Phosphorus recovery from waste biomass is becoming increasingly important, given that phosphorus is an exhaustible non-renewable resource. For the recovery of plant nutrients and production of climate-neutral fuel from wet waste streams, hydrothermal carbonization (HTC) has been suggested as a promising technology. In this study, digested sewage sludge (DSS) was used as waste material for phosphorus and nitrogen recovery. HTC was conducted at 200 °C for 4 h, followed by phosphorus stripping (PS) or leaching (PL) at room temperature. The results showed that for PS and PL around 84% and 71% of phosphorus, as well as 53% and 54% of nitrogen, respectively, could be recovered in the liquid phase (process water and/or extract). Heavy metals were mainly transferred to the hydrochar and only <1 ppm of Cd and 21–43 ppm of Zn were found to be in the liquid phase of the acid treatments. According to the economic feasibility calculation, the HTC-treatment per dry ton DSS with an industrial-scale plant would cost around 608 USD. Between 349–406 kg of sulfuric acid are required per dry ton DSS to achieve a high yield in phosphorus recovery, which causes additional costs of 96–118 USD. Compared to current sewage sludge treatment costs in Switzerland, which range between 669 USD and 1173 USD, HTC can be an economically feasible process for DSS treatment and nutrient recovery.

Hydrothermal carbonization (HTC) technology can convert various types of waste biomass into a carbon-rich product referred to as hydrochar. In order to verify the potential of hydrochar produced from stabilized sewage sludge to be an adsorbent for bacterial pathogen removal in water treatment, the Escherichia coli’s removal efficiency was determined by using 10 cm sand columns loaded with 1.5% (w/w) hydrochar. Furthermore, the removal of E. coli based on intermittent operation in larger columns of 50 cm was measured for 30 days. Since the removal of E. coli was not sufficient when the sand columns were supplemented with raw hydrochar, an additional cold-alkali activation of the hydrochar using potassium hydroxide was applied. This enabled more than 90% of E. coli removal in both the 10 cm and 50 cm column experiments. The enhancement of the E. coli removal efficiency could be attributed to the more hydrophobic surface of the KOH pre-treated hydrochar. The idle time during the intermittent flushing experiments in the sand-only columns without the hydrochar supplement had a significant effect on the E. coli removal (p < 0.05), resulting in a removal efficiency of 55.2%. This research suggested the possible utilization of hydrochar produced from sewage sludge as an adsorbent in water treatment for the removal of bacterial contaminants.

Soil moisture sensors can help to reduce the amount of water needed for irrigation. In this paper we describe the PlantCare soil moisture sensor as a new type of heat dissipation sensor, its calibration and the correction for temperature changes. With the PlantCare sensor it is possible to measure the matric potential indirectly to monitor or control irrigation. This sensor is based on thermal properties of a synthetic felt. After a defined heating phase the cooling time to a threshold temperature is a function of the water content in the synthetic felt. The water content in this porous matrix is controlled by the matric potential in the surrounding soil. Calibration measurements have shown that the sensor is most sensitive to −400 hPa and allows lower sensitivity measurements to −800 hPa. The disturbing effect of the temperature change during the measurement on the cooling time can be corrected by a linear function and the differences among sensors are minimized by a two point calibration.

In recent years, promising developments in the hydrothermal carbonization (HTC) of sewage sludge, as well as the potential to reclaim phosphorus and nitrogen, have emerged. In this study, the HTC of digested sewage sludge (DSS) was investigated for the downstream production of heavy metal (HM)-free fertilizer and the use of freeze concentration (FC) as a novel technology for process water treatment. To obtain clean fertilizer, phosphatic acid extracts were first treated with ion-exchange resins to remove dissolved HM, as well as phosphorus precipitating agents (i.e., aluminum and iron). Over 98% of the aluminum (Al) and 97% of the iron (Fe) could be removed in a single treatment step. The purified extract was then used for the precipitation of HM-free struvite crystals, with P-recovery rates exceeding 89%. Process water (PW) makes up the largest share of the two main HTC-products (i.e., hydrochar and PW) and is very rich in organic compounds. Compared to evaporation or membrane separation, FC is a promising technology for concentrating solutes from PW. Separation experiments resulted in the recovery of over 90% of the dissolved compounds in the concentrate. In our study, the concentrate was later utilized as an ammonium source for struvite precipitation, and the subsequent aerobic digestion of the remaining ice water resulted in an 85% reduction in chemical oxygen demand (COD) in 15 days.