Conversion of Leaching Pits into Septic tanks

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  • Mayor610
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Conversion of Leaching Pits into Septic tanks

Dear all,

We as Detailed Engineering Design consultants in Eastern Asian  a low income group country have been assigned the task for converting existing leeching pits at household (HH) level into septic tank. 
As we all know there are two constraints at HH level, finance and the availability of space for locating septic tank. Anyone having experience of undertaking this gigantic task at a District level by using PPP Model would do us a great favour by sharing his /her valuable experience.

Thanks & regards,

Mayor Kumar
Water Supply & Sanitation Specialist
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  • paresh
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  • Budding WASH researcher, especially interested in governance, public policy, finance, politics and social justice. Architect, Urban & Regional planner by training, Ex. C-WAS, India. I am a patient person :)
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Re: Conversion of Leaching Pits into Septic tanks

Hi Mayor.
Curious about the need to replace pits with septic tanks on a large scale.  Sharing experience from identifying pits in Alleppey, Kerala. We are yet to get them replaced.  

In small towns and villages, terms are often used interchangeably, that is all type of  on-site systems are referred to as septic tanks.  We found the same in Alleppey during our interactions with masons and contractors. We then conducted a household survey and found that most households have a pit. This was followed by a more detailed survey of identifying households that had space for a new OSS. This exercise needed a layout plan for each plot that needed a replacement. 

Interestingly, we did all this by engaging university students in summer and winter schools (reports available here ). They were involved in household surveys and then identifying locations where new OSS could be built for each of the plots. The Government has in-principle agreed to finance such replacement for a few hundred households along the canals based on certain criteria like plot size, income, etc. But as I said earlier, we are yet to implement it. 

Curious to know if this confusion in terminology used is also prevalent where you are working and if yes, how do you plan to distinguish pit from septic tanks.

Regards
paresh
Paresh Chhajed-Picha
Researcher at Indian Institute of Technology - Bombay, India
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  • raogk
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Re: Conversion of Leaching Pits into Septic tanks

Dear Manoj ji

Perhaps conversion of all leach pits into septic tanks may not be viable due to spatial constraints. This is because, if you want to convert the leach pits into septic tanks, you need the effluent to be treated either through a soakage pit or drainfield trenches for further aerobic digestion of the organic matter and nitrification. This is because the septic tank provides only primary treatment to the wastewater wherein the BOD and TSS levels are reduced only by about 40%-60% of those in the influent. Hence, you either have to go for biodigester toilets, or have space for construction of a soak pit (of course you have options for modified soakage pit with some vegetation doing some goo work reg. nutrient removal) or for dispersion of the effluent and further treatment through drainfield trenches. Since the effluent has to percolate through the drainfield and the mechanism involves filtration, adsorption and biological oxidation, the soil should have adequate permeability to allow percolation for even the drain fields to succeed. Hence a one solution fits for all may not hold good in this context. There is always the possibility of contamination of drinking water sources if they are nearby. 

Hence in my view, a biodigestr toilet may be the viable alternative both in terms of capital and maintenance aspects.

Hope this will help your exploration.

Warm regards
G Kondala Rao
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  • JKMakowka
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Re: Conversion of Leaching Pits into Septic tanks

In one of the projects I previously worked in we converted some existing old leaching pits (after emptying and "guestimating" remaining infiltration capacity) into soak-pits for newly installed (plastic) septic-tanks. This was mainly done due to space constraints and not an ideal solution I think.

Trying to seal the bottom of a previously used leaching pit to work as septic tank chamber is probably too complicated to be worth the effort, but maybe it could be done with a plastic membrane insert?
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  • Mayor610
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Re: Conversion of Leaching Pits into Septic tanks

Hi ! Kris.

I agree with you.


Best regards,

Mayor
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  • Mayor610
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Re: Conversion of Leaching Pits into Septic tanks

Hi ! Mr Rao,

Are bio- digester toilets really successful in the field where one can utilise the generated.bio-gas.

With regards,

Mayor Kumar 
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  • Mayor610
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Re: Conversion of Leaching Pits into Septic tanks

Hi ! Paresh,

Here we have the advantage in the sense that the Master Plan stipulates that it is the leaching pits that  are existing on ground.

