Processing organic waste into biogas and biofertiliser requires extensive pre-treatment before the waste is clean and sufficiently homogeneous for gas and fertiliser production.
In Oslo, both solid and liquid waste has since commissioning in 2012 been delivered to the Romerike Biogas plant (RBA) for pre-treatment. Now, Ramboll has been appointed by the city of Oslo to optimize the existing plant design and processes in order to obtain a more resilient facility and biogas production.
Treating 50,000 tonnes of Norwegian waste
Approx. half of the food waste received at the plant originates from Oslo’s households, the rest from Norwegian businesses and bordering municipalities. A crane lifts the solid waste from the waste bunker into the plant’s first pre-treatment step. Liquid waste is pumped directly to the buffer tanks, and bypasses the pre-treatment steps for the solid waste.
- The plant is designed to receive up to 50,000 tonnes of waste annually which is pre-treated and subsequently converted into biogas. The biogas is then upgraded into Liquefied BioGas (LBG) and distributed via gas tankers or pressure cylinders to be used for transport. Pre-treatment of this scale has rarely been seen before, and it functions as a sustainable way to replace natural gas, says Reno Munksgaard, Project Manager and biogas expert at Ramboll’s energy division.
The carbon neutral biogas produced is used for Oslo buses and as eco-friendly biofertiliser for agricultural use. The 50,000 tonnes of food waste provides sufficient biogas for 135 buses, and enough biofertiliser for 100 medium-sized farms.
The project supplier is the Norwegian company Cambi AS which is behind the plant’s thermal hydrolysis process (THP), which involves pressure boiling and sterilisation of the food waste substrate.
Glass, stone and metal form a tough diet
Ramboll has signed a contract with the City of Oslo and is currently looking into the areas needed for improvement. This involves minimising wear and tear and maintenance on the plant with the aim of optimising design, energy consumption and production capacity.
- There are certain “lessons learned” since take-off in 2012. Pre-treatment of organic waste is a major challenge primarily due to large quantities of glass, stone and metal found in the waste-mix which constitute a “tough diet” even for a pre-treatment plant. Effective and early separation of sand, sediments and other undesired objects is crucial to wear on equipment, pipes and tanks; making it necessary to design the plant accordingly, says Reno Munksgaard.
A gas boiler is currently used to heat up the biogas plant and evaporator, but Ramboll is looking into replacing it with a wood chip-fired boiler which would make a much cheaper solution adding further value to the project.
Additionally, Ramboll has offered a solution that includes separating clean plastic waste from residual waste at the RBA plant, thus freeing up treatment capacity at the waste-to-energy facility. The solution results in increased profits from treatment of additional amounts of waste and from sale of clean plastic for recycling. For comparison; one tonne of clean plastic takes up the same waste treatment capacity as two tonnes of waste.
The Oslo cycle-based waste management system
To meet the challenges of climate change, Oslo introduced its cycle-based waste management system and biological treatment of source separated food waste.
- Household waste is divided into three bags – green for food waste to make biogas, blue for plastic to be recycled and white for the remains that goes to the waste plants. The cycle emphasises the reuse and recycling of resources in waste and contributes to a better climate, says Reno Munksgaard.
The biogas plant at Romerike is a major contributor to fulfilling Oslo’s ambitions for cleaner air and less CO2 emissions.Besides the Romerike biogas plant, the cycle-based system involves a new incinerator line at Klemetsrud providing renewable electricity and heat to the district heating network and two optical sorting plants separating the blue and green bags from residual waste.