Phosphorus leakage causes eutrophication. Ecofiltration Nordic develops silicate based filters that capture phosphorus from sewage water and runoff from farmland, so it can be returned to productive land. The material can also be used to restore eutrophic waters.

Phosphorus is an element used in the genetic material (RNA, DNA) of both plants and animals, and is vital for all living cells. In the human body, phosphorus is involved in the cell metabolism, and is a component of bones and teeth. Fertilizer is the primary commercial application of phosphorus compounds, but they are also found in various other products such as toothpaste, foodstuffs and laundry detergents.

The natural filter media Polonite consists mostly of silicon dioxide (SiO2) and calcium oxide (CaO). The material has excellent capabilities of capturing phosphorus from sewage water. Photo: Ecofiltration Nordic.

Large amounts of phosphorus leach from farmland and sewages, contributing to eutrophication. The recurrent algal blooms and oxygen depletion in the Baltic Sea are partly caused by a surplus of phosphorus; about 30 000 tonnes of it reach the Baltic annually. Meanwhile, the global supply of phosphorus is limited, and a deficit of the resource would have a large impact on the future food production.

Ecofiltration Nordic AB has developed a reactive filter media that makes it possible to remove and recover the natural resource from sewage and runoff water, reducing the amount of nutrients that reach streams, lakes and seas. The purification and recycling technology is based on more than 20 years of research conducted by Professor Gunno Renman at the Royal Institute of Technology (KTH) in Stockholm.

Natural filter media

”A circular economy would both recover the substances for reuse, and limit the amount of chemicals being released to the environment. This calls for simple, robust and cost efficient technology, and our reactive filters have exactly those properties”, says Anders Berggren, interim CEO of Ecofiltration Nordic AB.

”Our natural filter material is a reactive sorbent, containing calcium, that quickly reacts with and absorbs the substance to be removed. The Polonite product is based on a silicate mineral from Poland. The raw material is porous, easily mined and has excellent capabilities of capturing phosphorus from both sewage water and runoff water from farmland. Positive calcium ions on the surface of the filter media bind the negative phosphate ions; a reactive “phosphorus trap”, so to speak. Thanks to the large active surface area – 500 kg of Polonite provides a reactive surface equivalent to 40 tonnes of regular sand – the material is also able to more or less eliminate bacteria and odour.

”After a brief period of drying, saturated Polonite can be used as an excellent fertilizer without further after-treatment. The captured phosphorus is slowly released – at the same pace plants are able to absorb it. Furthermore, the filters also contain calcium and silicon, which are important nutrients for soil and plants”, Anders Berggren says.

Private sewage filters

”Everyone with a private sewer is required to filter out phosphorus from the waste water. The reactive filter media offers several advantages: it is a natural product, so the home-owner does not have to buy chemicals or estimate proper dosage. There are no moving parts in the filter, nothing that can break, making them maintenance-free. The operating cost is low compared to chemical phosphorus removal, since less sludge has to be removed”.

”The individual sewer is upgraded with a phosphorus filter at the end of the cycle, as a tertiary treatment, before the water is released to the recipient. Installation and operation is hassle-free. The only maintenance that is required is to replace the filter material every two years”.

Water restoration

Phosphorus from agriculture runs off from fields down into drainage ditches, from where it continues to spread into streams, lakes and seas. The greatest part of this run-off occurs in the spring, when snow and ice melts, and when it rains in the fall.

”We have developed a method to restore eutrophic water. By using our filter technology, large amounts of phosphorus can be captured either by pumping out the water, filtering it through filter media, and then releasing it back to the water source again, or by treating the bottom sediment, which often is depleted in oxygen. The captured nutrients can be recycled back into local fields as fertilizer when the filter is changed”, Anders Berggren says.

Large scale wastewater treatment

The reactive cleaning process minimizes the release of phosphorus from private sewage. After a couple of years in use, the filter material is saturated with phosphorus. It can then be reused as fertilizer and soil improvement. Illustration: Ecofiltration Nordic AB.

Reactive filters offer a sustainable alternative or complement to traditional chemical precipitation, primarily in smaller wastewater treatment plants. Such phosphorus filters are installed as end polishing, and can flexibly be installed stationary or mobile. Benefits include savings on precipitation chemicals and expensive sand filter beds. The material can also be used for sludge treatment. The Polonite Stabilizer product is used in thickening and stabilization of sludge as a competitive alternative to lime.

”In collaboration with six British waste water treatment plants, Polonite was recently evaluated at the Somerton plant (21 000 population equivalents). The purpose of this pilot experiment was to compare the Polonite filter bed technology with chemical precipitation. The British environmental agency is about to put stricter regulations in place for small and medium-sized waste water treatment plants, regarding phosphorus treatment. We also evaluated how well Polonite would remove bacteria. The results were positive and it is clear that the reactive filters are working well in both small scale and large scale applications”, Anders Berggren says.

Polonite is also finding use as an alternative to innovative biological purification technology. Read more in the article ”Natural cycle purifies wastewater”, describing how Alnarp Cleanwater Technology uses plants and microorganisms to clean private sewers.

The article was published in january 2017.