Rate of filtration of septage through sand filter beds and technology for septage management

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  • HAPitot
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Re: Rate of filtration of septage through sand filter beds and technology for septage management

Dear Krishan, dear Pawan,

If I am talking of constructed wetlands for sludge disposal, I am talking about vertical flow wetlands which are constructed similar to sand drying beds only that they are planted with swamp plants (macrophytes) or at least water tolerant plants and care is taken that they don't ever get completely dry (even though that is sometimes difficult to achieve). These wetlands will usually have a large freeboard above the planted surface where the sludge can accumulate (sometimes more than one meter high in height). As the wetland is used, the solids will accumulate in that freeboard for years and slowly fill it up. Because of the time involved, the solids will also degrade and minaralize, so that the accumulation is actually much less than the actual volume of deposited solids. And the plants will grow into these solids and turn it into their rooted zone. That explains that the permeability of the layer is very good and loading can be high. So, what you actually end up in the freeboard is an accumulation of humus with plants growing in it.

When it comes to emptying a wetland, usually after years of operation, you take it out of operation for 6 months (something, of course, you have to plan for in the design of the facility), so that the pathogens are going to die due to the usually intense sunlight and the lack of nutrients, and then you can empty what was originally the freeboard above the original surface, then you plant the plants back into a remaining layer of humus, and start loading again. I'd call that very efficient!

In the case of Adjumani, which is a small (35 000 inhabitants) country town in northern Uganda, we had three wetlands of (originally) about 20 m3 capacity. One was meant as a back up, the other two could take at least 8 loads per day from one truck that would be operating in the area. After some days, the truck would go to a different area, and the wetlands could be resting for weeks and even months until another truck comes along. As I was saying, we've never seen the limit of what the wetlands can take, and it's only now that one of the basins is getting full.

Concerning the effluents from the wetlands - little was actually coming out, most of it was evaporated, and what was coming out was seeping into an adjacent land somewhat larger than the wetlands themselves. There, agricultural usage would have been possible, for example growing of bananas or papayas, but because of security reasons and the intrusion of cattle, this has never been successful - the fence of the facility got torn down by the local shepherds who apparently thought it nice to graze their animals on the beautifully green wetland areas - some of the many problems we had at that facility. Technically, the facility has far exceeded our expectations.

I hope that has given you some explanations and insight.

Kind regards,

H-A


PS: At the moment, I am working on a Susana case study on the sludge treatment in Adjumani, which will be giving more details.
Hanns-Andre Pitot
M.Eng. Environmental Pollution Control
presently in Seesen, Germany
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  • pkjha
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Re: Rate of filtration of septage through sand filter beds and technology for septage management

Drear All
I got information from the book Faecal Sludge Management by EAWAG/IWA published in 2014. I am thankful to Florian for his suggestion to go through the book. There are other books/ reports on different aspects of FSM published by EAWAG/SANDEC.
Kevin has rightly mention regarding loading of 100-200 kg TS /m2 of filter bed. There is also suggestion that there should be 200 mm of FSM load in filter bed. It appears to be more practical. Under different books/ reports / papers there are some differences in data and design parameters, for Filter bed and Constructed wetlands. This could be due to local environmental conditions, characteristics of FSM etc.
Removal of pathogens from leachate through sand filter is mainly due to physical barrier. I don’t have any evidence in this regard as sought by HAP. If anybody has evidence, may like to provide.
Constructed wetland is also one of the options for FSM. The problem is safe removal and reuse of biosolids. HAP may like to inform the process of removal of biosoilds based on his experience.
Regards

Pawan
Pawan Jha
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Re: Rate of filtration of septage through sand filter beds and technology for septage management

Hmm, maybe I am confused as well, but wouldn't a wetland be quickly overloaded when flooding it with high strength settled sludge (which is what sludge drying beds are designed for)?

Edit: Well... strictly speaking septage should have a similar strength as regular waste-water, but in most cases septic tanks are not emptied before there is a large sludge accumulation as the supernatant waste water/"effluent" is continuously discharged to a solids free sewer, secondary on-site treatment or is illegally infiltrated to the same effect.

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Re: Rate of filtration of septage through sand filter beds and technology for septage management

Dear all from my side!

