THICKENING ACTIVATED SLUDGE WITH SUSPENDED AIR® FLOTATION (SAF®)

Summary of three case studies

Harold Leverenz, Ph.D., P.E.1*, George Tchobanoglous, Ph.D., P.E., NAE 2, and Christina M. Skalko, P.E.3

¹ Research Engineer, Department of Civil and Environmental Engineering, University of California, Davis, Davis, CA,

² Professor Emeritus, Department of Civil and Environmental Engineering, University of California, Davis, Davis, CA

³ Project Manager, Short Elliott Hendrickson, Inc.

* Corresponding author

ABSTRACT

In the practice of wastewater treatment system design, process selection is often constrained by factors such as operational cost, performance, and physical footprint. A new process for the clarification and thickening of waste activated sludge (WAS) incorporating colloidal gaseous aphron (CGA) technology has proven to be highly effective with a small footprint. Technically, an aphron is defined as a gas or liquid phase encapsulated by a surfactant film. Since their initial identification and formulation, aphrons have been used extensively in a number of chemical process and gas and oil drilling applications. The generation and use of CGAs for thickening of waste activated sludge (WAS) and other wastewater applications has been pioneered with the development of the Suspended Air® Flotation (SAF®) process. The purpose of this paper is to: (1) provide background on CGAs flotation technology, (2) identify applications of CGA in wastewater treatment, (3) discuss thickening of WAS with CGA, and (4) present findings from three case studies where a legacy DAF process for WAS thickening was replaced with a SAF® process to increase capacity or address operational challenges, typically within the same flotation tank footprint. The case studies demonstrate the versatility of the SAF® process for (a) ability to process the most challenging feedstock, including stored WAS; (2) enhancement of the digestion process and elimination of digester foaming; and (3) high capacity and ease of operation, reducing operation needs.

Key Words

Colloidal gas aphrons, waste activated sludge, wastewater solids management

1.    CGA FLOTATION TECHNOLOGY

Current aphron technology is based primarily on the research of Professor Frank Sebba who worked on formation of fine bubble aphrons under atmospheric conditions (Sebba, 1971,1987). Technically, an aphron is defined as a gas or liquid phase encapsulated by a surfactant film. Fine bubble aphrons were first identified as micro foams, but later the name gas phase aphron was changed to colloidal gas aphron (CGA). Once the CGA suspension is formed, it is stable and applied to water at concentrations ranging typically from 0.5 to 1.5 mg/L. One of the most striking observations was that the micron sized CGA bubbles do not coalesce if they are immersed completely in water and that when they collided, the momentum is not sufficient to break the encapsulating surfactant film. It was also observed that the resulting concentration of surfactant is not sufficient to generate foam. Based on these properties, CGAs have wide application in the chemical process field as well as in gas and oil drilling applications. Additional details on these applications may be found in Molaei and Waters (2015) and Sebba (1985). The current full-scale implementation of the aphron technology for wastewater applications, developed by Heron Innovators, is known as the Suspended Air® Flotation (SAF®) process. Wastewater applications of the SAF® Process are considered in the following section.

2.    APPLICATION OF THE SAF® PROCESS IN WASTEWATER TREATMENT 

To date, the principal applications of the SAF® process have been for the removal of algae from wastewater treatment pond effluents and for sludge thickening, the subject of this paper. In the treatment of pond effluent, it has been possible to achieve effluent turbidity values of 0.5 NTU or less. The SAF® process is being used in a variety of other wastewater applications, including primary treatment, filtration of filter backwash, and secondary clarification, especially in industrial applications (Tchobanoglous and Leverenz, 2024). The principal components of the SAF® process are illustrated on Figure 1. The SAF® process uses an externally generated suspension of micron-sized (about 7 to 25 µm) air bubbles. Each of the bubbles is coated with a thin film of an electrically charged chemically active surfactant such as a soap film, either anionic, cationic, or nonionic, depending on the application. In water and wastewater applications it has been possible to achieve 40 to 50 percent volumetric air content. Because the CGA bubbles are charged, they are attracted readily to oppositely charged wastewater solids to be removed. Operationally, polymer and aphrons are injected into the influent, upstream of a mixer, before entering the flocculation portion of the flotation tank (see Figure 1). The flocculated solids then move to the flotation portion of the tank where they float to the surface and are removed by skimming. Clarified underflow passes under a baffle and over a weir into the effluent launder.

