Kinetic Studies on Readily Biodegradable Substrates by the Anaerobic digestion process

Redzwan, Ghufran (2001) Kinetic Studies on Readily Biodegradable Substrates by the Anaerobic digestion process. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The aim of the study was to evaluate the possibility of using a high rate anaerobic process to convert the soluble hydrolysis and acidification products from a first phase solid substrate anaerobic digester treating the organic fraction of municipal solid waste (OFMSW). To achieve this a comparative evaluation of the kinetics methods for predicting the effluent (S_e) soluble substrate concentration was undertaken. The methodology was developed by conducting anaerobic treatability studies on a readily degradable soluble wastewater, this was then extended to studies using a solids free leachate derived from the hydrolysis and acidification of OFMSW, and finally to a mixed soluble/suspended solids phase leachate produced by a high rate hydraulic flush bioreactor treating OFMSW.

Using the readily degradable wastewater two sets of experiments were undertaken to assess the kinetics of both a batch operation and fed-batch operation. These treatability studies were carried out at a laboratory scale in stirred tank reactors. Batch operation using this wastewater was inherently unstable and it was demonstrated that the problems were due to both a nitrogen deficiency in the wastewater and a lack of natural buffering capacity. Stable operation could be maintained by supplementing the buffering capacity by daily addition of NaHCO₃ and NH₃HCO₃ at 250-400 mg.I^-1.d^-1 and 10 mg.I^-1.d^-1 respectively to the batch system, this also provided sufficient nitrogen to maintain a healthy bacterial population. The kinetics of the batch reaction were best described using the equation S_e = S_o^kt (S_o = influent; t=time), the constant k was equal to 0.02h^-1 under normal opening conditions. For batch operation, an estimate of the maximum gas production (G_m) could be make using the specific function G=G_m^k/t (G= gas production). Statistically, this gave a better estimate of G_m than other known methods, in addition the method developed was more straightforward.

For the fed-batch reactor treating the readily degradable wastewater, a Michaelis-Menten kinetic approach was adopted and the reaction was proven to be of a first order, except at high loadings (>1.3 kg.m³.d^-1). The effluent soluble substrate could be predicted with confidence using the equation S_e= S_o / [1 + kHRT] (HRT - hydraulic retention time). An organic loading rate (OLR) of 1.4 kg.m³.d^-1 could be treated at a removal efficiency (E_r) of 92%. For a fed-batch operation, the constant k in the first order model when applied to the readily biodegradable wastewater was equal to 1.25d^-1. The coefficients r_m and K_m for the Michaelis-Menten kinetic equation were equal to 1700 mg.l^-1.d^-1 and 310 mg.l^-1.