16.3 Wastewater Lab: Nutrients, Alkalinity, Bacteria, Toxicity Testing and Lab Accreditation

The Nitrogen Family

Always report nitrogen species "as N" so results add up. A profile of ammonia, nitrite and nitrate along an aeration basin shows where nitrification is happening and whether alkalinity or DO is limiting.

Ammonia ISE tips: add the alkaline ionic-strength adjuster to raise pH above 11 (converting ammonium to ammonia gas the membrane can sense). Calibrate with standards that bracket the sample, stir at the same rate for every sample, and replace the membrane when the slope drifts.

Phosphorus

• Orthophosphate (reactive phosphorus) is measured directly by the ascorbic acid method. Molybdate and antimony form a blue complex that is read on a spectrophotometer.
• Total phosphorus requires persulfate digestion first, to convert polyphosphates and organic phosphorus to orthophosphate.
• Process use: comparing ortho-P in anaerobic and aerobic zones confirms biological phosphorus release and uptake. Filtered versus unfiltered results show how much phosphorus leaves as particles, which tertiary filtration can capture.

Alkalinity

Alkalinity is the water's capacity to neutralize acid, measured by titrating with standardized sulfuric acid to an endpoint near pH 4.5:

$$\text{Alkalinity (mg/L as CaCO}_3\text{)} = \frac{\text{mL titrant} \times N \times 50{,}000}{\text{mL sample}}$$

Example. A 100 mL sample takes 4.6 mL of 0.02 N H2SO4 to reach pH 4.5.

Alkalinity = 4.6 × 0.02 × 50,000 ÷ 100 = 46 mg/L as CaCO3

Nitrification destroys about 7.14 mg/L alkalinity per mg/L of ammonia-N oxidized. Low-alkalinity Oregon wastewaters can crash in pH, so many operators keep at least about 50 mg/L residual alkalinity in nitrifying basins. Track influent and effluent alkalinity daily when nitrification is required.

Effluent Bacteria

• Sample handling: sterile bottles containing sodium thiosulfate for chlorinated effluent; ice; 8-hour holding time from collection to the start of analysis for wastewater coliform and E. coli.
• Methods: membrane filtration on selective media, or enzyme-substrate tests (for example Colilert-18 with Quanti-Tray MPN). These are read as most probable number per 100 mL. Dilutions are needed for undisinfected samples.
• Calculations: permits often use a geometric mean, the antilog of the average of the logs, for monthly limits. One high result raises a geometric mean far less than it raises an arithmetic mean.

Process Control Tests at the Bench

• Settleometer (30-minute settling) and sludge volume index for activated sludge.
• Oxygen uptake rate (OUR) and specific oxygen uptake rate (SOUR): DO decline in a stirred, sealed sample over time, divided by volatile solids for SOUR. Used for activated sludge health and aerobic digester vector-attraction tests.
• Microscopic examination for protozoa, filaments and floc structure.
• Volatile acids and alkalinity for anaerobic digesters.

Whole Effluent Toxicity (WET)

WET tests measure the combined toxicity of effluent to living organisms:

A failed WET test usually triggers accelerated testing and a toxicity reduction evaluation to find the cause, such as ammonia, chlorine residual, metals or surfactants.

Lab Quality and Accreditation

• Wastewater compliance analyses must use approved 40 CFR Part 136 methods, holding times and preservation, plus the QA/QC your permit requires: blanks, duplicates, spikes and calibration checks.
• Drinking water compliance samples in Oregon go to laboratories accredited under the Oregon Environmental Laboratory Accreditation Program (ORELAP), the OHA program under OAR 333-064. Cyanotoxin analyses, for example, must use an ORELAP-accredited lab or the DEQ laboratory.
• Keep bench sheets, calibration logs and instrument maintenance records. They are legal records that support every DMR value.

Worked Example: Is There Enough Alkalinity to Nitrify?

Influent ammonia-N is 30 mg/L, the permit target is about 1 mg/L, and influent alkalinity is 220 mg/L as CaCO3.

• Ammonia-N nitrified ≈ 30 − 1 = 29 mg/L
• Alkalinity consumed ≈ 29 × 7.14 ≈ 207 mg/L as CaCO3
• Alkalinity remaining ≈ 220 − 207 = 13 mg/L

That leftover is far below the residual many operators aim to keep (commonly about 50 mg/L or more), so pH would sag and nitrification would stall. Options include feeding alkalinity (lime, caustic, sodium bicarbonate or magnesium hydroxide), or adding an anoxic zone. Denitrification there recovers about half of the alkalinity used in nitrification.

Sample Preservation for Nutrients (40 CFR 136, Table II)

Acid-preserved samples are reported as nitrate plus nitrite, not as separate nitrate or nitrite results. Filter orthophosphate samples in the field, because cells keep taking up and releasing phosphate after collection.