7.2 Effluent Disinfection, Dechlorination & Polishing

7.2 Effluent Disinfection, Dechlorination & Polishing

Following secondary clarification and nutrient removal, treated wastewater retains enteric pathogens including bacteria (Salmonella, pathogenic E. coli), viruses (enteroviruses, norovirus), and protozoans (Giardia, Cryptosporidium). Disinfection destroys these pathogens prior to discharge into Illinois surface waters. However, chemical disinfection agents, particularly chlorine, are acutely toxic to fish and aquatic organisms. Municipal facilities must balance pathogen inactivation against chemical residuals and dissolved oxygen standards.

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1. Illinois EPA NPDES Disinfection Standards

Under Title 35 of the Illinois Administrative Code (Subtitle C) and delegated Clean Water Act provisions, the Illinois EPA establishes microbial and residual discharge criteria:

• The effluent standard (35 Ill. Adm. Code 304.121(a)): effluents discharged to all general use waters must not exceed 400 fecal coliforms per 100 mL, unless the Illinois EPA determines that an alternative effluent standard applies. This single number — not a geometric mean plus daily maximum pair — is the Illinois effluent standard, and it is the figure to answer with on a state exam.
• Alternate and seasonal limits: under 304.121(b), the Agency sets alternate effluent standards through the NPDES permit program consistent with the water quality standards at 35 Ill. Adm. Code 302.209 and 302.306, applied year-round or seasonally based on documentation the discharger provides.
• Disinfection exemptions (35 Ill. Adm. Code Part 378): Illinois lets qualifying dischargers stop disinfecting. Waters used for public and food processing supply are year-round protected and must meet the 2,000 per 100 mL standard of 302.306 at any intake. Seasonally protected waters — large streams and rivers and lakes and ponds supporting primary contact, pooled areas of small streams, and streams flowing through or adjacent to parks or residential areas — must meet the 200 per 100 mL standard of 302.209(a) during the months of May through October. That May-through-October window is why most Illinois plants disinfect seasonally. Unprotected waters (average depths of two feet or less with no deep summer pools, physical obstacles to contact, or adjacent land uses that discourage contact) are not subject to the fecal coliform standards at all.
• Escherichia coli ($E. coli$): some Illinois permits incorporate E. coli limits, commonly a geometric mean of $\le 126\text{ CFU/100 mL}$, reflecting the federal recreational criterion.
• Total Residual Chlorine (TRC) Limits: Chlorine damages fish gill membranes and causes asphyxiation. Permitted Water Quality-Based Effluent Limits (WQBELs) for TRC are typically $0.03\text{ to } 0.05\text{ mg/L}$ daily maximum, with monthly averages often set at non-detect ($< 0.02\text{ mg/L}$), mandating chemical dechlorination whenever chlorine is used.

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2. Wastewater Chlorination & Contact Chamber Hydraulics

Chlorine is delivered as chlorine gas ($Cl_2$) through vacuum chlorinators or as liquid sodium hypochlorite bleach ($NaOCl$, 12.5%).

Chloramination in Wastewater

Because municipal effluent contains background ammonia-nitrogen, injected chlorine reacts rapidly with ammonia to form chloramines (combined chlorine):

$NH_3 + HOCl \to NH_2Cl\text{ (Monochloramine)} + H_2O$ $NH_2Cl + HOCl \to NHCl_2\text{ (Dichloramine)} + H_2O$

Wastewater disinfection relies on combined chloramines rather than dosing to breakpoint. Monochloramine is a slower disinfectant than free chlorine, requiring longer detention times, but it generates substantially fewer regulated trihalomethanes. Secondary effluent typically requires an applied chlorine dose of $5\text{ to } 15\text{ mg/L}$ to satisfy initial demand and maintain a residual of $1.0\text{ to } 2.5\text{ mg/L}$ entering the contact chamber.

Contact Chamber Hydraulic Design

Inactivation kinetics depend on disinfectant concentration ($C$) and contact time ($T$). The Chlorine Contact Chamber (CCC) must satisfy strict hydraulic criteria:

• Plug-Flow ($PF$) Hydraulics: True plug-flow is mandatory. Short-circuiting allows pathogenic bacteria to escape prematurely. Chambers require a length-to-width ratio ($L/W$) exceeding $10:1\text{ to } 40:1$.
• Cross-Baffling: Concrete serpentine channels with over-and-under or round-the-end baffles ensure uniform velocity and eliminate dead zones.
• Detention Time: Illinois Recommended Standards for Sewage Works require a minimum contact time of $15\text{ to } 30\text{ minutes}$ at peak hourly flow (or $\ge 30\text{ to } 45\text{ minutes}$ at average design flow).
• Sludge Management: Settled solids in contact basins decompose anaerobically, consuming chlorine, generating sulfide odors, and elevating fecal coliform counts. Basins must be periodically drained and washed down.

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3. Dechlorination Chemistry & Overdosing Hazards

To comply with sub-0.05 mg/L TRC limits, chlorinated effluent is chemically neutralized before discharge.

