5.6 Wastewater Characteristics, Organic and Hydraulic Loading, Diurnal Flow & Septicity

Knowing your influent before you treat it

Every process control decision in a wastewater plant starts from the same question: how much of what is arriving, and when. WPI's Treatment Process Evaluation and Adjustment content area is 38 percent of the Wastewater Treatment Class I exam, and its calculations are all built on influent characterization.

Composition of typical domestic wastewater

Raw domestic wastewater is roughly 99.9 percent water. Everything that matters is in the remaining 0.1 percent.

A commonly used planning figure is roughly 0.17 to 0.20 pounds of BOD5 per person per day and a similar mass of suspended solids, with 60 to 100 gallons per capita per day of flow.

The oxygen demand family

• BOD5 is the oxygen consumed by microorganisms degrading organic matter over five days at 20 degrees Celsius. It measures the biodegradable fraction and takes five days to report — its central operating limitation.
• CBOD5 excludes nitrogenous demand by adding a nitrification inhibitor, so it isolates carbonaceous demand. Many Colorado permits are written on CBOD5 for exactly this reason.
• COD oxidizes chemically with dichromate in about two hours and captures both biodegradable and non-biodegradable organics. It is always higher than BOD.
• The COD to BOD ratio is a useful fingerprint. Domestic wastewater typically runs about 1.5 to 2.5. A ratio well above 3 signals a large non-biodegradable or industrial fraction. Once the ratio is established for a specific plant, same-day COD becomes a practical surrogate for the five-day BOD result.
• Total organic carbon (TOC) is faster still and correlates well once calibrated.

Solids

• Total solids equals total suspended solids (TSS) plus total dissolved solids.
• TSS splits into volatile suspended solids (VSS), burned off at 550 degrees Celsius and representing the organic and biological fraction, and fixed suspended solids, the mineral ash.
• Settleable solids, measured in an Imhoff cone in mL/L after one hour, indicate what primary clarification will remove.
• In domestic wastewater, VSS is normally about 70 to 80 percent of TSS. In mixed liquor the MLVSS to MLSS ratio is a proxy for how much of the sludge inventory is active biomass, and it typically runs 0.70 to 0.85 — it falls when grit or industrial inerts accumulate.

Loading, and why it is not concentration

Concentration alone tells you nothing about whether the plant is overloaded. Load is what matters, and it comes from the pounds formula:

lbs/day = flow (MGD) x concentration (mg/L) x 8.34

Worked example. A plant receives 1.85 MGD with an influent BOD5 of 235 mg/L.

lbs BOD5/day = 1.85 x 235 x 8.34 = 3,626 lbs/day

If the aeration basin holds 0.60 MG at 2,400 mg/L MLVSS, the biomass inventory is 0.60 x 2,400 x 8.34 = 12,010 lbs, and the F/M ratio is 3,626 divided by 12,010 = 0.30 per day. Concentration alone could not have told you that.

Organic loading rate is expressed as pounds of BOD per day per 1,000 cubic feet of basin volume, and hydraulic loading as flow per unit area or volume. Both are compared against design values to judge whether an upset is a loading problem or a process problem.

Flow patterns and what they do to the plant

Domestic influent follows a diurnal curve: a low overnight, a sharp morning peak, a midday dip, and an evening peak. Small systems see wider swings than large ones because there is less averaging.

• Peaking factors of 1.5 to 3 times average daily flow are normal, and higher in small systems.
• Peak hydraulic flow washes solids out of clarifiers; peak organic load starves the aeration basin of oxygen. They do not always coincide.
• Infiltration and inflow superimposes wet-weather flow that is high in volume and low in strength, so the concentration falls while the mass load may barely move. A plant that suddenly sees influent BOD drop from 220 mg/L to 90 mg/L after a storm has an I&I problem, not a customer problem.
• Seasonal variation matters greatly in Colorado: ski and resort communities can see populations swing by a factor of five, and cold winter influent slows biological rates precisely when the load is highest.
• Industrial contributions arrive as batches. A single food processor or brewery discharge can double the organic load for a few hours. This is why pretreatment programs and significant industrial user permits exist, and why Regulation 100 raises collection system classification when three or more significant industrial users are present.

Septicity

Wastewater held too long without oxygen goes septic. Sulfate-reducing bacteria in the slime layer of a full-flowing sewer or a long force main reduce sulfate to hydrogen sulfide. The consequences run through the whole plant:

• Odor complaints at the headworks, at manholes, and at the force main discharge.
• Crown corrosion in gravity sewers, where H2S is oxidized biologically to sulfuric acid on moist concrete above the water line.
• Blackened, foul influent that has already consumed its own oxygen, arriving with an immediate oxygen demand.
• Depressed dissolved oxygen at the head of the aeration basin and a higher air requirement.
• Toxicity: hydrogen sulfide has an OSHA permissible exposure limit of 20 ppm as a ceiling and is immediately dangerous to life or health at 100 ppm.

Indicators an operator watches are influent color and odor, dissolved oxygen at the headworks, sulfide concentration, and oxidation-reduction potential.

Sampling that represents the day

• A grab sample is a single sample at one instant. Required for pH, temperature, dissolved oxygen, total residual chlorine, and bacteria, all of which change or degrade quickly.
• A composite sample blends aliquots over 24 hours. Flow-proportional compositing, where aliquot size or frequency tracks flow, gives the truest daily average and is what permits normally require for BOD and TSS.
• Time-proportional compositing takes equal aliquots at equal intervals and is acceptable only where flow is relatively constant.
• Samples must be iced to 6 degrees Celsius or below during collection and transport, and holding times observed: 48 hours for BOD, 28 days for preserved COD, 7 days for TSS.

Putting it together

Characterization drives everything downstream. Influent BOD and TSS set the F/M ratio and the sludge production rate. Influent ammonia and alkalinity set the nitrification capacity and whether supplemental alkalinity is needed. Influent phosphorus sets the chemical dose or the EBPR design. Flow variability sets the need for equalization. And the difference between influent and effluent, expressed as a percentage, is the removal efficiency the permit is written against:

Percent removal = [(influent - effluent) / influent] x 100

A plant with 235 mg/L influent BOD and 12 mg/L effluent BOD achieves (235 - 12) / 235 x 100 = 94.9 percent removal, comfortably above the 85 percent secondary treatment requirement.