I have an anaerobic reactor which needs to be fed 20 ml of sludge everyday. (Working volume: 300 ml) for 15 days hydraulic retention time. I'd like to feed it alternate days rather than every day, a blunt calculation says feed 40 ml for every 2 days. I guess this will change my HRT, I'd like to know your thoughts and let me figure out a way to address this.
If you assume a nearly instant (lets say up to a few minutes) perfectly mixed reactor, then your calculation is correct because the AVERAGE retention time = Vol / Inflow rate. On a plug flow reactor things are much different.
The key here is the term "average", meaning this is a statistical value, not the real value. Now, if we think of the inflow as collection of very small unitary volumes, then each unitary volume will have in reality it's own residence time. Some unitary volumes will very quickly leave your working volume and some will stay there for a much longer time, and you will end up with a statistical distribution of residence times that will average 15 days.
I think a look at this papers, specially the first one may help you:
https://link.springer.com/chapter/10.1007%2F978-3-540-77278-1_7
Article Hydrodynamic behavior of a combined anaerobic-aerobic system...
But in general, as you lower your replacement volume (and therefore the shorter time interval), the distribution will tend to be wider (heavily tailed), more spread apart.
To help you get some intuitive understanding of this, imagine and extreme situation where you would replace let's say 299 ml every 14.95 days, what would be your guess is the residence time?.....somewhere close to 15. Now this 1 ml remaining from the first volume will be mixed with the new 299 volume and by the next time you remove this, most of the first 1 ml will be gone. In other words the probability that this original 1 ml remains more than 15 days goes down very very quickly. Now try to do the same mental experiment but with a small replacement volume e.g. the original 20 ml. I will leave that up to you.
The HRT depends upon production of Biogas(production) and also following factor
1. pH value
2. Methane contents
3. VFA component
The HRT you can judge with methane production, according to that you can find out HRT.
HRT(Days)=Reactor Volume(m3)/Flow rate(m3/day)
Your HRT meaning mean Hydraulic Retention time will not change, but your Anaerobic system will most likely not be very happy since you go more and more towards a Feast / Famine systems and this will not be of advantage for all bacteria in your system and might lead to imbalance between the functional groups which can be causing i.e. pH drop because the produced VFA are not removed fast enough. Anaerobic system with typically 3 - 4 functional groups of bacteria that depend on each other generally prefer stable conditions with low variations in inlet load and Ideal wice you should rather go for an automated pump solution feeding at least 4 times a day if you want optimised kinetics from the system.
I suggest, try to fed settled residue (centrifuged) of the total volume of the feed sludge and discard the supernatant. This wii help to maintain the HRT without the impact of feed shock.
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