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Wastewater Treatment Process
What are the Major Water Pollutants?
There are several classes of water pollutants, which are usually caused by human activities:
What is a Acid Rain?
Rainwater typically has a pH of about 5 to 6, a naturally neutral and slightly acidic liquid. During precipitation, rainwater dissolves gasses (such as carbon dioxide and oxygen). Because our industries now emit great amounts of acidifying gasses (including carbon monixide and sulphuric oxides), these gasses are also dissolved into rainwater, which cause the pH of precipitation to fall to 4 or below.
When a pH is below the neutral 7, it becomes acidic. The lower the pH, the more acidic. When rain has a much lower-than-normal pH, it is called acid rain. Acid deposition typically has a pH below 4; seriously acidic conditions can create a pH as low as 1.5. When people die of acid deposition, it is usually caused by excessive mucous production in the bronchi, leading to choking from a lack of oxygen or to a heart attack.
Sludge particles produced in wasterwater by growth of organisms in aeration tanks. The term “activated” is based on the fact that particles are inundated with bacteria, fungi and protozoa.
ACTIVATED SLUDGE PROCESS
A biological wastewater treatment process that speeds up waste decomposition. When activated sludge is added to wastewater, it is aerated and agitated for a set amount of time, then allowed to settle out by sedimentation.
Condition where oxygen is present.
Condition where oxygen is absent (or sufficiently depleted).
ATPs (adenosine triphosphate) are molecules found in and around living cells. ATP test rapidly measures actively growing micro-organisms by detecting ATP.
Biological Oxygen Demand. The measure of oxygen used by microorganisms to decompose water, BOD is a test used by wastewater treatment plants as a measure of the oxygen-depletion effect of a waste contaminant. The 5-day BOD is the universally-accepted standard used to measure relative pollution effect. It measures the amount of dissolved oxygen consumed by biochemical oxidation of waste contaminants over a 5-day period. [BOD and COD lab tests are both used as wastewater quality indicators to determine if specific wastewater will have a significant adverse effect on fish or aquatic plant life.]
Sludge bulking occurs when the sludge does not settle properly and fails to separate out in the sedimentation tanks. Bulking is primarily caused by the growth of filamentous bacteria.
The long solid strands that result during sludge bulking. Similar to floc, filamentous bacteria have much greater volume and surface area than conventional floc and are very slow to settle.
Process of agitating wastewater to induce the small suspended particles to join together into heavier particles (termed “floc”) so they settle out.
Activated sludge mixed with raw wastewater
Rotating Biological Contactor is a biological process used in a secondary wastewater treatment plant. The RBC process uses 3-4m plastic disks mounted on a motorized shaft. When partially submerged in the pretreated wastewater, the RBC shaft is rotated so the disk continually introduces oxygen into the wastewater along with a biological medium. Most RBC designs include a minimum of 4 or 5 module sets (in parallel or in a series) to obtain the desired nitrification of wastewater.
In wastewater treatment, a sedimentation basin (settling basin or clarifier) is a large tank with a slow waterflow that allows floc to settle at the bottom. Normally, the sedimentation basin is located near the flocculation basin – as long distance transfer could cause the floc to break up. The amount of floc that settles out of the water depends on the basin’s depth and the amount of time retained in the basin.
The solids that settle out during wastewater treatment processing.
The liquid that is removed from settled sludge. It commonly refers to the liquid between the sludge on the bottom of the tank and the scum on the surface.
UV Light is used to ensure water is free from harmful organisms. When UV light is exposed to the water, the DNA (in any living matter within the water) absorbs the light. UV light cross-bonds with the DNA structure causing it to vibrate to the point of rupture and quickly ceasing normal cell functions. Ultra Violet (UV) light is now a standard practice in most municipal wastewater treatment processes.
ASCE/EWRI 2-06 is intended for use by engineers in preparing specs for compliance testing and by manufacturers in developing performance information.
STANDARD FOR MEASUREMENT OF OXYGEN TRANSFER IN CLEAN WATER
ASCE/EWRI Standard No. 02-06
“Standard 2-06 is a revision of ANSI/ASCE 2-91 and provides the current methods for measuring the rate of oxygen transfer – from diffused gas and mechanical oxygenation devices to water. This standard is applicable to laboratory scale oxygenation devices with water volumes from as low as a few gallons of water to full scale systems with huge water volumes (, such as those typically found in activated sludge wastewater treatment processes.
“This test method is based upon removal of dissolved oxygen (DO) from the water volume by sodium sulfite followed by reoxygenation to near the saturation level. The DO inventory of the water volume is monitored during the reaeration period by measuring DO concentrations at several points selected so that each point senses an equal tank volume. The method specifies a minimum number, distribution and range of DO measurements at each point. The data obtained at each determination point are analyzed by a simplified mass transfer model to estimate the apparent volumetric mass transfer coefficient and the saturation concentration. Non-linear regression is employed to fit the model to the DO profile measured at each point during reoxygenation. Estimates are adjusted to standard conditions and the standard oxygen transfer rate is obtained.
“A procedure based on the clean water test results is recommended for estimation of oxygen transfer rates under process conditions. Various components of power consumption are defined and methods for measurement of gas rate and power consumption by the oxygenation device are given. Energy efficiency of the oxygenation device is evaluated as the mass rate of oxygen transferred per unit power consumed.”
Source: Oxygen Transfer Standards Committee of the American Society of Civil Engineers, New York:ASCE, 978-0-78440-848-3 or 0-78440-848-3, 2007, 48 pp.