Modeling Stormwater Runoff Reduction from Rainwater HarvestingIntroduction to Fixed-Film Bio-Reactors for Decentralized Wastewater Treatment

By Andrew M. Jenkins, E.I. and Darrell Sanders, P.E.

December 2012

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Clean water repeatedly has been stated as the “Oil of the 21st Century.” Subsequently, decentral-ized on-site wastewater treatment system usage has become an emerging trend in various North

American industries. This emerging trend has produced a wide variety of treatment methods and plant designs, with biological treatment as the cornerstone.

HistoryBiological wastewater treatment technology has

advanced tremendously since its early roots in the late 1800s. The introduction of synthetic media into treatment processes occurred in the 1950s, resulting in the extended use of fixed-growth concepts, such as fixed film.

Continued research and experience led to the develop-ment of high-rate systems, such as biological aerated filters, synthetic trickle-bed filters, integrated fixed film activated sludge (IFAS) systems, rotating biological contactors (RBCs), moving-bed biofilm reactors (MBBRs) and various hybrid systems that combine the advantages of both suspended-growth and fixed-film processes. Many of these innovative systems provided improved efficiencies in treatment and power consumption. The basic concept behind all of these advancements was the enhancement of the biological process, improved treatment efficiency and sustainability. (References 6)

The use of fixed-film media in aerated reactors, commonly referred to as IFAS, is an established concept with increas-ing popularity. IFAS has helped the wastewater treatment industry by increasing the treatment capacity and nutrient removal capabilities of existing activated sludge facilities.

Furthermore, both fixed and free-floating types of media systems have been developed and perfected during the past several decades. Free-floating, fixed-film systems use a sponge or plastic media. More popular is the use of plastic media systems, which typically utilize small cylindrical biofilm carrier elements. The free-floating media systems can be applied in IFAS system designs. A pure MBBR process differs from IFAS in that there is no return activated sludge, but the MBBR still is a fixed-film process. When MBBR is combined, the traditional IFAS system is considered to have enhanced properties due to increased surface area for biofilm growth.

Fixed-film vs. suspended-growth processAlthough the basic metabolic processes that biological

systems use to remove carbon and nutrients in wastewater treatment plants (WWTPs) are the same for fixed-film and suspended-growth systems, there are some inherent differ-ences that provide several advantages and some challenges for the application of fixed-film processes.

Suspended-growth systems are comprised of biological flocs; but theoretically, all dissolved wastewater substrate, made of organic matter and targeted biological nutrients, is available to all cells. With fixed-growth systems, the substrates must diffuse through the biofilm layers to become available. This transport of substrate from the bulk liquid through the stagnant boundary layer, and into the biofilm through the process of diffusion, becomes a limiting factor. End products of the metabolic reactions must diffuse in the reverse direc-tion. Thus, a cross-section through a fully-developed biofilm will exhibit varying environmental and kinetic characteristics.

Introduction to Fixed-Film Bio-Reactors for Decentralized Wastewater Treatment By Andrew M. Jenkins, E.I. and Darrell Sanders, P.E.

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Learning ObjectivesAfter reading this article you should be able to:•Understand the basic history and key differences of

fixed-film bioreactor process designs for decentralized wastewater treatment systems.

• Learn key differences with commonly-used treatment processes.

•Understand differences between free-floating and fixed-media systems.

• Possess basic understanding of fixed-film biological reac-tor processes.

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Introduction to Fixed-Film Bio-Reactors

A single biofilm may have aerobic, anoxic and anaerobic processes occurring, and the substrate that becomes limiting will change through the depth of the biofilm. With this, it is evident that fixed-film processes are quite complex to model (References 6).

Some advantages generally associated with fixed-film processes include:

•Reducedoperatingandenergycosts;•Smallerreactorvolume;•Minimizedneedforsettlingcapacity;•Operationalsimplicity;and,•Reducedsludge.

Design considerations include:

• Potential clogging of themedia system as a result of inadequate screening;•Excessivegrowth,whichcouldplugthemediasystem

or cause free-floating media to sink; and • Inadequatemixingorshort-circuiting,resulting in inef-

ficient use of the media.

Today, four fixed-film processes are commonly utilized in decentralized wastewater treatment plants. These four tech-nologies are discussed to provide history and understanding of key differences between a trickling filter, rotating biological contactor (RBC), integrated fixed-film activated sludge (IFAS) and moving-bed biofilm reactor (MBBR) process.

Trickling filterThe emergence of the trickling filter was aided by the

development of an effective means of distributing the flow to the filter media. In 1887, an experiment station was estab-lished in Lawrence, Mass., to study stream pollution and sewage treatment. Experiments conducted there formed the basis for modern artificial biological methods. Thomas Caink’s 1897 patented invention using continuous aerobic principles and Frank Pullen Candy’s 1901 rotating-arm sprin-kler invention advanced the Lawrence Experiment Station concept into a rotating-arm distribution system driven by water jets.

