Effect of NH₃ and organic loading on the inhibition of mesophilic high-solid digestion

BACKGROUND The construction of small digesters will become more important for regions with demographic increase and limited areas. Savings of digester volume and a subsequently smaller footprint can be realized by the treatment of a more thickened raw sludge (high-solid digestion). However, this operation can lead to instabilities of the anaerobic process due to rising ammonium and ammonia levels. The aim of the study was to identify the limit of saving digester volume by increasing the total suspended solids (TSS) of the treated raw sludge. RESULTS The effects of high TSS and ammonia levels in the digested sludge were investigated in lab-scale reactors. Ammonium nitrogen (NH4-N) levels were set by dosing a urea solution as well as by feeding a higher concentrated raw sludge. Chemical oxygen demand (COD) removal declined from 64 to 54% with NH4-N levels rising from 2000 to 3200 mg L-1. However, the anaerobic biodegradation was not completely interrupted, which indicates the possible adaptation of anaerobic bacteria on high NH4-N levels. CONCLUSION From the results, it can be concluded that dewatering the raw sludge up to 7% TSS leads to an optimum benefit with maximum savings in digester volume. The benefits of the operation with high solids are a lower energy demand for heating up the raw sludge and reduced construction costs (-20%) due to smaller required digester volumes. (c) 2019 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.