IFAS systems add the benefits of Fixed Film systems into the suspended growth Activated Sludge process. Activated Sludge has process flexibility and provides a high degree of treatment. Fixed Film processes are inherently stable and resistant to organic and hydraulic shock loadings. Placing Fixed Film media into Activated Sludge basins combines the advantages of both of these systems.
The
additional biomass provided by placing Fixed Film media directly into
the suspended growth reactor does not increase clarifier solids loading
(a factor that often limits the treatment capacity of existing Activated
Sludge systems).
IFAS technology addresses the need for increasing Activated Sludge plant
capacity, with little or no added tankage, because of the additional fixed
biomass. The fixed biomass also contributes to the ability of the process
to respond to organic or hydraulic shock loads and to recover from upsets.
There are several types of media used to fix the biomass in the Activated Sludge basin. They include “Dispersed Media” entrapped in the aeration basin, and “Fixed Media,” such as structured sheet media or knitted fabric media, that is placed in the aeration basin.
The use of Submerged Fixed Film in the biological treatment of wastewater has been in practice for well over 60 years. Early work included the “Contact Aeration” process used in the 1930’s and 1940’s. In those days, asbestos panels were vertically suspended over a perforated pipe aeration grid. The process was staged with intermediate clarifiers, had no return sludge capability, and the total Hydraulic Residence Time (HRT) was typically 1.7 to 3 hours.
This process was stable and responded well to load fluctuations without significant operator attention. However, with no Return Activated Sludge (RAS) provision, it lacked the range of control of Activated Sludge. Also, the fixed panels did not facilitate oxygen diffusion, good mixing, or energy efficiency. Eventually, this concept gave way to current Activated Sludge practices.
In
the following decades, hundreds of installations employing Submerged Fixed
Film were introduced internationally, although relatively little work
was completed in the US. The small footprint and ease-of-operation of
Submerged Fixed Film systems were the primary benefits driving the use
of this technology.
In the 80’s and 90’s, work began in the US on the integration of Fixed Film and Activated Sludge technologies. And because of today’s increasingly stringent effluent requirements, high tankage expansion costs, and reduced funding options, increased attention is focusing on IFAS technology solutions, both in the US and internationally.
Several types of media available for IFAS systems fall into two categories:
Fixed Media IFAS Systems
Fabric media is available in a web configuration (AccuWeb)
or in a rope-like material. These flexible materials are typically attached
to rigid frames or assembled into modules that are placed within the activated
sludge tank.
PVC sheet media, commonly used in trickling filters, may be supported in frames within an aerated tank.
Dispersed Media IFAS Systems
Dispersed systems may use porous sponges or plastic finned-cylinder shapes
that are suspended or float (depending upon material density) in the activated
sludge tank.
| TYPES OF IFAS MEDIA | |||
| FIXED-IN-PLACE TYPES | ADVANTAGES | DRAWBACKS | |
 |
Fabric Web-type (AccuWeb) |
• Simple to install |
• May foul if rag removal is inadequate |
 |
Rope-type | • Rapid upgrade • No material losses |
• Material breakage and entanglement • Field assembly needed • May foul if rag removal is inadequate |
 |
PVC Sheet Media (Trickling Filter Media) |
• Rapid upgrade • No material losses |
• Structured media may impede mixing • May foul if rag removal is inadequate • Potential plugging from excess biomass |
| DISPERSED TYPES | ADVANTAGES | DRAWBACKS | |
 |
Polypropylene Finned Cylinders |
• Excellent
mixing • May eliminate RAS |
• Media losses (washout or abrasion) • Aeration devices and screens may foul |
 |
Sponges | ||
| APPLICATIONS OF IFAS SYSTEMS | |
| PROCESS | DESIGN CONSIDERATIONS |
| New
Plant Construction |
• Design tank and aeration
system geometry to incorporate fixed or dispersed media |
| Existing
Plant Retrofits |
• Add
pre-fabricated fixed modules to aeration tanks. • Evaluate aeration for increased BOD removal and biomass respiration. • Add equipment for dispersed media reactor, if selected. |
| Nitrification/ Denitrification Conversions |
• Add
biomass to increase Solids Retention Time (SRT) to value needed for
nitrification and convert to appropriate BNR process. • Add modules to provide more biomass without additional tankage. |
Biomass Effective Area
The enhanced treatment provided by IFAS is related to both the amount
of biomass growth on the media surface and the activity of the biomass.
