| Front page | | Contents | | Previous | | Next |

Experience with collection and treatment of organic waste in the municipality of Århus

Summary and conclusions

Background

In the spring of 2001, Aarhus Municipality introduced a new waste system for refuse collection where the collected bio waste is treated at a common biogas plant.

The project presents results from the waste system for bio waste introduced at app. 135,000 households in the Municipality.

The objective of the project is to contribute actual experience and data from planning, implementation and operation of a bio waste system for bio gasification in a full-scale system.

A number of investigations have been carried out, including three waste analyses, investigations of the state of the bags after collection in a refuse collection truck (with and without compression, respectively) and an investigation of the total flow (collection and optical screening).

The project forms part of the full-scale test implemented by the Danish Environmental Protection Agency concerning collection and bio gasification of bio waste. The project presents the tests in the Municipality of Copenhagen and six other municipalities in the metropolitan area and the collection tests made in the Municipalities of Kolding and Aalborg.

The Waste System

Before choosing a waste system, the Municipality made a point of establishing a system that was identical for all types of residential buildings. In the Municipality of Aarhus there is a large share of multi-storey buildings. Many multi-storey buildings have a rubbish chute and it has been attempted to ensure that it is not more difficult for these households to use the waste system than for households in one-family houses.

In connection with the applied waste system for refuse collection the citizens (besides separating paper and glass) have to organise the waste in a bio waste fraction to be packed in the distributed green plastic bags and in a residual waste fraction to be packed in the distributed black plastic bags or other bags. To facilitate the subsequent optical screening the bags have to be closed with a knot.

The collected green and black plastic bags are then sorted at the optical screening plant in a bio waste fraction that is collected in containers and transported to a common biogas plant located in the town of Spørring, and in a residual fraction that is transported by a conveyor belt directly to the incineration plant.

Please see the following flow outline.

Look here!
  

Sorting criteria and information

When determining the sorting criteria a starting point was taken in the sorting criteria that have been used in previous tests concerning the collection of bio waste in Aarhus Municipality. These sorting criteria were adjusted on the basis of discussions with the staff at the common biogas plant. Changes in the sorting criteria were mainly carried out in connection with garden waste.

The sorting criteria appear from Appendix A.

In two citizen inquiries, 95 % of the interviewees said that "to a very large degree" or "to a large degree" they know and understand the sorting criteria.

The three waste analyses carried out also show that the households that sort waste observe the sorting criteria. Sorting errors ascertained in the green plastic bags by and large originates solely from a few green bags that had been used for residual waste or from the fact that sorting had not at all been effected.

The waste analyses that were carried out (and other investigations) also showed that the slogan "remember the knot" worked. Nearly all of the distributed bags (green as well as black) had been closed.

In addition to external information e.g. material distributed to the households, through TV spots, advertisements etc. and internal information such as published material and info meetings, a "call centre" was established where telephone inquiries from citizens were received.

In the course of 28 weeks, the call centre received more than 11,000 telephone calls. It was not possible for the three employees at the call centre to receive and answer so many calls by themselves, and therefore the customer department at "Århus Kommunale Værker" provided backup assistance.

Collected amounts

The collected amounts have been continuously registered.

The specification method has meant that it is not the amount of bio waste that is collected at the households that is specified, but the amount of bio waste that after optical screening, bag opening and sieving leaves the optical screening plant.

As appears from Tabel 0.1 and Tabel 0.2 there is a great difference between the amount of bio waste that is collected at the households¹¹ and the amount of bio waste that leaves the optical screening plant.

Tabel 0.1
Share of bio waste in relation to the total amount of waste.

On the basis of the waste analysis results, waste investigation after collection in refuse collection trucks and test of the collection system and optical screening, the collected amount of bio waste is assessed at between 24 % and 37% of the total collected amount of waste.

The specification of the amount of bio waste that is removed from the optical screening plant and transported to the biogas communal plant has during continuous registration been estimated on an average to amount to 11 % of the total collected amount.

