​Until fairly recently the simplicity and reliability of fixed-film systems was seen as a trade off against the quality of the treated effluent produced. However, with the increasing need for sewage treatment in remote and unserviced areas, attention has been given to the equipment and design of fixed-film systems. Clearedge has worked with process engineers in the design and operation of our systems - as a result we can offer systems designed and certified by professional process engineers should the client so wish.

 

FIXED-FILM AERATION vs. ACTIVATED SLUDGE

Most municipal sewage systems utilise an activated sludge process which is the most common process used in large sewage treatment systems. The activated sludge process is efficient but complex. There are several parameters which need to be controlled to produce the required treatment:

 

• Sludge age

• Concentration

• Oxygen levels

• Inflows of food

 

Where skilled operators are not available these parameters are controlled with sophisticated probes and computers. In addition, the process produces large amounts of excess sludge which has to be disposed of.

 

Our preference in remote areas and situations where operator skills are limited, is for the fixed film process.

 

In simple terms, the activated sludge process is a soup of bacteria stirred and aerated continuously. Effluent is fed in and consumed by the bacteria which grow and reproduce – producing an ever increasing population of bacteria (sludge).

 

This bacterial population has to be retained and regulated. This involves recycling and wasting sludge from the clarifier. The wasted sludge has to be disposed of by drying or digestion – both of which have associated odour problems. In addition any high flows will wash the bacterial soup out through the system.

 

The fixed film process utilises the same bacteria, but they are provided with a large surface area (matrix) on which to fix. The oxygenated effluent is passed over the matrix and the bacteria extract the nutrients from the water, thus cleaning it.

 

Because the bacteria are fixed there is no need for recycling of sludge and the process produces much less sludge than the activated sludge process. The sludge that is produced is returned to the anaerobic pre-treatment section at the head of the works to be digested – giving water, carbon dioxide and methane as byproducts. This recycle to an anoxic zone is used to release Nitrogen, derived from the breakdown of Ammonia, to the air. A pump on a timer is all that is needed for the control of this process.

 

THE EQUIPMENT

A unique feature of our plant is the Whale Tooth™ aeration system. This is a patented system which uses pumps and venturis, and avoids the necessity of specialised equipment such as blowers, and any small pipes or diffusers.

 

This ensures low maintenance costs and readily available, inexpensive backup parts.

 

Our fixed film systems can be built into virtually any structure or tank – from standard plastic tanks, containers, to concrete structures.

 

Our systems have been designed with no specialised parts or components, the only moving parts are standard sewage-duty pumps which are readily available.