While it might seem that the amount of biomass should be directly proportional
to the measured surface area of the media, this approach can be very misleading.
It is the effective area that is most important.
In dispersed systems, for example, overgrowth of the biomass on the porous media limits the diffusion of oxygen and nutrients to the bacteria. Also, abrasion from the normal tumbling action of the media in the reactor can remove the slimes from the surface, which reduces the effective area.
In fixed systems, the bacteria can grow outward from the fixed surface, thus increasing the effective area. The effectiveness of PVC sheet media in IFAS systems may be limited by mixing and the possible plugging of certain passages within the modules.
Media LocationBrentwood studies show that the location of the media within the reactor has a limited effect on the relative amount of biomass growth on the media. In one study of an aerobic tank 132 feet long x 17 ft wide x 30 ft deep, samples taken along the length of the reactor only varied by ± 10% of the average.
A simple, but conservative, method of estimating the amount of media needed is to consider the additional amount of biomass needed to achieve a required improvement in treatment by conventional means. Next, determine the amount of media necessary to support the growth of that amount of biomass.
The amount of additional biomass needed can be estimated in a conventional fashion by using either the Food:Micro-organism ratio (F/M) or Solids Retention Time (SRT). For instance, if a 20 % increase in BOD loading is desired, a 20 % increase in biomass is needed to maintain a targeted F/M. Similarly, if it is determined that a 1/3 increase in SRT is required to support nitrification, the biomass must be increased by (approximately) one-third. The resulting mass can be converted directly into media requirements.
Example:
Current Plant Performance: 2 MGD Plant; 12 hr HRT; F/M = 0.10;
BOD = 200 mg/l
Treatment Goal: Increase flow by 20% at same F/M
Design Objective: Provide sufficient media to add 20% equivalent MLSS to existing system.
Calculate Current MLSS: Current Mass (lbs) = BOD lbs/day ÷ F/M, day = 200 ppm x 2.0 mgd x 8.34 lbs/gal ÷ 0.1 = 33,360 lbs
20% Increase = 0.2 x 33,360 lbs = 6672 lbs
| EQUIVALENT MLVSSMETHOD RESULTS (TO TARGET F/M) | ||
| MEDIA TYPE | BIOMASS GROWTH | MEDIA REQUIRED |
| Sponges | 120 mg/sponge(1) | 25,000,000 sponges |
| Finned Cylinders | 6-10 kg/m3 (1) | 300-500 m3 |
| Rope | 6.6 gm/m (1) | 460,000 m |
| Fabric (AccuWeb) | 40-60 lbs/1000 ft2 (2) | 133,000 ft2 |
(2)Brentwood Industries Research
Example:
Current Plant Performance: 2 MGD Plant; 12 hr HRT; F/M = 0.10;
Influent NH -N = 20 mg/l
Treatment Goal: Remove Ammonia to 1.0 mg/l
Design Objective: Provide sufficient media to oxidize NH .
Calculate required Ammonia removal: Ammonia Removed = (20-1) ppm x 2 mgd x 3.78 l/gal = 144 kg/day
Rope Media Required = 144 kg/day ÷ 0.6 kg/1000 m/day = 240,000 m
| KINETIC METHOD RESULTS | |||
| MEDIA TYPE | SPECIFIC REMOVAL RATE | CONDITIONS | MEDIA REQUIRED |
| Sponges | .4-.8 kg/m3/day | 10-12° C | 360 m3 |
| Finned Cylinders | .4-.8 kg/m3/day | 10-12° C | 360 m3 |
| Rope | .6-.8 kg/1000 m/day | 10-12° C | 240,000 m |
| Fabric (AccuWeb) | 4.5-6 lbs/1000 ft2/day | 10-12° C | 70,400 ft2 |
Brentwood Industries can help model your process. Using the BioWin 32 model(3), existing or proposed tanks can be divided into discrete anaerobic, aerobic, and anoxic zones to achieve removal of BOD, NOD, TN and P. The effect of dissolved oxygen, tank volumes, feed and recycle flow rates on effluent parameters can be modeled.
