3.1 Source Water Quality, Raw Water Intake & Rapid Mixing

3.1 Source Water Quality, Raw Water Intake & Rapid Mixing

Surface water treatment in North Carolina begins with an understanding of raw water characteristics and the hydraulic facilities that withdraw, screen, and prepare source water for chemical clarification. Because North Carolina encompasses three distinct physiographic provinces—the Blue Ridge Mountains, the Piedmont Plateau, and the Atlantic Coastal Plain—surface water supplies exhibit substantial regional diversity in hydrology, mineralogy, and chemical composition.

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1. North Carolina Surface Water Hydrology & Watershed Characteristics

North Carolina is divided into 17 major river basins, including the Cape Fear, Neuse, Yadkin-Pee Dee, Catawba, Tar-Pamlico, Roanoke, and French Broad. The vast majority of the state's population relies on surface water supplies impounded in municipal reservoirs or withdrawn directly from run-of-the-river intakes.

Piedmont Red Clay Mineralogy

The Piedmont province is underlain by intensely weathered crystalline bedrock that has produced deep residual soils known as Ultisols (commonly the Cecil soil series). These soils are rich in kaolinite, halloysite, and illite clays coated with hydrous iron oxides (giving Piedmont soils their characteristic red color).

When eroded into surface waters, these clays break down into sub-micron colloidal particles (0.001 to 1.0 µm in diameter). Because of their microscopic size and negative surface electrical charges, Piedmont clay suspensions possess negligible gravitational settling velocities. A 0.1 µm clay colloid would require decades to settle just one foot by gravity alone. Without chemical coagulation to neutralize surface charges and agglomerate particles into macro-flocs, these suspensions remain turbid indefinitely.

Naturally Low Alkalinity

A defining chemical feature of North Carolina surface water is its naturally low total alkalinity, typically ranging between 10 and 30 mg/L as CaCO3. In mountain headwaters and sandy coastal streams, alkalinity frequently drops below 10 mg/L.

Alkalinity measures the water's buffering capacity to neutralize hydrogen ions (H⁺). Because primary coagulants such as aluminum sulfate (alum) and ferric salts are acidic metal salts that consume alkalinity during hydrolysis, unbuffered NC waters experience severe pH depression when coagulants are dosed. If alkalinity is depleted below 20 mg/L as CaCO3, the coagulation process fails: coagulant metal remains in soluble ionic forms, floc will not form, and corrosive, acidic water enters the distribution network. Operators must routinely supplement raw water alkalinity with hydrated lime (Ca(OH)2), caustic soda (NaOH), or soda ash (Na2CO3).

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2. Reservoir Dynamics: Stratification, Turnover & Algae Blooms

Many municipal water treatment plants in North Carolina draw from man-made impoundments such as Lake Norman, Falls Lake, B. Everett Jordan Lake, and Kerr Reservoir. Deep reservoirs undergo distinct seasonal physical and chemical transformations that directly govern intake operations.

   WINTER MIXING (Uniform 4°C - 8°C)             SUMMER STRATIFICATION
=====================================     =====================================
 Surface: Fully Oxygenated, Uniform Temp   EPILIMNION: Warm (25°C-30°C), High DO,
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                Algae, Daytime High pH
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~    -------------------------------------
                                           METALIMNION / THERMOCLINE:
                                              Rapid Temp Drop (> 1°C / meter)
                                          -------------------------------------
 Bottom: Fully Oxygenated, Low Metals      HYPOLIMNION: Cold (10°C-15°C), Anoxic
                                              (DO = 0 mg/L), Soluble Fe²⁺/Mn²⁺,
                                              H2S, Low pH, High Coagulant Demand
=====================================     =====================================

Thermal Stratification

During late spring and summer, intense solar radiation warms the upper water column while the deeper water remains cold. Because warm water is less dense than cold water, reservoirs establish three distinct vertical zones:

1. Epilimnion: The warm, well-mixed, circulating upper layer (typically 22°C to 30°C in NC summers). It remains saturated with dissolved oxygen (DO) through atmospheric exchange and photosynthetic activity.
2. Metalimnion (Thermocline): The transitional middle layer characterized by a steep vertical temperature gradient—defined as a temperature decrease of at least 1.0°C per meter of depth. It functions as a physical barrier preventing vertical circulation between the upper and lower layers.
3. Hypolimnion: The cold, dense, stagnant bottom layer. Cut off from sunlight and atmospheric reaeration, biological decomposition of settling organic matter consumes all available dissolved oxygen, driving the hypolimnion into an anoxic (anaerobic) state.

