Energy-efficient ammonia recovery from digestate — First-of-its-kind technology in Japan developed under NEDO program

The New Energy and Industrial Technology Development Organization (NEDO) announced on March 2 that four organizations—KIMURA CHEMICAL PLANTS CO., LTD., Kobe University, Nobels, and FT Biopower Co., Ltd.—have been working on the development of a "Technology for High-Efficiency Ammonia Recovery from Wastewater Containing Low-Concentration Ammonia Using Membrane Separation and Distillation" through its subsidy program. NEDO also announced that the four organizations have successfully developed Japan's first process for recovering ammonia from methane fermentation digestate in an energy-efficient manner.

This is the outcome of NEDO's subsidy project, the "Program to Develop and Promote the Commercialization of Energy Conservation Technologies to Realize a Decarbonized Society / Practical Application Development / Development of Technology for High-Efficiency Ammonia Recovery from Wastewater Containing Low-Concentration Ammonia Using Membrane Separation and Distillation," which began in fiscal year 2022. The four organizations conducted demonstrations in collaboration with the Sewage Works Department, Public Construction Projects Bureau, City of Kobe from May 2024 and with the Commerce and Industry Department, City of Nagaoka, Niigata Prefecture from June 2025 to achieve the development of this recovery process-the first of its kind in Japan.

The demonstrations confirmed that the energy efficiency from an optimal combination of FO (forward osmosis) membrane technology and heat pump distillation technology, along with the cost-effectiveness of an ammonia recovery process that uses slaked lime, offers significant advantages over conventional technologies.

A bench plant was designed and fabricated based on beaker-scale test results, long-term demonstration tests were conducted using real liquid (methane fermentation digestate), and the resulting data was incorporated into simulation software developed by KIMURA CHEMICAL PLANTS CO., LTD. This concretely demonstrated both the energy efficiency and economic viability of the technology, and a clear outlook for the social implementation of this project was established.

Using digestate produced by methane fermentation of sewage and food waste, demonstration tests were carried out with the bench plant. Based on the condition of treating digestate (T-N: approximately 1,000 ppm) at a throughput of 297 t/d, it was estimated that CO₂ emissions could be reduced by 133 kL (crude oil equivalent) per year compared with conventional methods, with an equipment depreciation period of 21 years as an economic indicator, and an annual ammonia recovery of 106 tons (N).

Methane fermentation facilities that use sewage sludge and biomass such as livestock manure, food waste, and food processing wastewater as feedstock generate large volumes of digestate containing 0.1% to 0.3% ammonium ions as liquid waste.

Currently, nitrogen concentrations are typically reduced to around 10 ppm through aeration and denitrification by the activated sludge process before discharge. However, this treatment presents the challenge of consuming large amounts of electricity to power it.

In recent years, efforts have been made to use digestate as fertilizer. However, some of such attempts have faced problems such as difficulty securing nearby farmland, rising transportation costs, groundwater contamination from nitrogen compounds not absorbed by crops, and foul odors during digestate storage.

Furthermore, an international research team focusing on planetary boundaries has reported that the release of nitrogen compounds as chemical substances into the environment has already exceeded the safe operating limits for human activity, reaching a dangerous level.

For these reasons, minimizing the environmental release of nitrogen compounds and building a nitrogen-cycling society is an urgent challenge shared by the entire world. The recovery process developed and demonstrated in this project opens the way toward further greenhouse gas emission reductions in the biogas power generation sector.

Going forward, the four organizations expect to propose a centralized model in which digestate collected from multiple sites is first concentrated and reduced in volume using FO membranes, then gathered at a single location for ammonia recovery by distillation, and to proceed with the social implementation of this process. Further developments are eagerly anticipated.