Septic tank and cesspool sludge
Material description and quantity estimations
Approximately 242,000,000 tonnes of rural sludge that is not treated within the wastewater sewer system, i.e., septic tank and cesspool sludge, is produced in Finland every year. This rural sludge represents approximately 30% of the total amount of communal sludge (Sitra 2007.)
When it comes to the regions of the six largest cities in Finland, there are particularly many rural areas in the Turku region, and therefore a relatively great amount of septic tank and cesspool sludge is produced there when compared with the other regions. At the regional level (i.e., in Southwest Finland), approximately 100,000 m3 of septic tank and cesspool sludge is produced in the Turku region every year. According to estimates, 28,000 m3 of this remains uncollected (Varsinais-Suomen Liitto 2017). The majority of the collected sludge is water and, depending on the source, 5–20%, i.e., a total of 5,000–20,000 m3 of the material, consists of utilisable dry matter (Sitra 2007, Valonia 2014a). According to estimates, the septic tanks and cesspools constantly contain approximately 15 tonnes of phosphorus and 121 tonnes of nitrogen. The amount would be sufficient for fertilising 1,000 hectares of field in Southwest Finland (Valonia 2014b).
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). Biogas can be produced from sludge by digestion. Biogas can also be utilised in either heating, the combined production of heat and electricity, or as transport fuel.(Sitra 2007.) The nutrients contained by sludge can be recycled using them locally in field application. Sludge can also be pretreated, composted, and, eventually, sold as a fertiliser product.(Valonia 2014b.) Furthermore, sludge can also be treated with pyrolysis, after which the incinerated sludge ash can be used as 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).
The majority of the sludge produced by the properties in Southwest Finland is transported to wastewater reception points for treatment or to Gasum Oy’s biogas plant in Topinoja, Turku, for digestion. At the biogas plant, electricity and district heat is produced with the aid of the biogas. The solid residue is refined into soil and soil improver. Nutrients are also recovered, in the form of nitrogen concentrate, from the reject water obtained from the centrifuge (Gasum 2017).
Septic tank and cesspool sludge is municipal waste referred to in the Finnish Waste Act for which the waste management responsibility falls to municipalities. The septic tanks of the permanent dwellings and holiday homes not included in the sewer system must be emptied at least once a year. Cesspools are emptied when necessary. When sludge wells are emptied and the sludge is treated appropriately, their environmental impact is significantly reduced. Private companies perform the collection of sludge on the basis of the residents’ notifications. Since notifying the relevant company of the matter is the responsibility of the residents, there is a risk that the wells are not emptied regularly or at all. In the worst case, the uncollected sludge may be released to the environment or treated within the black economy. (Lounais-Suomen jätehuolto 2017.)
Thus, there are many innovation needs related to septic tank and cesspool sludge, with the aid of which their collection and treatment could be rationalised:
- Separation of dry matter and water already before transportation
- Centralised coordination or organisation of transportation
- Increase in the emptying rate of septic tanks and cesspools
- Accurate statistical monitoring of the sludge transports, tanks, and treatment
- Treatment solutions that are as effective as possible
The innovation needs are mainly related to the rationalisation of transport, as heavy road traffic produces significant carbon dioxide emissions and transports incur expenses. For communal sludge, the maximum profitable transport distance is 150–250 km (Pöyry 2008). Effective transportation or possible local utilisation of wastewater sludge reduces the total number of kilometers, which saves money and the environment.
For instance, separating dry matter and water already before transportation would reduce the need for transportation. By improving the statistical monitoring and registration methods, transport could be performed in a more comprehensive and effective way. For instance, in 2012, only approximately 30% of the transport entrepreneurs operating in Southwest Finland had registered themselves in the waste management register and the register data concerning the emptying of sludge tanks only covered approximately 22% of all the properties located outside the sewer system (Valonia 2013).
Sludge treatment must be coordinated well at the reception points and plants to which several transport entrepreneurs transport sludge. Coordination guarantees the sufficiency of both storage space and input material. Sludge transport costs can be reduced by planning the transportation routes carefully. Communication should be improved, so that the arrival of sludge at the treatment plant can be timed and planned economically in relation to the transport equipment in use (Sitra 2007).
Business-related challenges and opportunities
In Finland, there are a few entrepreneurs who treat the rural sludge they have collected and spread it on their fields as fertiliser. Sludge that has been on-farm treated under the right circumstances can be safely and economically placed on fields, and the entire process is profitable and environmentally friendly. Sludge recovery reduces the need for using inorganic fertilisers and thus improves Finland’s fertiliser self-sufficiency. In rural areas, the disposal on fields is a more inexpensive option than transporting the sludge to a reception point and paying the related fees. On the basis of the experience gained during the Putsareista pellolle project, it can be stated that the value of the plant nutrients in sludge remains low in relation to the costs incurred by the operations. The nutritional content of rural sludge proved to be surprisingly low. The operations do, however, have the potential to improve rural business opportunities. (Valonia 2014b.)
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., 2011). 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).
References (mainly in Finnish)
Gasum 2017. Verkkosivut. Turun biokaasulaitos.
Elomaa T. 2015. Kartoitus Kaakkois-Suomen jätelietteistä, lietteen tuottajista ja käsittelymenetelmistä. Diplomityö. Lappeenrannan teknillinen yliopisto,
Kangas A., Lund C., Liuksia S., Arnold M., Merta E., Kajolinna T., Carpén L., Koskinen P. ja Ryhänen T. Energiatehokas lietteenkäsittely SUOMEN YMPÄRISTÖ 17 | 2011
Lounais-Suomen jätehuolto 2017. Lietteiden keräys.
Pöyry Environment Oy. 2007. Lietteenkäsittelyn nykytila Suomessa ja käsittelymenetelmien kilpailukyky –selvitys.
Roponen M. 2013. Sako- ja umpikaivolietteiden nykytilanteen ja ongelmien kartoitus. Case: Jätelautakunta Kolmenkierto. Lahden ammattikorkeakoulu, Opinnäytetyö
Ruuhela 2017. Puhdistamolietteen käsittelyn hankinnan laatukriteerien kehittäminen. Varsinais-Suomen elinkeino-, liikenne- ja ympäristökeskus. RAPORTTEJA 18 | 2017
Valonia 2013. Julkaisematon selvitys sako- ja umpikaivolietetyhjennysten järjestämisestä Turun seudun jätehuolto oy:n osakaskunnissa. Turku 2013. Turun kaupunkiseudun jätehuoltolautakunnan arkisto.
Valonia 2014a. Haja-asutuksen sako- ja umpikaivolietteiden peltolevitys. Youtube-kanava.
Valonia 2014b. Haja-asutuksen sako- ja umpikaivolietteiden tilakäsittely ja käyttö maataloudessa. Putsareista pelloille -hankkeen opas
Varsinais-Suomen liitto 2017. Varsinais-Suomen materiaalivirtojen potentiaali kiertotalouden näkökulmasta.