Secondary and Tertiary Wastewater Treatment
Secondary treatment uses biological treatment processes. Microorganisms convert nonsettleable solids into settleable solids. Sedimentation typically follows, allowing the settleable solids to settle out. It removes approximately 85% of the BOD and TSS in wastewater. Secondary treatment for municipal wastewater is the minimum level of treatment required by the Clean Water Act. It includes:
1. Activated Sludge- This is the most common option in secondary treatment. It starts with aeration that encourages the growth of microbes in the waste. The microbes feed on the organic material, which then allows solids to settle out. Bacteria-containing "activated sludge" is continually recirculated back to the aeration basin to increase the rate of organic decomposition.
A closer look reveals how air is release from the bottom of the tank.
2. Trickling Filters- These are open tanks of "coarse media" (rocks about 3 inches in diameter -- usually stones or plastic, and not sand!) about 3-10 ft. deep. Wastewater is sprayed into the air (aeration), then allowed to trickle through the coarse media. Despite its name, trickling filters do not actually filter the water. Instead, the larger stones serve as a surface on which microbes grow. These microbes, attached to and growing on the stones, break down organic material in the wastewater. Trickling filters drain at the bottom; the wastewater is collected and then undergoes sedimentation.
3. Lagoons- (oxidation ponds or stabilization ponds): this method uses ponds to treat wastewater. Algae grow within the lagoons and utilize sunlight to produce oxygen, which is in turn used by microorganisms in the lagoon to break down organic material in the wastewater. Wastewater solids settle in the lagoon, resulting in effluent that is relatively well treated, although it does contain algae. These are slow, cheap, and relatively inefficient, but can be used for various types of wastewater. They rely on the interaction of sunlight, algae, microorganisms, and oxygen (sometimes aerated).
After primary and secondary treatment, municipal wastewater is usually disinfected using chlorine (or other disinfecting compounds, or occasionally ozone or ultraviolet light). An increasing number of wastewater facilities also employ tertiary treatment, often using advanced treatment methods.
Tertiary treatment (sometimes called "advanced treatment" ) includes any level of treatment beyond secondary treatment. These processes can be physical, biological, or chemical. Processes for treating drinking water can be adapted to tertiary treatment. They can include: filtration, activated carbon adsorption, and coagulation.
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