5.6 Wastewater Laboratory Analysis: Nutrients, Metals, WET & Regulatory Testing
Key Takeaways
- Total nitrogen equals total Kjeldahl nitrogen plus nitrite nitrogen plus nitrate nitrogen, and total Kjeldahl nitrogen itself equals organic nitrogen plus ammonia nitrogen.
- Total phosphorus requires a digestion step to convert condensed and organic phosphorus to orthophosphate, whereas orthophosphate is measured directly with no digestion.
- Whole effluent toxicity testing exposes test organisms to graded effluent dilutions and reports acute results as an LC50 and chronic results as a no observed effect concentration or an IC25.
- Regulatory compliance analyses must be performed by approved 40 CFR Part 136 methods in a laboratory accredited under the Virginia Environmental Laboratory Accreditation Program, while process control tests need not be.
- The volatile acids to alkalinity ratio is the key anaerobic digester control test, with a healthy range of roughly 0.05 to 0.15 and souring indicated above about 0.3 to 0.4.
Wastewater Laboratory Analysis: Nutrients, Metals, WET & Regulatory Testing
The ABC Wastewater Class IV Laboratory Analysis area carries 15 items and, notably, zero calculation items - meaning these questions test procedure, interference, interpretation, and quality control rather than arithmetic. The outline names bacteriological, biological, chemical, physical, and advanced analytical methods, plus process control and required regulatory testing.
1. Nitrogen Series
The relationships are worth memorizing as equations, because permit forms and exam items both use them.
TKN = Organic nitrogen + Ammonia nitrogen Total nitrogen (TN) = TKN + Nitrite-N + Nitrate-N Organic nitrogen = TKN - Ammonia nitrogen
| Analyte | Common methods | Notes |
|---|---|---|
| Ammonia (NH3-N) | Ion-selective electrode; phenate colorimetric; salicylate; distillation followed by titration | Distillation removes most interferences; preserve with sulfuric acid to pH below 2 and cool to 4 degrees C; 28-day holding time |
| TKN | Sulfuric acid/catalyst digestion converting organic N to ammonium, then ammonia determination | The digestion is the whole test; incomplete digestion under-reports |
| Nitrite (NO2-N) | Diazotization colorimetric | 48-hour holding time, no acid preservation - acid converts nitrite; refrigerate only |
| Nitrate (NO3-N) | Cadmium reduction to nitrite then colorimetric; UV spectrophotometric screening; ion chromatography | Cadmium column efficiency must be verified; cadmium waste is hazardous |
| Total nitrogen | Persulfate digestion, or calculation from TKN plus nitrite plus nitrate | Chesapeake Bay permits are written on TN |
A diagnostic worth knowing: if effluent ammonia is low, nitrate is high, and TKN is low, the plant is nitrifying but not denitrifying. If ammonia is low, nitrate is low, and TN is low, both are working. If ammonia is high and nitrate is near zero, nitrification has failed - check DO, sludge age, temperature, alkalinity, and for a toxic inhibitor.
2. Phosphorus Series
Total phosphorus = Orthophosphate + Condensed (poly) phosphate + Organic phosphorus
| Analyte | Preparation | Use |
|---|---|---|
| Orthophosphate (reactive P) | Filtered, no digestion; ascorbic acid or stannous chloride colorimetric | Process control for chemical dosing - only ortho reacts with alum or ferric |
| Acid-hydrolyzable P | Mild acid hydrolysis | Converts condensed phosphates |
| Total phosphorus | Persulfate or acid digestion of an unfiltered sample to convert everything to ortho, then the same colorimetric finish | Permit compliance |
The exam trap is straightforward: a total phosphorus result requires a digestion; an orthophosphate result does not. Reporting an undigested result as total phosphorus under-reports, sometimes badly, at plants with significant organic phosphorus.
3. Alkalinity, pH and Digester Control
| Test | Method | Control range |
|---|---|---|
| Alkalinity | Titration with standard sulfuric acid to a pH 4.5 endpoint (bromcresol green indicator), reported as mg/L as CaCO3 | 50 to 80 mg/L residual in a nitrifying aeration basin; 1,500 to 5,000 mg/L in an anaerobic digester |
| Volatile acids | Titration between defined pH endpoints (typically the pH 5.1 to 3.5 distillation-free method), reported as acetic acid | 50 to 500 mg/L in a healthy digester |
| Volatile acids to alkalinity ratio | Calculated | 0.05 to 0.15 healthy; above 0.3 to 0.4 indicates souring |
| pH | Electrometric, calibrated with at least two buffers bracketing the sample | Analyze immediately - the holding time is effectively zero |
The VA/Alk ratio is the single most valuable digester test because it moves before pH does. Alkalinity buffers the accumulating acids, so pH stays near 7 until the buffer is nearly exhausted - by which time recovery is difficult. A ratio trending from 0.10 to 0.25 gives days of warning that pH alone would not.