Around the same time, a reciprocating distributor was developed for the rectangular filters, and engine- and elec-tric-driven mechanical drives were in use by 1904. The trick-ling filter process continued to evolve during the first half of the last century.

In the late 1950s, the combined corporate efforts from Mead, Fluor and Dow Chemical resulted in the development and application of both random and bundle synthetic media. Different media configurations, producing different surface areas, continue to evolve today and enjoy widespread use in many applications. Design professionals should consider

operational issues respective to loading rates and shock loading concerns with these designs (References 6).

Rotating biological contactorsDevelopment of rotating biological contactors (RBCs)

were evolved from trickling filters and influenced by the need to reduce power consumption for wastewater treatment. In the early twentieth century, RBC concepts emerged and expanded. The 1960s and 1970s saw major RBC plant growth, but this growth became stymied by several problems, including performance that didn’t meet design expectations, excess biomass accumulations, shaft breakage, loping of disks from unbalanced biomass weight and undesirable biological growths. Today, some of these problems have been resolved and there are many systems in operation, but the acceptance of RBCs as an effective treatment process has not returned to its former level among design engineers and owners.

While the first-generation RBCs were 50 percent submerged, the 1980s saw a version that was up to 90 percent submerged with the shaft driven by air. The intent of these submerged biological contactors (SBCs) was to decrease the loading on the shafts, improve biomass control and provide an opportunity to retrofit existing activated sludge basins. The SBCs were piloted for denitrification in an anoxic reactor, but have seen only limited applica-tion. Design professionals should consider footprint, capital costs and operational and maintenance costs associated with material fatigue to the rotating shaft and mechanical assemblies (References 6).

Integrated fixed-film activated sludge (IFAS)The integrated fixed-film activated sludge (IFAS) process is

a hybrid process that combines fixed-film and conventional suspended-growth activated sludge treatment processes. The basic intent of an IFAS process is to provide additional biomass within the reactor volume of an activated sludge process for the purpose of increasing the capacity of the system or upgrading its performance. IFAS offers a practical and often cost-effective approach to upgrade treatment performance for facilities with site constraints.

Figure 1 illustrates a partially-submerged RBC.

2nd stagedrum

interstageba�e

1st stagedrum

rotation

�ow velocity

While highly flexible, the IFAS process still is subject to biomass control through a

return activated sludge (RAS) system, and can be applied to almost any type of flow schematic and reac-tor configuration. It has been used primarily in the aero-bic zones of treatment processes to enhance biochemical oxygen demand (BOD) removal and nitrification.

The IFAS process has been used in both complete-mix and plug-flow reactors, with each type of reactor having its own special design considerations based on the type of media used. Complete-mix refers to a closed system where flows are controlled by a designed detention time; while a plug-flow reactor is one based on fluid passage through a tank with equal flow entering and exiting.

There can be confusion when free-floating media, like in MBBRs, is used as the fixed film because they both use the same type of media. The key difference between the two is IFAS incorporates return activated sludge (RAS) into the process and maintains mixed-liquor concentrations that are typical of a conventional activated sludge process. The incorporation of RAS creates operational complexity as it requires operators to possess a deeper understanding of the functionality of the biological process.

Advantages of IFAS systems include:

• Ability to phase-in additional capacity or improve performance by adding more media;• Additional biomass for treatment without increasing the solids loading on final clarifiers;• Higher-ratetreatmentprocessesarepossible,thusallow- ing greater treatment in a smaller space;• Improved settling characteristics (reduced sludge volume index);• Reducedsludgeproduction;• Simultaneousnitrificationanddenitrification;and,• Improvedrecoveryfromprocessupsets.

Design considerations include:

• Potentialforodor(whentankisdewatered);• Additionaloperatingappurtenances;• Needtorelocatemedia;and,• Increased head loss associated with media-retention screens.

A variety of media systems have been used, and several types have become standard in the industry. In general, the media types may be differentiated as either fixed or free floating.

Fixed media includes media that is woven into a rope or a hexagonal pattern. The fixed media is mounted on frames and remains stationary in the activated sludge basin.

Free-floating media may consist of either cuboids of a sponge material or small plastic carrier elements resembling wagon wheels. The biomass grows on the surface, but is abraded from the outside surface of the media, leaving the active biomass on the inside of the wheel. More recently, it also has been used successfully in pre- or post-anoxic zones to enhance denitrification.

