Palm oil’s waste stream is a fertilizer line — if mills tune the biology

Fresh palm oil mill effluent (POME) is ~95–96% water and ~4–5% total solids, including 2–4% suspended lignocellulosic debris (researchgate.net). Typical raw POME chemical oxygen demand (COD, a measure of oxidizable organics) runs 30,000–50,000 mg/L and biochemical oxygen demand (BOD, oxygen used by microbes) 25,000–30,000 mg/L, with total nitrogen (TN) ~750 mg/L and ammoniacal nitrogen (NH₃‑N/NH₄‑N) ~120–220 mg/L (scialert.net).

One tonne of crude palm oil (CPO) produces ~2.5–3.5 m³ POME and carries ≈750 mg/L TN and 220–120 mg/L NH₃‑N (scialert.net). That translates to roughly 2–3 kg N and ~0.4 kg P per ton CPO in the effluent. Conventional ponding removes most organics but often leaves nutrients behind; in Malaysia, final discharge from common pond systems has been reported at BOD ~60–500 mg/L and total suspended solids (TSS) ~100–200 mg/L, with ammonium above ecosystem limits (no Malaysian standard for NH₃ is met; for reference, Malaysia’s Class A limit is 10 mg/L NH₄‑N) (pmc.ncbi.nlm.nih.gov).

Regulatory baselines and design targets

Indonesia’s PermenLH 2014 sets a final discharge COD ≤350 mg/L for POME (researchgate.net), with no strict nutrient limits. In practice, aerobic biological nutrient removal (BNR, nitrification to oxidize ammonia and denitrification or microbial assimilation to reduce TN and P) is used in polishing stages to meet environmental standards and operational goals.

Aerobic BNR performance in palm oil mills

After anaerobic digestion, aerobic polishing can achieve substantial nitrogen and phosphorus removal. In a study of four full‑scale mills, an integrated chain — ponding plus anaerobic digesters and extended‑aeration activated sludge with fixed media — removed ~92.5% of TN and ~94.5% of ammoniacal‑N; the top‑performing system left only ~7.5% of feed TN, with phosphorus removal more variable (some systems achieved ~90% TP, others much less) (researchgate.net). These outcomes imply effluent total ammoniacal nitrogen (TAN) <100 mg/L and similarly low TN in the best cases (researchgate.net).

For mills standardizing on activated sludge, packaged activated‑sludge systems align with the extended‑aeration setups used in those high‑removal plants. Where designs prefer smaller footprints and tight control, sequencing batch reactors (SBR) and moving‑bed biofilm reactors (MBBR) are established pathways to full nitrification, matching the “more compact systems” cited in published studies (researchgate.net).

Operating levers: aeration, SRT, and pH

BNR requires sufficient dissolved oxygen and retention time for slow‑growing nitrifiers; conventional open aerated ponds often incompletely nitrify. Well‑controlled aerobic bioreactors following anaerobic digestion routinely attain >90% NH₄‑N conversion (researchgate.net). SBRs treating POME have delivered effluent NH₄‑N below 50 mg/L (often <10 mg/L) with proper pH control and sludge age management (researchgate.net).

Constructed wetlands augment polishing; a Napier grass high‑rate wetland removed ~62.3% NH₄‑N on average, further trimming residual ammonia (pmc.ncbi.nlm.nih.gov). Where fixed media are preferred, fixed‑bed bio‑reactors can mirror the “fixed media” extended‑aeration setups reported in high‑removal chains.

Design point: removal versus recovery

Mills targeting discharge limits often design for >90% TN removal, yielding effluent TN on the order of a few tens of mg/L (researchgate.net) (researchgate.net). However, those high‑removal rates come at the cost of “wasting” N and P that could otherwise be recycled. If the goal shifts to resource recovery, designers may opt for partial BNR — for example, limited aeration to only partially nitrify, or omitting an anoxic denitrification step — leaving nutrients for land reuse. Even with high nitrification, nitrate‑rich water remains suitable for land application because plants assimilate NO₃.

Vendors frame this tradeoff explicitly: a nutrient‑removal package is tuned for compliance; a biological digestion train can be tuned for recovery when paired with land application.

