The removal of Phosphorous (P) from effluents prior to discharge back into the environment to prevent the eutrophication of receiving waters has been a challenge for wastewater treatment engineers. The standard solution used by the Water Companies has been chemical dosing, using ferric to remove P through the co-precipitation and settlement of iron salts.
Tightening and more broadly applied P consents has increased the requirement for a P removal solution that does not require power, freshwater, chemicals, generate sludge or require sludge storage systems.
ARMPhos is a reed bed engineering solution comprising a reactive media set within a wetland technology engineered framework.
- A bespoke designed solution for each site to achieve compliance
- Construction/Installation and commissioning
- Process guarantees where appropriate and depending on site requirements
- Consultancy, advice and recommendations based on performance
- Maintenance services
- Refurbishment services
Mechanism of Action
The reactive media, based on apatite, offers two modes of action, an initial adsorption/crystallisation phase of phosphate on to the apatite which stimulates the second phase of crystallisation of calcium/phosphate complexes out of solution on to the adsorbed crystals.
French wetland Engineers could see the need for a low/no energy, low maintenance P removal system over 12 years ago to match the characteristics of their highly successful full sewage treatment systems used widely in France. Using a reactive media in a wetland media setting was the obvious solution so they investigated a whole series of different reactive medias with P removal capacity including those based on iron, aluminium and calcium in the form of Zeolite, Expanded clays, LECA, Steel slag and Pozzolana to name a few. Apatite (rock phosphate) which comprises calcium phosphate with other salts and cations proved to have the highest P reduction capacity.
To the market
Initial attempts at bringing this to the market were with apatite beads formed with cement binders to increase the granularity of the media to improve hydraulic conductivity. A number of systems were constructed in this format but issues with the cement binders affecting pH, and inhibiting the second crystallisation mechanism resulted in a reversion to the use of raw apatite, ARMPhos, which if graded offered:
- Acceptable hydraulic conductivity
- Greater reactivity/binding sites
- Minimal pH effects
It was noted that the smaller particle sizes, finer grains, performed better in terms of P removal which may be influenced by increased surface area though there is thought to be a higher proportion of apatite within the finer grains also. The media supplied by ARM Ltd with the ARMPhos solution is graded to an optimum balance of finer and larger particle sizes to provide excellent P removal capacity and good hydraulic loading capability also.
The removal mechanisms result in the formation of more apatite through the complexing of calcium and phosphorous in solution and subsequent crystallisation. The time to refurbishment is based on reduction in permeability, though results to date suggest this would be between 8-12 years or longer depending on the specific site. The resulting media would be removed but could be processed for reuse either in the same way or passed forward to other users of apatite such as fertiliser manufacturers.