PH adjustment
Adjusting the pH of water and wastewater is carried out by the addition of an acid or a base, depending on the target pH and process requirements.
Some activities, such as boiler operations and drinking water requirements, demand pH 7 neutral water.
Water or wastewater is generally regarded effectively neutralized if (1) it causes minimal harm to metals, concrete, or other materials, (2) it has little effect on fish and aquatic life and (3) it has no effect on the biological matter (i.e., biological treatment systems).
Excess alkalinity or acidity must frequently be neutralized in chemical industrial treatment.
One of the most important aspects of water neutralization is determining the nature of the compounds that generate acidity and alkalinity.
In general, this is accomplished in laboratory-scale investigations by preparing titration curves that illustrate the amount of alkaline or acidic material required to modify the pH of the target wastewater.
The nature of titration curves obtained in these experiments is critical in determining the proper chemical type and dose.
Methods used for pH adjustment should be selected based on costs associated with the neutralizing agent and equipment requirements for dispensing the agent.
Common Neutralization Treatments in industry
The application of neutralization varies per industry.
The most common application includes the neutralization of acidic waste from mining industries, chemical precipitation, water softening and wastewater coming out from electronic manufacturing plants, and coagulation and flocculation in wastewater treatment plants.
Neutralization is also required for treated wastewater if the pH of such water is found to be higher or lower than the permissible discharge limits.
The following application is for neutralization and pH adjustment in the industrial field.
Water Softening
As explained earlier, the hardness of water is caused by the presence of polyvalent metal cations.
The major disadvantages of using this type of water are the increased consumption of soap required to produce lather when bathing or washing clothes and the formation of scales in boilers if this hard water is used for generating steam.
Chemical precipitation is commonly employed to soften the water, where alkalis are added to the water to raise the pH and precipitate the metal ions in the form of hydroxides and carbonates.
Softened waters usually have high pH values in the range of 10.5 and are supersaturated with calcium carbonate and magnesium hydroxide.
For further use of such high-pH waters, acid neutralization is applied.
Adjustment of pH toward neutrality is accomplished either by re-carbonation or by adding sulfuric acid.
pH adjustment by re-carbonation
pH adjustment by re-carbonation can proceed in two different ways: one-stage re-carbonation or two-stage re-carbonation.
In one-stage re-carbonation, enough CO2 is passed only one time to drop the pH to the desired level.
When sulfuric acid is used in place of CO2 in one-stage re-carbonation, the process is simply called one-stage neutralization.
In two-stage re-carbonation, CO2 is added to water at two different points after excess lime treatment.
At the first point of addition, the CO2 is passed to precipitate calcium carbonate. In the next step, CO2 is added to adjust pH to acceptable levels.
Metal Precipitation
Metal precipitation through the formation of metal hydroxide is one of the common methods of metal removal in industries.
At high pH, most of the metal hydroxides are insoluble and come out of the solution in the form of metal hydroxide precipitates.
Metals are precipitated as hydroxides through the addition of lime or another base to raise pH to an optimum value [2, 3, and 4].
Metal carbonate sediments can also be formed once soluble carbonate solutions such as sodium carbonate are added to metal solutions.
Because pH is the most important parameter in precipitation, control of pH is crucial to the success of the process.
Mine Drainage
The wastewater coming out of mining industries is highly acidic due to the presence of sulfuric acid in appreciable quantities.
Acid water coming out of mining industries is one of the common problems prevalent in the United States and around the world.
When sulfide minerals, primarily pyrite (FeS2), are present in mine waste, they can produce acid mine drainage when they come into contact with water and air.
Pyrite oxidizes to release sulfuric acid into water, resulting in a pH decrease that can be lower than 2.
Most of the metals coming in the mine waste dissolve at this pH, resulting in water that is toxic to aquatic life.
Chemical treatment by neutralization and subsequent precipitation is often applied to Acid Mine Drainage.
