Why are certain processes in ecoinvent modelled with negative output?

Question

Hello,

I am modelling a case with ecoinvent db and my process uses wastewater so I added the "market for wastewater, unpolluted" dataset. It has, however, negative contribution to the final impact results (I am using the EuGeos db). 

I created a product system for the process for the wastewater on its own and calculated the results and they are positive (most of them). I noticed that the dataset is modelled with negative output: - 1 m³. My explanation is that this is because the wastewater is actually brought back to nature, i.e. by using wastewater we save a certain amount of virgin water. 

So, do I have to include the wastewater in my model also with negative sign (it is actually used in a process and is not returned to the environment)? How do we proceed with datasets with negative outputs like that?

Yana

Comments

Dear all,

I contacted ecoinvent providers about this issue and found out a forum thread (https://www.ecoinvent.org/support/ecoinvent-forum/topic.html?&tid=88) where the problem is discussed as software related both for SimaPro and GaBi. Is it possible that my negative results arise from the calculation procedure of openLCA?

I would really appreciate your feedback.

Yana

Answer 1

The negative outputs and inputs are due to the "double negative" waste modeling in ecoinvent: a process must have a product on the output side, therefore also a wastewater treatment plant which has the product (to be treated wastewater) on the input side in real life gets in ecoinvent to be treated wastewater on the output side, and in order to maintain the mass balance, this output flow is negative. Then, the process which is producing wastewater has it on the input side (while in real life, on the output side) so that both can be connected, and it has it also as negative flow to again preserve the mass balance. I admit this is not terribly intuitive, if you model your own process that has wastewater output, you also need to move it to the input side, with a negative sign.