Regular septic tanks vs Biodigester septic tanks

  • pkjha
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  • Working for over 30 years in the fields of sanitation, biogas from human wastes, septage management, waste water treatment in rural as well as urban areas in India and other developing countries.
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Re: Regular septic tanks vs Biodigester septic tanks

Dear Kevin and All
In my earlier posts on the topic, I raised similar queries and sought chemical and bacteriological analyses reports of treated effluent, but could not get. Earlier reports from the IIT (India Institute of Technology), Chennai on the performance of such toilets mentioned effluent quality not better than sewage. It was discussed at length. Treatment efficiency needs to be supported with design. As you pointed out there is no innovative designs to support better treatment.

Pawan Jha
Foundation for Environment and Sanitation
Mahavir Enclave
New Delhi 110045, India
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  • hoffma
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Re: Regular septic tanks vs Biodigester septic tanks

Hi James,

I had similar doubts when „self-cleaning-bio-digester“ for onsite treatment came to Latin American market, it was 10 years ago. Here, Biodigester are offered in different sizes, adapted to onsite RAW WASTEWATER treatment of 1 - 10 households (not only one latrine). Based on these experiences I can confirm that you are totally right with all your questions:

Biogas: as Tore already pointed out, this technology only applies to very small wastewater volumes with low methane building capacity. Without a doubt, that is exactly the same process as in Septic Tank (anaerobic or septic digestion) and important to note too, that Biogas production also depends on efficiency of the degradation process; higher efficiency means more liberation of methane.
Sludge: Raw wastewater treatment means solid separation and sludge retention, followed, in this case, by anaerobic degradation (sludge stabilization). So, “never need to exhaust” just means that solids were flushed out together with the effluent. It can be caused by hydraulic problems but also because it was never emptied.
Effluent: Septic Tanks remove 30-40% of BOD load; in this case higher enviromental temperatures have only a low influence, because in warm climates septic tanks are usually much smaller. In addition, the efficiency of anaerobic process is naturally limited; even more sophisticate anaerobic technologies will hardly remove more than 60-70% BOD (domestic wastewater). Therefore, there is necessarily still a relativly high concentration of organic matter in the effluent of EACH anaerobic treatment and it is important to note too, that these effluents have a blackish color and smell (H2S, NH3) or, if not, the treatment process probably did not work.

Pre-fabrication of onsite treatment tanks is an important market, but was happened in practice is, that the necessity to reduce costs, combined with poor regulation and lack of experience with the peculiarities of onsite sanitation often result in far too small treatment volumes. To compare: A Septic Tank for a household in cold climate (Germany) needs at least (4 pe) 6.000 L volume. In warm climate (Brasil) the maximum for a household (5 pe) is about 1.200 L and for the same situation the “bio-digester” is offered (Peru) with less than 600 L effective Volume. So, peak loads can hardly be absorbed. Totally agree with Pawan: efficiency needs design and biological treatment does not require the addition of effective bacteria, but the conditions that natural wastewater bacteria can work.

So, is a Septic-Tank-Digester still better than nothing? I would say that depends on the conditions, it may even be an appropriate solution if:
- if it fulfill common key design parameter for anaerobic treatment of raw domestic wastewater as: adequate sludge settling volume and hydraulic retention time and consideration of specific hydraulic load, peak load and climate condition.
- if onsite discharge (soil infiltration) of anaerobically treated effluent is possible without surface run off (risk of contact) and without risk for groundwater, walls etc. Otherwise, or in case of reuse, anaerobic effluent needs secondary treatment.
- if periodical emptying of sludge is possible, what means: appropriate access to sludge settling zone (preventing risk of contact) and “acceptable low risk” concerning finally disposal of treated sludge (after onsite drying or transport to central treatment).

Above all, of course, benefits depend on adequate training and awareness raising of user, especially if service is not available and user will be responsible for operation of the Septic-Tank-Digester.

I hope these experiences are helpful. Regards, Heike
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  • zigazie
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  • Holder of a Bsc in health promotion, Masters in public health- health promotion and Bill and Melinda Gates Foundation Scholar studying Msc in sanitation at IHE Delft Netherlands till April 2019
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Re: Regular septic tanks vs Biodigester septic tanks

Dear Dr Ashok

Thank you for your information on bio latrines. I am also interested in getting the designs you have been using. I am however keen to learn more about the quality of the effluent which you said goes into the drains. How safe is the effluent. Yes you mentioned that no smell no mosquitoes but what about the pathogens and BOD. I will be happy to hear from you as an expert because i have seen that most systems e.g in Bangladesh they also discharge the effluent into the drains straight from the septic tanks.

Ziggy Kugedera is a Holder of a Bsc in health pr4omotion, Masters in public health- health promotion and Bill and Melinda Gates Foundation Scholar studying Msc in sanitation at IHE Delft Netherlands till April 2019. Have 14 years experience in SBC, WASH and Public Health
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  • AjitSeshadri
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Re: Regular septic tanks vs Biodigester septic tanks

Dear SuSanA Members.

We definitely need to innovate and apply natural principles to sustainable practices.

When an initiative starts to perform to communities, its like a life cycle of a new born baby.. to child.. then an adult.. etc.
Just like prudent parents, a child is nurtured and guided..a wwt digester plant too follows a path.. till it gets a sustained status.. then this plant's design is propagated with add-on features.

Only when the wastes are converted to resources eg re use water, compost, bio gas Etc, a confidence is built and communities start to experience a " feel good factor ".

We can propagate role models - growing elephant grass, harvesting them for uses viz. feed for cattle, poultry, fish feed Etc. for commercial consideration..
All the above done on compliance of standards and rules Etc..

Local jargon to term some bio digesters on urban drains as " gutter gas plants " has been there for a long time.. But all these initiatives add values to communities well - living..

Well wishes..
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