4.1 Raw Water Intakes, Screening Equipment & Raw Water Pumping

The first equipment in the train

Everything downstream depends on getting raw water into the plant reliably and free of debris. The WPI Equipment Operation and Maintenance outline lists "raw water intake, screening, and pumping" as the first item an operator inspects, maintains, and operates, and a plant that loses its intake has lost everything.

Intake configurations

The multi-level intake tower is the operator's most valuable water quality tool. A stratified Colorado reservoir has warm, algae-bearing water at the surface and cold, anoxic water carrying dissolved iron, manganese, ammonia, and sulfide at the bottom. Mid-depth withdrawal usually gives the best water. During a summer cyanobacteria bloom you drop below the epilimnion; when the hypolimnion goes anoxic you rise above it; after turnover the whole column is mixed and port selection stops mattering until stratification re-establishes. Ports are exercised on a schedule so gates do not seize in position, and each port should have a sampling capability so the operator can compare before switching.

Intake velocity is a design constraint with an operational consequence. Approach velocities are kept low — commonly 0.5 ft/s or less at the screen face — to reduce debris and fish entrainment and to limit headloss. High velocity pulls leaves, ice, and organisms onto the screen faster than they can be removed.

Screening equipment, coarse to fine

1. Trash rack (bar rack). Vertical or inclined steel bars with clear openings of roughly 1 to 3 inches, stopping logs, branches, and large debris. Cleaned by mechanical rake or by hand. A plugged trash rack shows up as a rising differential between river and wet well level.
2. Traveling water screen. A continuous belt of wire-mesh panels, typically 3/8 inch mesh, that rotates through the flow and is washed at the top by a high-pressure spray wash discharging debris into a trough. Screens may be run continuously in high-debris conditions or intermittently on a timer or differential level setpoint. Rising differential across the screen is the primary operating indicator.
3. Microstrainer. A rotating drum covered in fine fabric, commonly 20 to 60 microns, that removes algae and fine organic matter. Also backwashed by spray. Microstrainers reduce algal loading on filters, but they blind quickly during heavy blooms.
4. Fish protection. Where required, fine-mesh screens with low approach velocity, behavioral barriers, or return systems.

Maintenance items to know: spray nozzles plug and must be cleaned or replaced; chain, sprocket, and bearing wear on traveling screens produces mistracking; screen panels corrode and develop holes that pass debris; and the debris trough must actually drain or it backs up into the screen.

Cold-weather intake problems

Two ice phenomena appear on Colorado exams because they genuinely shut plants down.

• Anchor ice forms on the streambed and on submerged structures in shallow, turbulent water.
• Frazil ice is the more dangerous. In turbulent, open water that has become slightly supercooled, ice forms as fine crystals suspended in the water column rather than as a surface sheet. Those crystals adhere to cold metal — trash racks, screens, and intake gratings — and build up rapidly. The signature is an intake that blocks solid on a clear cold night with no visible surface ice. Countermeasures include heating or coating the rack, backflushing warm water, air bubbling, and switching to an alternate intake.

Raw water (low-lift) pumping

Low-lift or raw water pumps move water from the intake structure to the head of the plant. They are usually vertical turbine pumps set in a wet well, or horizontal split-case pumps with a flooded suction.

Operating essentials:

• Never run dry. Vertical turbine pumps rely on pumped water to lubricate line-shaft bearings unless externally lubricated. Low wet-well level must trip the pump, and the level switch is safety equipment, not a convenience.
• Suction blockage is the hidden failure. If the screen plugs, wet well level drops, submergence is lost, the pump begins to cavitate or draw a vortex, and motor amperage falls rather than rises. Falling amperage on a raw water pump with rising screen differential is a debris problem, not a pump problem.
• Air relief and priming. Where suction lift exists, a priming system with vacuum pumps or foot valves is required, and it must be tested.
• Flow matching. The plant is a series of unit processes sized for a design rate. Raw water pumping is throttled, staged, or driven by variable frequency drives to hold a steady plant rate rather than swinging with demand — the clearwell and distribution storage absorb demand variation.
• Grit protection. Sand pumped from a river destroys wear rings, mechanical seals, and check valve seats. Presedimentation exists partly to protect this equipment.
• Instrumentation to trend: wet well level, screen differential, pump discharge pressure, flow, motor amperage, bearing temperature, and vibration.