Vacuum Filtration System for Water Quality Tests

I use gravimetric analysis a lot in my environmental engineering projects involving either water or air quality measurements. Gravimetric analysis (not to be confused with gravimetry) is a series of techniques where the amount of a substance is measured by determining its weight. The weight of the substances is determined typically after it has been removed from a solution by precipitation, vaporisation or most commonly filtration. I use this to 1.) determine the amount of suspended solids in water and 2.) measure the concentration of atmospheric particles (after they have been captured in tray of water). I did some research into buying a filtration system – fell off my seat at the cost – and so decided to build my own.

I hope that you find this Instructable useful if you want to do the same thing. I have some other projects youo might like at flowrc.co.uk

The main parts of the filtration system are a funnel for the water sample, a flat perforated plate to sit the filter paper on, a vessel for capturing the filtered water and a system for applying some pressure to speed up the process. To apply some pressure I have a vacuum hand pump that is supposed to be used to bleed car brakes. It works perfectly and cost less than £15 (say $20) on-line. I highly recommend getting something like this if you are filtering particularly mucky water and/or using filter paper with a small pore size.

Overall I estimate that in total the system costs around £40 or say $65 USD. The materials and equipment used are.

Materials

3 no. A4 sheets of 3mm thick acrylic

1 no. A4 sheet of 5mm thick acrylic – used for the flange plates

8 no. M8x25mm hex bolts and nuts

1 no. hose nipple with a screw thread and locking nut

1 no. tank tap

1 no. hand vacuum pump

Acrylic solvent (glue)

Silicon baking sheet

Equipment list

A laser cutter (sorry about that I realise not everybody has access to one).

The body of the filtration system is cut from clear acrylic (Perspex). I cut the acrylic using a laser cutter to get a really accurate (and water tight) joints. I appreciate that this is a specialist piece of kit, but if you live near a hack-space I’m sure they would cut this for you.

The templates of the different parts are shown here accompanied by a photo of what they look like when glued together. It took me a couple of goes with the solvent/glue before everything was water tight. I determined the dimensions of the unit considering the typically volumes of water I would be filtering (about 0.5 to 2L), the size of the filter paper (90mm in diameter) and the thickness of the acrylic. The unit is made from 3mm thick acrylic, except the flange plates. These are 5mm acrylic to give them extra strength.

The part of the system for collecting the filtered water has two fittings. One is a hose nipple for applying the pressure to the system and the other is a tap so you can empty the vessel without undoing all the bolts. I fitted the tap before I glued the vessel together because I had better access at that point. Both fittings slot through the acrylic and are secured on the other side by a locking nut.

In order to pressurise the system the connection between the bottom flange plate and the filter plate has to be air-tight. This was achieved making a gasket out of a sheet of silicon baking mat. It might also be a good idea to make a gasket for the top of the filter plate to ensure it is water/air-tight. I didn’t do this as the system seems to work just fine without it and I didn’t want to create a crevice for deposit to hide in.

The accuracy of measuring amount of suspended solids is all

about the quality of the filter paper, the accuracy of your scales that you need to weight the paper before and after filtering the sample and drying the paper to a consistent level.

There is a standard method for the determination of suspended solids in water. Here is my shortened method.

1. Weight the filter paper (W1)

2. Place the filter paper on top of the perforated plate.

3. Place the funnel on top ensuring that the bolt holes are aligned

4. Fit and tightened the bolts. Tightened opposite bolts together to ensure an even pressure over the plate. Be careful don’t over tightened the blots as the acrylic is quite brittle. You won’t need to use a spanner – finger tight will be fine.

5. Fit the hose from the vacuum pump

6. Wet the paper (with distilled or deionised water) and generate a small pressure

7. Note the volume of the sample (L1)

8. Pour in the sample and continually apply pressure as needed.

9. If the collection vessel gets full open the tap to drain (whilst releasing the pressure)

10. Continue until the entire sample has been filtered, washing down the sides of the funnel as necessary.

11. Remove the bolts

12. Remove the filter paper and place it in a safe place to dry (I use a vented tuperware box)

13. Weight the filter paper again (W2), say 24 hours later

14. Determine the weight of solids removed from the water by subtracting W1 from W2 (in grams or milligrams)

15. Divide the weight by the volume of sample to determine the amount of suspended solids in the water sample mg/l or g/m3 or whatever units you used. [(W2-W1)/L1]

Again there are many types of filter paper with different pore sizes and we have to take this into consideration when we interpret our results. In addition the conditions in which we stored and dried the filter may affect accuracy but for most purposes the results will be pretty reliable. Strictly speaking we should have dried the filter paper at 105oC for an hour before each weighting. A small warning if you don’t have scales that measure to milligram level you may have to filter quite a lot of water before you get a measurement that you can rely on!

Things I’d do differently,

· Also one of the bolt holes sits right over the hose nipple making it difficult (impossible) to remove without removing the nipple – I’d definitely change that!

· Also it can get a little top-heavy and unstable and could do with a little stand to keep it steady.

If you have any suggestions or questions then please drop me a note.