Hi Dean

As requested, I'll try to clarify my thinking on this. I should have been clearer that I was talking about effluent quality in terms of pathogens - although in the case of direct discharge to a drain or watercourse, the increase in solids content of the effluent is also an aspect of quality;

When discharge is to a drainfield or soakaway, my assumptions are as follows - although as I said in my first post, I can find little quantitative evidence on this.

1. Pathogen removal occurs mainly below the soakaway/drainfield - there are studies that show that reduction through the septic tank itself is usually less than 1 log. The greatest reduction occurs in the biomat that forms immediately below the bottom of the soakaway or drainfield but further effective reduction will also occur below this, particularly above the water table.

2. Failure to desludge the septic tank regularly will not in itself have a great effect on pathogen levels in the septic tank effluent and pathogen reduction in the ground will initially not be affected greatly. However, the carry-over of solids will increase significantly as the capacity of the septic tank reduces. My observations of small septic tanks connected to drains suggests that the material exiting the septic tank may consist mainly of partly digested fine particles.

3. The solids will eventually start to block the drainage paths out of the drainfield/soakaway so that infiltration capacity is reduced, eventually leading to surcharging of the whole septic tank system and flooding with dirty water. There seems to be limited information on how quickly this occurs in different situations. Work done for WSP in Indonesia suggested that leach pits could go for many years without desludging but that once desludged they needed follow-up desludging fairly frequently. This suggests that blockage of the drainage paths and consequent hydraulic overloading rather than sludge build-up was the main driver for having the pits emptied. These leach pits only received water from pour-flush toilets so the hydraulic loading was low. In contrast, similar but larger leach pits in Ethiopia, where all the household wastewater was discharged to the pit needed more frequent emptying.

4. My overall impression is that blockage of drainage paths leading to hydraulic overloading is the main mechanism for septic tank system failure. Flow through the tank itself will continue even when the tank is full of sludge, although solids reduction through the tank will be negligible. The likelihood of large solid objects blocking the outlet from the tank will presumably increase if desludging is neglected.

One point that supports this thesis is that 'septic tanks' connected to drains are common in Northern India, Pakistan and Bangladesh. These tanks are rarely if ever emptied and most cities in this region do not have extensive tank emptying services, unlike many cities in Africa. This suggests that tank blockage is not a huge problem and that wastewater continues to flow through the tanks even though they are presumably full of sludge and having very little treatment effect.

It would be interesting to know if others have any alternative takes on this

Kevin

Hi Kevin, could you elaborate please? Clearly if solids start getting through to the soakage field then the effectiveness of the soakage declines. But what occurs first:
1. Soakage field failure (lack of soakage caused by solids in effluent)?
2. Septic tank failure (blocked with surface solids)?

I suppose there is the capacity issue where this is reduced as solids build up on the bottom, reducing residence time... but I'm thinking that it doesn't matter what quality the post-septic tank effluent is, the tank is there to remove solids so the soakage field continues to operate over time. Therefore the only parameter determining outflow quality is effluent solids content. Is that correct?
cheers
Dean

I think it depends on what you mean by effluent, which in turn depends on what happens after the septic tank. If the effluent is discharged to a drain or watercourse, which is not recommended by standard texts but which is not uncommon in many countries, its quality will certainly deteriorate if tank desludging is delayed. The same is true of the effluent that flows from a septic tank to an associated drainfield or leaching system. Again, it is clear that neglect of regular desludging is likely to result in deterioration and eventual failure of the drainage/leaching mechanisms. The more difficult question is to say what effect this will have on the quality of the liquid that leaches into the soil. I suspect that the effect will not be great but have been unable to find any information on this.

I don't think that this answers the question but perhaps it helps to refine it. It would be good to hear from anyone who can throw further light on this

Hmm, yes this would be a good talking point... but technically so obvious that I would be surprised someone actually studied that (i.e. lots of effort to "prove" something so clearly described by the functioning principle).

Maybe a proxy value would be studies on tank size or number of baffles that are somewhat related to the functionality of a septic system as a settling tank?

Hi All,

I wanted to check with you regarding whether there is any study available showing that septic tank effluent quality is better when the septic tanks are desluged at more regular intervals like 2-3 years as compared to long desludging cycles like 8-10 years.

I recollect having seen such a presentation at FSM4, but unfortunately I am not able to locate this presentation.

This point is also important from the perspective that whether cities should go for regular/scheduled desluding or demand based desluding which are at much longer intervals.

Would request all of you, in case you are aware of such a study or finding, please do share.

Many thanks

Regards,
Aasim