I would agree with Christoph that constructed wetlands are more efficient in removing solids than sand filters (drying beds) at least in terms of area requirements. There is no time lost with dying of the solids, you can consistently load the wetlands, and there is not too much hassle with removing the solids. Pawan, you were arguing that sand beds are more efficient in removing pathogens - what evidence would there be for that claim?

As a little addition to this post: on the pic on the left you see me together with my friend Simon on the edge of an artificial wetland for sludge disposal, located in Adjumani, Uganda. We had (and still have) 3 small basins in parallel, for four truck loads each. I think we have never reached the limit of the capacity of these wetlands.

Kind regards,

H-A
Hanns-Andre Pitot
M.Eng. Environmental Pollution Control
presently in Seesen, Germany

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  • kevintayler
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Re: Rate of filtration of septage through sand filter beds and technology for septage management

Dear Pawan

The normal practice is to design sludge drying beds on the basis of the total solids loading. EAWAG suggest that the design loading figure should be 100 - 200kg total solids per square metre per year for hot countries, (as compared with 50kg TS/m2.yr for temperate countries. However, they quote actual loading rates of up to 300kg TS/m2.yr measured in some field investigations in West Africa. Books like Metcalf and Eddy and Crites and Tchobanoglous give similar figures. (The EAWAG book should be downloadable from www.sandec.ch/fsm_book. (See post by Linda Strande elsewhere on this forum)

For typical solids concentrations in the septage and assuming perhaps 70% of the sludge is liquid that percolates through the filter, the average filtration rate is likely to be of the order of 2 - 3 metres per year.

The normal recommendation is that the sludge is loaded to am initial depth of 200 - 300mm and the drying period is 10 -15 days. One interesting paper describes experimental work done in Sanaa, Yemen and suggests that shorter drying periods are possible (see iwtc.info/wp-content/uploads/2013/11/99.pdf) but this is in a dry climate. Rain will certainly increase the drying time required. Most of the drying occurs during the first few days and the time required depends to a large extent on the final solids content required.

I can provide more detail but I think this covers the main points

Hope this helps

Kevin
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  • pkjha
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Re: Rate of filtration of septage through sand filter beds and technology for septage management

Dear Mudghal

Treatment of septage along with waste water in a STP is one option where such Sewage Treatment plants of required capacity are available. Main problem is for the areas where no centralised treatment facility is available - in most of the urban municipalities in developing countries.
For low income areas sand filter beds with or without plantation or constructed wetlands are sustainable.
A recently released book on Faecal Sludge Management by IWA mentions different technologies for FSM. I got proper answers of my queries.
regards
pawan
Pawan Jha
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  • F H Mughal
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Re: Rate of filtration of septage through sand filter beds and technology for septage management

Dear Pawan,

I have not heard of using filter beds for septage treatment. In the wastewater plant that I designed back in 1982 (aerated lagoons) in Karachi, septage was mixed with raw wastewater, and the wastewater flowed through a series of unit treatment system - grit removal chambers, primary sedimentation tanks, aeration units, and secondary sedimentation tanks. The sludge from secondary sedimentation tanks was pumped to the sludge drying beds. The final treated effluent was used for irrigation, and the dried sludge was used as fertilizer and soil conditioner.

The objective that you mention: "Objective of the filtration of septage is to minimize organic loads and use effluent for further treatment for its safe reuse in agriculture purpose and also use of dried solid sludge for agriculture purpose,"
was fully satisfied - BOD and SS removals were more than 90 per cent.

Regards,

F H Mughal
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Karachi, Pakistan

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  • Florian
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Re: Rate of filtration of septage through sand filter beds and technology for septage management

pkjha wrote: sand filter bed for septage management


Hi Pawan,

In drying beds, sludge is poured onto beds in batch loads. Liquids percolates through the sand bed while the thickened sludge dries on the surface until it is dry enough to be removed (and the beds can be loaded again). In that process, drying takes much longer than the percolation of the liquids. Thus factors for design are not the filtration rate but rather the drying rate of the sludge, which is determined by sludge characteristics and climatic conditions, thus very variable.

For guidance on design of drying beds for septage, I guess you should look for the Sandec sludge book, or american septage management literautre.

