7.1 Biological & Chemical Nutrient Removal

7.1 Biological & Chemical Nutrient Removal

Nutrient enrichment in receiving waters fuels accelerated eutrophication, toxic algal blooms, and oxygen depletion in Illinois waterways such as the Illinois and Mississippi river basins. Under Illinois EPA National Pollutant Discharge Elimination System (NPDES) permits and Title 35 rules, wastewater utilities must control effluent nitrogen and phosphorus to safeguard aquatic ecosystems.

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1. Nitrogen Forms in Wastewater

Nitrogen occurs in municipal wastewater across four primary states:

1. Organic Nitrogen: Nitrogen bound within organic matter (proteins, amino acids, urea). In raw domestic wastewater, organic nitrogen typically ranges from $10\text{ to } 25\text{ mg/L}$.
2. Ammonia/Ammonium ($NH_3 / NH_4^+$): Formed when urea hydrolyzes and organic nitrogen deaminates. In water, it exists in a pH- and temperature-dependent equilibrium:

$NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$

At typical wastewater pH ($7.0\text{ to } 7.8$), over 95% exists as non-toxic ionized ammonium ($NH_4^+$). Above pH 8.5 or at elevated temperatures, the equilibrium shifts toward un-ionized ammonia ($NH_3$), which is toxic to fish at concentrations as low as $0.05\text{ to } 0.2\text{ mg/L}$. Raw municipal wastewater typically contains $15\text{ to } 35\text{ mg/L}$ $NH_3\text{-N}$. 3. Nitrite ($NO_2^-$): An unstable intermediate oxidation state, normally $< 0.1\text{ mg/L}$ in fully aerated effluent unless nitrification is inhibited. 4. Nitrate ($NO_3^-$): The fully oxidized inorganic state produced by complete nitrification. While non-toxic to fish at ambient levels, nitrate stimulates aquatic plant proliferation.

Analytical Classifications

• Total Kjeldahl Nitrogen (TKN): $\text{TKN} = \text{Organic Nitrogen} + NH_3\text{-N}$ (typically $25\text{ to } 60\text{ mg/L}$ in raw wastewater).
• Total Inorganic Nitrogen (TIN): $\text{TIN} = NH_3\text{-N} + NO_2^-\text{-N} + NO_3^-\text{-N}$.
• Total Nitrogen (TN): $\text{TN} = \text{TKN} + NO_2^-\text{-N} + NO_3^-\text{-N}$.

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2. Biological Nitrification: Two-Step Autotrophic Oxidation

Biological nitrification is an aerobic, autotrophic process in which specialized bacteria oxidize reduced ammonia sequentially to nitrite and nitrate, utilizing inorganic carbon ($CO_2$ or $HCO_3^-$) for cell synthesis.

• Step 1: Ammonia Oxidation: Mediated by Ammonia-Oxidizing Bacteria (AOB), primarily Nitrosomonas:

$2 NH_4^+ + 3 O_2 \xrightarrow{\text{Nitrosomonas}} 2 NO_2^- + 4 H^+ + 2 H_2O + \text{Energy}$

• Step 2: Nitrite Oxidation: Mediated by Nitrite-Oxidizing Bacteria (NOB), primarily Nitrobacter (and Nitrospira):

$2 NO_2^- + O_2 \xrightarrow{\text{Nitrobacter}} 2 NO_3^- + \text{Energy}$

• Overall Nitrification Reaction:

$NH_4^+ + 2 O_2 \to NO_3^- + 2 H^+ + H_2O$

Critical Stoichiometric & Operational Parameters

1. Oxygen Consumption: Stoichiometrically, oxidizing $1.0\text{ mg}$ of $NH_3\text{-N}$ consumes $4.57\text{ mg } O_2$ ($3.43\text{ mg}$ in Step 1 plus $1.14\text{ mg}$ in Step 2). Facilities adding nitrification must increase aeration blower capacity by 40% to 70%.
2. Alkalinity Consumption & pH: Step 1 releases hydrogen ions ($H^+$) that destroy bicarbonate alkalinity ($HCO_3^-$). Exactly $7.14\text{ mg}$ of alkalinity as $CaCO_3$ is consumed per $1.0\text{ mg } NH_3\text{-N}$ oxidized. Without adequate buffer, mixed liquor pH plummets. Optimum pH is 7.5 to 8.5; rates drop sharply below 7.0 and cease below 6.0. Operators dose lime ($Ca(OH)_2$), soda ash ($Na_2CO_3$), or caustic soda ($NaOH$) to maintain an effluent alkalinity $\ge 50\text{ to } 100\text{ mg/L}$ as $CaCO_3$.
3. Dissolved Oxygen (DO): Because autotrophs have lower oxygen affinity than heterotrophs, aeration basins must maintain a DO $\ge 2.0\text{ mg/L}$ throughout the oxic zone.
4. Temperature Sensitivity: Nitrification rates decrease by roughly 50% for every 10°C drop in temperature. In cold Illinois winter conditions ($<10^\circ\text{C}$), nitrifiers reproduce slowly; operators must increase Mean Cell Residence Time (MCRT) from summer targets of 5–8 days up to 12 to 20+ days to prevent nitrifier washout.

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3. Biological Denitrification: Anoxic Heterotrophic Reduction

Biological denitrification reduces oxidized nitrate to harmless nitrogen gas ($N_2$), which escapes to the atmosphere.

