Raw Water

What the raw water system does

A surface water plant draws from a river, lake, or reservoir through an intake structure, screens it, and pumps it to the head of the treatment process with low-lift pumps. The rate at which it does that is the plant production rate. Every process downstream is sized and paced for it: the coagulant dose is a ratio to it, the flocculation and sedimentation basins have detention times that depend on it, the filters have loading rates set by it, and the disinfection contact time is computed from it. Controlling raw water well means delivering the rate the plant needs, changing it slowly, and knowing what is in it.

Flow control

The plant flow setpoint usually comes from finished water storage: the clearwell level, or the level of the distribution tanks that the clearwell feeds. Falling level calls for more production, rising level for less, within a band that keeps the plant running at a few discrete rates rather than hunting. Many plants run a production schedule instead, with operators setting a rate for the shift and the clearwell absorbing the difference. Either way, the raw water flow controller holds the setpoint with a control valve on the low-lift discharge, a drive on the pumps, or a combination, and the pumps stage up and down as the setpoint moves.

• Ramp the setpoint. A step of 20 percent in raw water flow hits the flocculators and filters at once and shows up as a turbidity spike an hour later. Ramp at a rate the process engineer sets, often a few percent per minute.
• Stage pumps by the controller output, not by flow alone. Start the lag pump when the lead is at maximum for a time and the flow is still below setpoint; stop it when the lead is at minimum for a time.
• Protect the minimum flow. Filters have a minimum rate below which they do not work well, and some chemical feeds have a minimum. The controller has a floor.
• Hold flow through a source change. Switching intake ports or wells changes the quality, not the rate; the flow controller keeps the plant steady while chemistry is adjusted.

Instruments

Watching the source

Raw water quality changes with storms, seasons, lake turnover, algae blooms, and upstream events, and the change usually arrives before anyone has seen it. The online instruments trend continuously on the SCADA system, and the useful alarms are on rate of change as well as on absolute value: turbidity that doubles in an hour is a storm plume arriving, whatever the number is. Operators respond by adjusting coagulant dose, changing intake ports where the structure allows drawing from a different depth, and slowing production so that the process has more time. A plant with several intake ports uses them deliberately: a deeper port during a bloom, a shallower one during turnover, and the port selection and its reason are recorded.

Protecting the intake

• Low level: stop the pumps before they lose suction, and alarm early enough for the operator to reduce production first.
• Screen differential: clean on a differential or a timer; alarm on high differential and on screen drive failure. A blinded screen starves the pumps.
• Zebra mussel and biofouling control: chemical feed at the intake, paced to raw water flow, with feed failure alarms.
• Ice and frazil: intake heating or port changes in cold climates, with temperature as the trigger.
• Security: intrusion and door alarms on the intake structure, which is usually remote and unattended.