Attached Growth Biological Process – Wastewater Treatment


As regulations governing the treatment of wastewater treatment facilities continue to change, owners need a biological treatment process that is flexible. Increasingly, multiple forms of secondary treatment are needed to meet ever changing discharge limits and today’s high standards.

A secondary treatment option gaining popularity is the attached growth process where media is used to to grow and maintain populations of microbes, creating a biofilm. Aquarius Technologies, LLC (Aquarius) uses a proprietary plug flow process with multiple stages to progressively biologically treat the wastewater. Each treatment stage is filled with stationary biofilm media and has a unique environment for growing different microbial populations. Successive stages of biofilm media create separate microbial living conditions and food sources within the process, resulting in a highly effective food chain. By creating a food chain, biological solids are minimized. Reductions up to 80% in biological solids have been realized over suspended growth processes.

Here’s how it works

High food-to-microorganism (F/M) ratios in the initial stages of treatment process create feast conditions, which encourages the rapid growth of lower life forms. In the later stages, famine conditions and low F/M ratios favor higher life forms when the system has a plug flow pattern. The higher life forms consume the lower life forms and therefore reduce the overall biological solids production.

A fine bubble diffused aeration system is used to provide oxygen to the microbes, ensure each treatment stage is completely mixed and enable the biofilm media to be scoured for controlling the amount of biological growth.

Where can this innovative attached growth biological system be used? It’s ideal for new municipal and industrial biological wastewater treatment applications. But it can also be added to any existing wastewater treatment processes, such as aeration basins and oxidation ditches. In addition, it can be used to meet a variety of pretreatment, secondary and nutrient removal treatment objectives.

Comparison to suspended growth biological treatment

Conventional activated sludge treatment with suspended growth technology is the most common form of biological wastewater treatment in use today. In this approach, the wastewater is mixed with free-floating microorganisms that eventually gather into biological flocs that settle out of the wastewater creating waste activated sludge. A portion of the sludge is returned to the influent of the treatment process to retain microorganisms to perform the treatment.

Suspended growth biological treatment systems have a number of significant disadvantages:

  • Inability to handle highly variable waste stream characteristics (flow and/or loadings). As the flow increases, the microorganisms can get washed out into the clarifier. As the loading increases or decreases, the microorganisms aren’t able to feed on the contaminants any faster or slower leading to a reduced efficiency and potential settling issues.
  • Production of significantly more sludge, which adds operational costs for its handling, storage and disposal.
  • Requirement of additional energy, operational oversight and maintenance to produce desired effluent quality.


Of all types of biological wastewater treatment technology, an attached growth biological system has the lowest cost of ownership in the right application. The Nebula MultiStage Biofilm System produces up to 80% less sludge than suspended growth systems, resulting in big savings in sludge handling, storage and disposal costs. The process has no moving parts and therefore is very low maintenance. The biofilm media racks are maintained in-place during the air scour procedure and the media is estimated to last decades. The only replacement parts are associated with the diffused aeration system diffuser elements, which are typically replaced every seven to ten years.