Figure 1
The schematic flow diagram of the Suspended Air® Flotation (SAF®) process. Two modes of operation are possible, depending on the characteristics of the wastewater solids to be thickened: (1) without chemical flocculation, solid line, and (2) with chemical flocculation, dashed line.

3.    THICKENING WASTE ACTIVATED SLUDGE WITH CGA 

In typical activated sludge treatment process flow diagrams, as shown on Figure 2, WAS thickening can be used: (1) to thicken waste mixed liquor (see Figure 2a), where the solids retention time method is used to control the process, or (2) alternatively, settled activated sludge can be thickened (see Figure 2b). In either case, the rationale for thickening is to increase the solids concentration of the WAS, by reducing the liquid volume. Where anaerobic digestion is used, thickening of WAS is used: (1) to improve process performance, typically gas production, by increasing the solids retention time and (2) to enhance the digestion process stability by reducing the liquid volume of the digester feed. Several alternative thickening processes are used to thicken WAS including gravity thickening, belt filter, and dissolved air flotation. A comparison of WAS thickening operations with both DAF and SAF® in three case studies is presented in the following section.

Figure 2
Alternative configurations for secondary sludge wasting: (a) from aeration tank and (b) from secondary clarifier. Note that the primary clarifier may not be present in some cases. Adapted from Tchobanoglous et al., 2014.

While the flow diagram for the SAF® process is similar to that for dissolved air flotation (DAF), there are three fundamental differences between the SAF® technology and the DAF process: (1) aphrons are formed at atmospheric pressure whereas in the DAF process air is dissolved into the liquid under pressure; (2) aphrons can be encapsulated in variety of different films, either anionic, cationic, or nonionic; and (3) dissolved gas (air) bubbles which form once the pressure is released in the DAF process have a tendency to coalesce when they touch because the surface area of two small bubbles, when combined, is less than the surface area for the two smaller bubbles when separated. By comparison, aphron bubbles do not coalesce, an important difference.

There are also several important operational differences that have been observed, from the case studies discussed in the following section, between the aphron technology and the DAF process. Important operational differences include: (1) increased solids loadings, up to 40 lb/ft2•h, a ten-fold increase over DAF in most applications, (2) ability to handle high and variable concentrations of total suspended solids (TSS) up to about 1.6% TS is one example, (3) easier to operate with respect to establishing the operational set point as compared to the DAF process; (4) reduced process operational time in some cases; (5) lower energy consumption, by 90% in one example, in part due to reduced operational time to process solids ; and (6) potential for reduced process footprint. These features are highlighted in the following case studies.

4.  CASE STUDIES 

The SAF® process has been evaluated for thickening of WAS in numerous installations and in all cases the SAF® process has been demonstrated to provide consistent high performance and reliable operation. Comparison of the SAF® and DAF process for WAS thickening is best exemplified by three recent case studies (Bauer, 2024; Krauss, 2024; Zeller, 2023), presented in this section, where the SAF® process has been used to replace and upgrade an existing DAF facility. The use of CGA has allowed for significant improvements in TSS removal, capacity, operational time, operational cost, and solids management.

Topeka, KS (Oakland Wastewater Treatment Facility)

A recent case study on WAS thickening at Topeka, KS, was summarized by Tchobanoglous et al. (2022). The total solids content of WAS at the Oakland WWTP varies considerably, depending on the operation of biological reactors and clarifiers. The SAF® process was evaluated at the Oakland WWTP to replace legacy DAF units for thickening prior to anaerobic digestion to increase the solids content of WAS. For example, WAS pumped from secondary settling tanks with a content of 0.4 to 0.8 % solids was thickened to a content of 4% solids, resulting in a five-fold decrease in sludge volume and corresponding increase in digester retention time. In addition to achieving high WAS solids separation efficiency, based on onsite testing, the principal advantages of the SAF® process include relatively small process footprint, low power requirement, low chemical usage, and the ability to handle both aged/stored sludge and high concentrations of suspended solids (up to 16,000 mg/L in this study).

Performance data for the SAF® pilot testing with a mixture of stored WAS mixed with settled WAS are summarized in Table 1. Performance data were also collected during July/August and September/October 2021 operational periods (see Tchobanoglous et al., 2022). For the entire period of operation, the removal performance was essentially the same, however, the chemical usage was variable due to the age and properties of the stored WAS. It was noted that the SAF® process is able to operate under a wide range of operational conditions, with influent total suspended solids (TSS) ranging from about 5,000 to 16,000 mg/L and process loading varying from 13 to 46 lb/ft2·h for both settled and stored WAS. Typical solids loadings for the existing DAF units, when operational, were about 1 to 2 lb/ft2·h.

