I SUPPOSE IT DEPENDS ON SIZE OF REACTOR.
Do you have a more definitive question?

Hi Nirupama, that should be no problem, the settler effluent will still certainly be way above 350 mgCOD/l, make sure that no aggressive cleaning agents will be introduced with the cleaning water - kind regards, Nico

Dear Nico,

Apologies for the late response. Thanks again for your inputs, they are still very helpful. I have finally decided to use a 2-chambered settler as pre-treatment for a 3-chambered ABR treating about 6KLD wastewater. The only concern I still have is the fact that significant amounts of water is used for flushing and floor-washing, since we are considering automated, electronic toilets here. From your experience, do you think waste would be too diluted to require any pre-treatment? 

Thanks again for your responses.

Regards,
Nirupama

Dear Nirapama,
apologies for this late reply, I hope it is still helpful, I would assume that the wastewater (coming mainly from toilets) will have such high strength that you dont need to worry about the digester removing too much of the organics and therefore making the ABR less efficient. Based on our research ABRs can be considered efficient for feed conc above approx 350 mgCOD/L...you wont get below 1000 mgCOD/L with the biogasdigester Id assume. A digester makes only sense if the gas will be used, For sludge accumulation in digester or settler you can use the rates given in the Sasse DEWATS book, I am not aware of better estimations available - make sure to include the compaction over time curve into your calculations,
Best, Nico

Dear Nico,

Thank you for your inputs. I understand that the ABR system works better the higher the strength of wastewater. In that case, I was wondering if the biogas settler pre-treatment step would render the strength too low for the ABR step to work in an optimal manner. I was wondering if a simple settler step instead would suffice. If so, how frequently would it be de-sludged? Would really appreciate your thoughts on this.

Regards,
Nirupama

Dear Nirupama,

your pretreatment needs to be able to treat a continuous flow and provide hygienic separation of the waste from the public.
Typical thickening settling tanks are batch fed, uncoveredand require some sort of mechanical removal after the settling phase, also the
fresh blackwater will probably lead to considerable biogasproduction which will reduce the settling and lead to nuisance because of smells. Based on the investigations mentioned above and work done on biogas settlers in Zambia
(Laramee (2017). Assessing lifecycle costs and benefits of integrated sanitation andenergy recovery strategies in low-income countries . PhD Thesis. Stanford University. searchworks.stanford.edu ) with black water as input you can expect a COD reduction of 60 to 70% with a biogas settler.
Also consider that disinfection agents for toilet cleaning will reduce anaerobic activity.

All the best, Nico

Dear Nicolas and all,

Would settling thickening tanks be a good pre-treatment option?

What are your views?

Nirupama

Dear Mr. Nicolas,

Thank you so much for your response. I had already been going through the wonderful work on your PhD Thesis (as Mr. Taylar pointed me towards), and it has given extremely valuable inputs.  From your inputs and through a conversation with a friend who worked with CDD Society, Bangalore, I understand that a pre-treatment is necessary, since the waste from toilets is highly concentrated blackwater with high solids content, as you rightly pointed out. I will consider using a biogas digester as you suggested.

The toilets that the system is being designed for are 'eToilets' (by Eram Scientific Solutions Pvt Ltd., Trivandrum, Kerala), which are fully automated, including automatic flush and platform rinse features. You can find more details  here  . Hence there are no concerns regarding the availability of water in the cubicles. 

I'm sure further reading of your work is going to prove very important for my project. Thanks again for your inputs. 

Warm regards,
Nirupama  

Dear Nirupama,

in terms ofhydraulic load ABRs are well suited to treat wastewater flows way above your flows, in literature ABRs are said to be viable for up to 1.000 m3/d and larger applications have been mentioned on this forum
(forum.susana.org/anaerobic-baffled-react...water-since-may-2013).

About sludge accumulation: our research indicates rates of 3 to 10 L per cap per year
- however these were measurements on ABRs treating domestic wastewater pretreated
with either settlers or biogas digesters which leads to recommended desludging
intervals of 3 to 4 years of the rear chambers – you can find details here
Reynaud, N.& Buckley, C., 2015 Field-data on parameters relevant for design, operation
and monitoring of communal decentralized wastewater treatment systems (DEWATS),
Water Practice & Technology Vol 10 No 4

You should definitely include a pretreatment in your design, especially since your wastewater
is mainly high concentrated blackwater with high solid content which would quickly
lead to blockages if directly sent to an ABR – consider using a biogas digester
to make use of the energy with HRT of 24 h + (the Sasse book on DEWATS contains
guidance on how to design). That way you catch most solids in your pretreatment
where it stabilises and less solids accumulate in the ABR where further solids
are retained and the soluble fraction of your organics are dealt with by the anaerobic
microorganisms.