Biochemical Pathway & Organisms

Denitrification is performed by facultative heterotrophic bacteria (Pseudomonas, Alcaligenes, Paracoccus) under oxygen-depleted conditions:

$NO_3^- \to NO_2^- \to NO \to N_2O \to N_2 \uparrow$

Environmental Requirements

1. Anoxic Conditions: Dissolved oxygen must remain strictly $< 0.2\text{ mg/L}$. Because heterotrophs gain more metabolic energy using oxygen, dissolved oxygen represses nitrate reductase enzymes.
2. Organic Carbon Source: Heterotrophs require an electron donor (organic carbon), supplied by raw influent wastewater BOD (pre-anoxic) or supplemental carbon feeds (methanol $CH_3OH$, sodium acetate, or glycerin) in post-anoxic reactors:

$6 NO_3^- + 5 CH_3OH \to 3 N_2 \uparrow + 5 CO_2 + 7 H_2O + 6 OH^- + \text{Biomass}$

1. Alkalinity Recovery: Denitrification produces hydroxide ($OH^-$), generating $3.57\text{ mg}$ of alkalinity as $CaCO_3$ per $1.0\text{ mg } NO_3^-\text{-N}$ reduced. This recovers ~50% of the alkalinity consumed during nitrification, stabilizing basin pH.

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4. BNR Process Configurations

• Modified Ludzack-Ettinger (MLE): Uses a pre-anoxic zone upstream of the aerobic basin. An Internal Mixed Liquor Recycle (IMLR) pump returns nitrate-rich mixed liquor from the tail of the aerobic zone to the pre-anoxic zone at 200% to 400% of influent flow ($Q$). Influent wastewater BOD serves as the carbon source to denitrify the recycled nitrate, reducing aeration costs and recovering alkalinity.
• 4-Stage Bardenpho: Adds a post-anoxic zone and a re-aeration zone downstream of the primary oxic basin. The post-anoxic zone achieves endogenous or carbon-supplemented denitrification. The re-aeration basin (10–15 min HRT) strips nitrogen gas bubbles from the floc and elevates DO to $4.0\text{ to } 6.0\text{ mg/L}$, preventing rising sludge in final clarifiers.
• Step-Feed BNR: Splits influent feed across alternating anoxic and aerobic stages, supplying carbon for denitrification while eliminating high-rate IMLR pumping.

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5. Phosphorus Removal: Chemical Precipitation vs. EBPR

Municipal wastewater contains $4\text{ to } 10\text{ mg/L}$ total phosphorus (TP). Standard activated sludge incorporates only 1.5% to 2.0% P by dry cell weight, removing less than 30% of influent phosphorus. Meeting typical NPDES limits ($0.5\text{ to } 1.0\text{ mg/L}$) requires chemical precipitation or Enhanced Biological Phosphorus Removal (EBPR).

Chemical Precipitation

Multivalent metal salts react with soluble orthophosphate to form insoluble precipitates:

• Alum: $Al^{3+} + PO_4^{3-} \to AlPO_4 \downarrow$
• Ferric Chloride: $Fe^{3+} + PO_4^{3-} \to FePO_4 \downarrow$
• Lime ($Ca(OH)_2$): Forms calcium hydroxyapatite ($Ca_{10}(PO_4)_6(OH)_2 \downarrow$) at pH $> 10.5$.

Chemicals are dosed at the primary clarifier (pre-precipitation), mixed liquor channel (simultaneous precipitation), or tertiary filters (post-precipitation). Due to side reactions with alkalinity, actual dosage requires $1.5\text{ to } 2.5\text{ moles of metal per mole of phosphorus}$, increasing sludge production by 15% to 30%.

Enhanced Biological Phosphorus Removal (EBPR)

EBPR utilizes Phosphorus-Accumulating Organisms (PAOs), predominantly Candidatus Accumulibacter phosphatis:

• Anaerobic Zone (No DO, No Nitrate): PAOs assimilate short-chain Volatile Fatty Acids (VFAs, acetic acid) and store them intracellularly as polyhydroxyalkanoates (PHAs). The energy for VFA uptake comes from hydrolyzing stored polyphosphate bonds, which releases soluble orthophosphate ($PO_4^{3-}$) into solution, causing mixed liquor soluble P to surge.
• Aerobic Zone (Luxury Uptake): In the presence of DO, PAOs oxidize stored PHAs for growth and use the energy to take up soluble orthophosphate from solution. They store this phosphorus as polyphosphate granules up to 20% to 30% of their dry cell weight (compared to 1.5–2.0% in normal bacteria).
• Permanent Phosphorus Removal: Soluble P in the aerobic basin drops below $0.2\text{ mg/L}$. Phosphorus is removed from the system exclusively by wasting P-rich Waste Activated Sludge (WAS) from the secondary clarifier. Settled sludge must not remain under extended anaerobic conditions in clarifiers or thickeners, which triggers phosphorus re-release.
• EBPR Configurations: The A/O process (Anaerobic/Oxic) is used for P removal alone. The A2O process (Anaerobic/Anoxic/Aerobic) incorporates an intermediate anoxic zone and internal recycle to protect the upfront anaerobic zone from nitrate contamination while achieving simultaneous nitrogen and phosphorus removal.