Energy consumption for WAS processing was also compared for the two flotation technologies. For the same amount of WAS processed per day, the average energy required for the SAF® was 4.9 kWh/d while corresponding energy usage for the DAF equipment was 49.9 kWh/d, a 90% reduction. It is anticipated that the energy usage for newer DAF units would be lower. The aphron flowrate used in Table 1 is the volumetric flowrate of the CGA suspension.

Table 1

Summary of representative pilot test performance for SAF® operating on WAS combined with stored WAS (Pilot test, October 2020) at the Oakland WWTP in Topeka Kansas.^a

^a Data courtesy Zeller (2023).

For the pilot test, aphrons were generated using a surfactant solution which was being consumed at a rate of approximately 90 gal/month for processing stored WAS with an estimated age greater than 12 months. The consumption of the surfactant was set at a high level due to the challenging nature of processing the stored WAS. Even under the conditions of high CGA usage and aged stored sludge, no residual foaming or digestion issues were observed.

Warminster, PA (Warminster Municipal Authority) 

The Warminster case study highlights the potential impacts of improved thickening process performance on downstream solids processing. Prior to installation of the SAF® process at the Warminster facility, a legacy DAF process had been used for thickening of WAS prior to anaerobic digestion. The DAF process typically produced thickened solids in the range of 3.5 to 4.5%. The low concentration of solids in the thickened WAS directly impacted the solids retention times (SRT) in the anaerobic digesters. Also, during operation with DAF, periodic digester foaming events occurred due in part to mixing limitations within the existing digesters and the SRT variations resulting from thickening process control. Digester foaming is commonly associated with anaerobic process overloading and imbalances from operation at reduced or fluctuating SRT.

In mid-June 2023, the DAF process was replaced with a SAF® unit. After the SAF® was placed into service as a direct replacement for the DAF, thickened solids TS increased to 4.5 to 5.5%, resulting in a significant improvement in stability of the anaerobic digestion process (see Figure 3). The thickened solids concentration achieved by the SAF® process improved the digester SRT resulting in a reduction in an increase in digester alkalinity. It was noted that since the SAF® went into service, foaming has not been observed in the digesters. Foaming is not expected as the surfactant used is biodegradable and not present at concentrations that can contribute to any residual foaming following the flotation process. 

The SAF® process has also allowed the operators to navigate challenging operational conditions imposed by historic drought conditions. Low influent flows resulting from lack of normal rainfall levels has required the facility to take clarifiers out of service.

Operating with a single clarifier requires increased WAS flows and lower WAS solids concentrations. The SAF® process has the flexibility to accommodate a wide range of feed conditions making it possible to operate reliably through changes in plant influent. The SAF® process has also been effective in managing upset conditions associated with other elements of the facility. The ability to achieve high performance under a wide range of feed sludge conditions has made the SAF® an indispensable tool for managing the anaerobic digestion process.

Sauk Centre, MN (Sauk Centre Wastewater Treatment Facility)

The WWTF at Sauk Centre receives primarily domestic wastewater and has a typical flowrate of 0.43 Mgal/d. The WWTF consists of fine screen (Huber), grit removal (Pista), activated sludge process (Short Elliott Hendrickson, Inc), UV disinfection (Trojan Technologies), and WAS thickening (Heron). Thickened waste solids are stored in a

1.2 Mgal vessel for seasonal land application.

The SAF® process was brought in to replace an overloaded DAF process. Considerations for the DAF replacement included:

• Meet or exceed capacity needs, equivalent to 96,000 gal/d
• Fit within existing DAF footprint, existing tank 60 ft2
• Equivalent or lower operational cost (polymer, operator time, and electricity)

The SAF® process was implemented and began operation in July 2023 and the results to date have been impressive, as summarized in Table 2. Following conversion of the DAF tank to the SAF® process, thickening process operational time has been reduced by 80% from 24 h/d x 7 d/wk to 7 h/d x 4.5 d/wk. In addition, the volume of biosolids produced was reduced by 15% in the first partial year after the SAF® process began operation, increasing the storage capacity and reducing the overall volume for hauling and land application.

