Black & Veatch's Frank Rogalla looks at submerged aerated fixed-film systems
When high-rate processes first appeared for municipal wastewater treatment in the early 1980s, they were restricted to a handful of proprietary processes such as the biological aerated filter (BAF). Yet the need for compact technologies to upgrade existing activated sludge plants was growing, with new regulations to adapt to more stringent effluent standards, pressed either by European guidelines on urban wastewaters or regional nutrient removal targets.
Therefore, technologies that could adapt into conventional aeration basins were sought as a cost-effective rehabilitation of existing infrastructure to new requirements. One popular idea to improve treatment efficiency was to use hybrid processes, or integrated fixed-film activated sludge (IFAS) systems, where biofilm carriers are placed into aeration basins, thereby increasing the biomass concentration without additional load on the clarifiers. The first technologies used fixed carriers such as woven fibers (ringlace) and submerged trickling filter packings, but it proved difficult to assure even growth and sufficient aeration of the media when it was placed into activated sludge basins. Only a handful of plants were built, such as Annapolis in the US and at the Amperverband in Germany.
Fixed media as stand-alone reactors, without solids recycle, found some application as submerged aerated filters (SAF), such as in Halifax, Yorkshire, and in the Emscherverband, Germany, and in many smaller prefabricated units. In such systems, any type of solids separation, such as conventional clarifiers, can be used to capture the biomass eroded from the biofilm. To reduce footprint and effluent solids concentration, high-rate clarification can be applied, such as lamella with or without chemical enhancement, ballasted flocculation, dissolved air flotation and/or deep bed filters.
Mobile is more
In parallel, in the 1980s, mobile media such as foam cubes of around 1.5cm side length were developed in the Linpor and Captor processes, and close to 20 plants have been equipped since 1988. Two such plants where the foam cubes are mixed with the activated sludge in aeration basins with capacities around 60Ml/d are operating in Bavaria, Germany. The first US installation has recently been commissioned in Rhode Island and Captor has also been used in Fairfield, Connecticut since 1996. For Linpor, the first application was tertiary nitrification, as a separate reactor behind existing plants, without solids recycle. A 125Ml/d plant has been operated in that mode for more then ten years in Aachen, Germany.
The most popular variation of the moving bed biofilm reactor (MBBR) was developed at the Norwegian University of Science and Technology (NTNU) under the direction of Professor Hallvard Odegaard and was first marketed by the Norwegian Kaldnes company. The process utilises a cylindrical plastic carrier of about 10mm diameter to provide an environment in which bacterial populations and protozoa can grow, as shown in Figure 1. The medium for the support of biofilm growth consists of high-density polyethylene (HDPE) cylinders with a specific gravity between 0.91-0.96. This media has an effective surface area for biofilm growth of 500m2/m3 and is used in reactors at fill rates of up to 67%, giving a biofilm surface area of approximately 350m2/m3 of reactor. Newer variations increase the biofilm surface, as shown in figures 2 and 3.
The Kaldnes system may be used for a large range of population equivalents and can be designed for high-rate carbonaceous-only requirements or in combination with anoxic zones for nitrogen removal. Around 170 plants have been built since 1990, half of them for industrial applications. In Wellington, Moa Point, New Zealand, a plant of 200,000PE is designed mainly for BOD removal. The largest municipal units are tertiary denitrification reactors in Malmoe-Sjoelunda, Sweden and Olso-Bekkelaget, Norway, both treating more than 350,000PE. High-rate pre-treatment with the MBBR is used in two large UK plants in the Anglian Water Region, Corby (150,000PE) and Pyewype (191,000PE), that are receiving high industrial loads and are followed by conventional activated sludge. Early on in Corby high loading of the MBBR resulted in the formation of fungal balls in which the media was completely encapsulated. This created temporary difficulties as the media tended to sink or jam against the sieves. Reducing the loading solved the problem. If loads are kept below about 8kgBOD/m3 of reactor volume per day, fungal balling should not occur.
