Water Journal : Water Journal May 2011
resource recovery refereed paper 94 MAY 2011 water technical features Abstract This paper highlights the results of a model which predicts potential renewable energy generation from sewage sludge based on a number of variables. Sludge type was the most influential parameter of those tested, with primary sludge generating over twice the energy of secondary sludge. As the type and quantity of sludge produced is fundamentally influenced by the wastewater treatment process, it is suggested that they be looked at in combination in order to maximise synergism. Pre-treatment technology and highly efficient co-generation were the next most beneficial factors. Introduction Plants for anaerobic digestion of sewage sludge have been traditionally designed to reduce the levels of harmful bacteria, odour and quantity of sludge to enable it to be stored or used as 'biosolids'. Biogas was of secondary importance. With the drive towards a low carbon economy, the value of energy recovery has assumed greater importance and new designs as well as operation of established plants are targeted at greater recovery of methane. This paper presents the outputs of a mathematical model based on both actual plant performance and bacteriological kinetics, set up to predict the influence of a number of parameters on the generation of biogas and ultimately renewable energy. Methods The backbone of the model was originally derived from correlations developed from data from a number of anaerobic digestion plants worldwide and has been presented previously (Barber, 2005a). The model used these correlations to predict volatile solids destruction against a number of variables including: sludge quantity and type; digester retention time, dimensions; material of construction; temperature of operation, etc. The destruction rate yields a basic figure for biogas production. The previous work has been updated here to include more factors which influence energy generation. The biogas production is now further altered depending on hydraulic retention time using kinetic models presented by Tong and McCarty (1991) and temperature of operation (Chae et al., 2008), presence/ absence of pre-treatment; parasitic energy requirements of pre-treatment; flare; and biogas bypassing for injection to grid. The revised gas production data is put forward to an energy recovery section of the model which then calculates power generation based on co- generation using a combined heat and power plant (CHP). The model calculates the heat required by the digestion plant and compares it to the heat available from the CHP. If deficient, either the additional energy required to heat the digestion plant is calculated or a revised energy generation is determined if biogas use is preferred. Heat requirements for the plant take into account any diversion of gas away from CHP (e.g. to a gas supplier), therefore it is possible to determine the maximum diversion before auxiliary fuel is required to heat the digesters. Baseline Conditions The baseline conditions for the study are summarised in Figure 1. For anaerobic digestion an operating temperature of 35°C (typical of mesophilic operation) has been assumed. In addition, a retention time of 16 days has been used with a void (or dead space) of 20%. Void space is that within the digester which is unavailable and may be due to hydrodynamics or inert material or both. The sludge type and composition has been based on typical figures and wastewater treatment configuration. With respect to the Combined Heat and Power plant (CHP), the baseline assumes that the plant is available for use 85% of the time, and that energy used within the plant is converted to electricity and heat at a rate of 33% and 45% respectively. All the baseline conditions were altered as variables to determine their influence. Under these conditions, 10,000 tonnes dry solids per annum generate 0.76MW of electricity (MWe). Results Dry solids concentration The concentration of dry solids (DS) in sludge fed into digesters is normally limited to approximately 6% DS due to non-newtonian flow behaviour, which makes mixing and handling increasingly difficult above this figure (Dawson et al., 2009). If rheology can be changed, however, the dry solids fed to a digester can be increased to a point where ammonia concentration influences loading (Wilson et al., 2009). Concentrations in excess of 10% have been successfully digested at full-scale by pre-teatment using thermal (Panter, and Kleiven, 2005) and acoustic hydrolysis (Barber, 2005b). In this study, the impact of adjusting dry solids from 2% to 7% is demonstrated in Figure 2. WPF Barber Modelling the influence of various parameters COGENERATION POTENTIAL FROM THE ANAEROBIC DIGESTION OF SLUDGE TDSA: 10,000 DS%: 5% VS%: 75% Primary: 60% Secondary: 40% CHP Anaerobic Digestion HRT (days): 16 Dead Space: 20% H:W ratio: 1.2:1 Temperature: 35°C Availability: 85% Electrical conversion: 33% Heat conversion: 45% Sludge for further processing Figure 1: Baseline conditions used in the study.
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