Hi...

I wanted to discuss regarding emptying of septic tanks - mainly to assess their appropriate cleaning frequency.

In India there are codes and manuals which have standardized size and cleaning frequency of septic tank.These suggest a maximum interval of 2 years for cleaning of septic tanks and related sizes linked to loads.

But, during our study of onsite sanitation in a few cities in India, we have found that, septic tanks that are constructed on-site are generally over sized and their cleaning frequency is more than the prescribed limits of codes and manual (e.g. by the CPHEEO). The common septic tank cleaning frequency is around 7 to 10 years for such on-site constructed large septic tanks.

I would like to find out about the implications of large /oversized septic tanks which are cleaned at such long intervals. How would such long cleaning intervals affect the treatment of excreta waste in septic tanks.

Regards,
Aasim

Hello,

if septic tanks are indeed oversized, then longer cleaning intervals are are no problem. In the contrary, longer time of the sludge inside the tanks allows for better digestion. A problem might be that emptying after such long time may be more difficult, because sludge solidifies on the bottom and is difficult to remove.

An important question, however, is if the tanks are really oversized, or if only the cleaning intervalls are prolonged, and by this the septic tanks loosing partly their retention function by too much sludge accumulating in them. The latter is more often the case in reality, rather than the first. In many cases, the emptying is not imposed but left up to the owners. As removing sluge implies costs, the owners often have their tanks emptied only once some problems occur, e.g. something is clogged. That generally happens only long time after the design-intervall for emptying, and the septic tanks have lost treatment capacity long before.

Regards, Florian

Dear all,

My in-laws in the USA have septic tank and they de-sludge once about 1/4 of the first compartment is filled with solids (which happens to be about every 4 to 5 years). I am not sure how scientific this method is, but to me it makes sense to have a recommendation based on how much solids have accumulated, more than based on time passed since last de-sludging.

Further, if the septic tank is not de-sludged on time, this will not only reduce the effectiveness, but also the solids that are then washed out will clogg-up the material in the leaching field. Which will be expensive to replace.

rgds

Marijn

Hi Florian and Marjin,

First of all.. Thanks for the prompt reply.

The scenario here that we observed in few cities of India is that the septic tanks are really oversized (like 2 to 3 times the standard size mentioned in the norms) due to which the cleaning frequency is prolonged like 7 to 10 years. Due to this the treatment capacity of septic tank becomes non-functional. You have rightly mentioned that the cleaning is not imposed, but at the discretion of the owner.

It would be great if you could let me know what chemical/biological treatment process stops inside the septic tank due to such long prolonged cleaning intervals or accumulation of solids that would happen as rightly mentioned by Marjin

Thanks in advance...

Regards,
Aasim

Dear Aasim,

I am not an expert (maybe Elizabeth von Muench can correct me if I go wrong here) but to the best of my knowledge:

In a septic tank there are 3 layers: sludge on the bottom, then water and a layer of scum (brown foam) on top. Now the solids that you flush away in your toilet are broken down by bacteria in the sludge and liquid, but not everything can be broken down and this material accumulates at the bottom as the sludge layer.
I have not come across any description of this bacterial process stopping over time, if that is the problem you are referring to?

As Florian pointed out, this sludge layer compacts over time, so it may become (almost) impossible to pump out of the tank at some point. Over time this layer also becomes thicker and thicker, leaving less space for liquids.

If we design a septic tank we look at the retention time (the time required for water to flow through the tank, this time is determined by the inflow (how much water enters the tank) and the size of the tank. The retention time should be at least 24 hours for the biological breakdown and the settling of solids. Therefore if the tank is too small, or too full of sludge, the retention time becomes too short and the solids (faeces) that enter it are not fully broken down. This leads to pollution and a lot of solids just flowing through the tank and clogging up the leach field.

I hope this helps you, if not try to describe the problem in a bit more detail.

By the way, if you google "septic tank retention time" there should be a lot of information available.

rgds

Marijn

aasimmansuri wrote:


Hello Aasim,

Maaijn already explained well how a septic tank works. It's main function is to allow for solid particles to settle down to the bottom, where then they are digest (degraded) by anaerobic bacteria.
As Marijn said, the main design criteria is the hyraulic retention time, this is important for the small particles to have enough time to settle down to the bottom. Hyraulic retention time is basically the volume of the first chamber (m3), divded by the flow coming into the tank (m3/h). Now because sludge accumulates on the bottom, the liquid volume in the tank becomes smaller and the hyraulic retention time becomes also smaller. If sludge is not removed, the retention time will eventually become too small for the settling process to happen, and thus the septic tank loosing its main function.
For this reasons, septic tanks are designed with a certain sludge accumulation volume, and need to be emptied once the sludge reaches this volume. The standards normally give the estimated time (e.g. 2 years) when this happens, but of course it would be more accurate to measure the sludge level (as Marvijns in-laws apparently do).