Before selecting the media and process to be used, the characteristics of the media types must be understood and adequate data must be developed to support the manufacturers’ claims. Most information is available to date on mobile plastic media, where even simulation models are available in the current activated sludge software packages (GPS-X, Biowin).
Keeping it back
The key to the moving bed biofilm reactor is the outlet sieves to retain the carriers, with openings slightly smaller than the media side length of 10mm. Design has progressed from the flat stainless steel mesh sieves originally used, which are still in application for anoxic reactors. Horizontally-mounted wedge-wire pipe sieves with appropriate wire spacing are often used in the UK. In Scandinavia, vertically-mounted stainless steel perforated plate sieves are preferred, as illustrated in Figure 4.
The effect of these sieves, with their relatively small openings on plant hydraulics, must be evaluated because additional headloss is created especially at peak flows. It is also necessary to pay close attention to the mechanical means of maintaining the sieves in a free-flowing condition. On vertical screens, an ‘air-knife’ is used for this purpose, which causes a significant discharge of air flow near the bottom of each sieve.
The upward movement of air helps keep the small media elements off the sieve face. For horizontally-mounted sieves, no specifice air knife is needed, as the turbulence created by the diffused aeration system is sufficient to maintain the sieves in good operating order. Because of the media retention sieves, it is recommended to screen the wastewater properly to capture any material that might bind the media, plug the retention sieves, or become entrapped in the aeration basins as floating substances, such as plastics and hair.
Perforated screens of 3-6 mm are recommended, although a plant in Broomfield, Conneticut, has been operated for more than a year without fine screens in the inlet works. On other plants though, the headworks have been overwhelmed in wet weather conditions, leading to blinding of the media retention screens and provoking some overflow and loss of media from the aeration basins.
Make it mix
The other important design issue is appropriate aeration to mix and erode the excess biomass from the biofilm carriers. For the system to remain efficient, biofilm thickness has to be controlled, so detached biofilm is suspended within the reactor and leaves the reactor with the effluent. Therefore, in addition to providing oxygen transfer and adequate mixing to keep the solids in suspension, the aeration system must subject the carriers to sufficiently vigorous contact and abrasion to control the depth of bio-growth. Aeration is therefore by coarse bubble, using stainless steel laterals and a specific delivery of 1.7Nm3/m3/h should be sufficient.
Because of the depth of biogrowth on the carrier elements, treatment is more effective at higher residual dissolved oxygen (DO) concentrations in the bulk liquid, to allow oxygen to penetrate deeper into the biofilm and to maximise the amount of microorganisms participating in the degradation. Whereas conventional activated sludge systems are designed to carry 1-2mgDO/l, biofilm systems should be designed for a DO concentration of 2-5mgDO/l.
In addition to the experience derived mainly from Norway, Sweden and the UK, recent experience has been gathered in Canada and the United States to upgrade existing plants for nutrient removal. In addition to studies in Johnson County in Kansas, Franklin in Tennessee, the region of Peel in Ontario, Grand Rapids in Michigan, West Haven in Connecticut and Mattabasset District in Connecticut, full-scale demonstrations have been performed at Cheyenne in Wyoming, South Adams County in Colorado and Broomfield in Colorado
All evaluations showed IFAS could lead to reduced tankage requirements and lower cost and site utilisation. Since IFAS operates as an activated sludge process, operators of similar systems will be comfortable with it. Further, the settling characteristics of the MLSS are comparable or better than those of conventional aeration, needing no changes in final clarification.
Experience in Broomfield and Cheyenne show carbonaceous activated sludge plants can be upgraded to nitrification without basin expansion, while maintaining the original hydraulic capacity. Based on these case studies, and a media fill fraction of 50%, the IFAS equipment – plastic media, aeration system and sieves – will cost about £100/m3/d. MBBR technology tends to cost 50-100% more, because of the larger volume of basin and amount of carriers needed for the same degree of treatment. Media cost itself is around £1,000/m3.
Detailed experience from the UK, US and Germany on full-scale IFAS and MBBR performance will be presented at the upcoming CIWEM Seminar in Leeds on April 20, 2005 (contact firstname.lastname@example.org).
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