Bio Tower
Aerobic Effluent Treatment

Operatings Cost

The factors affecting operating costs in this case are:-

* Aeration Efficiency
* Effluent Pumping
* Sludge Recycling
* Sludge Treatment and Disposal

Aeration Efficiency

In the present form of the activated sludge process, oxygen in the form of air is brought into contact with the waste water either by mechanical agitation or by bubble diffusion. Typically, the oxygen transfer efficiencies for the process are very low, in the range 6-20%. This means that a considerable excess of air and mechanical power must be employed to ensure that adequ ate oxygenation is achieved.

The power consumption for aeration of an activated sludge system is 0.28 to 0.67 kwh/kg O2 . Hence the aeration cost of an Etapak system is less than 4% of an activated sludge system, for the same degree of treatment. With ever increasing costs for energy, the difference in power consumption for aeration translates into considerable savings when Etapak media is used.

Effluent Pumping

The flow of the effluent through an activated sludge process is generally by gravity and thus only small pumping costs are incurred in providing sufficient head at the process inlet. Etapak bio-reactors are usually 5-6 m tall and thus pumps must be employed to raise the effluent and any recycle flow to the liquid distributor. Whilst this pumping consumes some power, the quantities involved are small compared with the savings possible by elimination of activated sludge aeration costs.

Sludge Recycling

To ensure an adequate concentration of biological solids are present in the aeration basins of an activated sludge system, a portion of the settled sludge must be recycled. The highly dense and viscous nature of this sludge makes it difficult and expensive to pump.

The wastage of active cells from an Etapak bio-reactor is small and thus no recirculation of biological solids is required.

When the factors affecting power consumption are taken into consideration, the energy factor for an activated sludge plant is 0.9 to 2.0 kWh/kg BOD removed.

In comparison, the energy factor for an Etapak bio reactor is 0.1 to 0.3 kWh/kg BOD removed, thus the overall energy costs are only 10-15% of those for an activated sludge system.

Sludge Treatment And Disposal

The sludge produced from an activated sludge plant is 0.5 to 0.8 kg TSS/kg BOD removed. Whereas for an Etapak bio-reactor, the sludge production is only 0.1 to 0.50 kg TSS/kg BOD removed.

Not only is the volume of sludge produced important, but the nature and type has to be considered. Biological sludges produced by an activated sludge system can be very difficult to settle or dewater. In contrast, the sludge from the Etapak systems is stable and shows good settling properties.

Assuming only a nominal value for disposal and not taking into account the nature of the sludge, the sludge disposal costs for Etapak bio-reactors are generally only 20-65% of that for activated sludge plants.

Maintenance

To ensure that activated sludge units operate efficiently and reliably, regular maintenance and high quality technical supervision is required to ensure that all mechanical equipment and instrumentation is functioning correctly.

Etapak bio-reactors on the other hand require only occasional monitoring and maintenance to the point that several plants are completely unmanned and monitored by Telemetry systems, thus further reducing operating costs.