The pH range for point source discharge is set by the US EPA.
is in the range of 6–9. The alkali comparison for acid mine drainage is given in Table 1.
Table 1 Alkali Comparison for Treatment of Acid Mine Drainage
| Alkali | Formula | Molecular weight | Equivalent weight | Factor a |
| Ca Neutralizers | ||||
| Hydrated lime | Ca (OH)2 | 74.10 | 37.05 | 1.35 |
| Quicklime | Cao | 56.08 | 28.04 | 1.78 |
| Limestone | CaCO3 | 100.08 | 50.04 | 1.00 |
| Mg neutralizers | ||||
| Dolomitic lime | Mg (OH)2 | 58.03 | 29.15 | 1.72 |
| Na neutralizers | ||||
| Caustic soda | NaOH | 39.99 | 39.99 | 1.25 |
| Soda ash | Na2CO3 | 105.99 | 53 | 0.94 |
a Factor to convert CaO to CaCO3 equivalence
Metal Sorption
Activated carbons have successfully been used for metal removal.
They are typically utilized for suspended particle filtration as well as organic and metal adsorption.
Removal of heavy metal ions from waste streams by inexpensive recyclable bio sorbents has emerged as an innovative technique in the last two decades.
The primary benefit is the high removal efficiency for metal ions. This is commonly referred to as biosorption.
Biodegradation is usually not involved because most bio-sorbents are passive.
The term “biosorption” is used because bio sorbents are made from organisms like bacteria and seaweed.
Numerous studies have shown that the sorption of metal ions from aqueous solutions is strongly pH-dependent.
An increase in the solution pH results in a decrease of positive surface charge and an increase of negatively charged sites and, eventually, an increase in metal ion binding.
Normally the pH effect becomes less important when the pH is above 4–6. The metal ion adsorption onto activated carbon increases from 5% to 99% from pH 2.0 to 5.5.
Sorption experiments using calcium alginate beads (a bio sorbent) demonstrated that the metal removal percentages increased from 0 to almost 100% (for metal concentrations < 0.1 mM) from pH 1.2 to 4 and a plateau was established at a pH > 4.
Therefore, neutralization pretreatment must be performed if the initial pH value of the metal waste stream is less than 6.
pH Adjustment for industrial wastewater
In contrast to home or municipal sewage, where the pH range is normally 6.0-7.5, industrial wastewaters have a much wider pH range ranging from very acidic to very alkaline.
It should also be noted that a facility may generate various wastewater streams, some of which may be acidic while others may be alkaline.
As a result, it can be beneficial in terms of decreasing chemical consumption for pH adjustment by providing adequate equalization before pH correction so that the various wastewater streams can achieve a degree of pH adjustment through their interaction.
This is especially essential if the acidic and alkaline streams are not formed simultaneously.
Holding and mixing are now required activities. The challenges with pH do not usually emerge since it is a natural property of wastewater.
It should be highlighted that if the chemical breaking of oily emulsions and coagulation were required, pH might be altered and may need to be changed again before biological treatment.
Automatic pH correction can be a surprisingly tough task to complete successfully.
This is due, in part, to the difficulties of uniformly mixing a small amount of reagent with a big volume of wastewater.
This is even more challenging if the features of the wastewater, such as its discharge rate, alter fast.
It is impossible to overestimate the ford of good equalization or blending before pH correction.
Because many industrial wastewater treatment plants are small in size, sodium hydroxide is frequently favored over lime for pH adjustment of acidic wastewater.
Before injecting sodium hydroxide into the pH correction tank, a solution of sodium hydroxide would be created.
Lime built up as a slurry can be employed in industrial wastewater treatment systems where the chemical consumption is large enough to justify the additional handling facilities required.
Handling lime powder (the normal form in which it is provided to industrial wastewater treatment plants) involves safety criteria that small industrial wastewater treatment plant operators may not be able to meet.
