Superabsorbent Polymer Performance & Conductivity (and TDS)

We are often getting asked “How much Waste Lock® super absorbent polymer will I need?” The answer is — It depends on the character of the liquid being absorbed. As we know, SAPs absorb by means of a diffusion gradient built into the backbone of the polymer chains. One gram of Waste Lock® 770 will absorb about 450 grams of DI water. But start adding cations (Sodium, Calcium, Magnesium, etc…) to the water and absorbency starts to fall. One gram of the Waste Lock® 770 will absorb about 120-150 grams of tap/well water or 55 grams of a 1% NaCl solution.

The best way to gauge is to test a smal sample of waste with polymer to determine the absorbency ratio. If that is not possible — or if one is processing many batches of waste — measuring CONDUCTIVTY is an easy and effective way to determine the relative “challenging nature” of the waste liquid. It also allows waste processors to measure diffeerences between waste batches.

Conductivity is the ability of a liquid to conduct a current. It is the reciprocal of resistance (ohms) and is expressed as Siemens (S) or in many cases milli-Siemens (mS) or micro-Siemens (µS) per centimeter (cm).

The Conductivity of most DI Water is around 0.05 to 0.2 µS/cm. Distilled water is typically 0.4 to 0.7 µS/cm. Start adding cations and the Conductivity climbs rapidly:

Waste Water: 0.9 to 9 mS/cm (Note: milli-Siemens not µS/cm !!!)
Brackish Water: 1 to 80 mS/cm
Ocean Water: 50-55 mS/cm
10% NaOH: 355 mS/cm

The point here is to track the RELATIVE Conductivity of different waste streams. We sometime will get a call from a customer saying that “the super absorbent polymer is not working as good.” In practically every case, the customer’s waste has a much higher cation load (Conductivity) than an earlier waste batch. Hence, more sorbent is required.

Conductivity meters are easily and cheaply available on-line and many sell for as little as $100-$200. It is a fast, reliable, nondestructive, inexpensive and durable means of measuring the ionic character of a sample.

Another option is to measure Total Dissolved Solids (TDS). TDS is inferred from Conductivity and then expressed as PPM (mg/Liter). TDS is mainly used in fresh water systems and it counts both inorganic & organic constituents of the waste liquid. The relationship between Conductivity and TDS is:

2 µS/cm = 1 PPM of TDS (mg/L)

Either system allows a waste processor to evaluate new waste batches prior to processing and sorbent addition.

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