Chemical Reductions in the Hypolimnion

Under reducing conditions (negative oxidation-reduction potential, ORP), insoluble minerals deposited in bottom sediments undergo chemical reduction into soluble species:

• Insoluble Ferric Iron (Fe³⁺) is reduced to soluble Ferrous Iron (Fe²⁺).
• Insoluble Manganese Dioxide (MnO2) is reduced to soluble Manganous Manganese (Mn²⁺).
• Sulfate (SO4²⁻) is reduced by anaerobic bacteria into Hydrogen Sulfide (H2S), creating a rotten-egg odor.
• Organic nitrogen is converted to Ammonia (NH3).

If raw water is withdrawn from an anoxic hypolimnion, these reduced constituents enter the plant, consuming immense amounts of chlorine or potassium permanganate, causing taste and odor complaints, and passing through filters to precipitate as brown or black stains (MnO2) in customer plumbing.

Destratification and Fall Turnover

In autumn, ambient air temperatures drop and solar radiation wanes. The epilimnion cools, increases in density, and sinks. Autumn winds provide mechanical energy that disrupts the weakening thermocline, causing the reservoir to completely destratify in an event known as fall turnover.

During turnover, stagnant, anoxic bottom water surges throughout the entire water column. Water treatment plant operators experience an abrupt operational shock:

• Rapid loss of dissolved oxygen at all intake levels.
• Sudden spikes in soluble iron (Fe²⁺) and manganese (Mn²⁺).
• Severe taste and odor episodes.
• Skyrocketing chlorine demand, disrupting primary disinfection CT calculations.

Operators must anticipate fall turnover by monitoring vertical reservoir temperature and DO profiles weekly, switching intake gate elevations, and applying pre-oxidants (such as potassium permanganate, KMnO4, or chlorine dioxide) ahead of rapid mixing.

Algae Blooms and Diurnal pH Fluctuations

In warm, nutrient-rich waters—especially those designated by North Carolina as Nutrient-Sensitive Waters (NSW) such as the Neuse River and Jordan Lake basins—cyanobacteria (blue-green algae) proliferate during summer months. Common genera include Microcystis, Anabaena, and Aphanizomenon.

Algal blooms impact plant operations in four major ways:

1. Taste and Odor Metabolites: Algae release 2-Methylisoborneol (MIB) and Geosmin, earthy and musty compounds detectable by the human palate at concentrations as low as 5 to 10 nanograms per liter (ng/L or parts per trillion). These cannot be removed by conventional coagulation and require powdered activated carbon (PAC), ozone, or advanced oxidation.
2. Filter Clogging: Diatoms with rigid silica frustules (Asterionella, Synedra) blind filter media surfaces, shortening filter runs from 48 hours down to under 12 hours.
3. Diurnal pH Swings: During daylight hours, intense algal photosynthesis consumes dissolved carbon dioxide (CO2): CO2 + H2O + Sunlight → [CH2O] + O2 The removal of carbonic acid drives the raw water pH upward, frequently exceeding 8.5 to 9.2 by late afternoon. At night, photosynthesis ceases while respiration continues, releasing CO2 and dropping the pH back toward 7.0. Because coagulant chemistry is highly pH-dependent, operators must continuously trim coagulant and acid/base dosages to match diurnal swings.
4. Cyanotoxins: Harmful Algal Blooms (HABs) can generate hepatotoxins (microcystins) and neurotoxins (anatoxins), requiring vigilant cell counts and enzyme-linked immunosorbent assays (ELISA).

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3. Raw Water Intake Structures & Debris Screening

Intake structures withdraw raw water from the source and convey it to the low-service raw water pump station while excluding fish, floating debris, and heavy bed-load sediment.

