Chemicals for water and wastewater treatment

Municipal and industrial treatment plants use chemistry to meet discharge permits and protect equipment. Coagulants such as ferric chloride 40% and aluminum sulfate 50% neutralize the charge on suspended particles so they gather into floc that settles in the clarifier. Iron and aluminum salts also precipitate dissolved phosphorus, which is why many wastewater plants dose them for chemical phosphorus removal. Both consume alkalinity, so operators add sodium hydroxide or soda ash to hold pH where floc forms, and sulfuric or hydrochloric acid to bring high-pH streams down.

Sodium hypochlorite oxidizes iron, manganese and hydrogen sulfide. Sodium bisulfite or sodium metabisulfite removes residual chlorine before discharge, or ahead of reverse osmosis membranes, which chlorine damages. Membranes are cleaned periodically with citric or hydrochloric acid for scale and with caustic and a chelant for organics. Dosing errors show up as poor settling, solids carryover into the effluent and permit exceedances.

Most wastewater and industrial treatment runs on technical grade, and that is the right choice for bulk pH adjustment and coagulation. The exceptions come from impurities and end use. An acid high in iron fed ahead of a reverse osmosis system can foul the membranes, and an acid cleaner carrying metals adds load the plant was not designed to remove. Commercial hydrochloric acid quality varies by source, so check heavy metals on the COA when your permit has tight metal limits. Membrane makers often name the cleaning chemicals they accept. Drinking water plants usually require NSF/ANSI/CAN 60 certified product; check before you order, because the grade name alone does not tell you whether a product is certified.

Wastewater discharges fall under the Clean Water Act. Direct dischargers hold NPDES permits, and industrial users sending water to a municipal plant follow the EPA pretreatment program (40 CFR Part 403) and local limits. Drinking water treatment falls under the Safe Drinking Water Act, and most states require chemicals added to drinking water to be certified to NSF/ANSI/CAN 60. Plants storing chlorine gas or other listed chemicals above threshold quantities may fall under OSHA Process Safety Management (29 CFR 1910.119) and the EPA Risk Management Program, and EPCRA Tier II reporting covers hazardous chemicals stored above reporting thresholds. Keep an SDS for every chemical on site, and a COA for lots where your permit or quality plan asks for one; it is free on request.

• Switching strength without resetting the pump. Moving from sodium bisulfite 40% to 25% without raising the feed rate under-doses, and residual chlorine can reach the outfall.
• Changing acid sources without checking metals. A new hydrochloric acid source with more copper or other metals can push effluent over a monthly limit.
• Storing hypochlorite in sun or heat. Strength drops quickly, the oxidant dose falls short and decomposition builds chlorate. Store it cool and shaded, and rotate stock.
• Leaving 50% caustic in a cold tank. It starts to crystallize at about 54 °F and blocks feed lines. Heat the tank or switch to 25% for winter.