Residuals and corrosion control

Two jobs the plant cannot skip

A treatment plant makes more than clean water. It makes sludge it has to handle, and it has to keep the finished water from eating the pipes it travels through. Both show up on the exam, the first as residuals management and the second as corrosion control, which is a Lead and Copper Rule requirement, not an option.

The residuals a plant produces

The settling basins drop sludge, the filters produce spent backwash water, and softening or membrane plants make their own concentrated residuals. The plant thickens the sludge, usually by gravity, then dewaters it on drying beds or with a press, and disposes of the solids to a landfill, to land application, or to the sewer where allowed. Backwash water is often recycled back to the head of the plant rather than wasted, which recovers water but returns its load, so it is managed, not just dumped.

Corrosion control and the lead rule

Water that is corrosive dissolves lead and copper out of the pipes and fixtures between the plant and the tap, which is the problem the Lead and Copper Rule exists to solve. The plant controls corrosion by adjusting the water itself, raising pH and alkalinity so it is less aggressive, or by adding a corrosion inhibitor like orthophosphate that lays a protective film inside the pipe. The Langelier index points the direction, corrosive versus scale-forming; the plant's job is to land the water on the non-corrosive side. Optimal corrosion control treatment is what the rule requires a system to install and maintain.

Iron, manganese, and sequestration

Ground water often carries iron and manganese that stain laundry and fixtures and clog mains. The plant either oxidizes them, with chlorine, permanganate, or aeration, and filters out the solids, or, at low levels, sequesters them with a polyphosphate that keeps them dissolved so they do not stain.

Where operators lose points

• treating backwash water as waste when it is usually recycled
• reading a negative Langelier index as scale-forming when it means corrosive
• thinking corrosion control is optional rather than a Lead and Copper Rule requirement
• expecting a sequestering agent to remove iron when it only holds it in solution

Quick reference

• Residuals are sedimentation sludge and spent backwash water; thicken, dewater, dispose, and often recycle the backwash.
• Corrosion control adjusts pH and alkalinity or adds orthophosphate; it is a Lead and Copper Rule requirement.
• Langelier index: negative is corrosive, positive is scale-forming, near zero is balanced.
• Iron and manganese: oxidize and filter, or sequester at low levels.

Where this fits

The rule behind corrosion control is in the regulations guide. See the water treatment exam page for how this fits the whole test, and your state's page for what applies where you're licensed.