Fixed Film Activated Sludge

(Adapted from BRENTWOOD 2009)

Integrated Fixed-film Activated Sludge (IFAS) Technology provides for additional biomass within a wastewater treatment facility in order to meet more stringent effluent parameters or increased loadings without the direct need for additional tankage. Industry practice for upgrading wastewater treatment plants usually focuses on increasing the bioreactor volume to provide the additional bacterial population required to meet the system’s kinetic needs. However, designers often encounter clarifier solids loading limitations that put an upper limit on the amount of biomass that can be carried in the suspended growth system. The advantage of biofilm processes compared to activated sludge processes is that the anaerobic, anoxic and aerobic zones can be combined in a single stage (see also rotating biological contactor, and anammox). IFAS systems allow for the additional bacterial population to exist on a fixed surface, thereby eliminating the need to increase the suspended growth population.

Media Types

There are a number of different approaches to IFAS implementation but the various configurations fall into one of two basic types: “dispersed media” entrapped in the aeration basin, and “fixed media”, such as structured sheet media or knitted fabric media, fixed-in-place in the aeration basin (BRENTWOOD 2009).

The Process

(Adapted from AQWISE 2010)

The Integrated Fixed-film Activated Sludge (IFAS) process combines the advantages of conventional activated sludge with those of biofilm systems by combining the two technologies in a single reactor (see also rotating biological contactor, anammox or trickling filters). Typically, an IFAS configuration will be similar to an activated sludge plant, with biomass carriers (described above) introduced into carefully selected zones within the activated sludge process. This allows two distinct biological populations to act synergistically, with the Mixed Liquor Suspended Solids (MLSS) degrading most of the organic load (BOD), and the biofilm creating a strongly nitrifying population for oxidation of the nitrogenous load (NH₄⁺).

(Adapted from BRENTWOOD 2009)

IFAS vs. Conventional Activated Sludge

• For new installations, IFAS systems will generally require less volume and therefore have less capital cost than a conventional AS system.
• For retrofits of existing activated sludge (AS) systems to address increased capacity or improved biological nutrient removal (BNR), IFAS systems represent a cost-avoidance associated with the additional volume of that would otherwise be required for additional AS capacity.
• IFAS systems require little or no additional operational costs or operating staff over conventional AS. However, the need for oxygen supply remains.

Cost Comparison of Various IFAS Systems

• Dispersed systems require expenditures for additional components, such as media-retaining sieves, air knives, and/or pumps for sponge regeneration.
• Rather than solely using the media-specific surface areas a means of comparing various IFAS approaches, a true capital cost comparison of different IFAS media systems should look at the cost of removing a given NH₃-N load.
• Based on the best available systems today, the treatment cost per pound of NH₃-N removed per day in a fixed media IFAS system is approximately 1/3 less than the treatment cost per lb.of NH₃-N removed per day in a dispersed media IFAS system.

Operation and maintenance of fixed film systems is normally carried out by skilled labourers. The effluent water from the IFAS system should be clarified (see graph above) before it is discharged. Waste sludge still contains hazardous substances (e.g. heavy metals or pathogens). It should be further treated (e.g. anaerobic digestion or composting) or dewatered (e.g. mechanical dewatering or drying beds) and discharged correctly.

All IFAS systems, whether based on dispersed or fixed media, require adequate preliminary treatment design and operation. Primary clarification or fine screening will prevent ragging and material build-up on the media in the aeration basin and clogging of the dispersed media and retaining screens (BRENTWOOD 2009). All operational activities must be carried out by skilled labourers.