Phosphorus Recycling: Sustainable Resource Recovery from Sewage Sludge from 2029
From 2029, the recovery of phosphorus from municipal sewage sludge will be mandatory for operators of wastewater treatment plants. This development marks an important step towards the sustainable use of resources, as natural phosphorus deposits are limited and are mainly concentrated in a few countries such as Morocco, China, the USA and Russia. In Europe, there are only small deposits in Finland and Norway. The European Union has therefore classified phosphorus as a strategically important element and is calling for phosphorus recycling from sewage sludge in order to conserve resources and reduce dependence on imports from crisis areas.
Potential and methods of phosphorus recovery
The potential for recovering phosphorus from sewage sludge in Germany amounts to 50,000 tonnes per year, which corresponds to more than 40 per cent of current agricultural demand. Dried sewage sludge contains between 2 and 55 grams of phosphorus per kilogram. With an annual volume of 1.8 million tonnes of sewage sludge in Germany, phosphorus recycling provides a considerable resource. From 2032, phosphorus recycling will also be mandatory for smaller plants. People and animals ingest phosphates daily through food and excrete most of them. These phosphates can be found at the end of the water treatment process in municipal and industrial sewage sludge.
The direct agricultural use of sewage sludge as a cost-effective alternative to mineral fertilisers has largely been discontinued because, in addition to phosphorus, it also contains substances of concern such as heavy metals, pesticides, pathogenic germs, pharmaceutical residues and microplastics. The composition of this diverse mixture can vary greatly depending on the source and the season. In 2022, more than 80 per cent of municipal sewage sludge was already being thermally recycled; direct use as fertiliser is only approved in individual cases.
Technical Process for Phosphorus Recovery
The sewage sludge ash produced during thermal utilisation contains phosphorus in the form of poorly soluble compounds. According to the new Sewage Sludge Ordinance, at least 80 percent of the phosphorus contained in the ash must be recovered. In the simplest case, it can be used directly as a fertiliser, provided that the limit values according to the Fertiliser Ordinance are adhered to. Various thermochemical processes make it possible to separate heavy metals by adding alkali or alkaline earth chlorides at temperatures between 850 and 1000 degrees Celsius. The addition of alkali salts also improves plant availability by breaking down sparingly soluble minerals.
An alternative process uses diluted mineral acids to break down the ash, which also improves the plant availability of the nutrients. The addition of acid reduces the pH value and leads to improved nutrient release in the soil, especially in the case of alkaline sewage sludge. A combination with nitrogen compounds leads to a biologically valuable increase in the nitrogen-sulphur ratio due to the input of sulphate. The heat of this reaction has a positive effect on the subsequent drying processes.
The recovery of phosphoric acid by reacting the ash with concentrated sulfuric acid is particularly efficient. After extraction and distillation, a chemically pure phosphoric acid with an 85 per cent concentration is obtained. The remaining insoluble components, such as sand, silicates and gypsum, which accumulate as a filtration residue, can also be isolated and used as a valuable raw material in the cement and building materials industry. In fact, the resulting products can be recycled.
Source: trade journal ‘CAV’
Photo: Hien Phung