may be caused by several things. Table 1 shows a summary of the types and the causes for foaming.

These phenomena may be identified by color, concentration, and the amount of foam. Foaming phenomena was first reported in 1969; however, now, it is a prevalent problem in most activated sludge systems (Hug 2006).

The foam in an activated sludge system, which is observed on the surface of sedimentation tanks and aeration tanks, is formed with by floating biomass. In addition, the foam may be produced in anaerobic digesters (Hug 2006). Foaming is the distribution of gas bubbles in the liquid. The gas bubbles are necessary to establish the foam. Air in the aeration unit, nitrogen gas that is produced during denitrification in anaerobic conditions, methane and carbon dioxide that is produced in anaerobic digesters may cause foaming in units (Hug 2006). In a foam column (the part of the wastewater with foam), several different transitional structures may occur at different heights throughout the column length. Near the surface, high gas content structure is usually produced with the lowest gas content, the structure near the base of the column and the overall foam density decreasing with the height of the column (Pugh 2005). In the aeration basin, the foam usually consists of relatively large bubbles covered with sludge (bubble size 1–10 cm ( while in anoxic or anaerobic reactors or on secondary clarifiers stable layers with small bubbles and high solids concentration occur (Hug 2006). Most maintenance and operation personnel think that foam on the surface of an aeration basin means that there is Nocardia and Microthrix present; however, creating different types of foam are not only due to Nocardia and Microthrix. Some foams may contain too many solids while others do not, and some of them create low dissolved oxygen (DO) (Re & Young 2010). Normal activated sludge will have a light tan color and cover anywhere from 10 to 25% of the surface of the tank without any disruption occurring in the operation of the activated sludge system (Re & Young 2010). Low pH leads to the growth of fungi in activated sludge that will follow the dominant filamentous bacteria (Davis 2010). The filamentous bacteria are the cause of foaming problems. The stiff white foam gradually with production of activated biomass in the system will be removed, if other conditions such as pH and DO are controlled (Re & Young 2010).

Hence, predominance of one type of foam over another will depend on environmental conditions in the activated sludge plants. Therefore, it may be possible to improve this situation by appropriate operation of wastewater treatment plants and controlling environmental conditions (Ekama et al. 1985). The presence of foam in wastewater treatment plants decreases the efficiency of the treatment process, increases the solids in effluent, and results in pollution of an acceptable source of water. Large amounts of foam can result in a multitude of operational problems, including a reduction in plant performance, physical hazard to operators from exposure to pathogens and walkway obstruction, blockage of pipes, a reduction in oxygen transfer, freezing and possibly damaged mechanical equipment (Hug 2006; Fryer et al. 2011). A considerable fraction of the active biomass can be trapped in thick foam layers and therefore be excluded from the intended biological processes. This may limit the plant performance and makes controlling of solids and sludge retention time (SRT) more difficult (Hug 2006; Fryer et al. 2011). In the food industry, proteins, lipids, and surfactants play an important role in stabilizing the foam. Foaming in wastewater treatment plants should be stable and contain high concentrations of dissolved solids (Hug 2006). Using polyaluminum chloride (PAC) in foaming control due to filamentous bacteria is effective without any disruption in chemical oxygen demand (COD) removal and nitrification; however, the mechanisms for controlling the foam are not yet clear (Roles et al. 2002). Reducing the sludge age was effective in controlling foam caused by filamentous bacteria (Noutsopoulos et al. 2006). However, this method in wastewater treatment plants with nitrification would not be appropriate, because decreasing the sludge age removes both filamentous bacteria and the nitrifies (Mamais et al. 2011). The survey team at La Trobe University in 1997 studied foaming and the growth of bacteria that cause foaming. According to their research, bacteria that cause foaming tend to attach on the surface of oil and fat and use this as the substrate; therefore, when the available substrate are low, this bacteria will be growing very fast since these bacteria degrade oil and fat. To remove the foam in wastewater treatment plants, mechanical methods were recommended by some researchers (Griffiths & Stratton 2010). Inorganic coagulant and organic polymers, such as ferric chloride, ferrous chloride, PAC, hydrated aluminum sulfate, cationic polymers in controlling foaming of the activated sludge system were effective during 14 months. PAC and cationic polymers were used to produce dense sludge and resulting in the potential of foaming being reduced by 75–100% (Mamais et al. 2011).

The intention of this study was to find a practical and proper solution for removal of foam on the surface of aeration tanks and clarifiers, and also to increase the efficiency of an industrial wastewater treatment plant, which is located in Tehran.

The wastewater treatment plant of a fish-canning factory, which is located in Tehran, encountered the problem of foaming. The capacity of the treatment plant was 190 m³/day; however, the influent declined to 95 m³/day due to the foaming problem of the wastewater treatment. As described in Figure 1, first, wastewater passes through a screen and then enters into the chemical sedimentation tank. In the chemical sedimentation tank, 71 kg/day alum is injected to precipitate Total Suspended Solid (TSS) and decreasing biochemical oxygen demand (BOD) and COD. Afterwards, the wastewater from the chemical sedimentation tank flows to a chain of three biological treatment plants.

