Minimal loading to a WWTP

[MirJev]

Hi everybody!
I am designing a 1000 PE WWTP for a Sea Resort. I assumed extended aeration process and volumetric loading of 0.25 kgBOD/m3.d and got the needed volume of aeration basin of 240 m3.
My question is: What is the minimal volumetric loading or F/M ratio necessary for the process to work, i.e. what is the minimal number of people that need to be in resort throughout the year in order for the process to be maintained?
M&E book states that the typical design values for extended aeration are 0.1-0.3 kgBOD/m3.d. When I apply the lowest value of 0.1 to my 240 m3 basin I end up with the load from 400 PE, which is quite high, because I guess the Resort will not be 40% full in winter months.
I assume the values given in M&E are given for the highest loading. Same case in other books and standards that I use, they all make recommendations for the highest loading, and I haven't yet come across the data about the lowest possible loading needed for the biomass to survive and do its work.

Thank you in advance for your answers!

 

[ashtree]

Its all a matter of how much you can turn everything down and what your permit conditions are. I believe you don't have to worry about keeping the biomass alive as it will adjust to whatever the load is if managed properly. But if you have an aeration system for example that you cannot significantly reduce the airflow or maintain the correct DO under minimum load conditions then you will have problems. You may need to have multiple units that you can take off line in low flow periods. Another option would be if the peak load period is relatively short would be to design to be heavily loaded at those times(right at the maximum of the typical design range ) so that at periods of lesser load you are still within the range at more conservative values.

The other issue is how quickly the load changes. If you were operating at 10% load for 3 months and then went straight to 100% load for a weekend then back to 10% this would also cause issues. Extended aeration is relatively robust though because of the large process volumes.

You have not mentioned anything regarding licence conditions or influent quality but i would suggest that in general terms provided you address the turn down issues then 40% load would be okay with many plants operating at 30% or less. Low load operation has its challenges though.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

Resort facilities usually have multiple units so that excess capacity may be taken off line. RBC systems are common for these applications.

 

[springrise]

Problem with a too low loading is not that the bacteria die (as long as it's aerated) but rather that filamentous bacteria start to grow, possibly causing problems with sludge sedimentation. Also, while volumetric loading is useful design parameter you could take a closer look on F/M loading (with the same volume you can increase the loading by decreasing mixed liquor solids concentration).
Definitely consider modular design with parallel units as bimr suggested.
Get as precise information about winter period as possible and design the smallest unit to the winter loading.