Solids Handling and Disposal | Activated Sludge Process | Wastewater Treatment Plant

 Solids Handling and Disposal

Purpose

• Excess solids generated during the primary and secondary wastewater treatment processes should be removed and stabilized.
• Pathogens should be reduced. (It should be noted that pathogens are reduced but not necessarily eliminated.)

Stabilization Process Descriptions

Digestion

• Biological treatment of primary and secondary treatment sludge to minimize volatile solids (organic) concentration.

• Aerobic Digestion

1. Aerobic bacteria perform digestion in an aerated tank.
2. Capable of reducing volatile solids content by up to 50%.
3. The efficiency of Volatile Solid Reduction is affected by temperature and the length of time sludge remains in the digester.
4. It does not produce a usable gas end-product, such as methane gas, which is produced by the anaerobic digestion process.

• Anaerobic Digestion

1. Anaerobic bacteria digest waste in a heated, unaerated tank.
2. Depending on the temperature and length of time the sludge is digested, it is capable of reducing the volatile solids content by up to 50%.
3. Sludge in the tank should be heated to 95°F for maximum efficiency.
4. Methane gas is produced by the biological action of bacteria. Acid forming bacteria consume volatile organic solids, resulting in the formation of organic acids. The organic acids are then converted to methane gas by methane-forming bacteria.

Incineration

• Sludge can be burned if it does not cause air pollution.

Wet Oxidation

• Treatment method for sludge prior to water removal.
• Dewatering separates organic and inorganic material from the vessel effluent after air is added into liquid sludge, which is then heated and supplied into a pressure vessel where organic material is stabilized.

Lime Stabilization

• To raise the pH of untreated liquid sludge to 12.0 or higher, lime slurry is added.
• High pH prevents additional biological activity in the sludge, making it stable.

Post-Lime Stabilization

• After dewatering, it is used to improve or stabilize the sludge.
• In a blending unit typically located at the discharge end of the dewatering equipment, quicklime, calcium oxide (CaO), is blended with the dewatered sludge cake.
• Calcium hydroxide (Ca(OH)2) is formed when quicklime reacts with moisture left in the dewatered sludge cake. This reaction produces heat.
• Depending on the lime dosage, the resulting calcium hydroxide raises the pH of the dewatered sludge to 12 or higher.
• Pathogens and viruses are inactivated by high pH and high temperatures (140 to 150°F), and the dewatered sludge cake is stabilized to prevent further decomposition.

Dewatering Technologies

• It is usually performed after digestion and is required prior to incineration.
• The goal is to remove water from the sludge in order to reduce overall volume.
• The solids content rises from 1 to 4% Total Solids to 15 to 20% Total Solids.

Technologies

• Mechanical belt filter press with a low energy requirement
• Centrifuge – a mechanical device that requires a large amount of energy.
• Sludge drying beds – non-mechanical, no energy input; gravity and evaporation are used.