Algae terrarium: automatically empty bottles

You did (almost) everything correctly already: 1. Place terrarium which produces pWater bottles. 2. Construct bottle emptier nearby. 3. Set bottle emptier to accept polluted water 4. do not set it to sweeping only (cause then it will accept only bottles marked with 'k') 5. raise the priority of the emptier sufficiently high

Important is step 5, where you set the priority of the emptier (also important might be the distance between bottle and emptier). If your dupes have many tasks to do, you need to micromanage the priorities. You should also note that only one bottle can be emptied at the same time. So if you have 1000 bottles lying around, there will only be one transport task active at the same time (in that case you might want to add additional emptiers).

Later on you can also use an automated sweeper (conveyor tech) to empty the bottles for you.