Calculating the weight and volume in pay as you throw waste collection: the pros and cons of the two measuring methods.
In the pay as you throw collection process, “pay as you throw metering of the quantity of the wastes consigned is obtained by determining, as a minimum requirement, the weight or volume of the RUR (solid urban waste) consigned by each user to the waste management public service”, as set out by Article 4 of the Ministerial Decree dated 20th April 2017 on the pay as you throw tariff. Again, Article 6 of the decree sets out that “the quantity of waste consigned is measured by direct weighing with detection of the weight, or indirectly by detecting the volume of the wastes consigned by each user and can be: a) carried out on board the vehicle doing the collection through the identification of the container or bag; b) carried out by a device worn by the collection operator through the identification of the container or bag; c) built into the container used for collection; d) carried out at a collection centre”. In particular:
1. “In the case of direct weighing, the quantity of waste per type of waste produced by the individual user (RIFut) which is being weighed, is calculated as a sum of the recordings of the weight transferred (PESconf) for each user expressed in kilogrammes”. The weight of the RUR (residual urban waste) is measured by means of approved balances or electronic/computerized weighing systems on the elevators installed on board the collection truck.
2. “In the case of indirect weighing, the volume of wastes consigned is determined by the dimensions of the container put out by the user or the capacity of the bag consigned or taken by the user or the dimensions of the opening in the consignment of containers with a volume limiter”. Moreover, “the quantity of waste for the reference types, produced by the user (RIFut), can also be calculated as the sum of the product of the volume, expressed in litres, of the container assigned for emptying, or the bag taken or the accessible volume in the case of containers with volume limiters, multiplied by the specific weight coefficient (Kpeso)”. In order to determine the volume of the general waste consigned, the municipality distributes to the users bags or bins with a set capacity, e.g. a 50 litre bag or a 30 litre bin, which corresponds to a quantity of general waste that is fixed and has a set volumetric coefficient: to give a few examples, a container with a certain capacity can contain 5, 10 or 15 kg of waste. The number of times the tub is emptied or the number of bag consignments by the user is then identified and recorded; this operation is carried out by reading an RFID tag on the bag or bin using a fixed device mounted on the vehicle or a system worn by the operator.
Direct weighing v indirect weighing: comparison between the two methods for pay as you throw metering
Direct weighing and indirect (volumetric calculation) weighing must be compared to assess the pros and cons of the two methods for measuring general waste. This must take into account some fundamental elements:
♦ Installation on the vehicle
Installing scales or an electronic weighing system on board the collection truck requires a lot of time, whilst a couple of hours are enough for an antenna fixed on the vehicle. Scales require considerable ongoing maintenance and need to be calibrated frequently, every so many weighing hours, or otherwise they accumulate errors; on the other hand, maintenance for the antenna is reduced and there is no need to withdraw it. The electronics of weighing systems are extremely complex, as opposed to the RFID system, which is very simple. Finally, the manual handling – or the way that the bag is tossed or the tub emptied inside the vehicle – for direct weighing consists of lifting the bin mechanically and is a slow process; for the indirect one, tossing the bag or emptying the bin and is quicker.
♦ Installation on the operator
It is impossible to install a weighing system on the operator, whilst installing a volume detection system on a man is not only possible, but quick and easy.
♦ Street bin
Bins with built-in balance for weighing are very expensive, and consequently liable to be stolen, need continuous maintenance and frequent recalibration and have complex electronics; for volumetric calculation of the RSU, the bin has no balance and therefore the maintenance is reduced for the general waste container and re-calibration is unnecessary. Even in bins with controlled access, the electronics is not complicated, as the top cover opens with a badge or card for the user identification and the volumetric waste coefficient is known.
♦ Recycler/ecological platform
The recyclers, ecological islands or platforms are fitted out areas where separated wastes that do not fall into the normal collection cycle are consigned: bulky items, household appliances, electronic waste, grass cuttings, wooden boxes, etc. There are ecological platforms fitted out for weighing with load cells positioned under the collection containers, but they are expensive and complicated to install; it is much easier to calculate the volume of the material, e.g. wood, starting from its volumetric coefficient, which is set.
Consequently, the more efficient weighing method is the indirect one, i.e. volumetric calculation
For the pay as you throw metering of the general waste, direct detection of the weight is definitely the most accurate method, but it is a laborious process and requires too high a cost, both for fitting out and for the maintenance of the weighing systems.
Volumetric calculation is less precise than direct weighing, but it is the more efficient method, because it has process benefits and is fast and economical. The small weighing errors are compensated by increasingly accurate calculation models.
Volumetric calculation and collection containers: bag v bin
If a municipality implementing the pay as you throw tariff chooses the volumetric calculation for metering the RSU, should it prefer the bag or the bin? Both types of containers have their pros and cons, to be assessed on parameters such as cost, distribution, speed of collection and duration of the RFID tag.
The bag has a really low cost, on average 15 cents, but its financial impact is high: using one a week, there will be an annual cost of 7.8 euros (0.15 cent x 52 weeks). The rolls of bags can be distributed via a vending machine; bag collection is quicker, especially if the operator takes four or five in his hand and throws them all together into the truck body, bit simultaneous grabbing is not compatible with reading the RFID tag for the pay as you throw tariff system. It is not unusual to see roads full of bags, which has a negative impact on the cleanness and decorum of the city. Finally, less important is the duration of the RFID tag, which is linked to the short life of the bag.
A bin fitted with an RFID tag has an initial cost of 4 or 5 euros, much higher than the bag, but can last for up to 6 or 7 years, with a cost of little more than 1 euro a year. The distribution of bins is more complex, but, given the long life of the container, is much less frequent. Emptying the bins – which assumes that the operator collects one container at a time and tips its contents inside the truck body – is a slower process, but more accurate. The rows of lined-up bins guarantee greater order and cleanness for the urban environment. Particularly strong rigid RFID tags are put on the bins and these least a long time, as long as the container.
The best choice is a mixed system
For general waste, the material on which the pay as you throw tariff is calculated, the choice of the bag allows you to easily establish its contents, checking whether the user has separated correctly, but the bag is more delicate and prone to breaking when thrown; the bin, on the other hand, does not allow you to see the content inside bit is stronger by a long way. As regards the other waste types, the container is absolutely necessary for organic waste, for reasons of hygiene – the bag decomposes with heat, is ruined by the rain and subject to attacks by wild animals – and is recommended for class, which is heavy, splinters and if put in a bag can break or cut it. For plastic, it is more appropriate to use the bag, which is thrown together with its contents because the materials are the same; since plastic is voluminous, putting it into a bin would make the handling and storage operations tricky.
The choice of the most appropriate container depends on the material collected, the characteristics of the municipality where the pay as you throw collection is implemented, the type of users and the number of operators used. A mixed system definitely manages to best meet the differing requirements in an area.