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Energy Saving in Waste Water Treatment : Week 9/15/2011

Saving Electrical Energy in Waste Water Treatment

In last fortnight, we wrote about available methods for measurement of process variables & their usefulness to the user. We also discussed implementation of various aeration control techniques. Now let's see, how we can ensure proper measurement as well as control of aeration.

The Solution and Implementation

Implementation of aeration control strategies will depend on the type of aeration systems in use at the plant and also on the capabilities these systems offer for control. Most controls for mechanical aerators and blowers involve the use of variable frequency drives. A variable frequency drive adjusts the speed of electric motors by modulating the power being delivered. Motor speed can thereby be matched to the amount of work demanded. Systems with variable frequency control can provide a "soft start", thus avoiding high torque, current surges as well as stress on the motor systems. Control of diffused aeration systems will depend on the type of blower and strategy for the coordination of multiple blowers. Blower types include constant speed centrifugal blowers, usually controlled by modulating air inlet valves or inlet guide vanes. Positive displacement (rotary lobe) blowers are controlled using a VFD to modulate the output.

Multiple blowers are coordinated by providing either parallel control (which simultaneously adjusts the output of all blowers following addition or deletion) or cascaded control (which adjusts the output of individual blowers to a minimum or maximum output before adding or deleting the next blower needed to satisfy the overall aeration demand).

Aeration control implementation often requires custom software based on the specific processes, control strategy, aeration systems and safety factors that apply to a specific treatment facility.

Many operators believe that the objective of the treatment process is the creation of surplus dissolved oxygen in the effluent from the aeration basin. Actually, maintaining surplus oxygen in the effluent at an arbitrarily high set point is merely a convenient way to ensure that the wastewater has been fully nitrified (all ammonia has been converted to oxidized nitrogen), which can occur only after a substantial reduction in the carbonaceous demand has occurred. But this is at the expense of delivering more oxygen than is actually demanded by the process. The difference between the oxygen required to achieve process objectives and the oxygen delivered to meet a "target" set point represents a pool of energy savings that can be captured by using appropriate measurements within the process

Measuring | Monitoring | Controlling – a complete intelligent solution

It is possible to have an intelligent algorithm which controls blower operation such that it optimizes DO Variation on account of change in load either in terms of influent quantity or even the composition of effluent. Optimized and lower operation of blower indirectly increases the life of blower and its rotor as an added advantage in addition to the energy savings.

Fig-1 shows the schematic representation of such an intelligent system, while Fig-2 indicates the benefits of intelligent aeration control

Next fortnight we will be coming up with a concluding article. In this article, we will give you some insights on this concept from our experts.