Sulfur Dioxide ($SO_2$) Chemistry

Gaseous sulfur dioxide ($SO_2$) is a fast-acting reducing agent that reacts with free chlorine and chloramines in $< 30\text{ seconds}$:

• Reaction with Free Chlorine:

$SO_2 + Cl_2 + 2 H_2O \to H_2SO_4 + 2 HCl$

• Reaction with Monochloramine:

$SO_2 + NH_2Cl + 2 H_2O \to NH_4HSO_4 + HCl$

These reactions reduce chlorine to harmless chloride ($Cl^-$) while consuming approximately $2.8\text{ mg}$ of alkalinity as $CaCO_3$ per mg of chlorine neutralized.

Stoichiometric Ratios & Chemical Handling

• Dosage Ratio: Chemically, $0.90\text{ lb } SO_2$ neutralizes $1.0\text{ lb}$ chlorine residual. Utilities feed a direct $1.0:1.0$ mass ratio ($1\text{ lb } SO_2$ per $1\text{ lb TRC}$) to ensure complete compliance.
• $SO_2$ Safety Hazards: Stored under pressure in 150-lb cylinders and 1-ton containers, $SO_2$ gas is heavier than air (density 2.26 times air), toxic, corrosive in moisture, and suffocating. Piping threads differ from chlorine equipment to prevent cross-connections. Emergency response to 1-ton container leaks requires the Chlorine Institute Emergency Kit B adapted for sulfur dioxide.
• Liquid Alternatives: To eliminate gas inhalation hazards, utilities widely deploy liquid sodium bisulfite ($NaHSO_3$, 38%–40%) or dry sodium metabisulfite ($Na_2S_2O_5$):

$NaHSO_3 + Cl_2 + H_2O \to NaHSO_4 + 2 HCl$

Overdosing Hazards: Oxygen Scavenging

Dechlorination dosing requires compound-loop automated control (feedforward flow pacing and feedback residual trim). Overdosing poses a severe environmental danger:

$2 SO_3^{2-} + O_2 \to 2 SO_4^{2-}$

Excess sulfite acts as an aggressive oxygen scavenger: $1.0\text{ lb}$ of excess $SO_2$ consumes $0.25\text{ lb}$ of dissolved oxygen ($O_2$). Overdosing strips DO from the effluent, causing receiving stream fish kills and violating NPDES dissolved oxygen limits.

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4. Wastewater Ultraviolet (UV) Disinfection

Ultraviolet disinfection eliminates toxic chemical storage and chemical residuals, relying on physical irradiation.

Photochemical Germicidal Mechanism

Microorganisms are exposed to UV-C light at the peak biocidal wavelength of $254\text{ nm}$. Light penetrates cell walls and is absorbed by DNA and RNA, forming thymine dimers (covalent bonds between adjacent thymine bases). This prevents transcription and cellular replication. Cells are not lysed, but are rendered sterile and non-infectious.

Lamp Technologies & Channel Hardware

1. Low-Pressure High-Output (LPHO) Lamps: Monochromatic output at $253.7\text{ nm}$, operating at low internal mercury pressure and cooler temperatures ($100^\circ\text{C to } 200^\circ\text{C}$) with high electrical efficiency. Standard for small-to-medium municipal plants.
2. Medium-Pressure (MP) Lamps: Polychromatic output across $200\text{ to } 300\text{ nm}$, operating at high intensity and elevated temperatures ($600^\circ\text{C to } 900^\circ\text{C}$). High power use, but compact footprint.
3. Channel Hardware: Submerged lamp banks are housed within protective quartz sleeves in open concrete channels. Downstream water level is controlled by serpentine finger weirs or motorized gates to keep lamps submerged.

Operational Parameters

• UV Transmittance (UVT): Percentage of $254\text{ nm}$ light passing through a $1.0\text{ cm}$ water path. Secondary municipal effluent typically displays $60%\text{ to } 70%$ UVT. Turbidity, iron, or dyes depress UVT, requiring higher lamp output.
• Delivered UV Dose: $\text{Dose } (\text{mJ/cm}^2) = \text{Intensity } (\text{mW/cm}^2) \times \text{Time } (\text{seconds})$. Design target is $30\text{ to } 40\text{ mJ/cm}^2$ at end-of-lamp life.
• Quartz Sleeve Fouling: Calcium, iron, and slime precipitate onto hot quartz sleeves, blocking light. Systems utilize automated mechanical wiper rings with chemical acid flushes (citric or phosphoric acid).
• Photoreactivation: Certain bacteria possess enzymes that repair thymine dimers when exposed to sunlight ($300\text{ to } 500\text{ nm}$). Delivering a dose $> 30\text{ mJ/cm}^2$ ensures irreversible inactivation.

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5. Effluent Post-Aeration Polishing

Treated effluent leaving clarifiers, contact tanks, or UV channels often contains low dissolved oxygen ($1.0\text{ to } 3.0\text{ mg/L}$). Discharging low-DO water impairs stream habitats and violates NPDES permit limits, which typically mandate $\ge 5.0\text{ to } 6.0\text{ mg/L}$ DO.

• Cascade Aeration: Gravity-driven step weirs with a $4\text{ to } 8\text{ foot}$ total drop. Splashing water naturally entrains atmospheric oxygen without electrical energy.
• Diffused Air Basins: Submerged fine or coarse-bubble diffusers powered by blowers in a dedicated post-aeration tank.
• Jet Aeration: High-velocity liquid-air aspirating pumps installed in effluent discharge channels.