Certain types of chemicals are used to clarify wastewater in primary industrial treatment. They are categorized as follows:
– pH adjustment chemicals
– Coagulant chemicals
– Flocculent chemicals
pH adjustment chemicals
Chemicals used in wastewater treatment to adjust pH serve to modify the ionic charge of the wastewater.
A definition of pH is an expression of the intensity of the basic or acidic condition of wastewater. Mathematically, pH is the logarithm (base 10) of the reciprocal of the hydrogen ion activity.
The pH is expressed numerically ranging above 0 thru 14. A pH or 7.0 is neutral and pH below 7.0 is acidic and pH above 7.0 is basic or alkaline.
Modifying the ionic charge of wastewater changes the solubility of certain solids and also allows for physical solids to attract to each other.
Selecting chemicals to adjust wastewater pH requires considering the chemical makeup of the wastewater.
Certain basic or alkaline chemicals used to raise wastewater pH are below:
CaO (calcium oxide or lime), MgO (magnesium oxide), Ca (OH) (calcium hydroxide, a hydrated form of lime) or Mg (OH) (magnesium hydroxide) are the most commonly used chemicals because of availability, low cost, and high capacity. Sludge bulk (volume) is a major problem, but recovery is possible. Lime is usually high calcium and comes either as quicklime or hydrated. It comes in dry form and is mixed with water to form a slurry before use.
Sodium hydroxide (caustic soda). Sodium hydroxide (NaOH) is a convenient, controllable and commonly available chemical, but expensive. It is generally used for small or occasional application or where limitation of sludge deposits is sought. Caustic soda is available in liquid form in two concentrations, 50 percent NaOH, which begins to crystallize at 54 F (12 C). Therefore, it must be properly stored or diluted prior to use.
Also, 25 percent NaOH, is available and begins to crystallize well below freezing temperature. Caustic soda is also available in an anhydrous or dry state (solid, flake, ground, or powdered) at 100 percent concentration. In the dilution process, considerable heat is generated. Therefore, the rate of dilution and method of cooling must be carefully controlled so that there is no boiling or splattering.
Certain acidic chemicals used to lower wastewater pH are:
Sulfuric acid (H2S04) is the cheapest and most readily available. It is strongly corrosive, dense, oily and clear or dark brown (depending on purity). Sulfuric acid should be of the USP (United States Pharmaceutical) grade, free of heavy metals. It is available in a number of grades containing 60 to 94 percent H2SO4. In the 93 percent grade, it is noncorrosive to steel drums; however, upon dilution it is highly corrosive.
Hydrochloric acid (HCl) or muriatic acid is a clear or lightly yellow, fuming, pungent liquid. It is poisonous and may contain iron or arsenic. Hydrochloric acid should be obtained in the purified form (USP) and is shipped in glass bottles, carboys and rubber-lined steel drums, tank cars or trucks. It contains approximately 35 percent available hydrogen chloride. Fuming can be reduced by dilution to 20 percent HCL.
Where available, carbon dioxide (C02) or sulfur dioxide (SO) may be applied in gaseous form. Flue gases are accessible and economical for neutralization of alkaline waters in certain industries.
Also, waste streams with concentrated high or low pH may be used to adjust pH of other waste waters instead of using chemicals to adjust pH.
Coagulant chemicals are used in wastewater treatment to adjust pH and begin coagulating solids in the wastewater. Selecting chemicals to coagulant solids in the wastewater requires considering the chemical makeup of the wastewater.
Certain chemicals to coagulant solids in the wastewater are as follows:
Ferric Chloride, Aluminum Chloride (AlCl3), Aluminum Sulfate [Al2(SO4)3] and Ferric Chloride (FeCl3).
Use of coagulant chemicals to adjust pH and may supplement the pH adjusted with chemicals identified above in the pH adjustment chemicals section of this blog.
Flocculent chemicals are generally synthetic chemicals and have no effect on pH, however they are used in wastewater treatment to flocculate and separate solids from clarified wastewater. Selecting chemicals to flocculate and separate solids in the wastewater requires considering the chemical makeup of the wastewater. Chemicals to flocculate and separate solids in the wastewater using rapid mix and gentle agitation are categorized as different types of polymers that are identified here for your understanding:
– Anionic polymers have a negative charge to the wastewater and are applied with rapid mix and gentle agitation to achieve solids separation.
– Nonionic polymers have a neutral charge to the wastewater and are applied with rapid mix and gentle agitation to achieve solids separation.
– Cationic polymers have a positive charge to the wastewater and are applied with rapid mix and gentle agitation to achieve solids separation.
– Polymers are commonly purchased in a concentrated dry or liquid form and mixed in water to a specific percent solution (i.e. 2%) before use.
Control use of all of these chemicals with appropriate/compatible instruments and pumps.
Always consult your chemical Material Safety Data Sheets (MSDS) for proper storing and handling procedures.