The SAF® is noticeably easier to operate than the old DAF according to the operators in Sauk Centre. The old DAF startup time was in excess of 30 minutes each day. The SAF® starts up each morning with the push of a button on the Control Panel, saving the operators time every day for other duties at the WWTF. In addition, the room with the SAF® is much quieter than the DAF. The loud DAF pump that was used to inject air for the DAF was deleted and replaced with the SAF® Froth Metering equipment (SAF® Generator), allowing the operators to work in the space without the noise nuisance.

Table 2

Comparison of DAF and SAF® processes at Sauk Centre Wastewater Treatment Facility^a

^a Data Courtesy Bauer (2024)

The polymer usage has also greatly reduced from the conversion from the DAF to the SAF® at Sauk Centre. The DAF used 10.5 gallons/week and the SAF® uses 4.6 gallons/week. The operators are working on further reducing the polymer but are currently limited by their polymer pump with hopes to replace this in the future. A formal analysis of the electrical cost savings has not yet been performed in Sauk Centre. The reduced hours of usage alone between the DAF and SAF® every day mathematically results in a lower electric usage. The DAF 5 HP air pump was replaced with the SAF® Froth Generator recycle pump of 3 HP and some other electrical components under 1 HP. The overall solids capture efficiency was measured to be 99.7%.

5.    DISCUSSION

The thickening of WAS, discussed in this paper, is a relatively new application for the SAF® technology. The performance of the SAF® process for the thickening of WAS has been evaluated at several facilities where the process was installed as a replacement for existing DAF units that have exceeded their design life or not meeting performance expectations. The SAF® units have proven to be effective for thickening both stored and fresh WAS, adapt to a wide range of WAS concentrations and flowrates, and improve the operation of downstream digesters. Switching from legacy DAF to SAF® resulted in stabilization of the digestion process, which previously experienced upset conditions and foaming due to poor WAS thickening. The fact that the bubbles do not coalesce during the flotation reaction is a key reason that the SAF® process is effective. The bubbles remain in suspension for an extended period, resulting in a greater number of adsorption sites and creating more opportunities for particle adsorption and flotation. The advantages for the SAF® process that have been documented by others (Zeller, 2023; Bauer, 2024; Krauss, 2024) include rapid startup, variable load processing, reduced operating time, and reduced energy requirements, as compared to the conventional DAF process.

6.    SUMMARY 

Based on the test results obtained to date, the SAF® process is a viable alternative to replace legacy thickening processes, typically achieving improved performance and capacity within the same footprint. Because of the documented effectiveness of the SAF® process, as described in this paper and in other studies, it is anticipated that flotation with CGA technology will find broader application in the field of wastewater management and result in enhanced operations, reduced energy consumption, and reduced personnel requirements.

ACKNOWLEDGMENTS 

The authors would like to acknowledge the following individuals who contributed the case study data referenced in this article: Dan Zeller (Oakland WWTF, Topeka, KS), Glen Bauer (Sauk Centre WWTF, Sauk Centre, MN), and Ron Krauss (Warminster Municipal Authority, Warminster, PA).

REFERENCES

Bauer, G. (2024) Personal communication, Wastewater Supervisor at Sauk Centre Wastewater Treatment Facility, Sauk Centre, MN.

Krauss, R. (2024) Personal communication, Sewer System Superintendent, Warminster Municipal Authority, Warminster, PA.

Molaei, A. and K.E. Waters (2015) “Aphron Applications—A Review of Recent and Current Research,” Advances in Colloid and interface Science, 216, 2, 36-54.

Sebba, F. (1971) “Microfoams—An Unexploited Colloid System,” J. Coll. and Interface Sci., 35, 4, 643 646.

Sebba, F. (1985) “Separations Using Aphrons,” Separation and Purification Methods, 14, 1, 127-148. 

Sebba, F. (1987) Foams and Biliquid Foams – Aphrons, John Wiley and Sons, New York. 

Tchobanoglous, G., H.D. Stensel, R. Tsuchihashi, and F.L. Burton (2014) Wastewater Engineering: Treatment and Resource Recovery, 5th ed., Metcalf and Eddy I AECOM, McGraw-Hill Book Company, New York.

Tchobanoglous, G., and H. Leverenz (2024) “Versatility of the SAF® Process in Municipal Wastewater Treatment Systems,” (in preparation). 

Tchobanoglous, G., H. Leverenz, and D. Zeller (2022) “Application of the Suspended Air® Flotation (SAF®) Process for Thickening of Waste Activated Sludge (WAS),” Water Environment and Technology, 6, 36-41.

Zeller, D. (2023) Personal communication, Operations General Manager, Water Pollution Control Division, City of Topeka, KS.