Mills are weighing sprawling anaerobic–facultative ponds against closed, high‑rate digesters that slash land use, capture nearly all methane, and tighten process control — at higher upfront cost but with energy and carbon upside.
Industry: Palm_Oil | Process: Palm_Oil_Mill_Effluent_(POME)_Treatment
For decades, palm oil mills have relied on long series of open anaerobic–facultative ponds to treat palm oil mill effluent (POME). The catch: these earthen lagoons sprawl over tens of acres and vent nearly all of their methane to the atmosphere. Closed‑tank digesters and UASB (upflow anaerobic sludge blanket) reactors — high‑rate anaerobic systems that run in days, not months — promise a radically smaller footprint and near‑total biogas capture.
In a sector where land can be scarce and emissions carry growing costs, the trade‑offs are now squarely economic: low‑CAPEX ponds that forgo energy revenue versus compact digesters that demand more capital but generate kWh and carbon benefits.
Here’s how the options stack up on land area, biogas capture efficiency, process control, and cost — with case data and sources embedded for verification.
Land footprint and retention times
Traditional ponding systems occupy very large areas — tens of acres. Studies report typical pond systems of 30–45 acres (12–18 ha) per mill with total HRT (hydraulic retention time) ≈100–160 days (mdpi.com). These earthen lagoons are essentially unconfined and extended — often dozens of ponds — i.e., “large land area required,” making them impractical near built‑up areas (intechopen.com) (researchgate.net).
Modern high‑rate digesters are much more compact. A closed tank digester or UASB reactor processes POME in days rather than months, so for the same flow it requires only a fraction of the land — often on the order of one‑tenth or less of a pond system’s footprint. One modeling study assumed an anaerobic tank treating 1 m³ of POME per unit with 20–25 day HRT (mdpi.com), versus pond HRT >100 days (mdpi.com) — implying roughly 4–5× smaller volume, and hence area.
In practice, high‑rate UASB/reactors can treat given loads in the range of 1–5 day HRT (versus ≫50 days for ponds) (intechopen.com) (mdpi.com), often reducing footprint by ~10× or more. Exact savings depend on hydraulic loading and design: for example, a 30 t/h (tonnes per hour) mill’s anaerobic lagoon system was estimated at ~12–15 ha (mdpi.com), whereas a 1.9 MW (megawatt) UASB‑based digester at a 60 t/h mill can be constructed on only a few hundred m² of flat land (researchgate.net). Solutions marketed as wastewater biological digestion systems align with these compact, high‑rate approaches.
Methane capture and energy yield
Perhaps the largest benefit of closed digesters is gas capture. Traditional open ponds and tanks are uncovered, so most methane (CH₄) escapes to the atmosphere and cannot be utilized. Baseline studies report that “significant” quantities of CH₄ (≈5.5 kg CH₄ per tonne of POME) are produced but lost in open ponds due to lack of a gas‑retrieval system (mdpi.com).
By contrast, covered/high‑rate digesters are sealed and capture nearly 100% of the biogas produced. Commercial models suggest yields around 21 m³ biogas per m³ of raw POME (mdpi.com). At typical CH₄ content (~50–60%), this equates to roughly 10–13 m³ CH₄ per m³ POME (on the order of 200–300 MJ), or about 2.29 kWh of electricity per m³ gas (≈48 kWh per m³ POME) when used in a gas engine (mdpi.com).
For context, Indonesia generates ≈28.7 million tonnes of POME per year (esdm.go.id). If each tonne (~1 m³) yielded 21 m³ biogas, that is ~600 million m³ biogas/yr (or ~1.38×10^9 kWh of thermal energy) that could be captured. In practice, electric output would be lower due to generator efficiency. A Malaysian analysis showed that installing a biogas trap in a typical mill raised waste recycling from ~81.8% to ~99.99% and cut the supply‑chain GHG footprint from 0.814 to 0.196 tCO₂‑eq per tonne CPO (mdpi.com), illustrating the avoided CH₄ release and added energy value. Closed reactors convert almost all of that methane into useful fuel, often generating on the order of 100–200 kW per 100 t/d (tonnes per day) of POME.
Treatment performance and control
Open ponds rely on natural stratification and long detention, with no mechanical mixing or precise temperature control. In practice this means unsteady performance (subject to weather, variable feed, foam and scum formation) and only modest organic removal. Typical ponding systems achieve on the order of 50–75% COD (chemical oxygen demand) — or ~55–60% BOD (biochemical oxygen demand) — reduction (mdpi.com) (intechopen.com) before requiring downstream polishing.
By contrast, UASB and other high‑rate anaerobic reactors achieve much higher conversions (commonly 85–95% COD removal) in a single step. One review notes UASB/anaerobic filtration reactors routinely remove >90% of COD, with ~60% of that converted to CH₄ (intechopen.com). Anand filtration POME studies report up to 94% COD removal and 63% CH₄ in biogas at ~4.5 kgCOD/m³·day loading (intechopen.com).
