Wastewater Characteristics and Microbiology
What you must know
Industrial wastewater is described by physical, chemical, and biological parameters. Key measures include BOD (biochemical oxygen demand, a bioassay of biodegradable organics over 5 days), CBOD vs NBOD (carbonaceous vs nitrogenous demand), COD (chemical oxygen demand, dichromate oxidation, captures most organics including non-biodegradable), TOC, TSS (total suspended solids), VSS, pH, alkalinity, and nutrients (N and P). The BOD5:COD ratio estimates biodegradability; a ratio near 0.5 or higher suggests readily treatable organics, while low ratios hint at refractory or toxic compounds. Toxicity, salinity, temperature, and shock loads are common industrial traps.
Microbiology drives biological treatment. Heterotrophic bacteria oxidize organics; nitrifiers (Nitrosomonas, Nitrobacter) are slow-growing autotrophs sensitive to pH, temperature, and toxics. Filamentous bacteria, protozoa (ciliates, flagellates, amoebae), and metazoa indicate sludge health. Floc-forming bacteria create settleable flocs; filaments provide backbone but overgrowth causes bulking. Observe a mixed liquor sample: a healthy floc is compact with clear supernatant and active ciliates.
Traps in questions
- Confusing BOD with COD: BOD measures biodegradable fraction only.
- Ignoring nitrification inhibition in BOD tests (use ATU to suppress nitrogenous demand).
- Assuming all COD is biodegradable.
- Mixing up indicator organisms; fecal coliform vs total coliform.
Worked example / rule of thumb
If influent BOD5 = 800 mg/L and COD = 2000 mg/L, ratio = 0.4 - moderately biodegradable; expect good biological removal but check for refractory fraction. Rule of thumb: BOD5 ≈ 0.5–0.7 × COD for readily biodegradable industrial streams.