ABC Wastewater Treatment Class II cheat sheet

Confined space entry requires a permit, atmospheric testing before and during entry, continuous ventilation, an attendant outside, and rescue capability on site.

Hazardous atmosphere thresholds for entry: oxygen below 19.5% or above 23.5%, flammable gas at or above 10% LEL, H2S at or above 10 ppm, CO at or above 35 ppm.

Wastewater atmospheres are oxygen-deficient and may contain H2S, methane, carbon monoxide, and ammonia; H2S deadens the sense of smell above roughly 100 ppm, so odor is not a reliable warning.

Chlorine gas is heavier than air and accumulates in low areas; a 1 ppm chlorine level is the common occupational ceiling and 10 ppm is immediately dangerous to life or health.

Sodium hypochlorite and sulfuric acid must never be stored or mixed together; the reaction releases chlorine gas.

Digester gas is roughly 60-65% methane and is explosive between about 5% and 15% in air; no ignition sources near gas-handling equipment.

Lockout/tagout: isolate all energy sources, verify zero energy state by attempting startup or testing, and apply a personal lock before servicing equipment.

Hearing protection is required at 85 dBA time-weighted average; noise doubles in intensity for each 5 dBA increase, so 95 dBA is twice as intense as 90 dBA.

Bar screens remove large debris; coarse bar spacing is typically 1 to 2 inches and fine screens 1/4 inch or less. Head loss across a screen rises as it blinds, signaling cleaning.

Grit removal targets discrete inorganic solids with specific gravity near 2.65; a well-designed grit system removes grit while keeping putrescible organic solids in the flow.

Detention time in a primary clarifier is typically 1.5 to 2.5 hours at design flow; longer detention risks septic conditions and rising H2S.

Overflow rate = flow divided by clarifier surface area, expressed as gallons per day per square foot; typical primary clarifier design is 600 to 1200 gpd/sq ft at average flow.

Weir loading rate = flow divided by total weir length, in gallons per day per linear foot; high weir loading causes short-circuiting and solids carryover.

The classic rule of thumb: a 1 mg/L change in primary effluent suspended solids changes the secondary influent BOD by about 0.5 to 0.7 mg/L.

Sludge from primary clarifiers is typically 3 to 7% solids and is more readily digestible than secondary sludge.

Percent removal = (influent minus effluent) divided by influent, times 100.

F/M ratio = pounds of BOD applied per day divided by pounds of MLVSS in the aeration tank; conventional activated sludge runs roughly 0.2 to 0.4 per day.

Mean cell residence time (sludge age) = pounds of MLSS (or MLVSS) in the system divided by pounds of solids leaving per day in waste sludge plus effluent.

Food-to-microorganism control: raising waste sludge rate lowers MCRT and F/M, which reduces filamentous bulking risk; lowering wasting raises both.

Sludge volume index (SVI) = 30-minute settled sludge volume in mL/L divided by MLSS in g/L, times 1000; SVI of 100 to 150 mL/g indicates good settling, above 200 suggests bulking.

Return activated sludge rate is set to keep the sludge blanket in the clarifier low; typical RAS flow is 0.5 to 1.5 times influent flow.

Dissolved oxygen in the aeration basin is normally maintained at 1.5 to 3 mg/L; low DO favors filamentous organisms and low DO also slows nitrification.

Trickling filter sloughing follows the food supply; organic loading is expressed as pounds of BOD per day per 1000 cubic feet of media, and recirculation dilutes influent strength and maintains hydraulic wetting.

Nitrification consumes about 4.57 mg of oxygen per mg of ammonia nitrogen oxidized and drops alkalinity by about 7.14 mg/L as CaCO3 per mg/L of ammonia nitrogen.

CT = disinfectant residual concentration in mg/L multiplied by contact time in minutes; achieving the required CT is the design basis for pathogen inactivation.

Chlorine demand is the difference between the chlorine dose and the residual remaining after the contact period.

Free chlorine exists as HOCl and OCl-; the balance shifts with pH, and HOCl is the far stronger disinfectant, so lower pH improves disinfection efficiency.

Breakpoint chlorination: chlorine demand must be satisfied and combined chloramines oxidized before free residual appears; the breakpoint is where residual begins to rise in proportion to dose.

A minimum of 30 minutes contact time at peak flow with adequate baffling is a common regulatory basis for chlorine contact chambers.

Sulfur dioxide or sodium bisulfite is used for dechlorination; roughly 1 mg/L of sulfur dioxide destroys about 1 mg/L of chlorine residual.

UV disinfection depends on transmittance and lamp intensity; suspended solids, iron, and hardness scale on sleeves all shield organisms or reduce dose.

Typical secondary effluent limits: BOD and TSS at 30 mg/L monthly average for conventional plants, 20/20 for many state permits, and 10/10 for advanced treatment.

Solids loading rate for a gravity thickener is expressed as pounds of solids per day per square foot; feed dilution with water can improve thickening of secondary sludge.

Anaerobic digestion is commonly staged in the mesophilic range of 95 to 98 degrees F; volatile solids destruction of 40 to 55% is typical.

