Small-scale sewage sludge dewatering: when a filter bag is the right tool
In large treatment works, sewage sludge dewatering is a job for machinery. Centrifuges, screw presses and chamber filter presses run continuously there, are designed for a steady inflow and pay for themselves on volume.
There are plenty of situations where none of those conditions apply. Package treatment plants with periodic desludging. Decentralised plants at industrial sites, campsites, hotels and residential developments. Municipal works with a side stream or an incident to deal with. Plants where the press is down for a week and the sludge keeps arriving.
Key points at a glance
In those cases the question is not which machine is best but what you do when there is no machine. A sewage sludge dewatering bag is a workable…
The sludge is pumped from the desludging or holding tank into a free-standing filter bag . The bag stands on made-up ground above something that will catch the…
This is the most important point with sewage sludge, and it is regularly underestimated. Municipal sewage sludge consists largely of fine, water-binding organic flocs. Unconditioned, that material gives…
Dewatering sewage sludge when a press does not pay
In those cases the question is not which machine is best but what you do when there is no machine. A sewage sludge dewatering bag is a workable answer.
How sewage sludge dewatering in a filter bag works
The sludge is pumped from the desludging or holding tank into a free-standing filter bag. The bag stands on made-up ground above something that will catch the filtrate. Water leaves through the textile area, the solids stay behind and consolidate over time.
Because the bag is open at the top and has no fixed inlet connection, it can be filled with the pump equipment you already have. No modification to the plant is needed, though you do need a suitable standing area and a settled route for the filtrate.
Once dewatering has gone far enough, the bag is lifted by its strap loops and sent for recovery or disposal together with its contents.
Conditioning matters more than the filter fabric
This is the most important point with sewage sludge, and it is regularly underestimated. Municipal sewage sludge consists largely of fine, water-binding organic flocs. Unconditioned, that material gives up water reluctantly, whatever filter medium you put it in.
With suitable conditioning, usually a polymer, the behaviour changes markedly: the flocs become larger and more stable, a more permeable filter cake forms, and the water finds a route. Where bag dewatering does not work, our experience is that conditioning is the cause more often than the bag is.
Which product at which dose is a question for the sludge, and it sits with the plant operator or the chemicals supplier. The filter medium comes from us, the process chemistry from upstream.
What gravity dewatering can and cannot do
Achievable dry solids and dewatering time depend on the sludge, on conditioning and on standing time, and cannot be stated in general terms. What can be said:
Gravity dewatering removes free and loosely bound water. It does not reach the figures of a pressure process, because no pressure is applied. The advantage is not in the result per batch but in the fact that no plant has to be kept on site at all.
How far it goes in your case is quickest to establish with a single bag: fill it with a real batch, record the standing time, weigh the bag, scale the result to your annual quantity. That one measurement replaces any general figure.
Which sludge dewatering bag suits which quantity
Which bag suits you follows in the first instance from the quantity per desludging run.
| Quantity per run | Sensible starting point |
|---|---|
| up to about 100 litres | Size M, 0.125 m³ |
| 100 to 400 litres | Size L, 0.5 m³ |
| 400 to 700 litres | Size XL, 0.8 m³, if the height suits |
| over 700 litres | Size XXL, 1.25 m³ or several bags |
With sewage sludge, density after dewatering is usually well below that of mineral sludges, so working load is less often the limiting factor than volume. The rule still stands: weighing once beats estimating three times. The calculation is set out in the size guide.
When to stay with the machine
Where there is continuous inflow through the day. Where dry solids figures are set contractually or by regulation. Where quantities run to cubic metres a day. Where there is no floor area for bags to stand for as long as they need.
In those cases bag dewatering is at most a supplement for downtime and side streams, not a replacement. For an assessment of a specific process, our industrial filtration consulting is the place to start.
Questions from operators of small and decentralised plants
Does it smell?
Sewage sludge smells, and dewatering changes little about that. With longer standing times and warm weather the smell can get stronger. A covered or lidded standing area and short standing times both help. Where neighbours are sensitive to odour, this point belongs in the planning.
Can the filtrate go back into the plant?
Returning it to the inlet is often the obvious route, because the filtrate carries an organic load. Whether and how that works in your plant is for the operator to decide on the basis of hydraulic and organic loading.
How long does a bag need to stand?
That depends on the sludge and on conditioning; the process guide explains the factors. Plan the standing area generously so that you are not lifting under time pressure.
Are the bags suitable for digested sludge?
Digested sludge generally dewaters better than raw sludge. That is only binding after a trial with your own material.
What about waste classification?
Bag and contents are disposed of together. How the dewatered sewage sludge is classified and recovered follows the applicable requirements and the analysis for your plant. The operator settles that with the waste contractor.
Still unsure which filter or next step fits?
Send us the medium, dimensions and application. We will help you decide whether a standard product is suitable or whether the case needs a custom solution.
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