BOD and Suspended Solids Removal | Activated Sludge Process | Wastewater Treatment Plant

 BOD and Suspended Solids Removal

Wastewater is a complex mixture of many different components. Suspended particles, biodegradable organics, pathogens, nutrients, refractory organics, heavy metals, and dissolved inorganic salts are all major contaminants in wastewater.

In general, the first three contaminants - suspended solids, biodegradable organics, and pathogens - are the most concerning.

The challenge of wastewater engineering is to design, build, and properly operate treatment plants that protect receiving waters (for health and aesthetic reasons) while remaining cost effective. When one considers the massive amount of wastewater generated, this is no small task. In fact, technology exists to convert even the most contaminated water to high quality drinking water (or even better!). However, the costs would be prohibitively expensive.

To meet this challenge, we rely primarily on two low-cost phenomena: gravity settling (sedimentation) and biological oxidation (activated sludge).

The major elements of this typical plant are well known to all of us, but it is important to remember that the two basic phenomena - settling and biological oxidation - are at the heart of this plant.

If such a thing exists, typical wastewater would contain about 220 mg/l of suspended solids (SS) and 220 mg/l 5 day biochemical oxygen demand (BOD,.). Because preliminary treatment is intended to remove large objects, virtually no BOD and SS are removed during this step. During primary sedimentation, settling removes approximately 50% of the SS. Furthermore, because some BOD is associated with particles that can settle out, approximately 35% of the BOD is removed. After primary treatment, our typical wastewater contains 110 mg/l SS and 140 mg/l BOD. This is rarely enough effluent quality to control water pollution. As a result, additional treatment is required following primary sedimentation. Secondary treatment accomplishes this.

The BOD left over after primary sedimentation is made up of both soluble and particulate organic matter. Proteins, carbohydrates, and lipids are examples of soluble organics. The particulates (SS) are mostly organic in nature and biodegradable once hydrolyzed (or dissolved). As a result, BOD and SS could be considered food for microorganisms such as bacteria, fungi, and protozoa. This is the issue and "solution" of water pollution control. When untreated wastewater is discharged into receiving water, microorganisms degrade the organics through natural processes that take many days. As a result, the water loses oxygen, resulting in unpleasant odours and sights.

The "cure" is to use the same natural processes that take days in carefully engineered and operated reactors to carry out microbial reactions in hours by pumping oxygen into the reactors.