Measurements of suspended solids – How does the salt content and the quantity of rinsing water affect the result?

According to the directions for waste incineration, NFS 2002:28, the total suspended solids in waste water from waste incineration plants shall be analysed through daily random sampling measurements or through representative flow proportional twenty-four-hour samples. According to the SS-EN 872:2005, several plants that analyse suspended solids have difficulties to comply with the terms for suspended solids. This was of concern first of all to the plants dimensioned for high chloride content in the outgoing process water. This report is a translation of the Swedish report “Mätning av totalt suspenderat material – Hur påverkar salthalt och mängden sköljvatten resultatet?” (F2010:02).

Avfall Sverige – Swedish Waste Management has for several years tried to raise awareness to this problem and has in a joint working group with representatives from Swedish waste incineration plants tried to achieve a better understanding of this phenomenon. The key question is if the water from the plants concerned really has a high content of suspended solids or if there are other reasons behind the high values observed.

The results of the study indicate that the most important factor is the salt content and the quantity of rinsing water used in the analysis. According to standards, all samples shall be rinsed with 2 x 20 ml of water. If the salinity exceeds 1000 mg/l, an additional 3 x 50 ml should be used. The way this is per-formed in practice varies among laboratories, and no laboratory makes regular measurements of the salt content in samples. If not specifically mentioned at the sample delivery, most laboratories use the standard quantity of rinsing water of 2 x 20 ml.

The samples that were taken also indicate that with really high salinity the 190 millilitres stipulated by the standards are not sufficient. In such cases, it is likely that additional rinsing water is needed. In order to be sure, further measurements are necessary.

A relatively simple method to estimate the salinity of the sample is to measure its conductivity. Making conductivity measurements of process water from incineration plants a routine in laboratories could thus be a first step towards a more uniform execution than there is today, as the treatment differs quite a bit as to the quantity of rinsing water between labs.   Conductivity could also be used to indicate the quantity of rinsing water at especially high salt contents, if further studies show that this is necessary.

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