Moving-bed biofilm reactors (MBBRs)In the last 20 years, the moving-bed biofilm reactor

(MBBR) has been established as a simple-yet-robust, flex-ible and compact technology for wastewater treatment. The increase in usage by many of the industry leaders exemplifies the large gains MBBR systems have had in providing successful results for BOD, ammonia oxidation and nitrogen removal applications. These are designed to meet a wide range of effluent quality standards, including stringent nutrient limits.

MBBRs use specially-designed plastic carrier media elements for biofilm attachment that are held in suspen-sion throughout the reactor by turbulent energy imparted by aeration, liquid recirculation or mechanical mixing energy. In most applications, the reactor is filled between one-third and two-thirds full with carriers. MBBR does not incorporate a return activated sludge (RAS), but is still considered a pure fixed-film process.

These systems provide less complex operations while generating less sludge than conventional activated sludge or IFAS processes. As such, a single MBBR system as a single-pass, plug-flow design, is much simpler to operate because it requires less operator input, control and need for experience or understanding of the functionality of the biological process. The key differentiator for moving-bed technology when compared with other biofilm systems is that it combines many of the advantages of activated sludge with the advantages offered by biofilm systems,

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Introduction to Fixed-Film Bio-Reactors

Free-floating biofilm carriers, with diameters smaller than a quarter, host biomass growth in an MBBR treatment design.

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while simultaneously trying to minimize the drawbacks of each.

Like other submerged-bed biofilm processes, MBBRs help to promote a highly-specialized active biofilm that is well-suited for the particular conditions in a reactor. This highly-active specialized biomass results in high volumetric efficiencies and increased process stability, resulting in a more compact reactor.

Unlike most other submerged-bed biofilm processes, MBBR is a continuous flow-through process, eliminating the need for backwashing of the media to maintain throughput and performance; thus, headloss and operational complex-ity of the treatment step is minimized.

Moving-bed reactors can offer much of the same flexibil-ity and flow-sheet simplicity as activated sludge processes, allowing multiple reactors to be configured in a flow-through series arrangement to achieve multiple treatment objectives (i.e. BOD removal, nitrification, and pre- and post-denitrification). This occurs without the need for inter-mediate pumping. Unlike suspended-growth processes, biological performance in the MBBR does not depend on the solids separation step because most of the active biomass is retained continually in the reactor. The solids concentration leaving the reactor with the treated flow is at least an order of magnitude lower in concentration. As a result, MBBRs are compatible with a variety of different separation techniques, not just conventional clarifiers.

MBBR versatility allows the technology to be considered in a variety of different potential reactor geometries. For upgrades at existing plants, this makes MBBRs well suited for retrofit installation to existing tanks.

Most of the research and development of plastic-carrier moving-bed reactors was conducted at the Norwegian University of Science and Technology in the 1980s, motivated by an international effort to reduce point-source discharges

of nitrogen to the North Sea. Since then, numerous manufacturers have developed a variety of carrier configurations using different geometries, materials and manufacturing techniques.

The MBBR process design can utilize a single reactor or several reactors in series. When designed in series, each section of reactors can be designated for a specific treat-ment function within the greater treatment scheme. The reactors promote the development of a specialized biofilm oriented toward a treatment goal based on the conditions set within the reactor (e.g., available electron acceptor and available electron donor).

This compartmentalized approach results in a

rather simple and straightforward design, whereby one or more complete-mix reactors are oriented in series, each with a specified treatment purpose. When the biomass concentration on MBBR carriers is presented in terms of an equivalent suspended solids concentration, values typically are 1,000 to 5,000 mg/L of suspended solids. Yet, when performance is assessed on a volumetric basis, results show that removal rates can be much higher than those compared with suspended-growth systems. This added volumetric MBBR efficiency can be attributed to the following:

1) High overall biomass activity resulting from effective control of biofilm thickness on the carrier due to the shear imparted by the mixing energy (e.g., aeration);

2) Ability to retain highly-specialized biomass specific to the conditions within each reactor, independent of the overall system solids residence time (SRT); and,

3) Acceptable diffusion rates resulting from the turbulent conditions in the reactor.

Introduction to Fixed-Film Bio-Reactors

Figure 2 illustrates a decentralized MBBR system with free-floating media.

