Keeping construction of an anaerobic digester (AD) going is never entirely easy but it’s particularly difficult in times of COVID-19 lockdown — especially when the project team and equipment providers are scattered around Europe and the UK. In Finland, lead contractor EcoProtech OY has managed to push construction toward the finish line at a site about 120 miles north of Helsinki that is owned by Pirkanmaan Jätehuolto Oy (PJO), a company servicing 17 municipalities around the city of Tampere and responsible for regional waste management. The AD facility is being added on to the existing PJO yard trimmings windrow composting facility located close to a new wastewater plant planned by Nokian Vesi Oy. The site is near the town of Nokia, famed as the original home of the mobile phone company of the same name. The development is about 9 miles west of the city of Tampere, the second largest population center in Finland and the largest inland city in the Nordic countries.
The project scope is to process commingled food waste and yard trimmings — as well as a range of sludges and dewatered biosolids transported by road from the wastewater treatment plant — to create biogas and then use the biomethane for vehicle fuel. At first glance the scope of the facility seemed self-defeating because requirements of the sludges and biosolids streams don’t mesh comfortably with the characteristics of the curbside-collected household organics. The solution is a twin-track hybrid facility with both a wet AD and a dry AD (HSAD) line with centralized control and coordination, together with an in-vessel composting (IVC) unit for solids finishing. The wet AD is a thermophilic EPAD system developed by EcoProtech and used successfully on a number of previous biosolids facilities. This is equipped with additional post-AD hygienization. The parallel thermophilic HSAD line is by Martin GmbH using the Thoeni TTV plug-flow dry digestion system with in-digester hygienization. After dewatering, the solid fractions will be biodried and matured in a 6-tunnel IVC system designed by Growing Designs Ltd and Target Renewables Ltd.
The scale and balance of inputs and outputs are highlighted in the At A Glance sidebar. Dewatered sludges will be delivered to an enclosed bunker; liquids will be delivered by road tanker to on-site holding tanks. The EPAD system is relatively high dry solids (up to 14%) for a “wet” system. The solids stream will be delivered to bunkers and either slow-speed shredded (yard trimmings including Christmas trees) or depacked (curbside collected domestic food waste). The computer-regulated mix will be loaded via a feed bunker and feed mixer to the TTV high solids digester with the combined input at about 30% dry solids. The existing yard trimmings composting site already produces a good quality compost. Additional compost products from the IVC will contain food waste and sludge digestates. Clean overs or other amendments will be used depending on the need for structure in the IVC composting process to achieve a good product suitable for post-composting screening.
The local area is relatively flat with a range of agricultural land to which digestate can be applied. But Finland is a cold country so all products must be storable and highly stabilized to the European Rottegrad IV standard or better. Products are estimated at approximately 50/50 high quality agricultural material and horticultural-grade products after screening.
The biomethane after upgrading will mostly fuel PJO waste collection trucks at an on-site fueling station with an option to export gas via a tanker truck docking station or gas-to-grid connection. Construction started in autumn 2019 aiming for commissioning in late 2020. Despite inevitable delays due to COVID the project team still expects to be online around the end of 2020.
Robin Szmidt is Director of Target Renewables Ltd.