All IFAS systems require adequate preliminary treatment design and operation. Primary clarification or fine screening will avoid ragging and material build-up on the media in the aeration basin and clogging of the dispersed media and retaining screens.
Mixing
Proper mixing is required for solids suspension, substrate transfer, and
oxygen diffusion.
Dispersed media, such as sponges and polypropylene cylinders, are suspended by the flow induced by the aeration system. Most systems require a roll pattern commonly provided by coarse bubble diffusers. The mixing should not be too vigorous or biomass could be eroded from the media.
The open configuration of fabric-type media (such as AccuWeb) allows unimpeded flow with any diffuser type and layout.
Hydraulic Profile & Volume Displacement
For dispersed systems, the volume displaced by the media can impact the
HRT and should be considered when sizing the aeration basins. Also, the
screen used to retain the media will increase head loss in the aeration
basin.
For fabric-type media designs, hydraulic profile impact and basin volume displacement are not significant.
Oxygen Transfer
Sufficient oxygen must be available to satisfy the demand of the additional
biomass and BOD oxidized. In many retrofitted plants, excess oxygen transfer
capabilities exist and require little or no modifications.
Data indicates that fixed media increases oxygen absorption efficiency by increasing bubble retention time. Nonetheless, maintaining conventionally established parameters will provide a safety factor, unless site-specific oxygen transfer testing indicates otherwise.
Equipment Access
IFAS installations should be designed to allow for removal of media for
proper maintenance of diffusers, mixers, and valves.
With dispersed media, a provision needs to be made to gather, move, and store media during basin access. Spare basins are potential sites.
Fixed media modules may be lifted out or moved aside to access the necessary equipment.
To
view a video of wheels being installed on a Brentwood AccuWeb module at
the Windsor Locks installation, click the picture. NOTE: QuickTime 5.0
or newer ir required to view this video. Download
QuickTime.
The mobility of the media in dispersed systems requires consideration. A retention screen is required to keep the media in the aeration basin. The aeration basin screens can blind with organic growth or rags, and they can also concentrate inert solids in the aeration basin. Sponges tend to compact against the screen, requiring a coarse bubble aeration system to “scrub” the sponges off the screen. Dispersed media also tends to collect in the downstream portion of the tank and must be airlifted back to the front of the reactor.
Sponges eventually build up solids and then settle, accumulating in the bottom of the basin. Periodically squeezing the sponges reduces the buildup. (One method to reduce solids buildup is to pump the sponges through a submersible pump on a periodic recycle loop.)
Sponges also exhibit material loss due to abrasion and require regular replenishment. Literature indicates a 1 to 2% per year replacement rate, with some reports indicating as much as 10% per year.
Fabric media may be subject to breakage, depending on the strength of the material. Individual strands may break between 30 lbs. and 140 lbs, depending on the specific media. AccuWeb media, for example, employs a hexagonal interlocking knit which provides a unified matrix with strengths exceeding 1000 lbs. per ft2 of media. The strand break strength may have a significant effect on frame design and handling considerations.
Elongation
Spiral-wound rope-type media may elongate and require a method for re-tensioning
on the support frames after some period of operation.
Web-type fabric media, such as AccuWeb, is self-tensioning and does not elongate over time.
Fouling
Structured sheet media systems are most effective when used with complete
diffuser coverage across the basin floor. The diffusers provide optimum
mixing and help prevent fouling and plugging.
Fabric-type media tends to move and flex with the liquid currents and will routinely shed excess biomass.
While direct economic comparison of the various IFAS processes can only be made on a case-by-case basis, some general comparisons can be made.
• In general, IFAS systems require less tankage and therefore have lower capital cost
• Modular systems allow for incremental additions of modular components, which may alleviate short- and medium-term financing requirements.
• Dispersed systems require expenditures for additional components, such as media retaining sieves, air knives, and/or pumps for sponge regeneration.
