Evaluation of a new technology to pre-sort waste for digestion

The waste separator installed at the biogas plant in Vaarst-Fjellerad in Jutland, named a "dewaster", squeezes a visibly clean, organic pulp, ready for digestion, from the biodegradable waste from households and businesses collected separately. The installation effectively separates plastics and other unwanted materials, but it has no effect on the content of heavy metals and other xenobiotic substances in the waste. The installation consequently does not eliminate the need to sort carefully waste at the source, if the quality of the fertiliser product is to be ensured.

The effect of the dewaster is judged partly on the basis of the mass-balances for different parameters, and partly on the basis of the concentration of xenobiotic substances and particles in the squeezed raw material.

Background and purpose

In trying to meet the targets for increased digestion of source-separated waste from households the pre-treatment of waste before the digestion process has particularly created problems. Existing methods to ensure pure fertiliser, free from pieces of plastic etc. have proven ineffective and expensive.

The municipality of Aalborg is prepared to exploit the resources in organic waste from households and businesses, and today it delivers biodegradable waste from 2,600 households and a flower trader to a separate digester at the farm-based plant in Vaarst-Fjellerad, 30 km. south of Aalborg. At the plant a robust and simple pre-sorting unit, named a "dewaster" has been developed and tested. This machine divides the waste and separates unwanted components in one process.

The dewaster consists of a reception tank to mix the waste, and a dry-matter press, run by a 10 kWh electric motor. The press squeezes the wet, soft part out of the waste as raw material for the digester, leaving the rest fraction to pass through as reject. The reject to be incinerated consists of the plastic-bags and paper-sacks in which the waste has been collected and incorrectly sorted items. The structure of the raw material facilitates optimal gas production.

To justify the whole concept of collection and treatment of biodegradable waste from the city, the municipality of Aalborg has required relative certainty of the possibility to produce fertiliser of the necessary quality to allow it to be spread on farmland.

With this background an investigation has been carried out of the dewaster’s effect in separating unwanted materials and the remaining content of heavy metals, xenobiotic and alien substances and items, as well as nutrients in the fertiliser.

The investigation

Based on 4 loads of supplied waste in the examination period, January 2000, a quantitative and qualitative determination has been carried out of 1: the raw waste, 2: the squeezed raw material for digestion and 3: the reject.

The following parameters for analysis have been chosen for the qualitative determination:

Organic matter, nutrients (nitrogen and phosphorus), heavy metals (cadmium and nickel), xenobiotic substances (DEHP and NPE)

Moreover the occurrence and characteristics of unwanted materials (plastic-bags, incorrectly sorted items etc.) were identified via a sorting process. The raw material was centrifuged and sieved to identify the size and characteristics of the particles included in the raw material.

In addition, the consumption of electricity by the pre-treatment was measured.

Due to limited resources the investigation has only been able to take a representative set of samples and has only had the analyses done by one laboratory.

In view of the difficulties connected with analyses of an inhomogeneous material such as waste, a certain reservation must be taken concerning the validity of the results.

Main conclusion

The dewaster effectively separates a suitable raw material for digestion and gas-production, leaving a fertiliser with a very low content of contaminants in the form of particles (larger than 2 mm) from the waste.

Besides paper-sacks, plastic-bags and incorrectly sorted items the reject contains some organic material. The reject accounted for 30-50 % of the 4 sample situations. This quantity can probably be considerably reduced by changing the choice of packaging, extending storage of the raw waste, and through technical modifications.

The method has no effect concerning the occurrence of heavy metals and xenobiotic substances in the waste. Concentrations of these substances in the raw waste are repeated in the reject, as well as in the raw material with a possible tendency for an increased level of DEHP in the raw material when the waste is stored in the reception tank for longer periods. Taking account of the low number of samples and analyses there seems to be a contradiction between extended storage (in all probability resulting in more organic mass in the raw material), and the aim to keep a low level of DEHP in the raw material.

By July 2000 and 2002, the waste from Aalborg contained low levels of heavy metals, but worrying levels of DEHP and NPE relative to the "cut off" values.

The investigation has identified possible sources of this contamination among the incorrectly sorted items.

The consumption of electricity in the pre-sorting process made up to 5 % of the energy resource in the delivered waste.

Project results

The table states how much raw waste (source-separated, biodegradable waste from households and the flower trader in Aalborg) was supplied in the four sample situations and the amount of the reject. The difference is the raw material produced for the digester.

The smaller amount of reject in the first sample situation is considered to be due to extended storing of the waste supplied (due to operational problems).

The raw waste and the reject were sorted manually to characterise the content of plastic-bags used by the kitchen separation of waste, and incorrectly sorted items.

The paper, together with the unidentified mass, constituting by far the larger part, essentially originates from the waste-sacks used by some of the households to collect waste. The proportion of unwanted items (plastic-bags and incorrectly sorted items) is 2-3 times greater in the reject than in the raw waste, coinciding with the fact that the reject constitutes 30-50 % of the raw waste.

In most samples for hand sorting flowerpots, coloured plastic packaging, glass, rags, and polypropylene materials were found.

Flowerpots and coloured plastic packaging from other investigations are known to be potential sources of DEHP and NPE in fertilisers or compost derived from household waste. When the sources have been traced, efforts to increase the quality of the collected waste could start here.

The content of alien items in the raw material for the digester was also determined by centrifuging and sieving. In this way, some inorganic material in the form of sand and gravel (which is not a problem in the fertiliser) was found. Organic dry matter constitutes 80-85% of the material. The raw material is almost free from plastic and other larger alien items. The particles in the raw material are small in that only a very little is caught by a sieve with holes of 2 mm.

By analysing the content of organic matter, nutrients, heavy metals and xenobiotic substances, the following values were found, indicated as intervals for the values in the 4 sample situations.

Heavy metals: The limit values for concentration in the raw material of cadmium and nickel are kept by a good margin.

Xenobiotic substances: For DEHP, one of the four samples showed a level above the cut off value from 01.07.2000. For NPE, the limit value is kept but the lowering the limit value to 10 mg/kg dry matter from 01.07.2002 may cause problems.

There is no clear difference between the levels found in raw waste, reject, and raw material. Therefore, the dewaster has neither a positive nor negative effect on the content of heavy metals and xenobiotic substances in the fertiliser end product.

To ensure the quality of the fertiliser, an effort against the sources of the xenobiotic substances in the waste collected should to be launched.

There may be a causal connection between the longer storage time of the first sample load in the reception tank and the detection of the highest level of DEHP in this sample, but the data-material is too limited to say anything for sure.

Nutrients: The dewaster has no clear effect on the content of nutrients in the raw material. The loss of nutrients is more or less equivalent to the loss of organic mass through the reject.

The consumption of electricity by the pre-treatment process was between 25 and 31 kWh per tonne of treated waste. If it is assumed that for each tonne of supplied waste, 100 m³ biogas can be produced with an energy-content of 650 kWh, it corresponds to around 5% of the energy resource of the waste used in the pre-treatment.

The project was carried out following a previous project, financed by the Danish Energy Agency, to develop the dewaster. Besides the parties of the present project, Reno-Nord, the inter-municipal waste company of Northern Jutland, participated in the development of the dewaster, as the reception tank and the dewaster are planned to be located at the incineration plant of this company.

The present project furthermore constitutes the first part in a larger full-scale project to collect, pre-treat, and digest biodegradable waste from restaurants, institutions, supermarkets and food stores in Aalborg. This project is expected to be finished by May 2001.

During this period collection will be changed from waste-sacs to skips.