Korrespondenz Abwasser, Abfall Thema: Kreislaufwirtschaft oder Klärschlamm
Impact-Factor 2012 ???
Barbara Schritz
CHEMICAL DIGESTION OF SEWAGE SLUDGE BIOCHARS TO INCREASE PHOSPHATE
PLANT AVAILABILITY
Biochar, char, pyrolysis, chemical pulping, Phosphate-recycling, regional resource management, nutrition
management
Summary
Global trading and the extension respectively the specialization of agricultural enterprises makes it harder to
close regional nutrient cycles. The reservoirs of qualitatively high valuable rock phosphates are limited and
concentrated in just a couple of countries. It is reasonable for regional nutrient return to produce phosphate
fertilizers from regional sources such as sewage sludge.
Not all chemical bonds of phosphorus are equally plant available. Therefore rock phosphates are chemically
digested with acids to transform tertiary phosphate compounds which are not plant available to primary or
secondary phosphates which can be used directly by plants. To prevent wasting of resources only plant
available phosphorus should be used as fertilizer.
Waste volumes can be reduced through the carbonization of sewage sludge which goes along with a
phosphorous concentration increase in the char. Carbonisation with the PYREG® system has the additional
benefit of a decentralized sanitation of the sludges. Equally to rock phosphates are those phosphorous
compounds not soluble to be used by plant. According to the phosphorous digestion of rock phosphates the
sewage sludge chars were treated with acid sulfur (H2SO4), hydrochloric acid (HCl), nitric acid (HNO3) and
formic acid (CH2O2). The solubility was tested with several acknowledged extraction methods.
As a first step the four acids were mixed with the char (0,5ml acid for 1g of char) to identify the phosphorus
digestion potential of the acids. The digestion with sulfuric acid was most promising because the formic acid
solubility was raised to 39% in comparison to the untreated char. In a second step to little acid application
rates were chosen (2,5%, 5%, 10% w acid / w char) which resulted in a decline of the solubility with
ammonium citrate and citric acid for all four acids. Probably the pH was not reduced sufficient to enable a
digestion of the phosphorous. As a third step the ideal sulfuric acid amount was ascertained with a wider grid
under surveillance of the pH. The pH was reduced below 2 with a sulfuric acid in a concentration of 50%,
resulting in a water solubility of the phosphorous of 72%. Whereas the untreated char had a water solubility
lower than one percent. Analysis show a positive digestion of phosphorus with sulfuric acid, whether the
phosphorous truly is available to plant will be tested in a field trial.