Bio Tower
Aerobic Effluent Treatment

Introduction (Etapak)

The Principle of Etapak Fixed Film Bio-Reactors

The process of ‘biological oxidation’ relies upon naturally occurring micro organisms present in the waste water adsorbing and subsequently oxidising dissolved organic pollutants from the waste water in the presence of oxygen.

Effluent is evenly dispersed over the surface of Etapak media by either a rotating or reciprocating distributor. Oxygen is transferred by diffusion from the air (which is continually passing through the media void spaces) in to the effluent, which trickles over the media surface. This oxygen supports living micro organisms which feed on the dissolved organic matter in the effluent and grow as a film over the surface of the media.

The biological film is alive and continually growing but,in order for it to stay alive, both the polluting matter,which is the food source, and oxygen must diffuse through its surface to reach all the aerobic micro organisms. Beyond a certain film thickness, the concentrations of food and oxygen become too low to sustain life. At that point the aerobic organisms starved of food and oxygen die. The dead layer or ‘biomass’ then breaks away (sloughs) from the media surface and is washed down through the Etapak bed. New biofilm now begins to grow on the media surface, becoming progressively thicker, until food can no longer diffuse through it to those layers near the media surface, which therefore die producing more biomass. Thus the cycle is perpetuated.

Whilst the above description is an over-simplif-ication of the physical and chemical process which occur in a fixed film bioreactor, it does indicate the properties a fixed film media must exhibit to achieve effective treatment.

1.2 Activated Sludge Systems The activated sludge process is a biological waste water treatment process in which a mixture of waste water and biological sludge containing a spectrum of micro organisms is agitated and aerated. The biological solids are subsequently separated from the treated waste water and returned to the aeration process to maintain the required mass of micro organisms for a given application.

The activated sludge process derives its name from the particular biomass formed when air is continuously injected into the waste water. In this process, micro organisms are mixed thoroughly with the organics, under conditions that stimulate their growth through the use of the organics as food. As the micro organisms grow and are mixed by the agitation as food. As the micro organisms clump together (flocculate) to form a suspended culture of biomass termed ‘activated sludge’. In practice, the waste water flows continuously into an aeration tank where air or oxygen is injected to mix the activated sludge with the waste water and to supply the oxygen needed for the suspended biomass to break down the org- anics. In conventional activated sludge systems, the waste water is aerated for 6-8 hours. The mixture of activated sludge in the aeration tank, termed ‘mixed liquor’, then flows from the aeration tank to a secon- dary clarifier, where the activated sludge is settled out. A portion of the settled sludge is returned to the aeration tank to maintain the high population of micro organisms necessary for rapid breakdown of the organics. Because more activated sludge is produced than can be used in the process, some of the return sludge is diverted to the sludge handling system for treatment and ultimate disposal.

In the evaluation of a process, the three most important questions are :

1. Does the process work?
2. What is the capital cost?
3. What is the operating cost?

With numerous plants operating world-wide by us & by our collaborators, we can offer process designs to various specifications with confidence.

Without conducting a detailed design for a specific application, it is difficult to give exact capital and operating costs. However, in considering the choice between an Etapak fixed film system or an activated sludge system, the important points affecting both the installed capital and the operating costs are discussed here.