Table 2 represents the characteristics of the wastewater treatment plant, including volumes, hydraulic retention times, overflow rates, dimensions of the aeration tanks, and clarifiers, which are shown in Figure 1.

Figure 2 presents the surface of one of the aeration tanks before foam is created. Since in this wastewater treatment plant, the fish, which used in the manufacture of fish canning, strongly influence the quality of effluent, the lack of suitable control of the chemical or biological parameters might cause foaming problems in the wastewater treatment plant. Figures 3 and 4 show stable and chocolate color, thick foaming problem in surface of the aeration tank and clarify tank of this wastewater treatment plant, respectively.

Table 3 illustrates the chemical characteristics of the wastewater treatment plant in different stages of the treatment 1 month before the foaming problem. The results are the mean of a triplicate sample collection. As indicated in Table 2, the chemical characteristics of the effluent of stage 1 were acceptable; however, the COD, BOD, and TSS of effluent in stage 2 and stage 3 increased compared to stage 1. The primary design and operation of the treatment process before our adjustment in this wastewater treatment plant indicated some drawbacks in the process, such as sludge putrefaction, inappropriate pH, and disturbance in the treatment procedure. In addition, the amount of return sludge to stage 2 was high and using stage 3 was not necessary for treatment. The operation of the wastewater treatment was weak. Therefore, the amount of COD in stage 2 and stage 3 has increased.

Effective universal strategies are not available for controlling the wastewater treatment plant. Specific measures need to be carried out according to the causes of foaming including the organisms involved and operational conditions. To improve the efficiency of the treatment plant and remove the foaming problem, in the first step pH was controlled and adjusted to between 6.5 and 8.5 in the system by adding lime. For coagulating the sludge and suspended solid in wastewater, adding 150–500 mg/L lime is needed (Eckenfelder 2003). Therefore, to remove foam on the surface of the tanks 47.5 kg of lime per day (500 mg/L) was added. The addition of oxidizing agents like chlorine will kill filamentous bacteria (chlorine also kills other bacteria) (Jenkins 2004; Rossetti et al. 2005; Hug 2006). Therefore, the addition of lime was preferred.

For removing foam to improve the efficiency of the treatment plant the flow rate of return sludge in the second stage was decreased by about 20%. In addition, return sludge to aeration tanks was received from the sludge collection tank instead of the sedimentation tank (clarifier) to prevent adding putrefied sludge to the aeration tank until there was an improvement in the conditions in the wastewater treatment plant. The aeration tank of the third stage was removed from the wastewater treatment system; it was used as a final clarifier, and the interval of discharging sludge in this unit was reduced to prevent sludge from putrefaction. To remove microorganisms that are producing foaming, foam created on the surface of the sedimentation tank was collected manually.

Stable and thick light brown foam on the surface of the aeration and the sedimentation tank was a serious problem in this wastewater treatment plant. Table 4 indicates the wastewater quality when the foam was observed on 16 December 2012. The results are the average sample measurements during a month after the foaming problem. At this time, the DO and BOD to nitrogen ratio were measured and found to be in the normal range; therefore, the pH reduction and inappropriate operation were the reasons for foaming.

The third stage of the aeration did not induce any changes in the status of the wastewater quality; therefore, the third stage was removed from the treatment plant system. It was used only as a final sedimentation (clarifier) basin. To control pH, 47.5 kg lime per day was added to the chemical precipitation unit, because when pH become lower than 6, the growth of fungi and filamentous bacteria may increase (Davis 2010). During 14 days of adding lime and adjusting pH, the amount of pH and COD was monitored on 18 December 2012 to 02 January 2013 (Table 5).

As indicated in Tables 4 and 5, COD in the wastewater after aeration in the second stage was increased instead of decreasing. Aeration in the second stage due to the relatively good quality of wastewater needed less biological treatment so the rate of return sludge reduced about 20%. In addition, as indicated in Tables 4 and 5, the COD of effluent in the second stage was increased; this might be due to the high rate of return sludge to the aeration tank of stage 2 and returning putrefied sludge to the aeration tanks. To solve this problem and to prevent putrefaction, sludge of this unit was collected and discharged in fewer intervals.

Additionally, foaming on the surface of the aeration tank in the second stage was more severe. The brown foam on the surface of tanks may be due to the high SRT of sludge (Re & Young 2010). According to visiting the treatment plant, the reason for this increase was an imbalance between the amount of return sludge and effluent flow. The rate of return sludge flow was 100%. Therefore, the large amount of sludge in the second and third stages can reduce the quality of wastewater. To reduce SRT and improving the efficiency of the wastewater treatment plant the settled sludge in stage 1 and stage 2 was transmitted to the sludge collection unit and then returned to the aeration tank rather than being directly transmitted from the sedimentation tank to the aeration tank. Therefore, this method prevents transmission of the old sludge from the sedimentation tank to aeration tank.

