Bio Tower
Aerobic Effluent Treatment

Capital Cost

The factors affecting capital cost in this case are :

* Volume of Plant
* Aeration Equipment
* Instrumentation and Control

VOLUME OF PLANT

The total installed capital cost is to a large extent dependent upon the size of plant required, which in turn is controlled by the permissible organic loadings for the required degree of treatment. Generally, the volume required for an activated sludge plant is less than an Etapak fixed film bio-reactor. However, activated sludge tanks require far more land area and are built in concrete, whereas Etapak fixed film bio-reactor configuration can be selected to suit site conditions and can utilize modern low cost glass lined steel tanks. Thus the apparent advantage of a smaller volume yields no saving on overall capital cost.

AERATION EQUIPMENT

As a consequence of low oxygen transfer efficiencies, large mechanical aerators and blowers are required in activated sludge units, which are very expensive in terms of both capital and operating costs. For Etapak fixed film bio-reactors, the natural draught caused by the small differences in densities of atmospheric air and of the air within the packed bed is sufficient for ventilation. Where it is considered prudent to install fans, for example bio-reactors treating effluents with process derived odours, the oxygen transfer is such that the cost of installing fans is negligible.

INSTRUMENTATION AND CONTROL

As previous stated, an activated sludge system requires maintenance of a balanced mixed liquor. The system is therefore very susceptible to shock organic and hydraulic loading, which can unbalance the mixed liquor. This is particularly likely to occur when dealing with industrial effluents prone to large discharge quantity and strength variations throughout a working day. Variations in waste water flow rate can cause a large portion of the active solids to be displaced to the classifier, leaving insufficient micro organisms in the aeration tank to handle are incoming organic loads. Therefore, complex instrumentation is necessary to monitor and control the system.

Etapak systems have a proven track record as being robust in withstanding shock organic and hydraulic loads, consequently, complex instrumentation is not required.

When all the factors that influence the capital cost are considered, the installed capital cost of an activated sludge plant is generally higher than that of an Etapak bio-reactor system.