Tidbits about Wastewater Treatment

Wastewater is water produced from all the manmade industrial and commercial activities. Containing impurities, wastewater can be quite hazardous and toxic to both people and animals, not to mention the environment. In order for wastewater to be cleaned, once again, for re-use in the environment, it needs to go through a process called wastewater treatment.

  • What are the toxic items found in wastewater?

There are several sorts of nasty contaminants that can be found in wastewater. They range from the biological contaminants, such as parasites and bacteria, to the chemical contaminants, such as pesticides and drugs, and other manufactured contaminants, such as arsenic and ammonia.

Wastewater produced from industries that deal with the production or use of iron, for example, includes products such as ammonia and cyanide. Coal processing in coking plants also produce wastewater, water filled with hazardous pollutants like benzene, cyanide, ammonia, anthracene, naphthalene, cresols, phenols, and other complex natural compounds, jointly referred to as “polyaromatic hydrocarbons” (PAH).

  • How is wastewater treated?

There are a few different methods of wastewater treatment utilized. Each of these methods is an extremely complex procedure or process of removing unwanted impurities, processing it in order to result in a wastewater stream that is desired by anaerobic bacteria, which would, in turn, transform it into useful matter, which can then be safely disposed of for environmental reuse.

Standard actions associated with wastewater treatment:

(1) Solids Removal

The beginning point in all wastewater treatment methods is the extraction of all solids. This process occurs when the majority of the solids found in wastewater are filtered out, only leaving the waste liquids. But the elimination of solids employs different techniques, the most popular being known as sedimentation. The wastewater is typically left untouched until the solids, particularly the larger solids in the batch, have settled at the bottom, forming a slurry or sludge. The finer solids– or those that have relatively fine densities–are removed by utilizing purification or ultra-filtration strategies. One alternative method often employed is flocculation, which is the task of using aluminum salts or poly-electrolytes for stronger removal potential.

As explained by the MRWA, “Flocculation, a gentle mixing stage, increases the particle size from submicroscopic microfloc to visible suspended particles. Microfloc particles collide, causing them to bond to produce larger, visible flocs called pinflocs. Floc size continues to build with additional collisions and interaction with added inorganic polymers (coagulant) or organic polymers. Macroflocs are
formed and high molecular weight polymers, called coagulant aids, may be added to help bridge, bind, and strengthen the floc, add weight, and increase settling rate. Once floc has reached it [sic] optimum size and strength, water is ready for sedimentation.

Design contact times for flocculation range from 15 or 20 minutes to an hour or more, and flocculation requires careful attention to the mixing velocity and amount of mix energy. To prevent floc from tearing apart or shearing, the mixing velocity and energy are usually tapered
off as the size of floc increases. Once flocs are torn apart, it is difficult to get them to reform to their optimum size and strength. The amount of operator control available in flocculation is highly dependent upon the type and design of the equipment.”

(2) Oil and Grease Removal

The next stage of drainage treatment is the removal of oils and greases from the drainage. This can be done utilizing skimming devices and works for oils in open water surfaces. For hydraulic oils and the majority of oil types, which include soluble or emulsified components that cannot be removed by simple skimming, the addition of solvents and surfactants are frequently employed.

Explained by George R Alther, “Removing oil and grease from wastewater is relatively simple and cheap. Process industries should take the lead in addressing these questions, so that they are ready when the law takes effect. Furthermore, if they design their system such that the wastewater can be recycled, they actually can use this new law to save costs and lessen public concern about discharge of contaminated wastewater.

Effective removal of O&G requires an understanding of emulsions, mechanical versus chemical. Furthermore, the operator must know how to break emulsions and how to test effective treatment methods in the laboratory. He then must know how to remove the now mechanically emulsified oil most economically (i.e., how to coalesce the oil droplets effectively and how to reduce the O&G content to non-detect) so that recycling of the wastewater is feasible. This means being familiar with post-polishing techniques, particularly the use of organically modified clays (organoclays). Organoclays remove oil and grease from water at seven times the rate of activated carbon.”

(3) Organics and Acid Removal

The last of drainage treatment is the removal of soft or hard organics, acids, and alkalis, as well as toxic products. There are numerous techniques involved, with increasing complexity. Some techniques at this stage of drainage treatment include distillation, incineration, vitrification, adsorption, garbage dump disposal, and chemical immobilization. For more information on this subtopic, we invite you to read the popular article Treatment Technologies for Organic Wastewater.

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