Short-term decision making

With short-term decisions we mean decisions that need to be made for the upcoming days, weeks or months to at most a year ahead. During the year, it happens that the requirements change due to for example the quality of the waste. This change should be compensated by decreasing or increasing the supply. However, sometimes M&S can not change the pace of the supply due to the contracts with the clients. When this is the case, there are three possible situations:

• Unable to decrease supply

Overow: The supply is higher than the requirements which leads to an in- crease in the utilization of the buer. If the buer is full, the waste needs to be diverted to TOP or send to an alternative location (competitors).

• Unable to increase supply (Current Situation)

decrease in the utilization of the buer. If the buer is empty, the waste needs to be retrieved from TOP, which in the current situation is possible. • Unable to increase supply (Situation without TOP)

Underow: The supply is lower than the requirements which leads to an decrease in the utilization of the buer. If the buer is empty, the incinerator will stop, which leads to costs as shown in Table 2.4. Supplies should be increased as soon as possible.

As said, Twence has contracts with its clients that provide them with a margin of 10% by which they can over- or under supply the agreed amount over the entire year. The decisions about whether to divert or retrieve the incoming waste and wood is made in a consultation with all involved employees. This consultation takes place two times per week and consists of a delegation from M&S and a delegation from Operations. Together they take care of the planning of the internal logistics. In the current situation these planners have the responsibility to ensure an ecient ow of waste or wood towards the desired destination, while making sure that the entities are neither over supplied as under supplied.

In Figure 2.11 we see the decisions that need to be made regarding orders that are planned in. The gure gives an overview of the situation in which Twence has to deal with a possible overow.

Figure 2.11: Decisions when waste arrives at Twence Same structure holds for line 3 (interchange A and B)

From Figure 2.11 we can conclude that in the situation of overow, we have three options. We state the decisions for deliveries planned on line 3. The decisions are the same with planned deliveries on line 1+2, only the lines need to be interchanged in the following statements.

• Delivery is planned on incineration line 3, but risk of overow arises: 1. Reschedule delivery to line 1+2

2. Adjust delivery amount in consultation with client 3. Divert delivery to TOP

With little modications we can immediately list the decisions in a situation of underow:

• Delivery is planned on incineration line 3, but risk of underow on incineration line 1+2 arises:

1. Reschedule delivery to line 1+2

2. Adjust delivery amount of line 1+2 planned deliveries in consultation with client

3. Retrieve waste from TOP

In the current situation, these are the decisions that are made and they are based on mass. Just as with the long term decisions, we will point-wise state these decisions:

• On which line should the waste be scheduled?

• When is diverting necessary based on the level of waste in the buer? • When is retrieving necessary based on the level of waste in the buer? • When is retrieving necessary based on the age of the waste in TOP? • When should waste be rescheduled to the other line?

• When should waste be send to an alternative location?

Also the short term decisions can be summarized for the current situation. The results are shown in Table 2.8.

Decision AEC

Schedule waste on which line Line 1+2 or Line 3

Diverting If the level of tons of waste in the buers is above 85% of the total capacity Retrieving If the level of tons of waste in the buers is below 15% of the total capacity

or if TOP Waste is about to become older than 3 years Rescheduling If the level of tons of waste in one of the buers is

above 85% or below 15% of the total capacity Alternative location If buers and TOP are full

or if diverting capacity is reached

Table 2.8: Taken decisions in current situation

As discussed in Section 2.2.3 about the inventory development, the thresholds, rep- resented by 85% and 15% in Table 2.8, are very important. In the current situation, there is no clear view on what these optimal thresholds are. We discussed some of the ndings from a report by Numan, but these ndings also do not provide clear guidelines. From personal observations we concluded that the thresholds are now mainly based on human estimates. When the buer is almost full (85%) we divert, and when it looks almost empty (15%), we retrieve. In our opinion, with a clear research about what the optimal thresholds are, Twence could save a lot of costs from unnecessary diverting or retrieving.