With regards,

Mayor
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  • raogk
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Re: Conversion of Leaching Pits into Septic tanks

Dear Manoj ji
May not be feasible at individual level, but may be considered at large scale (aggregated level).

Warm regards
G Kondala Rao
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  • JillHass
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Re: Conversion of Leaching Pits into Septic tanks

Dear Mayor Kumar,

Good to hear that you are converting unlined pits into septic tanks. The additional primary treatment will support the protection of groundwater and public hygiene. Your greatest challenge with be concerning what to do with the enormous amount of liquid generated and requires disposal, or ideally reuse.

There was a similar project which I designed and installed in Jordan at a Syrian refugee camp where septic tanks were installed to replace illegal pits but no disposal fields were installed -- instead the liquid portion of the sewage flowed by gravity to larger collection tanks (Phase I) and then to a wastewater treatment plant (Phase II).

For your situation, these septic (really interceptor) tanks can be installed near households and collect raw sewage from single households or multiple households; the preference is to have sewage collected as near to the source as possible so that the tanks are not too deep under ground. However, in lieu of installing disposal tanks near each HH or cluster of HHs, I would recommend effluent sewers (small diameter variable sewers or SDVS, or small bore sewers, etc.) to collect the liquid fraction of the sewage and have disposal of this liquid in one area away from the community -- the required space for each septic tank to have a discharge field would require considerable space and the maintenance for individual systems can be a burden to individual households whereas having a communal discharge area allows for smaller space (when compared to cumulative area of numerous HH systems) and for maintenance workers to access easily. The liquid waste is high in nitrogen that farmers can reuse this nutrient-rich liquid as fertilizer.

These effluent systems can be financially subsidized by government funding, and/or financially supported by the HHs using them as they are cost effective. They are appealing to funding bodies since there is municipal public works being installed to demonstrate long term infrastructure solutions; in fact the first small bore sewer was installed through funds from the World Bank
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. The capital costs for installation are typically lower than with traditional gravity sewer systems and the operation/maintenance costs are considerably lower.

Happy to discuss further with you if you need support.

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  • Mayor610
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Re: Conversion of Leaching Pits into Septic tanks

Hi ! Jill Hass,

Thanks a ton for sharing valuable to the point information I was looking for. Let me discuss with my colleague.

With regards,

Mayor Kumar
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  • drtkdas47
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Re: Conversion of Leaching Pits into Septic tanks

If the  water table is. Lesser than 2 meter ,then it is advisable to convert /retrofit Single Leach Pit  and Septic  Tank toilets (when tank is away from toilet structure )to Twin Leach Pit Toilets .When one pit is filled up ,other pit is used and filled up pit is. Left undisturbed for 1-1.5 years ,by which time the fecal sludge gets decomposed to compost .Hence ,in twin leach pit toilet there is no need to lift un-decomposed  fecal sludge ,when filled up.
In Rajasthan of India pilot initiative has been taken up and 900 engineers have been trained jointly by UNICEF and SBM(G)Directorate .
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  • goeco
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Re: Conversion of Leaching Pits into Septic tanks

There are key variables that should be clearly defined for this discussion. How do we define "sewage" and "effluent"? Household wastewater from a plumbed household, with a flush toilet?
Pits are generally for non-flush sewage (urine and feces) disposal and they generate sludge which requires removal at a cost.
Septic tanks are generally for flush toilets and household wastewater. Septic tanks really only remove the solids from the wastewater flow, so effluent volume is the same as the influent and requires disposal. They also generate sludge which requires removal at a cost.
Small-bore (simplified) sewers require water as the carrier for household wastewater/sewage/effluent. They then require centralised treatment and disposal (to either waterways or to land).
What system are we discussing here please? What is the driver for the conversion? Are households being transformed to plumbed water supply and flush toilets? Are these households with plots of land? Can I safely assume that wherever the system is, it must mitigate contaminant leaching into groundwater or water bodies?
Dean Satchell, M For. Sc.
Vermifilter.com
www.vermifilter.com
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