Tabel 0.2 shows the amount of bio waste and residual waste that can be estimated on the basis of continuous registration and investigations that were carried out.

The kg per household cannot be specified in connection with the investigation of waste after collection with a refuse collection truck, since the number of households from which waste has been collected during the investigations is unknown.

Tabel 0.2
Amount of bio waste and residual waste estimated according to continuous registration and investigations.

The three accomplished waste analyses showed a collection potential¹⁴ for the bio waste fraction of in average 4.2 kg per week at households in one-family houses and of 2.8 kg per household per week at households in multi-storey buildings.

Figure 0.1 shows the total flow in connection with the collection and treatment system. The flow has been exemplified with approximate amount indications when the collected amount of waste from households was 1,000 kg. The amount is divided into bio waste and residual waste according to the results of the waste system test that was carried out in September 2002.

Figure 0.1
Total waste flow incl. treatment.

Collection

In order to minimize possible damage to the collected bags, Aarhus Municipality in their tender material for collection of refuse determined the normative loading of a refuse collection truck to be max. 400 kg per m³.

As tenders are invited for collection tasks the winning refuse collectors can take that norm into account by purchasing trucks with greater cargo carrying capacity and at the same time take the situation into consideration when scheduling the route.

The weight limit of app. 4.5-5 tons in ordinary refuse collection trucks with an additional structure of app. 12-13 m³ involved considerable changes for the current refuse collector, partly in connection with the existing car park and partly in connection with the planning of the route. The reduced waste amount in the trucks resulted in a major change that by and large involved all districts.

Treatment

The collected waste is sorted at the optical screening plant in a bio waste fraction that is collected in containers and transported to the common biogas plant in Spørring and in a residual fraction that is transported by a conveyor belt directly to the incineration plant.

Unpacked waste such as large and small items creates problems at the optical screening plant, since the unpacked waste hampers the operation of the plant. Such items are removed manually by the staff working at the plant, and currently a waste amount corresponding to 30-40 containers is sorted out every week.

The buffer capacity in the discharge hall is based on an even supply of waste during opening hours - i.e. Monday to Friday from 7 a.m. to 2 p.m./3 p.m.

It has been necessary to let the waste stay during the night in five or six refuse collection trucks, so that the optical screening plant could be supplied with waste already from early morning.

The system allows eight refuse collection trucks to discharge waste simultaneously in the discharge hall. Nevertheless, queues or discharge problems arise now and then, when too many refuse collection trucks arrive at the optical screening plant at the same time.

The problem has partly been remedied in co-operation with the refuse collector by shifting the discharge time of the individual refuse collection trucks and using the possibility to discharge waste on the floor in the discharge hall.

The investigation of two waste loads at the optical screening plant showed:

	that the weight of the green bags at arrival at the optical screening plant amounted to 29 % and 30 %, respectively, of the total waste amount.
	that only app. 50 % of the green bags were intact at arrival at the optical screening plant.
	that app. 25 % of the green bags had been destroyed so much apart that waste was hanging out of the bags.
	that 73 % and 53 %15, respectively, of the amount of bio waste that was registered at arrival at the optical screening plant appeared again at the bag opening and sieving belt (after optical colour screening).
	that only app. half of the amount of bio waste that was registered at arrival at the optical screening plant appeared again after the total treatment at the optical screening plant (optical colour screening, bag opening and sieving).
	that app. 25 % of the green bags appeared again on the belt at the incineration plant.

The common biogas plant produces biogas with a mixture of slurry, organic industrial solid waste and bio waste – normally with an average methane percentage of 72 – 75 – highest for the bio waste line.

It is expected that a total of 3,900,000 m³ methane gas (Nm³Ch₄) will be produced each year, of this, app. 1,600,000 m³ on the bio waste line.

The matching biomass is expected to consist of 125,000 tons of livestock manure, 17,000 tons of bio waste from private households and 13,000 tons of organic industrial solid waste – giving a total of 155,000 tons that is spread on farm land in the form of degassed biomass.

