Improved anaerobic treatment of salty wastewater

Gepubliceerd op
16 november 2017

Waste water containing high concentrations of dissolved salts is unsuitable to be treated in conventional anaerobic wastewater treatment plants. The salt present harms the bacteria responsible for cleaning. By optimizing process conditions, Ph.D. scientist Dainis Sudmalis managed to have a functional system to clean wastewater containing salt. By understanding the mechanisms involved, the scientist hopes to have full-scale operative wastewater cleaning systems, able to deal with salty waste water in the future.   

Many waste water streams contain substantial amounts of salt. For example, leather tanning industries, vegetable pickling factories and cheese manufacturers produce waste water with high amounts of dissolved salt. In some countries, like Hong Kong, even common households produce salty wastewater, when salt water is used to flush toilets. Worldwide, wastewater is often treated using the Up-flow Anaerobic Sludge Bed (UASB) reactor, developed by former ETE Professor Gatze Lettinga in the early 1970’s. The revolutionary system replaced aerobic by anaerobic wastewater treatment and proved to be simpler, more cost-effective, while energy (biogas) and nutrients could easily be recovered. Essential for effective UASB operation are the microbial granules that are formed inside the reactor. These granules consist of densely packed microbes, that are too heavy to be washed out. However, salts present in wastewater may be toxic to microorganisms and interfere with the formation of these granules. For example, sodium may be linked to interference with binding forces keeping the microbial aggregates together. ‘When sodium is present at concentrations above 8 grams per liter, these microbial granules are not formed or fall apart’, says Dainis Sudmalis. ‘The up-flow velocity of wastewater subsequently washes these microbes out, resulting in a dysfunctional reactor.’ It is very common that wastewater contains more than 8 grams of salt per liter. To prevent reactor failure, a more robust principle, resistant to dissolved salts, had to be found.

Diminished reactor performance

Not only negative effects of salt in wastewater towards granule formation contribute to reactor failure. When microorganisms are exposed to osmotic stress, they cope with these conditions by increasing their own osmotic pressure and take up osmo-protectants, like potassium. However, potassium interferes with the enzyme machinery of microbial cells. Therefore, eventually organic molecules, like amino acids need to be taken up or produced inside the cell. This is an energetically costly process, resulting in diminished reactor performance and less biogas production.

Bacterial granules in a UASB laboratory set-up
Bacterial granules in a UASB laboratory set-up

During a first experiment, Sudmalis examined granule formation in wastewater containing salt, while optimizing reactor conditions. ‘Based on literature, we optimized many reactor variables suitable for non-saline reactor conditions’, Sudmalis explains. ‘We enhanced the amounts of organic substrates to be treated per day, up-flow velocity, as well as organic compound mixtures and concentrations, to create a better environment enabling many species of bacteria to grow.’ With all these factors optimized, the only stressor for the microbes was high salinity. To the scientist’s surprise, substantial amounts of granules were formed, despite the salt present. Possibly, the microbiota managed to take up osmo-protective compounds to cope with high salinity conditions.

wastewater substrates will allow microbial activity, but no granule formation. Therefore, we will measure reactor performance as well as granule formation under a variety of conditions.’ In addition, the effect of salt fluctuations on granule formation will be investigated.

Deal with salty wastewater
Together with Paques BV Sudmalis will investigate if the findings from his lab research can be successfully applied to a full-scale reactor running at elevated salinity. Sudmalis: ‘Eventually, we hope to fine-tune and control the optimal reactor conditions in such a way that we can deal with salty wastewater.’

Scanning electron microscopy image of a bacterial granule formed at 20 grams of sodium per liter (Photo: Dainis Sudmalis)
Scanning electron microscopy image of a bacterial granule formed at 20 grams of sodium per liter (Photo: Dainis Sudmalis)

This project is part of Water Nexus.

Key publication: Sudmalis et al. 2018. Fast anaerobic sludge granulation at elevated salinity. Water Research 128, 293-303.