CW to treat septic tank effluent for a school in Botswana

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  • Agas
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Re: CW to treat septic tank effluent for a school in Botswana

Dear Detlef and others who have contributed to this thread.
Sorry for the long silence and thanks for the suggestions and advice.
The project requires an EIA (Environmental Impact Assessment) before we can proceed with the next step which is detailed design.
The EIA has been underway since August last year and we hope it will be completed in the next 4-6 weeks. However, it is hard to predict, and they can drag on for a long time. We have an environmental engineer who is helping us with the design, and we are considering the different suggestions that were raised here. I hope that we can respond with more details soon.

best regards,
Agas
Agas Groth
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Re: CW to treat septic tank effluent for a school in Botswana

Dear Agas,

What is your progress on your communal wwtp (CW)?

Best Regards,

Detlef
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  • AjitSeshadri
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  • Marine Chief Engineer by profession (1971- present) and at present Faculty in Marine Engg. Deptt. Vels University, Chennai, India. Also proficient in giving Environmental solutions , Designation- Prof. Ajit Seshadri, Head- Environment, The Vigyan Vijay Foundation, NGO, New Delhi, INDIA , Consultant located at present at Chennai, India
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Re: Reply: CW to treat septic tank effluent for a school in Botswana

Dear JF and LetLef,
Nice info on CWs and Dewats - indeed an apt combo.
For all physical remedials settlers- anaerobic tanks with or without filters are used.
Can be followed by more ultra- fine filtration done.
We used next CWs in float form in 3 phases
Initial process in CWs done till fairly compliant effluent is reached say BOD5 @ 30ppm
In Final stage, CWs in Floats use Elphnt grass sp. grown, cropped for fodder use Etc
All P, C and micro-bio parameters in limit and compliant per PB standard s Then recycled water taken for secondary water uses in complex- flush water for toilets, garden irrigation Etc.
As all are NbSs, no handles, just visit once 6mths Etc

Well wishes for prospects 
Prof AjitSeshadri, 
Vels University
Chennai India 
 
Prof. Ajit Seshadri, Faculty in Marine Engg. Deptt. Vels University, and
Head-Environment , VigyanVijay Foundation, Consultant (Water shed Mngmnt, WWT, WASH, others)Located at present at Chennai, India

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  • Juan Francisco De León Ibarra, Master en Ingeniería Ambiental con especialidad en Tratamiento de aguas residuales, estudió Ingeniería Civil en la Facultad de Ingeniería de la Universidad Nacional Autónoma de México (UNAM). Durante 20 años ha dedicado su labor profesional a trabajar en pro del acceso a los servicios de agua y saneamiento en comunida
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Re: Reply: CW to treat septic tank effluent for a school in Botswana

Dear Letlef.

In my experience implementing CW  (20 years) the sludge quantity for schools,restaurants or hauseholds are always different!!.. So I only use literature standards as reference.

 I really recommend   to measure the flow and the waste water quality ( Q= ?? L/d, COD =  ? = mg/L , BOD5 = mg/L) ... 


For your reference I make fast calculations for your Settler/ septic tank using the DEWATS spreadsheet..

Best regardsJFrancisco
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Re: Reply: CW to treat septic tank effluent for a school in Botswana

Dear Juan,

Thanks for your suggestion.

Just to have a better understanding about the sludge volumes involved in this possible septic tank-sludge reuse example:

I would start with ~50 ml (settled dry matter) per p.e. and day (50 ml/p.e./d) = ~250 p.e. x ~50 ml/p.e./d = ~12,500 ml/d (~12.5 l/d) x 365 days/a = ~4,562 l/d (~4.5 m3/a for ~250 p.e.). 

Experiences and University studies (BSc, MSc. & PhD-work) found out: After resting/settling sludge more them ~5-6 years in septic tanks ~1/3 of settled sludge volumes "disappears" by internal anaerobic processes over the years = ~3 m3/a remain under "could" climatic condition. Maybe less under "warm" climatic condition!?

If all septic tanks gets dislodged every ~6 years by ~2/3 (~2 m3) of settled sludge (keeping ~1/3 (~1 m3) sludge volume in order to keep biological processes in septic tank in takt).

 ~6 years x ~2 m3/a = ~12 m3 settled (dry) sludge could be processed in a humification plant every ~6 years.

Just guessing, pumped out with water content (steering the settled sludge with water to be able to pump out) , we might talk about ~25 m3 watery sludge every ~6 years , or more.

Would you follow this simple sludge calculation?

