Wastewater Treatment Operator Class III cheat sheet

Treatment train order: preliminary (screens, grit removal) -> primary clarification -> secondary biological treatment -> tertiary polishing -> disinfection -> outfall.

Typical untreated domestic wastewater: BOD5 200-250 mg/L, TSS 200-240 mg/L, total nitrogen 35-45 mg/L, total phosphorus 6-10 mg/L.

BOD5 is roughly 65-70% of ultimate carbonaceous BOD; BOD5 = BODU x (1 - 10^(-k*t)) with k about 0.10 per day at 20 deg C gives BOD5/BODU near 0.68.

Nitrification consumes about 4.57 mg O2 per mg NH4-N oxidized and destroys about 7.14 mg alkalinity as CaCO3 per mg NH4-N.

Nitrification rule of thumb: at 15 deg C reliable nitrification needs SRT above roughly 10-15 days, DO of 2 mg/L or more, and at least 50 mg/L residual alkalinity.

Denitrification requires an anoxic zone (no free DO but nitrate present) plus a carbon source, and recovers about 2.86 mg O2 equivalent per mg NO3-N reduced.

Batch growth curve phases: lag -> log (exponential) -> stationary -> endogenous decay; activated sludge is deliberately held in the log-to-stationary transition.

Indicator organisms are total coliform, fecal coliform, and E. coli; E. coli is the preferred indicator of recent fecal contamination.

MLVSS/MLSS for healthy activated sludge is typically 0.70-0.80; a sharp drop signals accumulation of inert or inorganic solids.

Preliminary treatment components: bar screens or comminutors, grit chambers, flow measurement, and sometimes flow equalization or pre-aeration.

Screen openings: coarse bar screens 6 mm and larger, fine screens 1.5-6 mm; headloss across a clean bar screen is typically 0.15-0.3 m and rises as rags accumulate.

Grit classification: 65 mesh (about 0.2 mm) and larger, specific gravity about 2.65; removed by velocity control near 0.3 m/s or by aerated/vortex grit chambers.

Surface settling rate (overflow rate) = Q / A; primary clarifier design values are 30-50 m3/m2-d (800-1200 gpd/ft2).

Primary clarifier design: hydraulic detention 1.5-2.5 h at design flow, and weir loading below about 250 m3/m-d (10,000-20,000 gpd/ft) to limit solids carryover.

Expected primary removal: 50-65% of TSS and 25-40% of BOD5, producing primary sludge at about 4-6% total solids.

Percent removal = ((influent - effluent) / influent) x 100.

Septic-condition rule: if primary effluent is dark, odorous, or high in sulfide, shorten detention, increase sludge withdrawal, or add pre-aeration/chemicals.

Clarifier solids loading rate = ((Q + Qr) x MLSS) / A; exceeding design capacity raises the sludge blanket and effluent TSS.

F/M = (Q x BOD entering) / (MLVSS x aeration tank volume); conventional activated sludge 0.2-0.5 per day, extended aeration 0.05-0.15 per day.

MCRT (SRT) = solids mass in aeration tank plus clarifier / (solids wasted per day + solids lost in effluent per day).

SVI = (30-minute settled sludge volume in mL/L x 1000) / MLSS in mg/L; 80-150 good settling, 150-200 marginal, above 200 bulking.

Typical operating ranges: MLSS 2000-3000 mg/L conventional, 3000-5000 mg/L extended aeration; RAS 50-150% of influent flow; DO 1.5-2.5 mg/L.

Oxygen demand is about 1.0 kg O2 per kg BOD removed for carbonaceous oxidation, plus 4.57 kg O2 per kg of ammonia nitrogen nitrified.

Secondary clarifier design: overflow rate 16-30 m3/m2-d, solids loading about 100-150 kg/m2-d, sidewater depth 3-4.5 m, sludge blanket held below 0.5-1.0 m.

Bulking decision rule: if SVI exceeds 200, check low DO, low F/M, nutrient deficiency, septic influent (sulfides), or selectors; options include RAS chlorination or adding a selector zone.

Trickling filter loading: rock media 0.08-0.4 kg BOD/m3-d with 1-4 m3/m2-d hydraulic loading; plastic media 0.4-1.6 kg BOD/m3-d; recirculation ratio = Qr / Q.

Fixed-film process family: trickling filters, rotating biological contactors, moving bed biofilm reactors, and integrated fixed-film activated sludge.

Free chlorine speciation: HOCl dominates below about pH 7.5 and is roughly 80 times more germicidal than the hypochlorite ion OCl- that dominates at higher pH.

CT = disinfectant residual (mg/L) x contact time (min); required CT increases with higher pH, lower temperature, and more resistant organisms (viruses, Giardia, Cryptosporidium).

Chlorination rule: keep a measurable residual, typically 0.5-2.0 mg/L, after 20-30 minutes of contact, and dechlorinate where chlorine is toxic to the receiving water.

Chlorine demand = chlorine applied - chlorine residual after contact; breakpoint chlorination is the dose beyond which residual rises almost linearly with dose.

UV disinfection uses 254 nm light at roughly 30-40 mJ/cm2 for secondary effluent; it requires low TSS and high UV transmittance, and leaves no residual or disinfection byproducts.

Alternative disinfectants: ozone (forms bromate, needs an off-gas destruct unit) and chlorine dioxide (forms chlorite); both are stronger oxidants than free chlorine.

Typical secondary effluent limits: BOD5 30 mg/L, TSS 30 mg/L, with 85% removal of each; fecal coliform often 200 cfu/100 mL as a monthly geometric mean.

Un-ionized ammonia (NH3) is the form toxic to fish and its fraction rises with pH and temperature; permit limits and toxicity tests are applied to the un-ionized fraction.