Now back to your question: If your septic tanks are 2-3 times bigger than needed, in principle it is ok to also prolong the emptying intervalls accordingly 2-3 times. The processes in the septic tank will NOT stop. In the oversised tank, sludge has more room to accumulate, retention time will still be ok, and solids still can settle down. So from what you describe, I'd say your situation is ok.

But of course I'd like to caution again. To be sure if your tank is really oversised, you also need to check how many users are connected to the tank. More people in the house require a bigger tank.

To check the function of a septic tank, you can measure the suspended solids content in the effluent of the septic tank. If the tank works, this should be very low.

Best, Florian

Dear all,

We have the same situation in Raipur, Chhattisgarh, India. The size of the septic tanks is limited by the space available rather than guidelines, therefore they are in general oversized. We talked to a local civil engineer and he had not even heard of the CPHEEO guidelines which are based on the Indian Standards. I think they idea is: If I have the space why not use it, so I do not have do desludge it frequently, however economic consideration will also play are role. Definitely it is intersting how guidelines/standards get ingnored.

Regards,
Moritz

Hello,
I doubt very much that the septic tanks are oversized. What do you call oversized? A day retention time would be nice. From what I know in LA the septic tanks have desludgeing intervals which are considerably larger than under European conditions, unfortunately not always, but situations where no grey water goes to the septic tank help a lot (just practical observation by operating a fecal sludge truck, no scientific investigation). We do have many situations where there is not even an access to the tank for desludgeing, we have to break the concrete and put in a pipe.
When doing recommendations it is very recommendable to suggest at least a two chamber tank. That gives you a bit more security. When the first chamber is full, you (should) call the service. In practice the service is called when the flushing does not work any more as it should.
All the best Christoph

Dear Aasim and all,

let me give you some examples and information from Vietnam;

- septic tanks are mandatory for all buildings
- there are no propper design standards in place (to my knowledge). Moreover, the issue of grease traps (widely badly needed) is totally ignored
- house owners never empty their tanks voluntarily or regularly. Once the kitchen is flooded with waste water, they call the emergency service (expensive). Reportedly, many people have NEVER cleaned their septic tanks
- there is a long and painstaking discussion between international donors whether septic tanks should be supported / financed. No solution can be found despite so many conferences, seminars etc on this issue
- The city of Haiphong however has embarked on a very new concept: emtying of septic tanks is free of charge for all house owners, every 5 years. Costs are covered by the waste water tariff. Emergency services, however, have to be paid.

@ Aasim: I would be very much interested in you local design standards for septic tanks. Do you have an english version you could share? Thanks

Regards
Frank

Hi Frank!!!
nice to hear from you. And thanks for the info.
The concept of emptying every 5 years the septic tank seems to very interesting. A bit similar to the concept in Durban where the pits are emptied for free every 5 years but seems to be feasible only for cities with a good financial background.
Yours
Christoph

Just some quick comments about an interesting topic.

I think that the appropriate pump out of septic tanks is a problem worldwide. It’s interesting to hear about the Vietnamese solution to the issue. I look forward to hearing more about the success of the approach.

Regarding the situation in India, I wonder if there is a change in the way that people are using water in houses in urban areas? Are there flush toilets, high flow showers, more greywater produced? Is it all diverted to the septic tank or is it sent to a grease trap. Were the guidelines designed with these water uses in mind or have the flows and practices changed?

I also believe that the over sizing of the septic tanks is probably only a benefit from a treatment perspective. Other anaerobic treatment approaches such as 'baffle reactors' have been used effectively in India and they rely on an extended retention time. It is also interesting to note the work of companies like 'Orenco' in the United States who have some research moved towards larger septic tanks with less frequent pump out times (5-7 years I think).

That said, effective regular pumping out is crucial to any septic tank system as mentioned previously in the discussion.

Chris

Frapoberlin wrote:


Hi Frank,
that is very interesting to read. Is there more info available, eg. if and how exactly this empyting every 5 years is beeing implemented.
I visited Haiphong about 10 years ago and already then they had interesting solutions for septic tank desludging, e.g. small vacuum tugs to enter the narrow vietnamese lanes...
As for vietnamese standards, I believe to remember that the ministry of construction prescribed 3-chamber tanks of a certain volume per person, but I do not have this as hands, so I am not totally sure about this... (in reality, there are rarely 3-chamber tanks being built).

Florian