                       RAW WATER INTAKE STRUCTURE

  Source Water Surface
  ~~~~~~~~~~~~~~~~~~~~
        |             [Trash Rack / Bar Rack] (2"-4" spacing)
        |                  |       [Traveling Water Screen] (3/8" mesh)
        |                  |            |         [Low-Lift Raw Pump]
   Upper Gate (O2 Rich)    |            |                 |
        |======> [ ] ----> |            |                 |
                           |   =====>   |   ===========> [PUMP] ===> To Plant
   Lower Gate (Cold)       |            |                 |
        |======> [ ] ----> |            |                 |
  ~~~~~~~~~~~~~~~~~~~~
  Sediment / Bed Load

Intake Configuration Types

• Intake Towers: Concrete towers located offshore in deep reservoir water equipped with multiple inlet sluice gates positioned at different depths (e.g., shallow, middle, deep). Multi-level gates allow operators to select the cleanest water layer—avoiding surface algae blooms and avoiding anoxic bottom sediments.
• Shoreline Intake Wells: Concrete wet wells constructed directly into riverbanks, utilizing gravity intake conduits extending into the river channel.
• Submerged Cribs: Heavy timber or concrete structures anchored to the riverbed or lake bottom, covered with rip-rap and coarse grating, conveying water to onshore wet wells via gravity pipelines.

Screening Systems

1. Bar Racks (Trash Racks):
   • Heavy, parallel steel bars spaced 2 to 4 inches (50 to 100 mm) apart, oriented vertically or inclined at 30° to 45°.
   • Purpose: Intercept large floating debris, logs, tree branches, and trash before they enter pump suction lines.
   • Cleaning: Manually raked at small facilities; mechanically raked with automated differential-head rake carriages at larger plants.
2. Traveling Water Screens:
   • Located downstream of bar racks within the intake wet well.
   • Composed of continuous revolving wire-cloth mesh baskets with square openings of 1/4-inch to 3/8-inch (6.4 to 9.5 mm).
   • Operation: Screens rotate vertically on drive chains. Accumulated debris (leaves, small fish, twigs) is carried upward out of the water channel, where an overhead header of high-pressure water spray nozzles (60 to 100 psi) blasts the debris into a sluice trough for disposal.
   • Activation: Controlled automatically by a differential head sensor (initiating wash cycle when head loss across the screen exceeds 2 to 4 inches of water) or by an automated timer.

Clean Water Act Section 316(b) & Fish Protection

Federal regulations under Clean Water Act §316(b) and North Carolina Division of Water Resources (NC DWR) rules establish strict engineering standards to protect aquatic organisms:

• Impingement: Occurs when fish and macroinvertebrates are trapped against the outer surface of intake screens by the force of the flowing water.
• Entrainment: Occurs when small organisms, fish eggs, and larvae pass through the screen openings into the pumps and treatment plant processes.

To minimize mortality, regulatory standards limit the maximum through-screen intake velocity to 0.5 feet per second (fps) (0.15 m/s) under design low-water conditions. In designated sensitive aquatic habitats or endangered species reaches (such as waters harboring the Cape Fear shiner), through-screen velocities may be restricted to ≤ 0.3 fps, and specialized wedge-wire screens with slot openings of 1.0 to 3.0 mm and automated air-burst cleaning systems are mandated.

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4. Fundamental Raw Water Quality Parameters

Accurate, continuous monitoring of raw water parameters is critical for proactive process control. Operators must never treat raw water as a constant; weather events in North Carolina can alter source water quality within hours.

Stokes' Law and the Temperature Effect

Settling velocity (vs) of suspended particles is governed by Stokes' Law: vs = [g * (ρp - ρw) * d²] / (18 * μ)

Where:

• vs = settling velocity of the particle (ft/s or m/s)
• g = acceleration due to gravity (32.2 ft/s² or 9.81 m/s²)
• ρp = mass density of the particle
• ρw = mass density of water
• d = diameter of the particle
• μ = dynamic viscosity of water

Water viscosity is inversely proportional to temperature: water at 4°C (winter) is nearly twice as viscous as water at 25°C (summer). As viscosity (μ) increases in winter, settling velocity (vs) drops substantially. Particles settle slower in sedimentation basins, and chemical dissolution/hydrolysis reactions occur at reduced rates. Operators treating cold winter water must adjust operations by increasing coagulant doses, adding coagulant aids (polymers), or reducing plant hydraulic throughput to increase detention time.

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5. Rapid Mixing (Flash Mixing) Principles & Hydraulics

Rapid mixing—also termed flash mixing—is the unit process where chemical coagulants are introduced into the raw water stream and dispersed with extreme violence and speed.

