PURGE ASSEMBLIES FINE BUBBLE DIFFUSERS

It is well accepted in the industry that Fine Bubble Diffusers require purging of the condensate and liquids that may enter the aeration piping in order to maintain long-term performance. Those purge assemblies are designed to function as moisture removal devices in order to protect the diffusers, and to assure proper distribution of air into the piping system. Location of those purge assemblies is sometimes questioned in order to deliver superior performance. Environmental Dynamics International manufactures multiple types of purge assemblies to remove condensation moisture and/or solids from the system. We manufacture systems that can be employed as a continuous purge including a part of the piping system itself with a diffuser located at a low point in the system to expel this water and/or solids. Manual purge assemblies with a reservoir and an aspirating air lift action to the top of the tank are also employed. Functionally these two systems work very similar, and both types require proper location in the diffuser grid to deliver adequate performance. In all piping systems for aeration there is a drop-pipe to deliver air to the bottom of the basin. Typically, there will be a subheader at the wall, which will have the air distributed along one side of the basin or a subheader that goes across the basin. Air laterals are attached to this main distribution piping to deliver air to all the diffuser elements in the system. A typical arrangement would be as shown in the drawings below. Normal operation of a system would suggest that there can be minor amounts of condensation that will occur in the system during operation that must be removed. Long shutdown periods will also allow the water to gradually infiltrate the system and can even flood the entire underwater aeration system piping. All of these conditions must be considered and designed for quick removal of this material on aeration startup. The location of the purging system should be such that it will accomplish the following objectives:

1. Immediately start to remove water from the system when the air turns on.
2. It should be located at the lowest point in the system to remove as much of the condensate and/or solids as possible during normal operation.

3. It should function to prevent discharge of any significant amount of liquid or solids through the Fine Bubble Diffuser units! This is a critical item in selecting the location and function of the purge assembly. 

4. The assemblies should be located so that all of the moisture comes to a single point for removal at that purge assembly. 

The above design requirements suggest that the purge assembly should be located at the drop pipe itself! This is a critical factor and location of the purge near the drop pipe will allow the air subheader to act as the sump, because the invert of the subheader is less than the lateral piping. This pump assures maximum water pool available for purging the water out of the system. In addition, having the purge located near the air inlet drop pipe, means that the purge begins operation almost immediately as soon as air is applied to the system! Having the purge assembly operate almost immediately when the air is applied to the system is critical to preventing excess solids and liquid from being pushed through the diffuser units creating fouling or deterioration in overall performance. Having the purge assembly near the drop pipe allows the airlift action to begin immediately upon receipt of air, this will drop the water level in the subheader piping and in the laterals immediately. This will allow air to travel over the top of the water in the piping and discharge from the diffusers without pushing water through the diffusers! This ability to drop the water level in the piping and prevent the water from being pushed through the diffusers is a huge asset in assuring long-term performance. Having the purge assembly and the air inlet to the purge near the drop pipe creates the maximum performance capability. Historically, people have believed that the air supply pushes the water to the far end of the system and the purge should be located at the far end. This is (contrary to) the actual hydraulics and it is clear that the purge system near the drop pipe will give the better performance. Purges located at the end of the laterals (system) have a significant amount of water solids pushed through the diffusers before the purge operates. Locate purge assemblies near the drop pipe with the subheader as the sump for the best value and best performance.