Lime is commonly used because it is less expensive than sodium hydroxide.
When using lime, it is important to remember that it is slower than sodium hydroxide.
This means that the response tank must be expanded to accommodate the longer hydraulic retention times required.
Typically, a minimum of 20 minutes of HRT is permitted.
The contents of the reaction tank are combined using either a mechanical stirrer or air.
Sulphuric acid is a common chemical used to adjust the pH of alkaline wastewater.
The rationale for the selection is mainly cost.
Sulphuric acid may be substituted for hydrochloric acid if the downstream processes contain anaerobic processes and relatively significant amounts of acid are required.
Because sulfates can be decreased in the anaerobic process, odorous and corrosive hydrogen sulfide is emitted along with the anaerobic reactor’s gaseous emissions.
An example of a pH correction station at a pharmaceutical factory
The pH correction station has two chambers designed to deal with the high pH of the incoming wastewater.
At some times, the latter had reached 9.5 or higher.
It should be emphasized that the relationship between pH and the required reagent flow to effect change is highly nonlinear.
Because of the pH scale’s logarithmic structure, this occurs.
A pH unit shift represents a tenfold difference in acidity or alkalinity.
The tank’s two chambers were set up in a series, with the first chamber being smaller than the second.
The pH in the first chamber was controlled over a reasonably broadband with a bigger dosing pump, whereas the pH in the second chamber was adjusted over a considerably narrower band with a smaller dosing pump.
This method was required to avoid the pH swings that would have occurred if a single adjustment chamber with a big dosing pump had been utilized.
The containers used in an industrial wastewater treatment plant for the various unit processes can be fully excavated, partially excavated, or erected at ground level.
The decision as to which level a vessel should be placed is frequently based on the desired hydraulic grade line so that flow through an industrial wastewater treatment plant can be maintained, to the greatest extent possible, without the aid of additional pumping after the lift at the plant’s start.
While the preceding is a common criterion for determining the level, it is not always the case.
Excavated vessels were found at the pH correction station.
The key rationale for doing so, in this case, was to preserve an acceptable degree of aesthetics for the owner.
A bioprocess is used in the treatment train of many industrial wastewater treatment plants.
Bioprocesses are sensitive to pH and work best within a fairly limited range of 6.5-7.5.
While pH values outside of this range do not always result in a harmful situation, the bioprocess may be impeded or certain microorganisms may be favored over more desirable microorganisms.
Thickener sludge is one probable outcome of the latter occurrence.
Failure to manage pH adequately has been identified as the root cause of a surprisingly significant proportion of industrial wastewater treatment plants failing to produce wastewater of the required quality from both bioreactors and Physio-chemical processes.
References
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[3] J. P. Chen and H. Yu, Lead removal from synthetic wastewater by crystallization in a fluidized bed
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[4]. L. K. Wang, Y. T. Hung, H. H. Lo, and C. Yapijakis (eds.), Handbook of Industrial and Hazardous Wastes Treatment. Marcel Dekker, Inc., NY, NY. (2004).
[5]. US EPA, Design Manual—Neutralization of Acid Mine Drainage, U.S. Environmental Protection Agency, Municipal Environmental Research Laboratory, EPA-600/2-83-001, U.S. Environmental Protection Agency Technology, Cincinnati, OH, 1983.
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[8]. J. P. Chen and L. Wang, Characterization of a Ca-alginate based ion exchange resin and its applications in lead, copper and zinc removal. Separation Science and Technology, 36(16),3617–3637 (2001).
[9]. J. P. Chen, L. Hong, S. N. Wu, and L. Wang, Elucidation of interactions between metal ions and Ca-alginate based ion exchange resin by spectroscopic analysis and modeling simulation, Langmuir, 18(24), 9413–9421 (2002).
[10] Industrial Wastewater Treatment, NG Wun Jern, National University of Singapore, Imperial College Press 2006, p (54-56).