The Sub-Second Coagulation Window

When metal coagulants like alum (Al2(SO4)3) or ferric chloride (FeCl3) are injected into water, they do not remain as simple salts. Within 0.1 to 1.0 second, the metal cations undergo rapid hydrolysis, forming a sequence of short-lived, highly charged cationic complexes (such as Al(OH)²⁺, Al2(OH)2⁴⁺, and Al8(OH)20⁴⁺).

These positively charged intermediate species are responsible for charge neutralization—adsorbing onto negatively charged clay colloids and collapsing their electrostatic repulsion. If the coagulant is not dispersed uniformly throughout the entire raw water volume within this sub-second timeframe, the intermediate complexes react with water molecules instead, precipitating into neutral metal hydroxides (Al(OH)3). Once precipitated, charge neutralization efficiency drops drastically, requiring much higher chemical dosages to achieve "sweep coagulation."

                 MECHANICAL RAPID MIX BASIN

          Raw Water Inflow ====> [ Baffle ]
                                     |
                                     v
             Coagulant Feed Line ---> *  (Dispersed directly below impeller)
                                  |======|
                                  |  ||  |  Vertical Turbine Impeller
                                  | [==] |  (High Speed, Radial Flow)
                                  |  ||  |  G = 700 - 1,000 s⁻¹
                                  |======|
                                     |
                                     v
                     Rapidly Mixed Flow to Flocculation Basins
                     (Detention Time: 10 to 60 Seconds)

Rapid Mix Technologies

1. Mechanical Mixers: A dedicated square or circular basin equipped with an electric motor driving a high-speed radial turbine or axial-flow hydrofoil impeller. Chemical feed lines discharge directly beneath the impeller eye to ensure immediate contact with maximum turbulence. Detention times range from 10 to 60 seconds.
2. In-Line Static Mixers: Flanged pipe segments containing internal, motionless, alternating helical or geometric mixing vanes. The flowing water is divided, sheared, and rotated across the elements. Benefits include zero moving parts, zero electrical power consumption beyond pipe friction loss, and near-instantaneous mixing (detention time < 2 seconds).
3. Hydraulic Jumps: Coagulant is introduced immediately upstream of a standing hydraulic jump formed in an open channel (such as a Parshall flume or overflow weir). The abrupt dissipation of kinetic energy creates severe turbulence that thoroughly blends the chemical without mechanical drives.
4. Pump Suction Injection: Coagulants are injected directly into the suction manifold of raw water low-lift pumps. The high-speed pump impeller serves as the mixer. While energy-efficient, this approach can accelerate pump impeller erosion and makes dosage trimming less responsive.

The Camp and Stein Velocity Gradient (G-Value)

Turbulence intensity in water treatment mixing processes is quantified by the Velocity Gradient (G), developed by Thomas Camp and P.C. Stein: G = sqrt( P / (μ * V) )

Where:

• G = Velocity gradient in reciprocal seconds (s⁻¹)
• P = Power dissipated in the water in foot-pounds per second (ft·lb/s) or Watts (W)
• μ = Dynamic viscosity of water in lb·s/ft² or Pa·s
• V = Basin volume in ft³ or m³

For rapid mixing, the velocity gradient must be intense: G = 700 to 1,000+ s⁻¹. The product of the velocity gradient and detention time (G · t) represents the total mixing energy input:

• Rapid Mix G: 700 to 1,000+ s⁻¹
• Rapid Mix Detention Time (t): 10 to 60 seconds
• Rapid Mix G · t Product: 10,000 to 40,000 (dimensionless)

Detention times exceeding 60 seconds in rapid mix basins are undesirable: prolonged violent agitation shears early micro-floc aggregates and wastes electrical energy.

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6. Jar Testing Procedures & Process Optimization

The jar test is the foundational bench-scale laboratory procedure used by water treatment plant operators to evaluate and optimize coagulation, flocculation, and sedimentation. Because raw water quality fluctuates with rainfall, temperature, and seasonal turnover, the jar test simulates the full-scale treatment plant on a miniature scale, allowing the operator to adjust chemical dosages before poor-quality water compromises plant operations.

