Hi Jonathan.

Yes, the figures are guideline average figures per user. If a toilet is used by a large number of people the accumulation rate per person is likely to be higher, simply because less time will pass before the pit is full. The longer the sludge is in the pit, the lower the accumulation rate on a per person per year basis.

From what I have seen, if sludge is dry and dense it is a waste of time to add water to it in order to facilitate vacuum pumping. Only if the sludge is borderline pumpable (or suckable) then it would be worthwhile adding some water, but not too much is needed (10 or 20% by volume max).

If sludge is too dry for pumping without the addition of large amounts of water, it will be much more cost effective to simply dig it out by hand.

Regards

Dave

Many thanks Dave, this is very useful information. I will download and have a look at the reports. Presumably these figures don't take into account the extent of sharing - the are guideline average values ? One of the other issues to consider is the fact that the accumulation rate does not directly correspond with the volume of sludge if water is added to remove the sludge. Do you have any guidance about the amounts of water that are added during the desludging process ? best regards. Jonathan

For interest I have two reports from work done in the 90s by the CSIR for the WRC in South Africa. Gavin Norris looked at sludge accumulation rates in pits and septic tanks, and came up with a guideline of 0.07 l/c/d (= 25.6 l/c/yr) for pits and 0.08 l/c/d (= 29.2 l/c/yr) for septic tanks. After all the work we have done more recently we would not disagree with these numbers. Only the trash component pushes the number up.

Alan Wright produced some guidelines for septic tanks. He quotes 1987 data from de Villiers (see Tables 12 and 13) which puts the sludge accumulation rate at 30 litres/c/yr after 8 years. For shorter periods the rate is higher, and it will also be higher depending on anal cleansing materials.

Dear Aasim,

that is a very interesting document, I didn't know this, thank you. Very complete, 30 years old and probably we've not added really much knowledge since then...

You're right, the 230 l/capita and year refer to septage, collected sludge.

But rather than taking this number, the Indian guideline should have picked this sentence of the document: "In light of the significant variation in septage generation rates from one locality to another, every effort should be made to obtain actual records of septage quantities (i.e., from existing treatment plants recieving sepatae, or from local haulers) for a particular service area."

Another useful bit is "An alternative method of estimating septage quantities is to multiply the number of septic tanks in the service area by the average pumpout volume per unit (i.e., the total volume of a typical septic tank divided by the average number of years between pumpouts).

Regards,
Florian

Hi Florian,

Thanks for the quick response.

I think the EPA norms refers septage to both the solid and liquid material that is pumped out of the septic tank.

Please find attached the EPA document which suggests to take septage generation rates as 230 litre/capita/year (Refer Section 3.2.1 on pg 18 of the document). This nos is based on the septic tank data of US Census department, average septic tank volume and septage volume pumped out every 3 years.

The document also refers to Norwegian guidelines which suggest 250 l/captia/year and Swedish guidelines recommends 225 l/capita/year.

Thanks..

Regards,
Aasim

Dear Aasim,

I copy here the full quote of the relevant paragraphe from your (interesting!) document:

Adopting the (U.S. EPA, 1984) estimate of septage generation of 230 litres/year and an average household size of four, the septage generation/household would be 920 litres/year. Alternatively, assuming an average septic tank volume of 3 m3 and emptying of septage when one-third of the septic tank is filled with settled solids, the volume of septage emptied would be 1 m3.

I don't know the EPA guideline myself, so I am not sure if the figures refers to sludge or sediment accumulation inside the septic tank (for which the figure would be quite high) or the sludge + water removed from the tank and brought to the disposal site (septage). The figures sound realistic for the latter, so I suppose the EPA guideline refers to this one.

I wouldn't really agree to the way of the estimation in the second sentance in the quote from your document (eventhough the result isn't way off the mark). When a septic tank is emptied, normally the whole content (sediment + liquid phase) is sucked out from the first chamber tank, not just 1/3 (sometimes even more water is added to loosen the sediment). So say the tank is 3 m3, the first chamber 2 m3. So we have 2m3 of septage. If the tank is empied every second year, we come to the same average 1m3 per household and year. If the tank is empied less frquenquently, of course the average volume collected is less.

But this is just talking about volumes, not about concentrations of suspended matter, COD etc. These are best calculated based on local data from wastewater, also considering which wastewater streams normally go into the septic tanks. Using EPA values is probably not the best idea, as conditions in the USA might be quite different from the Indian situation.

Best regards,
Florian

Hi All,

There is an interesting collection of figures for sludge accumulation rates being discussed here.

We have a recent Advisory from Govt. of India on Septage Management, which quotes a figure of septage generation as 230/litres/capita/year (adopted from U.S. EPA 1984). So for a HHs size of 4 the annual septage that is generated is around 920 litres/year.

following is the link to the document (refer pg.17) : www.urbanindia.nic.in/programme/uwss/Advisory_SMUI.pdf

The numbers that are coming up in the discussion forum seems very less as compared to the numbers in the advisory.

It would be great to get inputs from the forum on these numbers suggested in the advisory.

Many thanks.

Regards,
Aasim

Hi, interesting topic, in addition to Florian’s observations these comments:

- Fecal sludge (FS) from Pit Latrines and FS from Septic Tanks normally are entirely different; especially in contamination rates with urban waste, water content and stabilization/degradation level.

- Key parameters for the volume and quality of FS (in case of Pit and Tanks) are: specific storage volume and exfiltration rate: i) Sealed (lined) or unsealed pits produce different FS volumes with different compositions; and ii) there is also a difference between FS from sealed Septic Tank (solid free effluent is drained) and tanks, which seep through the walls.

- In case of Pits it is very important if – and how much - flushing water is used; and in case of Septic tanks (always water toilets), it is important for FS volume and qualities if blackwater is mixed with greywater or not.

