3.6 Wastewater Characteristics, Influent Loading & Industrial Pretreatment

Wastewater Characteristics, Influent Loading & Industrial Pretreatment

The wastewater outline places "preliminary treatment processes" and process evaluation at the top of a 42-item content area, but everything in that area starts with knowing what is in the influent. Process control is fundamentally the management of a mass balance, and the mass entering the plant is the input to every calculation that follows.

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1. Typical Domestic Wastewater Strength

Per capita planning factors: approximately 0.2 lb BOD5 per person per day and 0.2 lb TSS per person per day, with about 0.026 lb total nitrogen and 0.005 lb total phosphorus. A community of 12,000 people therefore produces roughly 2,400 lb/day of BOD - a number an operator can use to sanity-check whether a suspicious influent result is real.

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2. The Oxygen Demand Family

The COD:BOD5 ratio is a diagnostic. Raw domestic wastewater typically runs 1.9 to 2.5. A ratio much above 3 means a large fraction of the organic load is not biodegradable - almost always an industrial contribution - and no amount of aeration will remove it. Because COD returns in two hours and BOD5 in five days, most plants establish a site-specific COD:BOD correlation and then use COD for daily process control.

CBOD versus BOD in permits. Most modern VPDES permits set CBOD5 limits precisely because a nitrifying plant's effluent would otherwise show high BOD5 from ammonia oxidation in the bottle rather than from unremoved carbon.

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3. Nitrogen and Phosphorus Fractions

Total Kjeldahl Nitrogen (TKN) = organic nitrogen + ammonia nitrogen. Total Nitrogen (TN) = TKN + nitrite-N + nitrate-N.

In raw domestic wastewater, roughly 60 percent of TKN is already ammonia and the rest is organic nitrogen that hydrolyzes to ammonia during treatment - which is why effluent ammonia can exceed influent ammonia at a plant that is not nitrifying.

Phosphorus arrives as orthophosphate (immediately available), polyphosphate (from detergents, hydrolyzing to ortho), and organic phosphorus. Only orthophosphate is directly removed by chemical precipitation, so a plant dosing alum or ferric measures ortho, not total, for dose control.

Alkalinity is a process control parameter

Nitrification consumes 7.14 mg of alkalinity as CaCO3 per mg of NH3-N oxidized. Denitrification returns about 3.57 mg/L, or half of what nitrification consumed.

Worked example. Influent alkalinity 180 mg/L, ammonia 28 mg/L, full nitrification, no denitrification.

• Alkalinity consumed = 28 x 7.14 = 200 mg/L
• Available = 180 mg/L - so the plant runs out of buffer, pH crashes, and nitrification stalls.
• With 60 percent denitrification: recovered = 0.6 x 28 x 3.57 = 60 mg/L, so net consumption is 140 mg/L, leaving 40 mg/L residual - still below the 50 to 80 mg/L residual normally recommended to hold pH near 7. Supplemental alkalinity (sodium bicarbonate, lime, magnesium hydroxide, or caustic) is required.

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4. Flow Patterns

Diurnal variation. Domestic flow peaks in the mid-morning and again in the early evening, with a minimum in the early hours. Peak hourly flow commonly runs 1.5 to 2.5 times average daily flow at a medium plant, and higher at small ones. Load peaks lag flow peaks slightly, and the two do not perfectly coincide.

Peaking factors matter because process control uses averages. An F/M ratio computed on average daily flow says nothing about whether the aeration basin is oxygen-limited at 10 a.m.

Wet-weather flow. Infiltration and inflow can multiply flow several-fold while diluting concentration. The signature is easy to read:

During heavy I&I, the danger is solids washout: high flow raises the clarifier surface overflow rate and solids loading rate, lifting the sludge blanket over the weirs and taking the plant's biomass inventory with it.

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5. Septage and Hauled Waste Receiving

Many Virginia plants accept septage. It is an operational hazard disguised as a revenue stream.

• Septage is roughly 20 to 40 times the strength of domestic wastewater - BOD often 5,000 to 8,000 mg/L, TSS 10,000 to 15,000 mg/L.
• A single 3,000-gallon load at 6,000 mg/L BOD delivers 3,000 x 8.34 x 6,000/1,000,000 = 150 lb of BOD in minutes. At a 0.5 MGD plant carrying 835 lb/day, that is an 18 percent daily load in one slug.
• Controls: a dedicated receiving station with screening and rock trap, metered slow bleed-in over hours rather than dumping, hours-of-acceptance limits, manifest tracking of hauler and source, and pH and sulfide screening before acceptance.
• Watch for struvite formation and for hydrogen sulfide odor in the receiving area.

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6. The Industrial Pretreatment Program

Publicly owned treatment works with significant industrial contributions administer a pretreatment program under 40 CFR Part 403, delegated in Virginia through DEQ.

Three tiers of standards

1. Prohibited discharge standards (general and specific). No discharge may cause pass through or interference. The specific prohibitions ban:
   • Pollutants creating a fire or explosion hazard, including any discharge with a closed-cup flashpoint below 140 degrees F;
   • pH below 5.0 (unless the works is designed for it);
   • Solids or viscous pollutants that obstruct flow;
   • Oxygen-demanding pollutants at a rate that causes interference;
   • Heat causing the influent to exceed 104 degrees F (40 degrees C) at the treatment plant;
   • Petroleum oil, non-biodegradable cutting oil, or mineral oil products in amounts causing interference or pass through;
   • Pollutants that create toxic gases, vapors, or fumes in quantities that may cause acute worker health and safety problems;
   • Trucked or hauled waste except at designated discharge points.
2. Categorical pretreatment standards. National, industry-specific numeric limits for categories such as electroplating, metal finishing, organic chemicals, and centralized waste treatment.
3. Local limits. Derived by the POTW itself from a headworks loading analysis that back-calculates the maximum influent concentration the plant can accept while still protecting the effluent permit, the biosolids quality criteria, worker safety, and the collection system.

Program mechanics

• Significant industrial users (SIUs) are permitted individually, with self-monitoring and reporting requirements and periodic POTW inspection and sampling.
• Baseline monitoring reports are required when a categorical user begins discharging or a new standard takes effect.
• Fats, oil, and grease (FOG) programs for food service establishments - grease interceptor sizing, cleaning frequency, and manifesting - are the most operationally valuable part of the program for most Virginia utilities, because FOG drives sanitary sewer overflows.
• Enforcement response plans define escalating action from notice of violation through fines and termination of service.

Why the operator cares. Interference by an industrial user shows as a plant upset the operator must diagnose: sudden loss of nitrification (metals or a toxic organic), filamentous bloom (a high-carbohydrate or low-nutrient waste), foaming (surfactants), digester gas production collapse (a toxicant carried through to the digester), or biosolids failing metals ceiling concentrations - the last of which can shut down land application entirely.