                           LABORATORY JAR TEST APPARATUS

     [ Variable Speed Controller: 0 - 300 RPM ] [ Digital Timer ]
        |           |           |           |           |           |
       [|]         [|]         [|]         [|]         [|]         [|]
     Jar #1      Jar #2      Jar #3      Jar #4      Jar #5      Jar #6
     Control    15 mg/L     20 mg/L     25 mg/L     30 mg/L     35 mg/L
    [ 2.0 L ]   [ 2.0 L ]   [ 2.0 L ]   [ 2.0 L ]   [ 2.0 L ]   [ 2.0 L ]
    =====================================================================
                [ Fluorescent Base Illuminator / Light Table ]

Equipment and Reagents

• Gang Stirrer: A precision six-paddle stirring machine equipped with flat paddle blades (1" × 3"), variable speed motor (0 to 300 RPM), digital tachometer, and illuminated base.
• Square B-Ker Jars: Six 2.0-liter square acrylic beakers (often called Gator Jars or B-Ker beakers). The square geometry acts as built-in baffles, preventing liquid vortexing and simulating the non-circular hydraulic flow patterns of actual plant basins far better than standard cylindrical glass beakers.
• Chemical Stock Solutions: Accurately prepared primary coagulant, polymer coagulant aids, and acid/base reagents. Standard practice is to formulate a 1.0% stock solution (10,000 mg/L), where 1.0 mL added to 1.0 L of sample equals an exact dose of 10.0 mg/L.

Step-by-Step Jar Test Protocol

1. Sample Collection: Collect 15 to 20 liters of fresh raw water directly from the plant intake. It is essential that the test begins immediately so the sample temperature does not drift toward room temperature. Testing warm water will produce erroneously optimistic settling results that cannot be replicated in a cold plant.
2. Baseline Characterization: Measure and record raw water turbidity, pH, total alkalinity, temperature, and true color.
3. Jar Setup: Fill each of the six jars with exactly 2.0 liters of raw water.
4. Chemical Dosing Schedule: Establish a logical dosing matrix across the jars. For example, if evaluating alum dosage:
   • Jar 1: 15 mg/L
   • Jar 2: 20 mg/L
   • Jar 3: 25 mg/L
   • Jar 4: 30 mg/L (current plant dose)
   • Jar 5: 35 mg/L
   • Jar 6: 40 mg/L
5. Rapid Mix Phase: Lower paddles into the jars. Start the stirrer at 100 to 300 RPM (G ≈ 700–1,000 s⁻¹). Rapidly inject the calculated chemical doses simultaneously into all six jars below the water surface. Run rapid mix for exactly 60 seconds.
6. Flocculation Phase (Tapered Mixing): Reduce paddle speed to simulate the plant's flocculation stages:
   • Stage 1: 40 to 50 RPM for 10 minutes.
   • Stage 2: 25 to 35 RPM for 10 minutes.
   • Stage 3: 15 to 20 RPM for 10 minutes. Observe and record the elapsed time (in seconds) until initial visible "pinpoint floc" appears in each jar.
7. Sedimentation Phase: Stop the stirrer, raise the paddles completely out of the water, and initiate the timer. Allow quiescent settling for 15 to 30 minutes (calibrated to reflect the plant's actual basin surface overflow rate).

Qualitative and Quantitative Evaluation

• Floc Appearance Time: Fast floc formation (< 90 seconds in Stage 1) indicates sufficient coagulant dosage and proper charge neutralization.
• Floc Size and Structure: Visual rating using standard indices (pinpoint, small, fair, good, large, feathery vs. dense compact agglomerates).
• Settling Velocity: Measure the time required for the bulk floc blanket to settle through the liquid column (e.g., inches settled per minute). Fast, distinct settling blankets indicate dense, easily settled solids.
• Supernatant Clarity: Using a pipet or the jar's built-in sampling siphon port (located 2 inches below the liquid surface), draw 50 mL of clarified supernatant from each jar without disturbing settled sludge. Measure:
  • Settled water turbidity (NTU)
  • Final pH
  • Residual total alkalinity (mg/L as CaCO3)
  • True color (PCU)

Interpreting Results

The optimal coagulant dose is not simply the dose that yields the lowest turbidity. The operator must select the lowest chemical dose that produces settled water turbidity under target limits (typically < 1.5 to 2.0 NTU for conventional plants), achieves clear supernatant with minimal pin-floc, maintains residual alkalinity above 20 mg/L as CaCO3, and minimizes sludge generation and chemical costs.