The Ultimate Combination of Sustainable Biosolids Treatment Technologies

The Hawaiian island of Oahu has nine wastewater treatment facilities operated by the City and County of Honolulu (CCH), two of which treat the majority of the flows on the island, the Honouliuli Wastewater Treatment Plant (Honouliuli) and the Sand Island WWTP (Sand Island). CCH is currently under consent order to upgrade Honouliuli and Sand Island to full secondary treatment by 2024 and 2035, respectively. In addition, CCH is converting Honouliuli into a regional biosolids processing and drying facility for many of the island’s treatment facilities. The design of resource recovery facilities in Hawaii has a unique set of project drivers as compared to the mainland. Specifically, power costs are very high (0.25 to 0.35 per kW-hr) on Oahu as are solids disposal costs [220/wet metric tonne (≈200/wet ton)]. The high operating costs of these facilities open up design alternatives that may not have reasonable payback periods on the mainland, but may be reasonable in Hawaii.The CCH is considering treatment systems for the Honouliuli project that will have lower annual operating costs and approach energy neutrality. Key design considerations for the liquid treatment processes were A-stage treatment to divert carbon from the liquid to solids streams, managing nitrification (ammonia based aeration control) to reduce aeration requirements, and advanced energy-saving aeration systems. The solids treatment systems are critical in approaching energy neutrality with the goal to both minimize biosolids production and maximum biogas production. The design analysis included the consideration of thermal hydrolysis process (THP) and enhancing the existing anaerobic digestion process, along with the usual dewatering and the inclusion of drying facilities at this WWTP. If THP is selected, digested and dewatered biosolids from the other treatment plants will be combined with thickened Honouliuli biosolids and treated by this process. Extensive process modeling demonstrated that passing the imported solids through THP and additional digestion will provide additional biogas that can be used for combined heat and power (CHP) energy recovery. Additionally, the THP processed biosolids will achieve a significantly higher percent solids in the dewatered product feeding the dryers, thus enabling the dryers to be downsized. Waste heat from the CHP system will be the primary heat source for the low-temperature belt dryer. No biogas is expected to be needed for drying since the belt dryer process can use almost all of the heat produced by a CHP system. Excess biogas or natural gas will be used to supply the steam generation needs of the THP process. Energy mass balance calculations confirmed that the planned water resource recovery system will significantly reduce net energy demands and approach, but not reach energy neutrality.