Analysis of sludge pretreatments for conventional anaerobic digestion wastewater treatment plants
Creators: name: Bordeleau, Etienne Louis
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2010
Subjects: subject: Engineering, Sanitary and Municipal.
subject: Engineering, Environmental.

description: Pretreatment of primary sludge, waste activated sludge, and comingled sludges has become of substantial interest for the improvement of the rate limiting hydrolysis step during anaerobic digestion. As primary sludges are already easily degraded, waste activated sludges provide the most noticeable improvements in anaerobic digestion (AD) as a result of sludge pretreatment (SPT). A vast literature and theory review was conducted to establish high-potential treatment and result ranges. The average wastewater compositions for total chemical oxygen demand (COD) and total solids (TS) were 16-43 and 13-40 g/L, respectively. Sludge pretreatment percent solids solubilisation ranges for all common SPT technologies were 13-42% for both particulate COD and total suspended solids (TSS). Subsequent anaerobic digestion enhancement ranges were percent reductions in total COD and TS of 24-55 and 22-46%, respectively, as well as biogas production increases of 9-43%. The ranges shown here were fairly representative of those observed for all SPTs. Additional SPT technology information for full-scale design was also gathered. A MS Excel spreadsheet wastewater treatment plant model (referred to herein as PretrAD) capable of comparing control and SPT incorporation scenarios was created. The PretrAD output was verified against a third party commercial model. PretrAD allows the user to input data on performance of all unit processes found in a typical biotreatment wastewater treatment plant (WWTP). Various return flow paths and other attributes of a WWTP can be specified and provide versatility and comprehensiveness. A database of typical performance data for all unit processes was compiled and used in PretrAD and for comparing performance of WWTPs with and without SPT. The stirred ball mill, high pressure homogenizer, and ultrasound technologies were mechanical processes with sufficient comparable data for inclusion in PretrAD. Conventional heating and microwave heating technologies were included as thermal processes. Alkaline and ozone technologies were included as chemical processes. Finally, the common conventional heating + alkaline (thermochemical) technology was also included alongside a purely financial assessment of a proposed microwave heating + alkaline technology incorporation scenarios. PretrAD was then used to evaluate treatment benefits by varying a number of performance parameters with and without SPT. Operating parameters that were changed were normal and low heat recoveries at the anaerobic digester, normal and doubled sludge disposal distances, low-peak influent flow regimes, and low to high SPT energy demands (evaluated with identical solubilisation results). Control and SPT scenarios were performed for all variables and both mesophilic and thermophilic AD. Overall net WWTP treatment costs based on energy inputs and recoveries along with chemical inputs were compared for the various scenarios. It was found that an all average influent flow regime represented (within 5%) all annual flow regime combinations of low, average, and high flows and their associated quality variations. Following the basic comparisons of control and SPT scenarios, additional runs were conducted with increases of 25 and 50% for the energy demands of each SPT. Mesophilic AD scenarios always had lower final costs than thermophilic AD scenarios under identical treatment parameters. Practical (cost-effective) and impractical (higher than control costs) scenarios were found for all SPTs except thermal and thermochemical processes. Thermal and thermochemical processes were always practic31 and always impractical, respectively, when compared to control scenarios using identical conditions. When different scenario conditions were compared, both thermal SPTs were deemed impractical when high cost results were compared to low cost results. Solids loadings and heat recoveries were the most cost-influential variables of PretrAD. Other important qualitative results were not incorporated into the evaluation. They include but are not limited to dewatering improvements, pathogen reduction, anaerobic digester vessel size reduction, and hydraulic residence time reductions for the anaerobic digester. Inclusion of these parameters could render some SPTs practical for scenarios where they were deemed impractical on a pure cost of treatment basis. Furthermore, full-scale incorporation drawbacks such as additional unit costs, operation and maintenance demands, and actual throughput capacities could render some practical scenarios impractical. The constructed PretrAD model has been proven effective for the rapid determination of treatment plant costs related to SPT incorporation. A tool such as this is vital for the site-specific analysis of SPT technologies.