Best, Florian

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Re: Rate of filtration of septage through sand filter beds and technology for septage management

Dear Christoph
One filter bed will be designed for total loading of septage for one day. 10 filter beds will be used one after another. It will help Sun dry of solid part on sand filter beds. In case of high rain fall areas more filter beds will require. At present, there is no such data either of rate of filtration rate or Sun drying rate for septage.
Treatment of filtered waste water will be carried out through any technology- WSP ( Waste Stabilisation Pond)or other technologies. That's not a problem.
I'll go through the papers you have mentioned. Thanks for sending such relevant papers.

regards
pawan
Pawan Jha
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Re: Rate of filtration of septage through sand filter beds and technology for septage management

Dear Pawan,
I did not get you right I guess.
Let me describe what I understood.
10 beds in parallel with 20 mm of coarse sand for a loading of 200 mm (in total?) per day of raw?? wastewater.
Followed by a WSP (don´t now what is that) or a anaerobic or aerobic filter.
Is that right?
I would do it the other way round in order to avoid clogging.
First a treatment and for helminth eggs in the end the filter beds. In this case these publications might be interesting - they are all older but valid:
BOUWER H.; RICE R.C.; LANCE, J.C. & GILBERT, R.G. (1980). Rapid-infiltration research at Flushing Meadows Projekt, Arizona. Journal Water Pollution Control Federation 52. 2457-2469.
BOUWER, H.; RICE, R. C. & ESCARCEGA, J. C. (1974). High-rate land treatment I: Infiltration and hydraulic aspects of the Flushing Meadows project. Journal Water Pollution Control Federation 52. 834-843.
ELLIS, K.V. & AYDIN, M.E. (1995). Penetration of solids and biological activity into slow sand filters. Water Research. 29. 1333-1341.
GUILLOTEAU, J. A.; LESAVRE, J.; LIENARD, A. & GENTY, P. (1993). Wastewater Treatment Over Sand Columns. in: Proceedings of the 2nd IAWQ Conference on Small Wastewater Treatment Plants. 28 - 30.06.1993, Trondheim. 153-160
GUPTA, R.P.& SWARTZENDRUBER, D. (1962). Septic tank effluent percolation through sands under laboratory conditions. Soil Science Soc. Americ. Proc. 26. 6-10.

best whishes
Christoph

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Re: Rate of filtration of septage through sand filter beds and technology for septage management

Dear Chritoph

Thanks a lot for your inputs with posting a useful paper.
I am preparing a design for a sand filter bed with 200 mm coarse sand and 200 mm loading of septage per day. Bottom design of the filter bed is suitable for creating partial vacuum for high filtration rate. Hope it will solve the daily filtration of loaded septage. For precaution (in case of clogging)there would be up to 10 filter beds to be used in sequence. It will help sun dry of septage. However, it is not clear whether the designed filter bed area will be sufficient for filtration of septage volume taken up to 200 mm.
Filtered effluent will be treated through WSP or contact media anaerobic or aerobic methods for its safe reuse in agriculture.
Use of constructed wetlands for septage is more problematic for its reuse in agriculture purpose. Moreover, settle sludge remains a major problem to handle.
I would like to have any data / report on sand filtration system for septage.
regards

pawan
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Re: Rate of filtration of septage through sand filter beds and technology for septage management

Dear Pawan,

the answer is very difficult for sandfilter for wastewater treatment as quality varies very much. Particularly I would not use sand filter as they will clogg always or you have to set up a maintenance cycle (for pealing of the surface).

I use always constructed wetlands. Attached you find the long version of an article from 97 (sorry – very old but still very valid and I use the findings until today almost unchanged)

Platzer, Chr. & Mauch, K. (1997). Soil Clogging in Vertical-flow Reed Beds - Mechanismns, Parameters, Consequences and ..... Solutions?. Wat. Sci. Tech. Vol 35, No. 5. pp 175-181.

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In the published version we had to cut out several general observations due to page limitations but it might be interesting for you the part around figure 1. Therefore I attached the long version which is not nicely corrected and everything perfect – please see the content not the form.

I do vertical flow wetlands (which would be somewhat close to your case) as a rule of thumb with less than 200 mm a day constant load in subtropical conditions. You could as well do 1000 mm in certain moments as the hydraulic capacity normally is not the limitation. Limitation is the pore clogging by biological activity over the time.
As explained in the article, resting periods help to restore hydraulic conductivity – but planted filters I guess not sand filters.

Working about infiltration with sand filters the publications of Bouwer, Rice et al (see literature) might be helpful as well but that is treated wastewater.

Hope that helps
Christoph
P.S. I forgot, pulse loading is absolutly crucial and intervals of rest and load as well

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