Biomontan tests acrylamide-free flocculant for sand and gravel washing

In process water treatment, the fine particles are removed by sedimentation in large settling tanks, clarifiers or lamella separators with subsequent dewatering. The addition of polymeric flocculants (pFM) improves the agglomeration of the fines into larger particles and enhances the sedimentation rate and drainage behavior. State of the art for years has been the application of synthetic, macromolecular polymer flocculants that contain small amounts of acrylamide (not polymerized due to production) as a residual monomer.

1 Quo vadis – stricter requirements for environmental standards

Legal regulations are imposing increasingly strict guidelines on the use of chemicals in the environmental sector. The EU Drinking Water Directive, which came into force following the conclusion of a 2-year transitional period on 12 January 2023 and obliges the EU states to comply with the new requirements for the quality of drinking water, sets the limit for the acrylamide content of drinking water at 0.1 µg/l as acrylamide is classified as “carcinogenic” according to the WHO. In addition, for the use of polyacrylamide-containing polymer flocculants, the standards EN 1407:2008 [2] and EN 1410:2008 [3] set the maximum permissible residual monomer content at < 200 ppm.

2 Sand and gravel washing with polymer flocculants (PF)

In the scope of a project for a new wet screening plant with integrated process water treatment, the authorities expressed concerns with regard to the use of conventional flocculants. It is necessary to ensure that in the event of sudden water discharge (like, for example, on account of a leak) from the process water treatment system, the concentration of acrylamide as specified in the EU Drinking Water Directive is not exceeded. However, since no relevant literature is available regarding the accumulation of acrylamide in the circulated water of wet screening systems, Biomontan Produktions- und Handels GmbH was tasked by the project applicant to conduct a relevant investigation.

3 Testing the accumulation of acrylamide in circulated water

With the currently available products with drinking water approval (low content of non-polymerized acrylamide), the limit stipulated in the EU Drinking Water Ordinance is complied with in the case of single use. In practice, however, process water is recycled more than once to minimize the amount of freshwater required, with a flocculant being added to improve sedimentation in each cycle. While this is largely bound in the separated solids, a small amount of flocculant remains in the circulated water. As a result, theoretically, an increasing amount of acrylamide can concentrate in the washing water.

In order to check the actual extent of this enrichment, the circulation of the process water was reproduced in the laboratory: 10 l washing water was flocculated and sedimented with the addition of 2 ppm (2 g/m³) of a PF with drinking water permit. After sedimentation, the clear water phase was decanted, and replenished with the freshwater to the original volume (approx. 4 % freshwater) and fine sediment was added so the dry substance was around 4 % again. Then it was mixed and again flocculated with 2 ppm PF. This sequence was repeated 25 times, so that the clarified water was theoretically completely replaced with freshwater by the end. From every fifth repeat cycle (cycle 5, 10, 15, 20 and 25), a sample was taken and the acrylamide content was analysed at an accredited analytical laboratory by means of HPLC-MS (High-Performance-Liquid Chromatography with mass spectrometry).

The laboratory tests have shown that for a single dose (2 g PF/m³), the acrylamide content in the circulated water remains under 0.02 µg/l, but already from the 5th cycle, the acrylamide limit of 0.1 µg/l stipulated in the EU Drinking Water Directive was exceeded.  After 25 repeats and complete substitution with freshwater, the value at 2.4 µg/l was well above the limit (Fig. 1).

4 Alternatives?

For several years, a wide range of different starch-based flocculants has been on the market, but have rarely actually been used due to their significantly higher consumption and the poorer performance compared to conventional flocculants. Despite steady further development and intensive research, the currently available flocculants based on renewable materials still lag behind conventional polyacrylamides in terms of performance [1].

For this reason, Biomontan has taken a different approach. Since 2022, the company has been offering a new flocculant with the name M-FLOC^® DAF 800 that does not contain any polyacrylamide and is comparable with conventional products in terms of performance.

5 PNEC value

The PNEC (predicted no effect concentration) is the term used for the predicted concentration of a generally environmentally hazardous substance up to which no impact on the environment can be determined. As long as the concentration in the environment remains under this value, no adverse effects should be observed. For acrylamide, the PNEC in freshwater is 30 µg/l, while for the monomer of the new PF, the PNEC is 2000 µg/l (Table 1). This means that a much higher concentration in the environment is rated as being non-hazardous.

The outstanding feature of the acrylamide-free flocculant M-FLOC^® DAF 800 is that an alternative monomer is used as a basic building block instead of the environmentally hazardous acrylamide. Currently, there are no limits defined for this monomer, but the PNEC for this monomer is almost 100 times higher than that for acrylamide.

6 How effectively does the product work?

In classical sedimentation tests, flocculation behaviour and sedimentation rates were tested with different flocculants. The acrylamide-free flocculant was compared with conventional and starch-based flocculants. Previous tests have already shown that biopolymers require a much higher quantity than conventional flocculants (8 – 10 times the amount). In comparison, the dosage of M-FLOC^® DAF 800 was significantly lower (Table 2).

The settling rate of the treated washing water, which is of essential importance for treatment in sedimentation basins, reveals the major advantage of M-FLOC^® DAF 800 compared to a starch-based PF.  Although the sludge settles slightly more slowly, after 25 seconds the sediment has sunk to 180 ml and thus 18 % of the starting volume. In comparison, the value for the acrylamide-containing PF was 120 ml or 12 % and that for the starch-based PF was 500 ml or 50 % (Fig. 3). Furthermore, the turbidity after treatment with M-FLOC^® DAF 800 was much lower than with the starch-based PF (Fig. 4).

In summary, the usage of the acrylamide-free product can lead to a slightly higher dosage and some compromises in performance, but these remain at an acceptable level. For autumn 2023, field trials at two gravel plants are planned where the technical capability of the M-FLOC^® DAF 800
product is to be tested.