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Sewage sludge
Material description and quantity estimations
Sewage sludge refers to sludge produced in the wastewater treatment process that contains the material removed during screening and desanding (Laitinen ym., 2014b). Sewage sludge can be treated in a variety of ways, such as composting, digestion, chemical treatment, thermal drying, or incineration. Sludge can also be utilised as a fertiliser, biogas, or soil improver (Roponen 2013). The most common treatment method is digestion, which combines both material and energy recovery. The digestion process that mainly takes place at a mesophilic temperature (+ 35–37°C) is in use in Finland. In addition to reject water, biogas utilised in energy production and stabilised sludge (Kangas ym., 2011) are also created as end products of the digestion process. The stabilised sludge can be dried and refined into soil products and fertilisers by composting (HSY 2017a; Laitinen ym., 2014a). Furthermore, sludge can also be treated with pyrolysis, and the resulted incinerated sludge ash can be used as a raw material in the production of concrete, glass, and ceramics. Sludge ash can also be used as a sorbent and in paving, earth construction, soil improvement, treatment of refinery sludge, or the stabilisation of dredging masses (Elomaa 2015).
Approximately million tonnes of sewage sludge, which equals to approximately 150,000 tonnes of dry matter, is produced in Finland every year (Blomberg & Toivikko 2015). The amount of sludge produced at wastewater treatment plants is expected to increase in the future along with the population growth and the development of treatment technology (Turunen 2016). Since reducing the amount of produced waste is not really a possibility, sewage sludge must, in the light of the waste hierarchy, be primarily directed to be used in agriculture (Nurmi 2010; Turunen 2016). This way, sewage sludge can be used to replace mineral fertiliser products, and the nutrients bound into the sludge, most importantly the diminishing usable phosphorus, can be retained in the food production cycle (Nurmi 2010). Despite this, in Finland, composted sludge is mainly utilised in landscaping. As a result, the nutrients often get washed away to surface waters, where they cause eutrophication (Laitinen ym., 2014a; Nurmi 2010).
In 2016, approximately a total of 87,000 tonnes of dried sewage sludge was produced in the capital area. Of that amount, approximately 30%, i.e., 26,000 tonnes, was dry matter (HSY 2017a). In the Turku region, the corresponding amount was 48,000 tonnes, of which 21.9%, i.e., 10,000 tonnes, was dry matter (Leino 2016). In the Tampere region, produced sewage sludge amounted to approximately 27,000 tonnes, of which 31.2%, i.e., approximately 8,000 tonnes, consisted of dry matter (Tampereen vesi 2016). In the Oulu region, the amount of produced sewage sludge was 36,000, of which 22%, i.e., approximately 8,000 tonnes, was dry matter (Oulun Vesi 2016).
Innovation needs
Even though nutrients can be recovered during the digestion process as part of the produced compost and soil products, the innovation needs related to the wastewater treatment process and sludge recovery have been identified in all of the six largest cities in Finland.
- Rationalisation of the nutrient cycle
- Rationalisation of the removal of harmful substances, such as pharmaceutical residues
- New technologies for the removal of microplastics
Oulun Vesi is currently engaged in a project related to nutrient recycling (https://www.oukapalvelut.fi/kehittamishankkeet/Hankekortti1.asp?ID=717). The project is supported by Business Finland, and its purpose is to obtain a long-term service agreement for sludge treatment (10 years). The aim is to carry out the procurement via an innovation partnership procedure, the objective of which is precisely to identify new service packages and create new business opportunities. (Alanärä 2017.) In addition, a method in which nutrients would be recovered directly from wastewater is being developed in the governmental key project RAVITA carried out by HSY (HSY 2017b).
Business-related challenges and opportunities
The agricultural use of sludge is not an unproblematic solution. Instead, it is significantly hindered by, e.g., the high heavy metal concentrations in sludge, pathogens, organic harmful substances, endocrine-disrupting chemicals, flame retardants, the high costs of agricultural use, and the prejudices against sludge recovery (Moisio 2017; Rantanen ym., 2008). In addition, the nutrient content of mineral fertilisers is typically higher than the nutrient content of organic fertiliser products, such as composted sludge (Postila ym., 2017).
The conditions for using sewage sludge in agriculture have, however, improved, as the heavy metal concentrations in sludge have significantly reduced over the years. The concentrations of organic harmful substances, which have become the latest concern, are also relatively low in Finnish sewage sludge when compared with the concentrations in the other Nordic countries (Kasurinen ym., 2014). In order for sludge to be able to be increasingly utilised in agriculture in accordance with EU’s waste hierarchy, the innovation needs related to the development of a product that is better suited for the hygienically safe treatment and agricultural use of sewage sludge must be resolved first (Nurmi 2010; Turunen 2016). For instance, thermal drying, with the aid of which sludge can be processed into a hygienic soil improvement product, has shown promising results as a method (Nurmi 2010). For the promotion of sustainable sludge treatment, it is essential that the prejudices related to sludge recovery are eliminated to create a viable market and that additional studies are conducted on the risks and opportunities related to the use of sewage sludge (Nurmi 2010; Turunen 2016).
Ruuhela’s study (2017) compared the composting, digestion, thermal drying, pyrolysis, and incineration of dehydrated sewage sludge. Carbon footprint reviews proved that the best method is digestion, since it produces more energy than it consumes. The produced energy can be used, e.g., in the combined production of electricity and heat or the production of biomethane. The energy balance of sludge incineration was also positive, but in that method, all nitrogen is lost from the sludge, which makes this process a poorer option than digestion.
The economic profitability of digestion often depends on the price of the produced electricity (Kangas ym., 2017). The best possible profitability for biogas produced by digestion is gained in transport use, where it amounts approximately to €50/MWh. The second most profitable option is the combined production of electricity and heat, which produces approximately €10–15/MWh. The least profitable option is the conversion of biogas into heat, which produces approximately €10/MWh (Pöyry Environment Oy 2007).
There is still rationalisation potential in the wastewater nutrient recovery, as only approximately 3% of sewage sludge is used in plant production. Even though all sewage sludge is processed, the methods have not been designed from the point of view of nutrient recycling. It must also be assessed how the nutrients in sewage sludge can be utilised if sludge reception jeopardises the image of farmed products and complicates their marketing. The costs of advanced processing methods have limited their use in the treatment of wastewater and sewage sludge (Marttinen 2017). The price of phosphorus recovery varies between 2–20 euros/kg, and the price of nitrogen recovery is at least twice the combined price of nitrogen removal and ammonia production using the Haber-Bosch process (Repo 2016). Reaching the sewage sludge-related goal set in the Government programme requires the development and testing of processes and the implementation of new methods. (Marttinen 2017).
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