JAR TESTING | index| back| next|

PUTTING JAR TESTING INTO PERSPECTIVE

In my view jar testing is a very useful analytical method and art used to determine mostly qualitative characteristics of a given water. Most of the time jar tests are performed to determine chemical dosage to clarify or soften a water, and obtain a rather subjective view of flocculation and floc settling character. The actual purpose may be to optimize the treatment scheme in an effort to improve water quality somewhat, or it may be to test a treatment schemes applicability, or if something within the equipment is affecting treatment. A skillful and experienced analyst can determine parameters which can be useful in the design of a proposed machine. When a machine already exists the tests should be designed to use machine parameters which are applicable. For example, if the machine size and flowrate are such that a flash mix period of 30 seconds exists it is rather foolish to flash mix for a minute in the test. Frankly, this concept has always seemed obvious to me but I have never seen another analyst consider it. I have seen a good deal of wasted effort put into jar testing.

Alright, I have touched on alot of territory above, and have only presented ideas which only provoke questions in the readers mind. Certainly, questions are what it takes to prepare a plan before jar testing can be undertaken. If the right questions, clearly defined questions, are not asked than no clear answers or data will be derived from the tests.

Let's face it, unless you're a knowledgable engineer and/or chemist or an operator with lots of analytical experience and real technical knowledge you're probably not going to be able to ask the right questions, and therefore not be able to develop a worthwhile jar test plan. This is, of course, the reason why very little is learned from many jar test endeavors. I hope that those of you who have seen jar tests performed and not yield any or very little tangible results are getting the idea.

Obviously, if I'm going to say much more about jar testing I will have to offer some sort of solution, not to mention that I will have wasted your time and mine. I will offer two. The first will take the form of a generalized plan and analysis which may help enable you to develop ideas so you can ask the right questions and hopefully develop a good jar test plan.

DEVELOPMENT OF JAR TEST PLAN AND ANALYSIS OF CLARIFIER, WATER, AND THE PERCEIVED PROBLEM

In order to begin you will need the following:

  1. Detailed prints of the clarifier in question, specifying dimensions of the clarifier internals. Equipment operating manual.
  2. Complete analysis of the raw water, or at least relevant cations and anions; Ca, Mg, Na, Fe and/or Mn if applicable, CO3, HCO3, Cl, SO4, and other anions to approximately balance the cations, pH, turbidity, and temperature at the sample point.
  3. Specifications of all chemicals, and their feed systems, which are fed to the clarifier.
  4. Access to the jar test machine which will be used for the tests.
  5. Access to the operators, and their log book, to help to precisely characterize the perceived problem.
It will probably be most difficult to precisely characterize the problem so begin there. Since this could be anything, observation and discussion will be the immediate tools. If the problem is intermittent or obscure in nature characterization may require "hovering" over or close by the machine for long periods. If the problem cannot be precisely defined, be assured that it will not be solved. After one or two extra shifts at the plant the investigator should have had sufficient time to collect all required data and become familiar with it. If the problem has not been clearly defined the investigator should have at least strong suspicions about it.

As I have stated a good plan for jar testing must be formulated. An outline of that plan could take the form of a data collection sheet. The sheet I like to use is presented here. A quick look at the sheet should evoke questions about how the jar test machine will be used. The raw water data is self evident, flash and slow mix rpm's is not as clear, although it might be to some who use a pre-determined procedure.

For me the following list of questions is raised here:

  1. What size sample and container will I use?
  2. What will be the flash mix rpm and the time of flash mix?
  3. What will be the slow mix rpm and the time of slow mix?
These are design questions and the answers must be based on the existing clarifier in question and how it can best be related to the jar test machine to be used. I will deal with each question individually, but only discuss the analysis. No calculations will be shown, and I will assume the reader is familiar with how clarifiers and jar test machines operate.

I am most familiar with the Phipps & Bird Jar Test Machine. However, I see no reason why other similar devices cannot be used. What is most important is that it be a variable speed device with a mixing paddle, preferably like the one in your clarifier. I also prefer to use a 600 ml. beaker and a 500 ml. sample for three reasons. Obviously, this means I need less sample, (many procedures call for liter or larger containers and a liter sample) and it also gives me a nice round number settling depth, about 4". The third reason is most important and the one which is probably least realized. A well designed draft tube or mixing chamber in a clarifier will have only a small paddle clearance with the wall. This is to create more turbulence and behave like a pump. A draft tube and impellor is a vertical lift pump. I hope you see what affect we want to create here, that is, to simulate the large equipment as much as possible. I highly recommend this system.

If you are not an engineer or good at solving problems you won't be able to do the next part or the slow mix for that matter. E-mail me and we can discuss it.

The flash mix time and speed should also as much as possible simulate the equipment. Now you'll need the detailed print, specs on the impellor drive and/or operating manual. With this said I hope you can see what you have to do.

  1. Calculate the maximum tip speed for both the clarifier and the jar test machine. The jar test machine is 236 ft./min.,quite slow actually, and good numbers for a well designed clarifier are 320 ft./min. for clarification and 440 ft./min. as a lime softener. (these are maximums) Determine the low end and make a linear graph and you will have tip speed for any impellor RPM or speed control setting.
  2. Determine the detention time in the mixing chamber or draft tube. A well designed machine is about two minutes, one minute is probably enough if the water never gets really cold, less than 40°F. Once I saw a machine with a DT of only 20 seconds. It did not operate well.

Okay we have flash mix speed and time, or do we? You will have to determine the optimum mixer or recirculator speed for the clarifier. This was discussed in Operation Mode for the clarifier. Also, if you find that the jar test machine cannot produce the tip speed you need as produced in the clarifier just set the jar test machine to maximum. The main concern is that we don't do a jar test study based on better conditions than those we can reproduce in the clarifier. We have to try out jar test theory in the clarifier, and we will eventually give it our best shot there. Jar testing takes hours, clarifier operational performance testing usually takes at least days.

Slow mix time is detention time of the flocculation zone. Another way to define it is as the volume of water in this zone divided by the flowrate. Finding slow mix speed can be difficult or easy depending on our method. We will use the easy method.

  1. Calculate the slow mix time. Twenty minutes is a good number for our well designed unit.
  2. A rule of thumb slow mix speed is the lowest speed which will suspend all of the floc during the slow mix period. This will depend on the size and type of floc formed during flash mix.

Now that we know the physical parameters for the jar tests we can move on to the next section which will discuss the actual procedure.

The second solution, while still complex and difficult in some ways can remove the aforementioned technical problems from your considerations. That solution is here.

Jar Testing
Last updated 1/23/98
Email: [email protected]

Hosted by www.Geocities.ws

1