All the Best
Detlef 
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  • Juan Francisco De León Ibarra, Master en Ingeniería Ambiental con especialidad en Tratamiento de aguas residuales, estudió Ingeniería Civil en la Facultad de Ingeniería de la Universidad Nacional Autónoma de México (UNAM). Durante 20 años ha dedicado su labor profesional a trabajar en pro del acceso a los servicios de agua y saneamiento en comunida
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Re: Reply: CW to treat septic tank effluent for a school in Botswana

I have taken note of the valuable points raised by my colleagues regarding to the constructed wetlands, and I concur with their perspectives. However, I would like to draw attention to an aspect that seems to have been overlooked: the collection and treatment system for the Fecal Sludge in all 30 septic tanks. In light of this, my contribution to the ongoing discussion is the suggestion that you take it into consideration.In areas with arid weather conditions, I recommend employing a Sludge drying bed in conjunction with co-composting of the kitchen biowaste. Conversely, in tropical environments, I propose considering the utilization of Sludge drying reed beds while recirculating the lixiviate back into the first CW.


Please accept my best regards.
JFrancisco

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Re: Reply: CW to treat septic tank effluent for a school in Botswana

Dear Euiso and dear Ajit & other experts in the field,

May you and others kindly add as experts in the field your second or probably third practical opinion/advice/know-how to the advantages of Agas's future project + others & my self could learn from this constructive "Open Source" exchange too, by not having a competition on the subject.  Keep in mind Agas and many others are not practical experts in the field, so you may use therefore "plain" English ;-).

All the Best

Detlef Schwager
Dipl.-Ing.(FH) & MSc. Tropical Water-Engineering
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  • AjitSeshadri
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  • Marine Chief Engineer by profession (1971- present) and at present Faculty in Marine Engg. Deptt. Vels University, Chennai, India. Also proficient in giving Environmental solutions , Designation- Prof. Ajit Seshadri, Head- Environment, The Vigyan Vijay Foundation, NGO, New Delhi, INDIA , Consultant located at present at Chennai, India
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Re: Reply: CW to treat septic tank effluent for a school in Botswana

Dear Researcher Agas:
Can ideally go for Dewats type design to remedy Sewage water and take for reuse also.
Both processed water, sludge and plant foliage from CWs can be reused by communities  for monetary good.

With best wishes, do well
Prof Ajit Seshadri 
The Vigyan Vijay Foundation 
Prof. Ajit Seshadri, Faculty in Marine Engg. Deptt. Vels University, and
Head-Environment , VigyanVijay Foundation, Consultant (Water shed Mngmnt, WWT, WASH, others)Located at present at Chennai, India

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  • Agas
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Re: Reply: CW to treat septic tank effluent for a school in Botswana

Dear Professor Choi,
Thank you for the advice. We will take this into consideration as we plan our project.
best regards,
Agas
Agas Groth
Director, Camphill Community Trust, Botswana
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Re: CW to treat septic tank effluent for a school in Botswana

Dear Agas,

Thanks for the details.

My very practical approach towards CW-WWTP design is always very conservative, not trying to “save” every little m2, m3 and $...

Your ~30 m3/d for ~250 p.e. is at a “communal level” for a CW-WWTP!
 
By all means, your main emphasis should be the protection of your drinking water sources, the boreholes against this large amount of WWTP effluent.  Kindly add the locations of the boreholes to your map.

Keep away this later punctual distributed high amount of effluent from your boreholes, please far as possible! Do not take this "stuff" too lightly in the longer term, I mean e.g. pathogenic E-coli (Escherichia coli)! Do you have any treatment in place for the raw borehole water? Do you have any information about the hydrogeological underground condition of your boreholes and your general area? Groundwater flow direction?  Groundwater flow direction do not always following the surface water flow direction...

Seeing the contour lines and location of your future CW-WWTP, unfortunately it will be very unlikely to fed your future CW-WWTP by gravity.  A simple gravity approach with less technical components is very unlikely. You will need at least 2 submerged wastewater lifting pumps, who are running alternating automatically.  

Having pumps enables you to use the more efficient vertical flow CW (good aerobic conditions), them horizontal flow CW (more anaerobic & only partly aerobic condition) to have an CW-WWTP effluent with a good max. 100 mg/l COD or 25 mg/l BOD.

“My” vertical flow CW’s make 10-40 mg/l COD under much colder climatic condition by quality planning and construction with permanent and hard quality controls, using 4 m2/p.e..

2 parallel installed vertical flow CW beds with total very conservative ~3 m2/ p.e. should be installed. Total ~750 m2, each bed ~ 380 m2. The beds should be fed by alternation every 7 days. This means always one bed should stay dry 7 days.  This bed alternation is typical for a communal level CW-WWTP.  Following my experiences the bed "in charge" should get only small ~5 mm for pre-treated effluent (by septic tank and others) distribution not heavy 20 mm distribution as stated in standards. (mm = l/m2). In practical terms ~380 m2 x 5 l/m2 = ~1,900 l (~2 m3) per distribution of pre-treated effluent to one bed.   Any permanent effluent distribution to a CW bed is ok for small private level (scale) CW's only.  For a communal WWTP scale you have to use industrial approaches and equipment quality. NO DIY-standard!!!

