15.2 Physical & Chemical Water Analysis
Key Takeaways
- Turbidity (NTU), pH, temperature, and conductivity are core physical/related measurements for treatment control and distribution awareness.
- Key chemical tests include alkalinity and hardness (often as mg/L as CaCO₃), chlorine residual by DPD, fluoride, iron, manganese, and phosphate.
- Colorimetric methods use color intensity proportional to concentration; titrimetric methods use titrant volume to an endpoint.
- Calibration, blanks, standards, clean cells/electrodes, and reagent quality are required for trustworthy numbers.
- Process-control data run the plant; compliance data prove legal limits—methods, labs, and reporting rules differ.
15.2 Physical & Chemical Water Analysis
Quick Answer: Operators use physical and chemical tests for both process control and compliance. Core drinking-water and treatment tests include turbidity (NTU), pH, temperature, conductivity, alkalinity, hardness, chlorine residual (DPD), fluoride, iron, manganese, and phosphate. Understand colorimetric vs titrimetric methods, keep instruments calibrated with standards, and never confuse a quick plant control test with a formal compliance result unless methods and documentation match.
Physical and chemical analyses are the day-to-day language of water treatment plants, distribution residuals, and many wastewater process checks. Exam questions mix what the test measures, typical units, method family, and why the result matters for public health or plant control.
1. Physical Parameters Operators Live With
Turbidity (NTU)
Turbidity measures light scattered by particles in water and is reported in nephelometric turbidity units (NTU). It is a critical filtration performance and pathogen barrier indicator in drinking water. High finished-water turbidity can signal filter breakthrough and elevated microbial risk.
| Context | Operator focus |
|---|---|
| Raw water | Storms, algae, and source changes spike turbidity and coagulant demand |
| Settled water | Jar-test and process optimization feedback |
| Combined filter effluent / individual filters | Compliance and filter run management |
| Distribution | Unusual turbidity may indicate main breaks, flushing needs, or intrusion clues |
Turbidimeters need calibration, clean sample cells, and proper warm-up. Bubbles and scratched cuvettes create false highs.
Temperature
Temperature affects reaction rates, disinfectant CT, biological activity, gas solubility, and customer complaints. Record temperature with residual and pH when the method or plant SOP requires it. Many chemical equilibria (including corrosion indices) are temperature-sensitive.
Conductivity / Specific Conductance
Conductivity estimates the water’s ability to carry current, related to dissolved ionic content (TDS proxy). Useful for:
- Source blending and saltwater intrusion trends (Florida coastal wells)
- RO/membrane feed and permeate monitoring
- Process upsets that change ionic strength
Conductivity is not a direct substitute for every ion-specific analysis, but it is a fast trend tool.
Color and Odor (related physical aesthetics)
True/apparent color and odor tests support taste-and-odor programs (Chapter 6 themes). They are often process-control oriented unless a specific secondary standard program applies.
2. Core Chemical Parameters
pH
pH is the hydrogen-ion activity scale (acidic <7, neutral 7, basic >7 for pure water at standard conditions). It controls:
- Coagulation chemistry and coagulant performance
- Disinfection effectiveness and hypochlorous acid/hypochlorite speciation
- Corrosion control and metal solubility
- Softening and precipitation reactions
Measure pH with a calibrated meter (or approved method). Buffer calibration (commonly two- or three-point) is non-negotiable. Temperature compensation matters for accurate readings.
Alkalinity
Alkalinity is the water’s acid-neutralizing capacity, usually expressed as mg/L as CaCO₃. It is dominated by bicarbonate/carbonate/hydroxide species in natural waters. Operators use alkalinity for:
- Coagulation and pH stability
- Lime softening control
- Nitrification alkalinity demand in wastewater (related plant chemistry)
- Corrosion control planning
Alkalinity is classically determined by titration to specific pH endpoints (e.g., phenolphthalein and total alkalinity endpoints).
Hardness
Hardness is primarily Ca²⁺ and Mg²⁺, reported as mg/L as CaCO₃. Hard water causes scale; soft water can be more corrosive if not properly stabilized. Hardness testing (titration or calculation from calcium/magnesium) guides softening decisions and distribution stability programs.
Chlorine Residual (DPD Method)
Free and total chlorine residuals are central to disinfection control. The DPD (N,N-diethyl-p-phenylenediamine) colorimetric method is the industry standard for many plant and field residual tests:
| Residual type | What it represents | Why operators care |
|---|---|---|
| Free chlorine | HOCl/OCl⁻ available for disinfection | Primary disinfection strength in free-chlorine systems |
| Combined chlorine | Mostly chloramines | Dominant residual in chloraminated systems |
| Total chlorine | Free + combined | Compliance and process total residual checks |
DPD produces a pink color proportional to chlorine; compare with a colorimeter/spectrophotometer or comparator. Interferents (high manganese, extreme monochloramine situations, etc.) require method awareness. Calibrate or verify with secondary standards and follow reagent shelf life.
