Inside the race to strip nitrogen and phosphorus from food‑plant wastewater

Food and beverage (F&B) effluents don’t trickle; they surge. Typical influent chemical oxygen demand (COD, a proxy for organic load) can run from a few g/L up to >30 g/L (www.mdpi.com), with total nitrogen (TN) often in the 0.5–2.0 g/L range and total phosphorus (TP) from ~0.01 up to 0.5 g/L depending on the sub‑sector (www.mdpi.com).

That is why plants are designing for regulatory targets like effluent TP ≈0.2 mg/L and TN ≈15 mg/L for high‑quality waters in Indonesia, while keeping day‑to‑day operations stable. Biological nutrient removal (BNR) weaves anaerobic, anoxic and aerobic zones into the secondary stage to pull this off: nitrification converts ammonia to nitrate, denitrification reduces nitrate to N₂, and enhanced biological phosphorus removal (EBPR) drives microbial phosphorus uptake. In EBPR, polyphosphate‑accumulating organisms (PAOs) take up volatile fatty acids (VFAs) in an anaerobic zone and release inorganic phosphate, then “luxuriously” uptake phosphate in oxic conditions, achieving ~80–90% P removal (www.mdpi.com). The operational challenge is balancing carbon availability, retention times, and reactor configuration to hold effluent N and P at spec.

A2O configuration: volumes and performance

The A₂O (anaerobic–anoxic–oxic) process lines up dedicated zones in that sequence. At steady state, return activated sludge (RAS) from the secondary clarifier — rich in nitrates — is recycled into the anoxic zone for denitrification. High‑strength influent organics supply VFAs: in the anaerobic zone PAOs take up VFAs and release stored phosphate; in the anoxic zone denitrifying heterotrophs convert nitrate to N₂ using organics; in the oxic zone autotrophic nitrifiers convert NH₄⁺ to NO₃⁻ and PAOs take up excess phosphate.

Designers often allocate anaerobic volume at ~10–15% of total BNR volume, anoxic ~20–30%, and oxic ~50–60%. A₂O can be retrofitted by partitioning an existing activated sludge basin, keeping tankage moderate while relying on internal return flows (typically RAS to anoxic, and possibly a nitrified recycle to the anaerobic zone if VFAs are low). Performance at full scale typically shows ~70–90% TN removal, with effluent TN often 5–10 mg/L. An EPA review reported A₂O effluent TN ~7.3–9.0 mg/L and TP ~0.03–0.98 mg/L across several installations (nepis.epa.gov), with COV (coefficient of variation) 16–42%, reflecting variable carbon loads.

In practice, A₂O alone often yields ~0.5–1 mg/L TP in effluent (80–90% removal) under good conditions (www.mdpi.com). For tighter TP targets (≪0.5 mg/L), plants typically supplement with chemicals. The main operational caveat is carbon limitation if influent COD/N is low — a condition less common in VFA‑rich F&B wastewaters.

Five‑stage Bardenpho design and results

The Bardenpho process extends A₂O with an additional anoxic–oxic sequence: influent → anaerobic → anoxic → aerobic → anoxic → aerobic → clarifier, with RAS returned to the last anoxic tank. The extra anoxic stage boosts denitrification capacity, while two aerobic stages enable sequential nitrification steps and additional PAO uptake. External carbon — methanol or VFAs — may be added to the second anoxic zone if COD is insufficient (nepis.epa.gov), typically metered by a dosing pump. The trade‑off: roughly 20–30% more reactor volume and more internal recycle pumping; extra headwork may be needed to recycle nitrified mixed liquor between oxic and anoxic zones.

On performance, five‑stage Bardenpho excels at very low nutrients. Case studies show effluent TN in the range 0.9–5.6 mg/L and TP 0.06–1.09 mg/L when combined with post‑treatment such as chemical addition (nepis.epa.gov). In Clearwater, FL, a full‑scale five‑stage Bardenpho with chemical polishing achieved ~2.3 mg/L TN and 0.11 mg/L TP (nepis.epa.gov). Even without chemicals, Bardenpho typically attains lower TP than A₂O, often approaching a few‑hundred μg/L. EPA data indicate multi‑stage Bardenpho can reliably push effluent TP below 0.2 mg/L when polished; for context, one study logged 0.13 mg/L TP after A₂O plus filter, and Bardenpho plants commonly report ~0.1–0.2 mg/L TP (nepis.epa.gov) (nepis.epa.gov). The drawback is complexity and capital, but the payoff is often >90% TN and >90% TP removal where ultra‑low effluents are required.

Other BNR layouts and intensification

Other schemes matter too. The University of Cape Town (UCT, also called Phoredox) arranges anaerobic → anoxic → aerobic with internal recycles (anoxic → anaerobic, aerobic → anoxic) to promote EBPR (nepis.epa.gov). Sequencing batch reactors (SBRs) can implement EBPR via a timed anaerobic feed followed by anoxic/aeration phases, offering flexibility in compact footprints.