Probably you need a receiving tank for the treated effluent too, to be able to pump your final effluent to every direction in need and far away from your boreholes.

I was in hope south of your boundary the waste land is yours too. Is there any possibility to make probably use for fire wood production or just percolating there the effluent of the CW-WWTP?

I hope it is not too disappointing for you, as you where in hope for a much simpler gravity fed horizontal flow CW?!

Waiting for your reply to take it further.

Detlef


PS: What are the general conditions around this ~30 septic tank with soak pits or pipe drainage? Any messy puddles? Any disturbance by this septic tanks towards any borehole? Are you forced by authority to take your waste water treatment to a much higher level or is it "just your hobby" ;-)?
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  • Agas
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Re: CW to treat septic tank effluent for a school in Botswana

Dear Detlef,

Thanks for your post with some detailed questions.
I will try to answer them as best I can.

- I am just guessing, you have ~30 already existing septic tanks, right?
Yes, they are all existing, many quite old.

- Are you in a sloppy area? I found your x,y-geolocation, so I could determine your existing slope by my self. I made a contour line map with   contourmapcreator.urgr8.ch/
Yes, as you see the slope is from NW to SE and about perfect for gravity flow.
The treatment works will be located in the SE corner of our plot. 

- Please mark your ~30 septic tanks, kitchen and your original plan on the provided "map" and send it back. Your CW's will be in the southern part of your large property by gravity fed?
I have attached a sketch plan showing the pipelines to collect the effluent and the location of the CWs.
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- Any wells for human consumption in the South? 
We have five boreholes, the nearest is approx. 30m from the location for the CWs. The Static Water Level is approx. 20-30m below ground level.

- "using two horizontal flow reed beds" in line or parallel? 
In parallel. The intention is that when they need cleaning, one can continue to be used while one is cleaned.

- How many people you like to serve with the project? Always 100% during the year? Average water consumption per day and per year?
see attached file
  These are the design figures. Actual are considerably lower, but allowing for increase in numbers. The number of people living on site is much less during holidays which are approx. one month, three times a year.

- Any kitchen or other difficult effluents included?
No, grey water is mostly disposed of in mulch basins (based on OasisDesign.net)

- Do you like to make any use of the treated effluent? Keep in mind planted CW in your hot dry climate "loose" via plants evaporation nearly all the water, the effluent output is very small or even nil.
Yes, we will use it to irrigate citrus orchards, but it will supplement borehole water, so losses are not critical.

- How you or your suppliers could make sure the gravel and sand are very clean washed and have the right grain sizes (good grading curve)?
We can probably mix purchased sieved gravel with river sand to achieve the required grading.

Many thanks,
Agas

 
Agas Groth
Director, Camphill Community Trust, Botswana
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Re: CW to treat septic tank effluent for a school in Botswana

Dear Agas,

Easter-Greetings from Zanzibar.

Let me give you just my practical experiences without starting a deep theoretical discussions.  Please provide some basic technical information about your project, in order to do so.

- I am just guessing, you have ~30 already existing septic tanks, right?

- Are you in a sloppy area? I found your x,y-geolocation, so I could determine your existing slope by my self. I made a contour line map with  contourmapcreator.urgr8.ch/

- Please mark your ~30 septic tanks, kitchen and your original plan on the provided "map" and send it back. Your CW's will be in the southern part of your large property by gravity fed?

- Any wells for human consumption in the South? 

- "using two horizontal flow reed beds" in line or parallel? 

- How many people you like to serve with the project? Always 100% during the year? Average water consumption per day and per year?

- Any kitchen or other difficult effluents included?

- Do you like to make any use of the treated effluent? Keep in mind planted CW in your hot dry climate "loose" via plants evaporation nearly all the water, the effluent output is very small or even nil.

- How you or your suppliers could make sure the gravel and sand are very clean washed and have the right grain sizes (good grading curve)?  

Until I receive your answers please see in the meantime at www.aqua-verde.de many pictures about CW, ABR and ponds constructions and design considerations (use auto translation for the German language texts). "Pictures say more them 1,000 words"...

Keep all as simple as possible and with some economical long term benefit!

See you again.

All the Best

Detlef
 

PS: Do not become a "slave" of your future system... ;-)
www.aqua-verde.de, AquaVerde Ltd. Zanzibar
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