Conversion of Leaching Pits into Septic tanks

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  • Mayor610
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Re: Conversion of Leaching Pits into Septic tanks

Hi ! Jill Hass,


Thanks for your detailed input as regards the small bore system. We have yet to make up our mind whether to adopt SBS.

Regards,

Mayor Kumar
Water Supply & Sanitation Specialist

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  • JillHass
  • Water & sanitation expert, with 30+ years experience in construction design & implementation management, and specialized in emergency response due to climate change or conflict/violent settings. Previously CTO for technology firm (patents, product development & commercialization); Consulting Engineer in asset management of municipal infrastructure
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Re: Conversion of Leaching Pits into Septic tanks

Great points, all of them.

I completely agree that nomenclature is critical so that weare in alignment with what is being discussed and we are sharing ideas
accurately describing the same system; however first I want to clarify that
small bore (simplified) sewerage networks function without flush toilets. I have designed and installed large systems in multiplecountries where squat toilets are employed -- the water for anal washing, urine
and sink drainage are sufficient to convey solids through household plumbing to
the communal tanks. In an installation in Jordan, over 16,000 households were
connected to a small bore/effluent system and almost none had flush toilets; in
fact most households connected their former pit latrines to the network.

Small bore systems (aka: effluent sewers) can be connected toany treatment system, or connected to a subsurface discharge field for effluent
disposal. In humanitarian emergencies, although not ideal, having the effluent
discharged into the open environment far away from human settlements can protect
public hygiene and reduce the exposure to waterborne diseases. Many different terms exist, some erroneouslyrepeated on the internet. I would define the following:
  • -      Septic tanks are primary treatment vessels, oftenemployed without actual connection to a subsurface discharge field, as with the
    small bore (simplified) sewerage systems. For this reason, for effluent sewers I
    prefer the terms to call the tank that knocks out solids as: interceptor tank,
    settling tank, or primary treatment tank. A septic tank would be defined
    correctly if it is indeed connected to a downstream subsurface discharge field
    within one property lot.
  • -      “Effluent” is the liquid fraction of wastewater(supernatant), typically decanted from sewage in settling tanks. This
    supernatant contains fecal waste and should be handled as contaminated liquid;
    however effluent has only small sized solids and can be conveyed using shallow
    sloped piping because steep slopes to create high scouring velocities to “float”
    larger solids downstream does not apply. Effluent can be conveyed within nearly
    flat graded systems without blockages from which gravity systems suffer.
  • -      *Effluent is often erroneously referred to as solelythe discharge from wastewater treatment plants with secondary or tertiary
    treatment methods; however septic tanks provide settlement for primary
    treatment and the discharge is also somewhat treated effluent.
  • -      “Blackwater” is any sewage that contains toiletwaste that includes urine and feces.
  • -      “Greywater”is sink, bathing/shower and clothes washing spent water. Greywater can be segregated
    from blackwater within household plumbing when different drainage piping from
    different sources of spent water (toilet, sink, etc.) are separated, but once
    mixed together, all wastewater containing any toilet waste contamination is
    considered blackwater
  • -      Greywateris often erroneously viewed as innocuous, but greywater can turn septic in
    certain conditions within 12-24 hours; as such, it should be handled as
    contaminated water. Greywater can also contain fecal contamination from bathing
    babies in sinks, diaper cleaning and soiled clothes washing.
  • -      ** however greywater is often erroneously labeled as the supernatant (liquidfraction) of mixed sewage/blackwater which creates confusion when greywater
    recycling/reuse is proposed since supernatant/effluent contains fecal
    contamination.
  • -      Effluentsewers are also called: alternative sewers, simplified sewers, small bore
    sewerage networks, alternative sewers (too generic since there are 4 types),
    small diameter variable gradient or SDVG, septic tank effluent gravity or STEG
    systems, etc.
All properlyengineered effluent sewer systems will preserve groundwater protection and
prevent contamination with water bodies. In effluentsewer systems with treatment plants included in the downstream assembly before
discharge, the local environment is better protected for several reasons. The most significant is because the deep trenching
for traditional gravity sewers create underground conduits for stormwater
transfer that can surcharge a treatment plant during heavy rains.
You are correctthat desludging of interceptor tanks is required; however the anaerobic
digestion that occurs over long periods of time in these tanks reduces the
overall sludge volume. Compared to the very short digestion time in most centralized
treatment facilities (2h to 30d) and the desludging requirements for holding
tanks (2d to 7d), the removal of preconditioned sludge from the effluent sewers’
interceptor tanks can be scheduled every 120d to 365d, depending on the sizing
of the tanks. However, the overall cumulative sludge volume will be less from the
effluent sewers’ interceptor tanks because there is sufficient time and space
needed to optimize anaerobic digestion and turn sludge into biogas in a
decentralized effluent system.I haveconnected communities that use flush toilets and pit latrines to effluent sewer
systems and they work very well, are economical to install and have lower
operating costs and maintenance demands than other systems. In most regions,
effluent sewers work extremely well and are more appropriate than conventional
gravity sewers. 
Global Technical Director, WASH
Relief International
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M: +1 (647) 268-4620
S: j.l.hass

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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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  • 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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  • 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
Water Supply & Sanitation Specialist

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  • JillHass
  • Water & sanitation expert, with 30+ years experience in construction design & implementation management, and specialized in emergency response due to climate change or conflict/violent settings. Previously CTO for technology firm (patents, product development & commercialization); Consulting Engineer in asset management of municipal infrastructure
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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.
Global Technical Director, WASH
Relief International
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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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  • 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
Water Supply & Sanitation Specialist

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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 
Water Supply & Sanitation Specialist

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Re: Conversion of Leaching Pits into Septic tanks

Hi ! Kris.

I agree with you.


Best regards,

Mayor
Water Supply & Sanitation Specialist

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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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  • 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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