Disinfection byproducts of concern are trihalomethanes and haloacetic acids formed when chlorine reacts with organics; chloramines lower THM formation but are weaker disinfectants.

Sludge sources and typical concentrations: primary sludge 4-6% total solids, waste activated sludge 0.5-1.5% total solids, blended thickened sludge 4-6% total solids.

Thickening selection: gravity thickener for primary sludge at 16-32 kg/m2-d solids loading; dissolved air flotation for waste activated sludge, reaching 3-5% total solids.

Anaerobic digestion: mesophilic at 35-38 deg C with 15-20 day SRT and 40-50% volatile solids reduction; thermophilic at 50-55 deg C with shorter SRT and Class A potential.

Digester stability indicators: pH 6.8-7.4, alkalinity 2000-3000 mg/L as CaCO3, volatile acids below about 200-300 mg/L, and volatile acids/alkalinity ratio below 0.1.

Biogas yield about 0.8-1.2 m3 per kg volatile solids destroyed, containing 60-65% methane and 30-35% carbon dioxide; H2S must be removed before engine use.

Dewatering performance: belt press or centrifuge cake typically 15-30% total solids; polymer dose is optimized by capillary suction time or jar testing.

Class A biosolids: fecal coliform below 1000 MPN/g total solids and Salmonella below 3 MPN/4 g, plus a vector attraction reduction option; Class B allows fecal coliform below 2 x 10^6 MPN/g total solids.

Land application rate is an agronomic rate set by crop nitrogen demand and biosolids nitrogen content, not by hydraulic loading.

Solids mass balance: kg dry solids per day = flow (m3/d) x concentration (kg/m3); use it to set wasting rates and digester loadings.

Manning equation: V = (1.486/n) x R^(2/3) x S^(1/2) in fps, or (1/n) x R^(2/3) x S^(1/2) in SI; n about 0.013 for concrete and PVC, 0.015 for vitrified clay.

Velocity criteria: 0.6 m/s (2 ft/s) minimum self-cleansing velocity at design flow, and roughly 3-4.5 m/s maximum to limit erosion and hydrogen sulfide release.

Sanitary sewer design flow uses a peaking factor (about 4 for small tributary areas, decreasing as population grows) plus an infiltration/inflow allowance.

Surcharge rule: when flow exceeds full-pipe capacity the hydraulic grade line rises and manholes may overflow, so relief capacity or storage is required.

Force main velocity 0.9-1.5 m/s (3-5 fps); provide air-release and vacuum-relief valves at high points and flushing connections at low points.

Pump affinity laws at constant impeller diameter: flow proportional to speed, head proportional to speed squared, power proportional to speed cubed.

Water horsepower = (gpm x total head in ft x specific gravity) / 3960; brake horsepower = water horsepower / pump efficiency; wire power = brake horsepower / motor efficiency.

NPSH available must exceed NPSH required; a shortfall causes cavitation, shown by noise, vibration, pitting, and reduced head.

Wet well design limits pump starts to roughly 6-12 per hour; the operating volume between lead and lag levels sets the cycle time.

BOD5 = ((D1 - D2) - (B1 - B2) x f) x 300 / mL of sample, where D is DO in the diluted sample, B is DO in the seed blank, and f = seed in sample / seed in blank.

BOD acceptance criteria: dissolved oxygen depletion of at least 2.0 mg/L and residual DO of at least 1.0 mg/L, otherwise adjust the dilution.

TSS: filter through a 1.5-micron glass fiber filter, dry at 103-105 deg C, desiccate, and weigh; VSS: ignite at 550 deg C and record the weight loss.

Settleable solids: 1 liter in an Imhoff cone for 60 minutes; report the settled volume in mL/L.

COD: dichromate digestion at 150 deg C for 2 hours; results in about 2-3 hours, correlates with BOD, but also oxidizes non-biodegradable matter.

DO measurement: Winkler azide modification titration is the reference method; membrane and luminescent electrodes require calibration and adequate sample flow.

Chlorine residual: DPD colorimetric or amperometric titration; free chlorine is distinguished from total chlorine by adding iodide.

Microscopy: stalked ciliates indicate mature stable sludge, free-swimming ciliates indicate high soluble BOD or low MCRT, and filamentous organisms or Nocardia indicate bulking or foaming.

Sampling rule: flow-weighted 24-hour composites for BOD and TSS; grab samples for pH, DO, chlorine residual, and fecal coliform.

Permit-required confined space: contains a hazardous atmosphere, engulfment, entrapment, or other serious hazard; entry requires a permit, attendant, atmospheric testing, and a rescue plan.

Acceptable entry atmosphere: O2 19.5-23.5%, LEL below 10%, H2S below 10 ppm, CO below 25 ppm; test before entry and monitor continuously while inside.

Hydrogen sulfide: OSHA PEL 20 ppm with a 50 ppm ceiling, IDLH 100 ppm; it is heavier than air, and high concentrations deaden the sense of smell.

Flammable and toxic limits: methane LEL 5% and UEL 15%, alarm at 10% LEL; chlorine IDLH 10 ppm; ammonia IDLH 300 ppm.

Lockout/tagout: isolate every energy source, apply locks and tags, and verify isolation by attempting startup before work begins.

NPDES permits set effluent limits, monitoring frequency, and discharge monitoring report (DMR) submission; noncompliance that may endanger health or the environment requires 24-hour reporting.

Bypass and upset provisions require oral notice within 24 hours and a written report within 5 days where the permit or rule requires it.

SSO/CSO reporting: notify the regulatory agency, issue public notice as required, and record volume, location, duration, cause, and corrective action.

Records retention: DMR and monitoring records at least 3 years, biosolids records under 40 CFR 503 at least 5 years; operator certification is state-issued with renewal and continuing education requirements.