As indicated in Table 5, adjusting pH improves effluent quality; however, it was not in the range of the discharge limit of the Department of Environment (Iran) yet. Light brown foam was observed on the surface of the aeration and sedimentation tanks. This indicated that filamentous bacteria was not removed from the system, and conditions for their growth were available. To control the growth of fungi and filamentous bacteria, pH in the sedimentation tank is held above 6.5 and to adjust it lime was used. For rapid removal of microorganisms that caused this problem, the foam on the surface of the sedimentation tank was collected manually. After applying these strategies, foaming on the surface of the aeration and sedimentation tanks was removed. The procedure we applied in the wastewater treatment plant during 18 days caused the decline of COD in effluent from 500 to 65 mg/L, which indicated about an 87% reduction in COD. As presented in Table 6, the wastewater treatment plant operated well after adjusting the pH by using lime and decreasing the rate of returning sludge flow rate by about 20%.

Therefore, the condition of this wastewater treatment plant was improved by adding 500 mg/L (47.5 kg/day) lime to the chemical sedimentation tank. For removal of foaming and adjusting pH to between 6.5 and 8.5, lime was added. The efficiency of the treatment processes also increased, and the COD of effluent went from 500 to 65 mg/L in 18 days. In other studies, researchers have used organic polymers and inorganic coagulants such as ferric chloride, ferrous chloride, PAC, hydrated aluminum sulfate, and cationic polymers for removal of foaming; however, they are very expensive and they are not available in some countries (Roles et al. 2002; Mamais et al. 2011). The advantages of using lime instead of organic polymers and inorganic coagulants are the cost of the lime, its effectiveness, availability and ability to increase the efficiency of the treatment process related to the afore-mentioned materials. Lime has often been used for chemical precipitation, sludge conditioning, and sludge stabilization (Turovskiy & Mathai 2006); however, in this research, we used lime for removal of foaming, adjusting pH, and solving the putrefaction of sludge.

We estimated the operational costs of lime for foaming control and compared it to other coagulants used by Mamais et al. (2011) (the operational costs were calculated for 95 m³/day of wastewater treated). Table 7 indicates the comparison between operational cost of lime and other coagulants.

As presented in Table 7, the operational costs of lime are less than other coagulants. We also used lime to remove foaming and bulking in another aerobic activated sludge wastewater treatment package on a construction site (Qom Monorail project) in January 2014. By using lime for 10 days, foaming and bulking was removed and the COD of effluent went from 380 to 55 mg/L. Therefore, using lime instead of other coagulants is recommended in this paper. Using lime has some advantages and disadvantages, which are as follows:

Advantages of using lime

1. simple operation;

2. low cost;

3. availability;

4. effective for removal of foaming;

5. appropriate for biological treatment;

6. increasing the pH (appropriate for systems with low pH) (Turovskiy & Mathai 2006).

Disadvantages of using lime

1. produces large volume of sludge (Turovskiy & Mathai 2006);

2. not suitable for systems with high pH;

3. hard dewatering of sludge with lime (Turovskiy & Mathai 2006).

The following main conclusions are drawn:

1. The SRT in this plant might be so high because of the high detention time of sludge in the sedimentation tank and the lack of discharge of excess sludge. Hence, stable, thick, brown foam was observed in the sedimentation tank. Transmission of sludge into the sludge collection unit and then return to aeration tanks removed filamentous bacteria and foaming from the system.

2. To deal with the foam of filamentous bacteria (Nocardia) the pH was decreased to the minimum amount. To control foaming, pH range was 6.5–8.5 by adding 47.5 kg per day. The results showed that by increasing the pH to 6.5, the foaming problem is gradually solved, and the efficiency of the plant is improved.

3. By adding 500 mg/L (47.5 kg/day) of lime into the chemical sedimentation tank, the foam was removed from the wastewater treatment plant. Therefore, using lime to remove foam may be a practical method, which can be used by operators, and it can be used in other wastewater treatment plants without any disturbance to the biological process.

4. To improve the efficiency of treatment in the second stage and remove foam on the surface of the units, control of return sludge rates according to wastewater quality were effective. Due to the relatively good quality of the raw wastewater in the second stage, the rate of return sludge in the aeration tank of the second stage was reduced by about 20%; therefore, during 18 days, the COD of effluent declined from 500 to 65 mg/L, which describes about an 87% reduction in COD.

5. The estimated costs of using lime for removal of foaming were about 0.0029 euros/day (0.004 dollars/day) for treatment of the wastewater. Application of lime to remove foaming in activated sludge treatment plants had lower costs compared to other coagulants.