Influence of Post- and Pre-Acid Treatment during Hydrothermal Carbonization of Sewage Sludge on P-Transformation and the Characteristics of Hydrochar

Abstract

Phosphorus (P) recovery from alternative P-rich residues is essential to meet the growing demands of food production globally. Despite sewage sludge being a potential source for P, its direct application on agricultural land is controversial because of the obvious concerns related to heavy metals and organic pollutants. Further, most of the available P recovery and sludge management technologies are cost-intensive as they require mandatory dewatering of sewage sludge. In this regard, hydrothermal carbonization (HTC) has gained great attention as a promising process to effectively treat the wet sewage sludge without it having to be dewatered, and it simultaneously enables the recovery of P. This study was conducted to analyse and compare the influence of acid (H2SO4) addition during and after HTC of sewage sludge on P leaching and the characteristics of hydrochar. The obtained results suggested that despite using the same amount of H2SO4, P leaching from solid to liquid phase was significantly higher when acid was used after the HTC of sewage sludge in comparison with acid utilization during the HTC process. After HTC, the reduction in acid-buffering capacity of sewage sludge and increase in solubility of phosphate precipitating metal ions had a greater influence on the mobilization of P from solid to liquid phase. In contrast, utilization of H2SO4 in different process conditions did not have a great influence on proximate analysis results and calorific value of consequently produced hydrochar.

Process Waters from Hydrothermal Carbonization of Waste Biomasses like Sewage Sludge: Challenges, Legal Aspects, and Opportunities in EU and Germany

“…The results showed that water only had a minor effect on extraction, whereas the addition of sulfuric acid can leach significant amounts of phosphorus into the process water during HTC [44]. Shettigondahalli et al [56] again conducted HTC trials with sewage sludge and the addition of sulfuric acid before and after HTC. The authors observed that, in contrast to acid leaching after HTC, a significantly lower concentration of phosphorus was converted from the solid phase to the liquid phase by the addition of sulfuric acid before HTC [56].…”

“…Shettigondahalli et al [56] again conducted HTC trials with sewage sludge and the addition of sulfuric acid before and after HTC. The authors observed that, in contrast to acid leaching after HTC, a significantly lower concentration of phosphorus was converted from the solid phase to the liquid phase by the addition of sulfuric acid before HTC [56].…”

“…Similar effects could be observed with the HTC of sewage sludge. Phosphates were transferred and accumulated into hydrochar alongside several heavy metals [56], whereas nitrogen is transferred to the process water as ammonium nitrogen and organic nitrogen [97,98]. If PW contains high nutrient loads, recovery may be worthwhile.…”

Effect of process parameters on phosphorus conversion pathways during hydrothermal treatment of sewage sludge: A review

“…A large number of studies in Table 1 have also investigated the effect of pH adjustment on P speciation and migration during HT treatments of SS [7,33,48,52,66,72,75,81,91,[103][104][105][106]. Indeed, adsorption, precipitation, and other P-related chemical reactions are considerably affected by pH [107].…”

“…NaOH is generally used to increase the medium pH [48,52,66,72,81,91,103]. Acid solutions used in the selected works include hydrochloric acid [7,52,66,72,91], nitric acid [33,48,104], sulfuric acid [75,103,105], formic acid [103], acetic acid [81,103], and citric acid [106].…”