Reject that solely originates from the source screened refuse collection currently amounts to app. 15 % of the bio waste supplied by the optical screening plant.

Economic aspects

The common biogas plant was established in 1993-1995. Up to the decision to implement a new waste system in Aarhus, expenses amounting to 49.9m DKK (current prices) have been paid.

As part of the decision to implement the new waste system the following capital expenses have been paid from 1999-2001 (current prices):

The operating budgets have been adjusted several times; most recently in May 2002 in connection with the optical plant and in September 2002 in connection with the common biogas plant.

In connection with both plants, the budgets have been adjusted to a level that is believed to be realistic in relation to the operating experience from the first year the waste system has been in operation. So far, the adjustments apply to 2002 and 2003.

The operating costs of the optical screening plant appear from Tabel 0.3.

Tabel 0.3
Operating budget of the optical screening plant for 2002 and 2003 in 1,000 DKK.

The expenses for removing waste comprise removal of residual waste for incineration as well as bio waste for bio gasification.

The operating costs of the common biogas plant appear from table Tabel 0.4.

Tabel 0.4
Operating budget of the biogas communal plant for 2002 and 2003 in 1,000 DKK

Additional expenses

1.0m DKK have been allocated annually for information activities etc. To that must be added 0.8m DKK that were allocated for extraordinary information activities in 2002. 8.7m DKK have been reserved annually for bags and the distribution of bags. Out of that amount 1.5m DKK are used for distribution.

Unit costs

The budgets are based on the treatment of 60,000 tons of source sorted waste annually (from this 6,000 tons bio waste). Concurrently with changes in amounts a number of costs are also changed - primarily the payment for incineration of residual waste and the treatment fee for the biogas plant. The latter is result of the fact that larger amounts of bio waste also give larger amounts of biogas and with that larger earnings for the biogas plant.

Costs per tons of source sorted waste are:

in case of 6,000 tons bio waste per year:	1,140 DKK
in case of 17,000 tons bio waste per year:	986 DKK

The 17,000 tons of bio waste was the amount originally anticipated. The treatment price for incineration of waste amounts to 590 DKK (incl. government tax).

Consequences for refuse collection fee and district heating rates

The refuse collection fee increased quite a lot following the decision to introduce a new waste system; from 1,764 DKK in 1999 for a one-family house with a 190 litre waste bin that is emptied every second week, to 2,299 DKK in 2002 (incl. 100 DKK for plastic bags). Converted into fixed prices the increase has been almost 19%.

Optimisation of the waste system

Aarhus Municipality continuously tries to optimise the waste system.

An effort is made to improve the quality of the collected waste, so that it is not necessary to sort out unpacked waste on the optical screening plant manually to the same extent as today.

An attempt is made to limit the extent of unpacked waste by introducing a new large-scale waste system in the entire municipality as on 1^st November 2002. In addition, buildings with common reception material (against payment) will get the opportunity to replace the existing lids on the 660 litre containers with a "lid in the lid". That would prevent large items from being placed in the containers.

In addition, from October 2002 general sanctions will be introduced in connection with serious sorting errors. The refuse collectors are not required to empty reception material containing bulky waste items, hazardous waste, electronics waste or broken bricks, stones or gravel. In each individual case the house owner will be asked to remove the incorrectly placed waste before the next day of collection. If that does not happen a special collection will be implemented and will cost 560 DKK incl. VAT.

Attempts are also made to increase the amount and improve the quality of the bio waste that is supplied to the common biogas plant.

That will take place among others by distributing stronger green plastic bags (40µ m instead of 33µ m) to the households in one-family houses in mid-November, so that less green plastic bags will be destroyed when they arrive at the optical screening plant. In that way, less bio waste will be lost in the plant.

In co-operation with the plant supplier, an attempt is also made to reduce the amount of green bags that are not removed for bio gasification in the optical screening plant, but instead continue to the incineration plant.

Finally, an attempt is made – in co-operation with the plant supplier – to improve the cleaning system of the sieve.

To replace the sieves a waste press will be installed in the course of 2003.