Sometimes Average isn't Good Enough: Understanding the Impact of Variability on Energy Recovery from Digester Gas

This technical paper and presentation will share results from an evaluation of digester gas utilization options at the Muddy Creek Wastewater Treatment Plant (MCWWTP), Winston-Salem, NC. The evaluation revealed the importance of considering daily variation in digester gas production on the economic viability of on-site energy production. The MCWWTP uses digester gas to heat boilers which produce hot water for anaerobic digestion process heating and facility heating. Excess digester gas not required for process heating may be used to fuel two (2) reciprocating internal combustion engine (RICE) driven blowers. The RICE driven blowers are preferentially operated during week day on-peak demand periods to reduce the MCWWTP electricity consumption and operating costs. The facility's electricity rate contains a significant demand charge, $13.69 and $7.57 per kW each month, for the on-peak demand periods during the summer and winter respectively. However, the electricity rate has a relatively low energy charge, $0.0626 and $0.0382 per kWh during the on and off peak periods, respectively. The evaluation identified $48,000 of annual demand reduction savings when considering the annual average digester gas production. However, when historical daily variability in digester gas production was considered the facility was not expected to achieve any of the demand reduction savings. This paper will expand on the effect of digester gas variability on the MCWWTP's long term biogas utilization plan.