Foam dispute bubbles away
Dow Corning's C Stevens and J Pollack refute STW silicone side-effects
The use of silicone antifoams during sewage and other wastewater treatment has prompted questions as to whether such materials could impair biological degradation efficiency.
Silicone antifoams used in wastewater treatment plants are usually emulsions with polydimethylsiloxane (PDMS) as the primary silicone component. The use of silicones in industrial applications and consumer products means they are discharged in trace quantities to sewer systems. Monitoring of PDMS at eight STWs in the USA showed influent concentration levels ranging from 87 – 373ppb. Levels vary depending on the type of industry served by the sewer system. In areas where there is a high density of textile mills, the concentration of silicone-containing materials may reach 700ppb.
Because of its very low water solubility and high molecular weight, PDMS does not biodegrade during sewage treatment. This finding is based on long-term activated sludge incubation experiments with C-labelled PDMS that show no breakdown to carbon dioxide or other volatile products over a 70-day incubation time. In addition, all C-labelled material was associated with the sludge fraction. In a similar experiment, C-labelled PDMS was added to activated sludge for two months under both aerobic and anaerobic conditions. Most of the PDMS was found to be associated with the sludge and there was no sign of biodegradation, as shown by the absence of carbon dioxide containing C.
Although non-biodegradable, PDMS meets the requirement for >90% elimination during wastewater treatment because it binds tightly to sewage sludge. Laboratory studies and monitoring data show consistent removal efficiencies of more than 94%, with effluent concentrations being less than the 5ppb analytical detection limit. At these low observed concentrations, compliance with STW discharge consents is not in jeopardy.
Analysis of European sludge samples taken in the 1980s revealed average PDMS concentrations of 350ppm (dry weight basis). More recent analysis of sludge at eight STWs in the USA showed a wider range of PDMS sludge concentrations (122- 5,000ppm), the higher concentration reflecting discharges from textile mills. The results also showed that digested sludges will frequently contain higher PDMS concentrations due to the loss of carbonaceous material during digestion. Conversely, lower PDMS levels are found in thickened and composted sludges as they are typically mixed with sawdust or other compostable material.
Despite these studies concerns still exist that silicones could form a layer over the bacteria, inhibiting bacterial activity and treatment efficiency. To investigate this very high concentrations of PDMS, up to 10,000ppm of dry weight of mixed liquor suspended solids (MLSS), were fed into a pilot-scale activated sludge treatment plant. PDMS loadings had no effect on the operating parameters (pH, suspended solids, sludge volume index, and specific oxygen uptake) or physiological activity of the sludge microflora in the model activated sludge units. Sludge digestion operating parameters (suspended solids, gas generation, and pH) were also unaffected.
In addition to this experimental evidence, PDMS antifoams have been used for many years for foam control in STWs. Silicone products such as Dow Corning Antifoam RD Emulsion, Dow Corning 1510 Silicone Antifoam and Dow Corning Antifoam 2210 have all been tested in an activated sludge respiration inhibition test (OECD Guideline 209), and show no adverse effects at concentrations up to 500mg/l. The recommended application rate for silicone antifoams is typically less than 15ppm.
The presence of PDMS in digested sludge does not present a problem when using sludge as a soil additive or for composting, because once the PDMS is added to soil it starts to degrade. The rate at which this occurs depends on the type of soil and conditions. A number of studies show application of sewage sludge containing PDMS to agricultural land does not lead to a build up of PDMS in the soil. Degradation rates can range from days to weeks. The degradation process is natural, including clay-catalysed hydrolysis of the PDMS followed by oxidation and biological degradation. The ultimate degradation products are carbon dioxide and inorganic silicates.
When sewage-sludge amended soils are used for agricultural purposes, there has been no observed effect on seed germination and plant growth or survival. This finding is based on experiments in which soil cores are taken, amended with PDMS-dosed sewage sludge, then cropped successively with wheat and soyabean. No leaching of PDMS was detected, and over a seven-month period, up to 50% of the applied PDMS was found to degrade.
When sewage sludge is incinerated, all the organo-silicon materials will decompose to form inorganic amorphous silica, water vapour and carbon dioxide. As no chlorinated compounds are present, no hazardous chlorinated by-products can be formed.
There is much evidence that silicone antifoams can be safely used for foam control in STWs plants. Toxicity studies confirm no bacterial inhibition even when tested at concentrations of 500ppm, which is far above recommended usage levels. There is also evidence to show that no silicone component of the antifoam will be present in the final effluent except at very trace concentrations, less than the detection limit of 5ppb, because it binds tightly to particulate matter.
It is therefore removed with the sewage sludge and ultimately be degraded to silica, water and carbon dioxide when the sludge is applied to soil or incinerated.