Membrane bioreactors (MBRs) and hybrid IFAS/MBBR systems intensify biomass concentration and nitrification capacity in small footprints, and can incorporate anaerobic selectors for P removal. In F&B plants, high‑rate MBRs (flux 30–40 L/m²·h, i.e., permeate flow per membrane area) have achieved ~80–95% COD and TSS removal (pmc.ncbi.nlm.nih.gov). These configurations are available as integrated membrane bioreactors or as add‑on biofilm carriers in moving bed bioreactors. Overall, trends favor converting a conventional activated sludge process into A₂O/Bardenpho or using compact units (MBR/IFAS) to meet stringent nutrient limits.

Chemical phosphorus precipitation and polishing

When EBPR alone cannot hit low TP, exogenous precipitation steps in. Metal salts — typically ferric chloride or sulfate, aluminum sulfate, or lime — react with soluble phosphate to form insoluble FePO₄, AlPO₄, or Ca₃(PO₄)₂, which are removed by sedimentation or filtration. Stoichiometrically, about 2.0–2.5 g Fe or 1.2–1.5 g Al is required per g‑P removed.

Effectiveness is high: under moderate dosing (e.g., 6 L/h FeCl₃ for ~6 mg/L influent‑P) removal can reach ~80% or more (iwaponline.com). In one full‑scale study, conventional ferric dosing removed ~78% of total P — down to ~1.3 mg/L effluent from 6 mg/L influent (iwaponline.com). For even lower targets, higher doses or combined tertiary filtration are used. In practice, chemical addition is often a tertiary “polishing” step after BNR: adding FeCl₃ downstream of an MBR or aeration basin can reduce effluent P from ~0.5 mg/L to <0.1 mg/L.

Compared to EBPR, chemical methods are less sensitive to influent organics or process control and can reliably hit very low P limits. The trade‑offs are additional chemical sludge and reagent cost; in the cited case, doubling FeCl₃ dose was required to meet a 2 mg/L discharge standard (iwaponline.com), illustrating diminishing returns. Integration strategies include targeting ~0.5 mg/L TP via EBPR, then using ~5 mg/L FeCl₃ to drive final P to <0.2 mg/L; some systems even recover precipitate as struvite.

Operationally, many plants dose metal coagulants ahead of a sedimentation step in a clarifier, or use tertiary filtration — for example, polishing through a cartridge filter — to capture fine precipitates.

Design levers and regulatory targets

Across configurations, biological systems like A₂O or five‑stage Bardenpho can remove >90% of N and P in F&B wastewater, with effluent TN often <5 mg/L and TP on the order of 0.1–0.5 mg/L under good conditions (nepis.epa.gov) (nepis.epa.gov). Bardenpho generally outperforms simpler A₂O at the cost of added volume and controls. Where regulations push TP below 0.2 mg/L, plants layer in chemical precipitation to remove ~80–90% of remaining P (iwaponline.com).

The consistent through‑line is design discipline: retention times, internal recycles, solids retention time (SRT), and carbon dosing must match the high organic loading and nutrient content of F&B effluents. Plants increasingly package these choices into integrated nutrient removal systems or retrofit existing tanks to meet evolving discharge limits.

Sources and case studies

Key data are drawn from full‑scale case studies and reviews: an EPA reference document shows A₂O effluent TN ~7.3–9.0 mg/L and TP ~0.03–0.98 mg/L across installations (COV 16–42%) and five‑stage Bardenpho with chemicals achieving TN ~0.87–5.59 mg/L and TP ~0.06–1.09 mg/L, with examples such as ~2.3 mg/L TN and 0.11 mg/L TP at Clearwater, FL (nepis.epa.gov) (nepis.epa.gov). A study of EBPR notes typical 80–90% P removal (www.mdpi.com).

A technical‑economic comparison reports ferric precipitation removing ~78% of total P at moderate dose — down to ~1.3 mg/L effluent from 6 mg/L influent — with higher doses and filtration needed for tighter limits (iwaponline.com). For influent characterization, see: Nikolić, Mijić, and Mitrović, “Characteristics of Food Industry Wastewaters and Their Potential Application in Biotechnological Production,” Processes 2025, 13(8), 2401 (www.mdpi.com).

Additional details appear in U.S. EPA, “Municipal Nutrient Removal Technologies Reference Document, Vol.1” (EPA832‑R‑08‑006, 2008), which also cites A₂O effluent TP at 0.025–0.98 mg/L in select cases (nepis.epa.gov); Abdoli et al., “A Review of the Efficiency of Phosphorus Removal and Recovery from Wastewater…” Water 2024, 16(17), 2507 (www.mdpi.com); and an MBR case study showing COD/TSS removals ~84–95% at high MLSS (Ng et al., Membranes 2021, 11(6), 456) (pmc.ncbi.nlm.nih.gov).