Consider the use of an anaerobic filter after the ABR and polishing with wetlands and ponds
Also makesure in your design that water is available within the cubicles, I have seen cases
in India where users of public toilets were supposed to carry flush water from
a central tab which did not happen (enough) so that pipes and system blocked quickly

I hope thishelps – all the best for your project! Nico

Dear Prof. Ajit,

Thank you so much for your detailed notes. With the insights provided by you and Mr. Taylar, I understand that a 1 year desludging period maybe too long, and at the same time, monthly desludging may be impractical and may even disturb the biological processes in the reactor. I can base my design on the valuable information that you have provided in this thread. 

I agree with your observation regarding the appropriateness of the DEWATS system in a developing country like ours. I hope to be able to master the design of many of these systems, incorporating practical, context-specific aspects and ensuring its sustained use. The forum has been extremely important and insightful for me in this regard.

Thanks again for you response.

Warm regards,
Nirupama Nair

Dear Ms. Nirupama Nair and others,

We note that you are working on the design of Anaerobic Baffled Reactor- ABRs for flows such as 10m3/day upto 50m3/day for public
toilets.-  the basis of the tank sizesand the suitability of locating them, tanks inter spaced within the Toilets facility- could be a Community Toilet Complex- CTC or a collection of individual toilets made with needed fixtures Etc., done at bus stops, stations and road junctions at the outer peripheries of the city or toilets in plotted
and flatted houses.                                                                                      
We will now discuss on the apt size selection of sewage holding tanks --  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 long period. Annual de-sludging frequency is a bit long, can be suited for those toilet complexes having minimal flush water and water used for self-cleaning and toilet cleaning only. Toilet- pan’s drain to pits is done a steeper gradient, and water used for clean up after each use too is minimal- 2 litres per use.
Mostly done in manual mode, upon sensing the septic- tank is full.

The success of the sanitation model is the one that totally utilises all the wastes- solids  incl liquid and gaseous emissions
ie solids on re use as compost/ manure, recycled water for secondary uses viz, flush- toilets, irrigating gardens. Gases released in the ABRs are useful energy potential.
                                                                                                                                 
Solids realized after each usage would be in the range 100g to 300g per person per day, its volumes to be 50ml to 150ml per person per day. Sg is considered as 1.5 – 3.0,  avrg- 2.0.                                                                                                                                 
Water discharges are in variance ie from 2 lit to 20 lit, per person perday, Gases emanating are Bio-gas mainly methane with others.
When the water usage is minimal, ideally the ABRs would function as Bio-digesters,
Now coming to the design of the FS holding/ septic tank and its capacity, it can be arrived at dependent on the flush water being released.

A  House hold avrg 4 px, say 10 HH in a flatted complex has 40 px, times    5 lit per day per person (1 litre urine included)  amounts to 200 litres per day.  Hence for 30 days it is 6000 lits / 6 kl.  Annually (usually not considered) is 72 kl./ cbm.
Other wash waters from bathing, washing, kitchen may be 50 to 100 litres per day per person, is not included.
High-end HHs would consume higher volumes.

It is evident that if water usage is less, it is ideal for use on Bio Gas Plants-BGPs (BOD as high as 1000- 2000ppm), on higher use of water
(BOD is low 300- 500 ppm, or lower ) can be used as recycled water after a STP process,

For your query you have asked,  more info. may be sourced in any reference book, pl build your specific fundamentals based on the above notings given. 
Also I note that Mr. Kevin Tayler Sir has also responded, and has referred to the ever famed Herr. Pedro Kramer of BORDA and CDD,

Let me go thro that response, and  will supplement my response. I have been for many years, disciples of BORDA, DEWATS System, and have profound respect for the DEWATS Systems used. Would feel that they are the most ideal for a developing country such as ours.
  
Well wishes,         

Dear Mr. Kevin,

Thank you so much for your response. It was extremely insightful. I had previously not given much thought into the fact that monthly desuldging would probably disturb the biological processes in the reactor, and that it was probably not necessary as well. As a budding Sanitary Engineer, these insights are extremely valuable for me. I will definitely try to gather more details from the resources that you have pointed me towards.  When I find more practical insights into the desludging requirements of an ABR system treating waste from public toilets, and hence the effect it has on the reactor size, through these resources and through my own experiments when I am able to do them, I will share them through this thread. I will also try to get in touch with CDD, as you suggested. 

Thanking you once again for response. 

Warm regards,
Nirupama