Yet more toilet system considerations

I’ve been surprised by the number of comments generated by my toilet and blackwater tank posts.  I’d assumed the cassette and composters would have left some remarks about our choice of pump-out but this didn’t occur. 

Bruce (nb Sanity Again) and Peter (nb Kelly-Louise) both made pertinent comments about my plan which had the positive effect of making me re-think the setup.

As Bruce rightly pointed out we would never be able to fully empty our tank and as; at 6”; it is quite shallow and this might mean a significant volume of waste wouldn’t be removed.  He also mentioned potential problems with a complex vacuum macerating system.  Peter also provided some useful information about his own Tecma system along with a copy of the manual.

I started to re-think the location and alignment of the tank but then recalled my earlier correspondence with Chris Wells of nb Belle.  He has a similar underfloor tank and hadn’t experienced any problems.  If we were to move the tank to above floor level then it would take up valuable storage space.  This is the major reason why I located it under the floor and one of the reasons why I specified a 20mm thick baseplate (eliminate the ballast).  I’ve decided to stay with the original underfloor tank.  However Bruce’s comments have made me realise I should consider planning to empty the tank when the water tank in the bow is almost empty.  This will increase the “bow up” trim of the boat thereby maximising the opportunity to empty the majority of the contents of the blackwater tank.

The vacuum system is complex and it has always concerned me I hadn’t identified a location in Waiouru for the vacuum generating system.  My sole reason for selecting a vacuum system was to reduce the volume of required “flushing” water.   Peter’s toilet system also uses very little water and has another advantage.  A powerful turbine pump.  It is capable of pumping to a height of 11m and a distance of 90m.  That is more than we need on Waiouru.  After reading the owners manual he emailed me I realised the toilet was very similar to the one we had on our last Black Prince hire boat.  As a result I’ve concluded Bruce was right about the vacuum system and Peter’s advice regarding his own toilet is the likely way to proceed.  Oh; the Tecma is also cheaper than a vacuum system!

Don’t read any further if you get squeamish about toilet tank capacity calculations and human waste.

Having calculated the capacity of the tank at 400 litres I wanted to know the average number of days between pump-outs.  The proposed toilet has a two flush system.  One for liquid only, and the other for solids.  The technical manual provides the amount of flush water for each cycle.   I can calculate the daily frequency of toilet use by observing our own habits.  Where I ran into difficult was finding a reliable source for the average daily volume of human solid and liquid waste.  I could measure the liquid, but the solids proved more problematic.  There was a wealth of information on Google regarding weight but little about volume. It’s the volume that is critical to the calculations.  Eventually I found a source which provided the answer (0.5565 litres for those of you who want to know).  It then became a simple matter of creating a spreadsheet with the fixed and variable data.  The toilet tank should last a maximum of 40 days between pump-outs.