With a closed system one can actively control conditions: mesophilic temperature (≈35°C), pH buffering, short hydraulic retention, and internal mixing. For Indonesian conditions, design guides recommend mesophilic operation versus higher‑but‑costly thermophilic regimes (organicsbali.com). pH buffering and chemical control are typically metered, which aligns with the use of a dosing pump in engineered plants. In contrast, an open pond “requires a large land area” and its “control and monitoring systems are difficult because of the pond size” (researchgate.net). In sum, high‑rate digesters achieve faster, more consistent treatment (days rather than months) and yield higher effluent quality (tens of mg/L BOD/COD) with a single anaerobic step.
Cost–benefit and payback
Open pond systems have very low capital and operating costs — no sophisticated equipment, just earthworks. Models show a conventional pond system might cost on the order of US$4 M CAPEX for a medium‑sized mill, with OPEX ~US$120k/yr (mdpi.com). By contrast, a closed lagoon digester is roughly 1.5×–2× more expensive (∼US$6–9 M CAPEX) with higher annual O&M (~US$180–270k) (mdpi.com) because of pumps, mixers, and gas handling. (These figures come from a 2025 case study model — actual cost scales with size and geography — but illustrate the order of magnitude.)
On the benefit side, the additional cost yields biogas (and electricity) revenues plus any carbon credits. A techno‑economic study of a 30 t/h mill found an anaerobic digester plus power plant (using a lagoon digester model) cost ∼US$1.98 M, giving an IRR ≈12.5% and payback ≈6.6 years (researchgate.net). Larger installations are even more attractive: a 60 t/h mill with a 1.9 MW UASB‑CHP (combined heat and power) unit achieved IRR ~29.7% (3.7 yr payback) in one analysis (researchgate.net). By contrast, a low‑flow case in Indonesia saw IRR drop to ~6.8% and payback ~10.8 yr when feedstock was insufficient (researchgate.net) — highlighting that obtaining enough POME is key.
In short, piped/covered digesters have double‑ or greater CAPEX but unlock the energy value of POME. Open ponds, while cheap, forgo this revenue: they produce no billable biogas and simply incur disposal costs (and GHG liabilities). Quantitative analyses (e.g., ROI, payback, CO₂ abatement cost) consistently find that, despite higher upfront costs, modern digester systems can pay for themselves via energy sale and avoided emissions (researchgate.net) (researchgate.net).
Environmental and policy trajectory
From an environmental standpoint, closed digesters clearly outperform ponds in methane mitigation. In Indonesia, only ~30 MW of the 1.5 GW POME‑to‑power potential has been developed as of 2018 (esdm.go.id), meaning most mills vent methane untreated. Harnessing POME gas reduces GHG by factors: one study estimated 572–693 thousand tonnes CO₂e per year could be avoided if Indonesia’s methane were captured (aspekpir.org).
Regulations increasingly push towards capture: current Indonesian standards require POME discharge ≤100 mg/L BOD, but experts note even “treated” effluent still carries nutrients (K, P, NH₄) that harm ecosystems (aspekpir.org). They advocate shifting the paradigm — for example, using partially treated POME on land and capturing methane (aspekpir.org) (aspekpir.org) — rather than relying solely on extensive ponding. Where nutrients are a concern, engineered options for nutrient removal are part of the wider treatment toolbox.
In practice, many mills are converting to covered/high‑rate systems (especially where land is scarce or greenhouse commitments exist). Malaysia now requires all mills to capture biogas, and companies have installed covered digesters closing the loop (mdpi.com). The downside is the cost and technical know‑how; the upside is quantifiable: one Malaysian case reported a 2.3% revenue increase and major GHG cut just by adding a biogas capture system (mdpi.com).
Bottom line
Open pond systems win on simplicity and low CAPEX/OPEX, but suffer very high land use, low treatment rates, and essentially zero methane capture (biogas is vented as emissions). Compact anaerobic digesters/UASB reactors require far less land (often >90% footprint reduction), achieve higher pollutant removal, and — critically — capture nearly all methane for energy use. That yields direct benefits (electricity, reduced fuel costs, carbon credits) but at higher capital cost and more complex operation. Where operators pursue engineered control — temperature, mixing, and pH buffering — they align with closed‑system practice and ancillary equipment common to biological digestion. The choice is an engineering/economic trade‑off: if land is cheap and environmental constraints lax, ponds may suffice; but where land is limited or methane must be controlled, high‑rate systems yield far greater benefits on a per‑hectare and per‑tonne basis (mdpi.com) (mdpi.com).
Sources: Peer‑reviewed and industry analyses of POME treatment comparing ponding vs advanced anaerobic systems (mdpi.com) (mdpi.com) (intechopen.com) (intechopen.com) (mdpi.com) (esdm.go.id) (aspekpir.org) (researchgate.net) (researchgate.net), including Indonesian case studies and regulatory reports.
