Hi Nirupama

You raise an important point and here are some initial thoughts in reply.

First, I spoke to Pedro Kraemer from BORDA many years ago and he said that there is a maximum size for ABRs.  Unfortunately, I did not record the figure that he gave and subsequent searches of the literature did not reveal any clear guidance on this. If Pedro or someone from BORDA sees this, perhaps they can advise.

I do think that sludge accumulation will be an issue although that is not necessarily an issue confined to large ABRs.  There is a lot of useful field information on the PhD thesis by Nicolas Reynaud, much of it based on Indian conditions.  It can be downloaded from  https://www.susana.org/en/knowledge-hub/resources-and-publications/library/details/2415 .  
* Comment by moderator (EvM): The PhD thesis has been discussed on the forum here .

I don't think that monthly desludging would be practical and it would probably disturb the biological processes in the reactor.   I don't think that it would be necessary for a reactor treating wastewater from a public toilet although the relatively high proportion of solids would suggest that the desludging interval would have to be shorter than normally assumed.  A few years ago, I saw a couple of public toilets with ABR treatment underneath installed on the outskirts of Bangalore. One had not been used much because people had plots big enough to install their own on-plot toilets but the other was next to a public market and bus terminal and had a good level of usage.  The women's group that ran it were certainly facing the challenge of desludging after a few years - perhaps about 3 although I do not have the details. 

CDD designed a DEWATS system, including ABR treatment for septage in Devanahalli in Karnataka and it may be that they can provide information on their experience with desludging.  I would expect the desludging needs of a septage treatment plant to be greater that for a plant treating wastewater from a public toilet but their experience should be relevant. The publicly available material does not seem to say much on the subject of desludging so you will probably have to try to contact them directly.

I hope that this is helpful. Let me know if you need clarification on anything

Best wishes

Kevin

Hello everyone.

I am a graduate student in Sanitation Science, Technology and Management at BITS Pilani, K K Birla Goa Campus, India. I have been working on the design of ABRs for flows such as 10m3/day upto 50m3/day for public toilets. If a de-sludging interval of 1 year is considered, a considerable volume of the tank has to be allotted for the accumulation of sludge during this period. And since public toilets cater to thousands to ten-thousands of users per day, the volume that has to be earmarked for sludge accumulation for comes out to be considerably large. I was hence wondering if an ABR is ideal for such large person equivalent. Or am I missing something here?

To reduce the tank footprint, I was wondering if monthly de-sludging can be considered, if we are able to provide automatic de-sludging pump connections to the tank. But is monthly de-sludging ideal or practically possible?

I would be extremely grateful if you could provide any practical insights into my query.

Thanks and warm regards,
Nirupama Nair