The Stuttgart Process (Germany)

The replacement of maize (Zea mays L.) by cup plant (Silphium perfoliatum L.) as biogas substrate and its implications for the energy and material flows of a large biogas plant

“…According to the approach by Meyer et al ., the P 2 O 5 content in the permeate (struvite) is approximately 25%–28%, which is close to the composition of pure struvite . For the calculation of struvite extraction potentials at the EPH, phosphate content of 25% in the solid recyclate is assumed accordingly.…”

“…In the beginning, citric acid is added to the hydrochar to initiate phosphorus leaching at a pH of 2–5 . The acidic sludge is filtered and the permeate is precipitated in a further reactor where the struvite precipitation takes place.…”

“…The acidic sludge is filtered and the permeate is precipitated in a further reactor where the struvite precipitation takes place. The retained material, which still contains considerable amounts of P, is used again in the process of acid leaching . NaOH is added to the permeated substance in the third reactor to increase the pH to 8–9 and mixed with the ammonium‐rich process liquid originating from the HTC treatment.…”

“…The choice and concentration of acid, base, and magnesium source in the two process steps as well as the influence of the pH are topics of current research . They influence the recovery time and the rate of recovered struvite, and are very relevant cost factors . The related literature would have to be reviewed and the process parameters adapted for the case of EPH.…”

“…During HTC treatment the P contained in the digestate can be bound in the sanitized hydrochar and leached into the process water once its pH is reduced. By struvite precipitation and an additional filtration step, 60%–90% of the phosphate (as P 2 O 5 ) can be removed as a solid while the P content in the remaining liquid is reduced …”