Activated sludge process – Upgrade with IFAS

After explaining how the activated sludge process works in part 1 we are now taking a closer look on how to increase the wastewater treatment capacity of ASP with IFAS systems.

IFAS stands for integrated fixed film activated sludge and describes the combination of fill media attached growth systems with the activated sludge process. The fill media can either be added to the aeration tank as free floating chips or in the form of retrievable fill media cage units. In both cases the fill media fulfills the same purpose: To provide additional surface area within the aeration tank for attached biofilm growth. The available surface area per fill media block can be up to 70ft2/ft3 (220m2/m3) so that depending on the water level the tank base area can be increased more than 500 times.

After some time, microorganisms start to colonize on the fill media surface as attached biofilm. Very similar to the microorganisms in the activated sludge the attached biofilm will start to break down organic matter and oxidize ammonium NH4+ to Nitrate NO3-. Compared to conventional activated sludge systems the MLSS can be increased from 3,500mg/l to up to 5,000mg/l which equals a capacity increase of about 50%! Of course, with increasing BOD removal rates more oxygen is required as well so that fixed bed biofilm units are equipped with diffusers for additional oxygen supply.

Furthermore, fixed bed biofilm systems have some advantages compared to the conventional activated sludge process. The attached biofilm on the fill media surface is less sensitive for influent peaks or interruptions of oxygen supply which makes an IFAS system reliable and comparable easy to operate. Also, the oxygen supply to the attached biofilm can be directly adjusted by the operator or even completely automized with DO (dissolved oxygen) control. The benefit for the customer is lower energy costs because of appropriate oxygen supply.

IFAS systems are way more affordable than aeration tank additions and are therefore a good alternative to new build tanks. To settle down the increased sludge amount, tube settlers can be added in the secondary clarifiers as well.