10.7 Nutrient Series, Metals Sampling, Whole Effluent Toxicity & Electrode Measurements (DO, pH, ORP, H2S)
Key Takeaways
- Total Kjeldahl nitrogen equals organic nitrogen plus ammonia nitrogen, and total nitrogen equals TKN plus nitrite plus nitrate.
- Ammonia toxicity is driven by the un-ionized fraction, which increases sharply as pH and temperature rise.
- Dissolved metals require filtration through a 0.45 micron filter before acid preservation, while total metals are preserved and digested without filtration.
- Whole effluent toxicity failures trigger a toxicity identification evaluation and a toxicity reduction evaluation, and ammonia, chlorine residual, surfactants, and industrial metals are the usual causes.
- Dissolved oxygen probes must be altitude corrected in Colorado, where saturation at 5,300 feet is roughly 83 percent of the sea-level value, and ORP distinguishes truly anoxic from truly anaerobic zones where a DO probe cannot.
The analytes a wastewater permit actually names
WPI's wastewater Laboratory Analysis outline requires operators to collect samples for, conduct, and interpret data from bacteriological analyses, biological analyses (BOD, CBOD, WET), chemical analyses (COD, nutrients, metals), and physical analyses (pH, temperature, DO, settleable solids), and to operate and maintain instrumentation for DO, pH, H2S, and ORP. BOD, COD, and TSS are covered in the wastewater laboratory section; this section covers the nutrients, metals, toxicity, and electrode measurements that make up the rest of the list.
Nitrogen series
Nitrogen appears in wastewater in several forms, and each is separately meaningful.
| Form | What it is | Significance |
|---|---|---|
| Organic nitrogen | Nitrogen in proteins, urea, amino acids | Hydrolyzes to ammonia |
| Ammonia nitrogen (NH3-N) | Free ammonia plus ammonium | Toxic to fish; exerts oxygen demand; the substrate for nitrification |
| Total Kjeldahl nitrogen (TKN) | Organic nitrogen plus ammonia nitrogen | The reduced nitrogen available for nitrification |
| Nitrite (NO2-N) | Intermediate of nitrification | Rising nitrite signals incomplete nitrification or nitrification in the distribution system |
| Nitrate (NO3-N) | Fully oxidized nitrogen | Denitrification substrate; drinking water MCL of 10 mg/L as N |
| Total nitrogen | TKN plus nitrite plus nitrate | The permit parameter under nutrient rules |
Two relationships to carry: TKN equals organic nitrogen plus ammonia, and total nitrogen equals TKN plus nitrite plus nitrate. Also note that ammonia toxicity is driven by the un-ionized fraction, which increases sharply with rising pH and temperature — the same total ammonia concentration is far more toxic at pH 9 and 25 degrees Celsius than at pH 7 and 10 degrees.
Methods. Ammonia by ion selective electrode, salicylate or Nessler colorimetry, or distillation and titration. TKN by digestion followed by ammonia measurement. Nitrate and nitrite by cadmium reduction, ultraviolet absorption, or ion chromatography.
Phosphorus
- Orthophosphate is the soluble, reactive, immediately available form and is what a chemical precipitation or EBPR process is controlled on.
- Total phosphorus requires digestion to convert condensed and organic phosphorus to orthophosphate before measurement.
- The standard colorimetric method is the ascorbic acid method, producing a blue complex read at 880 nanometers.
- For process control, an operator commonly runs orthophosphate on a filtered sample to see what the biology or chemistry has actually removed, and reports total phosphorus on an unfiltered sample for compliance.
Metals
Metals reach a wastewater plant from industrial users, from corrosion, and from stormwater. They matter for three reasons: they can be toxic to the biological process, they are limited in biosolids under Regulation 64 and Part 503, and they may carry water-quality-based effluent limits.
- Sample handling is where metals results are won or lost. Samples are collected in acid-washed containers and preserved with nitric acid to pH below 2 to keep metals in solution and off the container walls.
- Total metals requires acid digestion of the sample; dissolved metals requires filtration through a 0.45 micron filter before preservation. Getting this order wrong is a common and invalidating error.
- Methods are atomic absorption spectroscopy or inductively coupled plasma, which is why metals are sent to a laboratory rather than run at the plant.
Whole effluent toxicity
WET testing exposes test organisms — typically the water flea Ceriodaphnia dubia and the fathead minnow Pimephales promelas — to a series of effluent dilutions.
- Acute tests measure survival over 24 to 96 hours and report an LC50, the concentration lethal to 50 percent of organisms.
- Chronic tests measure survival plus growth or reproduction over about 7 days and report a NOEC (no observed effect concentration) or an IC25.
- A failed test triggers a toxicity identification evaluation (TIE) to find the cause and a toxicity reduction evaluation (TRE) to fix it. At domestic plants the usual suspects are ammonia, residual chlorine, surfactants, and metals from an industrial user.
- Sample handling matters enormously: WET samples are chilled, delivered fast, and, where chlorine is present, the residual is addressed per the method, because otherwise the test simply measures chlorine.
Electrode measurements
| Measurement | Instrument notes |
|---|---|
| Dissolved oxygen | Membrane (Clark) probes consume oxygen and require flow across the membrane; luminescent (optical) probes do not, and have largely replaced them. Calibrate in water-saturated air, and correct for altitude — a critical point in Colorado, where saturation at 5,300 feet is roughly 83 percent of the sea-level value |
| pH | Two-point calibration bracketing the sample; slope 95 to 105 percent; store in KCl storage solution |
| ORP (oxidation-reduction potential) | Reported in millivolts; used to identify aerobic, anoxic, and anaerobic zones in BNR. Strongly positive indicates aerobic, near zero to slightly negative indicates anoxic, and strongly negative indicates anaerobic and sulfide-forming conditions |
| Hydrogen sulfide | Field colorimetric or ion selective electrode for liquid; fixed and personal gas detectors for the atmosphere |
| Temperature | NIST-traceable verification, correction factor recorded |
ORP is the most underused of these. Because a BNR process depends on maintaining genuinely anoxic conditions in the denitrification zone and genuinely anaerobic conditions in the phosphorus release zone, ORP tells the operator directly whether those zones are what they are supposed to be — something a dissolved oxygen probe reading zero cannot distinguish.
A wastewater sample shows organic nitrogen of 8 mg/L, ammonia nitrogen of 22 mg/L, nitrite of 0.4 mg/L, and nitrate of 6 mg/L, all as nitrogen. What are the TKN and total nitrogen values?
An operator needs a dissolved metals result. What is the correct sample handling sequence?
Why must a dissolved oxygen probe calibration be altitude corrected at Colorado plants?