At the other end of the headworks installation is the RAS (Return Activated Sludge) mixing box. Raw wastewater is mixed here with activated sludge pumped back from further along in the treatment process (we'll get to that on the next page of the tour) prior to moving to the aeration basins. It is this sludge that does the majority of the work in the activated sludge process. The sludge carries literally millions and millions of aerobic and facultative bacteria, and it is these bacteria which do the actual work in this stage of the process. The first step is to get them mixed in with the raw water, and that is accomplished here. The raw water is flowing in from the bottom of the shot below, while the sludge is being pumped up into the center. Unfortunately, here is an example of bad design which has plagued the plant since it went online in 1998. This setup encourages the aerated wastewater to flow into the lower channel while most of the return sludge is directed into the upper. Recently we installed a mixer in the lower left hand corner of the box to promote better mixing, and so far it seems to have paid off nicely, with better-balanced D.O. (dissolved oxygen) levels in the twin aeration basins..
Once mixed, the wastewater/sludge mixture (known as "mixed liquor") passes through a pair of gates on its way to the aeration basins. The gates can be adjusted to control the amount of flow each basin gets, which is very useful during rain-induced high flows, and one of them is shown below.
Below are pictures of the plant's twin aeration basins. Each one holds 330,000 gallons of mixed liquor, and the racetrack shape can be clearly seen here. This is a variant of the activated sludge process called an "oxidation ditch". You can see from the placement of the mixers (one on each end of each channel) that the design will yield an oxygen-poor zone in the second half of each leg. This is designed to allow facultative bacteria in the mixed liquor to break down nitrates in the wastewater and therefore helping to reduce the possibility of an algal bloom or the unwanted proliferation of some other form of aquatic plant life in the receiving stream.
Here below are two good shots of the mixers used in the basins. They serve two main purposes: to keep a sufficient level of dissolved oxygen in the mixed liquor and to keep the mixed liquor mixed. Without either of these two elements, the activated sludge process will not work. In 2006 we installed a pair of VFDs (variable frequency drives). These drives run at remotely-controlled varying speeds, giving the mixed liquor just enough aeration and mixing to maintain recommended D.O. levels in the basins and helping save on plant electricity costs.
From the aeration basin, the mixed liquor moves into one of two clarifiers. Clarifiers are deep tanks designed to give the activated sludge portion of the mixed liquor plenty of time to settle out of the mixture, leaving clear, nearly-completely-treated water at the top -- hence the name. The sludge is pumped out of the bottom of each clarifier (more of that on the next page) and the water passes out of the clarifiers and onto the disinfection process.
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