Increasing biogas yield through addition of food waste

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1. Introduction

The Moosburg wastewater treatment plant has one 634,008 gallons anaerobic digester, which was designed to process a maximum 23,775 gallons/day of activated sludge at 3.5% solids. The AD is currently processing 17,170 gallons/day of sludge and producing 39decatherms/day of energy from biogas. The operators would like to better utilize their anaerobic digester to achieve energy self-sufficiency by supplementing their excess AD capacity with high energy food waste. Finsterwalder Umwelttechnik, Yield’s European partner, was brought in to simulate & analyze the impact of adding food waste to their anaerobic digesters. The final step was to design the optimal technical and financial solution for the plant.

Study Objectives

  1. Determine how to increase the overall capacity of the digester
  2. Determine how much food waste can be digested with this excess capacity considering the operating parameters of the mixing equipment and nitrogen load.
  3. Design a method to remediate the extra nitrogen that is created by digesting the food waste.
  4. Determine the expected gas output to assist in sizing the gas generator equipment.
  5. Determine the sensitivity of the system to failure

Methodology

Traditional methods for analyzing & optimizing the Anaerobic Digestion process consist of experimenting with lab equipment or making process changes in a working plant. This has proved to be very costly and time consuming.

Finsterwalder has developed a computer simulation called BioTip that simulates the anaerobic digestion process. This model and its conclusions have been validated by the data from many actual WWTP anaerobic digesters. The simulation is able to manage multiple organic streams of material with different loading schedules. Based on the compositions (carbohydrates, proteins, fats, water content) of the material the simulation will calculate the biogas yield, methane yield, concentration of organic acids, the PH value, the buffer capacity, organism concentration and the growth rate. The simulation models the four concurrent AD processes (Appendix A) Hydrolysis, Acidogenesis, Acetogenesis, and Methanogensis in 1 minute increments until the system achieves steady state or fails. Retention time has often been difficult to predict, as the AD system is very complex. The model can work with complex inputs and schedule and accurately predict steady state, which aids in determining retention time.

This study utilized the BioTip to simulate two process changes to the Moosburg Anaerobic Digester. One of the areas of investigation was the thickening of the sludge to increase gas production and create excess capacity in the AD. The impact of the addition of 24.2 tons per day of food waste (the available food waste) was also studied.

Simulations Outputs:

  1. Biogas Production
  2. Methane Production
  3. Concentration of Volatile Organic Solids
  4. Buffer Capacity – ability of system to keep pH stable

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