PumpTechnology

Moving bed

media

Primary Treatment / Flow EQ

Secondary Treatment / IFAS-MBBR Bioreactor

Influent

Effluent

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References and Resources

1. Hegemann,W.(1984)“ACombinationoftheActivatedSludgeProcesswithFixed-FilmBiomasstoIncreasetheCapacityofWasteWaterTreatmentPlants,”Water Science & Technology, Vol. 16: 10-11, pages 119–130.2. Ødegaard,H.;Rusten,B.andSiljudalen,J.(1999)“TheDevelopmentoftheMovingBedBiofilmProcess—fromIdeatoCommercialProduct,”European Water Management, Vol. 2: 2, pages 36-43.3. Ødegaard,H.;Rusten,B.andWestrum,T.(1994)“ANewMovingBedBiofilmReactor—ApplicationandResults,” Water Science & Technology, Vol. 29: 10-11, pages 157-165.4. Reimann,H.(1990)“TheLINPOR®ProcessforNitrificationandDenitrification,”Water Science & Technology, Vol. 22: 7-8, pages 297–298.5. Royal Commission on Sewage Disposal. (1908) Royal Commission on Sewage Disposal, Fifth Report, London, United Kingdom.6. Water Environment Federation (2010) Biofilm Reactors, WEF Manual of Practice No. 35, pages 211-231.

Andrew M. Jenkins E.I., is Director of Plastics Product Management for Contech Engineered Solutions, including Magellan, Contech’s packaged, decentralized wastewater treatment system. He holds a B.S. in Engineering from Missouri University of Science and Technology, and has 15 years experience in petroleum, mining and civil engineering. He may be contacted at ajenkins@conteches.com.

Darrell Sanders, P.E., is Chief Engineer for Contech Engineered Solutions. He holds a B.S. in Civil Engineering from the University of Cincinnati and MBA from the University of Dayton. He has been a registered Professional Engineer in Ohio since 1996. Sanders has served on committees for the NCSPA, AASHTO, ASTM and Uni-Bell. Contact him at dsanders@conteches.com.

MBBR media selectionHallvardØdegaardet.al.analyzedtheinfluenceofthecarriersizeandshapeonperformance,especiallyrelatedtohighly-

loaded plants working on municipal wastewater (References 3). The results demonstrated that MBBRs should be designed based on surface area loading rate (SALR), and measured, in part, by carbonaceous biochemical oxygen demand (CBOD) in grams per square meter per day (g/m2/d). Whiletreatmentrequirementsdrivefixed-filmsystemdesign,SALR,dissolvedoxygen(DO)concentrations,BOD,NH3

oxidation (nitrification) and denitrification are just a few additional considerations that should be identified in order to estab-lish the most effective system design.

It also has been demonstrated that the shape and size of the carrier did not seem to be significant as long as the effective surface area is the same. The plastic media has proven to have a long service life with systems in operation with no media degradation that requires replacement or replenishment (References 2).

ConclusionThe wastewater industry has evolved during many decades of general practice and research. Currently, there is an emerg-

ing trend toward solutions based upon fixed-film growth, including MBBR and IFAS, in decentralized wastewater treatment applications. Trickling filter, RBC, IFAS and MBBR are currently the most widely used fixed-film design types by the majority of solution providers within the industry. Increasing need for compact designs, and systems with the lowest total installed cost and reduced life-cycle operational and maintenance costs continues to drive the development of these bioreactor solutions.

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Introduction to Fixed-Film Bio-Reactors

The above picture shows untreated water (left) vs. the same water source treated using MBBR process (right).

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Introduction to Fixed-Film Bio-Reactors

1. What is one difference between a fixed-media system and a free-floating system?

a) Hexagonvs.octagonshapeb) Use for denitrification vs. nitrificationc) Mounted vs. unmountedd) Submerged 50 percent vs. 90 percent

2. What benefit(s) can a MBBR system provide versus typical fixed-media systems?

a) No need to backwashb) Highvolumetricefficienciesc) Not dependent on solids separation stepd) All of the above

3. A single biofilm may have which process(es) occurring?

a) Aerobicb) Anaerobicc) Anoxicd) All of the above

4. Free-floating media are applied in _______ configurations:

a) MBBR and RBCb) IFAS and MBBRc) IFAS, MBR and MBBRd) IFAS, RBC and MBBR

6. How often are MBBR plastic media carriers replaced or replenished?

a) Once per yearb) No media degradation requiring replacementc) Once per 10 years of operationd) Dependent on the system’s design

7. Name an advantage of an IFAS design:

a) Improved recovery from process upsetsb) Additional biomassc) Odor reductiond) 90 percent submerged rotating contactor

8. What is a dissolved wastewater substrate?a) Dissolved solidsb) The supporting structurec) Dissolved organic matter and targeted nutrientsd) None of the above

10. Where did many of the original studies on plastic carriers (e.g. MBBR) occur in the 1980s in order to reduce Nitrogen discharges?

a) Spainb) United Statesc) Norwayd) Japan

5. MBBR systems gain added volume efficiencies due to? a) Increased oxygen b) Returned activated sludgec)Highoverallbiomassactivity d) Odor reduction

9. Which should be considered for an RBC design? a) Material fatigue of the rotating shaft and

mechanical assembly b) Capital costs c) Footprint d) All of the above

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