Volatile solids reduction percent = (feed VS minus digested VS) divided by (feed VS minus digested VS lost), computed on a mass basis using the Van Kleeck formula.

Digester gas production is roughly 12 to 18 cubic feet per pound of volatile solids destroyed, at about 60 to 65% methane.

Digester pH should stay near 7.0; a drop below 6.8 signals acid accumulation and impending souring, often from organic overload or temperature shock.

Volatile acid to alkalinity ratio above about 0.3 indicates an unstable digester; a healthy digester typically runs 0.1 to 0.2.

Belt filter press or centrifuge performance is judged by cake solids, solids capture, and polymer dose; capture percent = (solids in cake divided by solids in feed) times 100.

Conditioning with polymer or inorganic coagulant improves dewatering by neutralizing surface charge and bridging particles; overdosing polymer can blind media and raise cost without gain.

Total dynamic head = static head plus friction head plus velocity head; a pump operating far right of its curve is typically moving more flow than design and drawing higher amps.

Cavitation produces noise and vibration from vapor bubbles collapsing at the impeller; causes include low suction head, a clogged suction line, or a fluid too warm for the available NPSH.

Affinity laws for centrifugal pumps: flow varies with speed, head varies with the square of speed, and power varies with the cube of speed.

Motor current unbalance should be kept under about 10%; higher unbalance causes overheating, and a single-phasing condition shows as high current on the remaining phases.

A motor's full load amps (FLA) is on the nameplate; a reading above nameplate FLA at rated voltage indicates overload, misalignment, or a closed discharge valve.

Wet well level control: pumps should start and stop on level setpoints with enough differential to prevent short cycling, which overheats motors.

A flow meter reading that disagrees with a weir or flume measurement should be verified by checking for a partially blocked primary element or air in the sensing line.

Dissolved oxygen probes require membrane or sensor maintenance and calibration; a probe reading a constant value regardless of blower output is likely fouled or failed.

Preventive maintenance schedules are driven by manufacturer intervals and operating hours; vibration and oil analysis catch bearing wear before failure.

A centrifugal pump that loses prime after shutdown usually has a suction leak, a faulty foot valve, or air entrainment in the suction line.

Excessive vibration on a blower or pump commonly traces to misalignment, an unbalanced impeller, worn bearings, or a loose baseplate.

Filamentous bulking shows as high SVI, a poor settling blanket, and low effluent clarity; corrective actions include chlorinating the return sludge, adjusting F/M, and correcting low DO or nutrient deficiency.

Pin floc (small, weak floc) in the secondary clarifier often follows low F/M, young sludge, or toxic shock and produces turbid effluent with low TSS settling.

Clarifier solids washout can come from hydraulic overload, a rising sludge blanket, or denitrification floating sludge; rising sludge from denitrification appears as clumps floating to the surface.

A digester losing gas production and pH while volatile acids climb is going sour; stop feeding, check mixing and temperature, and restore alkalinity before resuming feed.

Root cause analysis should separate the failure mechanism from the trigger; repeat failures at the same point indicate a design or installation problem, not operator error.

BOD5 is measured over 5 days at 20 degrees C in the dark; the standard seed correction accounts for oxygen demand from the seed itself.

BOD test results require a depletion of at least 2 mg/L and a residual DO of at least 1 mg/L for the result to be valid.

Total suspended solids are determined by filtering a measured volume through a glass fiber filter, drying at 103 to 105 degrees C, and weighing the residue.

Volatile suspended solids are found by igniting the dried TSS residue at 550 degrees C; the loss on ignition represents the volatile fraction.

Composite sampling should be flow-paced rather than time-paced when the goal is to characterize mass loading, because flow varies through the day.

Grab samples are appropriate for pH, chlorine residual, and dissolved oxygen because these parameters change rapidly and cannot be preserved.

Sample preservation: pH and residual chlorine must be analyzed immediately; BOD and ammonia are preserved by chilling to 4 degrees C and acidifying to pH below 2 for ammonia.

MLSS is measured by the same filtration and drying method as TSS, and MLVSS by ignition; the ratio MLVSS/MLSS is typically 0.7 to 0.85 in activated sludge.

A discharge monitoring report (DMR) is submitted on the schedule in the permit, commonly monthly, and reports the monitoring results against permit limits.

A violation is a measured value exceeding a permit limit; a violation that is not reported within the permit's timeframe can escalate to a separate reporting violation.

The bypass and upset provisions of the Clean Water Act provide an affirmative defense only if the operator notifies the authority and meets the permit's conditions.

Records such as calibration logs, chain-of-custody forms, and DMRs must be retained for the period the permit specifies, commonly at least 3 years.

A permit limit expressed as a monthly average applies to the arithmetic mean of all samples in the calendar month; a weekly or daily maximum is a separate limit.

Noncompliance reporting generally requires notifying the agency within 24 hours for certain events and following up in writing within 5 days, per permit language.

The National Pollutant Discharge Elimination System permit is the controlling document for effluent limits, monitoring frequency, and reporting deadlines.

Operator certification requirements, including continuing education hours, are set by the state certifying authority rather than the federal rule.