High-rate Thermophilic Bio-Methanation of the Fine Sieved Fraction From Dutch Municipal Raw Sewage

Most wastewater treatment plants (WWTPs) are trying to reduce fossil fuel consumption to a minimum. For achieving energy neutrality at conventional activated sludge systems a multistep approach is required that firstly addresses the optimization of energy use in aeration, pumping of sludge and recycle flows, and sludge dewatering. Secondly, the recovery of chemically bound energy should be maximized, requiring an upgrade of sludge digestion facilities when anaerobic digestion (AD) is applied. Thirdly, at those WWTPs that are so far not equipped with AD, such as (small scale) extended aerated biological nutrient removal plants, cost-effective AD systems need to be implemented. The reviewed paper particularly addresses the third category.

An efficient way of minimizing oxidation of filterable matter in extended aeration tanks is the recovery of cellulose-rich slurries from raw sewage with a fine mesh (< 500 μm) sieve. Subsequent, on-site biomethanation of the recovered fine sieved fraction (FSF) at high dry solids content, could significantly contribute to minimize fossil energy requirements at these WWTPs.

A new study published in the journal, Applied Energy, researched the bio-methane potential (BMP) of FSF from municipal raw sewage, which is an energy-rich material that contains mainly cellulosic fibers originating from toilet paper. The research was led by Professor Jules B. van Lier, Dr. Dara S.M. Ghasimi and Dr. Merle de Kreuk from Delft University of Technology in the Netherlands. Other scientists involved in this study are, Dr. Sung Kyu Maeng from Sejong University and Dr. Marcel H. Zandvoort at Waternet, Amsterdam (the Netherlands).

For AD, thermophilic (50-60⁰C) or mesophilic (30-40⁰C) conditions can be chosen. Mesophilic AD of organic solids is often reported as the most convenient, stable and reliable form of substrate conversion leading to stable methane production rates. However, mesophilic hydrolysis rates are lower than thermophilic conversion rates, since the rate of many, if not most, (bio)chemical reactions double as the reaction temperature increases by 10⁰C. Moreover, the high temperature in thermophilic reactors will increase substrate solubility and decrease bulk liquid viscosity, leading to an improved mixing performance and thus increased digestion rates.

It’s well known that at a fixed solids retention time (SRT) a thermophilic digester produces more methane per weight of biomass per time unit than the mesophilic counterpart. In addition to SRT, factors such as substrate loading potential, bio-methanation production rates, as well as maximum substrate conversion rates, are parameters required for the design and operation of an AD plant. Moreover, the inoculum to substrate ratio (R_i/s) is considered a crucial parameter for the operation and design of batch-wise or plug flow operated AD processes.

For implementation of the experiment, FSF was collected from a 350 μm mesh fine sieve at WWTP Blaricum, the Netherlands, and stored at 4⁰C prior to perform BMP tests. Thermophilic inoculum was obtained from a plug flow dry anaerobic composting (DRANCO, OWS, Brecht, Belgium) digester, operated at an SRT of 15 days and treating mainly vegetable, fruit and yard wastes with a dry matter content of about 35%. Mesophilic inoculum was taken from an anaerobic digester of a WWTP (Harnaschpolder, Delft, the Netherlands) that treats both primary and secondary sludge with a maximum solids content of 5% and which was operated at an SRT of 22 days. Both inoculum and FSF were fully characterized and the BMP of FSF was assessed. Results were used to evaluate the potentials for full-scale application considering a horizontal plug flow reactor with recirculation taking the following aspects into account: specific heat capacity of FSF, heat and temperature control, heat exchanger, heat requirement, heat loss, electrical energy requirement for mixing and pumping and energy consumption for FSF digestate dewatering and sludge drying.

Under mesophilic conditions, proper BMP assessment was only possible applying Ri/s ratios of 3 and higher, leading to a BMP value of about 300 mLCH₄/gVS_added. Under thermophilic conditions, all applied R_i/s ratios, even the very low ratios of 0.5 and 1, resulted in a good degradation of FSF. Obtained BMP values were slightly higher than those obtained under mesophilic conditions.

The observed specific methane production rate under thermophilic conditions was always higher than under mesophilic conditions, at all applied R_i/s ratios. Highest observed specific methane production rates were 67 and 189 mLCH₄/gVS_added ( 0.19 and 0.54 gCOD/gVS_inoculumd) for mesophilic and thermophilic conditions, respectively.

Maximum biogas production rates of 1 and 6.2 m³/m³d were calculated based on an average methane composition of 53% and 57% for thermophilic and mesophilic conditions, respectively.

Calculations for applying full-scale FSF (TS ≈ 23%) digestion showed the potential for a net recoverable energy of 287 MJ/ton FSF and 237 kWh electric/ton FSF. In these calculation the following energy consuming units were taken into account: fine sieving operation (132 kWh/d), digester operation (29 kWh/d and 445 MJ/d as heat), FSF dewatering from 9% to 20% TS        (34 kWh/d) and sludge drying (3133 MJ/d as heat).

This study showed that thermophilic adapted sludge is more appropriate for FSF biodegradation than mesophilic sludge, owing to higher apparent hydrolysis rates, higher specific methane production rates, higher bio-methane potential and possibility to apply lower inoculum-substrate ratios.