This article reviews the unit operations in a wastewater treatment plant, and describes the fundamental biological and chemical principles behind treatment technologies.
Detailed design of industrial effluent treatment plants is a matter best left to specialists. Being able to design a chemical plant or an oil refinery does not mean that you are qualified to design a facility to treat the aqueous effluent generated by that plant or refinery. However, it is useful for all chemical engineers to be knowledgeable about the basics of the discipline to be able to discuss it intelligently with designers and suppliers.
Effluent treatment has its own unit operations and jargon, which you need to understand to specify the plant, but there are also several concepts that chemical engineers may be unfamiliar with. The two areas that chemical engineers tend to find most challenging are water chemistry and biology.
This article describes the key unit operations in an industrial effluent treatment plant, defines some of the terminology used by water specialists, and explains the relevant water chemistry and biology.
Despite its ubiquity, water is strange stuff. Of particular interest to the water engineer is its ability to solubilize substances with varying degrees of dissociation into ions, which underlies pH and related properties, such as buffering.
The concentration of hydrogen ions in water is expressed as pH, which is approximately equal to the negative base-ten logarithm of the hydrogen ion concentration. As a reminder, the scale runs from 0 (very strong acid) to 14 (very strong alkali). A pH of 7 indicates a solution is neutral, neither acidic nor alkaline.
The most common requirement for discharging treated effluent to the environment is that it must be neither too acidic nor too alkaline. Acceptable ranges are set by environmental regulators, usually from around pH 5 to pH 9, to avoid killing aquatic life.
Strong acids such as hydrochloric acid are highly dissociated in solution — that is, the vast majority of dissolved HCl is in the ionic form (H+ and Cl–). It does not take a lot of hydrochloric acid to obtain a solution at pH 1. However, a weak acid such as acetic (ethanoic) acid is poorly dissociated, with a far smaller proportion of its hydrogen ions available in solution. No amount of added acetic acid will produce a solution at pH 1.
The strength of acids or alkalis is not defined by their concentration, and solutions of both weak and strong acids do exhibit some concentration-related pH change. A 0.01-M solution of HCl has a pH of 2, and a pure 1-M solution has a pH of 0.1. For comparison, a pure 0.01-M acetic acid solution has a pH of 3.4, and a 1-M solution has a pH of 2.4.
Effluent streams are not pure solutions, and the contaminants in them interact in complex ways. A pump to deliver sufficient acid (or alkali) to neutralize the alkali (or acid) present in an effluent cannot be designed based on first principles using pH as a measure of the amount of hydrogen ions to be added or removed. This is mainly because ions present in water interact with added acid or alkali to prevent the pH from changing — a phenomenon known as buffering. Carbonic acid (H2CO3) and its derivatives — hydrogen carbonate (HCO3–) and carbonate ion (CO32–) — are the most common buffering agents in natural waters, although there may be other buffers in industrial effluent.
To understand how pH might not change when acid or alkali is added to a carbonate-buffered system, it may help to think of carbonic acid as being a weak acid and hydrogen carbonate a weak alkali, both of which keep pH stable if a small amount of acid or alkali is added. The higher the concentration of the buffering substances, the stronger the effect.
Take, for example, the acetic acid discussed earlier. Even though a 1-M solution may have a pH of only 2.4, it has just as many hydrogen ions as a 1-M solution of hydrochloric acid (although not all of acetic acid’s hydrogen ions will be in solution). A 10-M solution of acetic acid has ten times as many hydrogen ions, but is still only pH 2.4, because acetic acid is such a weak acid and it has poor dissociation.
There are often many weak acids and bases in an effluent, and water is itself amphoteric — i.e., it is both a weak acid and a weak base. Accurately estimating the number of available hydrogen ions to determine acidity, or the equivalent number of hydroxyl ions to determine alkalinity, is practically impossible.
Buffering in most process effluents can be accounted for by considering the interactions between carbonic acid, hydrogen carbonates, and bicarbonates, as well as pH. In water engineering, carbonates, bicarbonates, and hydroxides taken together constitute alkalinity, which is measured by titration against a strong acid with one of two indicators. Methyl orange changes color from red to orange-yellow at pH 4.6, at which point all three alkaline ions are neutralized. This measure of alkalinity is known as total alkalinity or “M Alk.” Phenolphthalein changes from colorless to pink at pH 8.2, at which point only the hydroxide ions have been neutralized. This estimate of alkalinity is known as “P Alk.”
M Alk and P Alk can be used along with pH to determine the amount of acid or alkali required to generate a specified pH. You do not need to be able to perform the necessary calculations in order to specify a system, but giving the plant supplier M Alk, P Alk, and pH enables them to accurately estimate dosing pump sizes. These three values are commonly not available to the designer, so conservative rough estimates must be made.
Readers who want to carry out these calculations themselves can find a good account of how to do so in Ref. 1.
Volumetric flows of effluent vary over time, because flows through the main plant vary greatly. The plant may run...
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