Indonesia is the world’s largest palm oil producer, and the industry is moving fast to capture methane from palm oil mill effluent (POME) — with strong backing from BRIN, GAPKI and the Indonesian Biogas Association (ABGI). What that same push is about to surface, mill by mill, is a second problem: concentrated ammonia in the digestate left behind once methane capture succeeds.
With an estimated 600 mills nationwide and roughly 125 million tonnes of POME generated every year, Indonesia’s palm oil sector represents one of the largest untapped biogas resources in the world. As open lagoons are progressively replaced by covered anaerobic digesters to capture that methane, mills are solving a genuine emissions and energy problem. But anaerobic digestion concentrates ammoniacal nitrogen rather than removing it.
Why Anaerobic Digestion Concentrates Ammonia Instead of Removing It
POME contains substantial organic nitrogen — proteins and other nitrogenous compounds carried over from fruit processing. During anaerobic digestion, microbial breakdown converts much of this organic nitrogen into ammoniacal nitrogen (NH4+/NH3).
Unlike carbon, which leaves the system as biogas, nitrogen has nowhere to go — it stays in the liquid phase, and as the digester reduces overall liquid volume through solids separation and dewatering, that nitrogen becomes progressively more concentrated in the remaining reject water and digestate. The result is a liquid stream that can carry several thousand mg/L of ammonia-nitrogen — a load most mills’ existing pond-based treatment infrastructure was designed decades before AD was part of the picture, and was never sized to handle.
A Different Problem Than Municipal Digestate
POME digestate differs from municipal sewage sludge digestate in one important respect: it typically carries proportionally less phosphorus relative to its nitrogen load.
This matters practically, because it means struvite precipitation — a nutrient recovery technology often discussed for municipal digestate — is a poor fit here; without sufficient phosphorus, struvite crystallisation cannot capture a meaningful share of the ammonia present. Nitrogen-specific recovery, rather than a phosphorus-driven process, is the appropriate technology match for POME.
Why This Matters Now, Not Later
Industry commentary on Indonesia’s POME-to-biogas rollout has noted that deployment is currently held back partly by the absence of a binding national regulatory framework specific to POME biogas. That is expected to change as the sector scales — following the same pattern already seen in landfill leachate regulation (Permen LHK No. 59/2016) and general industrial effluent standards (Permen LH No. 5/2014, which already includes a palm oil sector annex covering BOD, COD and oil-and-grease parameters).
As biogas infrastructure becomes standard across the industry, ammonia-specific requirements for digestate are a reasonable expectation within the current regulatory trajectory. Mills that install AD capacity without planning for the ammonia side-stream risk building a compliance gap into their own investment, rather than solving it at the design stage when it’s cheapest to address.
The Recovery Opportunity : OTAR (Organics Thermal Ammonia Recovery)
Ammonia stripped from POME digestate does not have to be destroyed. Depending on configuration, OTAR® can recover it as ammonium hydroxide, or — notably relevant for Indonesia — as ammonium sulphate (ZA), a fertiliser already distributed at national scale through Pupuk Indonesia’s subsidised fertiliser program.
Indonesia’s domestic nitrogen fertiliser production currently meets only a fraction of national demand. A mill that recovers its own nitrogen as ammonium sulphate is not just solving a compliance problem — it is contributing, at a small scale, to the same domestic fertiliser supply story government is trying to strengthen (see our companion article on Indonesia’s fertiliser self-sufficiency for the fuller policy picture).
Integrating with Existing Biogas Infrastructure
For mills already investing in biogas infrastructure, adding ammonia recovery at the digestate dewatering stage is a natural extension rather than a separate capital project. It uses the same waste heat source (the biogas engine itself, or the boiler serving it) that is already on site.
This is precisely the Recycle Prevention configuration: treating the concentrated side-stream at its source, before it ever recirculates ammonia back into the mill’s main effluent treatment line and degrades the performance of ponds and digesters that were never designed for that additional load.