• IFAS systems require little or no additional operation costs or operator attention over conventional activated sludge.
| WWTP UPGRADE ESTIMATES (1) | ||
| METHOD | COST | COMMENTS |
| U.S. Rope Media | $1,075,000 | Includes frame and media |
| Japanese Rope Media | $1,200,000 | 273,000 meters of unassembled material |
| Sponge Media | $1,400,000 | 48 million foam pads, screen air lifts, air knife, annual foam pad replacement (± 10%), operations management |
| Rotating Biological Contactor Media |
$1,800,000 | 12 RBCs, operational costs |
| Finned Cylinder Media | $2,300,000 | 2000 m3 carrier elements, wedge wire pipe assemblies, oxygen analyzer, field service |
| Conventional | $3,500,000 | Add 25% more aeration and final clarifier, associated equipment |
IFAS
Case Study
In 1997, the City of Greensboro, NC had difficulty in consistently meeting its winter NH -N limits at its North Buffalo Plant due to loading and increasingly stringent effluent requirements. Greensboro needed to increase their biomass inventory to allow consistent nitrification without the significant capital investment of additional tankage.
As a full-scale demonstration of the capabilities of IFAS technology, they installed AccuWeb fabric media modules in one train of their four train, 16 MGD plant. Nine AccuWeb frames required a 3-man crew with a crane five days to install, without interrupting plant operations or dewatering the activated sludge basin.
According to Arthur White, Water Reclamation Manager, Greensboro Department of Water Resources, “We have consistently met our limits since the addition of the AccuWeb System. The AccuWeb train averages 24% better ammonia removal than the other three trains, even with the common clarifiers and return sludge piping. We also rebound much quicker after hydraulic surges than we did prior to AccuWeb.”
Contact Brentwood for more information on this and other AccuWeb installation results.
Log on to The Water Environment Research Federation’s website www.werf.org for their March 2000 publication entitled Investigation of Hybrid Systems for Enhanced Nutrient Control, a compilation of 15 case studies and an overview of IFAS Technology, Moving Bed Biological Reactors, and Membrane Biological Reactors.
Insufficient Biomass Inventory in the Aeration
Basin/Overloaded Clarifiers
IFAS systems can increase the effective MLSS in an aeration basin by as
much as 3000 mg/l. This additional biomass can offset the need for additional
aeration basin capacity.
Furthermore, IFAS can also be designed to specifically “off-load” clarifiers by shifting an appropriate portion of the biomass to the fixed film. This is particularly effective in applications with limitations on the clarifier solids loading, which often limits the MLSS content of the aeration basins. Additionally, this may eliminate a need for filters behind “overworked” clarifiers.
Hydraulic & Organic Shocks
Biomass populations in IFAS aeration basins are not susceptible to biomass
washouts during hydraulic surges as they are in suspended growth systems
without IFAS, because they are fixed in place. Additionally, the fixed
biomass acts as a “re-seeder” to get the system returned to
normal operations quickly after such a surge. System nitrification is
restored much quicker when a large mass of nitrifiers is retained on the
fixed film. The fixed film component also provides a degree of treatment
while the suspended biomass is rebuilding. This can mitigate or prevent
permit excursions based upon the amount of fixed film in the system.
The depth of biomass provided on fixed film also resists organic shocks better than does suspended biomass.
Process Stability
By increasing the bacterial population (Mass) with the fixed film component
for a given loading (Food), the F/M ratios are lowered. Alternatively,
loadings may be increased while maintaining F/M ratios. Typically, lower
F/M systems are more stable than higher F/M systems.
Solids settling may also improve. Reductions in Sludge Volume Index (SVI) of from 200 to 144 ml/g TSS and from 150 to 75 have been seen. Significantly, the SVI variability is also reduced with IFAS systems. This helps stabilize MLSS control. Lower SVI allows more concentrated RAS (Return Activated Sludge), reducing the return sludge flow rate, saving power, increasing hydraulic retention time in the aeraton basin, and reducing solids load on the clarifier.
IFAS system studies and reports from owners and engineers consistently demonstrate reduced sludge production. Studies indicate that reduction in sludge production or wasting rates is expected where F/M levels are reduced, or where the waste sludge solids concentration is higher.
The fixed biomass increases the SRT (sludge age), promoting nitrification over simple suspended growth systems. During cold weather and where lower compliance limits are imposed, the added biomass improves the performance of nitrifying plants, or even allows non-nitrifying plants to nitrify. Recent research indicates that autotrophic bacteria tend to grow more readily on fixed film surfaces than in a suspended growth environment.
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