Agriculture affects both the quantity and the quality of water available for other purposes, which becomes problematic, especially during increasingly frequent severe droughts. This requires tapping into the resources that are typically neglected. One such resource is a by-product of anaerobic digestion, in which moisture content typically exceeds 90%. Application of hydrothermal carbonization process (HTC) to this residue could partially remove organic and inorganic material, improve dewatering, decrease the overall solid mass, sanitize the digestate, change its properties, and eliminate problems related with emissions of odors from the installation. However, a significant gap still exists in terms of the dewatering of the hydrochars and the composition of the effluents. This work presents results of experimental investigation focused on the removal of organic compounds from the HTC effluent. Results of qualitative and quantitative analysis of liquid by-products of HTC of the agricultural digestate showed that acetic acid, 3-pyridinol, 1-hydroxyacetone, and 1,3-propanediol were the main liquid organic products of the process. Application of ultrafiltration process with the use of 10 kDa membrane for liquid HTC by-product treatment allows for the reduction of chemical oxygen demand up to 30%, biological oxygen demand up to 10%, and dissolved organic carbon up to 21%.

The organic fraction of waste is increasingly used for biogas production. However, the fermentation process used for this purpose also produces waste in the form of digestate in addition to biogas. Its liquid fraction can, among other things, be a source of water, but its recovery requires many advanced technological processes. Among the first in the treatment train is usually coagulation/chemical precipitation. Its application changes properties, including the size and zeta potential (ζ) of the fractions that have to be removed in subsequent processes. Changes in particle size distribution and ζ potential occurring in the liquid fraction of municipal waste biogas plant digestate and solutions after coagulation/chemical precipitation with FeCl₃·6H₂O, PIX 112 and CaO were analyzed. The particle size distribution of the raw digestate was wide (0.4–300 µm; up to 900 µm without ultrasound). The median particle diameter was about 12 µm. The ζ potential ranged from −25 to −35 mV in the pH range 5–12, and the isoelectric point (IEP) was at pH 2. The best treatment results obtained with the use of. 10 g FeCl₃∙6H₂O/dm³ shifted particle size distribution towards finer particles (median diameter: 8 and 6 µm, respectively, before and after ultrasound). The ζ potential decreased by about 5–10 mV in the pH range 2.5–12 without changing IEP. An amount of 20 g/dm³ of FeCl₃∙6H₂O caused the disappearance of the finest and largest fraction. d₅₀ was about 21.5 µm (17.3 µm after ultrasound). An amount of 20 g/dm³ of FeCl₃∙6H₂O generated a positive high electrokinetic potential in the range of pH 1.8–5. The IEP appeared at pH 8, and after reaching about −5 mV it again became positive at pH about 11.

New regulations aimed at curbing the problem of eutrophication introduce limitations for traditional ways to use the by-product of anaerobic digestion—the digestate. Hydrothermal carbonisation (HTC) can be a viable way to valorise the digestate in an energy-efficient manner and at the same time maximise the synergy in terms of recovery of water, nutrients, followed by more efficient use of the remaining carbon. Additionally, hydrothermal treatment is a feasible way to recirculate recalcitrant process residues. Recirculation to anaerobic digestion enables recovery of a significant part of chemical energy lost in HTC by organics dissolved in the liquid effluent. Recirculating back to the HTC process can enhance nutrient recovery by making process water more acidic. However, such an effect of synergy can be exploited to its full extent only when viable separation techniques are applied to separate organic by-products of HTC and water. The results presented in this study show that using cascade membrane systems (microfiltration (MF) → ultrafiltration (UF) → nanofiltration (NF)), using polymeric membranes, can facilitate such separation. The best results were obtained by conducting sequential treatment of the liquid by-product of HTC in the following membrane sequence: MF 0.2 µm → UF PES 10 → NF NPO30P, which allowed reaching COD removal efficiency of almost 60%.

One of the processes that can serve to valorise low-quality biomass and organic waste is hydrothermal carbonization (HTC). It is a thermochemical process that transpires in the presence of water and uses heat to convert wet feedstocks into hydrochar (the solid product of hydrothermal carbonization). In the present experimental study, an improvement consisting of an increased hydrophobic character of HTC-treated biomass is demonstrated through the presentation of enhanced mechanical dewatering at different pressures due to HTC valorisation. As part of this work’s scope, flashing-off of low-quality steam is additionally explored, allowing for the recovery of the physical enthalpy of hot hydrochar slurry. The flashing-off vapours, apart from steam, contain condensable hydrocarbons. Accordingly, a membrane system that purifies such effluent and the subsequent recovery of chemical energy from the retentate are taken into account. Moreover, the biomethane potential is calculated for the condensates, presenting the possibility for the chemical energy recovery of the condensates.

Geosmin and 2-methylisoborneol (MIB) are the most common causes of unpleasant odours in drinking water. A method was proposed to detect and recognise these compounds in water and determine their concentrations. The method utilises commercial solid-state gas sensors and data analysis. Sample preparation plays an important role. The aqueous solution is converted into a gas sample using a specially designed dynamic headspace. The responses of the sensors are recorded during exposure to headspace vapours in a dynamic mode. The best limit of detection for geosmin, LOD = 6.20 µg/L, was attained with a TGS2602 sensor. The best limit of detection for MIB, LOD = 0.52 µg/L, was attained with a TGS826 sensor. Geosmin and MIB recognition was 100% successful based on TGS826 and TGS2602 response classifications. Geosmin and MIB concentrations were effectively determined in solutions containing one or both compounds. The respective mathematical models utilised the responses of TGS826 and TGS2602. The smallest concentration prediction error was RMSE = 2.19 µg/L (for geosmin) and RMSE = 0.33 µg/L (for MIB). The study demonstrated the application potential of non-specific gas sensors for the early warning monitoring of geosmin and MIB presence in water. Further studies are needed to develop a system that can be tested in field conditions.