Category: Engineering

Sludge Retention Time

Sludge Retention Time

10/24/25

“How can we quantify the average amount of time microorganisms remain in a biological treatment system?”

Many wastewater treatment systems use microorganisms in sludge for treatment. However, these microorganisms will stay in the system only for a certain period of time. So, how can we determine how long, on average, these microorganisms will remain in these systems? Well, what if we were to take the mass of microorganisms present and divide it by the rate microorganisms leave the treatment system? This Sludge Retention Time governs wastewater treatment reactor behavior with the equation Sludge Retention Time = Volume of Reactor Chamber * Mixed Liquor Suspended Solids/(Waste Sludge Flow Rate * Waste Sludge Solids Concentration * Effluent Sludge Flow Rate * Effluent Sludge Solids Concentration). The effluent sludge solids concentration is frequently negligible, so the term is often taken out.

Food to Microorganism Mass Ratio

Food to Microorganism Mass Ratio

10/23/25

“How can we compare the amount of mass a wastewater treatment plant needs to clean versus the potential it can provide?”

Wastewater treatment plants will have to remove a certain amount of organic compounds, as quantified by the BOD loading rate. At the same time, they have a potential measured by the Mixed Liquor Suspended Solids multiplied by the volume. The ratio between these two variables measures the amount of waste that needs treatment versus the capabilities of the system, much like the “food” that needs to be consumed versus the “hunger levels” of the system. In fact, wastewater experts call this relationship the Food to Microorganism Mass Ratio in light of the metaphor.

Volumetric Substrate Loading Rate

Volumetric Substrate Loading Rate

10/22/25

“How can we quantify the amount of organic matter entering a reactor over a given amount of time and volume?”

Reactors treat water to make it usable for people. We can use the BOD loading rate to quantify how much organic matter is entering a reactor over a given time. However, we also might want to know how much treatment is necessary not just per time but also per volume to make it independent of reactor size. We can simply quantify this using the Volumetric Substrate Loading Rate, which is equal to a reactor’s BOD loading rate divided by its volume.

BOD Loading

BOD Loading

10/21/25

“How can we quantify the amount of organic matter entering a treatment system over time?”

Every day, wastewater treatment plants use microorganisms to break down organic matter in water. Plant operators need to estimate how much organic matter is entering a treatment system over time to design their system properly. But how can we do this? Well, we know that we already have a unit, biological oxygen demand, to estimate organic pollutant concentration. And we know that the influent water stream will contain a certain concentration of organic contaminants. So what if we were to multiply the influent volume flow by its BOD concentration (milligrams of oxygen microorganisms need to use to consume pollutants per liter)? Well, this will give us a variable called BOD Loading, measured in mass of oxygen needed for pollution cleaning per time, that plant operators can use for design calculations.

Sludge Dewatering

Sludge Dewatering

10/20/25

“Why do we need to remove water from sludge?”

Sludge is a mixture of solid waste and water created after an industrial or water treatment process. The more water a body of sludge has, the more volume and weight it will take up, and the more difficult it is to handle. Draining water from the sludge will make it lighter and easier to operate. Sludge Dewatering is an essential tool for sludge operators.

Image credit: https://caryloncorp.com/

Perimeter Runoff Control

Perimeter Runoff Control

10/19/25

“Why do we need to protect the perimeter of our construction site from releasing sediment with stormwater runoff?”

Construction operators are obligated to apply best management practices to prevent stormwater runoff sweeping sediment from their sites from entering water bodies. One of the most important aspects is ensuring the perimeter of the site is controlled. Perimeter Runoff Control strategies keep sediment inside the site. Silt fences and sediment basins are examples of perimeter runoff control. The National Pollutant Discharge Elimination System Construction Stormwater Permit requires construction operators to implement perimeter runoff control before beginning soil disturbing activity as part of their stormwater pollution prevention plans. Construction operators can temporarily dismantle perimeter control infrastructure to allow vehicles to enter the site, as long as the infrastructure is restored as soon as possible.

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Electrodialysis

Electrodialysis

10/17/25

“How can we use electric potential to desalinate water?”

Saltwater is much more common than freshwater in the world. Consequently, we could greatly increase our water supplies by converting saltwater to freshwater. But traditional reverse osmosis techniques are incredibly energy-intensive. Is there an easier way to desalinate water? Well, we know that salts have an electrostatic charge. So what if we were to place the water stream between a cathode and an anode and insert an alternating series of membranes that only allow cations or anions to pass? Well, this will create streams of purified water, separated from saltwater. This process, called Electrodialysis, is a smart way to desalinate water and make it useful for people.

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Air Stripping (Water Treatment)

Air Stripping (Water Treatment)

10/13/25

“How can we use air blowers to remove volatile organic compounds from water?”

Toxic volatile organic compounds (VOCs) might seep into water during the water cycle. According to Henry’s Law, low air pressure can remove these VOCs from water. So what if we were to push water into a tower with air from a giant air blower coming from the bottom and move it down while exposing as much surface area as possible to the low-pressure air to filter out the VOCs? This is the idea behind Air Stripping, a common method for treating VOCs in human-use water.

Image credit: https://hydroquipinc.com/

Mass Transfer

Mass Transfer

10/12/25

“How does mass move from one phase/substance to another?”

Substances in different phases or substances may come into contact with each other. This can cause the mass of a certain substance in one phase or concentration to move to another, such as through diffusion (one substance moving from a single concentration in another to uniform distribution), convection (mass transport by bulk motion of a surrounding fluid), absorption (chemical penetration of one substance into another), or distillation (separation of different components by different boiling points). This Mass Transfer is foundational to understanding how chemical engineering systems operate.