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1.5: Solids Handling Processes

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  • Learning Outcomes

    • Understand where solids from the wastewater treatment are generated
    • Compare the different ways to thicken wastewater solids


    For many years the solids leaving the wastewater treatment plant were referred to as sludge. The term sludge is still used but typically refers to the untreated raw sludge leaving the primary clarifiers or the waste activated sludge (WAS) leaving the secondary clarifiers. However, once the sludge has been treated, it can be converted to biosolids. Biosolids can be used for many beneficial purposes such as compost, fertilizer, and landfill cover. In the liquid portion of wastewater treatment, the goal was reduction of organic matter. Much of that organic matter ends up in the sludge. In biosolids, it’s that organic matter that can be used for composting. There is also an ample amount of nitrogen and phosphorus which are key components in fertilizer. That organic matter has a lot of potential energy. During the digestion treatment stages, this energy can be captured and used to power other portions of the treatment process.

    Primary Treatment Sludge Thickening

    Gravity thickening is commonly used to thicken primary solids. The purpose of thickening is to reduce the water content. This makes a more concentrated sludge that will require smaller digesters in the next treatment stage. Gravity thickening works very similarly to primary sedimentation. However, instead of starting with raw wastewater we are starting with concentrated sludge. In the primary sedimentation tank, the raw wastewater will increase from around 1% solids to up to between 4% and 6% solids. Starting with the higher percent solids concentration and putting it through a similar treatment process can further increase the solids concentration to 10% or greater. By increasing the percent solids the sludge becomes thicker. It’s getting thicker because the water is being removed.

    Similar to the sedimentation process gravity thickeners rely on the heavy solids in the sludge to settle by gravity to the bottom of the thickener. As the solids settle the water will be separated out. The tank will become stratified with larger amounts of solids on the bottom of the tank and layer of water with less solids on top. The top layer is referred to as the supernatant and the bottom layer is called subnatant. To optimize the performance of a gravity thickener it’s critical to have “fresh” primary sludge. Sludge that is becoming or has already gone septic will not settle as easily as newer sludge. Therefore, controlling the rate of pumping from the primary sedimentation tank to the gravity thickener must be closely watched. If the sludge is going septic in the primary tanks then the pumping rate will need to be increased. This will lower the detention time and keep the sludge from going septic. However, if it’s pumped too quickly then the percent solids might be decreased which is also not optimal. Another way to deal with septic sludge is to add chlorine to reduce biological activity.

    Secondary Treatment Sludge Thickening

    When thickening secondary sludge or a mixture of secondary and primary sludge, a dissolved air flotation thickener (DAFT) is commonly used. A DAFT works by taking a portion of the clear subnatant leaving the DAFT and pressurizes the water with an air compressor. The water is then conveyed back into the DAFT where it is exposed to atmospheric pressure. The difference in pressure causes lots of tiny air bubbles to be released, just like opening a soda bottle. The tiny air bubbles will float to the top of the DAFT. The incoming sludge will not be able to settle to the bottom because of the rising air bubbles. Therefore, the solids remain on the top layer and the bottom layer is clearer water. The solids will be skimmed off and sent to a hopper where it will be pumped to the next stabilization treatment stage. The clearer water on the bottom will be sent back to the headworks to be further treated in the liquid portion of the treatment process.

    A DAFT can typically create a solids concentration between 4% and 8%. The return water is pressurized between 40 and 70 psi. A key parameter to a well operated DAFT is the air to solids ratio. A common air to solids ratio is around 0.02 to 0.04. If not enough air is supplied to the tank, the solids will settle to the bottom. Other operational parameters are the speed of the skim arm. Operators should set the speed to slowly skim the thickened sludge into a hopper. If the skim arm speed is set too high it will thin out the solids and reduce the percent solids concentration. There should be about a 1 to 3 foot blanket of thickened sludge on the DAFT. Polymer is often added to aid the coagulation of solids. This also causes the solids to bind together and make larger particles that will more easily float to the top of the unit.

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