Conversion of Leaching Pits into Septic tanks


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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
  • -      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. [font=Arial, sans-serif]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.[/font]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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S: j.l.hass
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  • Mayor610
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  • Water Supply & Sanitation Specialist
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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.


Mayor Kumar
Water Supply & Sanitation Specialist
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