4. Metals
| Method | Principle | Typical use |
|---|---|---|
| Flame atomic absorption (FAA) | Sample aspirated into a flame; a hollow cathode lamp measures absorption at an element-specific wavelength | mg/L-level metals; one element at a time |
| Graphite furnace AA | Electrothermal atomization | Low microgram-per-liter levels |
| ICP-OES (atomic emission) | Argon plasma excites atoms; emission measured | Multi-element, moderate detection limits |
| ICP-MS | Plasma plus mass spectrometry | Multi-element, part-per-trillion detection; the modern standard for biosolids and trace metals |
| Cold vapor AA | Mercury reduced to elemental vapor | Mercury only |
| Hydride generation | Volatile hydride formed | Arsenic, selenium |
Sample handling for metals: collect in acid-washed plastic or glass, preserve with nitric acid to pH below 2, and hold up to 6 months (mercury 28 days). "Total" metals are digested before analysis; "dissolved" metals are filtered through 0.45 micron before preservation. Filtering after acidification redissolves particulate metal and invalidates the dissolved result.
Metals matter to an operator in three places: pretreatment program compliance, biosolids Part 503 limits, and effluent water-quality-based limits. A metals excursion in the biosolids and a metals excursion in the effluent generally point to the same industrial user.
5. Whole Effluent Toxicity
VPDES permits at many Virginia plants include whole effluent toxicity (WET) testing, which asks a question no chemical-specific limit can: does the whole effluent, with all its interactions, harm aquatic life?
| Test type | Duration | Organisms | Endpoint |
|---|---|---|---|
| Acute | 24 to 96 hours | Ceriodaphnia dubia, Pimephales promelas (fathead minnow), Daphnia | LC50 - the concentration lethal to 50 percent of organisms; also reported as TUa = 100/LC50 |
| Chronic | 7 days | Ceriodaphnia dubia (survival and reproduction), Pimephales promelas (survival and growth) | NOEC (no observed effect concentration), IC25; TUc = 100/NOEC |
Tests run a dilution series plus a control, with rigorous requirements for control survival, reference toxicant testing, and dilution water quality.
When a test fails, the permit generally requires a Toxicity Reduction Evaluation (TRE), often beginning with a Toxicity Identification Evaluation (TIE) that manipulates the sample - filtration, aeration, pH adjustment, EDTA chelation, C18 solid-phase extraction, sodium thiosulfate addition - to identify the class of toxicant. Common culprits at municipal plants: residual chlorine (identified by the thiosulfate manipulation), ammonia (pH manipulation), surfactants, and metals (EDTA manipulation).
6. Process Control Versus Regulatory Testing
This distinction determines what method, what laboratory, and what documentation is required.
| Process control testing | Regulatory compliance testing | |
|---|---|---|
| Purpose | Operating decisions | Permit reporting |
| Method | Any method the operator trusts - settleometer, centrifuge spins, quick COD, field DO | Must be an approved method under 40 CFR Part 136 |
| Laboratory | The plant bench | Must be accredited |
| Documentation | Operator log | Full chain of custody, QC data, and reporting |
| Consequence of error | A bad process decision | A permit violation or a false report |
Virginia accreditation. Laboratories performing analyses for compliance with Virginia environmental programs must be accredited under the Virginia Environmental Laboratory Accreditation Program (VELAP), administered by the Department of General Services Division of Consolidated Laboratory Services under 1VAC30-46, based on the TNI standard. VELAP accreditation is field-of-testing specific: a plant lab may be accredited for BOD, TSS, pH, and residual chlorine while sending metals, nutrients, and WET to a contract laboratory.
A caution about the boundary. A plant may run any test it likes for process control, but the moment a result is used on a DMR it becomes a compliance result and must meet every Part 136 and accreditation requirement. Reporting a process-control number on a DMR is a data integrity problem, not a technicality - and under 18VAC160-30-320 it exposes the licensee personally.
An effluent sample is analyzed for phosphorus without a digestion step and reported as total phosphorus on the discharge monitoring report. What is wrong with this result?
An anaerobic digester shows a volatile acids concentration of 640 mg/L as acetic acid and an alkalinity of 2,000 mg/L as calcium carbonate, with pH still at 7.0. How should the operator interpret this?
A chronic whole effluent toxicity test fails, and a toxicity identification evaluation shows that toxicity disappears when sodium thiosulfate is added to the sample. What toxicant class does this implicate?