Biowaste and wastewater sludge management
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Case HSY (Finland)
Helsinki Region Environmental Services HSY is a municipal waste and wastewater management company.
Biowaste treatment
In HSY operational area, biowaste collection began in 1993 and firstly concerned 10 apartment properties. Now regulation has changed and the collection expands already to 1-4 apartment properties.
Biowaste management (anaerobic digestion and composting of digestate) is organize in the Ämmässuo Ecoindustrial center Ekomo.
HSY’s treatment of biowaste is based on partial decomposition, where biowaste is digested and composted. The most appropriate part of biowaste is always directed to both treatment processes. Partial digestion is a more ecological treatment method than composting thanks to biogas production. More than 50,000 tonnes of biowaste are brought to Ämmässuo every year from both households and companies.
Pre-treatment of biowaste
Biowaste is directed to the shredder and screened. Fine matter (< 80 mm) is transported to a biogas plant for digestion. Larger, or rough, material ends up directly in the composting process. Around 50 per cent of biowaste is directed to the digestion process and 50 per cent to direct composting.
Biogas plant
HSY can efficiently recover the energy contained in biowaste in their own biogas plant. A thousand kilograms of biowaste generates an average of 200 cubic meters of biogas, which means 4.4 million cubic metres per year. It generated 9.5 GWh of electricity and 8.6 GWh of heat to be utilized at HSY’s own biogas power plant. Digestate generated in the digestion process is transferred to the composting process. This will restore humus, coal and valuable nutrients, such as phosphorus and nitrogen, to the cycle.
Composting process
A suitable mixture is made of rough biowaste, digestate and support material (different types of wood chips). The support substance circulates several times in the process, and the average amount of virgin support material needed during the year is only 4,000 tonnes. After the plant treatment, compost generated from the mixture is transferred from the interiors of the composting plant to outdoor seams for post-maturation. Compost packs are turned every two to three weeks to enable compost to mature effectively. Compost is matured in the field for as long as it can be found to be mature using a test method approved by the authorities. The total duration of the industrial composting process in Ämmässuo is 6 months, on average.
Around 17,000 tonnes of compost is generated in Ämmmässuo during the year. It is used as soil raw material for landscaping or as soil improvers in agriculture. About 20-25% of the ready-made soil sold is compost.
Wastewater sludge treatment
Composting site Metsäpirtti for wastewater sludge was founded in 1994 and receives annually approx. 85 000 tonnes of sludge produced on two wastewater treatment plants.
During composting process, municipal sludge is mixed with stabilizing mediums (peat, stable horse manure or biofibre) to produce homogenic mixture which is then packed to compost windrows. Composting takes about 6-9 months. Composts are aerated with the help of turning gear 4-6 times before they are ready. End products are fresh compost to agriculture, soil improvement compost to green building and garden soil. Agricultural compost is transported without cost within a 60 km radius.
About 800 tons of phosphorus and 800 tons of nitrogen is recycled annually in Metsäpirtti composted soil products. In addition to nutrients, the organic compounds of sludge are recycled.
Case Alytus Region Waste Management Center (Lithuania)
The Alytus region waste management center was established in 2002. Its shareholders are 7 municipalities and it serves altogether 165 000 inhabitants by treating 80 000 tonnes of municipal waste/year. The turnover of the waste management center is 7,2 million EUR.
The work consist of the center covers all the spectrum of the waste management including waste collection, recycling/Reusing, education, collection of fees, composting, landfilling.
Biowaste recycling process in the center consins of four phases:
- Pre-treatment
- Anerobic digestion
- Composting
- Separation of impurities
What makes the center unique is a mechanical treatment plant, that treats i.a. biowaste, which input is ~30 000 t MSW/annually. The output of the plant is the following: compost/energy production-51%, incineration-28%, overlaying of a dumpster-5%, recycling-3%, landfilling-13%
The biological treatment plant situated in the same area treats ~ 20 000 t of biowaste annually by anaerobic digestion. The outputs of the plant are compost, electricity and reduced CO2. Biowaste streams are green waste and wood chips, food waste, liquid biowaste and 20-80 mm fraction from MSW.
The final product that is made from the biowaste is compost.
Case Tallinn Waste Recycling Center (Estonia)
The Tallinn Waste Recyling center was opened in 2003 after old landfill was closed. Its main operations are: production of RDF, waste transport, bottom-ash treatment and biowaste composting.
The center treats 20 000 – 30 000 tons of biowaste per year, including category III animal waste (fish, meat etc.). It deals with tenders from Tallinn, regions near Tallinn, and also private clients as customers. Since separate collection it can be seen a positive rise in amounts, but a high content of biowaste in mixed waste should still to be improved.
The treatment process includes: crushing, screening, processing in windrows (raise the temperature, cover with coretex membrane) including automatic aerating and mixing, aftertreatment and screening with 10mm screen, removing plastic and wood
The main challenge is dealing with impurities of the treated waste: approx. 50% non-biodegradable plastics; also the waste can contain glass and plastic packaging.
The final product of the center is a certified compost, which is produced 6000-9000 tons annually. The compost is strong and needs to be mixed: 70% soil and 30% compost.The buyers of the product are mainly private companies, private persons and farmers.