City of Honolulu, Hawaii Sand Island Egg-Shaped Digester Start-Up Considerations

The City and County of Honolulu contracted with Synagro Technologies, a privately owned company, to Design, Build, and Operate (DBO) an “In-Vessel Bioconversion Facility” (Facility) located at the Sand Island Wastewater Treatment Plant in Honolulu, Hawaii. The Facility consists of an Egg-Shaped anaerobic digester, centrifuge de-watering, and a heat drying and pelletization facility. The project is highly dependent on the successful operation of the digester to condition the municipal sludge and to produce biogas in sufficient quantities in order to minimize the purchase of fuel oil. Initial digester operations consisted of the micro-digester start-up technique utilizing seed sludge from an on-island source, and maintaining the hydraulic retention time between 18 and 25 days. Initial process parameters that were monitored to evaluate digester health consisted of pH, alkalinity, volatile acids, biogas CO2, and biogas H2S. Shortly after introduction of the Sand Island WWTP feedstock, alkalinity and pH began to drop along with a corresponding rise in volatile acids. Hydrogen sulphide concentrations in the biogas, while always elevated, began a rapid ascent into the “extremely inhibitory” range. Immediate corrective action was taken including a reduction of Sand Island feedstock, addition of supplemental seed material, control of pH and alkalinity, and chemical addition to address elevated sulphides. As a result of this action, digester operations stabilized and demonstrated continuous improvement. Continuing the aggressive corrective actions, however, was not in the long-term interest of the project. An understanding of the toxic and/or inhibitory conditions within the digester needed to be identified and acceptable long-term corrective action required implementation. The long term corrective actions, which include ferric chloride addition for H2S control and potassium hydroxide addition as a micro-nutrient supplement, have completely reversed disastrous early conditions and allowed the facility to consistently produce high quality biosolids pellets for the Hawaiian market.