Removal of Organic Pollutants from Effluent of Anaerobic Digester Using Hydrochars Produced from Faecal Simulant and Sewage Sludge

Hydrochars produced from hydrothermal carbonisation of faecal simulant (FS) at 180 °C for 30 min and sewage sludge treated via CAMBI process at 165 °C for 30 min were used for adsorption of organic pollutants in effluent from an anaerobic digester (AD). The adsorption potential of the hydrochars investigated was compared with that of a commercial powdered activated carbon (PAC). It was found that the CAMBI and FS hydrochars were effective only after chemical activation. KOH activation increased chemical oxygen demand (COD) removal from 33.0 to 59.6% and 75.2% for FS and CAMBI, respectively. Extra activation with HCl improved the adsorption efficiency of FS, increasing COD removal to 79.3%, but not for the CAMBI hydrochar even though its surface area was increased. Acidic pH aided organics removal for both hydrochars. PAC adsorption capacity was the highest (> 90%), and this was not affected by pH. All the adsorbents could successfully remove the organics in a very short time as 30 min. The optimum dosage of the hydrochars to reach a high uptake of organics was 30.0 g/L. The adsorption reaction followed a pseudo-second-order kinetic model for all the hydrochars. The adsorption onto the hydrochars correlated well with Temkin isotherm model and followed a type II and IV isotherm type, except PAC which was described by the Langmuir isotherm with a high adsorption capacity of 400.0 mg/g. The study demonstrated that hydrochars of FS and CAMBI activated with KOH are efficient and sustainable adsorbents for the removal of organics from AD effluent.