Comparing PAM Grades for a Sludge Dewatering Trial

The method below fixes the variables that decide the outcome and leaves the polymer as the only difference. Apart from a balance, a drying oven and, if available, a turbidity meter, it needs no laboratory equipment.

Start with the sludge, not the price

Sludge is the variable that changes most during a trial and gets recorded least. Take both grades from the same sampling point, at the same moment in the process cycle, and write down what that point is: raw sludge, thickened sludge, or sludge that has already passed a conditioning step. The same stream before and after thickening is not the same material, and a sample held overnight is different again.

Record feed solids, whether the sludge has already received lime, ferric salt or another conditioner, the temperature at the time of the test, and any change in the upstream process during the trial. If the trial has to run on stored sludge, run every grade on the same stored sample and record the holding time, the temperature and whether the container was sealed. A comparison between fresh sludge and yesterday's sludge measures storage, not polymers.

Set one dosing basis for both grades

Three different numbers are all called the dose, and mixing them up is the most common reason a comparison cannot be repeated:

· Product dose: grams of product as delivered, per cubic metre of sludge or per dry tonne of solids. This is what the invoice is calculated on.

· Active dose: grams of active polymer per dry tonne of solids. This is what does the work.

· Solution concentration: how much product in how much water, which decides what the dosing pump actually delivers in a minute.

Powder and emulsion grades do not carry the same proportion of active polymer, and the proportion differs between grades and between batches. Take the active content from the technical data sheet or the COA for the grade in front of you, not from what the product family usually contains.

Prepare the stock solutions at the same concentration, with the same water, at the same mixing speed and for the same time, and use them within the same interval after preparation. Polymer solutions change with time and temperature, and a solution mixed the previous afternoon will not behave like one mixed ten minutes ago. Put the concentration and the age of the solution on every run sheet.

Keep the mechanical conditions identical

Dewatering is a mechanical result as much as a chemical one. A grade that looks weak in a beaker can be the better grade in a centrifuge, and the reverse also happens. Keep the conditioning step identical: same device, same speed setting, same duration, and write the setting as a number rather than as fast or slow. Keep the dewatering step identical as well, whether that is the same g-force and spin time in a centrifuge, the same belt speed and pressure on a belt press, or the same vacuum and cycle on a Buchner funnel. Run the grades in one session with one operator, and where possible code the samples so that the operator does not know which grade is in the jar.

Read four results, not one

A trial produces one number that everyone looks at and three more that decide the cost. All four belong on the sheet.

Table 1 — How to read the four results that decide a PAM comparison

Floc size is worth noting but not scoring. Large flocs that break up as they enter the machine are worse than smaller flocs that survive the shear, and appearance alone cannot tell the two apart.

Score the grades against what you pay for

The delivered price per kilogram is not the comparison. What matters is the cost of dewatering one dry tonne of solids, which combines the product dose with the price. Agree the weights before the trial begins, so that the scoring cannot afterwards be arranged to favour the grade someone already prefers.

Table 2 — Scoring sheet for two grades, completed from the same run data

Where most comparisons go wrong

· Different sludge batches used for the two grades, with no retained sample to check the difference.

· Solutions of different concentration or age, compared as though they were the same.

· One dose per grade, chosen from the previous trial, when the optimum may sit elsewhere on the ladder.

· Judging by floc size while ignoring the filtrate.

· Comparing price per kilogram of product instead of cost per dry tonne of solids.

· Changing the machine setting halfway through because the cake was running wet.

The trial log

One sheet per run, filled in at the bench rather than reconstructed afterwards.

Table 3 — Trial log, one sheet for each run

What to send with your inquiry

For a grade recommendation we need the conditions the trial has to run under:

· Sludge type and source — municipal, paper, textile, food processing or another stream

· Feed solids and the dewatering equipment you operate

· The polymer form and dose you use now, if any

· The cake solids or disposal constraint the plant has to meet

· Trial quantity, delivery terms and destination port

With those we can send the grade options that suit the form you can handle, the technical data sheet and COA for each grade, and a sample size that matches your trial design. We do not quote a fixed dose for a sludge we have not tested; the dose is what your trial establishes.

A grade wins on the sheet, not in the beaker: the same cake with less active polymer, or a cleaner filtrate at the same dose.