Fluoride
Fluoride may be naturally present in Florida groundwater; adding fluoride to a Florida public water system has been prohibited since July 1, 2025 (s. 403.859(8), F.S.), so Florida fluoride testing now tracks natural background against the 4.0 mg/L primary MCL and 2.0 mg/L secondary standard rather than a fluoridation setpoint. Operators monitor dose and residual with colorimetric, ISE (ion-selective electrode), or other approved methods. Overfeed is a public-health emergency scenario—accurate measurement and fail-safes matter more than “set and forget.”
Iron and Manganese
Iron and manganese cause colored water, staining, and taste issues and can foul filters and softeners. Methods are often colorimetric. Results guide oxidation, filtration, and sequestration strategies (Chapter 6). Sample preservation and filtration decisions (dissolved vs total) must match the question you are asking.
Phosphate
Phosphate testing supports corrosion control (orthophosphate residuals), wastewater nutrient removal, and chemical feed verification. Methods are typically colorimetric (e.g., ascorbic acid methods for orthophosphate). Know whether you need ortho vs total phosphorus (digestion required for total).
3. Colorimetric vs Titrimetric Concepts
| Approach | How it works | Common examples |
|---|---|---|
| Colorimetric | Reagent forms a colored complex; color intensity ∝ concentration (Beer’s law) | DPD chlorine, many iron/manganese/phosphate kits, some fluoride tests |
| Titrimetric | Titrant is added until an endpoint (color change or pH) is reached; volume of titrant relates to concentration | Alkalinity, hardness, some chlorine tests historically, some COD variants |
Operator implications:
- Colorimetric tests need clean cells, correct wavelength/filter, and valid calibration curves or factory curves.
- Titrations need accurate burets/digital titrators, correct indicators, and careful endpoint recognition.
- Both need reagent quality, proper sample volume, and awareness of interferences.
Portable field kits are excellent for process control when used within their range and maintained. They are not automatically equivalent to a certified lab’s compliance method unless approved and documented as such.
4. Calibration, Standards, and Good Lab Habits
| Practice | Purpose |
|---|---|
| Calibration with standards | Sets instrument response to known concentrations |
| Blank | Corrects reagent/background color or instrument zero |
| Secondary standards / gel standards | Day-to-day checks between full calibrations (where allowed) |
| Spike/check standards | Verify recovery and ongoing accuracy |
| Documentation | Log calibrations, lot numbers, and out-of-control events |
| Maintenance | Clean electrodes, replace membranes, service turbidimeters |
pH meters: calibrate with fresh buffers bracketing the expected sample pH. Store electrodes per manufacturer guidance.
Spectrophotometers/colorimeters: zero with blank; verify with a standard in the working range.
Turbidimeters: calibrate with formazin or approved standards; keep sample cells pristine.
If a standard reads wrong, stop and fix the measurement system before adjusting plant chemical feeds based on a bad number.
5. Process Control Samples vs Compliance Samples
This distinction appears constantly on exams and in real plants:
| Feature | Process control | Compliance |
|---|---|---|
| Purpose | Run the plant today (coagulant dose, residual, filter run) | Demonstrate legal/regulatory limits are met |
| Methods | Often plant SOPs, kits, online analyzers | Approved methods; certified lab when required |
| Frequency | High (continuous/hourly/shift) | Per monitoring schedule / permit |
| Reporting | Internal logs and SCADA | Official reports to FDEP/primacy agency |
| Consequence of error | Process upset | Violation, public notice, enforcement |
Online chlorine and turbidity analyzers are powerful process tools. They still need verification against grab methods. A compliance turbidity or residual value must follow the applicable rule’s method and quality requirements.
6. Putting Numbers to Work (Treatment Examples)
| Result trend | Typical operator interpretation |
|---|---|
| Settled turbidity rising | Coagulant/flocculation/sedimentation problem |
| Filter effluent turbidity spike | Breakthrough, air binding, rate change, media issue |
| Free residual collapsing in distribution | High demand, nitrification (chloramines), leak/intrusion, underfeed |
| pH drop after coagulant | Expected acidifying effect—check alkalinity buffer |
| Iron rising in finished water | Oxidation/filtration failure or distribution release |
| Conductivity jump in wellfield | Blending change or possible saline influence |
Numbers without context are trivia. Numbers with location, time, and process knowledge become control.
7. Safety and Sample Integrity Notes
- Wear PPE with acids (alkalinity titration reagents, metal preservatives).
- Do not return unused sample to the process stream if contaminated with reagents.
- Match container and hold time from Section 15.1 for any sample leaving the plant.
- Keep reagents in date; degraded DPD reagents under-read chlorine—dangerous for public health.
Physical and chemical testing is not “extra work.” It is how licensed operators see invisible risks and keep Florida systems in control.
Turbidity is reported in NTU and is most valuable to drinking-water operators because it:
The DPD method is commonly used in water treatment plants to measure:
Alkalinity is typically determined by which general analytical approach and reported as what unit basis?
Which statement best distinguishes process-control testing from compliance testing?