Bottom Solids
Some solids will settle on the bottom of the DAF separation chamber. When this occurs, these solids need to be removed frequently. Allowing these solids to accumulate on the bottom of the tank can become septic (anaerobic). Septic sludge can cause the wastewater pH to decrease as well as produce sulfides which is one of the new Pollutants of Concern in certain areas of the United States. In addition, septic sludge becomes more difficult to dewater.
There are two ways to determine the volume of sludge on the bottom of the separation chamber. The technical method to identify the volume of sludge on the bottom of the DAF is to use a Sludge Judge.
The Sludge Judge (manufactured by Nasco) is a sludge sampler that can handle sludge concentrations up to 5% solids. The Sludge Judge comes with three 5-foot clear polycarbonate sections with a bottom foot.
Insert the Sludge Judge into the DAF separation chamber and pull up on the tool. Pull the Sludge Judge out of the separation chamber to view the volume of sludge settled on the bottom of the tank. Take a few more measurements at different cross-sections of the tank to determine a bottom sludge profile.
If a Sludge Judge is not available take a 10ft length of conduit and stick it into the separation chamber. If there is a build up of solids there will be resistance as you push the conduit to the bottom of the tank
Another method to determine the depth of sludge in the DAF is to run your hand along the side of the DAF separation tank. Starting from the top, move your hand down towards the bottom of the DAF. As you start to move your hand downward, the cooler temperature will indicate where the bottom solids level starts to accumulate. When you hit the real cold spot alongside of the DAF that’s where the most concentrated solids are located. The bottom sludge blanket should not exceed twelve inches. When your hand starts at the top of the tank and it is already cold there is a serious bottom sludge build up.
Why Remove These Solids?
Depending upon the DAF manufacturer, the aeration pump may be located near the bottom of the separation chamber. If the bottom solids layer is too thick these solids may be drawn into the suction side of the aeration pump reducing the microbubble generation that will negatively affect flotation. If these solids are dense such as settled sand, these particulates can cause abrasion to the pump impellor(s).
Additionally, if the bottom solids layer becomes too thick it will be difficult to remove these bottom solids. Some DAFs include augers to move the sludge off of the bottom. If the solids are too thick the auger will bore through the solids but actually not move solids along the top of the auger. It acts as a drill burrowing a hole through the sludge rather than moving the sludge out of the bottom of the DAF.
Some DAF manufacturers install multiple drains along the bottom of the DAF that can automatically or manually drain the bottom sludge for short periods of time to prevent a significant bottom solid build up.
These bottom solids should be pumped into a sludge storage tank to be dewatered on-site or processed off site as a liquid waste. Frequent bottom blow downs prevent the costly DAF draining and clean out. By doing frequent blow downs during normal operations, this will minimize expensive system shutdowns, system draining, and solids pumping by an outside sludge disposal contractors.
Quite often, buildup of solids has become bad enough that the contractor has to use water to move the sludge to the pumps to transfer into the tanker trucks further increasing costs because water is now being hauled. These costs also commonly include Confined Space (Hazardous Duty) Fees. Doing daily DAF bottom blowdowns greatly reduce these costs.
Many wastewater operators have become quite creative in bottom solids management. One operator made a pool vacuum type of apparatus that allowed him to pull solids off of the bottom of the DAF. The suction line was connected to the suction side of the air operated diaphragm (AOD) pump. This sludge is pumped into the sludge storage tank and the solids would be dewatered through the dewatering device.