Using the Jar Stirrer
The Jar Test Method
The purpose of the jar test is to use the results that it produces to effectively optimise the performance of a water treatment plant. Essentially the jar tester is a series of 4 or 6 mini clarifiers. The essence of getting the best out of a jar test is to know how to relate the results obtained back to the operation of the treatment plant.
1 ppm = 1 mg/l = 1g/m31000 litres = 1 m3 (1 cubic metre)
Most jar testers have 1 Litre jars. For this reason a 10,000ppm Alum solution is the ideal strength for most jar test methods. Alum solutions can be made up using the following methods:
- Solid AlumSimply weigh out 10g of the solid alum with an accurate balance and dissolve into 1 litre of water (distilled or deionised if possible).
- Liquid Alum (as delivered)The solution strength of liquid Alum supplied by IChem Ltd is 47% by weight. That means 1 tonne of liquid Alum has 470kg of Solid Alum dissolved in it. The Alum molecule Al2(SO4)3 aq weighs more than the water molecule H2O, a tonne of liquid Alum is not 1000 litres. It is in fact 757.5 litres. To express delivered liquid Alum in terms of parts per million we have:470,000 grams of Alum in 0.7575 cubic metres of water
470,000 grams / 0.7575 m3 = 620,500 ppm
To dilute this down to 10,000 ppm you should approximate by diluting 16mls into 1 litre of water, (distilled or deionised if possible), or:620,500ppm x 16 / 1000 = 9928ppm = approx 10,000ppm
Poly Aluminium Chloride
- SoliPACAs with solid alum, weigh out 10g of poly aluminium chloride and dissolve into 1 litre of water.
- LiquiPACThe LiquiPAC is supplied as 35% solution. Like the alum molecule, the PAC molecule [Al2(OH)5Cl2.(6SO4)]n, weighs more than water. For the delivered tonne of LiquiPAC you have:350,000 grams of PAC in 0.8333 cubic metres of water
350,000 grams / 0.8333 m3 = 420,000 ppm
To dilute this down to 10,000ppm you should approximate by diluting 24mls to 1 litre of water:420,000ppm x 24 / 1000 = 10,080ppm = approx 10,000ppm
The typical polyelectrolyte dose in most water treatment plants is around 0.1ppm. Due to the low doses,the ideal solution strength to use in a jar test is only 100ppm. Such a weak solution is often difficult to make up without the aid of a modern analytical balance. For the operator the best way to make up a 100ppm is to dilute down some polyelectrolyte from the dosing tank.
Say you add 2kg of polyelectrolyte to a 1,000 litre tank.
You have a:
Then dilute 50mls to 1 litre of water:
10,000ppm solutions of Alum are stable for at least a week and should be stored in a dark, cool place or a fridge. 10,000ppm PAC should be prepared daily as it deteriorates on storage. For this reason, during full plant operation, IChem Ltd recommend that LiquiPAC be dosed neat to avoid this problem. A 100ppm Polyelectrolyte solution should be kept no longer than two days and also stored in a dark, cool place or fridge.
The Jar Test
As much as possible the jar test is a mimic of the treatment plant. As every treatment plant is different, every jar test should also be different. The operator prior to doing a jar test should first have calculated the delay times between each chemical addition point. For ease of explanation however, we will outline the jar test for a “typical” plant that is often encountered. ie:
|Coagulant dose pt polyelectolyte dose pt||3 mins|
|Polyelectrolyte dosing to entering clarifier||1 min|
|Resident time in clarifier||1 hour|
- Fill the tester jars with the raw water to be tested.
- Set the jar tester speed to approximately 100rpm. Typically, this is when there is a small vortex forming in the jars. Too fast a speed will break up the flocs as they form. Too slow a speed will not mimic the plant and will not produce sufficent mixing of the coagulant. The coagulant is Alum or PAC. With a 10,000ppm solution strength and a 1 litre jar, a 1ml addition to the jar equals a dose of 10ppm.While the jar tester is going, add different quantities of coagulant to each jar.
ie: 0, 10, 15, 20, 25, 30, 35ppm Alum
- Allow the jar tester to run for 3 minutes and then reduce the speed down to a slow stir of around 30rpm.
- Run at this slow speed to allow the coagulant only floc to develop fully. This can take up to 30 minutes though typically 10 minutes is common, then stop the jar tester.
- If the floc that has developed looks even with good water clarity, pick the jar with the best result to set the plant coagulant to. If you have underdeveloped, or pin floc repeat t different doses until the best result is obtained. Remember that pin floc can result from either over or under dosing.
- Once the optimum coagulant dose has been determined the polyelectrolyte dose can then be found. Firstly, dose the optimum coagulant dose to every jar ( fresh raw water ) and run at 100rpm for 3 minutes.
- After 3 minutes add the polyelectrolyte. Remember, 1ml = 0.1ppm. Typically, expect doses around 0.05ppm to 0.2ppm.Run at 100rpm for another minute and then reduce to a slow speed of around 30rpm.
ie: add 0.5, 1.0, 1.5, 2.0, 2.5, 3.0mls of 100ppm stock soln.
- Again, allow the floc to develop then stop. The best polyelectrolyte dose will result in larger flocs than coagulant only. Floc should be even and the settling rate enhanced.
- Having obtained optimum coagulant and polyelectrolyte dosage now work these doses back to the plant’s operation.
From the jar test you will have obtained two optimum dosages:
Xppm of Coagulant (Alum or PAC)
Yppm of Polyelectrolyte
To relate these doses back to the plant you will need to be able to time your dosing pumps, know the strengths of your dosing solutions and also the flow rate of water through the plant.
Lets assume we have a plant producing 200 m3/hr of water requiring 20ppm Alum and 0.15ppm Poly.
The Alum tank is 20% by volume Alum and the polyelectrolyte is at 0.2%.
Pump Rate = Coagulant Dose (g/m3) x Flow rate (m3/hr) / Alum tank strength (g/l)
ie: Pump Rate = 20 g/m3 x 200 m3/hr / 200 g/l = 20 l/hr
Pump rate = Polyelectrolyte Dose (g/l) x Flow rate (m3/hr) / Polyelectrolyte Tank Strength (g/l)
ie: Pump Rate = 0.15g/l x 200m3/hr / 2g/l = 15 l/hr