- The exfiltration efficiency from pit/tank depends on the specific soil characteristics but soil capacity also can be saturated by time. Exfiltration also can change into infiltration, forinstance in case of rainfalls/ increasing groundwater level. These are the most common reasons, when Pits or Septic Tanks needs to be emptied with high frequency, the result is less stabilized and diluted FS.

- Low water consumption and/or high exfiltration rates result in dense fecal sludge, so that in case of vacuum extraction a lot of water is needed and the product is diluated.

Characteristics of fecal sludge hardly can be compared without this background information. In my experiences, fecal sludge can be very different in volume, concentration and onsite stabilization level even if in the same region and the same onsite storage facilities (as pits or tanks), but certainly in different regions and use of different onsite facilities, different water consumption, different soil characteristics or different climates and so on.

Regards Heike

Hi,
interesting collection of figures for sludge accumulation rates here!

I just would like to point out one issue that has not been mentionned yet.

Sludge accumulation rates can be used to estimated sludge volumes produced in a certain area. However, the volume of sludge produced in an area is not necessarly equal to the volume of sludge collected, and the latter is the relevant parameter for designing sludge management systems.

The volume of sludge collected may be quite different from the sludge produced for various reasons:
- septic tanks may be improperly designed or built and not accumulate the expected quantity of the produced sludge
- septic tanks or latrines may not actually be emptied in regular intervals, or are not emptied at all
- not all sludge in vaults or pits is removed during the emptying as often part of the solidified sediment remains in the facility
- not all sludge that is removed by companies or persons will be delivered to the treatment facility; part of the sludge will be dumped elsewhere.
- the sludge may be diluted during removal, volume and concentration be changed

An alternative way for estimation of sludge volumes to be managed would be not to use sludge accumulation rates at all (or only use them for estimating a kind of theoretical maximum of sludge produced), but estimate sludge collection rates using data from sludge collection companies, private collectors, etc. A tricky thing is of course to quanitfy the impact the designed improvement measures will have on the sludge collection rate.

Best regards,
Florian

Dear Lukas, Remi, Freya and Dave,

Just a short email to express my appreciation to your responses to my posting requesting information about quantities of faecal sludge produced per capita per day. You have provided some very useful information. Many thanks.

It would be great to have a 1 page SuSanA technical briefing note on this because the unit volumes used or engineering design purposes can obviously have a major implicaiton when considering sanitation infrastructure requirements for a city.

best regards,

Jonathan

Hi all

Our work has been referred to in the posts above, but after all was said and done we came up with the figures of 40 litres/person/year as a planning figure, but 60 litres is not a bad figure to use if users are putting a lot of trash in their latrines.

Since doing that work we have monitored a number of pour flush latrines which have been on trial for the last three years. With these there is no trash, and there is obviously a litre or two of water introduced to the pit with each use. The median sludge accumulation rate we see with these latrines is about 23 litres/person/year (from memory), i.e. half of what we saw with the pit latrines. That is quite a significant difference. I note that this accords with Freya's figures below.

As Freya notes, there is a lot of variability in filling rates.

Regards

David Still

Hi Jonathan,
As introduction, I’m a sanitation consultant currently based in Indonesia, working on urban sanitation projects for the Water and Sanitation Program of the World Bank.

Last year I did a study for WSP in Indonesia on the sludge accumulation rates in on-site systems, to be presented at the WEDC conference later this year. On-site systems in Indonesia had sufficiently different variables to those studied in previous research to warrant a detailed study. Additionally, compared with the research in South Africa where pits were filling fast, in Indonesia demand for pit emptying is very low.

Typical systems were: single pits with unsealed base; wet contents due to water used for flushing and anal cleansing and groundwater infiltration into pits; almost no rubbish (due to water cleansing and goose neck water sealed toilets). Most systems had never been emptied, with an average age of 16 years. Only blackwater enters pits, greywater is typically discharged to drain.

Settled sludge was measured in 107 systems combined with a household questionnaire of those plus another 100 households where the pit was inaccessible to measure. The mean accumulation rate is 25 l/p.yr and median 13 l/p.yr, ranging from 0 to 130 l/p.yr. Variables that influenced accumulation were occupants (linear increase), years accumulation (reduced rates for increasing years), emptying frequency, size of system and whether it had an outlet (rather than leaching from base).

As background information I found very good research by Still and Foxon in South Africa on accumulation rates in predominately dry pits, some other good papers are listed below and highlight that accumulation rates are dependent on local conditions.

Let me know if I can provide any more details.
Kind Regards,
Freya Mills

• Still, D., & Foxon, K. (2012). Tackling the Challenges of Full Pit Latrines. South Africa: WRC.
• Still, D. (2002). After the pit latrine is full….What then? Effective options for pit latrine management. WISA Biennial conference. Durban: ICC.
• Norris, G. (2000). Sludge Build-Up in Septic Tanks, Biological Digesters and Pit Latrines in South Africa. South Africa: WRC.
• Moores, J. (2002). Septic Tank Pumping Frequency Project. Nova Scotia, Canada: Dalhousie University.
• Franceys, Pickford & Reed (WHO), 1992, A Guide to the Development of On-site Sanitation. Geneva: WHO.
• WHO. (1982). Emptying On-Site Excreta Disposal Systems in Developing Countries: An Evaluation of the Problems. Switzerland: WHO.
• Howard, T. (2003). Solids Accumulation Rates for On-Site Sewage Treatment and Disposal Systems: A Focus on Charlotte County Florida.
• Buckley, C. F. (2008). Scientific support for the design and operation of ventilated improved pit latrines (VIPS) and the efficacy of pit latrine additives. South Africa: Water Research Commission Report.