What is ‘Purification’?
Accorging to environmental legislation in force in Sweden, anyone operating a wastewater treatment plant has a responsibility not to disseminate phosphorus, municipalities as well as private house owners not connected to the grid. Authorities have specified requirements for treatment efficacy (purification limits). These limits, and especially the application of them, are not always clear.
Authorities are defining levels of purification, but what does it mean in reality? Is it really ‘purification’ we are talking about, or would the more relevant term be ‘buffering’ or ‘retention’?
Many on-site treatment plants to day consists of some sort of soil or sand/gravel bed. Often for ground infiltration, or otherwise a bed with a watertight layer at the bottom and an outflow. Sometimes a cross-over between the two where water not possible to infiltrate is let out through an outflow pipe.
Phosphorus is an element in the periodic table. Therefore it cannot be dissolved and unlike e.g. Nitrogen, phosphorus cannot easily be transformed into a gas phase. So what does purification actually means when it comes to phosphorus? There are basically three ways to deal with phosphorus in wastewater:
1. Disseminate wastewater into some kind of bed.
The porosity and natural micro-biology in the bed-material will be able to buffer a certain amount of phosphorus. After a couple of years the bed material will be saturated and the phosphorus will continue to the ground surrounding the bed. Here, it might stick in the soil, at least for some time, until rainfall and the wastewater itself pushes the phosphorus further. Sooner or later, it is likely it will mix with the groundwater. It is impossible to state when and what will happen in each specific case, but there is a strong probability that it will, eventually, end up in water basins (like the Baltic Sea) where it will add to the eutrophication. But is this ‘purification’? We would argue that ‘Buffering’ or ‘Retention’ are more accurate words to describe the function. In any case, this phosphorus is lost forever.
2. Apply chemicals to the wastewater to chemically bind phosphorus
In this process, known as ‘chemical precipitation‘ or ‘flocculation’, certain chemical compounds, based upon aluminum or iron, are added to the wastewater. These chemical compounds will create flocks of phosphorus that will be gathered together with sludge separated in the first treatment step.
To make the process work, you need to dose the chemical compound accurately; in practice this means in proportion to the actual flow. Adding too little chemicals means some phosphorus will pass through the system untreated. Adding too much means unreacted chemicals will leak out of the system, into the environment. Most important, however, is to make sure that there is a continuous replenishment of chemicals; if the system runs dry on chemicals there will be no phosphorus binding at all. None whatsoever! If chemicals are always available, and the system always well-tuned, phosphorus will be caught, i.e. the wastewater will be purified from phosphorus. But there is very little recycling potential in this phosphorus; partly because the chemical binding will be very strong, and secondly because it will be mixed with sludge (only approx. 20% of wastewater sludge is recycled in Sweden).
3. Use of Reactive filter media, Polonite®
Polonite is entirely based upon a natural mineral called Opoka, mainly composed of calcium silicate. The Opoka is excavated in a quarry. It is crushed and sieved into suitable fractions, 2 – 6 mm, and made reactive by exposing it to heat. ‘Reactive’ means it’s pH has been increased. In this way it will effectively bind phosphorus in the water passing through the material.
New Polonite® will have a pH >12 and it will have a strong phosphorus binding function down to pH around 8. Wastewater is passing through the Polonite® media as a natural flow, propelled only by natural gravitation. No electricity or other external resources are needed, which translates into a very robust solution for taking care of phosphorus in the wastewater. Unlike solutions based upon chemical precipitation, there is no risk of the system ‘running dry’, but a slow process over several months where the phosphorus reduction is gradually going down.
When the Polonite® is saturated, the entire volume is replaced with new. Thanks to the higher pH, pathogens are effectively eliminated as water pass through the filter media. Saturated Polonite® consist of lime-mineral and phosphorus. After drying in the air, it’s easily spread on farmlands. Most of the phosphorus is quickly released to the soil.