Treated POME as fertigation input

Treated POME retains appreciable N, P, and K and can substitute part of chemical fertilizer. Anaerobically‑treated POME (supplemented with chicken manure) supplied “substantial NPK,” boosting plant growth by >50% versus controls; the authors note “the treated POME exhibits substantial NPK nutrients” and could act as a low‑cost partial substitute (researchgate.net) (researchgate.net). Haryani et al. (cited there) found that applying ~5 ton POME/ha to Napier grass significantly increased biomass (researchgate.net).

Field evidence from Sumatra showed direct land application of POME markedly increased oil palm yields: treated plots produced about 38 fresh fruit bunches (FFB) per month (≈40% more trees bearing fruit) and ~791 kg/38FFB per month (≈44% higher bunch weight) compared to unfertilized controls; while that trial used raw POME, it demonstrates fertilizer value, and even if a fraction of nutrients remains after treatment, significant responses are likely (iopscience.iop.org).

How much fertilizer can POME replace?

Rough calculations underscore the potential. If 1 t CPO yields ~3 m³ POME at ~0.7–0.75 g/L N, that’s ~2.1–2.3 kg N per ton CPO. At ~20 t CPO/ha/yr productive yield, that corresponds to ~40–45 kg N/ha/yr from effluent. Indonesian guidelines recommend roughly 200–300 kg N/ha/yr for mature palms, so POME could supply ~15–20% of N needs (akvopedia.org). Phosphorus and potassium also contribute; as a reference point, ~0.08 kg P₂O₅ per m³ raw POME is cited in industry summaries.

Beyond nutrients, long‑term POME use can improve soil physical and chemical properties (raising cation exchange capacity and pH on acidic soils) (scialert.net), and studies report POME reuse can raise soil pH and electrical conductivity (EC), enhancing nutrient availability (palmoilmagazine.com).

Application logistics and soil safeguards

Land‑application systems range from plantation pipelines and tanker trucks to injection “bio‑bore” holes. Typical rates mirror irrigation, e.g., 20–30 mm per week, with dilution or TSS separation where needed to avoid waterlogging. Constraints include ensuring pathogens or phytotoxins are absent (mature pond effluent is usually safe) and preventing salinity build‑up. Because POME derives from palm biomass, heavy metal concerns are minimal compared to sewage effluent.

Data from Malaysia indicate plantation soils can adsorb 60–77% of POME ammonia within days, reducing runoff risk (scialert.net).

Business math: fertigation as cost control

Recycling POME as fertigation can cut fertilizer costs. The Lampung trial’s 40–44% yield boost from POME alone suggests substantial nutrient contribution; even a 20–30% lift, allowing for application inefficiencies, would justify replacing a similar fraction of fertilizer expense (iopscience.iop.org). As a planning example, a mill producing 1,000 m³ POME/day (≈100 t CPO/day) could irrigate ~100 ha with ~10 mm/day of effluent, supplying ≈7 kg N/ha per irrigation; scheduled through the dry season, this would cover significant N before each vegetative flush.

For operations standardizing equipment to enable either path, upstream anaerobic stages pair naturally with anaerobic/aerobic digestion systems, while downstream aerobic polishing can be dialed via SBR or MBBR to favor compliance or recovery.

A practical BNR‑plus‑reuse playbook

Where effluent is land‑applied rather than discharged, systems need not meet the strictest TN discharge limits. Operators can test post‑aeration effluent; values such as 100–200 mg/L TN and 50–100 mg/L K represent per‑cubic‑meter fertilizer value of ~0.1–0.2 kg N and 0.05–0.1 kg K. Spreading a few cubic meters per palm per application can replace a portion of fertilizer bags. Monitoring soil nitrate guards against salt build‑up.

In short, aerobic BNR in POME treatment can remove >90% of ammonia and total nitrogen — effluent NH₄‑N below 50 mg/L (often <10 mg/L) is achievable with the right sludge age and pH management — but mills may capture more value by leaving nutrients in and sending water back to the estate (researchgate.net) (pmc.ncbi.nlm.nih.gov).