Concentrate Systems

At the skid

The concentrate leaves the last stage through a control valve that holds the concentrate flow at the value that gives the design recovery. The valve sees the highest pressure in the plant on one side and near atmospheric on the other, so it is a severe-service valve with cavitation control, and its position loop is tuned slowly so that it does not fight the permeate flow loop. A pressure sustaining valve or an orifice downstream may hold backpressure on the last stage. The concentrate flowmeter is the second half of the recovery calculation and is checked against the permeate meter and the feed meter, because a recovery calculated from a bad meter scales the last elements.

Monitoring

Disposal routes

Surface water outfall

Discharge to a river, estuary, or ocean through a diffuser under a permit that limits flow, salinity, pH, dissolved oxygen, and toxicity. Pretreatment often includes degasification to strip hydrogen sulfide and carbon dioxide, aeration to raise dissolved oxygen, and pH adjustment. The controls run the degasifier, the aeration, and the pH loop, and hold the discharge within its limits with alarms and a divert.

Sewer

Discharge to a wastewater collection system under an agreement that limits flow, total dissolved solids, and pH so that the treatment plant is not upset. The controls meter the flow, hold pH, and alarm on the agreement limits; a storage tank may buffer the discharge to off-peak hours for the treatment plant.

Deep well injection

Injection below the drinking water aquifers into a confined zone under a permit. The controls hold the injection pressure below the permit maximum, meter the flow, monitor the annulus pressure that proves the well casing is intact, and alarm on any deviation; a loss of annulus pressure or a rise in injection pressure stops injection and the plant.

Evaporation ponds

In dry climates; the controls are pond level, freeboard, and the transfer between ponds, with leak detection monitoring under the liner.

Zero liquid discharge

Evaporators and crystallizers that turn the concentrate into solids; an industrial process with its own control system, rare in municipal plants because of cost.

Injection wells

An injection well is a regulated structure with a tubing string inside a casing and a sealed annulus between them, and the annulus pressure is the proof that neither has leaked. The controls monitor the injection pressure at the wellhead against the permit maximum, the flow and its total, the annulus pressure against its allowed range, and the annulus fluid level or volume; a deviation alarms and, beyond limits, stops the injection pumps and the membrane trains that feed them. Mechanical integrity tests on the well schedule are recorded, and the historian keeps every reading for the permit reports. A concentrate storage tank between the plant and the well lets the trains run through a short well outage and lets injection be steady rather than following train starts.

Alarms and records

• Flow above the permit limit or the disposal capacity: alarm, then reduce plant production.
• Quality outside the permit range: alarm, divert to storage where possible, then stop.
• Injection pressure or annulus deviation: alarm, then stop injection.
• Degasifier or aeration failure: alarm; discharge stops if the permit depends on it.
• Every parameter the permit names, historized at the resolution the permit implies, with the totalizers and the daily values reported from the historian.