Friday 10 February 2012

Anaerobic systems

I keep coming across more than quite a bit of confusion here! So let’s be clear. Septic tanks were first developed in France in the 1860s by Jean-Louis Mouras. Donald Cameron used them in England in 1895 (see his “Monster Septic Tank” here). What happened next? Well, Imhoff tanks came on the scene in 1906, though these were too big for individual use (prefabricated units are still available for up to 1000 p.e. − details here).

Quite a bit of research on septic tanks was done in the1950s by the US Department of Housing (important as around a third of US households are served by septic tanks) – the main emphasis was on how many compartments it should have. I’ve lost/can’t find most of these reports but my memory is that it was found that two compartments were better than one but that three weren’t better than two (nor were four or five, etc.).

Upflow filters were then developed in India to ‘polish’ septic tank effluents − see Upflow filters for septic tank effluents by V. Raman and N. Chakladar (Journal of the Water Pollution Control Federation 44 (8), 1552−1560, August 1972). I think these are excellent!

Then came the next septic-tank development: the UASB – upflow anaerobic sludge blanket reactor (developed by Professor Gatze Lettinga of Wageningen University in the 1970s): the influent is introduced across the reactor base and flows upwards, through the sludge layer, to the effluent weir; biogas can be collected if so desired. [Gatze Lettinga said to Barbara Evans, who works with me at Leeds, ‘why are you going to Leeds to work with Duncan Mara – he’s mad’. I take this as a compliment as I’ve “seen through” UASBs!]

A PhD student of mine, Professor Miguel Peña Varón, developed the high-rate anaerobic pond (HRAnP) to combine the efficiency of UASBs with the simplicity of anaerobic ponds (he also showed that conventional anaerobic ponds were ‘better’ than UASBs – see here).

Then came DEWATS − Decentralized Wastewater Treatment Systems – which typically comprise a ‘settler’ (a two-compartment septic tank) and an ‘anaerobic baffled reactor’ (ABR) (basically a baffled septic tank), followed by an anaerobic filter and a ‘planted gravel filter’ (i.e., a horizontal-flow constructed wetland). There’s the Decentralized Wastewater Treatment Systems (DEWATS) and Sanitation in Developing Countries - A Practical Guide which will set you back €48.75 plus postage (€12 outside Europe)!

On ABRs see The evaluation of the anaerobic baffled reactor for sanitation in dense peri-urban settlements, a WRC report dated 2006. Part of the report’s recommendations: “The ABR is able to provide better and more efficient treatment of wastewater than a septic tank. Therefore it is recommended that an ABR system can be used in any situation that is considered appropriate for a septic tank.” I don’t think I agree with this somewhat sweeping recommendation!

This is getting a bit too much into wastewater treatment (rather than sanitation per se), so why not have, for a small community a two-compartment septic tank (with the influent entering across the base of the first compartment), or for a larger community a high-rate anaerobic pond, then a secondary facultative pond + an aerated rock filter? Not sure I care much for UASBs, DEWATS or ABRs. Well, I thought I’d just mention it.