Clever Danes inject new life into waste management

Written by: RWW | Published:

Imagine UK householders no longer having to separate their waste at source and the biodegradable materials in black bag rubbish being turned into a high quality bioliquid that is suitable for biogas production. Fiction? Not according to Danish technology that has just been launched. Geraldine Faulkner reports.

Lego, Carlsberg, Bang and Olufsen, as well as bacon and sweet pastries are items that Denmark is best known for, but now there is a waste treatment technology which can be added to the mix, namely REnescience, an automated continuous running process which splits unsorted municipal household waste into recyclables and uses enzymes to create energy products.

And it’s not just municipal solid waste (MSW) that the process can handle, REnescience also treats a variety of other waste types such as fines from materials recovery facilities (MRFs) and food waste, i.e. wastes with a high level of biodegradable materials. 

DONG Energy says this means the procedure has a high capture rate of biomass along with the recycling of materials such as metals and plastics. And the beauty of it all is that the process requires no prior treatment such as source separation of shredding.

“REnescience is an alternative to sorting household waste, but with a significantly higher gas potential and opportunities for greater recycling,” states Thomas Dalsgaard, executive vice president, DONG Energy Thermal Power before adding: “By treating unsorted household waste with enzymes, the REnescience technology can convert more than 90% of the organic waste to a biofluid which can be transformed into biogas and used (after upgrading) in the natural gas grid or transport sector. Other parts of the waste, such as plastics and metal, will be cleaned and separated into various recyclable fractions.”

In a nutshell, the process goes like this: The docking station receives waste from waste trucks, the biodegradable part of the waste is converted into biogas and can be upgraded to e.g. green gas, fuel for transport or to generate green heat and power. Metals and plastics are sorted into separate fractions and cleaned; the cleaned fractions can be recycled and re-used.

The in-feed system mixes the waste with water and water is re-circulated from the back-end of the process. The heat reactor stabilises the mix of waste and water by pre-treating the waste mix and the enzyme reactor liquefies the biodegradable part of the waste. 

The hydrolysed waste is separated into three streams; bioliquid, 2D fraction (mainly plastic foils and textiles) and 3D fraction (which comprises mostly metals and plastics). The cleaned fraction can be recycled or used to generate green heat and power as refuse derived fuel (RDF). The fraction is cleaned and sorted to the required specification necessary for further sorting and recycling.

So what quantities are we looking at?

According to DONG Energy, a REnescience plant with a capacity of 80,000 tonnes of waste per year, corresponding to the annual waste volume of a large Danish municipality, would produce approximately eight million m3 of biogas from the waste. This corresponds to the annual natural gas consumption of around 5,000 households, and an annual CO2 reduction of up to 13,000 tonnes. 

The waste is expected to yield between 4,000-6,500 tonnes of plastic and 1,600-3,800 tonnes of metal for recycling.  

“These amounts vary from country to country, depending on the waste composition,” states a company spokesperson.

The point the company is keen to focus on regarding the biogas produced by REnescience is that it is said to be able to produce more than four times as much biogas as would be produced if the same amount of waste was sorted in advance and then converted into biogas in a traditional biogas plant. An added bonus is that the technology also enables water from the biofluid to be reused, resulting in 75% of the water being re-circulated in the process.

“We therefore refer to REnescience as a waste refinery,” says the company.

And that is not all. Along with the bioliquid and recyclable materials such as plastics and metals, the process also generates a storable solid fuel (refuse derived fuel) that is reported to have a high calorific value with low alkali content that can be used for energy recovery in green electricity and heat production, while the digestate resulting from the biogas production has the potential as fertiliser thanks to the nitrogen and phosphorus it contains.

“REnescience acts as ‘stand-alone’ or ‘front-end’ to various plant concepts while the high level of biomass captured in the bioliquid ensures that significantly more efficient plant concepts can be introduced to the market,” states the company. 

“REnescience sorts waste much more effectively than [people]. It sorts out the non-degradable parts (glass, wood, plastic, metals etc) and produces four to five times the amount of biogas from the biodegradable parts (food and paper waste) compared to sorting systems handled by individuals.” 

Essentially, as well as enabling a simpler collection system which requires no source separation, REnescience centralises the sorting of waste.

What else distinguishes the process? 

REnescience runs at lower temperatures than, say, other waste treatments like anaerobic digestion. The Danish system operates at 50° Celcius and at atmospheric pressure to separate potential recyclables in the MSW such as metals and plastics.

Also, DONG Energy is keen to point out that the technology could bring “significant” benefits to the UK waste industry in that the high quality bioliquid produced means REnescience could replace mechanical biological treatment (MBT) as one of the UK’s most favoured treatment processes.

What is the next step for the company?

Dalsgaard again: “DONG Energy is in dialogue with several different business partners in Denmark and abroad in order to build a full-scale REnescience plant.

“We’re experiencing a growing interest in the Netherlands and the UK, but also in China, the USA and the Middle East, there is an interest for this technology.” 

However, the executive vice president emphasises that he would like the first full-scale plant to be built in Denmark. “It would be a global showcase creating a demand for Danish know-how and competences to develop and construct similar plants,” states Dalsgaard.

DONG Energy says it has initiated analyses of the possibilities of constructing a full-scale plant in the Triangle Region in Denmark that would be able to handle the waste coming from up to 170,000 households.

“No other technology for processing mixed wastes that contain organic matter in this manner exists worldwide - a positive story for local communities and the waste industry,” adds Dalsgaard.

Fact File: The process

In the REnescience process, unsorted MSW is wetted and warmed to temperatures appropriate for enzymatic hydrolysis. Through enzymatic action, biodegradable materials are liquefied, which permits easy separation of non-degradable solids. 

Liquefaction is key in the REnescience process. Enzymes are added into the waste stream in a specially designed reactor. 

Biodegradable content of unsorted waste is liquefied and separated from non-degradable solids to create a pumpable bioliquid with high biomethane potential.

In the ballistic separation process, the treated waste is separated into three fractions - bioliquid, two-dimensional and three-dimensional solid fractions.

The 3D fraction is transported to a washing drum, where surface residues are washed off. 

Used wash water is re-circulated.

After the ballistic separator, the 2D fraction is transported to a press, where a liquid containing organic fibres and residues are separated from the 2D fraction. The liquid is then mixed into the bioliquid stream.

The 2D fraction is washed to improve storage stability and to retain as much biomaterial as possible. Used wash water is re-circulated back to the pre-treatment step.

After washing, a second pressing takes place to increase dry matter content.

When the bioliquid exits the ballistic separator, it is mixed with bioliquid from the first pressing of the 2D fraction. Further treatment depends on the intended use of the bioliquid. Depending on the client’s markets, the bioliquid can be used for a variety of purposes, for example, biogas production.

10th International Energy from Waste 2014

Ulrik Falkenberg from Dong Energy will be giving a presentation on REnescience at the 10th International Energy from Waste Conference in London on February 27. For more details of the conference, visit:

www.efwlondon.eu







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