Sewerage treatment plant

CarolynSinclair

We have just accepted an offer on our property. We need to replace the septic tank which is no longer working properly. The issue is the size of the replacement Sewerage treatment plant. Quotes are varying from 8 person to 11. We have a 3 bedroom house with attached annex with 2 bed and small kitchen and bathroom. Above the garage is a one open plan room with kitchen area and shower room. Do we just calculate as 6 bed in total (8 person) or 3 individual units making 11? Thank you to anyone who can advise - it’s frying my brain! 

gwynlas

Go for the higher number as whilst you might not need it at the moment it will future proof you for a time when you might sell.

ThisIsWeird

What's the difference in physical size, cost, and complexity?
If minimal in the scheme of your house value, then gwynlas it.

Section62

gwynlas said:

Go for the higher number as whilst you might not need it at the moment it will future proof you for a time when you might sell.

Not always a good idea - some types have minimum flow/volume levels below which the plant won't necessarily produce a compliant discharge.  In some cases there may also be an energy cost penalty installing a plant which is excessively oversized.

theoretica

CarolynSinclair said:

We have just accepted an offer on our property. We need to replace the septic tank which is no longer working properly.

Are you replacing the septic tank on the property you are leaving?  Or on one you are buying?

ThisIsWeird

Leaving, I understand.

CarolynSinclair

theoretica said:

CarolynSinclair said:

We have just accepted an offer on our property. We need to replace the septic tank which is no longer working properly.

Are you replacing the septic tank on the property you are leaving?  Or on one you are buying?

Thank you for replying - the one we are leaving. 

ThisIsWeird

And the difference in physical size, cost, and installation complexity between the two?
And, and as S62 asks, have the installers made any comments on potential drawbacks of 'oversizing', other than extra cost?
The room above the garage is, I presume, another complete living space with a bed?
Is there any potential for further development or extending?
Based on your current setup, I think it would imply around 9 persons if all bedrooms occupied - Ie first bedroom in each occupied by a couple, additional bedrooms more often by singles - but there's potential for going higher, especially if the 2-bed is holiday let.
In answer to S62's concern, I would simply ask the manufacturer if there were potential issues with 'under' use, but I cannot imagine how there could be, as many households go through large occupancy number changes at times, often ending up with just a couple.
Could you provide answers to the Q's, please?

Section62

ThisIsWeird said:

In answer to S62's concern, I would simply ask the manufacturer if there were potential issues with 'under' use, but I cannot imagine how there could be, as many households go through large occupancy number changes at times, often ending up with just a couple.

Of course the number of occupants will vary - which is why, if daily volume is important, there would be a range over which the unit is designed to operate efficiently.  The larger the unit specified (e.g. one for 11 people) then potentially the further from the design capacity if the household reduces down to two.  Larger capacity units are also often designed on the basis they will serve more than one property, so statistically* a unit designed for 12 people and (say) serving three dwellings (of four people) is less likely to have only 2 people using it on a regular basis.

*Foul sewerage design is largely based on averages and assumptions, not definite facts.  The unit won't explode showering everyone with untreated sewage if it is used by only two people, but before specifying a significantly oversized unit it is important to understand whether there would be performance issues if the daily flow regularly drops below a certain level.

theoretica

Is there a reason for not consulting the buyers? Are they buying to run BnB / multihouseholds, or a small family that likes lots of space?

CarolynSinclair

ThisIsWeird said:

And the difference in physical size, cost, and installation complexity between the two?
And, and as S62 asks, have the installers made any comments on potential drawbacks of 'oversizing', other than extra cost?
The room above the garage is, I presume, another complete living space with a bed?
Is there any potential for further development or extending?
Based on your current setup, I think it would imply around 9 persons if all bedrooms occupied - Ie first bedroom in each occupied by a couple, additional bedrooms more often by singles - but there's potential for going higher, especially if the 2-bed is holiday let.
In answer to S62's concern, I would simply ask the manufacturer if there were potential issues with 'under' use, but I cannot imagine how there could be, as many households go through large occupancy number changes at times, often ending up with just a couple.
Could you provide answers to the Q's, please?

Thank you for replying. No comments from installers at all about potential drawbacks from oversizing, and being honest it’s not something I even thought of. The room above the garage -  yes can have a living space and one double bed. No chance of any further extensions. 2 bed most likely will be let out by new buyers.  Based on our 3 bedroom house they said 5 person, but it’s whether the 2 bed annex is classed as an individual unit. Plus room above garage. I understand it all has to be signed off by building control to say it’s suitable for size of property.