Knowing When to Waste Sludge

How do I know when to waste sludge from my package treatment plant? This is a question that has plagued operators of small, extended aeration plants for years. With little or no equipment to help determine how much sludge is in the plant, we have to go to the method of using a settlometer. We use this tool to help determine the settling characteristics of the sludge in a plant, how much sludge is there and how much to waste.

A clear container that is about three times as tall as it is wide is best to use as a settlometer. Jars that would work include a lemonade pitcher, a gallon pickle jar or, in a pinch, a quart jar. Next, measure it into 10 equal sections (10 percent each). Go to the end of the aeration section of the plant right before it goes into the clarifier and get enough samples that will fill your settlometer to the 100 percent line. Stir the sample to make sure that it is completely mixed, without breaking up the floc too much. After stirring, take an approximate reading in percent of the drop in the sludge every 5 minutes for the first 30 minutes. Also, observe the sludge. It should form into a blanket or bed and settle as a mass, filtering the water up through the bed of solids as it sinks.

What color is the sludge? A dark brown or black sludge that settles in the first five minutes as discrete particles leave turbid water normally indicates very old sludge that needs to be wasted. If the sludge is a dark tan or a dark brown and settles slowly (between 35 and 50 percent in about an hour), you have just about the right amount of sludge and no wasting is necessary at this time. If it is a light tan or a very light brown, does not form into a solid blanket or bed and settles as individual floc particles, this indicates young sludge and needs more age. If the sludge is dark tan or brown and does not settle at all in the first 30 minutes, we need to look at something else.

If it does not settle in the first 30 minutes, you may have too much sludge or filamentous organisms, usually caused by grease. To determine which you have, start the process again. Before you add the sample from the aeration basin, fill 50 percent of your settlometer with nonchlorinated effluent, then fill it with sample. Stir it and allow it to settle. If it settles to about 50 percent after 30 minutes, you have too much sludge and need to waste. If it still does not settle after 30 minutes, you most likely have filamentous organisms, which is for another post.

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